GB2171328A - Gas chromatography - Google Patents

Abstract

In method for gas chromatography, analysis is by measuring changes in the amount of absorbed gas in the column. In analytical apparatus, carrier gas passes through tubes 1 and 2 containing absorbent material and through capillary restrictors 4 and 5. Sample to be analysed is injected into carrier gas entering tube 1. Transducer 6 monitors changes in pressure difference between restrictor entrances and hence changes in flow rate and amount of absorbed gas in tube 1. Areas on trace of flow rate differences recorded by chart recorder 7 give sample analysis as gas volumes. New method can be used simultaneously with conventional method using composition detector 3. <IMAGE>

Description

SPECIFICATION Improvements in or relating to gas chromatography This invention relates to a method and apparatus for analysing a mixture using techniques of gas chromatography.

Gas chromatography is in common use for the analysis of mixtures. In a typical arrangement a carrier gas flows through a long tube, known as a "column", containing material known as "packing". A sample of the mixture to be analysed is injected into the carrier gas at the column entrance and carried into the column. The packing will absorb the mixture by absorption, dissolution or the like temporarily. Different components of the mixture are absorbed by the packing to different extents. Absorbed molecules are almost stationary and only the molecules remaining in the carrier gas contribute to the motion of the sample. Thus the more strongly a component is absorbed the more slowly it moves and the sample becomes separated into its components. As the components eventually emerge from the column their presence in the carrier is detected by monitoring a physical property of the gas stream.For example the loss of heat from a hot wire or the ionization of a flame may be monitored. As an alternative to packing the column a capillary column having a thin layer of liquid on the interior surface of the column may be used, the liquid serving the same purpose as the packing. It has also been proposed to monitor density or viscosity variations in the gas by measuring a pressure drop at constant flow rate but neither of these techniques has proved commercially successful.

The apparatus may be calibrated by injection of known mixtures: this facilitates identification of components and enables more quantitative assessment of the constitution of the injected mixture to be made even though the apparatus is, in principle, non-quantitative.

Attention to detail is necessary to obtain good results. For example if a hot wire is used to measure the thermal conductivity of the gas it is important to control the temperature and the flow rate since both will affect the dissipation of heat from the wire. This is made more difficult because, as has already been recognised in the art, the flow rate of the gas downstream of the column varies.

This variation in flow rate is due to what is known as the "Sorption Effect". This effect can best be understood by considering a band of absorbable substance being driven through a column by a stream of non-absorbed gas. At the rear of the band the substance is being liberated from the packing and carried forward; at the front of the band it is being absorbed. The rate at which gas passes an imaginary stationary observer wfthin the band is equal to the gas flow rate behind the band plus the rate of desorption at the rear of the band.

Within the band the gas velocity is greater than outside. This is the Sorption Effect which was first described nearly thirty years ago.

While the band is within the column the outlet flow will remain constant if the inlet flow is constant. As the band emerges from the column the flow rate will increase.

Up to now the variation in flow rate has been recognised as a nuisance in the art and gas chromatographs have been designed so as to minimize the effect of it.

The invention is based upon the realization that changes in the amount of absorbed gas in the column, can be detected and used to provide a quantitative analysis of a mixture. The change in the amount of absorbed gas may be detected as a change in flow rate, as a change in pressure or by other means.

Thus the present invention provides a method of analysing a mixture using a gas chromatograph in which a sample of the mixture is injected into a carrier gas, the carrier gas and mixture are passed into a chromatograph column and variations in the amount of absorbed gas in the column are monitored and used to analyse the mixture.

In one particular form which has been the subject of experiment, the present invention provides a method of analysing a mixture using a gas chromatograph in which a sample of the mixture is injected into a carrier gas, the carrier gas and mixture is passed into a chromatograph column at a controlled flow rate and variations in the flow rate of the carrier gas downstream of the column are monitored and used to analyse the mixture.

The method of the invention has a considerable number of advantages. Because the flow rate variation is directly related to the quantity of a given component in the mixture, the invention readily provides a quantitative indication of the amount of the component in the mixture. Furthermore, this can be true even if the identity of the component is not known. Because the amount of each component emerging from the column can be quantitatively determined, it is also possible to detect whether any components have been retained in the column. The flow rate measurement does not impose the same minimum size requirement as for example a hot wire sensor and thus it is possible, if desired, to miniaturise the apparatus.Also, because it is only necessary to measure the flow rate of the gas (rather than heat it or burn it) there is no restraint imposed on the carrier gas that is employed and, for example, chlorine or oxygen could be employed as the carrier gas.

It is preferable that, rather than detecting the variations in the flow rate of the carrier gas by making an absolute measurement of the flow rate, the variations are detected by comparing the flow rate of the carrier gas downstream of the column with a reference flow rate; the reference flow rate may be the flow rate of the carrier gas upstream of the column or it may be the flow rate of gas downstream of a control chromatograph column.

Thus the invention further provides an apparatus for analysing a mixture of gases, the apparatus comprising: a first chromatograph column having an inlet and an outlet; a second chromatograph column having an inlet and an outlet; and a differential flow meter connected to the outlets of the first and second columns to measure the difference in flow rate of gas emerging from the first column and of gas emerging from the second column.

The difference in flow rate may be measured by any suitable means. For example a constriction may be provided in each of the gas flow paths downstream of the first and second columns and the pressure difference between points upstream of the constrictions measured. With this method of measurement a cheap and rugged detector may be employed. Also it is possible, if desired, for the sensing of the pressure to be carried out at a location remote from the columns; in such a case no significant delay will be introduced in the response of the detector since the pressure rise will be transmitted rapidly to the detector as soon as an absorbed substance leaves the column; this is of course in contrast to the conventional arrangement in which the detector does not respond until the substance that was absorbed in the column itself reaches the detector.

The present invention also provides a method of checking for imperfections in the packing of a chromatograph column in which a carrier gas, including at least one substance that is absorbed to some extent in the packing, is passed into the column and variations in the amount of gas absorbed in the column are monitored, for example by monitoring flow rate. With a perfect packing and a constant flow rate of the gas into the column, there should be no variation in the flow rate downstream of the column while the substance is passing through the packing; however if there is for exampie a gap in the packing then the substance will be momentarily liberated from the packing resulting in an increased flow rate downstream and will thereafter be reabsorbed resulting in a reduced flow rate downstream.

The substance that is absorbed to some extent by the packing may be the carrier gas itself.

The present invention also provides a method of testing the temperature control of an oven in which first and second chromatograph columns are placed in the oven, a gas is passed into each of the columns at a controlled flow rate and the difference in the flow rate of gas emerging from the first column and the gas emerging from the second column is noted. Conveniently the gas is passed into each column at the same rate. A change in temperature in one column but not the other would be reflected by a variation in the flow rates of the gas emerging from the columns.

The invention also provides a gas chromatograph apparatus comprising a first chromatograph column having an inlet and an outlet, a second chromatograph column having an inlet and an outlet, and a flow measuring device connected to the outlets of the first and second columns to measure the difference in the flow rate of gas emerging from the first column and of gas emerging from the second column.

The invention also provides a gas chromatograph apparatus comprising a chromatograph column having an inlet and an outlet and a flow measuring device connected to the inlet and the outlet of the column to measure the difference in flow rate of gas emerging from the column and of gas passing into the column.

The apparatus defined above may be used in any of the methods defined above.

The invention does not preclude measurements of physical properties of the gas being made and indeed it may be advantageous to make these measurements in addition. For example the apparatus might comprise first and second chromatograph columns and two hot wire elements associated with each column; one hot wire element of each column may be located in a position directly in the gas flow path where it is sensitive to flow rate changes and the other element may be located in a position out of the main gas flow path where it is relatively insensitive to flow rate changes and therefore detects the thermal conductivity of the gas; the four elements may be connected to a microcomputer which may interpret the readings from the four elements.

By way of example, a particular embodiment of the invention will now be described with reference to the accompanying drawings, of which, Figure 1 is a schematic diagram of a gas chromatograph apparatus; and Figure 2 is an example of results obtained from a chart recorder of the apparatus of Figure 1.

The gas chromatograph apparatus shown in Figure 1 comprises a first chromatograph column 1, a second chromatograph column 2, a katharometer 3, capillary flow restrictors 4 and 5 connected to the outlets of the first and second columns 1, 2 respectively downstream of the katharometer, a differential pressure transducer 6 connected between the upstream ends of the flow restrictors 4, 5 and a chart recorder 7 connected to the pressure transducer 6 and the katharometer 3.

It should be understood that the katharometer 3 is not a necessary part of the apparatus of the present invention but was included in the experimentai apparatus to enable a comparison to be made between the output of the katharometer and the output of the pressure transducer 6.

The first and second chromatograph columns 1, 2 and the katharometer 3 were housed in an oven and in the particular experiments to be described these components consisted essentially of a standard kathometer chromatograph known as the Pye Model 104.

In the particular example of the apparatus that was tested, each of the columns 1, 2 were of 4mm internal diameter, and packed with 13.29 of Linde 5A molecular sieve adsorbent giving a packed length of 1.5m. The particle size was in the range of 0.5 to 0.7mm. The flow restrictors 4, 5 were each 70cm long and had an internal diameter of 0.5mm.

With an oven temperature of 50"C, helium gas was passed through both of the columns 1, 2 at the same rate of approximately 25mls/min. Ap proximately 0.5ml of air was then injected into one column. Figure 2 shows the traces obtained from the chart recorder 7 connected to both the katharometer 3 and the differential pressure transducer 6.

The trace referenced 8 is the trace showing the output from the differential pressure transducer 6 and full scale deflection on this trace represents 1mm of differential water pressure.

The trace referenced 9 is the trace showing the output from the katharometer bridge circuit with a bridge current of 60mA and output unattenuated; full scale deflection on this trace represents 0.1mV.

It will be noted that the traces are offset from one another; this is because the pens of the chart recorder were spaced 5mm apart.

The katharometer trace 9 requires little explanation as it will be familiar to those skilled in the art.

The first peak referenced 10 is the oxygen peak and the second peak referenced 11 is the nitrogen peak.

The pressure transducer trace 8 will not however be familiar and will now be explained. It will be seen that there is first a large positive peak 12 which in this example goes off the scale; that peak is caused by the injection of the air into one of the columns. There then follows immediately a large negative peak 13 which in this example also goes off the scale; that peak is caused by the absorption of the bulk of the air by the packing.

The oxygen passes through the packing in the column more rapidly than the nitrogen which is more readily absorbed. As the oxygen reaches the end of the column and is liberated from the packing a positive peak 14 is shown by the chart recorder. Later the nitrogen reaches the end of the column and is liberated from the packing giving rise to the positive peak 15.

Because the pressure difference recorded by the pressure transducer 6 is directly proportional to the differential flow rate the areas under the peaks are themselves representative of the amounts of gas that were absorbed in the column. This is not of course true of the katharometer trace.

It will be noted that the apparatus also makes it possible to detect whether any injected air has remained in the column. If it has not then the total of the areas under the peaks caused by gas liberated from the column should equal the area under the peak caused by the initial absorption of gas in the column (provided none of the peaks go off the scale of the chart recorder as in the example of Figure 2).

On the pressure transducer trace 8, it will be noted that there are ripples in the trace between the peaks. These ripples are believed to be caused by small imperfections in the packing of the column causing momentary liberation and then re-absorption of the gases as they pass through the column. Any serious imperfection in the packing will cause much larger ripples than those shown and the invention thus provides a method and apparatus for checking the quality of such packing.

It is also possible to use the packing to check any local variation in temperature within the oven.

Because the differential pressure transducer measures very small flow rate changes it is sensitive to any variation in temperature of one column that is not identically copied in the other column.

While a specific form of apparatus and specific experiments with that apparatus have been described with reference to the drawings, it will be appreciated that many variations may be made to that apparatus within the scope of the invention.

While reference has for the most part been made to a chromatograph column containing a packing it should be appreciated that the invention is also applicable to a capillary column.

Also it should be understood that references in the specification to "gas chromatography" are to be interpreted in their usual broad way and they encompass vapour chromatography.

While in the described embodiment the mixture to be analysed is injected at one time into the carrier gas it will be appreciated that other arrangements may be employed. In particular the technique of frontal analysis, in which the mixture is continuously passed through the column with the carrier gas and the concentration of the mixture in the carrier gas is altered in steps, may be employed with the invention; references to injecting a sample of a mixture to be analysed should be construed accordingly.

Claims (15)

1. A method of analysing a mixture using a gas chromatograph in which a sample of the mixture is injected into a carrier gas, the carrier gas and mixture are passed into a chromatograph column and variations in the amount of absorbed gas in the column are monitored and used to analyse the mixture.

2. A method of analysing a mixture using a gas chromatograph as claimed in Claim 1 wherein a sample of the mixture is injected into a carrier gas, the carrier gas and mixture are is passed into a chromatograph column at a controlled flow rate and variations in the flow rate of the carrier gas downstream of the column are monitored and used to analyse the mixture.

3. An apparatus for analysing a mixture of gases, the apparatus comprising: a first chromatograph column having an inlet and an outlet; a second chromatograph column having an inlet and an outlet; and a differential flow meter connected to the outlets of the first and second columns to measure the difference in flow rate of gas emerging from the first column and of gas emerging from the second column.

4. An apparatus for analysing a mixture of gases as claimed in Claim 3 wherein a constriction is provided in each of the gas flow paths downstream of the first and second columns and the pressure difference between points upstream of the constrictions is measured.

5. An apparatus for analysing a mixture of gases as claimed in Claim 4 wherein the sensing of the pressure is carried out at a location remote from the columns.

6. An apparatus for analysing a mixture of gases as claimed in Claim 4 or Claim 5 wherein the constrictions are at locations that are remote and downstream from the first and second columns.

7. A method of checking for imperfections in the packing of a chromatograph column in which a carrier gas, including at least one substance that is absorbed to some extent in the packing, is passed into the column and variations in the amount of gas absorbed in the column are monitored.

8. A method of testing the temperature control of an oven in which first and second chromatograph columns are placed in the oven, a gas is passed into each of the columns at a controlled flow rate and the difference in the flow rate of gas emerging from the first column and the gas emerging from the second column is noted.

9. A method for measuring the change in the amount of gas absorbed in an absorbent in a chromatographic column when the temperature is changed in which flow measuring apparatus is connected to the inlet and also the outlet and the difference in the flow rate of gas emerging from the column and the gas entering the column is noted.

10. A gas chromatograph apparatus comprising a first chromatograph column having an inlet and an outlet, a second chromatograph column having an inlet and an outlet and a flow measuring device connected to the outlets of the first and second columns to measure the difference in flow rate of gas emerging from the first column and of gas emerging from the second column.

11. A gas chromatograph apparatus comprising a chromatograph column having an inlet and an outlet and a flow measuring device connected to the inlet and the outlet of the column to measure the difference in flow rate of gas emerging from the column and of gas passing into the column.

12. A gas chromatograph apparatus comprising first and second chromatograph columns and two hot wire elements associated with each column, one hot wire element of each column being located in a position in the gas flow path where it is sensitive to flow rate changes and the other element being located in a position out of the main gas flow path.

13. A gas chromatograph comprising a chromatograph column having an inlet and an outlet and at least two hot wire elements associated with the inlet and at least two hot wire elements associated with the outlet, at least one hot wire element associated with the inlet being located in a position upstream from the inlet in a position in the gas flow path where it is sensitive to flow rate changes and at least one of the other hot wire elements associated with the inlet being located out of the main gas flow path, and also at least one hot wire element associated with the outlet being located in a position downstream from the outlet in a position in the gas flow path where it is sensitive to flow rate changes and at least one of the other hot wire elements associated with the outlet being located out of the main gas flow path.

14. An apparatus comprising a chromatograph as in Claim 13 and a computer programmed to monitor the hot wire elements and to calculate the flow rates at the outlet of the chromatograph column.

15. An apparatus as in Claim 14 wherein the computer is programmed to calculate the compositions at the outlet of the chromatograph column.