GB2029635A - Mass spectroscopy of fluids leaving a chromatograph - Google Patents

Abstract

A sample 19 of fluid is supplied continuously or discontinuously into a vacuum chamber 5 where it is bombarded by short pulses of light from laser 2 focused on a point on the axis 9 of a time of flight mass spectrometer. In Figure 1, liquid is carried by a transporting strip 14 between spools 17, 18 and the light can be transmitted through strip 14 (Figure 2). In Figure 3 the fluid is, directed by a jet (30) to the focal point of the light on the transporting strip whereas in figure 4, droplets are intermittently directed to the focal point without the use of a transporting strip. <IMAGE>

Description

SPECIFICATION Analysing fluids leaving a chromatograph The invention relates to a method and apparatus analysing fluids leaving a chromatograph.

Chromatography is a known method for separating and detecting substances. Liquid chromatography is for example an analytical method that enables substances composed of chemically different components to be separated in the same solvent on the basis of their different flow times through a separating column. For this purpose the solution is forced at vary high pressure through a tube filled with a suitable, finely divided adsorption material (e.g.

Al203). Normally a continuous solvent stream flows through the separating column. The unknown substance is injected into the column at specific time intervals. The various components of the solution leave the separating column after different passage times (typically between 1 and 15 minutes). The time width of an individual line is of the order of a few seconds. Water, alcohol, benzene and the like are for example used as solvents. In order to detect the substance components, which are present in a large volume of solvent after they have left the separating column, a UV detector is most frequently used. The disadvantage of such a detector is that it is not substance-specific. In addition, it has a detection limit of at most 10-B g.

In gas chromatography the mobile phase corresponding to the solvent is a carrier gas; the stationary phase is as a rule a liquid which forms a thin layer on a porous filler located in the separating column.

It is known to combine liquid chromatography and mass spectrometry together. This can be achieved for example by applying the eluate leaving the chromatograph to a transporting strip or transporting wire and passing the latter via sub-atmospheric pressure locks through the mass spectrometer.

Before entering the vacuum, the solvent is evaporated. The subsequent ionisation ofthe substance under investigation can be effected by evaporation followed by electron bombardment ionisation, the transporting strip being suitably heated for this purpose (c.f. German Auslegeschrift 26 54 057). In this type and manner of ionisation organic molecules are as a rule largely destroyed. It is therefore not possible to obtain qualitative, let alone quantitative, information about the presence of organic, preferably high molecular weight organic substances, using this method.

German Auslegeschrift 26 54057 also proposes ionising the substance under investigation directly on the transporting strip, e.g. by ion bombardment or under the action of a high electric field. Such a method cannot be used in gas chromatography.

Moreover, it has at most the known sensitivity of secondary ion mass spectroscopy. Finally, an often undesirably high degree of destruction of high molecular weight substances always occurs.

The object of the present invention is to provide a method and apparatus for analysing fluids leaving a chromatograph using a mass spectrometer, in which the described disadvantages no longer arise.

In accordance with the invention this object is a method for analysing fluids leaving a chromatograph which comprises introducing a fluid sample continuously or discontinuously into a vacuum chamber bombarding the fluid sample in vacuo with a short pulse laser beam, and directing the ions thereby formed to a time of flight mass spectrometer for analysis.

If the fluid is a liquid, the introduction of the liquid and hence the substance under investigation into the vacuum chamber can be effected in a known manner, e.g. with the aid of a transporting strip or transporting wire, preferably after evaporating the solvent. The fluid, whether gas or liquid, can however, also be introduced directly i.e. without a transporting agent, into the vacuum chamber, and in this way the method can also be used in gas chromatography. On account of the pulsed ionisation and the use of a time of flight mass spectrometer this method is up to five powers often more sensitive than for example analysing an eluate using secondary ion mass spectroscopy. Compared with secondary ion mass spectroscopy, matrix effects are weak.

Special preparations of the substrate are not necessary.

Furthermore, the energy density produced by a laser pulse and which is responsible for the ionisation can be varied within wide limits by altering the focusing of the laser beam in the region of the sample, with the result that the energy density can be adjusted so that it does not destroy the molecular structure. Non-destructive analysis of even high molecular weight substances is thereby made possible. Surprising results are provided in particular by observing negative ions produced by electron accumulation on complete molecules or complex species. In this way it is possible to analyse organic substances having mass numbers up to 100 and above.

Finally, the desired information covering all masses is immediately available after each laser bombardment. When using other, spatially dispersed mass separation systems (quadrupoles, magnets or the like) it is necessary to ionise continuously over a prolonged period of time so that all masses, or at least the mass regiona of interest, can be investigated by continuously altering the electric or magnetic fields that are decisive for the separation of the masses. Compared with the method according to the invention, these other methods require not only a greater time but also the presence of a fairly large amount of the substance under investigation.

The invention also provides an apparatus for carrying out the above method, the apparatus comprising a chromatograph, a vacuum chamber, means for introducing fluid leaving the chromatograph into the vacuum chamber, means for ionising the fluid introduced into the vacuum chamber and comprising a laser as well as means for focusing pulses of laser light pulses onto the fluid into the vacuum chamber, and a time of flight mass spectrometer for detecting the ions produced.

Further advantages and details of the invention will appear from the following description, reference being made to the accompanying generally schema tic drawings in which: Figure 1 is a sectional view of an apparatus being a first embodiment of the present invention and adapted to carry out the method thereof; Figure 2 is a similar view of a second embodiment of apparatus, and Figures 3 and 4 are similar views of respectively still further embodiments of apparatus.

In all the figures a chromatograph 1 and a laser 2 are each shown diagrammatically as a block. The fluid under investigation passes from the chromatograph 1 via a line 3, preferably provided with a shut-off valve 4, into a vacuum chamber 5. The desired ionisation of the substance under investigation takes place in the vacuum chamber by means of laser light pulses, indicated by the beam path 6. The ions formed (positive or negative) are attracted in the direction of a time of flight tube 8 by an electrode 7 to which a suitable voltage is applied. Mass separation takes place in a known manner in the time of flight tube. The groups of molecules, which are separated in each case according to their mass, are recorded with the aid of an ion detector (not shown) and further known electronic equipment.

Tube sections 10 and 11 are arranged coaxially (on axis 9) with the time of flight tube 8. These tube sections are at different electrical potentials and form electrical fields that accelerate and also focus the ions.

The embodiment shown in Figure 1 serves to investigate substances leaving the chromatograph 1.

To this end, the liquid leaving the line 3 and metered for example by the valve 4 is applied to a transporting strip 14 which is led into and out of the vacuum chamber 5 through locks 15 and 16. A conveying strip 14 is provided in the embodiment illustrated, and is unwound from the spool 17 and wound onto the spool 18.

Of course, any other type of transporting strip or transporting wire can be selected. In the case of endless conveying strips it is necessary to clean the transporting strip before it is re-fed into the vacuum chamber.

The sample layer (dotted line 19) on the transport ing strip 14 is ionised by laser light pulses. In order to introduce the laser light, a side arm 20 with a connecting flange 21, which is covered with a disc 22 transparent to laser light, is provided on the cham ber 5. A lens system 23 serving to focus the laser light is situated outside this side arm. The lens system 23 and the plane of the window 22 are dimensioned and aligned in such a way that the focal point of the laser light is situated where the axis 9 of the time of flight tube and the ion optics system meets the transporting strip 15. With such an arrangement the sample 19 can also be bombarded in incident light. The ions thereby formed are attracted in the direction of the time of flight tube 8, in the manner already described.

In the embodiment according to Figure 2 the sample 19 is analysed by transmitted light, in contrast to the embodiment according to Figure 1.

The components 20 to 23 are therefore arranged coaxial with the axis 9 of the time of flight tube on the opposite side of the transporting strip 14. In this embodiment, the transporting strip 14 conveniently consists of a material that is transparent to laser light. In the case of a non-transparent conveying strip 14 the strip itself must be bombarded and penetrated each time.

In the embodiment according to Figure 2 the vacuum chamber 5 has two compartments 25 and 26 in which different high degrees of vacuum can be maintained by means of the vacuum pumps 27 and 28. To this end, the electrode 7 lies adjacent to a partition 24 so that, with the exception of the central opening of the electrode 7, there is no connection between the two compartments. It is sufficient for example if, in compartment 25, in which the ionisation of the sample takes place, a fine vacuum (e.g.

10-' to 10-3 mbar) is maintained. In compartment 26 in which the time of flight tube is located, a high vacuum (e.g. at least 10-5 mbar) should prevail, so that the ions drifting in the time of flight tube do not collide too frequently with residual gas particles and thereby become lost. The high vacuum is generated by a high vacuum pump 28, to which a backing pump 27 is connected. The backing pump 27 serves simultaneously to maintain the fine vacuum in the compartment 25. in addition, the locks 15 and 16 may be connected to the backing pump 27; for the sake of simplicity, the relevant connecting lines are not illustrated.

In the embodiment shown in Figure 3 the line 3 opens out directly into the chamber 5, and its mouth is formed as a nozzle 30. The axis 31 of the nozzle 30 as well as the components 20 to 23 defining the direction of the laser light path are arranged and dimensioned in such a way that the axis 31, the axis 9 of the time of flight tube, and the focal point of the beam path 6, intersect. This preferably occurs in the region of a strip 32 which is led through the vacuum chamber 5 in a similar manner to the transporting strip 14 in the embodiments according to Figures 1 and 2. Such an arrangement has the advantage that it is also suitable for analysing gases. The ionisation of the substance under investigation (gas or liquid) is promoted on account of the fact that a plasma is produced in the region of the surface of the strip by means of the laser beam.Of course, it is not absoiutely essential that the surface onto which the gas beam is directed and on which a plasma is produced, moves. A stationary surface (metal or dielectric substance) can also be used.

To begin with, the substance being investigated, preferably gases, can also be adsorbed on the strip 32. The axis 31 of the nozzle 32 must then be aligned so that the beam of the substance under investigation strikes the strip 32 before it is bombarded with the laser light 6. In the embodiment shown in Figure 4, the substance under investigation is introduced into the vacuum chamger 5 in a discontinuous manner. The line 3 is connected to the vacuum chamber 5 by the nozzle 34. The nozzle 34 is arranged so that the exiting substance is directed roughly perpendicular with respect to the axis of the laser light 6. A connection pipe 35 for a vacuum pump 36 is arranged opposite the nozzle 34 so that the entering substance can be sucked out again as quickly as possible.

In order to permit a discontinuous entry of the substance under investigation and present in the line 3, the wall of the line 3 is provided with a membrane 36 associated with a piston 37 that can be moved backwards and forwards by means of an actuating device 38. A pressure pulse can be produced in the line 3 by pressing in the membrane 36 for a short time by means of the piston 37, resulting in a brief entry of fluid under examination through the nozzle 34 and into the vacuum chamber 5 (indicated by the droplets 39). Since at the moment of a laser light pulse a droplet (or also a small gas cloud if the fluid under investigation is a gas) must be present in each case at the focal point of the laser beam it is expedient to synchronise the pressure pulses with the frequency of the laser light. A control device 40, which is connected by control lines 41 and 42 to the laser 2 and the acltuating device 38, serves for this purpose. The control device is designed so that it can be adjusted in such a manner that a phase shift can be produced between the frequency of the laser light and the pressure pulses. An optimum coincidence of the laser light and the droplets 39 can be effected in this way.

The frequency of the laser light may, as is known, lie in the UV region, in the visible region or even in the IR region. The pulse duration is conveniently in the ns region, e.g. 20 ns. The transporting strips 14, 32 conveniently consist of aluminium if it is not possible to use non-transparent strips. Aluminium interferes only slightly in the arrangement of organic substances.

Claims (18)

1. A method for analysing fluids leaving a chromatograph which comprises introducing a fluid sample continuously or discontinuously into a vacuum chamber bombarding the fluid sample in vacuo with a short pulse laser beam, and directing the ions thereby formed to a time of flight mass spectrometer for analysis.

2. A method according to Claim 1, wherein the fluid is a liquid, the liquid is introduced into the vacuum chamber on a transporting strip ortrans- porting wire, and the solvent is at least largely evaporated before entry into the vacuum chamber.

3. A method according to Claim 2, wherein the transporting strip is of aluminium and the analysis of the liquid on the strip is carried out by indicent laser light.

4. A method according to Claim 2, wherein the transporting strip is substantially transparent to laser light, and the analysis of the liquid on the strip is carried out by transmitted laser light.

5. A method according to Claim 1, wherein the fluid (liquid or gas) is introduced into the vacuum chamber without using a transporting agent.

6. A method according to Claim 5, wherein the fluid is applied by means of a nozzle onto a restricted surface region of a substrate onto which the laser beam is also focused.

7. A method according to any one of the preceding claims, wherein the fluid is introduced discontinuously into the vacuum chamber and the pulse sequence of the laser beam is synchronised therewith.

8. A method according to Claim 7, wherein at least part of the fluid leaving the chromatograph is introduced to the vacuum chamber by a line, and pressure pulses are produced in the line at a frequency equal to that of the laser beam.

9. An apparatus for carrying out the method according to Claim 1, comprising a chromatograph, a vacuum chamber, means for introducing fluid leaving the chromatograph into the vacuum chamber, means for ionising the fluid introduced into the vacuum chamber and comprising a laser as well as means for focusing pulses of laser light pulses onto the fluid into the vacuum chamber, and a time of flight mass spectrometer for detecting the ions produced.

10. An apparatus according to Claim 9, wherein a transporting strip and low pressure locks are provided for transporting fluid in the form of a liquid from the chromatograph into the vacuum chamber.

11. An apparatus according to Claim 9, wherein a nozzle is provided for directly introducing the fluid into the vacuum chamber, and that nozzle and the means for focusing the laser light pulses are dimensioned and aligned in such a way that the fluid stream and the focal point of the laser beam intersect on a substrate.

12. An apparatus according to Claim 11,wherein the substrate is a transporting strip, preferably of aluminium.

13. An apparatus according to Claim 11, wherein the means for introducing the fluid comprises a fluid line and a nozzle leading into the vacuum chamber, and means are provided to produce pressure pulses in the line.

14. An apparatus according to Claim 13, wherein the means for producing pressure pulses in the line comprise a membrane in part of the wall of the line and a piston for reciprocating the membrane.

15. An apparatus according to Claim 14, including means for regulating the pressure pulses together with the laser light pulses.

16. An apparatus according to any one of claims 9 to 15, wherein the vacuum chamber includes two compartments at different pressures, the time of flight tube of the time of flight mass spectrometer is mounted in the compartment at the lower pressure, the other compartment is arranged in front of the inlet opening to the time of flight mass spectrometer, and the fluid is ionised in the latter compartment.

17. A method for analysing fluids leaving a chromatograph according to Claim 1 and substantially as hereinbefore described.

18. The apparatus for analysing a fluid constructed, arranged and adapted to operate substantially as hereinbefore described with reference to any one of the Figures of the accompanying drawings.