GB2156072A - Mass spectrometer inlet system - Google Patents

Abstract

Method and apparatus for introducing a sample for analysis into a pyrolysis mass spectrometer which reduces the volume of air entering the system with each sample and enables more control over the temperature than can be achieved by Curie point heating. The sample is placed on a slide (16) which is sealed against the mouth of the spectrometer inlet system. The slide (16) carries a heater either in the form of a heating element (80) or in the form of a heat absorbing material which may be heated by an external energy source, the slide in the latter case being transparent.

Description

SPECIFICATION Mass spectrometer inlet system The present invention relates to an inlet system for a mass spectrometer and in particular for a pyrolysis mass spectrometer.

In a mass spectrometer, ions of a sample are analysed based upon their mass or massto-charge ratio in order to determine the ions present and their relative proportions. It is therefore necessary in various forms of mass spectrometer to be able to introduce the molecules to be analysed into the evacuated interior of the apparatus without affecting the vaccum within the apparatus.

In the prior art, the introduction of pyrolysate into the mass spectrometer relies upon a trap with an entry which may be opened to atmospheric conditions to allow the sample to be entered and which is then closed to seal off the sampleom the atmosphere. The trap is then evacuated by a purposely provided pump, and when the desired vacuum conditions are reached a valve is opened to connect the trap to the interior of the mass spectrometer.

When the sample is under vacuum conditions, it is pyrolysed and the pyrolysate passes through an expansion chamber and a capillary tube to form a molecular beam for ionisation and subsequent analysis in the mass spectrometer. The need for the expansion chamber stems from the fact that on pyrolysis the pressure within the trap increases substantially and a large volume expansion chamber is required to broaden the pressure pulse so that the molecules travelling down the capillary tube for analysis should do so at a controlled rate, the differential pressure being excessive in the absence of such an expansion chamber.

The prior art system as described above, requires a complicated arrangement of pumps and gates which is expensive and cumbersome to implement. Also because of the large amount of air entering the system with each sample, a considerable time is taken to reestablish a vacuum.

A proposal has already been made in copending Patent Appln. No. 831 5956 to mitigate these disadvantages. In the latter Patent Application, there is proposed an inlet system for use in a pyrolysis mass spectrometer which comprises a capillary tube for the introduction of pyrolysate for analysis into the ion source of the spectrometer, an expansion chamber arranged at the end of the capillary tube, an aperture formed in the expansion chamber and sealingly connectable to a tube containing a sample to be analysed, a valve closure member slidably disposed within the expansion chamber for opening and closing the aperture, a pumping line for evacuating the expansion chamber to low vacuum after communication is established between the expansion chamber and the sample tube and means for sealing off the pumping line from the expansion chamber after a low vaccum has been achieved in the latter chamber to permit evacuation of the expansion chamber to a higher vacuum by way of the capillary tube.

The above proposal reduces the volume of air that must be pumped out of the vacuum system before analysis of each sample and therefore speeds the analysis considerably.

However, the incorporation of the sample in a tube means that the volume of air in the tube must inevitably enter the system and this limits the operating speed.

The method of pyrolysing the sample in the above application involved Curie point heating of a boat arranged within the sample tube, on which boat the sample was deposited. The ferromagnetic boat is heated by a heating coil which must surround the boat and this necessitated the use of sample tubes.

Curie point heating relies on the change in magnetic properties of materials when their Curie temperature is reached. If the heating is being caused by magnetic hysteresis, the heating effect stops when the Curie temperature is reached. This enables the sample to be heated very rapidly to a predetermined temperature but the temperature cannot be varied and is dictated by the material used in making the boat.

The present invention seeks to provide a method of sample introduction into a pyrolysis mass spectrometer which permits greater advantage to be taken of the fast analysis facilitated by the proposal in the above copending application and which provides greater control over the tenperature of pyrolysis than Curie point heating.

According to a first aspect of the invention, there is provided a method of introducing a sample for analysis into a pyrolysis mass spectrometer, in which the sample is mounted on a slide, the slide is urged against the mouth of the inlet system to seal against the said mouth and form part of the vaccum envelope of the spectrometer, and the sample is heated by a heater carried by the slide.

The heater may either be an electric heating element through which a current is passed or a film of energy absorbing material capable of being heated by a radiant energy source arranged outside the envelope of the spectrometer.

In accordance with a second aspect of the invention, there is provided a sample carrier for introducing a sample to be analysed into a mass spectrometer, the carrier comprising a slide, means for enabling the slide to seal against the mouth of the inlet system of the spectrometer and heating means on the slide for generating or absorbing energy in order to pyrolyse the sample for analysis.

Preferably, the slide carries an electrical heating element through which a current is passed to heat the sample, the element being deposited as a thick film onto the slide. In order to enable a seal to be established with mouth of the inlet system, a raised ring of the same thickness as the thick film constituting the heater element may be provided around the heating portion of the element to seal against a flat surface on the mouth of the inlet system.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows an inlet system generaly similar to that described in Application No.

8315956, Figure 2 shows a sample carrier in accordance with a fist embodiment of the invention, and Figure 3 shows as second embodiment having an alternative form of heating: In Fig. 1, there is shown a capillary tube 10 through which molecules are introduced into the ion source of a mass spectrometer for analysis. The mass spectrometer need not be considered in detail within the context of the present application but it is mentioned by way of background that the inlet system in Fig. 1 is intended for use with a quadrupole mass spectrometer in which the ions are accelerated and pass down a quadrupole structure which acts as a filter only permitting ions of a certain mass-to-charge ratio to reach a collector electrode. The inlet system may however be employed with any other form of mass spectrometer.

The capillary tube 10 is surrounded by a heating jacket 1 2 and opens at its upper end as viewed into an expansion chamber 14 which is sealed by a slide 1 6 carrying a sample.

In addition to carrying the sample, the slide 1 6 supports heating means for pyrolysing the sample. The heating means may, as shown in Fig. 2, comprise a heater element 80 deposited as a thick film on the slide 16. The slide must seal against the mouth of the inlet system and in order that the raised film should not interfere with sealing, the heating portion designated 80a is surrounded by annular portion 80b which may seal against an O-ring located in a groove surrounding the mouth of the inlet system. By appropriate dimensioning of the heating portion 80a and the annular portion 80b it is possible to ensure that most of the electrical power is dissipated in the heating portion 80a.Contact pads 80c lie outside the vacuum chamber and are contacted by resiliently biassed contacts pins recessed into the outer surface of the expansion chamber (not shown in Fig. 1) Alternatively, the contacts on the slide may be brought to the opposite surface either by passing around the edges of the slide or by the use of plated through holes whereupon the contact pins may form part of the backplate used to urge the sample slide against the mouth of the inlet system.

Fig. 3 shows a transparent slide 1 6 in which a central area on which the sample is placed is coated with a black heat resistant material 90 which may be heated by means of a radiant energy source disposed outside the vacuum system. The energy source may be a laser beam but alternatively a heated filament may be optically imaged onto the heat absorbing coating 90.

The sample may thus be pyrolysed by externally applied energy or electric current and the temperature of the sample can be varied and controlled at will. Conveniently, the temperature is controlled by means of a feedback control system which varies the amount of energy reaching the sample in dependence upon the difference between the desired temperature and the actual temperature as measured directly.

In use, the sample on the slide is heated to the desired temperature. The vapour resulting enters the expansion chamber 1 4 and there expands to reduce its pressure, the reduced pressure serving to drive the vapour at a slower rate through the capillary tube 10 towards the accelerating electrode of the electrode structure of the spectrometer. The purpose of the heating jacket 1 2 is to prevent particles from being deposited on the inner surface of the capillary tube 10.

A piston 1 8 is reciprocable within the expansion chamber 14 an O-ring seal 20 is provided around the mouth of the aperture which is sealed by the sample slide 1 6. The piston 18 has a piston rod 22 which is formed with an axially extending recess 25 which acts as part of a valve for connecting the expansion chamber 14 to a rotary pump connected to a port 24. The end of the piston rod 22 is connected to a bellows 26 acting as a vacuum seal, the rod being movable by means of a motor 28.

The piston 18 when in its extreme right position as viewed seals against a second seal 30 so that the expansion chamber 14 is isolated from the rotary pump. When the piston moves slightly to the left, it seals off the end of the capillary tube 10 but also establishes communication between the expansion chamber 14 and the rotary pump through the recess 25 in the piston rod 22.

The piston is provided with a passage 32 so that it permits communication between the spaces on its opposite sides and consequently both the sample tube and the expansion chamber 1 4 are evacuated by means of the rotary pump.

When it is desired to introduce a sample into the spectrometer the piston 1 8 is first moved to the left to seal against the seal 20 at the same time sealing off the end of the capillary tube 10 so as not to permit entry of any gases into the spectrometer.

The sample slide 1 6 is now secured in position sealing against the outer surface of the mouth, a further O-ring 34 arranged outside the expansion chamber 14 being provide for this purpose. The piston 1 8 is then moved slightly to the right permitting some air to enter the expansion chamber 14 which until this time has been under high vacuum. It is noted however that because the sample carrier is in the form of a slide the quantity of air entering the system at this stage is minimal and little time is taken to re-establish the vacuum conditions within the spectrometer between samples. During the time that air is in the system, the end of the capillary 10 is closed off by the piston 1 8.

Such air as enters the system causes a rise in pressure but only for a short time because the rotary pump which has a high displacement rapidly reduces the pressure in the expansion chamber 14 to provide a low vacuum. Once a sufficiently low pressure has been reached in the expansion chamber 14, the piston 1 8 is moved to the right, sealing against the seal 30 and now isolating the expansion chamber 1 4 from the rotary pump and opening the capillary tube.The interior space of the spectrometer is permanently evacuated by means of a diffusion pump and the same diffusion pump will now take relatively little time to evacuate the expansion chamber 14 through the capillary tube 1 0. Once the vacuum within the expansion chamber 14 is sufficiently good, the sample is pyrolised to allow particles to enter the spectrometer for analysis.

When Curie point heating is employed to pyrolyse a sample, it is possible to determine the temperature with accuracy, as is necessary, but the analysis may only take place at a fixed temperature. It may be desired at times either to vary the temperature, depending on the nature of the sample to be analysed, or possibly to carry out the analysis in two stages, first at a low temperature at which the breakdown of the sample is incomplete and subsequently at a higher temperature.

The invention permits such analysis since the temperature is controlled during use by sensing the actual temperature and varying the amount of current passing through the heating element or the amount of energy incident upon the slide to achieve the desired analysis temperature. A measure of the temperature may be derived for example by measuring the resistance of the heater element before and during pyrolysis.

It is a further advantage of the invention that the sample for analysis may be prepared with greater ease by laboratory technicians.

One reason for this is that the slides are larger and more easily handled than small tubes and another is that laboratory workers are more familiar with slides, these being the usual method of mounting specimens for analysis.

A still further advantage of using slides for mounting the specimens is that it simplifies automated feeding of samples to a spectrometer. In this context it is noted that slides are easily stacked and may be fed one at a time to the spectometer by a fairly simple indexing mechanism. Also in such automated feeding, it is important to be able to identify the origin of each sample and by contrast with sample tubes, slides are also easily labelled.

Claims (8)

1. A method of introducing a sample for analysis into a pyrolysis mass spectrometer, in which the sample is mounted on a slide, the slide is urged against the mouth of the inlet system to seal against the said mouth and form part of the vacuum envelope of the spectrometer, and the sample is heated while supported on said slide.

2. A method as claimed in claim 1, wherein the sample is heated by passing a current through a heating element formed on said slide.

3. A method as claimed in claim 1, wherein the sample is heated by absorbing radiant energy on a heat absorbant coating formed on said slide.

4. A sample carrier for introducing a sample to be analysed into a mass spectometer, the carrier comprising a slide, means for enabling the slide to seal against the mouth of the inlet system of the spectrometer and heating means on the slide for generating or absorbing energy in order to pyrolyse the sample for analysis.

5. A sample carrier as claimed in claim 4, wherein the slide carries an electrical heating element through which a current may be passed to heat the sample, the element being deposited as as thick film onto the slide.

6. A sample carrier as claimed in claim 5, wherein in order to enable a seal to be established with mouth of the inlet system, a raised ring of the same thickness as the thick film constituting the heater element is provided around the heating portion of the element to seal against a flat surface on the mouth of the inlet system.

7. A sample carrier as claimed in claim 5, wherein a coating of heat resistant energy absorbing material is formed on the slide capable of being heating by means of a radiant energy source disposed outside the envelope of the mass spectrometer.

8. A method of introducing a sample for analysis into the inlet system of a pyrolysis mass spectrometer as claimed in claim 1 and substantially as herein described with reference to and as illustrated in the accompanying drawings.