Anal. Chem. 1983, 55,2275-2280 Walling, C. Acc. Chem. Res. 1975, 8 , 125. Phung, P. V.; Burton, M. Radiaf. Res. 1957, 7 , 199. Balakrlshnan, I.; Reddy, M. P. J . Phys. Chem. 1988%7 2 , 4609. Horning, E. C.; Horning, M. 0.;Carroll, D. I.; Dzidlc, I.; Stillwell, R. N. Anal. Chem. 1973, 45, 936. (8) Carroll, D. I.; Dzidlc, I.; Stillwell, R. N.; Haegele, K. D.; Horning, E. C. Anal. Chem. 1975, 47, 2369. (9) Dzidlc, I.; Carroll, D. I.; Stillwell, R. N.; Horning, E. C. Anal. Chem. 1975, 47, 1308. (IO) Hunt, D. F.; McEwen, C. N.; Harvey, T. M. Anal. Chem. 1975, 47, 1730. (11) Kambara, H.; Kanomata, I. Shitsuryo Bunsekl 1978, 2 4 , 229. (12) Kambara, H.; Kanomata, I. Anal. Chem. 1977, 49, 270. (13) Caldecourt, V.; Zakett, D.; Tou, J., Paper presented at the 30th Annual Conference on Mass Spectrometry and Allied Topics, Honolulu, HI, June 1982. (14) Caklecourt, V.; Zakett, D.. Tou, J. Int. J . Mass. Spectrom. Ion Phys. 1983, 49, 233. (4) (5) (6) (7)
2275
Beynon, J. H.; Cooks, R. G.; Amy, J. W.; Baltinger, W. E.; Ridley, T. Y. Anal. Chem. 1973, 4 5 , 1023A. Kondrat, R. W.; Cooks, R. G. Anal. Chem. 1978, 50, 81A. Slayback, J. R. 6.; Story, M. S. Ind. Res. 1981, 2 , 129. Story, M. S.; Stelmer, U.; Boltnott, C. A.; Sokolow, S.; Weissand, M.; Slayback, J. R. B., paper presented at the 29th Annual Conference on Mass Spectrometry and Allied Topics, Minneapolis, MN, May 1981. Schoen, A. E.; Zakett, D.; Korzenlowski, R. W., paper presented at the 29th Annual Conference on Mass Spectrometry and Allled Topics, Mlnneapolis, MN, May 1981. (20) Carroll, D. I.; Dzidic, I.; Horning, E. C.; Stillwell, R. N. Appl. Spectrosc. Rev. 1981, 17, 337.
RECEIVED for review March 8,1983. Accepted August 5,1983. This work was supported in part by the National Science Foundation Grant CHE80-11425.
Direct Liquid Introduction/Thermospray Interface for Liquid Chromatography/Mass Spectrometry Thomas Covey and Jack Henion* Equine Drug Testing and Research Program, New York State College of Veterinary Medicine, Cornell University, 925 Warren Drive, Ithaca, New York 14850
A new version of the thermospray LC/MS Interface is reported, It dlffers from those described previously in that It Is a dual purpose probe-type interface whlch Is introduced into the mass spectrometer vla the standard direct insertion probe inlet. The device is a dual purpose LC/MS interface and It can provide conventional DLI LC/MS, or the copper vaporizer may be heated electrically to produce thermospray ionization. The Interface provides the opportunlty to obtain both DLI and thermospray LC/MS results for a given problem such that the optimum mode may be chosen. Detection llmlts are currently about 100 ng In the thermospray mode for Involatlle, iabie compounds utillzkrg 2 mm 1.d. rnkrdbore LC columns at a flow rate of 150 pL/min.
The discovery of thermospray LC/MS by Vestal (1-3) and others (4, 5 ) appears to offer one of the most exciting developments toward a truly viable LC/MS interface. Historically, liquid chromatographers have been advised that LC/MS requires low or nonaqueous eluent compositions, restricted buffers or modifiers, and stable, relatively volatile compounds. Trace analysis (defined as low nanogram levels) of labile compounds on the moving belt (6) or heated concentrator wire (7) LC/MS interface appears impractical in some instances. However, there have been some impressive applications using the transport interface and its use in various areas continues to be of interest. The direct liquid introduction (DLI) LC/MS interface reported by McLafferty (8) requires an unfavorable split of HPLC effluent which precludes routine full scan DLI LC/MS trace analysis. The determination of labile biological metabolites using DLI/MS appears preferred over the moving belt (9),but unless micro LC/MS techniques are utilized ( 1 0 , l l )the technique still does not provide trace analysis capability. These and other approaches have provided an increasingly viable means Q€ accomplishing LC/MS, but routine sensitivity in many instances has not been comparable to that afforded by GC/MS. Many researchers involved with environmental
and toxicology studies require LC/MS detection limits better than has been commercially available. In addition, molecular weight and structural information from LC/MS mass spectra should be readily available from the fragile compounds that are so ideally suited for HPLC. Vestal (3),Edmonds (12),and Yergey (13) have reported impressive thermospray applications including dramatic LC/MS sensitivity and the ability to handle high molecular weight and labile compounds. These results were produced from similar but separate instruments utilizing the same design. The success of these results prompted us to implement the thermospray LC/MS concept into our existing LC/MS program. We report the construction and preliminary results of a new, dual purpose interface which provides both DLI and thermospray LC/MS capability from the same device. It is unique in that as a thermospray interface it is inserted or removed from the standard half-inch direct insertion probe inlet of a Hewlett-Packard 5985B GC/MS and also provides conventional DLI LC/MS. This device offers several new advantages which include: (a) dual purpose operation of either conventional or micro LC/MS in the DLI mode or thermospray operation from one device, (b) a removable thermospray interface which facilitates maintenance and conventional use of the solid probe inlet of the GC/MS, and (c) practical add-on to existing GC/MS instruments equipped with a chemical ionization (CI) source. These features combined with simplified construction offer an easy introduction to two of the most popular current approaches to practical LC/MS. EXPERIMENTAL SECTION In this DLI/thermospray probe interface (Figure I), a central microbore (0.004 in. i.d.) stainless steel tube transfers total LC effluent to the ion source of the mass spectrometer. The LC effluent may pass either through the thin (0.0005 in.) stainless steel diaphragm, F, using the DLI mode (filament on) or the diaphragm may be removed and the probe operated in the thermospray mode (filament off). In certain cases where the diaphragm pinhole is larger (10-30 pm diameter) and the eluent flow rate is reduced to 0.1-0.2pL/min the diaphragm may be used in either the thermospray or DLI mode. The diaphragm (Op-
0003-2700/83/0355-2275$01.50/00 1983 Amerlcan Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. SCALE
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Flgure 1. Dual purpose DLI/thermospray LC/MS probe interface: (A) central microbore (0.004 in. i.d.) through-put tube; (B) two 50-W 120-V ac cartridge heaters; (C)heated copper vaporizer; (D) thermocouple; (E) removable end cap; (F) stainless steel pinhole diaphragm.
timation, Windham, NH, or Precision Aperature, Ft. Wayne, IN) has a precisely centered “one shot” laser generated pinhole (usually 20 pm diameter for thermospray operation or 5 pm diameter for DLI operation) which is held firmly against the exit end of the microbore tubing at the probe tip. The threaded end cap accomplishes this by pressing the diaphragm against the machined copper probe tip surface. The metal-to-metal seal affords zero dead volume at the probe tip while permitting total transfer of LC effluent through the pinhole orifice into the CI ion source. The rounded end cap fits snuggly into the extended desolvation chamber described previously (14). If higher eluent flow rates are utilized ranging from 0.5 to 1.0 mL/min the pinhole diaphragm is removed for thermospray LC/MS experiments. The probe is constructed from three simple parts (Figure 1) which include the hollow, half-inch outside diameter probe shaft (Part No. 421225CT, Rainin Instrument Co., Woburn, MA), a threaded copper vaporizer housing the microbore stainless steel throughput tubing (0.004 in. i.d. X 0.062 in. o.d., Part No. 30211, Alltech Associates Inc., Deerfield, IL) (C),and the threaded end cap (E). The copper vaporizer is drilled to accommodate the microbore throughput tube (A), two 0.125 in. 0.d. X 1.250 in. 50-W heater cartridges (B) (Part No. ClE13, Watlow Inc., St. Louis, MO), and an iron-Constantine thermocouple (D) (Part No. Type J, 0.062 in. X 1.0 in., Watlow, Inc., St. Louis, MO). The heater cartridges are wired in series and powered by a conventional laboratory variable output transformer. A temperature feedback sensor would be preferable, but the transformer provided a simple, satisfactory power supply ranging from 0 to 120 V ac. The thermocouple leads were connected to a conventional analog thermocouple pyrometer. The probe shaft end of the vaporizer is threaded such that a vacuumtight stainless steel-copper junction is accomplished by tightening the threaded probe shaft to the vaporizer. This allows installation of the two cartridge heaters and thermocouple prior to assembly of the probe. The copper vaporizer tip is similarly threaded and machined smooth to accommodate the removable end cap. The end cap is constructed to center the pinhole diaphragm precisely over the microbore throughput tube and form a tight seal between the vaporizer tip and the diaphragm. This ensures efficient transfer of LC effluent through the pinhole. The microbore throughput tube is intimately silver soldered to the copper vaporizer to ensure efficient heat transfer from the heated vaporizer to the stainlesssteel tubing. The microbore tubing may be easily reopened after mchining the flat probe tip surface by the electropolish procedure described previously ( 1 1 ) . The cartridge heater and thermocouple wire leads were soldered to a four-pin BNC connector which was taken from the standard direct inserion probe supplied with this MS (Hewlett-Packard,Palo Alto, CA). The major modification to the Hewlett-Packard 5985B GC/MS was the incorporation of a 300 L/min rough pump on the GC side of the CI source similar to that described by Vestal (3). The GC was rolled on its track 12 in. away from the MS. Figure 2 shows the half-inch pump out line which mates to the GC port of the source block and allows increased pumping directly on the ion source. This allows the introduction of up to 1 mL/min of the aqueous LC effluent directly into the GC/MS. The only other change was that the holes for sample inlet and thermospray pump out on the CI ion source were drilled out to 1/8 in. diameter. The thermospray pump out line is constructed from a blank 2.5 in. Varian flange (D) which accommodates an outer tube (C) welded t o a symmetrical brass Swagelok union equipped with Teflon ferrules which allows the inner half-inch stainless steel tube (E) to slide in against the source block. This feature allows
Flgure 2. Schematic of DLI/thermospray inlet and pump out region of HP5985B GUMS: (A) to thermospray rotary pump; (B) Swagelok union equipped wlth Teflon ferrules; (C) outer tube welded to the high vacuum flange; (D) high vacuum flange; (E) half-inch stainless steel slide tube; (F) CI ion source; (G) MS analyzer housing; (H) extended desolvation chamber; (I) thermospray vaporizer; (J) half-inch direct insertion probe isolation valve; (K) DLMhermospray LC/MS interface. HP D L I LClMS INTERFACE
L J
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SPECTROMETER
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Schematic of overall DLI/thermospray LC/MS system.
tube E to be retracted from the source block during its removal. Tube E is connected via a copper half-inch tube to a 300 L/min Alcatel rotary pump. The rotary pump may be isolated from the MS during conventional CI operation by closing the half-inch ball valve (Worcester Valve, Part No. 10-1 V 411 BTE) which is installed in the thermospray pump-out line near the rotary pump. ~ thermocouple tube (not shown in Figure 2) was positioned A l / in. inside of tube E with its opening within in. of the ion source region to allow nominal measurement of source pressure during thermospray or DLI operation (Sargent-Welch, Catalog No. 28884-40F, Skokie, IL.). A liquid nitrogen cold trap was later installedjust before the rotary pump to trap excess solvents. This is a desirable addition which protects the rotary pump from abusive exposure to LC solvents. The liquid nitrogen cryopump which is usually used for DLI LC/MS (11) was also used in these experiments. It provides additional pumping for aqueous, low molecular weight solvents and appeared to favorably assist the thermospray rotary pump. Figure 2 shows the remainder of the MS system including the DLI/thermospray probe, K, inserted through the standard half-inch direct insertion probe inlet, J, into the extended desolvation chamber, H. The heated vaporizer, I, provides thermal energy to produce the thermospray heated vapor which enters the conventional CI ion source. When this probe is used for thermospray operation the emission current is set to 0 A and the electron energy to 10 eV (this is the lowest value allowed by the data system). With the other source tuning parameters set at normal CI values, these source conditions provide no external ionization from the standard filament, e.g., only thermospray ionization occurs. When the probe is used in the DLI mode, the emission current is set to 300 pA and the electron energy to 230 eV. With all other source values unchanged, one can then accomplish conventional DLI LC/MS. Thus the only difference between thermospray and DLI LC/MS is whether the filament of the ion source is off or on. Figure 3 shows an overall schematic of the DLI Thermospray LC/MS system. The chromatographic system consists of two Model 6000A pumps and a Model 660 solvent programmer (Waters Associates, Milfred, MA) modified for micro-LC equipped with a 5-pL loop injector (Rheodyne, Cotati, CA) connected via microbore tubing (0.004 in. i.d.) to a Brownlee RP-18 10 micron OD-222 2 mm i.d. X 22 cm microbore column equipped with a 3-cm RP-18 guard column (BrownleeLabs, Santa Clara, CA). The exit of the Brownlee microbore column is connected to a Waters Model 440 fixed wavelength (254 nm) UV detector equipped with a 1-pL microcell (Part No. 97212, Water Associates, Milfred, MA). The exit of the UV detector cell is connected to the DLI thermospray LC/MS probe interface via microbore tubing (0.004 in. i.d.). This arrangement offers the convenience of monitoring the
ANALYTICAL CHEMISTRY, VOL. 55, NO. 14, DECEMBER 1983
UV chromatographic trace prior to the MS total ion current chromatogram. The reversed-phase LC eluent consisted of 50/50 CH30H/0.05 M NH40Acoperated at flow rates ranging from 0.1 to 1.0 mL/min. Most of the DLI/thermospray work was performed using an eluent flow rate of 150 pL/min. The normal operating range of these 2 mm i.d. microbore LC columns is easily accommodated by the MS pumping system described above and provides short LC/MS run times. The delay time between UV detection observed on the strip chart recorder and MS detection of ion current observed on the real-time MS terminal averaged about 10 s. The choice of the 2 mm i.d. LC columns over 1mm i.d. microbore columns was dictated by the suggestion (15)that thermospray LC/MS may require a higher linear velocity of LC effluent through the vaporizer than provided by the lower flow rate of the latter. The mass spectrometer was operated with ita dual EI/CI source in the CI position following conventional tuning in the E1 and CI/CH4 modes. The source temperature was typically held at 250 "C. The details of tuning for thermospray and DLI LC/MS follow.
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RESULTS AND DISCUSSION Since these experiments were our first experience with thermospray ionization, we focused initially on obtaining ions from dissolved adenosine (10 ppm, in 50/50 CH30H/0.05 M NH,OAc) fed continuously into the mass Spectrometer. The HPLC system described above pumped the adenosine solution through the heated vaporizer at 0.15 mL/min. The flow rate was held constant while adjustments of vaporizer temperature, probe position, source pressure, source temperature, and source tuning were systematically varied. Considerable difficulty was encountered initially. Without an external source of ionization (e.g., filament ''offn) no combination of the above adjustments appeared to produce thermospray ionization. However, whenever conventional CI source conditions were utilized (e.g., filament "on"), DLI LC/MS was achieved and abundant ions for adenosine and the HPLC eluent were observed. Thus it appeared that DLI LC/MS conditions always produced ions but that thermospray ionization seemed to require much more rigid control of experimental conditions. Since it had been suggested (15) that the 0.004 in. i.d. microbore tubing may be too large for the micro-LC flow rate that we had chosen, we experimented with various sized pinhole diaphragms. The first evidence for thermospray ionization in this work came when an 18 pm pinhole diaphragm was utilized at a flow rate of 0.15 mL/min through the vaporizer heated to 225 "C. The ion current was very stable over a 7-h time period and the thermospray ion current could be made to appear and disappear reproducibly over a vaporizer temperature range of *25 "C as has been reported (3,13). When the thermospray probe was removed from the direct insertion probe inlet under actual thermospray conditions, it produced an audible, visible jet of high velocity vapor similar to that described by Yergey (16). This condition persists unless either the eluent flow rate or the vaporizer temperature is altered. Thermospray ionization utilizing eluent flow rates less than 0.3 mL/min appears to require the smaller orifice afforded by the pinhole diaphragm whereas higher eluent flow rates provide thermospray ionization directly from the microbore throughput tubing. Figure 4 shows a comparison of two characteristic ions from adenosine obtained from this LC/MS interface in both the DLI (upper) and thermospray (lower) LC/MS mode. Figure 4A shows all the relevant CI source tuning parameters for a typical DLI LC/MS experiment. Note in particular that the emission current is set at 300 PA and the electron energy at 230 eV. The ion current display in Figure 4A shows m / z 136 and 268 (M + 1)' for adenosine in the DLI LC/MS mode. It is clear that the fragment ion for adenosine at m/z 136 is much more abundant than the (M 1)' ion at m/z 268 even under the mild ionizing conditions of DLI.
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Flgure 4. (A) m l z 136 and 268 ions for 10 ppm adenosine under DLI LClMS ion source condltlons uslng 0.15 mLlmln 50150 CH,OH/O.O5 M NH,OAc. (B)m l z 136 and 268 ions for 10 ppm adenosine under thermospray LClMS ion source conditions using 0.15 mL/min 50l50 CH,OH/O.O5 M NH,OAc. The Ion source conditions differ from DLI LClMS only in that the emission current is 0 A and the electron energy is 10 eV.
Flgure 5. Thermospray extracted ion current profile for the (M 4- 1)' ions of theophylline ( m l z 181) and caffeine m l z 195) using a flow rate of 0.15 mL/min 50150 CH,OH/O.O5 MNH,OAc. The LC column was a reversed-phase Browniee 10 p m ODS-222 equipped with a 3-cm guard column.
Figure 4B, however, shows these same ions obtained in the thermospray ionization mode. The indicated emission current of 0 pA and 10 eV electron energy suggests that there was no contribution to ionization from an external source of ionizing electrons. In this thermospray experiment the m / z 268 (M + 1)+ion for adenosine is twice the abundance of the m/z 136 fragment ion. It appears, therefore, that these data corroborate the claim by others (1-5) that thermospray ionization is a very mild form of ionization. In fact, these data demonstrate that in this instance thermospray LC/MS is a milder form of ionization for adenosine than DLI LC/MS in the positive ion chemical ionization (PCI) mode using the same LC/MS interface and MS. The on-line thermospray LC/MS determination of the xanthine drugs theophylline and caffeine is shown in Figure 5. The eluent used was not optimum for the efficient separation of these compounds. The 10 ppm solution of adenosine was purged from the LC column with 50/50 MeOH/0.05 M NH,OAc until no detectable adenosine could be observed in either the DLI or thermospray ionization mode. An injection of 100 ng each of theophylline and caffeine produced
2278
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the ion current chromatogram shown in Figure 5. The marginal separation and considerable tailing of the caffeine component are due primarily to the unfavorable choice of LC eluent rather than the thermospray interface. It should be noted that the 100-ng levels of these compounds are readily observed by this technique and that the ion current is stable throughout the course of the experiment. The thermospray ionization LC/MS mass spectra for the chromatographicpeaks in Figure 5 are shown in Figure 6. As expected from very mild ionization of relatively stable compounds the (M 1)+ions at m / z 195 for caffeine and m / z 181 for theophylline are the predominant ions in their respective mass spectra. Although overly simplified mass spectra such as these are of little use for structural characterization by themselves, they would be very useful for obtaining daughter ion spectra by tandem mass spectrometry (LC/ MS/MS) (17). Figure 7 shows the results of a more rigorous comparison of ionization by conventional DLI LC/MS and thermospray LC/MS. We have studied the DLI LC/MS behavior of corticosteroids such as betamethasone for several years (11) and have been frustrated by the paucity of molecular weight information due to the facile loss of the C1, side chain. Figure 7A shows the typical positive ion chemical ionization (PCI) DLI LC/MS mass spectrum for betamethasone. No (M 1)+ a t m/2 393 is seen whatsoever due to the loss of 60 amu to produce the base peak of m / z 333. This behavior is characteristic for this class of compounds and is not improved by negative chemical ionization (11). In contrast, however, the PCI thermospray ionization LC/MS mass spectrum of betamethasone shown in Figure 7B displays the desired (M + 1)' ion as the second most abundant ion in this mass spectrum. The m / z 333 ion is still an abundant fragment, but there is now clear support for the molecular weight of this compound. The data in Figure 7 offer valid support for both the dual purpose nature of this new DLI/thermospray LC/MS interface and the very mild ionization condition provided by thermospray LC/MS as has been suggested (1-5). The spectra shown in Figure 8 were obtained from two separate injections of betamethasone with no experimental changes other than changes from the source emission/electron energy conditions shown in Figure 4A to those shown in Figure 4B. In a practical sense this interface may be used to acquire both DLI and thermospray LC/MS results on a problem of interest followed by utilization of whichever technique is preferred. It is possible that when the latter has become routine and better understood, thermospray combined with MS/MS may provide the best overall combination of molecular weight verification and structural characterization.
+
Figure 7. (A) PCI D L I LC/MS mass spectrum of betamethasone using the LC conditions of Figure 5. (B) PCI thermospray LC/MS mass spectrum of betamethasone using the LC conditions of Figure 5. A
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described for Figure 5. To further evaluate the thermospray LC/MS feature of this interface a variety of organic compounds were tested under flow injection analysis (FIA) conditions. This involved injection of the sample of interest via a microloop injector
ANALYTICAL CHEMISTRY, VOL. 55, NO. 14, DECEMBER 1983 A
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standard 5-pg solution of leucine enkephalin. This extracted ion current profile for the (M + 1)' ion at m / z 556 shows that a stable, abundant ion current signal was observed for the intact molecule. Figure 9B shows that some useful fragmentation ion current was observed in addition to the (M + 1)' base peak. The relatively abundant (M + 23)' ion is presumably due to the addition of a sodium atom to the parent leucine enkephalin. This natriation has been observed by others (2,19) and may offer additional diagnostic information when better understood. These data also show that labile compounds such as this brain opiod peptide may be characterized by this new ionization technique (1). Traditionally, peptides of this type have been determined by field desorption (FD) MS, but required extensive work up of complex biological samples prior to MS analysis of the sample (20). If thermospray ionization LC/MS can provide the powerful combination of on-line chromatographic separation with mild ionization, the technique could be very useful in the area of biological trace analysis. If structural characterization could be accomplished by MS/MS techniques on the abundant (M 1)+ions such as those presented in this work and that reported by others (5), many challenginganalytical problems could potentially be solved in a straightforward manner (21).
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CONCLUSIONS A dual purpose DLI/thermospray LC/MS interface has been demonstrated on a modified commercially available GC/MS. After initial tuning in both the DLI and thermospray ionization modes, either technique may be utilized by the simple change of two ion source parameters. Commercially available Brownlee microbore LC columns (2 mm i.d.1 equipped with a guard column provide fast, efficient separation of rather dirty samples and on-line mass spectrometer monitoring of the total LC effluent in the DLI LC/MS mode. The present system provides practical detection limits of about 25 ng in full scan split effluent DLI LC/MS mode and about 100 ng in the full scan thermospray LC/MS model. The latter detection limits should be improved significantly by future improvements in the system which are currently under way. These include a temperature feedback control which monitors the thermospray "plume" in the ion source, a smaller inside diameter microbore throughput capillary, and a heated extended desolvation chamber which will allow increased variation of the probe interface during operation. Impressive thermospray LC/MS detection limits have been demonstrated (3,12, 13) and we plan to achieve this necessary goal. The most important limitation of the present system is that the thermospray feature is not as routine or reliable as the DLI LC/MS feature. DLI LC/MS can be achieved under nearly any condition on a daily basis. However, there are times when we are unable to obtain any thermospray ionization even when we are duplicating previous successful experimental conditions. This is an unforunate fact that is apparently not without precedence (19) although some leaders in this field achieve routine operation (3, 12, 13). When thermospray LC/MS becomes as routine as DLI LC/MS has become in our hands, this dual purpose interface may be a useful means of accomplishing two of the currently most viable modes of achieving LC/MS. ACKNOWLEDGMENT The authors are grateful to M. Vestal, C. Edmonds, and A. Yergey for many helpful discussions and to G. A. Maylin for his continued support of this work. LITERATURE CITED (1) Blakely, C. R.; Carmody, J. J.; Vestal, M. L. Anal. Chem. 1980, 52,
.---.
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(2) Blakely, C. R.; Carmody, J. J.; Vestal, M. L. J . Am. Chem. 1980, 702, 5933. (3) Blakely, C. R.; Vestal, M. L. Anal. Chem. 1983, 55, 750.
SOC
.
2280
Anal. Chem. 1983, 55, 2280-2284
(4) Thomson, B. A.; Iribarne, J. V. J. Chem. Phys. 1979, 71, 4451. (5) Thomson. B. A.; Iribarne, J. V.; Dziedzic, P. J. Anal. Chem. 1982, 54, 2219. (6) McFadden, W. H.; Schwartz, H. L.; Evans, S. J. Chromafogr. 1978, 122, 389. (7) Christensen, R. G.; Hertz, H. S.; Meiselman, S.; White V. E. Anal. Chem. 1981, 53, 171. (8) Arplno, P. J.; Dawkins, B. G.; McLafferty, F. W. J . Chromafogr. Sci. 1974, 12, 574. (9) Qmes, D.; Devant, G.;Dixon D. J.; Martin, L. E. 2nd Workshop on LClMS and MSIMS, Montreux, Switzerland, Oct 21-22, 1982. (10) Henion, J. D.; Maylin, G. A. Biomed. Mass Specfrom. 1980, 7 , 115. (11) Eckers, C.; Skrabalak, D. S.; Henion, J. D., Clln. Chem. (Winston-Sa/em, N.C.) 1982, 2 8 , 1882. (12) Edmonds, C. G.; Pang, H.; McCloskey, J. A.; Blakely, C. R.; Vestal, M. L. Presented at 30th Annual Conference on Mass Spectrometry and Allied Topics, Honolulu, HI, June 6-11, 1982, Paper ROC 6. (13) Yergey, A. L.; Vestal, M. L.; Biakeiy, C. R. Presented at 30th Annual Conference on Mass Spectrometry and Aiiied Topics, Honolulu, HI, June 8-11, 1982; Paper MAP 12. (14) Sugnaux, F.; Skrabalak, D. S.; Henlon, J. D. J. Chromafogr. 1983, 264, 357.
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RECEIVED for review July 20, 1983. Accepted September 1, 1983. The authors thank the New York State Racing and Wagering Board Equine Drug Testing and Research Program for financial support. Part of this work was first presented at the 31st Annual Conference on Mass Spectrometry and Allied Topics, Boston, MA, May 8-13, 1983; Paper 343.
Gas-Nebulized Direct Liquid Introduction Interface for Liquid ChromatographyIMass Spectrometry James A. Apffel,* Udo A. Th. Brinkman, and Roland W. Frei Department of Analytical Chemistry, Free University, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
Evert A. I. M. Evers Department of Organic Chemistry, Free University, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
The design of a direct liquid introduction (DLI) LC/MS interface Is described. This system uses a jet of helium gas to aid the nebulization of the vaporizing LC effluent and sample Into the MS source. The effects of several operating variables, such as probe tip position, mobile phase composttion and flow rate, and source temperature are discussed. The performance of the unit is evaluated in terms of the variance Contribution to the total peak width, repeatability, linearity, detection limits and the applicability for a range of samples including phenyiurea herbicides, polycyclic aromatic hydrocarbons, drugs such as ciobaram and ranltidine, and catecholamines.
In the last several years, the field of on-line high-performance liquid chromatography/mass spectrometry (LC/MS) has grown from the desire for LC detector specificity comparable to that available in gas chromatography/mass spectrometry (GC/MS) into an analytically applicable technique (I). As an LC detector, the mass spectrometer offers a number of advantages over conventional detector systems, including simultaneous use as both a universal and specific detector and the capability of yielding molecular weight and confirmatory identification information. On the other hand, LC offers not only the possibility of separating nonvolatile and thermally labile compounds for subsequent MS analysis but also use of LC pretreatment, preconcentration, and cleanup techniques not practical with GC. Interfaces used in coupling these two analytical techniques fall into two basic categories: transport interfaces and direct liquid introduction (DLI) interfaces. A number of systems have been used in the DLI approach for introducing the LC effluent into the MS source. These include the formation of
liquid jets through either a viscous flow capillary (2) or 1-5 pm diaphragms (3) and vacuum nebulization interfaces (4). In the vacuum nebulization interface, the LC column effluent is nebulized from a capillary tip by a flow of gas into an intermediate vacuum chamber before entering the MS through an orifice (46). While the diaphragm systems have shown the most impressive results, the diaphragms themselves are expensive and are subject to wear (7). The narrow capillaries (
