GB2155664A

GB2155664A - Pyrolysis mass spectrometer

Info

Publication number: GB2155664A
Application number: GB08406773A
Authority: GB
Inventors: Thomas William Ottley
Original assignee: Prutec Ltd
Current assignee: Prutec Ltd
Priority date: 1984-03-15
Filing date: 1984-03-15
Publication date: 1985-09-25
Also published as: EP0177530A1; WO1985004282A1; GB8406773D0; AU4068085A; JPS61501664A

Abstract

Method of pyrolysing a sample for analysis in a mass spectrometer, which comprises providing a transparent window (10) in the vacuum envelope of the mass spectrometer for permitting the sample (12) to be viewed during pyrolysis. The blackbody radiation emitted during heating is monitored to provide an electrical signal indicative of the temperature and the heating source (20) is controlled in dependence upon the sensed temperature in such a manner as to maintain the pyrolysis temperature at a predetermined value.

Description

SPECIFICATION Pyrolysis mass spectrometer The invention relates to a pyrolysis mass spectrometer.

In a pyrolysis mass spectrometer, the particles used to form the ion beam are produced by pyrolysing a sample to be analysed. The degree to which the sample is broken down depends upon the temperature of the pyro lysis and it is therefore important to reguiate the temperature with precision.

One known way for pyrolysing a sample employs the principle of Curie point heating.

The sample is placed on a ferromagnetic boat and heated by passing current through a coil surrounding the boat. The losses caused by magnetic hysteresis raise the temperature of the boat rapidly but on reaching the Curie point, no further heating occurs due to the change in the magnetic properties of the material. Thus the boat and the sample could be raised rapidly to an accurately predetermined temperature. However, the temperaure is predetermined by the material use in the making of the boat and cannot be varied at will.

Other methods of heating sample in order to pyrolyse it have also been proposed but they suffer from the disadvantage that it is difficult to regulate the temperature.

The present invention seeks to provide a method of pyrolysing a sample which offers greater flexibility than Curie point heating in determining the temperature of analysis, yet permits the temperature to be controlled accurately.

According to the present invention, there is provided a method of pyrolysing a sample for analysis in a mass spectrometer, which comprises providing a transparent window in the vacuum envelope of the mass spectrometer for permitting the sample to be viewed during pyrolysis, heating the sample by a controllable heating source, monitoring the blackbody radiation emitted during the heating to provide an electrical signal indicative of the temperature and controlling the heating source in dependence upon the the sensed temperature in such a manner as to maintain the pyrolysis temperature at a predetermined value.

The sample may conveniently be mounted on a transparent slide or within a transparent tube forming part of the vacuum envelope of the spectrometer.

Preferably, the sample is supported on a substrate having a coating of predetermined emissivity. For example, the sample may rest on a glass slide having a coating of high emissivity or it may rest on a foil provided a coating of high emissivity. In the latter case, it is further preferred that the radiation from the side of the foil not in contact with the sample be used to indicate the pyrolysis temperature as the measurement is not then affected by the emissivity of the sample.

Though the heating may be effected by means of an electrical heating element, it is preferred that the energy from a radiant energy source disposed outside the vacuum envelope of the spectrometer be imaged onto the sample support in order to heat the sample. The source may be a laser source but because of the small size of the sample, it is sufficient and less expensive to image the filament of a bulb onto the sample carrier.

The frequency spectrum of the blackbody radiation is temperature dependent and at the temperature used to pyrolyse samples in a mass spectrometer the main energy falls in the infrared region of the spectrum. It is important to note therefore that the term "transparent" as used herein is intended to mean transparent to infrared radiation rather than visible radiation.

It is not the total amount of energy emitted in this frequency range that is indicative of temperature but the shape of the frequency spectrum. In the preferred embodiment of the invention, the emitted radiation is incident upon two detectors with different frequency characteristics, the relative outputs of the two detectors providing an indication of the temperature of pyrolysis.

The detectors may be of the same type but their frequency response altered by means of filters having different pass bands arranged in the apth of the incident radiation.

The invention will now be described further, by way of example, with reference to the accompanying drawing which is a schematic representation of apparatus for pyrolysing a sample in mass spectrometer.

In the accompanying drawing, a glass slide 10 carrying a sample 1 2 is urged into sealing engagement with the mouth of the inlet system 1 4 of a mass spectrometer. A suitable inlet system is described in Patent Application No. 8315956 but a detailed explanation of the operation of the mass spectrometer is not essential to the understanding of the present invention. It suffices to say that the particles resulting from the pyrolysis are formed into an ion beam which passes through a filter capable of discriminating between ions based upon their mass or mass-to-charge ratio. This enables an indication to be given of the atomic masses present and their relative proportions. The particles present are dependent upon the temperature of pyrolysis as this affects the degree to which the sample is broken down.

The slide 10 is formed with a coating of predetermined emissivity on which the sample 1 2 rests. The coating preferably has high emissivity and absorbance and must itself be capable of withstanding high temperature without releasing particles into the vacuum. A suitable material for the coating is oxidised nickel which meets these requirements but other materials such as tungsten may be employed.

In order to heat the sample 12, an image of the filament of a bulb 20 (preferably a tungsten halogen bulb) is formed on the coating on the slide 10, by means of an annular lens 22. The current through the filament of the bulb 20 is variable by a suitable circuit (not illustrated) to allow regulation of the heating.

At the centre of the lens 22, there is provided a concave mirror 24 facing the slide 10 and focussing the blackbody radiation emitted from the coating on the slide onto a split detector 26. The split detector is associated with two filters 28, 30 having differing pass bands. The relative amounts of energy sensed by the two parts of the split detector 26 indicate the temperature of the coating on the slide and hence the pyrolysis temperature. A shield 32 prevents the detector from being affected directly by the energy emitted by the bulb 20.

The glass used in the slide is of importance since some glasses are opaque to infrared.

The use of inexpensive borosicate glass precludes measurement much above wavelengths of 2um but this still permits measurement upto a temperature of 7400"C, which suffices for most applications.

The temperature control circuit typically comprises a first differential amplifier to measure the difference between the output of the split detector 26, the output of the differntial amplifier being representative of the actual temperature. A second differential amplifier may compare the output of the first differential amplifier with a preset voltage representing the desired temperature and produce an error signal which controls the current flowing through the filament of the bulb 20 and thereby alter the energy incident upon the sample.

It is seen in the case of the embodiment described above, the heating and temperature monitoring are performed from the same side of the sample by suitable design of the optical system. This need not necessarily always be the case. For example, if the sample is arranged on a foil disposed within a tube, the bulb 20 and its imaging optics may be arranged on one side of the sample while the detectors may be arranged on the opposite side or at right angles. This configuration reduces the possibility of the detectors being directly affected by the energy emitted by the bulb 20, thereby avoiding the need for the heat shield 32.

Claims

1. A method of pyrolysing a sample for analysis in a mass spectrometer, which comprises providing a transparent window in the vacuum envelope of the mass spectrometer for permitting the sample to be viewed during pyrolysis, heating the sample by a controllable heating source, monitoring the blackbody radiation emitted during the heating to provide an electrical signal indicative of the temperature and controlling the heating source in dependence upon the the sensed temperature in such a manner as to maintain the pyrolysis temperature at a predetermined value.

2. A method as claimed in claim 1, wherin the sample is mounted on a transparent slide or within a transparent tube forming part of the vacuum envelope of the spectrometer.

3. A method as claimed in claim 1 or 2, wherein the sample is supported on a substrate having a coating of predetermined emissivity.

4. A method as claimed in claim 3, wherein the coating is of nickel oxide.

5. A method as claimed in claim 3 or 4, wherein the temperature is measured by monitoring the radiation emitted by a part of the coating not covered by the sample.

6. A method as claimed in any preceding claim, wherein the sample is heated by imaging the energy from a radiant energy source disposed outside the vacuum envelope of the spectrometer onto the sample support.

7. A method a claimed in claim 6, wherein the source is the filament of an incandescent light bulb.

8. A method as claimed in any preceding claim, wherein the emitted blackbody radiation is incident upon two detectors with different frequency characteristics, the relative outputs of the two detectors providing an indication of the temperature of pyrolysis.

9. A method of pyrolysing a sample for analysis in a mass spectrometer, substantially as herein described with reference to and as illustrated in the accomapnying drawing.