Evaporative light scattering detector

Embodiments of the present invention are directed to evaporation light scattering detectors having an evaporative chamber having a wall that is in good thermal contact with a heat sink. The heat has a high thermal mass such that a change in temperature of the wall during an analysis is minimized.

FIELD OF THE INVENTION

This invention relates to improved evaporative light scattering detectors and methods of operating the same.

BACKGROUND OF THE INVENTION

Evaporative Light Scattering Detectors (ELSD) are commonly used in high performance liquid chromatography (HPLC) or supercritical fluid chromatography (SFC) because they are capable of detecting a wider variety of analytes than many other types of chromatographic detectors. Prior ELSD's comprise a nebulizer which receives the eluent from the chromatograph and generates an aerosol comprising droplets of the mobile phase. When an analyte elutes from the chromatograph these droplets will also comprise dissolved or suspended particles of the analyte. The aerosol generated by the nebulizer is passed into a heated desolvation region wherein the mobile phase evaporates leaving dry particles of the analyte. The particles then pass through a light beam and their presence is detected by measuring the scattered light from the beam.

The simplest prior ELSD's comprise a nebulizer, desolvation region and a light scattering region. An example of such a prior ELSD is given in U.S. Pat. No. 6,229,605 B1. Typically, the nebulizer is either a co-axial or cross-flow pneumatic nebulizer which uses a flow of inert gas to produce an aerosol from the eluent from the liquid chromatograph. This aerosol is generated directly in the desolvation region, which may comprise a heated drift tube, which solvent is evaporated from the aerosol. At the exit of the drift tube, desolvated analyte particles from the aerosol pass through a light beam, typically disposed perpendicularly to the direction of travel of the desolvated particles. Light scattered from the beam by the particles is detected by one or more detectors, (typically photomultipliers) disposed so that neither the particles themselves or the laser beam strike them. The signal from the detector(s) is amplified and fed to a display device such as a chart recorder or a computer for further processing.

The signal from the detector(s) is a measure of the quantity and size of analyte particles entering the laser beam, and hence a measure of the concentration of the analyte in the chromatograph eluent. As explained, an ELSD will produce a signal from almost all analytes providing that they are sufficiently involatile to avoid loss by evaporation in the drift tube. However, many factors affect the response of the ELSD, including the size and shape of the analyte particles, their chemical and physical properties, the nature and flow rate of the mobile phase, and the parameters of the nebulizer and drift tube, for example, nebulization gas flow and drift tube temperature. It is necessary to adjust these latter parameters carefully in order to obtain optimum performance, and the optimum values frequently depend on the mobile phase flow rate and composition, which can cause problems when gradient elution is employed.

It has been found that best results are obtained when the nebulizer generates an aerosol of relatively uniform droplet size. If it does not, large droplets may not be completely evaporated in the drift tube and may pass into the laser beam, generating signals even when no analyte is present in them. Increasing the drift tube temperature can help to evaporate those larger droplets, but risks evaporation of relatively volatile analytes because once the solvent is completely evaporated the temperature of an analyte particle may rise rapidly.

To mitigate the problem of droplet size, many ELSD's comprise a separate nebulizer chamber between the nebulizer itself and the heated drift tube.

The nebulizer chamber is typically unheated, so that the larger droplets in the aerosol separate out by condensation on the walls. A drain is provided to remove the condensed liquid. In some cases an impactor is provided, on which larger droplets in the aerosol will impinge and be lost, while smaller droplets are carried around it by the gas flow through the nebulizer chamber.

The average size of the droplets entering the heated drift tube is therefore smaller than it would be if the nebulizer chamber was not provided, which allows a lower drift tube temperature to be used while still completely desolvating the analyte particles. This in turn reduces analyte losses by evaporation. Use of a separate nebulizer chamber also facilitates operation with a supercritical fluid chromatograph, for example one using compressed carbon dioxide as a mobile phase.

A disadvantage of the additional chamber is, however, the loss of analyte that may be present in the larger droplets that condense in it, reducing sensitivity. Nevertheless, the majority of currently available ELSD's incorporate a separate nebulizer chamber. An example of such a commercially available ELSD is the Waters model 2420, (Waters Corporation, 34 Maple Street, Milford, Mass. 01757, USA). Other prior ELSD's comprising separate nebulizer chambers are described in WO 2004/077047, U.S. Pat. No. 6,362,880 and U.S. 2003/0086092 A1.

A typical application for an ELSD is for the HPLC analysis of complex mixtures of natural products. In these applications, the chromatographic separation may typically take between 20 and 120 minutes and large numbers of unknown compounds may be present. In such applications, the use of a UV-absorbance detector is risky because some of the unknown compounds may fail to be detected even when present in large concentrations, because they have no UV-chromophore. An ELSD has a more universal response and hence is more suitable in this application. Recently, ELSD's have begun to be used by chemists involved in the sythesis of new drug candidates by combinatorial chemistry. In this application, the HPLC analysis frequently last only 2 or 3 minutes. Unfortunately, prior ELSD's typically require as long as 15-20 minutes to stabilize at the commencement of an analysis, which increases the total analysis time by up to a factor of 10 and renders the use of the ELSD less cost effective in comparison with other detectors, despite its other advantages.

SUMMARY OF EMBODIMENTS OF THE INVENTION

It is an object of the present invention to provide ELSD's that are more stable in use than prior types. It is another object to provide ELSD's that have shorter stabilization times than prior types. It is a further object of the invention to provide a methods of operation of ELSD's which result in more stable operation and in a shorter stabilization time than prior methods.

In accordance with these objectives there is provided a detector for receiving the eluent from a liquid- or supercritical fluid-chromatograph in which a signal indicative of the presence of an analyte in said eluant is generated by the scattering of light by desolvated particles of said analyte, said detector comprising a nebulizer for generating an aerosol from said eluent in a chamber having a wall that is in good thermal contact with a first heat sink, said first heat sink having a high thermal mass such that the change in temperature of said wall during an analysis of said eluent is minimized.

Conveniently, the first heat sink may have a high thermal conductivity and may be comprised of a metal such as aluminium. The wall of the nebulizer and the first heat sink may comprise a single piece of high conductivity material, but preferably the wall is comprised of a chemically inert material such as stainless steel. In preferred embodiments the thermal mass of such a wall is kept to a minimum because its thermal conductivity is usually relatively low. The wall may be disposed in intimate thermal contact with the first heat sink to ensure that the temperature of the wall exposed to the aerosol is as close as possible to the temperature of the first heat sink.

In a particularly preferred embodiment, the nebulizer chamber may comprise an aluminium housing having a high thermal mass, the housing having a hollow cylindrical bore with a thin lining of stainless steel. In such an arrangement the aluminium housing may serve as the first heat sink and the lining of stainless steel may serve as the wall of the chamber.

In other embodiments the first heat sink may be fitted with a heat pump (for example, one or more Peltier effect devices) to transfer heat from it to a second heat sink. The second heat sink may be cooled by a fan. This arrangement may be used to cool the nebulizer chamber, prior to admission of eluent into the detector, to a sub-ambient temperature (for example, about 15° C.), as well as to help maintain the temperature of the chamber during an analysis.

In still other embodiments, a heater may be additionally or alternatively provided on the first heat sink. This may be used to help maintain the nebulizer chamber temperature when the nebulization of the eluent imposes a high cooling load, typically during the later stages of an analysis carried out at high eluent flow rates.

In another preferred embodiment, a temperature controller may be provided to maintain the temperature of the nebulizer chamber within specified limits. The temperature controller may control the power supply to any combination of the heat pump, fan and heater, where these are provided.

In yet another preferred embodiment, the invention comprises a nebulizer chamber as described above, a drift tube having an exit, said drift tube receiving and desolvating droplets of eluent from the nebulizer chamber, a light scattering chamber into which dry particles of analyte may pass from the drift tube, and a manifold surrounding said exit defining an annular space around said exit into which space a sheath gas may be introduced to confine said analyte particles within an annular flow of said sheath gas as they enter said light scattering chamber. The inventors have found that this reduces noise in the signal produced by the light scattered by the analyte particles.

The invention may also provide a method of detecting the presence of an analyte in the eluent from a liquid- or supercritical-fluid chromatograph in which the presence of an analyte in said eluent is indicated by the scattering of light by desolvated particles of said analyte, said method comprising nebulizing said eluent to produce an aerosol in a chamber having a wall, and minimizing the change in temperature of said wall during an analysis of said eluent by providing a first heat sink in good thermal contact with said wall, said first heat sink having a high thermal conductivity.

The invention may further provide a method of detecting the presence of an analyte in the eluent from a liquid- or supercritical-fluid chromatograph in which said presence is indicated by the scattering of light by desolvated particles of said analyte, said method comprising nebulizing said eluent to generate an aerosol in a chamber having a wall, and reducing the temperature of said wall at least prior to the analysis of said eluent and the generation of said aerosol.

Preferably, the temperature of the nebulizer chamber is reduced to a temperature below ambient temperature before the flow of eluent into the detector is started. Further, preferably the temperature may be reduced below 20° C. and most preferably to 15° C. or lower. The temperatures chosen may depend on the flow rate and composition of the eluent and may be chosen by experiment. Advantageously, a first heat sink may be provided in good thermal contact with the wall to stabilize the nebulizer chamber temperature, and further preferably the heat sink has a high thermal mass and a high thermal conductivity. Heat may be removed from the first heat sink to a second heat sink as previously described.

The inventors have found that providing a heat sink in contact with the wall of the nebulizer chamber stabilizes the temperature of the chamber and reduces the time taken for the detector to stabilize before an analysis can be commenced. Pre-cooling the heat sink as described may be carried out before the flow of eluent is started and may further reduce stabilization times and improve stability of the detector. The shortened stabilization times also reduce or eliminate the need to start the flow of eluent through the chromatograph and detector some time before an analysis is commenced, which is characteristic of prior detectors. This reduces waste of solvents.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The principal components of a preferred embodiment of an ELSD according to the invention are shown inFIGS. 1 and 2. Eluent from a liquid- or supercritical-fluid chromatographic column enters a nebulizer1through an inlet pipe2and is nebulized to produce an aerosol3(FIG. 3) inside a nebulizer chamber4. Nebulizer1is a coaxial-flow pneumatic nebulizer of the type used in prior ELSD's such as the Waters 2420 and need not be described in detail. Typically, a nebulizing gas flow rate of about 3 l/minute of nitrogen may be used for nebulizing a flow of 1 ml/minute of a mobile phase containing a high proportion of water.

Droplets of eluent produced by the nebulizer1are carried in the flow of nebulizing gas out of the nebulizer chamber4into a drift tube5that comprises a coil of thin-walled stainless steel tubing. Drift tube5is heated by an electrical heating tape (shown in part at6) wrapped around it. An oven enclosure7, packed with thermal insulation, encloses the drift tube5and heating tape6. The temperature of the drift tube5is typically maintained at a temperature selected by the user in the range 30-50° C. A suitable temperature control system is described below. As in conventional prior ELSD's, droplets entering the drift tube5undergo desolvation so that any analyte present in them emerges as a stream of dry particles from an exit8of the drift tube5into an optical scattering chamber9comprised in an optical bench10.

An optical bench10suitable for use in a detector according to the invention may comprise the bench used in the prior Waters 2420 ELSD.FIG. 2is a schematic drawing illustrating the principle of operation of such a bench. A quartz/halogen lamp11produces a beam of light44that is focused on a slit45by two condensing lenses46and47. The resulting beam is focused by a relay lens48to the centre of the scattering chamber9, into which analyte particles to be detected are introduced from the exit8of the drift tube5. The particles are introduced in a direction perpendicular to the plane ofFIG. 2. A stray light baffle49is also provided. Unscattered light leaving the scattering chamber9is absorbed by a light trap51, which also incorporates a photodiode (not shown). A signal from the photodiode may be used to monitor the intensity of the light entering the scattering chamber9and to ensure that the lamp11is serviceable.

Whenever an involatile analyte is present in the eluent entering the detector through inlet pipe2, dry particles of analyte are swept from the drift tube5into the scattering chamber9and cause light to be scattered from the incoming light beam at a range of angles dependent on the nature and size of the particles. At least some of the light so scattered travels in a beam52and is focused by a lens53on to a photomultiplier54via an adjustable mirror55. Light scattered in the opposite direction, that is along axis56, is absorbed by a light trap57.

The signal from the photomultiplier54may be amplified and digitized for further processing by a digital computer or microprocessor to yield a signal indicative of the quantity and size of analyte particles entering the scattering chamber9. The electronic apparatus and software used for this purpose may be similar to that used in a conventional prior ELSD, such as the Waters 2420.

FIG. 3is a sectional view of a nebulizer chamber4suitable for use in a detector according to the invention. It comprises a thin wall58of stainless steel formed into a cylindrical tube to define the chamber in which the aerosol3is formed. Wall58is in good thermal contact with a first heat sink59that comprises a rectangular-section block of aluminium having a high thermal mass surrounding the thin wall58. Aluminium is a particularly suitable material for the first heat sink59because it has a high thermal conductivity, which ensures that temperature gradients along and around the wall58are minimized. The inventors have found that minimizing these gradients improves the performance of the detector. Other materials, for example, copper, brass, or other alloys that have high thermal conductivity, may also be used for the first heat sink59. The wall58is preferably in good thermal contact with the first heat sink59over a substantial portion of its length. The inventors have found that minimizing the axial temperature gradient along the portion of the wall that is “visible” to the aerosol3also improves detector performance. Similarly, if the wall58is comprised of material having a relatively low thermal conductivity, for example stainless steel, it should have a relatively low thermal mass to ensure that the temperature distribution along its length is substantially determined by the first heat sink58. Conveniently, the wall58may comprise a 3-inch long cylindrical tube of stainless steel having a wall thickness of 0.035 inches. The first heat sink59may comprise a 1.5 inch square section aluminium block bored out to receive the wall58, as illustrated inFIG. 3. Heat sink59should extend as far as possible along the length of the wall58, subject to the presence of other components associated with the nebulizer chamber4, for example the nebulizer1.

The wall58may also be comprised of glass, quartz, or ceramic, but it may be more difficult to ensure good thermal contact between these materials and the first heat sink59, and the materials are typically more fragile.

It is also within the scope of the invention to manufacture the wall58and first heat sink59from the same material, for example, brass or ceramic having a high thermal conductivity, or even certain types of polymers or plastics. In this case, the two components may also comprise a single piece of material. The material should have as high a thermal conductivity as possible so that temperature gradients along it are minimized, and should have a high thermal mass, as discussed. These properties may conflict with the need to provide a chemically inert environment to surround the aerosol3, but for many applications, an acceptable compromise may be found.

Two Peltier effect devices60,61are attached to the first heat sink59, A second heat sink62, conveniently an aluminium block approximately 2 inches square, is attached to the other faces of the Peltier devices60and61. A fan63is provided to cool the second heat sink62. The Peltier effect devices comprise a heat pump64that removes heat from the first heat sink59and transfers it to the second heat sink62. A heater65may also be fitted to the first heat sink59.

In the complete detector assembly, the nebulizer chamber4may be orientated at a small angle relative to the horizontal, as shown inFIG. 1. This allows droplets of eluent that condense of the wall58to accumulate at the lowest point of the chamber. As in prior ELSD's such as the Waters 2420, a drain66configured as a siphon, is provided at this point, as shown inFIG. 1. This arrangement ensures that approximately the same level of condensed eluent is maintained in the chamber during operation, which improves stability. It may be necessary to fill the drain66with eluent prior to operation of the detector to ensure the best results.

FIG. 10is a schematic diagram of a temperature control system suitable for use in a detector as described. The system shown inFIG. 10is not an essential feature of the invention, but may be used to further enhance performance. A first temperature controller72receives a signal via connection69from a temperature sensor71mounted on the drift tube5. Controller72operates to maintain the temperature of the drift tube5at a predetermined value (chosen by the operator) by adjusting the power fed to the heating tape6via connection73. A computer or microprocessor74may receive an input from a user specifying the chosen temperature, and may send a signal on connection75to controller72to set the desired temperature. Alternatively, an operator may set a chosen temperature directly on the controller72. Typically, the drift tube temperature may be set in the range 30-50° C. according to the nature of the eluent and its flow rate. In the examples discussed below, a temperature of 48° C. was employed.

A second temperature controller67may receive a signal from a temperature sensor70mounted on the first heat sink59via connection68. Controller67may control the power provided to the Peltier effect devices60and61via connection76to adjust the rate at which they transfer heat from the first heat sink59to the second heat sink62. Additionally or alternatively, the speed of fan63may be controlled by controller67via connection77to vary the rate of cooling of the second heat sink62and thus control the temperature of the first heat sink59. In another embodiment, an additional temperature sensor (not shown) may be provided on the second heat sink62to provide a signal to controller67which may then control the speed of fan63to maintain the temperature of the second heat sink62approximately constant.

The temperature controller67may also provide via connection78an output to a heater65mounted on the first heat sink59. As explained, a preferred method of the invention involves the cooling of nebulizer chamber4below ambient temperature before the flow of eluent to the detector is started. Nebulization of the eluent tends to cause a further drop in temperature of chamber4(see examples discussed below) and in certain circumstances, it may be desirable to heat the chamber4rather than to cool it. If the signal from temperature sensor70indicates that the temperature of the first heat sink59is falling below the desired minimum, and the power supplied to the Peltier devices60,61and/or the fan63is already low or zero, controller67may provide power to the heater65in place of the Peltier devices60,61. It is within the scope of the invention for controller67to reverse the polarity of the power supplied to the Peltier devices60and61so that they transfer heat from the second heat sink62to the first heat sink59in order to increase its temperature. However, this mode of operation tends to reduce the lifetime of the Peltier devices and it is preferable to provide power to a separate heater and switch off the Peltier effect devices when additional heat input is required.

A computer74may receive an input from an operator that determines the desired temperature of the nebulizer chamber4and transmit this to controller67via connection79.

In a preferred method of the invention, eluent from a liquid- or supercritical-fluid chromatographic column flows through the inlet pipe2into a nebulizer1. A coaxial flow of nebulizing gas (for example, 3 l/minute of nitrogen) is introduced in a conventional way to generate an aerosol3(FIG. 3) inside a nebulization chamber4. The temperature of the chamber4is maintained as described above. Droplets from the aerosol3may be passed into a heated drift tube5where the solvent is evaporated. Dry particles of any analyte present in the eluent may be swept from the drift tube5into a light scattering chamber9where they may scatter light from a beam44to be received by a photomultiplier54, as described previously. In further preferred methods, the temperature of the nebulizer chamber4is stabilized by providing good thermal contact between its wall58and a first heat sink59. Heat sink59has a high thermal conductivity and a thermal mass sufficient to minimize temperature fluctuations during a chromatographic analysis. In yet further preferred methods, heat may be pumped from the first heat sink59to a second heat sink62by a heat pump64which may comprise one or more Peltier effect devices60,61.

In another preferred method, the temperature of the nebulizer chamber4is reduced below ambient temperature before and analysis is commenced, and preferably before the flow of eluent into the detector is started. This may be achieved by operation of a heat pump64that may comprise one or more Peltier effect devices60,61. Conveniently, the temperature may be reduced below 20° C., or most preferably to 15° C. or lower. Other preferred methods according to the invention may involve experimentally determining the most suitable operating temperature for the nebulizer chamber4before an analysis is commenced, for example by monitoring the change in temperature of the nebulizer chamber4when a gradient elution similar to that to be used for an analysis is run into the detector in the absence of an analyte.

Yet further preferred methods may comprise controlling the temperature of nebulizer chamber4by means of a temperature controller67. The method may involve controlling the power provided to Peltier effect devices60,61, a fan63on the second heat sink62, and/or a heater65on the first heat sink59, in order to maintain the temperature of the nebulizer chamber4approximately constant.

Referring next toFIG. 11, in a yet further preferred embodiment of the invention the exit8of the drift tube5is surrounded by a manifold80which defines an annular space81around the end of the tube where it enters the light scattering chamber9. A sheath gas (conveniently nitrogen or dry air) is introduced through an inlet82in the manifold80so that it flows through the annular space81around the drift tube and into the light scattering chamber9, as indicated by the arrows83. Analyte particles exiting from the drift tube are in this way confined within an annular flow of gas as they enter the scattering chamber. The inventors have found that this stabilizes the flow of particles into the scattering chamber9and reduces noise in the signal from the photomultiplier54which results from the light scattered by the particles.

Working Examples

FIGS. 4,5and6compare the variation in the temperature of the nebulizer chamber with time for two prior ELSD's and an ELSD according to the invention. Referring first toFIG. 4, curves12,13and14respectively show the temperature variation observed in the case of prior Waters 2420 detector, a prior Sedex 75 detector (Sedere Inc, 1206, River Road, Cranbury, N.J. 08512-9900, USA) and a Waters 2420 detector fitted with a nebulizer chamber as described above in place of the chamber originally supplied. In each case, the flow of eluent (1.8 ml/minute) was started at time=0 and the temperature of the nebulizer chamber was monitored for a period of three minutes. The drift tube temperature was maintained at 48° C. The composition of the eluent (approximately 90% water, 5% acetonitrile, and 5% of a 1% solution of trifluoroacetic acid in water) remained constant throughout these experiments.

In the case of the modified Waters 2420 detector (curve14) the chamber was cooled prior to the admission of eluent, as described above.FIG. 4clearly shows that the temperature of the chamber during the first three minutes of eluent nebulization remains substantially constant using a detector according to the invention (curve14) but drops significantly in the case of prior detectors (curves12and13).

After the 3-minute period illustrated inFIG. 4had elapsed, the experiments were continued by starting a gradient elution, during which the composition of the eluent was changed from 90% water, 5% acetonitrile and 5% of the trifluoroacetic acid solution to 0% water, 95% acetonitrile and 5% of the trifluoroacetic acid solution, over a period of 3.5 minutes. This period of gradient elution was followed by a period of 1.5 minutes during which the composition remained constant. The composition of the mobile phase was then very quickly returned to its starting composition (90% water) and held constant for a further 1 minute. The temperature of the nebulizer chamber for each of the three detectors was recorded and is illustrated inFIG. 5, in which curve15relates to the prior Waters 2420 detector, curve16to the Sedex detector and curve17relates to the modified Waters 2420 detector comprising a nebulizer chamber according to the invention. The changing heat load imposed on the nebulization chamber by the nebulization of an eluent of changing composition results in the chamber temperature of all three nebulizers falling, but the temperature variation of the detector according to the invention (curve17) falls by a smaller amount than it does for either of the two prior detectors (curves15and16).

One minute after the end of the period illustrated inFIG. 5, the gradient elution experiment described forFIG. 5was repeated.FIG. 6illustrates the observed temperature changes. The changing heat load due to the varying composition of the eluent again resulted in changes of the nebulizer chamber temperature for all three detectors, but as in the previous experiments the change was smaller in the case of the detector according to the invention (curve18) than it was for the Sedex detector (curve19) and the unmodified Waters detector (curve20). This means that the detector according to the invention, pre-cooled in the absence of a flow of eluent, is ready for use much sooner than the prior detectors, which require a stabilization time of some 15 minutes during which the eluent has to be passed into the detector.

FIGS. 7A-7Dfurther illustrate that a detector as described above is ready for use after a shorter stabilization time than that required for a prior detector. The figures show four chromatograms21-24(FIGS. 7A-7Drespectively) obtained using a Waters 2420 ELSD fitted with a nebulizer chamber as described above in place of its original chamber. Four consecutive injections of a sample comprising 0.5 mg/ml of flavone dissolved in dimethylsulfoxane (DMSO) were made. The first injection (chromatogram21) was made three minutes after starting the eluent flow into the ELSD, and the subsequent injections, chromatograms22-24, were each made immediately after the end of the analysis of the previous injection (approximately 3.5 minutes after its injection). The initial mobile phase composition was 90% water, 5% acetonitrile, 5% of a 1% solution of trifluoroacetic acid in water, and the composition was changed according to the gradient described in respect ofFIGS. 5 and 6above. InFIGS. 7A-7D, peaks29-32in chromatograms21-24respectively, are the flavone sample. Peaks25-28are due to the DMSO solvent. Because DMSO is volatile, it should be lost by evaporation in the nebulizer and drift tube of the ELSD. Any residual DMSO peak is therefore indicative of incomplete desolvation, and if large, an indication that the detector is not ready for use. Chromatograms21-24inFIGS. 7A-7Dexhibit only very small DMSO peaks, suggesting that desolvation is substantially complete even in the case of the first chromatogram24. This confirms that the detector is ready for use after only 3 minutes stabilization time.

The stability of a detector as described is further illustrated inFIGS. 8A-8E, which show five chromatograms33-37(FIGS. 8A-8Erespectively) obtained for 5 different injections of a sample of 0.5 mg/ml of flavone dissolved in DMSO. The volumes injected were 2, 4, 6, 8 and 10 μl for chromatograms33to37respectively. It can be seen that the DMSO peak is almost undetectable in every case, showing that desolvation remains complete and stable despite the high volumes of injected solvent.

FIGS. 9A-9Cshow that stable operation and substantially complete desolvation can be obtained for different solvents with a detector having a nebulizer chamber as described above. Chromatogram38(FIG. 9A) is the result of an injection of pure DMSO (that is, containing no analyte) recorded with a higher detector sensitivity, but using otherwise identical chromatographic conditions as the experiments shown inFIGS. 7A-7D. Only a small peak41due to DMSO peak is visible. Chromatogram39(FIG. 9B) is the result of a similar experiment in which the same quantity of isopropyl alcohol was injected, and shows a similar small peak42. Chromatogram40(FIG. 9C) is the result of a similar injection of acetonitrile, and exhibits a small peak43. In all cases, the small size of the solvent peaks indicates stable operation of the detector with substantially complete desolvation.