Sample preparation apparatus and method for elemental analysis spectrometer

Abstract

A sample preparation apparatus for an elemental analysis system comprising a sample combustion and/or reduction and/or pyrolysis arrangement for receiving a sample of material to be analyzed, and producing therefrom a sample gas flow containing atoms, molecules and/or compounds; a gas chromatography (GC) column into which the sample gas flow is directed; a heater for heating at least a part of the GC column; and a controller for controlling the heater. The controller is configured to control the heater so as to increase the temperature of at least the part of the GC column while the sample gas flow in the GC column elutes.

Claims

1. A sample preparation apparatus for an elemental analysis system, comprising: a sample combustion and/or reduction and/or pyrolysis arrangement for receiving a sample of material to be analyzed, and producing therefrom a sample gas flow containing atoms, molecules and/or compounds including N.sub.2 and CO.sub.2; a gas chromatography (GC) column into which the continuous sample gas flow is directed; a heater for heating at least a part of the GC column; and a controller for controlling the heater; the controller including a timing circuitry and being configured to control the heater so as to increase the temperature of at least the part of the GC column either (1) whilst CO.sub.2 in the continuous sample gas flow in the GC column elutes or (2) whilst the continuous sample gas flow in the GC column elutes and after the N.sub.2 and CO.sub.2 have passed the GC column.

2. The sample preparation apparatus of claim 1, further comprising a detector for detecting atoms, molecules or compounds that have passed through the GC column, and wherein the controller is configured to control the heater so that the temperature of at least the part of the GC column is increased after a first one or more species of atoms, molecules or compounds have passed the GC column.

3. The sample preparation apparatus of claim 1, further comprising a thermometer or thermocouple for monitoring a temperature of at least part of a GC column, wherein the controller is configured to control the heater so that the temperature changes substantially linearly between a start temperature T.sub.start and an end temperature T.sub.end.

4. The sample preparation apparatus of claim 1, further comprising a thermometer or thermocouple for monitoring a temperature of at least part of a GC column, wherein the controller is configured to control the heater so that the temperature changes substantially non-linearly between a start temperature T.sub.start and an end temperature T.sub.end.

5. The sample preparation apparatus of claim 1, further comprising a thermometer or thermocouple for monitoring a temperature of at least part of a GC column, wherein the controller is configured to control the heater so that the temperature change is partly linear and partly non linear between a start temperature T.sub.start and an end temperature T.sub.end.

6. The sample preparation apparatus of claim 1, further comprising a detector for detecting atoms, molecules or compounds that have passed through the GC column and a thermometer or thermocouple for monitoring a temperature of at least part of a GC column, and wherein the controller is configured to control the heater so that the temperature of at least the part of the GC column is increased after a first one or more species of atoms, molecules or compounds have passed the GC column across a first temperature range (T.sub.2T.sub.start), and wherein T.sub.2>T.sub.start.

7. The sample preparation apparatus of claim 6, wherein the controller is configured to control the heater so that the temperature of at least the part of the GC column is increased after a first one or more species of atoms, molecules or compounds have passed the GC column with substantially linearly, or substantially non-linearly, or with both linear and non-linear temperature changes, across the first temperature range (T.sub.2T.sub.start).

8. The sample preparation apparatus of claim 1, further comprising a detector for detecting atoms, molecules or compounds that have passed through the GC column and a thermometer or thermocouple for monitoring a temperature of at least part of a GC column, and wherein the controller is configured to control the heater so that the temperature in at least the part of the GC column changes across a second temperature range (T.sub.endT.sub.2), and wherein T.sub.end>T.sub.2 before a second one or more species of atoms, molecules or compounds have passed the GC column.

9. The sample preparation apparatus of claim 8, wherein the controller is configured to control the heater so that the temperature changes substantially linearly, or substantially non-linearly, or with both linear and non-linear temperature changes, across the second temperature range (T.sub.endT.sub.2) before a second one or more species of atoms, molecules or compounds have passed the GC column.

10. The sample preparation apparatus of claim 2, wherein the sample and/or reduction and/or pyrolysis arrangement is configured to generate N.sub.2, and CO.sub.2 or N.sub.2, CO.sub.2 and SO.sub.2.

11. The sample preparation apparatus of claim 1, further comprising a thermometer or thermocouple for monitoring a temperature of at least part of a GC column, and wherein the controller is configured to maintain the temperature of the GC column at a first, fixed temperature T.sub.start during a first period of sample analysis, to ramp the temperature of the GC column from the first fixed temperature T.sub.start to a second, higher fixed temperature T.sub.end over a second period of sample analysis, and to maintain the temperature of the GC column at the second, higher fixed temperature T.sub.end over a third period of sample analysis.

12. The sample preparation apparatus of claim 11, wherein the controller is configured to commence the ramping the temperature of the GC column from the first temperature T.sub.start to the second fixed temperature T.sub.end, at a predetermined time after combustion/reduction/pyrolysis of the sample.

13. The sample preparation apparatus of claim 11, wherein the first fixed temperature lies in the range of 35 to 90 degrees Celsius.

14. The sample preparation apparatus of claim 11, wherein the second fixed temperature T.sub.end is between 190 degrees Celsius and 300 degrees Celsius.

15. The sample preparation apparatus of claim 11, wherein the controller is configured to instruct the heater to cause the GC column to rise in temperature from the said first fixed temperature T.sub.start to the second fixed temperature T.sub.end over a period of around 1 to 3 minutes.

16. The sample preparation apparatus of claim 11, wherein the controller is further configured to ramp the temperature of the GC column down from the second higher fixed temperature T.sub.end to the first fixed temperature T.sub.start over a fourth period of sample analysis following the said third period of sample analysis.

17. The sample preparation apparatus of claim 16, wherein the controller is configured to instruct the heater to cause the GC column to drop in temperature from the said second fixed temperature T.sub.end to the first fixed temperature T.sub.start over a period of around 1 to 3 minutes.

18. The sample preparation apparatus of claim 17, wherein the controller is configured to commence the ramp down of temperature of the GC column from the second temperature T.sub.end to the first temperature T.sub.start, at a predetermined time after the GC column has attained the said second temperature T.sub.end during the said third period of sample analysis.

19. A sample preparation apparatus for an elemental analysis system, comprising: a sample combustion and/or reduction and/or pyrolysis arrangement for receiving a sample of material to be analyzed, and producing therefrom a sample gas flow containing atoms, molecules and/or compounds including N.sub.2 and CO.sub.2; a gas chromatography (GC) column into which the continuous sample gas flow is directed; a heater for heating at least a part of the GC column; and a controller for controlling the heater; the controller being configured to control the heater so as to increase the temperature of at least the part of the GC column either (1) whilst CO.sub.2 in the continuous sample gas flow in the GC column elutes or (2) whilst the continuous sample gas flow in the GC column elutes and after the N and CO.sub.2 have passed the GC column; wherein the controller is configured to maintain the temperature of the GC column at a first, fixed temperature T.sub.start during a first period of sample analysis, to ramp the temperature of the GC column from the first fixed temperature T.sub.start to a second, higher fixed temperature T.sub.end over a second period of sample analysis, and to maintain the temperature of the GC column at the second, higher fixed temperature T.sub.end over a third period of sample analysis; wherein the controller is further configured to ramp the temperature of the GC column down from the second higher fixed temperature T.sub.end to the first fixed temperature T.sub.start over a fourth period of sample analysis following the said third period of sample analysis; wherein the GC column is located within a housing, and wherein the apparatus further comprises a means for directing relatively cooler gas into the housing to expel relatively warmer gas within the housing.

20. The sample preparation apparatus of claim 19, wherein the housing comprises a plurality of walls, at least some of which define an internal channel for receiving the expelled relatively warmer gas and directing it out of the housing through one or more openings therein, and wherein the means for directing relatively cooler gas into the housing comprises a fan or a pump.

21. A sample preparation apparatus for an elemental analysis system, comprising: a sample combustion and/or reduction and/or pyrolysis arrangement for receiving a sample of material to be analyzed, and producing therefrom a sample gas flow containing atoms, molecules and/or compounds including N.sub.2 and CO.sub.2; a gas chromatography (GC) column into which the continuous sample gas flow is directed; a second GC column or a LC column wherein the sample to be analyzed and received by the sample combustion and/or reduction and/or pyrolysis arrangement has been generated by the second GC column or LC column by a chromatographic process from a sample supplied to the second GC column or LC column; a heater for heating at least a part of the GC column; and a controller for controlling the heater; the controller being configured to control the heater so as to increase the temperature of at least the part of the GC column either (1) whilst CO.sub.2 in the continuous sample gas flow in the GC column elutes or (2) whilst the continuous sample gas flow in the GC column elutes and after the N.sub.2 and CO.sub.2 have passed the GC column.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention may be put into practice in a number of ways and some specific embodiments will now be described by way of example only and with reference to the accompanying drawings in which:

(2) FIG. 1a shows a highly schematic arrangement of a part of a prior art elemental analysis isotope ratio mass spectrometer (EA-IRMS);

(3) FIG. 1b shows, again highly schematically, an arrangement of a part of a prior art gas chromatography isotope ratio mass spectrometer (GC-IRMS);

(4) FIG. 2 shows a plot of the detected output of the EA-IRMS of FIG. 1a, during simultaneous Nitrogen, Carbon and Sulphur (NCS) analysis of a sample of Sulfanilamide;

(5) FIG. 3a shows a highly schematic arrangement of a sample preparation section of an EA-IRMS in accordance with an embodiment of the present invention, having a system controller and a temperature varying GC column;

(6) FIG. 3b shows a highly schematic arrangement for varying the temperature of the GC column in FIG. 3a;

(7) FIG. 4 shows a flowchart illustrating the operation of the system controller and GC column of FIGS. 3a and 3b using a temperature increase during sample analysis;

(8) FIG. 5 shows a chromatogram of N.sub.2 and CO.sub.2 peaks obtained from a prior art EA-IRMS with an isothermal GC, using caffeine as a sample;

(9) FIG. 6 shows a first exemplary temperature profile that may be applied to the temperature varying GC column of FIG. 3a;

(10) FIG. 7 shows a chromatogram of N.sub.2 and CO.sub.2 peaks obtained from an EA-IRMS embodying the present invention, to which the temperature profile of FIG. 6 is applied during sample elution, again using caffeine as a sample;

(11) FIG. 8 shows a chromatogram of N.sub.2, CO.sub.2 and SO.sub.2 peaks obtained from a prior art EA-IRMS with an isothermal GC, using sulfanilamide as a sample;

(12) FIG. 9 shows a second exemplary temperature profile that may be applied to the temperature varying GC column of FIG. 3a;

(13) FIG. 10 shows a chromatogram of N.sub.2CO.sub.2 and SO.sub.2 peaks obtained from an EA-IRMS embodying the present invention, to which the temperature profile of FIG. 9 is applied during sample elution, again using sulfanilamide as a sample;

(14) FIG. 11 shows a third exemplary temperature profile that may be applied to the temperature varying GC column of the invention;

(15) FIG. 12 shows a fourth exemplary temperature profile that may be applied to the temperature varying GC column of the invention;

(16) FIG. 13 shows a fifth exemplary temperature profile that may be applied to the temperature varying GC column of the invention;

(17) FIG. 14 shows a sixth exemplary temperature profile that may be applied to the temperature varying GC column of the invention;

(18) FIG. 15 shows a seventh exemplary temperature profile that may be applied to the temperature varying GC column of the invention;

(19) FIG. 16 shows an eighth exemplary temperature profile that may be applied to the temperature varying GC column of the invention; and

(20) FIG. 17 shows a ninth exemplary temperature profile that may be applied to the temperature varying GC column of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(21) Referring first to FIG. 3a, a highly schematic arrangement of a sample preparation section of an EA-IRMS in accordance with an embodiment of the present invention is shown. Those components common to FIGS. 1a and 3a are labelled with like reference numerals.

(22) The sample preparation and combustion/reduction proceeds, in the embodiment of FIG. 3a, in the same manner as was described in the Background section above, in respect of FIG. 1a. To avoid unnecessary repetition, this part of the process will only be summarised here.

(23) A sample (not shown in FIG. 3a) is weighed and placed in a combustible capsule that is sealed and placed into an autosampler carousel 10 positioned above a combustion furnace 20. The autosampler carousel 10 injects the sealed sample capsule into the combustion furnace 20 under the control of a system controller 200. As before, Helium may be supplied to the autosampler 10 as a purge gas, and combustion in the combustion furnace 20 may be carried out in the presence of pulsed oxygen.

(24) Helium carrier gas is employed to carry the sample across an oxygen donor compound. The flow rate of the helium carrier gas is again optimally between 40 and 200 mL/min, but can be up to 1000 mL/min. The reaction zone in the combustion furnace 20 is typically held at a temperature between 400 and 1100 degrees Celsius, with an ideal range of between 900 and 1050 degrees Celsius.

(25) The resulting NO.sub.x, CO.sub.2, SO.sub.2 and/or H.sub.2O products are reduced in a reduction oven 30, which may be a separate component as shown schematically in FIG. 3a, or may form a part of a single, combined combustion/reaction unit.

(26) The reduction oven 30 is generally held at a temperature between 450-900 C. and the gases exiting that reduction oven are then directed through optionally a chemical trap 40 and a moisture trap 50, again as previously described; the order of the chemical and moisture traps 40, 50 may be reversed depending upon the reagents employed in each.

(27) The dried gaseous output of the moisture trap 50 is introduced into a GC column 60, for separation of the gases. The GC column 60 of preferred embodiments of the present invention will be described in further detail below, but in general terms, the GC column 60 may preferably incorporate a carbon molecular sieve.

(28) The GC column 60 of FIG. 3a is mounted within a GC chamber 250 whose interior is heated by halogen lamps 65. The halogen lamps 65 are controlled by a heater controller 68 which is connected to the system controller 200. A fan 240 draws ambient (cool) air into the interior of the chamber 250.

(29) FIG. 3b shows an embodiment of the GC chamber 250 of FIG. 3a, in schematic sectional view. The GC chamber 250 contains the GC column 60 which is positioned generally centrally of the GC chamber 250. The GC chamber 250 has outer side walls 64a, a base 64b and a closure 64c, each of which are formed of an insulating material. The inner surfaces of the outer side walls 64a and the closure 64c have a reflective coating. The outer side walls 64a are separated from the closure 64c by openings 260, 260.

(30) Extending in an axial direction of the GC chamber 250 are inner walls 66. The inner walls are also coated or formed from a reflective material. The inner walls 66 are spaced inwardly of the outer side walls 64a of the GC chamber 250 so as to define fluid channels 67 which communicate with a central region of the GC chamber at a first end proximal the GC column 60 and the base 64b, and which communicate with the openings 260, 260 at a second end. The halogen lamps 65 are mounted outwardly of the GC column 60, upon the inner walls 66, so that, in use, heat is radiated from the halogen lamps 65 towards the GC column 60. Electrical power is supplied from the exterior of the GC chamber 250 to the halogen lamps 65 via electrical standoffs 69 extending outwardly across the fluid channels 67. A gas supply inlet 71 and a gas outlet 71 are also provided which extends outwardly through the outer side walls 64a to the GC column 60 so that the sample and/or reference gases generated upstream of the GC column 60 (FIG. 3a again) can be introduced into the GC column 60 and leave the GC column 60 as gaseous output.

(31) The fan 240 is, as noted above in connection with FIG. 3a, mounted externally of the GC chamber 250 and, in use, draws ambient (relatively cool) air from outside of the GC chamber 250 and blows it into the central part of the GC chamber 250. The relatively cool air forces any relatively warm or hot air present in the vicinity of the GC column 60 to be expelled from the GC chamber 250 along the fluid channels 67 and out via the openings 260, 260.

(32) Rapid ramping up (heating) and down (cooling) of the temperature of the GC column 60 can thus be achieved. To achieve rapid heating, the system controller 200 sends a trigger signal to the heater controller 68 which applies electrical power to the halogen lamps in order to cause the temperature in the GC chamber 250 to be increased. The heater controller 68 may be programmed with one or many temperature profiles (some examples of which will be described in respect of later Figures) that cause the temperature of the GC column 60 to be ramped up to one or more temperature set points. The skilled person will recognise that proportional-integral-differential (PID) or other known feedback control techniques may be employed in order that the set point temperatures are reached without excessive overshoot or oscillations.

(33) The temperature may be ramped between first and second set points at a constant (or substantially constant) rate. The heater controller 68 may be configured to ramp between different set point temperatures at different constant rates, depending for example upon the experiment being carried out and the constituent compounds, molecules etc. Additionally or alternatively, the rate of temperature change between two set points may be non-linear, or may be linear over a part of the time and non linear at other times. It is moreover to be understood that the temperature gradient does not even need to be constantly positive between the two set points, provided only that, during elution of gases through the GC column, there is a net positive increase in temperature.

(34) For example, it appears that providing a small temperature change even at the start of the experiment, when the GC column 60 is eluting the N.sub.2 and CO.sub.2, can improve further the baseline separation. So the temperature ramp could start slowly and then increase in rate as the temperature of the GC column 60 rises.

(35) The arrangement described above in connection with FIG. 3b also allows rapid cooling of the GC column 60 between experiments. In particular the fan 240 and the arrangement of the outer side walls 64a and the inner walls 66, resulting in the fluid channels 67, allows the cool air blown by the fan 240 to rapidly purge the GC chamber 250 of warm or hot air in order to allow a lower starting set point temperature to be rapidly attained.

(36) Separated gases eluting from the GC column 60 are then conveyed through a thermal conductivity detector (TCD) 80 for weight percent measurements. After (non destructive) analysis by the TCD 80, the analyte gases are directed into an isotope ratio mass spectrometer for simultaneous measurement of .sup.13C, .sup.15N and .sup.34S values.

(37) In the IRMS (not shown in FIG. 3a), the combusted, reduced gases are ionized and passed through a magnetic sector analyser where they separate in space according to their mass to charge ratios. The resulting spatially separated ion species are detected at a plurality of Faraday detectors in a detector array.

(38) Techniques for ionization, separation and detection in the IRMS will be familiar to the skilled reader. The details of the IRMS do not in any event form a part of the present invention and will not be discussed further.

(39) Turning now to FIG. 4, a flow chart illustrating the steps carried out during Gas Chromatography is shown. At step 300, the system controller 200 sends a set/reset signal to the heater controller 68 of the halogen lamps 65 to hold or move the temperature measured at the GC column 60 to a start temperature T.sub.start. The start temperature T.sub.start of the GC column 60 is held between 45 and 80 degrees Celsius, but is ideally in the range of 50 to 70 degrees Celsius. In a most preferred embodiment, the system controller 200 sends a set/reset signal to the heater controller 68 so that the GC column 60 is held at 50 degrees Celsius for N.sub.2 and CO.sub.2 separation.

(40) Once the temperature of the GC column 60 is stabilized at the desired start temperature T.sub.start, a ramp up trigger signal is generated. This ramp up trigger signal may be generated based upon a predetermined timefor example, the ramp up trigger signal may be generated at a time t.sub.5 after the system controller has instructed the autosampler 10 to inject the sample billet into the combustion oven 20. The time t.sub.5 may itself be predetermined through factory or user calibration or may be user settable. Alternatively, the ramp up trigger signal may be generated based upon detection of a threshold gas flow rate of N.sub.2/CO.sub.2 at the GC column 220 and/or the GC chamber entrance, for example.

(41) At step 310 of FIG. 4, the heater controller 68 receives the ramp up trigger signal from the system controller 200 and commences a temperature ramp (step 320). In the preferred embodiment, this results in a rapid rise in the temperature of the GC column 60 from the start temperature T.sub.start (preferably 50 degrees Celsius) to an end temperature T.sub.end, which is (again in the preferred case) 150 degrees Celsius. By rapid rise is meant a change from T.sub.start to T.sub.end over several tens of seconds, and most preferably over 1 to 3 minutes.

(42) As noted above, the heater controller 68 controls the temperature of the GC column 60 so as to ramp up at a linear rate, a non linear rate, or a combination of the two.

(43) At step 330, once one or more temperature sensors in the GC chamber 250/GC column 60 (not shown in FIG. 3a or 3b) determine that the end temperature T.sub.end has been reached, the heater controller 68 of the halogen lamps 65 then controls the temperature of the GC column 60, so that the temperature of the GC column 60 is held constant at the temperature T.sub.end. The time over which the GC column 60 is held at temperature T.sub.end may, as with T.sub.start, either be pre-programmed within the heater controller 68 based upon factory or user calibration, or may be user selected, or may be based upon detection of a threshold of gas flow. As was explained in the Background section, SO.sub.2 elutes more slowly than N.sub.2 and CO.sub.2 so the system controller 200 may look for a threshold of SO.sub.2 gas flow into the GC column 60 for example.

(44) Once system controller 200 determines, based on a time, a user input or a threshold gas flow rate, that the GC column temperature is to be reset, a ramp down trigger signal is generated by the system controller 200 and sent to the controller 68 of the halogen lamps 65. This results in a rapid cooling of the GC column 60: see step 340 of FIG. 4. The temperature drop is (as with the temperature rise) typically tens of seconds and optimally 2 minutes. As explained in connection with FIGS. 3a and 3b above, rapid cooling is preferably facilitated by the use of the fan 240 which blows cool air into the GC chamber 250 in order to displace warm or hot air adjacent the GC column 60 The final temperature following ramp down is T.sub.start again.

(45) Once the temperature has reached T.sub.start, the control loop reverts to step 300 again, ready for a next sample to be loaded into the EA-IRMS by the autosampler 10.

(46) FIGS. 5, 7, 8 and 10 show chromatograms measured with EA-IRMS. For the sake of clarity, reference peaks are not shown in those chromatograms, and only the peaks of the isotope of the molecules having the highest abundance (N.sub.2: isotope mass 28 u, CO.sub.2: isotope mass 44 u and isotope mass SO.sub.2: 64 u) are shown.

(47) FIG. 5 shows a chromatogram of N.sub.2 and CO.sub.2 peaks obtained from a prior art EA-IRMS with an isothermal GC column, using caffeine as a sample. The left hand peak 100 in FIG. 5 arises from N.sub.2, whilst the right hand peak 200 is derived from CO.sub.2 Peak tailing is apparent in FIG. 5.

(48) FIG. 6 shows a first exemplary temperature profile that may be applied to the temperature variable GC column 60 of FIGS. 3a and 3b. The temperature profile of FIG. 6 is, in particular, applied to the GC column 60 by the heater controller 68 based upon a trigger signal from the system controller 200. It will be seen that the start temperature T.sub.start is 50 degrees Celsius and the heater controller 68 holds the GC column 60 at that temperature for 150 seconds. At that point, the heater controller 68 causes the power supplied to the halogen lamps 65 to be increased so that the GC column temperature rises in a linear manner from 50 up to 150 degrees Celsius over a period of 100 seconds. The heater controller 68 then maintains the GC column 60 at the upper set temperature T.sub.end of 150 degrees Celsius until the experiment is concluded. The temperature is then ramped back down again but this is not shown in FIG. 6. FIG. 7 shows a chromatogram of N.sub.2 and CO.sub.2 peaks obtained from the EA-IRMS embodying the present invention, such as is shown in FIGS. 3a and 3b, to which the temperature profile of FIG. 6 is applied during sample elution, again using caffeine as a sample. It will be seen that the N.sub.2 peak 128 and the CO.sub.2 peak 244 are each much narrower than in FIG. 5, with the peak tailing much reduced. The separation between the two peaks 128, 244 is thus greatly increased.

(49) The GC column employed to generate the chromatograms of FIGS. 5 and 7 contains a porous material. The pore mean diameter of the porous material is preferably larger than 50 Angstrom, particularly preferably larger than 65 Angstrom, and in the specific embodiment employed to obtain the chromatograms of FIGS. 5 and 7, is 70 Angstrom (1 Angstrom=1*10.sup.10 m).

(50) The material in the GC column has a a large surface area (preferably larger than 900 m.sup.2/g, particularly preferably larger than 1100 m.sup.2/g.) Again in the embodiment employed to obtain the chromatograms of FIGS. 5 and 7, the material in the GC column has a surface area of larger than 1100 m.sup.2/g.

(51) The GC column can be filled with spherical carbon. The GC column employed to obtain the chromatograms of FIGS. 5 and 7 is filled with a spherical carbon molecular sieve.

(52) The GC column is preferably filled with a spherical material having a diameter between 0.12 mm and 0.5 mm, preferably between 0.15 mm and 0.4 mm and particularly preferably between 0.2 mm and 0.35 mm. The GC column employed to generate the chromatograms of FIGS. 5 and 7 is filled with a spherical material having a diameter between 0.2 mm and 0.4 mm.

(53) FIG. 8 shows a chromatogram of N.sub.2, CO.sub.2 and SO.sub.2 peaks obtained from a prior art EA-IRMS with an isothermal GC, using sulfanilamide as a sample. The N.sub.2, and CO.sub.2 peaks 128, 244 in FIG. 8 are close together and again exhibit peak tailing; the tail of the N.sub.2 peak 128 runs into the leading edge of the CO.sub.2 peak 244. The SO.sub.2 peak 364 is broad with a FWHM (full width of half maximum) of around 60 seconds.

(54) FIG. 9 shows a second exemplary temperature profile that may be applied to the temperature variable GC column 60 of FIGS. 3a and 3b. The temperature profile of FIG. 9 is, in particular, applied to the GC column 60 by the heater controller 68 based upon a trigger signal from the system controller 200. It will be seen that the start temperature T.sub.start in the profile of FIG. 9 is 70 degrees Celsius and the heater controller 68 holds the GC column 60 at that temperature for 280 seconds. At that point, the heater controller 68 causes the power supplied to the halogen lamps 65 to be increased so that the GC column temperature rises in a linear manner from 70 up to 240 degrees Celsius over a period of 170 seconds. The heater controller 68 then maintains the GC column 60 at the upper set temperature T.sub.end of 240 degrees Celsius until the experiment is concluded. The temperature is then ramped back down again but this is not shown in FIG. 9. The benefit of this heat and cool strategy is based upon the strongly differing elution speeds of N.sub.2 and CO.sub.2 on the one hand, and SO.sub.2 on the other. As the three gases arrive at the GC column 60 with the latter held at T.sub.start (70 degrees Celsius for example), the SO.sub.2 is relatively slowly eluting over the column. Once the temperature is ramped up to T.sub.stop, the SO.sub.2 experiences a higher temperature and this reduces the SO.sub.2 elution time.

(55) Reduction in the SO.sub.2 elution time causes the peak in the resulting mass spectrum to be sharper and with minimal tailing. This beneficial effect is clearly seen in FIG. 10, which shows EA-IRMS analysis of the same sample (sulfanilamide) as was employed to generate the prior art isothermal mass spectrum of FIG. 8. Comparing FIGS. 8 and 10, the SO.sub.2 peak 364 at the right hand side of the chromatogram is seen to be much sharper. The temperature ramping scheme of FIG. 9 results in an SO.sub.2 peak width (full width at half maximum) of around 25-30 seconds (time is shown on the horizontal axis). This is nearly half of the peak width shown in FIG. 8 that employs isothermal GC, where the broad flat peak (full width at half maximum) there is around 60 s wide.

(56) The GC column used to generate the chromatogram of FIGS. 8 and 10 contains a porous material, again preferably with large pores (eg pore mean diameter greater than 50 Angstroms). The column is filled with a material having a large surface area, eg at least 900 m.sup.2/g. The filler is a polymer having a spherical shape and a silanised surface. The column is filled with a spherical material preferably having a diameter between 0.12 mm and 0.5 mm and in the specific arrangement employed to generate the chromatogram of FIGS. 8 and 10, it is between 0.15 and 0.35 mm. Overall, the total analysis time employing the scheme described above is less than 12 minutes, and all peak integration is concluded in around 9-10 minutes. Thus there is at least a 33% improvement in analysis time when changing the temperature of the GC column 60 during an analysis, relative to the prior art isothermal GC analysis (where, as discussed in the Background section, compromise times of 18 minutes are employed). A reduction in sample analysis time improves sample throughput and system productivity.

(57) A further benefit of the reduced analysis time is that the volume of Helium purge/carrier gas needed to complete each experiment can be reduced. A flow of helium gas only needs to be present during the sample analysis phase. At other times, the flow can be throttled. If the time taken to carry out each experiment can be reduced by a third, this offers the opportunity to save very significant amounts of helium over an extended period of use of the improved EA-IRMS device of the present invention. Reactor lifetime and chemical trap lifetime may also be extended when using a non-isothermal temperature profile, since the improved analytical and workflow procedures outlined above reduce the time per experiment, and provide an increased maintenance interval.

(58) One further surprising consequence of the use of a non-isothermal temperature profile during EA-IRMS is that simultaneous .sup.13C, .sup.15N and .sup.34S measurements, along with % C, % N and % S measurements, are achievable even for those bulk organic samples such as wood or bone collagen, where the ratio of Carbon to Sulphur can exceed 5000:1, preferably 7000:1 and particularly preferably 10,000:1. As a result, it is often not necessary to repeat an experiment multiple times (in order to obtain a statistically acceptable result), as can often be the case with isothermal GC analyses.

(59) Turning now to FIGS. 11-17, various different exemplary temperature ramping schemes are shown. In FIG. 11, the temperature gradient is constant (ie the slope is linear). In FIG. 12, the temperature gradient is non linear between the start and finish temperature, and in particular the rate of change of temperature is relatively low at the start and finish of the temperature ramping, reaching a maximum around half way between T.sub.start and T.sub.end.

(60) FIG. 13 illustrates the use of two plateaus with a linear gradient between the two. FIG. 14 by contrast employs a non-linear gradient between two plateaus, again with the rate of change of temperature being slowest towards the start and end temperatures T.sub.start and T.sub.end, and with the most rapid change being between those two temperatures.

(61) FIG. 15 employs two plateaus again, but this time has zero gradient at the start temperature T.sub.start up to t.sub.5 (to form the first plateau), a constant gradient between t.sub.5 and t.sub.7, then a non constant gradient between t.sub.7 and t.sub.6 and finally a zero gradient after t.sub.6 at the end temperature T.sub.end (to form the second plateau).

(62) FIG. 16 employs three plateaus rather than two, with a constant gradient between the first and second, and another constant gradient between the second and third plateaus (which may be the same as or different to the gradient between the first and second plateaus).

(63) Finally FIG. 17 employs three plateaus, but this time has zero gradient at the start temperature T.sub.start up to t.sub.5 (to form the first plateau), a constant gradient between t.sub.5 and t.sub.8, then a zero gradient (to form the second plateau) between t.sub.8 and t.sub.9, a non constant gradient between t.sub.9 and t.sub.6 and finally a zero gradient after t.sub.6 at the end temperature T.sub.end (to form the third plateau).

(64) Although some specific embodiments have been described, it will be understood that these are merely for the purposes of illustration and that various modifications or alternatives may be contemplated by the skilled person. For example, although a single GC column has been described, it will be understood that the invention is equally applicable to a system involving multiple (eg, 2) GC columns. In particular, it is possible to use a second (additional) GC or LC column before any combustion or reduction etc takes place. This allows the constituents of the sample to be chromatographically separated before they are each (potentially separately) combusted, reduced or otherwise. Each set of combustion or reaction products (eg N, C or S) can then be separately analysed using the temperature variable GC column 60 described above.

(65) It will of course be understood that the temperatures and ramping rates employed to generate the chromatograms of FIGS. 6 and 9 are exemplary in nature. In general terms, the parameters chosen (temperature(s); ramping rate(s); ramping rate profiles, ie linear, non linear or combined ramping rates; no, one, or multiple intermediate plateaus during ramping from start to finish temperatures, etc) will depend upon multiple factors such as (but not limited to) the sample to be analysed, the configuration (size, shape, phases etc) of the GC column 60, and so forth. The skilled person will have no difficulty in identifying and optimising the parameters. So, for example, although a starting temperature of 50 degrees Celsius was employed to generate the chromatogram of FIG. 6, a range of temperatures from around 35 degrees Celsius up to around 70 degrees Celsius, preferably a range of temperatures from around 45 degrees Celsius up to around 60 degrees Celsius may in fact be employed. Likewise, a range of end temperatures in FIG. 6 between around 120 and 190 degrees Celsius, preferably between around 135 degrees and 170 degrees Celsius may be used. The rate of temperature increase (indicated as 1 degree per second in FIG. 6 may be anywhere between around 0.5 degrees per second up to around 2 degrees per second. It will be understood that the rate of temperature increase needs to be correlated with the peak positions, and these are dependent upon both the sample and the GC column. Likewise in respect of FIG. 9, a range of temperature gradients between 0.5 degrees per second and 2 degrees per second is possible, the start temperature may be anywhere from around 35 degrees Celsius up to around 90 degrees Celsius, preferably anywhere from around 45 degrees Celsius up to around 70 degrees Celsius, and a range of end temperatures in FIG. 9 between around 190 and 300 degrees Celsius, preferably between around 220 and 270 degrees Celsius may be used.

(66) The foregoing embodiments employ an EA-IRMS to generate exemplary chromatograms, in order to illustrate the effects and benefits of the invention. It is however to be understood that the invention is not limited to such a spectrometer. Other forms of elemental analyser can be used and the benefits of applying a temperature variation to a GC column during analysis may be obtained. For example, the concept may be applied to a Thermal Conductivity Detector, a Flame Photometric Detector, a Flame Ionisation Detector, an Isotope Ratio Infrared Spectrometer, any Magnetic Sector Analyzer, or a Double Focussing Sector Mass Spectrometer.