GB2537148A

GB2537148A - Time of flight mass spectrometer

Info

Publication number: GB2537148A
Application number: GB1506033.8A
Authority: GB
Inventors: Kalinina Diana; Paul Daniel Lamb Richard
Original assignee: Kratos Analytical Ltd
Current assignee: Kratos Analytical Ltd
Priority date: 2015-04-09
Filing date: 2015-04-09
Publication date: 2016-10-12
Also published as: GB201506033D0; WO2016162658A1

Abstract

A time of flight mass spectrometer (TOFMS) having: an ion source 110; a detector 150; a variable voltage unit and a control unit wherein the control unit is configured to control the TOFMS to perform at least one acquisition cycle that includes operating the ion source to produce ions having a plurality of m/z values, wherein the ion source is operated so that ions having different m/z values strike the detector at different times; operating the detector to produce an output current representative of ions having different m/z values striking the detector; operating the variable voltage unit to apply a dynamic voltage waveform to the detector during the acquisition cycleso that the magnitude of the dynamic voltage waveform varies within the acquisition cycle, wherein the magnitude of the output current produced by the detector is dependent on the magnitude of the dynamic voltage waveform applied to the detector by the variable voltage unit. In one embodiment different voltages are applied to the detector at different times corresponding to different m/z values. This method improves the mass resolution of the mass spectrometer.

Description

TIME OF FLIGHT MASS SPECTROMETER

This invention relates to a time of flight ("TOP) mass spectrometer and in particular to the application of a dynamic voltage waveform to a detector in a TOF mass spectrometer.

A mass spectrometer is a well-known instrument commonly used for identifying a compound from the molecular or atomic masses of its constituents and/or to elucidate the structure of a molecule, by recording ions produced by ionising the compound/molecule.

An example TOF mass spectrometer 100 is shown in Fig. 1(a). This example TOF mass spectrometer 100 includes an ion source 110 configured to produce ions having a plurality of mass-to-charge ratio (m/z) values, a field free drift region 130 configured to separate ions produced by the ion source according to their m/z value, and a detector 150 configured to produce an output current representative of the relative abundance of ions having different m/z values striking the detector 150.

In the example TOF mass spectrometer of Fig. 1(a), the ion source 110 is a MALDI ion source that includes a laser 112 for ionising a sample carried on a sample plate by firing light at the sample. The MALDI ion source 110 may additionally include acceleration/extraction electrodes 114 for accelerating ions produced by the ion source and/or viewing optics 118 (which may include an illumination source) for viewing a sample under test. The TOF mass spectrometer 100 may include one or more ion optic components for manipulating ions produced by the ion source, e.g. for accelerating, decelerating, steering, deflecting, reflecting, focussing and/or re-focussing ions produced by the ion source 110. In the TOF mass spectrometer 100 of Fig. 1(a), the ion optic components include focussing elements 120 located at a (preferably optimal/optimised) position in the field free drift region 130.

In use, the MALDI ion source 110 is operated to produce ions having a plurality of m/z values by using the laser 112 to fire a pulse of light at a sample under test, which is located on a sample plate 116. Typically, prior to analysis the sample under test is kept at a constant high voltage of several kilovolts, normally up to 30 kilovolts. When the laser pulse is focused onto a compound sample (typically to a width of 100um ±50 um), the compound sample ionizes, and the ions leave the surface with a large spread of initial velocities. Focusing widths may vary for different applications from a few microns to a few hundreds microns.

In the depicted example, the acceleration/extraction electrodes 114 are used to accelerate the ions produced by the ion source towards the detector 150 such that ions having different m/z values strike the detector at different times. Typically this is achieved creating a potential difference between the sample and acceleration/extraction electrodes 114 by application of a high voltage pulse to the acceleration/extraction electrodes 114 and/or sample plate 116, which preferably occurs at an optimised moment in order to reduce the initial spread in velocities for ions having m/z values of interest. Normally the high voltage pulse is applied after some time interval after ions are initially produced by the ion source 110, i.e. after the laser pulse. Typically this interval is of several nanoseconds to several microseconds after the laser pulse. The accelerated ions then exit the MALDI ion source 110, typically through an exit electrode in the ion source (usually held at ground potential), to form an ion beam emerging into the field free drift region 130 towards the detector 150.

The ion optic components may include one or more sets of electrodes as necessary along the ion beam path, e.g. for providing directional correction and/or focusing. The focussing elements 120 are an example of such additional sets of electrodes.

Normally ions produced by the ion source will have almost the same kinetic energy, so that their velocity will be mass dependant. Therefore, the ions having different m/z values are caused to strike the detector 150 at different times. Measuring the flight time of the ions after the laser pulses (start) and current produced by the detector allows calculation of the m/z values, since ions with smaller m/z values strike the detector sooner than ions with larger m/z values.

The example TOF mass spectrometer 100' of Fig. 1(b) is similar to that of Fig. 1(a), but includes a series of reflecting elements 170, which may be used to extend flight path as well as to improve kinetic energy spread. For a conventional TOF mass spectrometer, the detector 150 will normally be equipped with a secondary electron multiplier (SEM) detector, such as a discrete dynode electron multiplier detector ("EM detector" herein) and a microchannel plate detector ("MCP detector" herein).

Some of the following paragraphs refer to an "effective gain", which is a concept that can be useful for understanding the dependence of the magnitude of the output current produced by a detector on the magnitude of the operating voltage applied to the detector. "Effective gain" is discussed in more detail below, though it should be appreciated that this parameter does not need to be actually measured in order to implement the present invention.

Most SEM detectors work by converting ions striking an impact surface into ("primary") electrons, which are then amplified into a larger number of ("secondary') electrodes in a cascade manner, which are subsequently collected by a collector. In case of an EM detector, the average number of secondary electrons that is produced from a single primary electron determines the gain at each dynode stage, and the total gain of the detector is a result of the primary electron amplification efficiency over the all dynode chains. In order to detect ions with low abundance, it is normally best to operate an EM detector at a higher gain values. However, the effective gain of a detector for ions of different masses is in general dependent on the chemical nature of the ion under analysis, as evidenced e.g. by [1], [2].

Typically a power supply is used to apply a constant operating voltage to an EM detector, wherein the magnitude of the output current produced by the detector is dependent on (among other things) the magnitude of the operating voltage. Thus, the magnitude of the operating voltage can be viewed as determining the gain (and indeed the effective gain) of the EM detector. Because of the nature of ions produced in TOF mass spectrometers, SEM detectors cannot typically measure the precise quantity of ions produced by an ion source since in general the amplification of the SEM detector cannot be characterised completely. In addition, in TOF mass spectrometers, the ion beam has a focusing characteristic that is mass and/or instrument design dependant and as a result affects the amount of ions of any particular mass impacting the detector. The magnitude of the output current produced by the detector for ions of a given m/z value is therefore dependent on a number of parameters such as the total amount of the primary electrons generated by those ions, and the magnitude of amplification gain of the primary electrons produced by the impacting ions at the constant operating voltage. For this reason, the operating voltage needs to be carefully set (sometimes referred to as "tuning" or "gain tuning") according to various requirements which are discussed in the following paragraphs.

The detector is typically one of the most stressed parts of a TOF mass spectrometer and the lifetime of a detector is strongly affected by e.g. the operating voltage used (which as noted above impacts the gain of the detector), the output current and the operating pressure levels. This results in sensitivity deterioration and/or contamination on the secondary emissive surface, an effect which is sometimes referred to as detector "ageing". Therefore, detectors in TOF mass spectrometers often require frequent attention, such as gain tuning (i.e. adjusting the operating voltage applied to the detector), in order to maintain instrument performance. This can be done by the user, by a service engineer or by automated gain adjustments between acquisition cycles via software. In newer generation instruments, where operational speed and throughput have increased markedly, detector failure is a more frequent occurrence, incurring substantial cost due to the need to replace the detector.

Another problem with the detectors typically used in TOF mass spectrometers is saturation. Saturation of a detector can occur when the output current (which can be referred to as the "ion signal") produced by the detector for ions having a particular m/z in a given acquisition cycle deplete the active (electron multiplication) surfaces of the detector and/or the current from the power supply so that the effective gain of the detector reduces, and therefore becomes lower for following masses with higher m/z values. Detector saturation is a particular problem for samples with low analyte concentration, or those with a high degree of impurities, or for samples with a wide mass range of interest (e.g. above 1000Da). Some samples may require high laser fluence, which can significantly increase the background chemical noise (especially in the low mass range < 800Da), causing the detector to saturate and underperform.

Historically, an advantage of an MCP detector (compared with an EM detector) was a fast response time, e.g. less than 1ns. For a TOF mass spectrometer, this corresponds to a higher mass resolution. However, a big disadvantage of an MCP detector is that they can easily saturate and it is generally thought to be not as robust in storage or in response to a low vacuum level. As it has been reported in [3], saturation of an MCP detector could reduce the intensity of the higher masses by as much as 80%. Saturation can also have an effect on the gain of an EM detector, although usually to a lesser extent than is the case for an MCP detector.

The latest improvements to EM detectors allow for the achievement of a response time that as fast as that of MCP detectors, e.g. less than 1ns. An EM detector is also generally more mechanically robust and lower in cost than a comparable MCP detector and, as a consequence, EM detectors are finding more widespread use in TOF mass spectrometers.

The geometry of a device such as an EM detector may vary, but a conventional design comprises a cathode, an anode, and a number of resistors and capacitors coupled to a number of intervening dynodes. A typical discrete-dynode electron multiplier has between 10 and 25 dynodes and jointly provides an operating gain of between 104 and 109, depending on the applied operating voltages typically between 1000-5000V.

Detector ageing, as discussed above, is a common problem of EM detectors used as a part of a detection system in a mass analysing systems with large variation of the impacting ion flux, such as TOF mass spectrometers, ion traps and mass filters.

Much research has been done to outline and resolve the problem of ageing of EM detectors, and improvements have been implemented in newer EM detectors, although the ageing process is something that cannot currently be avoided completely due to the nature of the dynodes generating the avalanche of electrons or its contamination. In applications such as TOF mass spectrometry, detector life expectancy can be improved to some degree by the use of new coatings and improved geometrical design. It has previously been reported that the primary cause of EM degradation over time has been due to carbon deposition on the surface of one or more dynodes that have been exposed to greater doses of secondary electrons, see [4]. It was also reported that the process of ageing is directly related to the total accumulated charge of electrons per unit area on a dynode surface. Consequently, a number of different approaches have been adopted to try to mitigate this problem: one, for example, being the use of active film technology, see [5].

However, whatever technology improvements are applied to detectors, it will always be possible to maximise the lifetime of the detector by reducing the total charge drawn from the detector over time.

Another characteristic of detectors typically used in TOF mass spectrometers is the dependence of the detector sensitivity not only on ion abundance, but also on the energy and speed of the ions at the impact surface of the detector (where ions are converted to electrons to be multiplied in the subsequent gain stages). For ions produced a MALDI ion source, ions impacting the detector are produced by the laser desorption method. Accelerated groups of ions strike the impact surface of the detector with different impact velocities, depending on the mass of the ions and the accelerating voltages. The effective gain of the detector is therefore strongly dependant on the mass range and is not constant, i.e. the effective gain is higher for the lower mass ions, which have a relatively high impact velocity, and lower for the higher mass ions, which have a significantly lower impact velocity. Due to the nature of MALDI sample preparation and ion production by a MALDI ion source, very large ion signals are produced in the low mass range, with high impact velocity ions, especially the ions from sample matrices; this significantly contributes to the ageing process of the electron multiplier. In addition, MALDI TOF mass spectrometers normally operate lasers with a power that is within a very small window around an optimum laser power, the so called 'laser threshold', and any increase in the laser power above this threshold tends to produce excessive ion signal that saturates the detector.

Most automated experiments that are performed with control over the laser power inherently have a degree of lag in the lowering of the power, so it is inevitable that saturating levels of ion concentrations will impact upon the detector during such experiments. Furthermore, the latest generation of mass spectrometers utilise lasers capable of supporting increased experiment repetition rates of up to 5kHz, meaning increasing instrumentation throughput and therefore it is only natural that detector life expectancy is further reduced. Advances in detector technology, unfortunately, have not as yet been great enough to completely mitigate for this.

All of the above factors strongly contribute to the detector ageing process.

Different approaches to improve the problem of detector saturation or/and ageing have previously been proposed, these techniques including changing the operating voltage applied to the detector between acquisition cycles ("gain control") or selective ion blanking.

For example, automatic gain control has been proposed in a manner that involves implementing automatic gain adjustments between acquisition cycles, upon receiving a predetermined threshold detection signal, see [6], or depending on the size average value of the acquired peak, see [7].

As another example, Selective ion blanking or detector bias gating has been proposed in a manner that involves operating the detector in a blanking mode prior to the arrival of the mass range of interest, to inhibit the detection of impacts of unwanted ions, see e.g. [3], [8], [9].

Unfortunately these existing approaches are only moderately successful and provide limited improvement on the detection efficiency and performance over the wide mass range. Also, in the case of automatic gain control, they have been found to be impractical in relation to TOF mass spectrometers, where mass spectra are affected by differences in the voltage applied to a detector between multiple acquisition cycles. Moreover, the response time for a fast signal fluctuations effect between ion generation events of such control could be limited in response time, that could cause loss of valuable signal or even sample, see [6], [7] Typically a detector in a TOF mass spectrometer is 'tuned' in order to achieve best operation in certain applications, especially those that are set to detect in the wide mass range. In this context, the operating voltage applied to a detector is normally set to a constant value thought to achieve best performance for detection of higher masses detectable or low abundant ions, with the compromise being increased noise and loss of resolution for the detection of lower masses. In some cases there are several tunings, with each tuning having a different detector gains set for operation only within certain mass ranges. In any case, the/each acquisition cycle used to produce a given mass spectra is performed so that there is a single, constant, applied operating voltage during each acquisition cycle. The operating voltage (or operating voltages, if there are several tunings) is re-tuned only after a certain amount of the natural detector ageing. It should be noted that, in this context, the tuning of operating voltage is a compromise between best achievable detected ion signal for a certain mass range and noise levels.

Note that higher laser fluence is often required in order to ionize higher mass ranges.

This immediately increases the background chemical noise and level of the signal in the lower and mid mass range. Most modern instruments, with automatic data acquisition based on the predetermined settings of the data quality, have intelligent control of the laser fluences that is auto-adjusts based on the detected signal level and/or signal quality. Usually response-reaction time to reduce laser power in order to lower the amount of the ions reaching the detector is limited and not always fast enough. It should be remembered that detector lifetime is a cumulative charge value and therefore prompt response in reducing the amount of unwanted signal is thought desirable by the present inventors.

In view of the above, the present inventors also believe that it would be desirable to significantly reduce the effect of saturation in the detector and hence reduce the effect of low mass ions on the detector gain.

The present inventors also believe that it would be desirable to maximise the lifetime of the detector by reducing the total charge drawn from the detector.

The present inventors also believe it would be desirable to find a practical approach to improve detector operational lifetime with increasing ion throughput without compromise on the performance, where the instrument is continuously operating at a high gain load, especially in the wide mass range applications with optimum gain settings for each mass region.

The present invention has been devised in light of the above considerations.

At its most general, a first aspect of the invention may provide: A time of flight ("TOF") mass spectrometer having: an ion source; a detector; a variable voltage unit; a control unit; wherein the control unit is configured to control the TOF mass spectrometer to perform at least one acquisition cycle that includes: operating the ion source to produce ions having a plurality of m/z values, wherein the ion source is operated so that ions having different m/z values strike the detector at different times; operating the detector to produce an output current representative of ions having different m/z values striking the detector; operating the variable voltage unit to apply a dynamic voltage waveform to the detector during the acquisition cycle so that the magnitude of the dynamic voltage waveform varies within the acquisition cycle, wherein the magnitude of the output current produced by the detector is dependent on the magnitude of the dynamic voltage waveform applied to the detector by the variable voltage unit.

By operating the variable voltage unit to apply a dynamic voltage waveform to the detector so that the magnitude of the dynamic voltage waveform varies within the acquisition cycle, the relative sizes of peaks representative of (e.g. the abundance of) ions having different m/z values can be changed compared with a more conventional arrangement in which a constant operating voltage is applied to the detector within the/each acquisition cycle. As discussed below in more detail, this may be useful to improve the ability of the TOF mass spectrometer to detect ions having certain m/z values and/or to compensate for an effective gain characteristic of the detector.

Preferably, the dynamic voltage waveform is configured so that, within the/each acquisition cycle: a first voltage is applied to the detector by the variable voltage unit at a first time when ions having a first m/z value are striking the detector such that the output current includes a peak representative of (e.g. the abundance of) ions having the first m/z value; and a second voltage, different from the first voltage, is applied to the detector by the variable voltage unit at a second time when ions having a second m/z value are striking the detector such that the output current includes a peak representative of (e.g. the abundance of) ions having the second m/z value.

In this way, the relative sizes of peaks representative of (e.g. the abundance of) ions having the first and second m/z values can be altered depending on the values of the first and second voltages, compared with a more conventional arrangement in which a constant voltage is applied to the detector within the/each acquisition cycle.

Note that a dynamic voltage waveform as defined above with reference to ions having first and second m/z values is distinguished from an ion blanking method, such as that disclosed in [8], since the dynamic voltage waveform as defined above is configured such that the output current includes peaks representative of ions having the first and second m/z values. In contrast, an ion blanking method such as that disclosed in [8] involves temporarily changing a voltage between specific dynodes of a detector such that an output current does not include a peak representative of ions having certain predetermined m/z values, regardless of whether ions having those predetermined m/z values are striking the detector or not.

Of course, achieving a dynamic voltage waveform configured so that, within the/each acquisition cycle, a first voltage is applied to the detector at the first time and a (different) second voltage is applied to the detector at the second time could be achieved in a variety of ways. For example, the dynamic voltage waveform could include a voltage ramp (see e.g. Fig. 2) so that different voltages are applied to the detector at two or more times when ions having different m/z values are striking the detector (see Fig. 4). But a skilled person would appreciate from the disclosure herein that the dynamic voltage waveform could equally have a more simple (e.g. step-like) or complex (e.g. with numerous rises and falls in magnitude) form, depending e.g. on the effect on the sensitivity of the detector being sought at different m/z values.

Preferably, the second m/z value is larger than the first m/z value and the second voltage is larger in magnitude (e.g. with reference to a local ground) than the first voltage. In this way, the magnitude of the voltage applied to the detector increases with m/z value during the/each acquisition cycle, e.g. so as to provide an increase in sensitivity at the second m/z value relative to the first m/z value. As discussed below in more detail, this may be useful e.g. to allow ions having high m/z values to be detected that would not otherwise be detected, e.g. due to an effective gain characteristic of the detector.

In other embodiments, the second m/z value may be larger than the first m/z value with the second voltage being smaller in magnitude than the first voltage. In this way, the magnitude of the voltage applied to the detector decreases with m/z value during the/each acquisition cycle, e.g. so as to provide a reduction in sensitivity at the second m/z value relative to the first m/z value. This could be useful e.g. if there is only a small number of lower masses ions and so it is desirable to increase the size of peaks occurring at lower m/z values (relative to those occurring at higher m/z values).

As a skilled person would appreciate in view of the disclosure herein, the dynamic voltage waveform could be configured to have a variety of different forms/shapes, depending e.g. on the effect on the sensitivity of the detector being sought at different m/z values.

For example, the dynamic voltage waveform could include a voltage ramp, wherein the magnitude of the dynamic voltage waveform gradually increases or decreases in magnitude.

For example, the dynamic voltage waveform could be defined with reference to third and fourth voltages, e.g. the dynamic voltage waveform could optionally be configured so that, within the/each acquisition cycle: a third voltage is applied to the detector by the variable voltage unit at a third time when ions having a third m/z value are striking the detector such that the output current includes a peak representative of (e.g. the abundance of) ions having the third m/z value; and optionally a fourth voltage is applied to the detector by the variable voltage unit at a fourth time when ions having a fourth m/z value are striking the detector such that the output current includes a peak representative of (e.g. the abundance of) ions having the fourth m/z value.

The third voltage may be different from the first and second voltages. The fourth voltage may be different from the first, second and third voltages.

Herein, when the voltage applied to the detector is described as having a magnitude, or as having a magnitude that increases, decreases, or otherwise varies, that magnitude may be defined with reference to a local ground.

Preferably, the control unit is configured to control the TOF mass spectrometer to perform multiple acquisition cycles.

If there are multiple acquisition cycles, then preferably the dynamic voltage waveform is substantially the same in each acquisition cycle.

If there are multiple acquisition cycles, then preferably the dynamic voltage waveform is synchronised to be produced at substantially the same time relative to an event occurring in each acquisition cycles, e.g. the firing of a laser by the ion source.

In this way, output currents produced during each acquisition cycle can avoid becoming mass shifted with respect to each other, which could lead to a loss of resolution.

The dynamic voltage waveform may vary in magnitude between a lower voltage value and an upper voltage value.

Preferably, the difference between the lower voltage value and the upper voltage value is at least 10 Volts, more preferably at least 30 Volts.

For a typical instrument, the difference between the lower voltage value and the upper voltage value may be 200 Volts or lower. However, larger differences may be suitable depending on the instrument used.

The dynamic voltage waveform may be defined with reference to a predetermined range of m/z values, which may correspond to a mass range of interest to a user of 20 the TOF mass spectrometer.

Preferably, the dynamic voltage waveform is configured so that, within the/each acquisition cycle: the voltage applied to the detector by the variable voltage unit is at an initial value during a first period of time occurring before ions having m/z values falling within a predetermined range of m/z values strike the detector; the voltage applied to the detector by the variable voltage unit varies in magnitude during a second period of time occurring whilst ions having m/z values falling within a predetermined range of m/z values strike the detector; the voltage applied to the detector by the variable voltage unit returns to the initial value during a third period of time occurring after ions having m/z values falling within the predetermined range of m/z values strike the detector.

Returning the voltage applied to the detector to the initial voltage during the third period of time may be useful to ensure that the initial voltage is the same for any subsequent acquisition cycles and may further be useful (if the initial voltage has a magnitude that is adequately low) to protect the detector from depletion caused by ions striking the detector that have large m/z values that are not of interest.

It should be apparent that if the voltage applied to the detector by the variable voltage unit is at the initial value at the end of the second period of time, the third period of time may have a zero length.

For the avoidance of any doubt, the voltage applied to the detector by the variable voltage unit could rise and/or fall in magnitude (e.g. with reference to a local ground) during the second period of time.

Note that it may take some time for the voltage applied to the detector by the variable voltage unit to return to the rest voltage during the third period of time, see e.g. the dashed line in Fig. 2(a).

The second period of time (during which the gain voltage applied to the detector varies in magnitude) may typically be 1000us or less, more preferably between 10us and 1000us.

The third period of time (during which the voltage applied to the detector returns to the initial value) may typically be between 10us and 1000us.

These time values may be appropriate for a typical instrument, but a skilled person would appreciate that very large or very small instruments may require different times.

For the avoidance of any doubt, the initial value could be 0 Volts in some embodiments, e.g. as might be appropriate if the voltage applied by the voltage unit is capacitively coupled to an operating voltage applied to the detector by a separate power supply (see below).

We note for completeness that, for a TOF mass spectrometer, there is in general a time offset between the production of ions and their detection, i.e. so ions produced within one acquisition cycle may be being detected at the same time as (or even after) ions are being produced within a subsequent acquisition cycle. Therefore, for the avoidance of any doubt, we note that the first, second or third periods of time referred to above (in relation to the voltage applied to the detector) within one acquisition cycle may occur at the same time as (or even after) ions are being produced within a subsequent acquisition cycle.

In other words, one acquisition cycle may start at the ion source before ions from an earlier acquisition cycle have been detected at the detector.

In some applications, the dynamic voltage waveform applied to the detector by the variable voltage unit increases in magnitude (e.g. with reference to a local ground) during the second period of time, e.g. so that the voltage applied to the detector by the variable voltage unit is lower in magnitude when ions having m/z values towards a lower (i.e. lighter) end of the predetermined range of m/z values are striking the detector compared with the voltage applied to the detector by the variable voltage unit when ions having m/z values towards an upper (i.e. heavier) end of the predetermined range of m/z values are striking the detector.

In some embodiments, the voltage applied to the detector by the variable voltage unit may gradually rise in magnitude from the initial value to a peak value during the second period of time.

For the avoidance of any doubt, the voltage applied to the detector by the variable voltage unit could be positive or negative, depending e.g. on the configuration of the mass spectrometer, e.g. on the charge on the ions produced by the ion source.

In some embodiments, the voltage applied to the detector by the variable voltage unit may gradually rise (or fall) in magnitude during the second period of time. However, the dynamic gain voltage waveform could take a more complex form during the second period of time, e.g. so that the gain voltage applied to the detector both rises and falls during the second period of time. The actual form will depend e.g. on the effect on the sensitivity of the detector being sought.

The initial value could be configured so that the output current is not able to include a peak representative of ions striking the detector when the voltage applied to the detector by the variable voltage unit is at the initial value. For example, the initial value could be set at a value so that secondary electrons cannot be produced by the detector, e.g. so as to achieve an "ion blanking" effect.

The detector may include: an impact surface configured to emit one or more electrons when struck by an ion; and one or more electron multiplication surfaces; and a collector; wherein the one or more electron multiplication surfaces are configured to convert electrons emitted from the impact surface into a larger number of electrons to be collected by the collector to provide the output current.

Two examples of detectors having an impact surface, one or more electron multiplication surfaces and a collector are a discrete dynode electron multiplier detector ("EM detector" herein) and a microchannel plate detector ("MCP detector" herein). However, the same principles could be applied to other types of detector in which similar considerations apply.

An EM detector typically includes an impact surface, a plurality of dynodes, and a collector, wherein the impact surface is configured to emit one or more electrons when struck by a particle (e.g. an ion). Typically, a first dynode is configured to convert the one or more electrons emitted from the impact surface into a larger number of electrons, wherein each subsequent dynode is configured to convert electrons emitted from a previous dynode into an even larger number of electrons, and wherein the collector is configured to collect electrons emitted from a last dynode to provide the output current.

In some EM detectors, the impact surface may be provided by a further dynode situated before the first dynode referred to above. Alternatively, the impact surface may be separate from the dynodes (e.g. as shown in Fig. 3). In most EM detectors, the dynodes are polished electrodes that are electrically coupled with a network of resistors, zener diodes and/or capacitors. Typically an operating voltage is applied to a front electrode of the EM detector (e.g. the impact surface, see below) and the network of resistors, zener diodes and/or capacitors is configured so that the voltage at each successive dynode reduces in magnitude towards the collector, which is usually biased at a local ground voltage. An example structure of an EM detector is described in more detail below with reference to Fig. 3.

From the above discussion, it can be seen that the dynodes of the EM detector provide the electron multiplication surfaces for an EM detector.

An MCP detector typically includes a plurality of channels. Typically, the wall/walls of each channel are configured to emit one or more electrons when struck by a particle (e.g. an ion produced by the ion source), and to convert the one or more emitted electrons into a larger number of electrons through further collisions with the wall/walls of the channel, wherein a collector is configured to collect electrons at an end of each channel to provide the output current..

From the above discussion, it can be seen that the wall/walls of each channel provide the electron multiplication surfaces for an MCP detector. The MCP detector may have a chevron type multiple plate structure.

Preferably, the variable voltage unit applies the dynamic voltage waveform across a majority of, more preferably all of, the electron multiplication surfaces of the detector.

As can be seen from the discussions below, not only is this generally more convenient to implement, but it is also thought to help change the sensitivity (i.e. effective gain) of the detector on a timescale that is able to change the relative sizes of peaks representative of ions having different m/z values within the/each acquisition cycle.

Applying the dynamic voltage waveform across all of the electron multiplication surfaces of the detector may conveniently be achieved by applying the dynamic voltage waveform to a front electrode of the detector.

A front electrode of a detector may be defined as an electrode of the detector that faces towards ions incident on the detector when the TOF mass spectrometer is in use. For an EM detector, a front electrode of the EM detector may be the impact surface, the first dynode, or a first entrance grid of the detector.

For an MCP detector, a front electrode of the MCP detector may be a front plate located at an input end of the plurality of channels of the MCP detector.

Preferably, the mass spectrometer includes a power supply configured to apply an operating voltage to the detector, preferably to a front electrode of the detector. The operating voltage may be for use by the electron multiplication surfaces of the detector in converting electrons emitted from the impact surface into a larger number of electrons. An EM detector or MCP detector typically requires such an operating voltage in order to operate.

In the case of an EM detector (see above), the operating voltage may be applied across the dynodes of the EM detector (typically, this operating voltage is divided between the dynodes, e.g. using resistors). This is normally achieved by applying the operating voltage to a front electrode of the EM detector.

In the case of an MCP detector (see above), the operating voltage may be applied along the channels of the MCP detector. This is normally achieved by applying the operating voltage to a front electrode of the MCP detector.

The operating voltage may typically be a negative voltage. The operating voltage may typically have a magnitude that 500 Volts or higher.

There are of course a variety of ways in which the variable voltage unit may apply the dynamic voltage waveform to the detector.

Preferably, the mass spectrometer includes a power supply configured to supply the detector with an operating voltage (see above), the variable voltage unit is separate from the power supply, and the variable voltage unit applies the dynamic voltage waveform to the detector by modulating the operating voltage applied to the detector by the (separate) power supply. More preferably, the variable voltage unit applies the dynamic voltage waveform to the detector by capacitively coupling the dynamic voltage waveform to the operating voltage applied to the detector by the (separate) power supply. This is a particularly efficient way of applying the dynamic voltage waveform to the detector.

However, in other embodiments, the mass spectrometer includes a power supply configured to supply the detector with an operating voltage (see above), the variable voltage unit is separate from the power supply, and the variable voltage unit applies the dynamic voltage waveform directly to the detector, e.g. by capacitively coupling the dynamic voltage waveform to an appropriate electrode/dynode of the detector.

In yet further embodiments, the mass spectrometer includes a power supply configured to supply the detector with an operating voltage (see above), and the power supply acts as the variable voltage unit. This could be achieved, for example, by configuring the power supply used to supply the detector with a variable operating voltage so as to apply the dynamic voltage waveform to the detector.

From the above discussion, it can be seen that the variable voltage unit may be the same as or separate from a power supply configured to supply the detector with an operating voltage.

A typical detector may have a "gain" associated with it, which may define the number of electrons collected by a collector of the detector for a single electron generated at the ion impact surface.

The present invention may be described/understood in terms of an effective gain, G, which is a concept that may be used to describe the dependence of the magnitude of the output current produced by the detector on the magnitude of the dynamic voltage waveform applied to the detector. The effective gain, G, may be defined according to the equation: !output = q N G (1) where 'output is the output current produced by the detector, N is the number of ions striking the detector per second and q is the charge of the ions striking the detector. As discussed elsewhere in this document, effective gain is generally not constant and tends to vary in a complex non-linear manner with respect to various parameters including the m/z value of ions striking the detector, voltage(s) applied to the detector, conversion efficiency of the impact surface, voltage(s) between dynodes of the detector. So effective gain would not be constant with m/z value even if (as is conventional) a constant gain voltage were applied to the detector.

For these reasons, effective gain is not a parameter that can be easily measured and a skilled person would readily understand that effective gain does not need to be quantified or measured in order to implement the present invention. Rather, effective gain is defined here simply to enhance a readers understanding of the present invention.

A detector may (qualitatively) have an effective gain characteristic associated with it, by which, for a constant gain voltage applied to the detector, the effective gain of the detector varies with the m/z value of ions striking the detector. As discussed below in more detail, it is common for a detector used in a TOF mass spectrometer (such as an EM detector or an MCP detector) to have an effective gain characteristic by which, for a constant gain voltage applied to the detector, the effective gain of the detector reduces as the m/z value of ions striking the detector increases.

Preferably, the dynamic gain voltage waveform is configured to compensate for an effective gain characteristic of the detector, e.g. so that a variation in effective gain with the m/z value of ions striking the detector is reduced (preferably substantially eliminated) compared to a constant gain voltage being applied to the detector, e.g. within a predetermined range of m/z values.

Additionally/alternatively, the dynamic gain voltage waveform may be configured to improve the ability of the TOF mass spectrometer to detect ions having one or more predetermined m/z values, e.g. if it is thought that ions having such m/z values are present in low numbers.

Preferably, the ion source is operated in conjunction with one or more other components of the TOF mass spectrometer so that ions having different m/z values strike the detector at different times. However, in a very simple TOF mass spectrometer, there may simply be an ion source and a detector without other components being located between the ion source and the detector.

The one or more other components of the ion source may, for example, include one or more ion optic components for manipulating ions produced by the ion source.

Thus, the mass spectrometer may include ion optic components for manipulating ions produced by the ion source. The/each acquisition cycle may include using the ion optics to accelerate, decelerate, steer, deflect, reflect, focus and/or re-focus the ions produced by the ion source.

The one or more other components may, for example, include a collision device (e.g. a collision cell).

Thus, the mass spectrometer may include a collision device. The/each acquisition cycle may include operating the collision device to fragment the ions to produce fragments of the ions produced by the ion source. Fragments of the ions produced by the ion source could additionally/alternatively be produced in the TOF mass spectrometer by natural fragmentation.

Thus, for the avoidance of any doubt, the ions striking the detector at different times within the/each acquisition cycle may be ions produced by the ion source and/or fragments of ions produced by the ion source (e.g. as produced by a collision device or by natural fragmentation).

In some embodiments, the variable voltage unit may include a voltage source. The variable voltage unit may include waveform shaping circuitry configured to produce the dynamic voltage waveform. In simple embodiments, the waveform shaping circuitry may include an RC circuit, but other (e.g. more complex) circuitry could readily be envisaged by a skilled person.

The TOF mass spectrometer may include a recording unit configured to record the output current produced by the detector during the/each acquisition cycle as a mass spectrum representative of the relative abundance of ions having different m/z values striking the detector. For the avoidance of any doubt, the output current may be amplified and/or converted to a voltage before being recorded, e.g. using an amplifier and/or oscilloscope.

If there are multiple acquisition cycles (see above), the recording unit may be configured to combine the mass spectra obtained from each acquisition cycle together, e.g. so as to form a combined mass spectra.

The control unit may include timing circuitry configured to be triggered by an event in the/each acquisition cycle, e.g. the firing of a laser by the ion source, e.g. so as to provide synchronisation between the event and the production of the dynamic voltage waveform.

The control unit may, for example, include a computer, but a control unit including e.g. analogue timing circuitry as described above could also be used.

The recording unit (if present) may be implemented by a computer.

Preferably, the ion source includes a laser for ionising a sample carried on a sample plate by firing light at the sample. Preferably, the laser is for ionising a sample by firing pulses of light at the sample. The light produced by the laser is preferably UV light, though IR light is also possible.

However, the sample may be ionised by other techniques.

The ion source may include acceleration/extraction electrodes for accelerating ions produced by the ion source, e.g. towards the detector such that ions having different m/z values strike the detector at different times.

The ion source may be a MALDI (matrix-assisted laser desorption/ionisation) ion source. In this case, the TOF mass spectrometer may be referred to as a "MALDI TOF" mass spectrometer. For a MALDI ion source, the sample may include biomolecules (e.g. proteins), organic molecules and/or polymers. The sample may be included in a (preferably crystalised) mixture of sample material and light absorbing matrix.

The TOF mass spectrometer may be used for microbial identification (e.g. bacterial/fungal identification), for example. A MALDI TOF mass spectrometer is particularly useful for this purpose.

A second aspect of the invention may provide a method of operating a time of flight mass spectrometer according to the first aspect of the invention. The method may include any method step implementing or corresponding to a time of flight mass spectrometer described above.

A third aspect of the invention may provide a method of modifying a TOF mass spectrometer, preferably so as to provide a TOF mass spectrometer according to the first aspect of the invention.

A method according to the third aspect of the invention may include configuring a control unit of a TOF mass spectrometer to control the TOF mass spectrometer to perform at least one acquisition cycle as set out in the first aspect of the invention.

The method may include installing a variable voltage unit in the TOF mass spectrometer.

The invention also includes any combination of the features described above except where such a combination is clearly impermissible or expressly avoided.

Examples of our proposals are discussed below, with reference to the accompanying drawings in which: Fig. 1 shows (a) an example linear TOF mass spectrometer and (b) an example reflecting TOF mass spectrometer in which the present invention may be implemented.

Fig. 2 illustrates figuratively a comparison of (a) voltage applied to a detector and (b) the resulting effective gain of the detector during an example acquisition cycle, where a first dynode of the detector has a constant operating voltage applied to it (solid lines) and has a dynamic voltage waveform applied to it (dashed lines).

Fig. 3 shows an example EM detector that includes a plurality of dynodes along with the voltage at each dynode of the detector when a dynamic voltage waveform is applied to a front dynode of the detector.

Fig. 4 shows a spectrum of the bovine serum albumin (BSA) protein, as measured with a MALDI mass spectrometer (a) wherein a dynamic voltage waveform was applied to the detector during each acquisition cycle and (b) where a constant operating voltage was applied to the detector within each acquisition cycle.

In general, the following discussion describes examples of our proposals that may result in an optimised voltage being applied to a detector of a TOF mass spectrometer, e.g. to permit a complete mass spectrum to be obtained for a wide mass range.

In the following discussion, the invention is exemplified with reference to the detector 150 of the example TOF mass spectrometer shown in Fig. 1(a), where the detector is assumed to be an EM detector, since the discrete dynode structure of an EM detector makes the understanding and implementation of the invention easier than for an MCP detector.

However, it will be clear to those skilled in the art that the invention could easily be applied to TOF mass spectrometers having other forms (e.g. the TOF mass spectrometer 100' of Fig. 1(b)) and that the detector 150 could have another form, e.g. an MCP detector, particularly an MCP detector having a chevron type multiple plate structure.

In accordance with embodiments of the present invention, a control unit (not shown) is configured to control the TOF mass spectrometer 100 of Fig. 1(a) to perform at least one acquisition cycle (preferably a plurality of acquisition cycles).

The control unit may, for example, be implemented by a computer or/and be part of the TOF instrument.

The/each acquisition cycle preferably includes: operating the ion source 110 to produce ions having a plurality of m/z values, wherein the ion source is operated so that ions having different m/z values strike the detector at different times; operating the detector 150 to produce an output current representative of ions having different m/z values striking the detector 150; operating a variable voltage unit (not shown) to apply a dynamic voltage waveform to the detector during the acquisition cycle so that the magnitude of the dynamic voltage waveform varies within the acquisition cycle, wherein the magnitude of the output current produced by the detector 150 is dependent on the magnitude of the dynamic voltage waveform applied to the detector by the variable voltage unit.

Thus, the voltage applied to the detector 150 varies within the/each acquisition cycle, e.g. for the/each laser shot used to produce ions having a plurality of m/z values, if the ion source 110 is a MALDI ion source.

As shown by the solid lines in Fig. 2, with a constant operating voltage being applied to the detector 150 within the/each acquisition cycle (Fig. 2(a)) the effective gain of the detector 150 reduces as the m/z value of ions striking the detector 150 increases.

This represents a conventional operating voltage setting for the TOF mass spectrometer 100.

As shown by the dashed lines in Fig. 2(a), a dynamic voltage waveform is applied to the detector within the/each acquisition cycle so that, within the/each acquisition cycle: the voltage applied to the detector by the variable voltage unit is at an initial value during a first period of time occurring before ions having m/z values falling within a predetermined range of m/z values strike the detector; the voltage applied to the detector by the variable voltage unit varies in magnitude during a second period of time occurring whilst ions having m/z values falling within a predetermined range of m/z values strike the detector; the voltage applied to the detector by the variable voltage unit returns to the initial value during a third period of time occurring after ions having m/z values falling within the predetermined range of m/z values strike the detector.

An idealised effective gain resulting from applying the dynamic voltage waveform of Fig. 2(a) to the detector 150 is shown by the dashed line in Fig. 2(b). In this "ideal" case, the effective gain of the detector 150 is substantially constant for ions having m/z values falling within a predetermined range of m/z values. However, this is only illustrative as there are various non-linearities in the variation of effective gain with m/z value and therefore effective gain would be difficult to measure in practice, let alone to achieve a substantially constant effective gain over the predetermined range of m/z value. Nonetheless, by having a dynamic voltage waveform that gradually rises in magnitude during the second period of time, the dynamic voltage waveform will, it is thought (qualitatively, at least) have the effect of compensating for an effective gain characteristic of the detector 150, since ions having a large m/z can be detected that would otherwise not be detected with a constant operating voltage being applied to the detector 150 (see Fig. 4, discussed below).

In this example, the predetermined range of m/z values is a mass range for which the TOF mass spectrometer 100 is optimised, and this is shown by the double-ended arrow in Fig. 2.

Note that in the dynamic voltage waveform of Fig. 2(a), voltages with lower magnitudes (e.g. with reference to a local ground) are applied to the detector 150 for the lower masses with high speed impact, and voltages with larger magnitudes (e.g. with reference to a local ground) are applied as the heavier masses with lower impact speed reach the detector 150. Since it is well known that the effective gain of an EM detector has a non-linear dependence in which the effective gain reduces with increasing mass, increasing the voltage in this manner allow for the variation in effective gain to be reduced (qualitatively, at least) for the m/z range of interest.

A skilled person will appreciate that, provided that adequate numbers of ions having first and second m/z values are striking the detector in the predetermined range of m/z values shown in Fig. 2 (where the second m/z value is larger than the first m/z value), then the dynamic voltage waveform shown by the dotted line of Fig. 2(a) is configured so that, within the/each acquisition cycle: a first voltage is applied to the detector by the variable voltage unit at a first time when ions having the first m/z value are striking the detector such that the output current includes a peak representative of the abundance of ions having the first m/z value; and a second voltage, larger in magnitude than the first voltage, is applied to the detector by the variable voltage unit at a second time when ions having the second m/z value are striking the detector such that the output current includes a peak representative of the abundance of ions having the second m/z value.

Since lower voltages are applied to the detector 150 whilst ions with lower m/z values are striking the detector 150, the cascade of secondary electrons produced for the lower mass ions is reduced, and the corresponding output current produced by lower mass ions is reduced, meaning less current is drawn from the detector 150 as a result of lower mass ions striking the detector 150. This effective reduced current accumulation caused by the lower mass ions will result in longer detector life, reduced noise and an improvement in low mass resolution, whilst using with a voltage applied to the detector 150 that is optimised for detection of larger masses (that might not otherwise have been detected with a constant operating voltage).

This allows for improved mass spectra, by avoiding high current pulses cascading through the dynodes caused by lower mass ions, and therefore helps to reduce damage and carbon sticking. Moreover, the detector 150 can be "tuned" for a wide mass range, with initial voltages applied by the variable voltage unit being configured for detection of lower masses of interest (whilst avoiding saturation being caused by the lower masses of interest, that might have been occurred if using a constant operating voltage), and with later voltages applied by the variable voltage unit being configured for detection of heavier masses of interest (that might otherwise have gone undetected if using a constant operating voltage). Thus, one dynamic voltage waveform tuning could be used for a wide mass range of interest.

As shown in Fig. 2(a), it is proposed to bring the voltage applied to the detector 150 back to an initial, i.e. pre-acquisition, value, with a view to protecting the detector 150 from the slower and higher masses that could reach it outside of the mass range of interest. In addition, returning the voltage applied to the detector to the initial value will allow the same dynamic voltage waveform to be used with the same timing on subsequent acquisition cycles.

Note that it would be possible to achieve an "ion blanking" effect (i.e. non-detection) of lower masses, e.g. by setting the initial value of the voltage applied to the detector to be below that required by the detector 150 to produce secondary electrons. That is, the initial value of the voltage applied to the detector 150 by the variable voltage unit could be configured so that the output current produced by the detector 150 is not able to include a peak representative of ions striking the detector when the voltage applied to the detector 150 by the variable voltage unit is at the initial value.

The variable voltage unit may include a voltage source and waveform shaping circuitry configured to produce the dynamic voltage waveform.

By way of (non-limiting) example, the waveform shaping circuit may include an RC circuit (including at least one resistance and at least one capacitance). The dynamic voltage waveform of Fig. 2(a) is an example of a dynamic voltage waveform that could be produced by the controlled charge and discharge of a waveform shaping circuit that includes an RC circuit. The RC circuit may be configured for specific settings of the TOF mass spectrometer 100.

Preferably, the variable voltage unit is configured to apply the dynamic voltage waveform to the detector under the control of a timing unit. The timing unit may include circuitry configured to be triggered by an event in the/each acquisition cycle so as to provide synchronisation between the event and the production of the dynamic voltage waveform, e.g. so that the voltage applied to the detector 150 by the variable voltage unit starts to vary whilst ions having a predetermined m/z value are striking the detector. The event could be an acquisition cycle starting trigger event, such as the firing of a laser or pulsed extraction of ions from the ion source 110 (as might be appropriate if the ion source 110 is a MALDI ion source).

As discussed above, after the voltage applied to the detector 150 has varied as required, the voltage applied to the detector 150 preferably returns to the same initial value, e.g. in order to avoid any degradation on the instrument performance as well as so as to avoid mass shift that could be caused by the dynamic voltage waveform being different for different acquisition cycles. As the detector ages (even though the aging rate is preferably reduced as a consequence of the techniques taught herein), the initial value of the voltage applied to the detector 150 by the variable voltage unit could be adjusted (e.g. increased) and the form of the dynamic voltage waveform (which in this case is a voltage ramp) could be adjusted to compensate for the change in the effective gain characteristic of the detector, if required.

Of course, the dynamic voltage waveform of Fig. 2(a) is just one example, and other (e.g. more complex) dynamic voltage waveforms are equally possible, e.g. so as to obtain a more uniform or an otherwise desired effective gain characteristic. Such dynamic waveforms may be produced using a timing circuit that includes circuitry that is more complex than the simple RC circuit, as proposed above.

However, regardless of the form of the dynamic voltage waveform, it is highly preferable that the dynamic voltage waveform is the same and applied with the same timing in each acquisition cycle, so as to avoid mass shift in the mass spectra produced from the acquisition cycles. Mass shift between acquisition cycles is particularly undesirable, since it may cause loss of resolution in a combined mass spectrum produced from the acquisition cycles.

In the specific example of Fig. 2(a), the dynamic voltage waveform can be viewed as a dynamic voltage pulse that starts at an initial value and then gradually increases with a slope of increasing negative voltage, up to a peak value that is -100V (or even -200V) larger the initial value voltage. The slope and peak voltage may of course depend on application, mass range and instrument design, sensitivity and performance, and is preferably 'tuned' in the initial instrument setup in order to achieve predetermined specifications.

The timing of the pulse could be defined according to a mass range of interest and ion flights characteristics. For example, the pulse start time could be set by a delay such that the pulse starts whilst ions having a lowest m/z value of interest are striking the detector 150, and so that the pulse ends whilst ions having a highest m/z value of interest are striking the detector 150. This will allow additional time for the initial voltages to relax to the initial (pre-pulse) voltage value. Typical values of pulse length for most instruments could be several microseconds to 100us, although this could vary, depending e.g. on the geometry of the instrument used.

Fig. 3 illustrates an example EM detector 150 that could be used in the TOF mass spectrometer 100 of Fig. 1(a).

This EM detector 150 could, for example, be configured to detect particles selected from photons, neutral molecule, as well as ions produced by the ion source 110 and/or fragments of such ions, which fragments could be produced e.g. in a flight tube of the TOF mass spectrometer 100, e.g. by natural or forced (e.g. CID) fragmentation methods..

As shown in Fig. 3, the detector 150 includes an impact surface 152 configured to emit one or more (secondary) electrons when struck by an ion.

The impact surface 152 may equally be capable of producing one or more secondary electrons when struck by other particles such as photons, or electrons, but that property of the impact surface is more useful for applications other than TOF mass spectrometers.

In general, each impact on the impact surface 152 will produce one or more secondary electrons which are accelerated toward the first in a series of n dynodes D1-Dn. Typically discrete dynode EM is made of a series of dynodes that could be described as electrodes having a good secondary emission properties. At each dynode, secondary electrons emitted by a previous dynode (or the impact surface 152, in the case of the first dynode D1) strike an impact surface of the dynode causing emission of secondary electrons which are accelerated to the next dynode in the chain. The dynodes D1-Dn are typically held at decreasing negative potential by a chain of resistors. Larger numbers of (i.e. multiplied) electrons from each dynode is accelerated to the next one with a (typically) lower potential defined by the design of the detector. After typically more than 10 dynodes, the cascade of the electrons has been multiplied by several orders of magnitude.

Finally, at the end of the chain of dynodes D1-Dn is typically an electrode, which may be referred to as an anode or collector 156, which collects electrodes from the last dynode to provide the output current from the detector 150 (this output current may be further amplified by a conventional amplifier). In this way the impact of a single ion or pulse of ions on the first surface can produce a very large pulse of output current from the detector 150. Typically this output current will be recorded, e.g. in a recording unit, e.g. by passing the output current through a resistor and recording the corresponding voltage pulse, e.g. using an oscilloscope or transient recorder.

At the end of the EM, the resulting amplified current may be defined according to Equation (1), above.

In general, when a charged particle strikes the impact surface 152 of the detector 150, one or more "primary" electrons are emitted from the impact surface 152 as a result of the impact causing such electrons to be released from atoms in the surface layer of the impact surface 152. The number of primary electrons released depends on the type and characteristics of the incident particle, its energy and other characteristics such as the composition of the impact surface. These primary electrons are then accelerated to a first dynode in the chain, which is typically held at a lower potential.

The accelerating voltage between each accelerating stage such as a dynode pair is typically produced by applying a high operating voltage to a front electrode of the detector 150, e.g. the impact surface, a first dynode or a first entrance grid, whereas the output current is provided by a collector at a back of the detector. Note that the collector may be held at a local ground potential in a non-floating configuration. The EM detector design typically allows voltages to be distributed across the dynodes in the detector for example using a "biasing" network 154 of resistors, capacitors and/or zener diodes to determine the voltage between each pair of dynodes in a controlled fashion. The voltage between dynode pairs and the amount of the secondary electrons determine an emission efficiency (which is in part determined by the operating voltage applied to a front electrode of the detector), which determines the gain of the detector (see above definition). Typical values of accelerating voltages between dynodes are vary from design to design. Typically, the voltage between dynode pairs within one chain could vary from several mV to 300V, and a typical operating voltage applied to the EM detector may be between lkv to 5kv.

As discussed above, conventionally, the operating voltage applied to a front electrode (e.g. the first dynode of) an EM detector is kept constant for some period of the instrument life e.g. for days weeks or months depending e.g. on the particular application and the sample throughput of the TOF mass spectrometer in which it is installed, until the sensitivity is dropped and the gain of the EM detector has to be increased to keep the performance. However, for the reasons described above, in a TOF mass spectrometer, this means that the voltage applied may be optimised only for a limited mass range only or it may be a compromise choice for performance across different mass ranges or for a wide mass range. By being optimised for a one mass range the operating voltage may be higher than would be optimal for another mass range such that the lifetime of the detector could be reduced.

Therefore, preferably, as described above e.g. with reference to Fig. 2(a), a dynamic voltage waveform is applied to the EM detector so that the magnitude of the dynamic voltage waveform varies within the acquisition cycle.

Conveniently, this may be achieved by modulating a constant operating voltage applied to a front electrode of the detector by a separate power supply, e.g. by capacitively coupling the dynamic voltage waveform to the operating voltage applied to the detector by the (separate) power supply. As discussed above, the dynamic voltage waveform is preferably applied to the detector in a manner that is synchronised with an acquisition cycle starting trigger event. As discussed above, this allows the voltage applied to the detector 150 to be optimised for a mass range of interest.

The dynamic voltage waveform could potentially be optimized for any mass spectrometer instrument settings.

Some example dynamic voltage waveforms 160 are shown in Fig. 3. Note how the scale of the voltage waveform decreases at each dynode stage due to the network 156 of resistors, zener diodes and/or capacitors.

In theory, the dynamic voltage waveform could be applied (e.g. by capacitive coupling) to the impact surface, first dynode, last dynode, or indeed to intervening dynodes of the detector. However, it is preferable in the case of this invention that the dynamic voltage waveform is applied to a front dynode or last dynode of an EM detector since this is practically easier to implement and would not require any changes to the detector itself.

More preferably, the dynamic voltage waveform is applied to a front electrode of the detector (e.g. by capacitively coupling the dynamic voltage waveform to an operating voltage applied to a front electrode of the detector, as described above). In this way the dynamic voltage waveform will influence the multiplication of electrons in the first of the dynode stages, where the potential difference between the dynodes is typically largest. Also, as discussed in more detail below, this is believed to help to allow the voltage to be changed across the dynodes quickly enough to change the sensitivity of the EM detector for ions within the each acquisition cycle having different m/z values, which is important since the overall sensitivity of the detector is limited by the response time of the whole dynode chain, which is in turn dominated by the slow response of the dynodes at the back of the detector.

In more detail, the last stages of the EM detector typically have capacitors in parallel with the resistors or zener diodes that store charge to supply the large current pulses generated by the ion signals. This capacitance has the effect of maintaining the gain voltage across the last stages which, in turn, would make it more difficult to change the effective gain of the detector by applying the dynamic voltage waveform to the back of the detector.

Also, by applying dynamic voltage waveform to a front electrode or first stages of the dynode chain, the size of the dynamic voltage waveform (i.e. difference between a lower voltage value and upper voltage value) can be much lower than would be needed were the dynamic voltage applied to the back of the detector (e.g. 10s of volts compared to 100s of volts), and also the effective gain can be changed quickly enough to vary with the time-of-flight and therefore with ion mass. This helps to allow effective gain to be varied in a mass dependent fashion during each acquisition cycle.

A typical EM detector in a TOF mass spectrometer has an inherent capacitance that would mean that a change in a gain voltage applied to a first dynode of the EM detector would take a certain time delay to take effect across all dynodes of the EM detector. On the face of it, this would make it difficult to change the voltage across all the dynodes quickly enough to change the sensitivity of the EM detector for ions within the each acquisition cycle having different m/z values. However, in practice, the inventors have found that an adjustment in sensitivity can be achieved by changing the voltage applied to a front dynode of an EM detector, even before enough time has elapsed for that change in voltage to take effect across all dynodes in the detector. Without wishing to be bound by theory, the inventors believe this is because the front dynodes with higher voltages set across the pair of the dynodes aren't typically coupled with capacitors towards the back of the detector and therefore can influence the sensitivity of the detector on a relatively short timescale. It is thought it would be more complicated to obtain a similar effect by applying the dynamic voltage waveform at the back of the detector, where dynodes are typically coupled with capacitors in parallel to the resistive network in order to prevent unwanted voltage spikes. As discussed above, typically the dynode pairs towards the back of the detector are held with very low voltages values across them and in addition tend to utilize zener diodes to fix the design desired voltages between dynode pairs.

So as to provide some example parameters, for the linear TOF mass spectrometer of Fig. 1(a) with 20kV total accelerating voltage, 1ms of the free flight time and mass range of interest from 2000Da to 30000Da, the pulse length ("second period of time" discussed above) is preferably up to 100usec. For the higher masses like Bovine serum albumin ("BSA") 66kDa or Immunoglobulin G ("IgG") 150kDa the pulse length ("second period of time" discussed above) is preferably up to 250usec. This numbers could be calculated specifically for each instrument setting or tuning, if required. As discussed above, the pulse could be arranged to increase the voltages only for a time window ("second period of time", discussed above) for a desired range of masses to be detected, with the initial voltage being set to effectively blank out those masses outside of the window. In this case, voltages within the window may be set e.g. to obtain a desired gain for the minimum and maximum mass and sensitivity requirements.

A typical MALDI mass spectrum of the protein bovine serum albumin (BSA) with and with detector gain control pulse is shown in Fig. 4.

In the mass spectrum of Fig. 4(a), a dynamic voltage waveform in the form of the pulse shown by the dashed line in Fig. 2(a), with delay of 50us to improve blanking and reduce background noise, was applied to a front dynode of the detector 150, i.e. so that the voltage was gradually increased during 150us to an additional value of about 60V.

A clear improvement of the signal detection at higher m/z values can be seen in Fig. 4(a) compared with the corresponding mass spectrum of Fig. 4(b) that was obtained using a constant operating voltage.

When used in this specification and claims, the terms "comprises" and "comprising", "including" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the possibility of other features, steps or integers being present.

The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

For example, a stacked MCP detector (multiple stage MCP assembly) may be used in place of the EM detector described above with a controlled circuit of applied dynamic gain voltages for the front electrode of the MCP stage in a similar manner. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

All references referred to above are hereby incorporated by reference The following statements provide general expressions of the disclosure herein.

REFERENCES: 1. U. Fehn. Isotope effect of ion-electron emission. int. J. Mass Spectrom. Ion Phys. 15,391 (1974).

2. U. Fehn. Variance of ion-electron coefficients with atomic number of impacting ions. Int. J. Mass Spectrom. Ion Phys. 21,1 (1976).

3. A. Westman, G. Brinkmalm, and D. F. Barofsky. MALDI Induced Saturation Effects in Chevron Microchannel Plate Detectors. Int. J. Mass Spectrom. Ion Proc., 169/170(1997): 79-87.

4. A. Cutter, K.L.Hunter, P.Paterson, R.Stresau. The "Ageing" Mechanism in Electron Multipliers and Operating Life. Technical Article SGE 72-A.

99. . -o -0 4 5. K.L. Hunter. Tips for optimising Multiplier Lifetime, Technical Article SGE 69A. http:

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Claims

CLAIMS: 1. A time of flight ("TOF") mass spectrometer having: an ion source; a detector; a variable voltage unit; a control unit; wherein the control unit is configured to control the TOF mass spectrometer to perform at least one acquisition cycle that includes: operating the ion source to produce ions having a plurality of m/z values, wherein the ion source is operated so that ions having different m/z values strike the detector at different times; operating the detector to produce an output current representative of ions having different m/z values striking the detector; operating the variable voltage unit to apply a dynamic voltage waveform to the detector during the acquisition cycle so that the magnitude of the dynamic voltage waveform varies within the acquisition cycle, wherein the magnitude of the output current produced by the detector is dependent on the magnitude of the dynamic voltage waveform applied to the detector by the variable voltage unit; wherein the dynamic voltage waveform is configured so that, within the/each acquisition cycle: a first voltage is applied to the detector by the variable voltage unit at a first time when ions having a first m/z value are striking the detector such that the output current includes a peak representative of ions having the first m/z value; and a second voltage, different from the first voltage, is applied to the detector by the variable voltage unit at a second time when ions having a second m/z value are striking the detector such that the output current includes a peak representative of ions having the second m/z value.
2. A TOF mass spectrometer according to claim 1, wherein the second m/z value is larger than the first m/z value and the second voltage is larger in magnitude than the first voltage.
3. A TOF mass spectrometer according to claim 1, wherein the second m/z value is larger than the first m/z value and the second voltage is smaller in magnitude than the first voltage.
4. A TOF mass spectrometer according to any of the previous claims, wherein the control unit is configured to control the TOF mass spectrometer to perform multiple acquisition cycles and the dynamic voltage waveform is substantially the same in each acquisition cycle.
5. A TOF mass spectrometer according to any of the previous claims, wherein the control unit is configured to control the TOF mass spectrometer to perform multiple acquisition cycles and the dynamic voltage waveform is synchronised to be produced at substantially the same time relative to an event occurring in each acquisition cycles.
6. A TOF mass spectrometer according to any of the previous claims, wherein the dynamic voltage waveform may vary in magnitude between a lower voltage value and an upper voltage value, wherein the difference between the lower voltage value and the upper voltage value is at least 10 Volts.
7. A TOF mass spectrometer according to any of the previous claims, wherein the dynamic voltage waveform is configured so that, within the/each acquisition 10 cycle: the voltage applied to the detector by the variable voltage unit is at an initial value during a first period of time occurring before ions having m/z values falling within a predetermined range of m/z values strike the detector; the voltage applied to the detector by the variable voltage unit varies in magnitude during a second period of time occurring whilst ions having m/z values falling within a predetermined range of m/z values strike the detector; the voltage applied to the detector by the variable voltage unit returns to the initial value during a third period of time occurring after ions having m/z values falling within the predetermined range of m/z values strike the detector.
8. A TOF mass spectrometer according to any of the previous claims, wherein the detector includes: an impact surface configured to emit one or more electrons when struck by an ion; and one or more electron multiplication surfaces; and a collector; wherein the one or more electron multiplication surfaces are configured to convert electrons emitted from the impact surface into a larger number of electrons to be collected by the collector to provide the output current.
9. A TOF mass spectrometer according to any of the previous claims, wherein the variable voltage unit applies the dynamic voltage waveform across a majority of, the electron multiplication surfaces of the detector.
10. A TOF mass spectrometer according to any of the previous claims, wherein the detector is a discrete dynode electron multiplier detector or a microchannel plate detector.
11. A TOF mass spectrometer according to any of the previous claims, wherein the dynamic voltage waveform is applied to a front electrode of the detector that faces towards ions incident on the detector when the TOF mass spectrometer is in use.
12. A TOF mass spectrometer according to any of the previous claims, wherein the mass spectrometer includes a power supply configured to supply the detector with an operating voltage, the variable voltage unit is separate from the power supply, and the variable voltage unit applies the dynamic voltage waveform to the detector by modulating the operating voltage applied to the detector by the power supply.
13. A TOF mass spectrometer according to any of the previous claims, wherein the variable voltage unit applies a dynamic voltage waveform to the detector under the control of a timing unit, wherein the timing unit includes circuitry configured to be triggered by an event in the/each acquisition cycle
14. A TOF mass spectrometer according to any of the previous claims, wherein the control unit may include timing circuitry configured to be triggered by an event in the/each acquisition cycle so as to provide synchronisation between the event and the production of the dynamic voltage waveform.
15. A method of operating a time of flight mass spectrometer according to any of the previous claims, wherein the method includes controling the TOF mass spectrometer to perform at least one acquisition cycle that includes: operating the ion source to produce ions having a plurality of m/z values, wherein the ion source is operated so that ions having different m/z values strike the detector at different times; operating the detector to produce an output current representative of ions having different m/z values striking the detector; operating the variable voltage unit to apply a dynamic voltage waveform to the detector during the acquisition cycle so that the magnitude of the dynamic voltage waveform varies within the acquisition cycle, wherein the magnitude of the output current produced by the detector is dependent on the magnitude of the dynamic voltage waveform applied to the detector by the variable voltage unit.
16. A method of modifying a TOF mass spectrometer that has: an ion source; a detector; a control unit; wherein the method includes configuring the control unit of a TOF mass spectrometer to control the TOF mass spectrometer to perform at least one acquisition cycle that includes: operating the ion source to produce ions having a plurality of m/z values, wherein the ion source is operated so that ions having different m/z values strike the detector at different times; operating the detector to produce an output current representative of ions having different m/z values striking the detector; operating a variable voltage unit to apply a dynamic voltage waveform to the detector during the acquisition cycle so that the magnitude of the dynamic voltage waveform varies within the acquisition cycle, wherein the magnitude of the output current produced by the detector is dependent on the magnitude of the dynamic voltage waveform applied to the detector by the variable voltage unit.
17. A method according to claim 16, wherein the method additionally includes installing a variable voltage unit in the TOF mass spectrometer.
18. A TOF mass spectrometer substantially as any one embodiment herein described with reference to and as shown in the accompanying drawings.