WO2003102573A1

WO2003102573A1 - Method and device for partializing a gaseous sample in chromatographic introduction

Info

Publication number: WO2003102573A1
Application number: PCT/EP2002/005800
Authority: WO
Inventors: Stefano Boursier Niutta
Original assignee: Dani Instruments S.P.A.
Priority date: 2002-05-29
Filing date: 2002-05-29
Publication date: 2003-12-11
Also published as: AU2002367992A1

Abstract

A method for partializing a gaseous sample in chromatographic introduction, using a multi-port valve (10) connected to a sample line containing the gas to be analyzed at a constant pressure (Pg), to a constant volume chamber (12), to the carrier gas feed and to the analytical column entry, characterised in that during the loading of the constant volume chamber (12) with the gas to be analyzed, the pressure of said gas to be analyzed is maintained constant downstream of said constant volume chamber at a value (Pvent) between the value of the pressure of the gas to be analyzed (Po).

Description

METHOD AND DEVICE FOR PARTIALIZING A GASEOUS SAMPLE IN CHROMATOGRAPHIC INTRODUCTION

This invention relates to a method and device for partializing a gaseous sample in chromatographic introduction. In gas chromatographic analyses, a gaseous sample is generally injected into the gas chromatograph column using the circuit shown schematically in Figures 1 and 2 and comprising a multi-port change-over valve 10. With reference to these figures, and in particular to that valve, in which the six ports are indicated by the numerals from 1 to 6, the port 1 and the port 4 are connected to an external or internal constant volume chamber

12 (loop) containing the gas to be analyzed, the port 2 is connected to the carrier gas, the port 3 is connected to the gas chromatographic system

(column), the port 5 is connected to a sample line, i.e. the source of the gas to be analyzed, and the port 6 is connected to discharge at atmospheric pressure.

During the loading step (see Figure 1) the six-port valve 10 is disposed such that in its interior the port 6 is connected to the port 1 , the port

2 is connected to the port 3 and the port 4 is connected to the port 5. By virtue of these connections the gaseous sample to be analyzed enters through the port 1 and leaves from the port 6, to discharge to the outside.

During the injection step (see Figure 2), the valve 10 is switched such that in its interior the port 6 is connected to the port 5, the port 2 is connected to the port 1 and the port 4 is connected to the port 3. By virtue of these connections the carrier gas for the gas chromatograph enters through the port 2, leaves from the port 1, passes through the loop 12 and urges the gas to be analyzed, contained within it, into the gas chromatograph at the entry to the gas chromatographic column through the port 4 and the port 3.

For the injection to be properly carried out, the same quantity of gaseous sample must always be injected into the gas chromatographic column; this is generally approximated by temporarily closing the discharge line connected to the port 5 and in this manner filling the loop 12 in slight overpressure. The discharge line is then opened, and when the gas has finished flowing, it can be assumed that it is stationary within the loop 12 at atmospheric pressure. It is evident that if the gas pressure within the loop is constant

(atmospheric pressure) and the loop temperature is constant, the gas quantity injected into the chromatographic column will be constant.

A drawback of this known method is that the molar quantity of gas to be analyzed, which is introduced into the gas chromatographic column, is that corresponding to the volume of the loop at the loop pressure (atmospheric pressure in this case) and at the loop temperature on the basis of the equation PV = nRT, and cannot be decreased without using split circuits, which do not ensure rigorous repeatability of the analyses. Consequently it is not possible to obtain high measurement accuracy and it is not possible to use different types of columns, in particular capillary columns which require smaller quantities of sample.

Another drawback is that the chromatographic analysis accuracy is based on the assumption that the atmospheric pressure is always constant, even though in reality this is not always true. An object of the invention is to eliminate these drawbacks by providing a method and device which enable rigorously accurate gas chromatographic analyses to be effected.

A particular object of the invention is to be able to partialize the gaseous sample to be analyzed, without the need to use splitting.

Another object of the invention is to be able to partialize the gaseous sample to be analyzed, practically without any limitation, so rendering the loop volume also suitable for direct injection into a capillary column.

Another object of the invention is to effect gas chromatographic analyses insensitive to the local atmospheric pressure value.

Another object of the invention is to effect gas chromatographic analyses using gaseous samples having the desired degree of dilution.

These and further objects which will be apparent from the ensuing description are attained, according to the invention, by a method for partializing a gaseous sample in chromatographic introduction as described in claim 1.

According to the invention, the device for implementing the method as described in claim 5.

Two preferred embodiments of the invention are described in detail hereinafter by way of non-limiting example with reference to the accompanying drawing, in which:

Figure 1 shows a circuit scheme of an injection device according to the traditional method during the loading step,

Figure 2 shows it during the injection step, Figure 3 shows a circuit scheme of an injection device according to the invention, and Figure 4 shows a modified embodiment thereof. As can be seen from the figures, the injection device according to the invention comprises a traditional six-port valve, indicated overall by 10. It is connected to a chamber 12 of known volume (loop), the inlets of which are connected to the ports 1 and 4.

The multi-port valve 10 is also connected to the carrier gas line at the port 2, to the gas chromatographic column at the port 3, and to a sample line which extends from the port 5 to a source of the gas to be analyzed which is at a constant known pressure P_g. This source can be a standard analysis vial 16, or any container 17 of gas to be analyzed, again at a constant known pressure P_g.

To the port 6 of the multi-port valve 10 there is connected the control point 22 of a pressure regulator, indicated overall by 18 and comprising an electronically controlled valve 20 able to increase its port when the pressure at the control point, positioned upstream of the valve, increases beyond its set valve Pve_nt, in order to return it to said set value.

The outlet of the valve 20 is directly connected to discharge, at a pressure P₀ < P_g.

The device of the invention operates in the following manner: During the loading step the multi-port valve 10 is positioned so that within its interior the port 6 communicates with the port 1 and the port 4 communicates with the port 5.

Under these conditions the gaseous sample, which originates from the line or from the vial 16, enters through the port 5, leaves from the port 4, fills the chamber 12, enters through the port 1, leaves from the port 6 and flows to the pressure regulator 18, which maintains it at the preset value P_vent (P₀ <

P ent *"- Pg)-

Specifically, during the initial loading step, until the value P_vent has been attained, the proportional valve 20 tends to remain closed so that the gas flow to be sampled progressively increases in pressure until it reaches the value P_vent- With increased pressure at the control point 22, the control system evidently causes the port of the proportional valve 20 to increase in size with exit of the excess gas to discharge, until the pressure at the control point 22 is returned to the value P_vent. When the pressure in the chamber 12 and in the vial 16 stabilizes at the value P_vent. the multi-port valve 10 is switched over to initiate the injection step.

Within the interior of the valve 10, this switching connects the port 6 to the port 5, the port 1 to the port 2 and the port 3 to the port 4. In this manner the carrier gas, which enters through the port 2, leaves from the port 1 and urges the gas contained in the chamber 12 to enter through the port 4 and to then leave from the port 3 towards the gas chromatographic column.

As the gas pressure within the chamber 12 is maintained rigorously constant at the value P_vent and the temperature is also maintained, as in traditional devices, at a rigorously constant value, the molar quantity (mass) of the gas which depends on the pressure, the volume and the temperature, according to the gas equation of state PV = nRT, is maintained rigorously constant. Moreover, because of the pressure regulator 18 practically any partialization ratio P_g/P ent can be obtained without using a split circuit, and hence with considerable accuracy and repeatability of the gas chromatographic analysis. The range of values of the pressure P_vent. and hence of the partialization ratio is virtually without limit; however if to obtain the required ratio it is sufficient for P_veπt to be equal to atmospheric pressure, the exit of the pressure regulator 18 can be connected directly to atmosphere (P₀ = atmospheric pressure); if instead the regulation pressure P_vent must be less than atmospheric pressure, the exit of the pressure regulator 18 must be connected to a vacuum system, which will be at a higher vacuum the greater the partialization ratio to be obtained.

Specifically, if a vacuum up to 10^"5 mbar is required, the exit of the pressure regulator 18 is preferable connected to a rotary pump or a diaphragm pump; if instead a vacuum up to 10^"6 mbar is required, a turbomolecular pump is preferably used; finally if a vacuum up to 10^"7 mbar is required, a diffusion pump is preferably used. In any event the depressurization system enables such high sample dilutions to be obtained as to enable the sample to be directly introduced into a capillary column.

In an advantageous additional embodiment, shown in Figure 4, the device of the invention also comprises a pressurization system for the vial 16 and in general for the source of gas to be analyzed. This pressurization system comprises an auxiliary gas container 24, for example of helium, and a pressure regulator indicated overall by 26 and comprising an electronically controlled valve arranged to open its port when the pressure P_press at the control point 22, positioned downstream of the valve, decreases.

The main purpose of this pressurization system is to obtain within the vial 16 a dilution of the sample to be analyzed. This pressurization system also enables the circuit to be scavenged during stand-by (i.e. when the port 5 of the multi-port valve 10 is no longer connected to the vial 16), so reducing the memory effect of the gas chromatograph. It also enables automatic tests to be effected, aimed at verifying that the vials used are properly sealed.

Claims

C L A I S

1. A method for partializing a gaseous sample in chromatographic introduction, using a multi-port valve (10) connected to a sample line containing the gas to be analyzed at a constant pressure (P_g), to a constant volume chamber (12), to the carrier gas feed and to the analytical column entry, characterised in that during the loading of the constant volume chamber (12) with the gas to be analyzed, the pressure of said gas to be analyzed is maintained constant downstream of said constant volume chamber at a value (P_vent) between the value of the pressure of the gas to be analyzed (P_g) and the value of the exit pressure (P₀).

2. A method as claimed in claim 1 , characterised by maintaining the exit pressure (P₀) at atmospheric pressure.

3. A method as claimed in claim 1, characterised by connecting the exit of said sample line to a vacuum system.

4. A method as claimed in claim 1, characterised by controlledly pressurizing the gas to be analyzed, before its introduction into the analytical column, with an auxiliary gas (24).

5. A device for implementing the method claimed in one or more of claims from 1 to 4, comprising a multi-port valve (10) connected to a sample line containing the gas to be analyzed at a constant pressure (P_g), to a constant volume chamber (12), to the carrier gas feed and to the analytical column entry, characterised by comprising at the exit from the sample line, downstream of said constant volume chamber (12), a pressure regulator (18) having an electronically controlled proportional valve (20) interposed between the control point (22) and the exit, which is at a pressure P₀ < P_g.

6. A device as claimed in claim 5, characterised in that the exit of the pressure regulator (18) is connected to atmospheric pressure.

7. A device as claimed in claim 5, characterised in that the exit of the pressure regulator (18) is connected to a vacuum system.

8. A device as claimed in claim 7, characterised in that the vacuum pump of the vacuum system is a rotary or diaphragm pump.

9. A device as claimed in claim 7, characterised in that the vacuum pump of the vacuum system is a diaphragm pump.

10. A device as claimed in claim 7, characterised in that the vacuum pump of the vacuum system is a turbomolecular pump.

11. A device as claimed in claim 7, characterised in that the vacuum pump of the vacuum system is a diffusion pump.

12. A device as claimed in claim 5, characterised by also comprising a pressurization system comprising a gas container (24), and a pressure regulator (26) with an electronically controlled proportional valve (28) having its control point positioned downstream of this latter in the gas flow direction.