Eluent Jet Interface for Combining Capillary Liquid Flows with Electron

A new interface based on an eluent jet in combination with a conventional gas chromatography momentum separator for use with electron impact mass ...
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Anal. Chem. 1996, 68, 675-681

Eluent Jet Interface for Combining Capillary Liquid Flows with Electron Impact Mass Spectrometry Charles E. Kientz,* Albert G. Hulst, Ad L. De Jong, and Eric R. J. Wils

TNO Prins Maurits Laboratory, P.O. Box 45, 2280 AA Rijswijk, The Netherlands

A new interface based on an eluent jet in combination with a conventional gas chromatography momentum separator for use with electron impact mass spectrometry is described. The formation of the eluent jet is based on radio frequency inductive heating. The aerosol formation in the interface is discussed in relation to commercial particle beam (PB) interfaces. The interface is tested in combination with electron impact mass spectrometry using flow injection analysis at flow rates in the range of 5-15 µL/ min commonly encountered in microcolumn liquid chromatography. Electron impact spectra at 1-10-ng levels are found to be comparable with reference spectra. In the single-ion mode, 50 pg of caffeine is detectable with a signal-to-noise ratio of 3:1. Contrary to many other PB systems, linear calibration plots are obtained in the tested range of 3-200 ng of caffeine. In 1984, Willoughby and Browner1 introduced a new mass spectrometer interface that allowed both electron impact (EI) and chemical ionization (CI) in combination with liquid chromatography (LC). Based on this principle, many commercially available particle beam (PB) interfaces are successfully being used for the analysis of molecules that are not compatible with gas chromatography/mass spectrometry (GC/MS) methods. The systems are based on eluent nebulization, either pneumatical or by thermospray, into a desolvation chamber which is connected to a momentum separator where the high molecular weight analytes are preferentially transferred to the MS ion source, while the low molecular weight solvent molecules are efficiently pumped away. The use of PB interfaces has shown great promise, since the EI spectra obtained contain the fragmentation patterns necessary for unambiguous identification. However, quantitative analysis has been limited by fluctuations in absolute response observed in intraand interday runs and nonlinearity at lower concentrations.2-9 Besides, commercial instruments are not compatible with capillary flows resulting from microcolumn LC (micro-LC) and capillary electrophoresis (CE). On the other hand, in modern analysis, these capillary separation techniques are convincingly being used. Much work has been devoted to the weaker point of these (1) Willoughby, R. C.; Browner, R. F. Anal. Chem. 1984, 56, 2626. (2) Wood, D. J. Spectrosc. Int. 1990, 2, 37. (3) Doerge, D. R.; Burger, M. W.; Bajic, S. Anal. Chem. 1992, 64, 1212. (4) Kim, I. S.; Sasinos, F. I.; Stephens, R. D.; Brown, M. A. J. Agric. Food Chem. 1990, 38, 1223. (5) Bellar, T. A.; Behymer, T. D.; Budde, W. L. J. Am. Soc. Mass Spectrom. 1990, 1, 92. (6) Brown, F. R.; Draper, W. M. Biol. Mass Spectrom. 1991, 20, 515. (7) Apffel, A.; Perry, M. L. J. Chromatogr. 1991, 554, 103. (8) Ho, J. S.; Behymer, T. D.; Budde W. L.; Bellar, T. A. J. Am. Soc. Mass. Spectrom. 1992, 3, 662. (9) Jedrzejewski, P. T.; Taylor, L. T. J. Chromatogr. 1994, 677, 365. 0003-2700/96/0368-0675$12.00/0

© 1996 American Chemical Society

techniques, i.e., the detection side. The detection strategy includes options as widely divergent as (chemi)luminescencebased derivatization or laser-based detection principles,10 the use of continuous-flow fast atom bombardment (CF-FAB) and electrospray interfaces in combination with tandem mass spectrometers11 and, in the case of LC, modification and application of detectors initially designed for GC, such as the flame photometric or thermionic detectors.12,13 The current benefits of miniaturized LC systems, such as improved permeability and column quality, a chemically more inert system, and economic advantages, are widely acknowledged. Moreover, the use of micro-LC with its flow rates of typically 1-15 µL/min vs conventional-size LC has several additional advantages regarding MS. Cappiello et al.14,15 demonstrated the following benefits of the use of liquid microflows with a PB interface: the increase in sensitivity and a negligible contamination of the pumping system, ion source, and mass analyzer by the solvent vapors. For the introduction of the liquid microflow, a specially designed microflow nebulizer was employed. In recent years, we have extensively studied the on-line coupling of micro-LC and flame-based GC detectors and the use of thermospray LC/MS for the trace-level determination of nonvolatile organophosphoric and alkylphosphonic acids.16-22 Recently, we accomplished the coupling of CE with flame photometric detection (FPD).23 These studies were carried out for the identification of chemical warfare agent degradation products needed in view of the recent Convention on Chemical Weapons. This Convention forbids the development, production, stockpiling, and use of chemical warfare agents. (10) Van den Beld, C. M. B.; Lingeman. H. In Luminescence Techniques in Chemical and Biochemical Analysis; Baeyens, W. R. G., De Keukeleire, D., Korkidis, K., Eds.; Practical Spectroscopy Series 12; Dekker: New York, 1991; p 237. (11) Niessen, W. M. A.; Tjaden, U. R.; Van der Greef, J. J. Chromatogr. 1991, 554, 3. (12) Kientz, Ch.E.; de Jong, G. J.; Brinkman, U. A. Th. J. Chromatogr. 1991, 550, 461. (13) Kientz, Ch. E.; Brinkman, U. A. Th. TrAC, Trends Anal. Chem. 1993, 12, 363. (14) Cappiello, A.; Bruner, F. Anal. Chem. 1993, 65, 1281. (15) Cappiello, A.; Famiglini, G. Anal. Chem. 1994, 66, 3970. (16) Kientz, Ch. E.; De Jong, G. J.; Verweij A.; Brinkman, U. A. Th. J. Microcolumn Sep. 1992, 4, 465. (17) Kientz, Ch. E.; De Jong, G. J.; Verweij, A.; Brinkman, U. A. Th. J. Microcolumn Sep. 1992, 4, 476. (18) Kientz, Ch. E.; De Jong, G. J.; Verweij, A.; Brinkman, U. A. Th. J. Chromatogr. 1992, 626, 59. (19) Kientz, Ch. E.; De Jong, G. J.; Verweij A.; Brinkman, U. A. Th. J. Chromatogr. 1992, 626, 71. (20) Wils, E. R. J.; Hulst, A. G. J. Chromatogr. 1988, 454, 261. (21) Wils, E. R. J.; Hulst, A. G. J. Chromatogr. 1990, 454, 523. (22) Wils, E. R. J.; Hulst, A. G. Fresenius J. Anal. Chem. 1992, 342, 749. (23) Sa¨nger-van de Griend, C. E.; Kientz, Ch. E.; Brinkman, U. A. Th. J. Chromatogr. 1994, 673, 299.

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Figure 1. Eluent jet interface: (A) fused-silica liquid introduction capillary (0.10 mm i.d. × 0.17 mm o.d.); (B) metal ring (3 mm length × 2.5 mm o.d.); (C) quartz tube (0.8 mm i.d. × 1.5 mm o.d.); (D) desolvation tube (3 mm i.d. × 6 mm o.d.); (E) induction coil.

For the coupling of liquid microflows to GC detectors, an interface was developed based on a plug-type liquid jet introduction directed into the flame of the detector. In the present study, the thermal formation of the liquid jet has been further improved by employing radio frequency (rf) inductive heating. This modification of the interface allows direct coupling to an MS via a common GC/MS momentum separator to perform PB-EI-MS. The liquid is introduced as a divergent aerosol in nearly all LC/MS interfaces, although many types of nebulization device are being used. In this study, however, a nonconventional eluent jet introduction is investigated in combination with quadrupole and magnetic sector mass spectrometers. EXPERIMENTAL SECTION Materials. Analytical-grade methanol was purchased from Merck (Darmstadt, Germany). Analytical-reagent grade ammonium acetate and ammonium formate were obtained from Aldrich Chemie (Steinheim, Germany). Throughout the study, deionized water (Milli Q water purification system, Millipore, Bradford, MA) was used. All solvents and solutions were filtered prior to use over 0.45-µm pore size filter disks from Millipore. Caffeine (1,3,7-trimethylxanthine), Amido Black (naphthol blue black), and naphthalene were supplied by Sigma (St. Louis, MO). Triethyl phosphate was obtained from BDH Ltd. (Poole, UK) and simetryn [N,N′-diethyl-6-(methylthio)-1,3,5-triazine] from Altech Associates (Deerfield, IL). Thiodiglycol [bis(2-hydroxyethyl) sulfide] was synthesized in our laboratory. Liquid Chromatography. The micro-LC system consisted of a Phoenix Model 20 CU pump (Fisons Instruments, Milan, Italy) and a Valco sample injection valve (VICI, Schenkon) provided with 60- and 200-nL internal volumes. Flow injections were carried out using fused-silica capillaries (0.1 mm i.d. × 300 mm L) supplied by Polymicro Technologies (Phoenix, AZ) Eluent Jet Interface. The eluent jet interface is the successor of interfaces originally developed for the coupling of micro-LC16-19 and CE23 with GC detectors. The liquid introduction process is based on a sharp temperature gradient at the tip of the fusedsilica liquid introduction capillary (see A in Figure 1). The sharp temperature gradient is induced by heating a small metal ring (B) mounted on the top of a small quartz tube (0.8 mm i.d. × 1.5 mm o.d.) providing helium cooling (C). These parts are inserted in (D), a wider quartz tube (3 mm i.d. × 6 mm o.d.; desolvation tube) with a 0.3-mm orifice at the end. The metal ring is heated by means of high-frequency induction, which is a noncontact method of heating electrically conducting materials. The induc676 Analytical Chemistry, Vol. 68, No. 4, February 15, 1996

tion heater (Fritz Hu¨ttinger Elektronik, Freiburg, Germany) produces a radio frequency (900-1100 kHz) current in the induction coil (E) placed on the outside of the desolvation tube. This generates an alternating magnetic field in the desolvation tube, which induces an electrical current in the small metal ring, developing sufficient heating power. The desolvation tube is inserted in an unmodified one-stage GC/MS jet separator (Fisons Instruments, Wythenshawe, UK) replacing the inlet jet. The desolvation tube is inserted in such a way that the liquid jet initiates at ∼10-mm distance outside the jet separator at ambient temperature. About 30 mm of the desolvation tube is inside the hot (200 °C) part of the jet inlet of the separator, while the end of the tube (0.3 mm nozzle) is placed close to the 0.25-mm nozzle of the outlet jet (glass) directed to the EI source. The helium flow is regulated by a pneumatic flow controller (Fisons). Mass Spectrometry. The experiments were carried out on a Nermag (Argenteuil, Paris, France) R10-10C quadrupole instrument equipped with a DEC PDP11/23 microcomputer and a VG70250S high-resolution magnetic sector instrument (Fisons Instruments, Wythenshawe, UK) combined with an HP 5890A gas chromatograph and a DEC PDP 11/73 microcomputer provided with the NIST/EPA/MSDC database.24 The accelerating voltage used by the VG 70-250S instrument was 4 kV. The ion source temperatures of both systems were set at 200 °C. The jet separator was connected directly onto the reentrant inlet port of the VG MS system. In the case of the Nermag spectrometer, the original GC/MS inlet was removed and a flange was constructed to connect the jet separator in such a way that the outlet jet was directed into the ion source. The jet separator inlet and outlet temperatures were set at approximately 200 and 250 °C, respectively. With the Nermag MS system, the helium cooling was of the same order of magnitude as that being used in combination with GC detectors, i.e., 50 mL/min. In the case of the VG 70-250S system, an excess of helium flow was adjusted in the range 5-200 mL/min. The jet separator was connected to a double-stage E2M12 rotary pump (Edwards High Vacuum International, Crawley, West Sussex, UK). RESULTS AND DISCUSSION Eluent Jet Aerosol Formation. The conversion of 0.5-3 mL/min liquid flow rates, which are used in conventional-size LC systems, into stable aerosols is well-known in analytical chemistry in the field of elemental analysis. There are ∼10 nebulizer types available, of which seven are being used in today’s PB interfaces.25 The majority uses a high gas flow (L/min), the so-called pneumatic nebulizer, which may be concentric or at right angles to the liquid flow and operates sometimes in combination with heat (thermal-pneumatic nebulization). Other types are based on thermospray, ultrasonic devices or employ a high electrical field such as electrospray. Ultrasonic nebulization and electrospray are often used in combination with high gas flows (i.e., pneumaticassisted electrospray or ultrasonic nebulization). All these nebulization methods have in common that a divergent disperse mist of droplets is formed. The droplet size may be undefined or well controlled, depending on the process involved. An example of well-controlled nebulization is the cross(24) The National Institute for Standards and Technology (NIST), Environmental Protection Agency (EPA), and Mass Spectrometry Data Centre (MSDC) database is supplied in Europe by The Royal Society of Chemistry, University of Nottingham, UK. (25) Creaser, C. S.; Stygall, J. W. Analyst 1993, 118, 1467.

Table 1. Divergence of the Jet at a Distance of 60 mm distancea (mm)

spot diam (mm)

spot shape

0-2 3 4 5

4.0-5.5 3.0-3.5 1.5-2.0 instable jet

clustered single single

a The end of the 100-µm-i.d. fused-silica liquid introduction capillary vs the bottom of the metal ring.

flow pneumatic type, which produces droplets with basically monodisperse sizes being ∼2d, where d is the liquid orifice diameter. This monodisperse aerosol generation interface (MAGIC) was the prototype of the PB interface. In modern PB interfaces, the droplets are introduced by various nebulizer types and are desolvated in a desolvation chamber. During the process of transport through the desolvation chamber, severe losses of analyte will occur due to the divergence of the aerosol jet, resulting in aerosol coagulation followed by settling and impact on the wall. To reduce the impact on the wall of the desolvation chamber, extremely large dimensions are required, often in combination with coaxial gas flows to prevent wall contact. In our new approach, this is unnecessary, because liquid plugs or droplets are transported in succession in a straight line directed to the 0.3-mm orifice of the jet separator. In our earlier work16-19 on the coupling of micro-LC with GC detectors, it was found that by combining heating and sufficient cooling, only a small zone (0 mm). The clustered spot probably results from satellite drops. The impact of large satellites on the TLC plate was observed when the end of the capillary was inside the tube of the helium cooling (at a position of 0-2 mm; see Table 1). A small spot was detected, indicating the stable transport of the nonvolatile analyte, when the end of the capillary was located about 3-4 mm through the 3-mm L metal ring. In another experiment, the liquid jet was observed by a microscope in a strong focused light beam against a dark background. Fused-silica capillaries with internal diameters of

Figure 2. A, liquid jet; B, pulsating spray.

25, 50, 75, and 100 µm were used. The process of jet formation was studied on varying the end of the introduction capillary (nozzle) toward the heated zone, the liquid flow rate from 2 to 15 µL/min, and helium gas flow rates from 50 to 200 mL/min. In this way, the whole process of jet formation and jet instability and formation of satellite drop was clearly observed and recorded by video. Stable liquid jets were observed with all the capillaries studied when the nozzle equaled or protruded within 1.5 mm from the top of the heated metal ring, which corresponds to 3-4.5 mm in Table 1 and confirms the results obtained with the pigment (see Figure 2A). A pulsating spray or the formation of satellites was observed if the tip position was inside or protuded more than 1.5 mm outside the metal ring. Figure 2B shows the pulsating spray when the nozzle is pulled back inside the metal ring. With 75- and 100-µm capillaries, the jet diameters at the tip measured at a distance of 2.5 mm from the nozzle were 70 and 94 µm, respectively, with a standard deviation of (8 µm (n ) 3), which corresponds with the capillary internal diameter. On varying the helium flow rate in the range 50-200 mL/min, again a negligible influence was found for all capillaries. Without the use of helium, the eluent jet formation was interrupted or disturbed. A minimum helium flow of ∼50 mL/min is required to obtain a stable jet. If the rf generator is switched off, the liquid collects at the tip of the capillary. After switching on the rf generator, the liquid jet is recovered within 20 s. The formation of essentially uniform-size liquid plugs was demonstrated using water-sensitive paper. Liquid plugs were deposited on the paper, when the paper was mechanically passed through the eluent jet at a speed of ∼0.20 m/s. Figure 3 shows the impact of the liquid plugs obtained with a 75-µm capillary. The long-term stability of the eluent jet was studied over several days using a relatively high 0.5 M ammonium acetate Analytical Chemistry, Vol. 68, No. 4, February 15, 1996

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Figure 3. Trace of liquid plugs or droplets.

concentration and proved to be very good. Clogging problems did not occur when the 75-and 100-µm fused-silica capillaries were used. However, with 25- and 50-µm internal diameters capillaries, increasing back pressures were observed due to clogging of the capillaries. In our earlier experiments, it was demonstrated that by interfacing liquid microflows to GC detectors, the relatively large 100-µm-i.d. fused silica tip is advantageous because clogging of the capillary seldom occurs. On the other hand, when smaller capillaries or orifices below 50 µm were used, clogging was a major problem, especially with buffer eluents. Nevertheless, the use of small orifices is essential for most other spraying devices, especially in combination with capillary liquid flow rates and low gas flow rates. For example, the MAGIC system, which operates in the flow range of 0.1-0.5 mL/min, already requires a 6-µm orifice. With the current eluent jet interface, however, the 75and 100-µm capillaries can be used with flow rates in the range of 5-15 µL/min. Interfacing to mass spectrometers via eluent jet or droplet jet introduction instead of nebulization is expected to show great promise in the future because there is less loss due to impact on the wall and settling, and additionally, the small jet can be directed exactly to the required spot, e.g., the skimmer. Therefore, future studies of other droplet jet devices are of interest. Recently, Hager et al.26 presented the use of a droplet jet to perform electrospray MS based on the phenomenon of disintegration of liquid jets by vibrating the capillary tip at a high frequency with a piezoelectric buzzer. Desolvation. Due to the small divergence of the aerosol, a desolvation chamber 1000-fold smaller (quartz tube, 40 mm length × 3 mm i.d.), at least in volume, may be used compared to commercial PB systems currently available, for example, at least 300 mm length × 40 mm i.d. In addition, the combination of decreased liquid and gas flow rates requires less effort of the pumping system of the momentum separator (i.e., the use of one rotary pump instead of two or three). When the droplets are transported through the desolvation tube, the solvent starts to evaporate due to the high temperature (200 °C) of the desolvation chamber. The high temperature as compared with commercial PB systems, generally operating at temperatures around 80 °C, is necessarily due to the short traveling time (∼0.1 s) of a droplet in the desolvation chamber used. In the experiments with the Nermag quadrupole instrument, the helium flow rate was adjusted to 50 mL/min. With the VG sector instrument, it proved to be necessary to introduce an additional concentric helium flow to prevent air leaks, causing a discharge of the accelerating voltage. (26) Hager, D. B.; Dovichi, N. J.; Klassen, J.; Kebarle, P. Anal. Chem. 1994, 66, 3944.

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Figure 4. EI mass spectrum of thiodiglycol (top) and library spectrum (bottom).

The increase in helium flow caused a minor decrease in response of a factor ∼2 on addition of 250 mL/min. Addition of ∼100 mL/ min proved to be sufficient. Momentum Separator. The influence of the distance between the 0.3-mm-i.d. nozzle of the desolvation tube and the 0.25mm-i.d. nozzle of the outlet jet of the separator was studied using the Nermag quadrupole MS and caffeine and triethyl phosphate as test compounds. At a short distance of ∼0.3 mm, solvent vapor carryover occurred as observed from the presence of methanol cluster ions in the spectra. At a distance of ∼2.5 mm, no cluster ion formation was observed. At a distance of 1.5 mm, the obtained spectra were still free from cluster ions, but the sensitivity to caffeine increased 2-fold as compared with a nozzle-skimmer distance of ∼2.5 mm. Performance of the System. Linearity and sensitivity were studied with caffeine because this chemical is widely used to demonstrate the specifications of LC-PB systems including commercial systems, allowing comparison of our results. EI spectra of several other compounds, such as simetryn, naphthalene, and triethyl phosphate, have been recorded to study the potential and limitations of the system in comparison with published data of other systems. Thiodiglycol, the decomposition product of the chemical warfare agent sulfur mustard, was studied as target compound. In the first example (Figure 4), it is demonstrated that, for the identification of the relatively small polar compound thiodiglycol, the background with a methanol-water (1:1) eluent is sufficiently low to allow scanning (1 s) from the relatively low mass limit m/z 35 to 200. The EI spectrum recorded with the Nermag quadrupole instrument gives additional information on

Figure 6. Flow injection analysis of 6, 12, and 60 ng of caffeine. Eluent, methanol/water (1:1); flow, 15 µL/min.

Figure 5. EI mass spectrum of simetryn (top) and library spectrum (bottom).

the cluster around m/z 45 and is comparable with the published NIST/EPA/MSDC reference spectrum. A second example recorded with the VG sector instrument under full-scanning conditions (m/z 50-500) shows the EI spectrum of the pesticide simetryn (Figure 5), which also shows a good resemblance to the NIST/EPA/MSDC reference spectrum. Linearity. In the literature, nonlinear calibration curves have been reported at low concentration levels under LC/PB-MS3-8 as well as under SFC/PB-MS conditions,9 resulting in poor detection limits, reduced dynamic range, and quantification difficulties. In a recent extensive review, Creaser and Stygall25 discussed these phenomena. Bellar et al.5 observed that the addition of ammonium acetate to the eluent or the presence of coeluting compounds, enhanced the analyte response and restored the system linearity. These effects were attributed to processes in the particle formation step. The coeluting material gives rise to particles larger than those formed from pure analyte, resulting in a higher momentum and better transport to the ion source. This phenomenon was described as “carrier effect”. This carrier effect was studied more extensively by others.5-7 Some additives enhanced the response; however, none worked well for all. Ho et al.8 found that most of the 13 compounds tested exhibited nonlinear calibration graphs even with ammonium acetate present in the eluent. However, coeluting internal standards were found to enhance the response of both analytes and standards. An alternative explanation for carrier effects was proposed by Doerge et al.3 No coelution enhancement was observed with [1,3,7-13C3]caffeine, instead of [3-13C1]caffeine, demonstrating that mass transfer effects and chemical complex formation do not affect PB transmission efficiency. Spectral overlap between native analyte and nonlinear responses caused the observed enhancement. Observed linearization of calibration plots was explained by the “enhanced”

amounts of analyte which pushed the detector response into its linear range. Creaser and Stygall25 finally concluded: “Although the results reported by Doerge et al.3 are compelling, the continued observation of carrier effects for many different compounds, co-eluents and additives, over a range of concentrations, on a variety of instruments and detectors, suggests that a true “high pass filter” effect does exist.” Recently, a paper described the particle size distribution of the aerosol and of the dried residual particles obtained with and without the presence of ammonium acetate.27 In all cases studied, a broad, polydisperse aerosol distribution was found. The ammonium acetate carrier effect was observed, however, not accompanied by the expected increase in particle diameter, suggesting an increase in the number of aerosol particles transmitted. The linearity of the eluent jet interface was investigated on both sector and quadrupole instruments. Contrary to the expected nonlinearity, linear calibration curves were obtained for caffeine in the investigated 3-200-ng range based on peak area as well as on peak height. The experiments were carried out using 60-nL flow injections at flow rates of 5 and 15 µL/min of methanolwater (1:1) and methanol-0.3 M ammonium formate (7:3) under full-scan conditions (m/z 50-200 and 35-500, respectively). Examples of the reproducibility and linearity are presented in Figures 6 and 7. In Figure 6, the results on the reproducibility of three subsequent injections of increasing amounts (6, 12, and 60 ng, respectively) are shown obtained with an eluent flow rate of 15 µL/min. In Figure 7 calibration curves of caffeine are shown obtained at a flow rate of 5 µL/min in the range from 3 to 60 ng (obtained with the quadrupole instrument) and up to 200 ng (inset, obtained with the VG sector instrument). The observed linear calibration plots were found to be independent of the eluent composition or addition of ammonium formate or ammonium acetate and flow rate or MS instrument used. This may be well explained by the process of droplet formation. In the abovediscussed nonlinearity of PB systems, a linearizing effect was explained by the particle formation; i.e., a larger particle or an increased mass results in a momentum gain and thus better transport through the separator and hence more material reaching the ion source. Doerge3 stressed the importance of enhanced amounts of analyte to push the detector into its linear range. With the eluent jet, both effects will occur; that means that with 75and 100-µm capillaries, large 75- or 100-µm-diameter liquid plugs or at least droplets are formed. The difference in volume as (27) Wilkes, J. G.; Zarrin, F.; Lay, J. O., Jr.; Vestal, M. L. Rapid Commun. Mass Spectrom. 1995, 9, 133.

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Figure 7. Calibration curves of caffeine: range 3-60 ng; eluent, methanol/water (1:1); flow, 5 µL/min; instrument, Nermag quadrupole MS. Inset: range, 18-200 ng; eluent methanol/0.3 M ammonium formate (7:3); flow, 5 µL/min; instrument, VG sector MS.

compared with thermospray, which has a mean droplet diameter of ∼2 µm,27 results in an increased particle, a momentum gain as well as an “enhanced” amount of analyte entering the ion source. We hypothesize that the observed nonlinearity or “high-pass filter” effect is caused by the type of aerosol formed. At a certain low analyte concentration when dealing with nonuniform size droplets, the absolute amount of analyte in small-size droplets will be too low to pass the momentum separator and to reach the detector. Thus, from a broad polydisperse aerosol distribution, a large part will not be detected at a certain low concentration. With a relatively high analyte concentration, even the smallest droplets may contain a sufficient amount of analyte to create adequate momentum to reach the detector. This may also explain that, in the early PB research by Willoughby and Browner,1 linear calibration curves were obtained on a variety of compounds. In their paper, much attention was given to the aerosol formation. They generated an essentially monodisperse aerosol distribution with the cross-flow pneumatic nebulizer (MAGIC) instead of other pneumatic nebulization methods which are commonly used in the commercial instruments resulting in a broad polydisperse aerosol distribution. Sensitivity. The lowest detectable amount of caffeine was 50 pg with a signal-to-noise ratio of 3:1 obtained with the quadrupole instrument in the single-ion-detection mode (m/z 194). Figure 8A shows the result of four respective 100-pg flow injections of caffeine in the single-ion-detection mode using a methanol-water 1:1 eluent and a flow rate of 5 µL/min. The EI spectrum of 3 ng of caffeine (Figure 8B) obtained under full-scan conditions is still acceptable as compared with the NIST/EPA/MSDC database showing five abundant ions: m/z 194, 109, 55, 67, and 82. The use of the VG sector instrument allows accurate study of the influence of the eluent and helium gas flow on the sensitivity of the system. In the MS configuration used, the GC and the dropletjet interface via the jet separator are both on-line coupled to the MS. A fused-silica capillary GC column is directly introduced into the ion source. Moreover, the jet separator can be closed by a dump valve, which allows simultaneous GC/MS analysis with and without the introduction of 5 µL/min methanol-(0.01 M) ammonium acetate eluent and helium flow introduced via the jet separator entrance. The introduction of eluent and helium caused a 2-fold decrease in sensitivity of the system. Without eluent introduction but with increased helium flow, the GC/MS sensitiv680 Analytical Chemistry, Vol. 68, No. 4, February 15, 1996

Figure 8. (A) Single-ion monitoring (m/z 194) of four respective injections of 100 pg of caffeine. Instrument, Nermag quadrupole MS; eluent, methanol/water (1:1); flow 5 µL/min. (B) EI mass spectrum of 3 ng of caffeine; conditions, see (A).

ity remained constant, even if the source pressure was adjusted above the pressure as obtained with liquid introduction by increasing the pressure in the momentum separator. This may indicate that the reduced sensitivity may be caused by a reduced ionization efficiency due to the eluent molecules/ions and may explain the increased sensitivity for micro- vs macroflows.15 When injections of caffeine and naphthalene were compared via GC/MS and via droplet jet-PB MS, it was found that there is still a severe loss of analyte, i.e., 20-fold in the case of caffeine and 40-fold for naphthalene. On the other hand, the obtained sensitivity of the eluent jet interface with the VG system is comparable with today’s commercially available PB systems while the Nermag quadrupole system was found to be even more sensitive. It is to be expected that the sensitivity of liquid jet PB MS will not match the sensitivity of GC/MS, because at least the measured factor of 2 has to be considered, due to the presence of eluent in the system. Besides, the GC column is mounted directly in the EI source, while in the LC mode, analytes dissolved in the eluent have to travel via the desolvation tube through the jet separator. However, the passage through the outlet jet was found to be critical and dependent on the temperature; i.e., high temperatures were required (250 °C, the maximum that can be adjusted) to obtain symmetrical peaks and maximum sensitivity. This is in accordance with results on temperature and design of the liner reported by Tinke et al.,28 who reported that a significant proportion of the particles were “sticking” to the transfer line walls, leading to decreased sensitivity and an impaired peak shape. Therefore, we think that, in our setup, the small dimensions of (28) Tinke, A. P.; Van der Hoeven, R. A. M.; Niessen, W. M. A.; Tjaden U. R.; Van der Greef, J. J. Chromatogr. 1991, 554, 119.

the outlet jet may still cause a part of the 20-40-fold loss of analyte. At the present state of development, improvements are possible concerning the transport through the liner of the jet separator to enhance transport efficiency. Therefore, future attention will be devoted to this critical part of the system. CONCLUSIONS With the current eluent jet interface in combination with a conventional GC jet separator, analytes are detectable at lownanograms levels under full-scan EI conditions, which is acceptable for many applications in micro-LC and comparable or even better than results obtained with today’s commercially available LC/PB-MS systems. The interface proved compatible with both a quadrupole and a magnetic sector instrument. Linear calibration plots were obtained independent of additives added to the eluent. The nonlinearity of present PB systems is suggested to be caused by the principle of nebulization, i.e., the resulting aerosol which may be polydisperse or monodisperse. From the research

presented in the literature, it is obvious that an increase in particle size, which may be an analyte with or without an additive, is essential for efficient PB operation. ACKNOWLEDGMENT The authors thank Dr. B. L. M. van Baar and Prof. Dr. U. A. Th. Brinkman of the Free University Amsterdam for helpful discussions concerning this work, E. C.M. van Daelen (TNO-PML) for his technical assistance in setting up the microscopy video system, and Dr. M. S. Nieuwenhuizen (TNO-PML) for proofreading and editing the manuscript.

Received for review September 1, 1995. November 30, 1995.X

Accepted

AC950892Z X

Abstract published in Advance ACS Abstracts, January 15, 1996.

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