GB2370517A

GB2370517A - Gas chromatography-mass spectrometer having different length chromatography columns

Info

Publication number: GB2370517A
Application number: GB0124485A
Authority: GB
Inventors: John Morrison; Francois Fourel
Original assignee: Micromass UK Ltd
Current assignee: Micromass UK Ltd
Priority date: 2000-10-12
Filing date: 2001-10-12
Publication date: 2002-07-03
Anticipated expiration: 2021-10-12
Also published as: GB0025018D0; GB0124485D0; GB2370517B; DE10151646B4; DE10151646A1

Abstract

A mass spectrometer comprises first and second chromatography 7,11, preferably gas chromatography, separation means and a mass analyser, wherein in one mode of operation output from the first chromatography means is transmitted to the mass analyser 16, and in a second mode, output from the second means is transmitted. Preferably the chromatography columns are of different lengths to allow sulphur dioxide to elute before carbon dioxide and nitrogen. Also disclosed is an elemental analyser and mass spectrometer, preferably an isotopic ratio mass spectrometer having modes of operation as above; an elemental analyser for combusting a sample, having chromatography columns of different length; an isotopic ratio mass spectrometer for determining carbon, sulphur, and nitrogen; and methods of mass spectrometry and isotopically analysing a sample using two gas chromatography means and a mass spectrometer.

Description

K 74066002.354

MASS SPECTROMETER

The present invention relates to a mass 5 spectrometer.

An elemental analyser is known which combusts a sample placed in a sealed tin container in a catalytic furnace at 1000 C. A small amount of oxygen is introduced into a flow of helium carrier gas as the 10 sample enters the furnace so that the highly exothermic oxidation of the tin container raises the temperature to about 1800 C. The helium carrier gas sweeps the gaseous combustion products into a second (reduction) reactor, from which nitrogen, carbon dioxide, sulphur dioxide and 15 water emerge. Water is removed and the nitrogen, carbon dioxide and sulphur dioxide which respectively comprise the nitrogen, carbon and sulphur present in the sample, are then separated by a chromatographic column and are quantitatively detected by a thermal conductivity 20 detector. The separated gases are then analysed by a stable isotope ratio mass spectrometer to determine the isotopic ratios of nitrogen, carbon and sulphur in the sample. The chromatography column used is typically 2 25 metres long with the result that nitrogen eludes after about 120 seconds, carbon dioxide after about 220 seconds, and sulphur dioxide after about 15 minutes (with a corresponding very broad elusion peak). The known apparatus therefore suffers from the problem that 30 it takes a long time to isotonically analyse the carbon, nitrogen and sulphur in the sample.

If it is only desired to determine the isotopic ratio of sulphur in the sample, then a shorter (a 80 cm) chromatography column can be used. Sulphur dioxide will 35 elute after a few minutes, but the chromatography column is too short to resolve nitrogen and carbon dioxide which co-elute. The N and C peaks are hence not resolved. Thus with the known apparatus if a chromatography

À. -

aÀ À À À À -

.. - À À column is used which is long enough to resolve carbon dioxide and nitrogen, then the system is correspondingly slow because the sulphur dioxide takes a long time to elate. If on the other hand a shorter chromatography 5 column is used so that sulphur dioxide elutes more quickly, then nitrogen and carbon are not resolved.

It is therefore desired to provide an improved mass spectrometer. According to a first aspect of the present 10 invention there is provided a mass spectrometer as claimed in claim 1.

Advantageously, the mass spectrometer according to the preferred embodiment is able to isotonically analyse a number of elements including sulphur, nitrogen and 15 carbon without a lengthy delay waiting for the sulphur dioxide to elute. According to the preferred embodiment, sulphur dioxide elutes in less than one minute and is then followed by nitrogen and carbon dioxide which elute in a further five minutes making a 20 total analysis time for all three species of around six minutes. This is nearly three times faster than the known apparatus, and therefore represents a significant advance in the art.

The apparatus is particularly suitable for 25 isotonically analysing organic samples.

Preferably, the mass spectrometer further comprises a sample conversion means arranged upstream of the first and second chromatographic separation means.

Preferably, the sample conversion means is an 30 elemental analyzer.

Preferably, the sample conversion means is arranged to combust a sample.

Preferably, nitrogen and/or carbon dioxide and/or sulphur dioxide are emitted by the sample conversion 35 means.

Preferably, the first chromatography means comprises a first gas chromatography column.

. .. - rr À À , ,,. Preferably, the first gas chromatography column has a length selected from the group comprising: (i) 1 m; (ii) < 90 cm; (iii) < 80 cm; (iv) < 70 cm; (v) < 60 cm; (vi) < 50 cm; (vii) < 40 cm; (viii) < 30 cm; (ix) < 25 5 cm; (x) < 20 cm; (xi) < 15 cm; and (xii) < 10 cm.

Preferably, the second chromatography means comprises a second gas chromatography column.

Preferably, the second gas chromatography column has a length selected from the group comprising: (i) > 1 10 m; (ii) > 1.1 m; (iii) > 1.2 m; (iv) 2 1.3 m; (v) 2 1.4 m; (vi) 2 1.5 m; (vii) 2 1.6 m; (viii) 2 1.7 m; (ix) 2 1 8 m; (x) 2 1.9 m; (xi) 2 2 m; (xii) 2 2.1 m; (xiii) 2 2.2 m; (xiv) 2 2.3 m; (xv) > 2.4 m; (xvi) 2 2.5 m; (XVii) 2 2.6 m; (xviii) 2 2.7 m; (xix) > 2.8 m; (xx) 2 15 2.9 m; and (xxi) 2 3 m.

According to a second aspect of the present invention, there is provided an elemental analyzer and a mass spectrometer as claimed in claim 10.

According to a third aspect of the present 20 invention, there is provided an elemental analyzer as claimed in claim 11.

According to a fourth aspect of the present invention, there is provided an elemental analyzer as claimed in claim 12.

25 According to a fifth aspect of the present invention, there is provided a mass spectrometer as claimed in claim 13.

Preferably, the ratio of the length of second chromatography column to the first chromatography column 30 is at least: (i) 2; (ii) 3; (iii) 4; (iv) 5; (v) 6; (vi) 7; (vii) 8; (viii) 9; (ix) 10; (x) 11; (xi) 12; (xii) 13; (xiii) 14; (xiv) 15; (xv) 16; (xvi) 17; (xvii) 18; (xviii) 19; and (xix) 20.

According to a sixth aspect of the present 35 invention, there is provided a method of mass spectrometry as claimed in claim 15.

According to a seventh aspect of the present

:: - _ _, :

invention, there is provided a method of isotonically analyzing a sample as claimed in claim 16.

According to an eighth aspect of the present invention, there is provided a mass spectrometer as 5 claimed in claim 17.

various embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Fig. 1 shows the apparatus in a first mode of 10 operation; Fig. 2 shows the apparatus in a second mode of operation; and Fig. 3 shows the apparatus in a third mode of operation. 15 In the discussion that follows, reference will be made to the analysis of a sample containing sulphur, nitrogen and carbon which is combusted so as to produce sulphur dioxide, nitrogen and carbon dioxide respectively. 20 Fig. 1 shows a preferred embodiment of the present invention. A sample 1 to be analysed is passed into the sample conversion means 2, typically an elemental analyser. The gaseous mixture leaving the sample conversion means 2, which is the product of combustion 25 and further treatment of the sample is entrained in a flow (not shown) of an inert carrier gas, typically helium, and enters the gas chromatography ("GC") segment 3 via an inlet tube 4. The tube 4 is connected to port no. 6 of a first, six-port, rotary sample valve 5. Port 30 no. 5 of the first sample valve 5 is connected via a first connecting tube 6 to the input side of a primary gas chromatographic column 7, which is relatively short (typically 20 cm). The outlet of the primary column 7 is connected to a thermal conductivity detector ("TCD") 35 8 and then via a second connecting tube 9 to port no. 2 of the sample valve 5. The first connecting tube 6, primary column 7, TCD 8 and second connecting tube 9

À.. ... À À

_.. f. _ . : together form a primary gas flow path.

Port no. 1 of the first sample valve 5 is connected via a third connecting tube 10 to the input of a secondary GC column 11 (which is relatively long, 5 typically 2m) and thence via a diluter 12 and fourth connecting tube 13 to port no. 1 of a second, four-port, rotary sample valve 14. The third connecting tube 10, secondary column 11, diluter 12 and fourth connecting tube 13 together form a secondary gas flow path.

10 Port no. 2 of the second sample valve 14 is connected via an open split 15 to the inlet of a mass spectrometer 16. Port no. 3 of this sample valve 14 is connected to port no. 3 of the first 6-port sample valve 5 via a fifth connecting tube 17. Port no. 4 of the 15 first sample valve 5 which is provided with a supply 18 of carrier gas (typically Helium at 20 ml/min) and port no. 4 of the second sample valve 14 is vented to air via a vent 19.

In the first mode of operation, as shown in Fig. 1, 20 the first sample valve 5 is aligned so as to connect port no. 1 to port no. 2, port no. 3 to port no. 4, and port no. 5 to port no 6 so that they are in fluid communication with one another. The second sample valve 14 is aligned so as to connect port no. 1 to port no. 2 25 and port no. 3 to port no. 4. Initially, in this first mode, the gas to be analysed enters the GC segment 3 via the inlet tube 4 and passes through sample valve 5 to the primary GC column 7. In this first mode, the outlet of the primary column 7 is connected via the TCD 8 to 30 port no. 2 of the first sample valve 5. This is connected to port no. 1 of the sample valve 5, which is connected to the secondary GC column 11. The outlet of the secondary column 11 is connected via the diluter 12 and fourth connecting tube 13 to port no. 1 of the 35 second (four-port) sample valve 14 and thence via the open split 15 into the mass spectrometer 16. The diluter 12 is provided so as to provide a means for

A. ? . ' - 6 adjusting the gas flow into the mass spectrometer 16 to take into account the relative abundances of carbon, nitrogen and sulphur in a typical sample. The speed of passage through the connecting tubes 6,9,10, 13,17 is 5 much greater than the speed of passage through the GC columns 7,11 so that for the present discussion, the time spent in the connecting tubes may be considered to be negligible compared with the time spent in the columns 7,11.

10 Due to the relative speeds of elusion of the different gases, and with appropriate dimensioning of the GC columns 7,11, there will come a point when the nitrogen and carbon dioxide have passed through the system as far as the secondary column 11, while the 15 sulphur dioxide remains in the primary column 7. In other words, the nitrogen and carbon dioxide have passed through the primary gas flow path and have entered the secondary gas flow path, whilst the sulphur dioxide remains in the primary gas flow path. At this point, 20 the apparatus is switched (i.e. the sample valves 5,14 are rotated) from the first mode to the second mode, the operation of which is described below.

In the second mode of operation, as shown in Fig 2, the first sample valve 5 is rotated through 60 , so as 25 to now connect port no. 1 to port no. 6, port no. 2 to port no. 3, and port no. 4 to port no. 5. The second sample valve 14 is rotated through 90 , so as to now connect port no. 1 to port no. 4, and port no. 2 to port no. 3.

30 In this mode, carrier gas from the inlet tube 4 passes via the first sample valve 5 and the third connecting tube 10 directly to the secondary column 11 so that nitrogen and carbon dioxide continue separating in column 11. The carrier gas passes via the diluter 12 35 and the fourth connecting tube 13 to be vented to air.

Meanwhile, the sulphur dioxide in the primary column 7 is now entrained in a second flow of carrier

.. . . .. o À l ' s - - 7 - gas which enters the first sample valve 5 via port no. 4 and which flows out via port no. 5, through the first connecting tube 6 to the primary column 7. The gas leaving the column 7 passes via the detector 8 and ports 5 nos. 2,3 of the first sample valve 5, via the fifth connecting tube 17 to port no. 3 of the second sample valve 14 and thence to the mass spectrometer 16 via port no. 2. The sulphur dioxide is then isotonically analyzed using the mass spectrometer 16 to determine the 10 isotopic ratio of sulphur in the sample.

After mass spectrometric analysis of the sulphur is completed, the apparatus is switched into a third mode of operation as shown in Fig. 3. In this mode, the first sample valve 5 remains in its second mode 15 alignment, but the second sample valve 14 is rotated back through 90 to its first mode alignment, so that the gas leaving the diluter 12 enters the mass spectrometer 16 via the open split 15. This gas contains nitrogen and carbon dioxide which have been separated by the 20 secondary column 11. The isotopic ratios of nitrogen and carbon may then be determined using the mass spectrometer 16. In this mode, gas leaving the first column 7 is vented to air.

When it is desired to perform another analysis, the 25 apparatus is set back into its first mode by rotating the first sample valve 5 back through 60 so that the apparatus is ready to begin another analysis cycle.

As described above, carbon dioxide and nitrogen flow first through the primary gas flow path 6,7,8,9 and 30 then through the secondary gas flow path 10,11,12,13 before entering the mass spectrometer 16 for analysis.

The sulphur dioxide, however, only passes through the primary gas flow path 6,7,8,9 and then via the fifth connecting tube 17 preferably directly to the mass 35 spectrometer 16. In this way the total analysis time is reduced to that required to separate the carbon dioxide and nitrogen in the secondary column 7, since the

A r À r À - 0 À À ÀÀ :. _ - 8 - sulphur dioxide is analyzed whilst the carbon dioxide/nitrogen separation is being carried out.

Advantageously, the sensitivity of the sulphur dioxide analysis is improved because the chromatographic peak 5 produced by the primary (short) column 7 is much sharper than that which would otherwise be obtained if the sulphur dioxide were passed through a longer chromatography column as in the prior art.

The rotation of the sample valves 5,14 is lo preferably controlled by control means 20 e.g. a computer. The computer may also be used to control the mass spectrometer 16. Conveniently, the computer may be programmed so as to switch the sample valves 5,14 after predetermined time periods. Detectors may be used (not 15 shown) to control the rotation of the sample valves 5,14 when particular gases are detected at particular points in the apparatus. It is also envisaged that the mass spectrometer 16 may be arranged to produce a signal when ions of a certain mass to charge ratio are detected.

20 This signal may be used as an input to the control means 20. The GO segment 3 may be a separate segment provided between a conventional elemental analyzer and a conventional isotopic ratio mass spectrometer.

25 Alternatively, the GC segment 3 may be incorporated into an elemental analyser, or alternatively it may be incorporated as an inlet means for a mass spectrometer.

Claims

À . ....

: - 9 74066002.354

Claims

5 1. A mass spectrometer comprising: a first chromatography separation means; a second chromatography separation means; and a mass analyser; wherein in a mode of operation the output from said 10 first chromatography separation means is arranged to be transmitted to said mass analyser and in another mode of operation the output from said second chromatography separation means is arranged to be transmitted to said mass analyser.

2. A mass spectrometer as claimed in claim 1, further comprising a sample conversion means arranged upstream of said first and second chromatographic separation means. 3. A mass spectrometer as claimed in claim 2, wherein said sample conversion means is an elemental analyser.

4. A mass spectrometer as claimed in claim 2 or 3, 25 wherein said sample conversion means is arranged to combust a sample.

5. A mass spectrometer as claimed in claim 2, 3 or 4, wherein nitrogen and/or carbon dioxide and/or sulphur 30 dioxide are emitted by said sample conversion means.

6. A mass spectrometer as claimed in any preceding claim, wherein said first chromatography means comprises a first gas chromatography column.

7. A mass spectrometer as claimed in claim 6, wherein said first gas chromatography column has a length

. . À . À . - 10 selected from the group comprising: (i) 1 m; (ii) < 9o cm; (iii) 80 cm; (iv) < 70 cm; (v) < 60 cm; (vi) < 50 cm; (vii) < 40 cm; (viii) < 30 cm; (ix) < 25 cm; (x) < 20 cm; (xi) < 15 cm; and (xii) < 10 cm.

8. A mass spectrometer as claimed in any preceding claim, wherein said second chromatography means comprises a second gas chromatography column.

10 9 A mass spectrometer as claimed in claim 8, wherein said second gas chromatography column has a length selected from the group comprising: (i) 2 1 m; (ii) 2 l.1 m; (iii) 2 1.2 m; (iv) 2 1.3 m; (v) 2 1.4 m; (vi) 2 1. 5 m; (vii) 2 1.6 m; (viii) 2 1.7 m; (ix) 2 1.8 m; (x) 15 2 1.9 m; (xi) 2 2 m; (xii) 2 2.1 m; (xiii) 2 2 2 m; (Xiv) 2 2.3 m; (xv) 2 2. 4 m; (xvi) 2 2.5 m; (xvii) 2 2.6 m; (xviii) 2 2.7 m; (xix) 2 2.8 m; (xx) 2 2.9 m; and (xxi) - 3 m.

20 10. An elemental analyzer and a mass spectrometer, preferably an isotopic ratio mass spectrometer, said elemental analyzer comprising: a first gas chromatography column; a second gas chromatography column longer than said 25 first gas chromatography column; and means which, in use, selectively transmits either: (i) the eluent from said first gas chromatography column to said mass spectrometer; or (ii) the eluent from said second gas chromatography column to said mass 30 spectrometer.

À. . . ...

s s 11. An elemental analyzer for coupling to a mass spectrometer, said elemental analyzer comprising: a first gas chromatography column; a second gas chromatography column longer than said 5 first gas chromatography column; and an output coupler for selectively transmitting, in use, the eluent either from said first gas chromatography column or from said second gas chromatography column to a mass spectrometer.

12. An elemental analyzer comprising: combustion means for combusting a sample to produce combustion gases including sulphur dioxide and/or carbon dioxide and/or nitrogen; 15 first and second gas chromatography columns of different lengths; and gas flow directing means wherein: (i) in a first mode of operation said gas flow directing means causes said first and second chromatography columns to be in 20 fluid communication; (ii) in a second mode of operation said gas flow directing means causes the output from said first gas chromatography column but not said second gas chromatography column to be in fluid communication with a mass spectrometer; and (iii) in a third mode of 25 operation said gas flow directing means causes the output from said second gas chromatography column to be in fluid communication with a mass spectrometer.

13. A mass spectrometer for determining the isotopic 30 ratios of at least carbon, sulphur and nitrogen contained in a sample, comprising: two chromatography columns wherein sulphur dioxide is arranged to elude from one chromatography column before carbon dioxide and nitrogen elute from the other 35 chromatography column.

14. A mass spectrometer as claimed in claim 13, wherein

- - r e t,\. À F ._ - 12 the ratio of the length of second chromatography column to said first chromatography column is at least: (i) 2; (ii) 3; (iii) 4; (iv) 5; (v) 6, (vi) 7; (vii) S; (viii) 9; (ix) 10; (x) 11; (xi) 12; (xii) 13; (xiii) 14; (xiv) 5 15; (xv) 16; (xvi) 17; (xvii) 18; (xviii) 19; and (xix) 20. 15. A method of mass spectrometry comprising: producing a plurality of gases from a sample; 10 separating some of said gases in a first chromatography column having an output; separating other of said gases in a second chromatography column having an output; and switching which output is connected to a mass 15 analyzer.

16. A method of isotonically analyzing a sample to determine the isotopic ratios of sulphur, carbon and nitrogen, comprising: 20 combusting said sample to product a plurality of combustion gases; separating some of said combustion gases in a first chromatography column; separating other of said combustion gases in a 25 second chromatography column; and selectively switching the outputs of said first and second chromatography columns to a mass analyzer.

17. mass spectrometer comprising an inlet system 30 comprising two or more chromatography columns of different lengths and valve means arranged so that sulphur dioxide is transmitted to a mass analyzer prior to carbon dioxide and nitrogen.