Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J49/00—Particle spectrometers or separator tubes
  • H01J49/26—Mass spectrometers or separator tubes
  • H01J49/34—Dynamic spectrometers
  • H01J49/40—Time-of-flight spectrometers

Definitions

• the present invention relates to time-of-flight (TOF) mass spectrometry, in particular to such type of a TOF mass spectrometer (TOFMS) that operates with an ion source emitting continuously in time.
• TOFMS TOF mass spectrometer
• the present invention specifically deals with a new concept of eliminating certain deficiencies of
• TOF mass spectroscopy is a special branch of mass spectroscopy; it is used to determine the chemical composition of a given sample.
• the sample to be analyzed can equally be gaseous, liquid or solid.
• a tiny part of the sample should be ionized and admitted into the vacuum chamber of a mass spectrometer apparatus which consequently separates the ions with respect to their mass/charge ratios.
• Most of the ions are single-charged, so the spectrum finally obtained is simply referred to as a mass spectrum. Separation of the ions can take place either in space, ie. ions of different mass/charge ratios arrive at various locations, or in time, ie. ions of different mass/charge ratios arrive at the same location, however, at different times.
• the present invention is based on the latter type of ion separation technique.
• all mass spectrometer apparatuses exploit electric and/or magnetic fields. That is why there is a need for generating ions from the sample; it is the charge of the ions that is able to interact with the fields applied.
• TOFMS apparatuses The operating principle of TOFMS apparatuses is as follows: in an ion source ions are generated which due to the acceleration effect of an electric field applied reach the same level of kinetic energy. As the ions have velocities inversely proportional to the square root of their mass/charge ratios, it takes them a definite time to fly over a given distance; the time of their flight is proportional to the square root of their mass/charge ratio. Hence, the time of the flight serves as the mass information sought.
• the time of the flight can be determined basically in two ways.
• the first possibility is that the launch of the ions takes place in a pulsed manner.
• the time of their flight can be determined as a difference between the time of arrival recorded and the time of launch.
• the pulsed type launch of ions can be carried out with either a pulsed ionization or gating of a continuously emitting ion source in a pulsed manner.
• the width of the launching pulse should be short in time because when the difference between the times of flight of two ions with neighbouring mass numbers is smaller than the width concerned, the spectrometer is not able to discriminate the two ions, ie. the spectrometer's resolution is limited by the width of the launching pulse.
• the second possibility is that the generation of ions takes place continuously and stationarily in time, ie. not in a pulsed manner.
• the obtained flow of ions is modulated in intensity by a cosine function of time at the beginning of the drift tube, and the frequency of the modulating signal is increased uniformly and continuously.
• the flow of ions is again subjected to a modulation in intensity.
• Both of the modulators are driven by the same signal.
• the magnitude of the current signal detected and its phase relative to that of the modulating signal are recorded for all the frequencies used for modulation.
• the TOF spectra are generated from the signal so detected by utilizing a Fourier transformation.
• 4,707,602 discloses a method and a TOF apparatus operating in the Fourier mode, ie. with a continuously emitting ion source instead of pulses of ions.
• the apparatus claimed contains an evacuable envelope, an ion source with a substance to be analysed emitting continuously, a drift tube of sub-atmospheric pressure, ion accelerating means in the form of a grid assembly and steering plates, detector means for detecting flow of ions, a modulator for modulating the flow of ions at a given modulation frequency, a frequency generator for providing uniform changes in the modulation frequency and transforming means for generating a mass spectrum of the substance under study from the signals recorded by the detector.
• the apparatus of US-4, 707,602 exhibits essentially two major advantegous features opposed to a TOFMS operating with a pulsed excitation of ions.
• the excitation is characterized by a duty cycle (percentage of the time when the excitation is active) of about 0,25, while that of an apparatus of a pulsed ion emission is only about 10 " .
• the difficulties emerging due to the space charge present within the flow of ions become much easier to handle as the space charge density of a flow of ions, due to a longitudinal spread-out, is much lower than what is observed in ion bunches present in apparatuses exploiting a pulsed type of ion emission.
• a TOFMS of the present type also exhibits some limitations and disadvantages with regard to the measurement and interpretability of data.
• the difficulties basically relate to the modulation of the intensity of the flow of ions.
• the time resolution is roughly equal to the period associated with the highest modulation frequency applied.
• One of the aims in a measurement is to reach the highest possible time resolution. This requires increasing the modulation frequency up to as high as possible.
• the longitudinal extension of the modulator ie. its dimension in the direction of the flow of ions
• the longitudinal extension of the modulator has a crucial effect on the maximal value of the modulation frequency of the flow of ions. If the effective wavelengths for the flow of ions are defined by the distances being covered during one period of the modulating signal at a given beam voltage by various ion species having definite mass/charge ratios, the shortest wavelength observable is just equal to the longitudinal extension of the modulator.
• the object of the present invention is therefore to eliminate, or at least to reduce the deficiencies of known TOFMS apparatuses used in Fourier mode, ie. to provide such an advanced TOFMS apparatus that is able on the one hand to operate at a modulation frequency being sufficient for ensuring a high resolution of the obtained mass spectra, and on the other hand to record TOF spectra free from undesirable higher harmonics being preclusive of interpreting the data.
• the aimed object of the present invention is reached in a first aspect by constructing of a TOFMS wherein the electronics comprises an analogue multiplier, and the output of the frequency generator is connected simultaneously (i) to the modulator and (ii) to a first input of the analogue multiplier, optionally via the broad band power amplifier; the output of the detector is connected to a second input of the analogue multiplier, optionally via the signal preamplifier; the output of the analogue multiplier is connected to a data acquisition channel of the controlling and processing unit, optionally via the low pass filter.
• the electronics comprises an analogue multiplier, and the output of the frequency generator is connected simultaneously (i) to the modulator and (ii) to a first input of the analogue multiplier, optionally via the broad band power amplifier; the output of the detector is connected to a second input of the analogue multiplier, optionally via the signal preamplifier; the output of the analogue multiplier is connected to a data acquisition channel of the controlling and processing unit, optionally
• the aimed object of the present invention is reached in a second aspect by preparing a modulator that is built up of a deflection unit and a pass unit separated by a certain distance;
• the deflection unit is prepared from two narrow sheet members each having a slit in the middle formed congruently, the sheet members are arranged one after the other in a transverse position relative to the path of the flow of ions and bound together via adjacent sides thereof, wherein one of the adjacent sides is insulating and the other one carries a pair of metallic deflection strips extending along the longitudinal edges of the slit formed therein, while the remaining free sides of the sheet members are metallic;
• the pass unit is prepared in the form of a single member made of a thin metal sheet having a slit in the middle being congruent with the slits of the sheet members of the deflection unit; and the metallic regions of the deflection unit and the pass unit are equally grounded.
• the modulator has a longitudinal extension comparable to the distance covered by the flow of ions during a period associated with the upper limit for the predefined range of frequencies.
• FIG 1 shows a schematic block diagram of an advanced time-of-flight mass spectrometer (TOFMS) according to the invention equipped with an analogue multiplier;
• Figure 2a is the longitudinal sectional view of the modulator used in the TOFMS according to the invention;
• Figure 2b shows front views of each building member of the modulator
• FIG. 3 is a further embodiment of the TOFMS apparatus according to the invention as used in the Example;
• Figure 4 is a plot of the current signal (in arbitrary units) versus the modulation frequency (in units of kHz) detected by the detector;
• FIG. 5 is the fast Fourier transform (FFT) of Fig. 4.
• Figures 6 and 7 are exemplary mass spectra derived from Fig. 5 by expressing the intensity on a linear and on a logarithmic scale, respectively, taken on ambient air.
• a TOFMS apparatus comprises an ion source 2 operating continuously within a vacuum chamber 1 , various ionoptical elements (not shown), an electrostatic reflector or an energy filter 3, a detector 4 preferably in the form of a secondary electron multiplier (SEM) and an electronics connected to the vacuum chamber 1 from the outside for control.
• the pressure of the residual gas in the vacuum chamber 1 is maintained below about 10 "6 mbar by means of a per se known pumping assembly (not shown) connected from the outside to the vacuum chamber 1.
• a per se known pumping assembly (not shown) connected from the outside to the vacuum chamber 1.
• the substance can be either combined with the ion source 2 or introduced to the ion source 2 from the outside in a manner (eg. through an introduction means) known in literature.
• a special modulator 5 (to be discussed later) with small longitudinal extension is placed in order to accelerate and modulate the flow of ions 7 therethrough. Behind the modulator 5 a free drift space 6 is formed.
• the electronics comprises a signal preamplifier 11 , an analogue multiplier
• a low pass filter 13 a controlling and processing unit 10 preferably in the form of a personal computer (PC), a frequency generator 9 of programmed frequency, preferably in the form of an oscillator, for sweeping over a predefined range of frequencies and a broad band power amplifier 8 for driving the modulator 5 at different modulation frequencies.
• the cut-off frequency of the low pass filter 13 is about several kHz.
• the frequency bandwidth of the broad band power amplifier 8 is equal to the highest modulation frequency applied. The latter ranges from about 1 MHz up to about several hundreds of MHz.
• the output of the detector 4 is connected directly to the input of the signal preamplifier 11.
• the output of the signal preamplifier 11 is directly connected to an input of the analogue multiplier 12.
• To a second input of the analogue multiplier 12 the modulating signal generated by the frequency generator 9 and optionally amplified by the broad band power amplifier 8 is fed.
• the output of the analogue multiplier 12 is connected to a data acquisition channel of the controlling and processing unit 10 via the low pass filter 13.
• a control output of the controlling and processing unit 10 is connected directly to the frequency control input of the frequency generator 9.
• the output of the frequency generator 9 is connected to the modulator 5 via the broad band power amplifier 8 in order to drive it.
• the TOFMS apparatus operates as follows.
• the flow of ions 7 generated by the ion source 2 travels through the energy filter 3, the modulator 5, and the drift space 6 and finally reaches the detector 4.
• the modulator 5 modulates the flow of ions 7 in intensity at a given frequency generated by the frequency generator 9.
• the modulated flow of ions 7 reaching the detector generates an electric signal therein.
• the signal which is a mixture of the first harmonic and many higher harmonics is led out from the vacuum chamber 1 and after having been subjected to a preamplification by means of the signal preamplifier 11 , it is fed into the analogue multiplier 12, instead of a customarily used demodulator.
• the signal is subjected to an analogue multiplication with the driving signal also applied to the modulator 5. Since after a proper adjustment the analogue multiplier 12 can be considered as an electronic device generating a pure cosine waveform, the multiplication results in an output signal of a pure cosine waveform, which means that all the higher harmonics are cancelled out or filtered from the signal. As a next step, this output signal is fed via the low pass filter 13 into the controlling and processing unit 10, which stores it for further processing and instructs the frequency generator 9 to generate the next driving signal of a higher frequency.
• the signal data acquisited are processed in a traditional way (ie. by being subjected to eg. a fast Fourier transformation) in order that a mass spectrum be generated.
• a traditional way ie. by being subjected to eg. a fast Fourier transformation
• Some illustrative spectra can be seen in Figs. 5 to 7.
• Figures 2a and 2b show a preferred embodiment of the modulator 5 used in the TOFMS apparatus according to the invention. It is basically built up of two components, a deflection unit 5a and a pass unit 5b, which are parallel to each other and arranged downstream in this order in the path of the flow of ions 7 perpendicular to the path. Furthermore, the two units are apart from each other by a distance ranging about 10 to 30 mm.
• the deflection unit 5a comprises two identical, generally circular members. Each member is made of a two-sided sheet of printed circuit board with a (deflection) slit 15 in the middle.
• the pass unit 5b is a single member made of a thin metal sheet with a (pass) slit 18 in the middle. It has also a circular shape in general and is grounded.
• the slits 15, 18 are aligned essentially congruently, their width and length is about 1 mm and about 10 mm, respectively.
• Fig. 2b the sides (denoted by A to F) of the members can be seen.
• hatching represents metallization of a certain area, while the areas without hatching denote insulator regions.
• one of the sides of the first member of the deflection unit 5a is fully metallized forming a screening plate 16, while the other side of the same member is without metallization, ie. it is left insulator.
• One of the sides of the second member of the deflection unit 5a is again fully metallized, so it also forms a screening plate 16, while a pair of deflection strips 14 extending parallel on both edges of the slit 15 is formed on the other side of the second member.
• the two members at issue are combined together by means of bonding the insulator side B and the side C carrying the deflection strips 14 together keeping the slits 15 aligned.
• the outer sides A and D of the deflection unit 5a ie. the screening plates 16 are grounded and serve to prevent the buildup of a stray electric field which in turn would distort the path of the flow of ions 7 approacing to and departing from the pair of deflection strips 14.
• the deflection strips 14 are narrow, ie. they are about 1 to 2 mm in width, and formed with a small surface area in order that their capacity relative to that of the screening plates 16 grounded be as small as possible for facilitating high frequency applications.
• the operation of the apparatus is demonstrated for ambient air introduced intentionally into the apparatus.
• the background pressure was kept as low as 10 "6 mbar, the air introduced represents 7x10 "6 mbar.
• the expected minor components are the usual water vapor and some carbon dioxide present at 18 and 44 atomic mass units (AMU), respectively.
• an embodiment of the apparatus according to the invention consists of the following major parts: a vacuum chamber with pumping means for providing low pressure therein, a source of ions embodied in the form of a gas introduction means and an ion gun for ionizing the molecules/atoms of the gas to be analyzed, an energy filter, an Einzel lens, a SEM detector, a modulator according to the invention with deflection control, an analogue multiplier (type 796), a high frequency drive amplifier, a controlled oscillator, a broad band low pass electrometer and a personal computer for controlling the setup and processing the data obtained.
• the Einzel lens serves to prevent beam divergency and to focus the flow of ions onto the detector. During measurements the vacuum is kept in the order of 10 "6 mbar.
• the ion gun provides a primary ion current of 100 nA at the beam voltage of 1 kV.
• the modulator in its original concept is simply a pair of deflection plates followed by an aperture as shown earlier in Fig. 2a.
• the intensity of the ion flow can easily be modulated by deflecting and conducting it through an aperture as it was already mentioned earlier.
• the actual measurement is carried out by increasing the modulation frequency from 1 MHz up to 8 MHz uniformly in 4096 steps.
• the output of the analogue multiplier is recorded for each frequency.
• a single cycle lasts for 3 ms, so the measurement takes about 13 seconds.
• the data collected are shown in Fig. 4 as a function of the modulation frequency.
• FIG. 5 shows spectra of air investigated up to 150 AMU with the intensity expressed on a linear and on a logarithmic scale, respectively.
• the nitrogen peak is roughly five times higher, which is the expected ratio, similar to natural air. Due to the application of the analogue multiplier electronics according to the invention, however, no higher harmonic peaks appear in the spectra, since the modulation applied is almost a perfect cosine one, in contrast to ion beam modulation carried out in the vacuum chamber.

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• Analytical Chemistry (AREA)
• Other Investigation Or Analysis Of Materials By Electrical Means (AREA)

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Citations (2)

• 2002
  • 2002-05-08 HU HU0201537A patent/HU224767B1/hu not_active IP Right Cessation
• 2003
  • 2003-05-08 AU AU2003230043A patent/AU2003230043A1/en not_active Abandoned
  • 2003-05-08 WO PCT/HU2003/000037 patent/WO2003096374A1/en not_active Ceased

Patent Citations (2)

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