Anal. Chem. 1993, 85, 3534-3539
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Investlgatlon of Cryopumplng for Enhanced Performance In Supercritical Fluid Chromatography/Mas Spectrometry J. David Pinkston* and Donald J. Bowling Miami Valley Laboratories, The Procter & Gamble Company, P.O.Box 398707, Cincinnati, Ohio 45239-8707
INTRODUCTION Supercritical fluid chromatography/mass spectrometry (SFC/MS) has been shown to be a powerful tool for complex mixture analysis. It has been used to characterize a wide variety of mixtures, most of which are relatively nonpolar and of limited volatility. Examples include derivatized oligosaccharides,192 ethoxylated surfactants,3-4ecdysteroids,6 derivatized poly(acry1ates)ls polynuclear aromatic hydrocarbons,' cyclic peptides,8vQ and poly(isocyanates).10 Other applications are found in recent monographs.llJ2 In this laboratory, we have most commonly linked opentubular SFC to a differentially pumped, triple-quadrupole mass spectrometer using the direct-fluid introduction interface.IA We have typically observed a dramatic drop in signalto-noise ratio (S/N) above SFC pressures of approximately 30.4-35.5 MPa (300-350 atm) while operating with mobilephase flow rates typical of 50-pm open-tubular columns (0.5-3 mL/min COzmeasured at room temperature and atmospheric pressure). Other investigators have reported similar effects in SFC/MS.IS This drop in S/N at higher SFC pressures is presumably related to the increased flow of SFC mobile phase (most commonly COZ)into the ion source when operated with a fixed-flow restrictor. It is unclear whether the primary detrimental effect of COz is upon the ionization mechanism (within the ion source) or upon the transmission of ions from the source through the mass analyzer to the detector (outside the ion source). The drop in S/N above the 30.4-35.5-MPa range was a relatively minor problem while the upper pressure limit of the SFC instrumentation was 40.5 MPa (400 atm). We have increased the upper pressure limit of our custom-built SFC systems to 56.7 MPa (560 atm)14in order to benefit from the superior performance provided by higher column-oven temperatures and more retentive stationary phases. This has
* Corresponding author.
(l)Pinkston, J. D.; Owens, G. D.; Burkes, L. J.; Delaney, T. E.; Millington, D. S.; Maltby, D. A. Anal. Chem. 1988,60,962-966. (2) Reinhold, V. N.; Shwley, D. M.; Kuei, J.; Her, G. Anal. Chem. 1988,60, 2719-2722. (3) Matsumoto, K.; Tsuge, S.; Hirata, Y. Shitsuryo Bunseki 1987,35, 15-22. (4) Pinkston, J. D.; Bowling,D. J.; Delaney, T. E. J. Chromatogr. 1989, 474, 97-111. (5)Raynor, M. W.; Kithinji, J. P.; Bartle, K. D.; Games, D. E.; Mylchreest, I. C.; Lafont, R.; Morgan, E.; Wilson, I. D. J. Chromatogr. 1989.467.292-298. ~... Pinkston, J. D.; Delaney, T. E.; Bowling,D. J. J. Microcolumn Sep. 1990,2, 181-187. (7) Berry, A. J.; Games, D. E.; Perkins, J. R. Anal. h o c . (London) 1986,23,451-463. ( 8 )Kalinoski, H. T.; Wright, B. W.; Smith, R. D. Biomed. Enuiron. Mass Spectrom. 1988,15,239-242. (9) Huang, E. C.; Jackson, B. J.; Markides, K. E.; Lee, M. L. Anal. Chem. 1988,60,2715-2719. (10) Blum, W.; Ramstein, P.; Grether, H.-J. J. High Resolut. Chromatogr. 1990,13, 290-292. (11) Lee, M. L., Markides, K. E., Eds. Analytical Supercritical Fluid Chromatography and Extraction; Chromatography Conferences: Provo, TTT WO -,1___". (12) Wenclawiak, B., Ed. Analysis with Supercritical Fluids: Extraction and Chromatography; Springer-Verlag: Berlin, 1992. (13) Kalinoski, H. T.;Hargiaa, L. 0. Presented at the 6th (Montreux) Symposium on LC/MS (LC/MS SFCIMS CZE/MS; MS/MS), Ithaca, NY, July 19-21, 1989. (14) Chester, T. L.; Bowling, D. J.; Innis, D. P.; Pinkston, J. D. Anal. Chern. 1990,62, 1299-1301.
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left a sizable gap between the region where SFC/MS S/N begins to drop and the maximum operating pressure of the SFC systems. We first made two changes to the SFC/MS system and operating proceduresin an attempt to narrow the gap between the pressure region where SFC/MS S/N begins to drop and the maximum operating pressure of the SFC system. The first change was the substitution of a 20-kV conversiondynode for the original 5-kV dynode located before the electron multiplier. The 20-kV dynode has been shown to yield superior response and higher S/N for high molecular weight species.15 (Generally, the species eluting above 35.5 MPa (350 atm) in most SFC applications have molecular weights above 10oO.) The second change was in our tuning procedure. Previously, the SFC pressure was held at 7.1-10.1 MPa (70100 atm) while tuning. We now set the SFC pressure at 20.330.4 MPa (200-300 atm) (depending on the particular application) while tuning. This improves the S/Nat higher SFC pressures. However, neither of these changes made the dramatic improvement in S/N that we were seeking. In previous work in this laboratory, Owens et al.16 demonstrated an -&fold improvement in signal by using cryopumping on a single-quadrupole mass spectrometer operated in the SFC/MS mode. The influence of cryopumping on SFC/MS performance was not studied in detail, nor was the system optimized (i.e., cryopumping in the source compared to cryopumping in the analyzer region) in this work. This previous work did suggest that cryopumping might improve the performance of our SFC/MS system and that performance improvements greater than those previously observed might be obtained by optimizing the cryopump configuration. We investigated the influence of various combinations of one, two, and three simple, liquid nitrogencooled cryopumps on the mass spectrometer analyzer pressure during simulated SFC/MS separations in order to optimize the cryopump configuration. We also compared SFC/MS spectra and chromatograms obtained with and without cryopumping using pure standards, standard mixtures, and real samples. Here we report the resulta of these experiments.
EXPERIMENTAL SECTION Instrumentation. Chromatography. Supercritical fluid chromatography work was performed with a custom-built SFC system based on a Varian Model 3400 gas chromatograph oven and a Model 8500 liquid chromatographypump (Varian, Walnut Creek, CA) modified for pressure control and programmed with an external computer." The system was used as previously describedleJ7except that the system was modified for operation up to 56.7 MPa (560 atm) rather than 40.5 MPa (400 atm1.l' Direct,on-column injection onto the SFC column was utilized, withno flow or timesplit,usingaModelECI4W.linjectionvalve (Valco, Houston, TX) with a 0.1-pL internal loop. A 1-m-long, 50-pm4.d. piece of untreated fused silica tubing linked the injection valve to the column. The column was a 10-m-long, (15) Schoen, A. E.; Syka, E. P. Presented at the 36thASMS Conference on Allied Topica, San Francieco, CA, June 5-10,198& p 843. (16) Owens, G. D.; Burkes, L. J.; Pinkston, J. D.; Keough, T.; S i m s , J. R.; Lacey, M. P. ACS Symp. Ser. 1988, No. 366,191-207. (17) Chester, T. L.; Innis, D. P.; Owens, G. D. Anal. Chem. 1985,57, 2243-2241. 0 1993 Amerlcan Chemical Soclety
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S, im source;
50-pm4.d. SB-Biphenyl-30 (Dionex, Sunnyvale, CA). The restrictor was a thin-walled,robot-drawn tapered capillary.'7J8The restrictor tapered from a 50-pm i.d. to a 4-pm aperture over 3.3 em. ThemobilephasewasSFC-gradeCO, (ScottSpecialtyGases, Plumsteadvil!e, PA). Chromatographic separations were typically performed using linear pressure ramps from 10.1 to 55.7 MPa (100 to 550 atm) at 0.5 MPa/min (5 atm/min). The SFC oven temperature was 120 "C. Mass Spectrometry. AModelTSQ-70triple-quadrupnlemass spectrometer (Finnigan-MAT, San Jose, CA) equipped with the standard, direct-introduction SFC/MS interface was used for this work. The standard TSQ-70 is differentially pumped by two Model TPH 330, 330 Lis turbomolecular pumps (Balzersl Pfeiffer, Liechtenstein) backed by a single, 300 L/min, Model 2012A rotary vane pump (Alcatel, Malakoff, France). In experimentawheretworoughingpumpa were used, thesecond pump wasa300L/min,Model1376 two-stage belt-drive pump (Sargenb Welch, Skokie, IL). The standard electron ionizationlchemical ionization (EI/CI) ion source was operated in El mode with an E1 gas chromatography (GC) ion volume and in CI mode with a CI-GC ion volume. In CI mode, 1%ammonia in methane (MathesonGasProducta,Twinshurg,OH) wm usedas thereagent gas. The base pressure in the analyzer manifold housing was (5-7) x 106 Pa ((4-5) x 1 W Torr), as measured with a BayertAlpert ionization gauge (uncorrected). In the CI mode, with no SFC mobile phase flowing into the ion source, the analyzer manifold pressure was (6-9) x lo-' Pa ((6-7) X 106Torr). The analyzer manifold pressure was 3 X 10-9 Pa (2 X 106Torr) with the SFC/MS interface in place and the chromatographic system at a mobile-phase pressure of 20.3 MPa (200 atm) hut reached (E91 X 10-9 Pa ((6-7) X 106 Torr) during the pressureprogrammed Chromatographic runs. Perfluorotributylamine (PFTBA) and tris(perhoronony1)s-triazine(PFNT) (PCRInc., GainesviUe,FL) wereusedtosupply anion current forcalibrationandtuning. PFTBAwasintroduced through the TSQ-70 calibration gas line. PFNT was introduced using the direct insertion probe at a constant temperature of 45 OC. Tuning was performed with the chromatographic system at a pressure of 20.3 MPa (200 atm). Typical mass spectrometer parameters for SFC/MS runs included electron energies of 70 eV for E1 and 200 eV for CI, an electron emission current of 200 SA, a source temperature of 150 OC, an analyzer manifold temperature of 70 'C, a conversion dynode voltage of 20 kV, an electrometer sensitivity of lW7A/V, and an electron multiplier voltage of 1500 V. The SFCIMS interface probe temperature was maintained at 120 "C and the probe tip beater at 350 O C . Quadrupole 3 was operated in the "low-mass range" mode (mass-to-charge ratio ( m / d 4-2000) at (18) Chwter, T.L. J. Chrornotogr. 1984,299,424-431.
a scan rate of 2 slscan over a range of m / z 90-1990 except where otherwiseindicated. Quadrupoles 1and2wereoperatedinradio frequency-only mode. Figure 1shows the location of the three liquid nitrogen-cooled cryopumps for the comparisons described in the subsequent paragraphs. The cryopumps operated by condensation of COz and other relatively less volatile species onto liquid nitrogencooled surfaces in the vacuum chambers. The cryopumps were custom-built by Finnigao-MATand adapted tothe TSQ-70maps spectrometer. Cryopump 1 was designed to fit the flange on the side of the ion source manifold region. The glass plate covering theanalyzerand ionsource regions was replaced with twosteinless steel plates, one over the analyzer and one over the ion source. Holes in the metal plates allowed insertion of the cooled surface of cryopumps 2 and 3 intu the analyzer manifold or ion source manifold regions. Thecooledsurfaceofcryopump 1 wascircular with a diameter of 7.0 cm. On this surface were two circular, 1.9-cm-highridges. One was at a 1.5-cm radius from the center while the outer ridge wasat aradiusof3.5cm. The cooled surfares of cryopumpr 2 and 3 were cylinders 7.5 cm in diameter and 1.0 cm in height. A screen, 9.0 cm in diameter, was suspended 0.5 cm belou each cylinder. In the mmparisonn described below, the cooled surfaces of cryopumps 1 and 2 were located in the ion source manifold region to the side of the source and above the source, respectively. The cooled surface of cryopump 3 was located in the analyzer region over quadrupole 1. When operating with cryopumping, we generally fill the cryopumps in the morning and top them off two to three times during the course ofan 8-h day. With the SFC pressure held at 55.7 MPa(550atm),themyopumpswillmaintaintheir pumping efficiency for approximately 20 h using a relatively fast flowing capillary restrictor (expanded CO2 flow rate of approximately 2 mL rnin). At least 8 h is required for the liquid nitrogen to boil off with the cryopumps completely filled. This time can he shortened by partially filling the cryopumps or hy manually removing liquid nitrogen from the pumps. As in dealing with any cryogenicequipment, caution should he exercised to prevent the condensation of oxygen on a cryogenic surface where organic vaporsmay havealsocondensed. Thecondensed COzispumped from the cryocooled surfaces as the cryopumps warm. The C02 sublimes over a period of - 2 h, during which time the mass spectrometer returns to ita base pressure. Sample Preparation. Ethoxylated allyl alcohol, with an averageethoxylatechain lengthof 11 (synthesized in-house) was heated at 80 "C for 1 h in a 1:l mixture of pyridine (Pierce Chemical Co., Rockford, IL)and N,O-bis(trimethy1silyl)trifluoroacetamide catalyzed with 1% trimethylchlorosilane (BSTFA with Iri TMCS Pierce) to form the trimethylsilyl (TMS) derivative. Theconcentration of this solution was 3.0'; (weight
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Flgure 2. SFC pressure vs analyzer pressure: A, SFClMS wlth no cryopumping in the C I mode; 6 ,SFClMS with no clyopumping in the E1 mode: m. SFClMS with cryopumping in the C I mode; 0. SFClMS with cryopumplng In the E l mode. of underivatized samplelvolume of derivatization solution). A poly(ethy1eneglycol) mixture (referred to hereafter as PEG-M) was treated in the same manner in order to form the TMS derivative. PEG-Mwasdesienedtocoverawidemolecularweieht range and is a mixture of PEG 200 ( 6 O c by weight). PEG ROO ( 8 ~ ) , P E C 4 0 0 ( 7 C ; ) , P E G 6 0 0 t 1 9 . ~ ~ ~ " lC1W(22.5~i),and oPEG PEG 1450 ( 3 7 %. )tSiema.St. Louis. MO). Theconcentration of . the PEG-M sample was.5.0%. A single-componentstandard was also prepared. A 95%-pure alkylethoxycarhoxylate (AEC)standard (synthesizedin-house), with the structure CHa(CHd.(OCH&Hz).OCHzCOOH (where n = 8 and x = ll),was heated as described above in 11pyridine and N-methyl-N-(tert-hutyldimethylsilyl)tritluoma~tamide (MTBSTFA; Pierce) to form the tert-hutyldimethylsilyl (TBDMS) derivative. Theconcentrationof the AEC standard solutionwas 0.06% (w/v). Miranate LEC (Miranol Chemical Co., Inc., Dayton, NJ),a commercialanionic surfactant mixture with componentssimilar in structure to the AEC standard, was derivatizedin 2 1 pyridine and MTBSTFA to form the TBDMS derivative. The concentration of the Miranate LEC solution was 1.5% (wlv). The constituents of a commercial personal-care product were separated into a variety of fractions. One of the fractions was derivatized in a manner similar to the Miranate LEC sample, as described above, to form the TBDMS derivative. ~~~~~~~~
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RESULTS AND DISCUSSIONS The two turbomolecular pumps of the TSQ-70 mass spectrometer were backed by one rotary-vane pump when delivered. We investigated the possibility that a dedicated roughing pump backing each turhopump might lower the analyzer manifold pressure a t higher SFC flow rates. A comparisonofthe analyzermanifoldpressurevsSFCpressure was performed with and without the additional roughing pump (as discussed in the Experimental Section). Unfortunately, the additional pump made virtually no difference in the analyzer manifold pressure as the SFC pressure rose. Ourattentionthen turnedtotheuseofcryopumpingtoreduce the mass spectrometer pressure. Effect of Cryopumping on Mass Spectrometer Pressure. Figure 2 shows the SFC pressure vs the analyzer manifold pressure for SFC/MS separations in both E1 and CI modes. Comparisons are shown for separations with and without cryopumping. The SFC pressures ranged from 10.1 (100 atm) to 55.7 MPa (550 atm). At an SFC pressure of 35.5 MPa (350 atm), the analyzer manifold pressure is near 5 X 103Pa (4 X 1WTorr) without cryopumping. The mean free path for nitrogen a t this pressure is -3 times the distance between the ion source and detector of the TSQ-70 (-60 cm). It is thus no surprise that we generally observed a drop in SIN above 30.4 MPa (300 atm) without cryopumping. The figure clearly shows that cryopumping significantly lowers
Noncryopumped
Cryopurnped
Flgure a. Comparison of the intensitiesof the ammonium adduct lon
the TBDMS-AEC standard. (A) The specha were collected at an elution pressure of 21.7 MPa (214atm). The analyzer pressure wlthout cryopumping was 2.8 X 10" Pa (2.1 X IOJ Torr), while the analyzer pressure with cryopumping was 1.3 X lo4 Pa (9.4 X lO-'Torr). (6) The spectra were collected at an elution pressure of 55.7 MPa (550 atm). The analyzer pressure without cryopumping was 8.8 X 103 Pa (6.6 X Torr),while the analyzer pressure with cryopumping was 1.7 X Pa (1.3 X Torr). of
the analyzer pressure in both E1and CI modes over the entire SFC pressure range, and especially a t high SFC pressure. Effect of Cryopumpingon theNH,CI Spectrumof an It = 8 AEC Standard. The effects of cryopumping on the mass spectrum of a pure compound introduced by SFC were evaluated using the TBDMS derivative of a pure (singlecomponent) AEC standard with an ethoxylate chain length of 8 and a carbon chain length of 12. This probe was introduced to the ion source through the SFC system a t low and high elution pressures with and without cryopumping. The effects of cryopumping on the intensity of the ammonium adduct ion are shown in Figure 3. In Figure 3A, a t an isobaric SFC pressure of 21.7 MPa (214 atm), cryopumping results in only a small increase in the ion's ahundance. To generate the results presented in Figure 3B, the pressure was raised immediatelyto55.7MPa (550atm)afterinjectiontosimulate the effect of cryopumping upon a species eluting at this pressure. Figure 3B shows a 5-fold increase in the [M + IS]+ ion ahundance because of cryopumping. AU other parameters remained constant when the cryopumped and noncryopumped runs were compared. The intensities presented in Figure 3 are drawn from the spectrum produced by averaging the mass spectral scans across the chromatographic peak. The results a t 21.7 (214 atm) and a t 55.7 MPa (550 atm) cannot he directly compared due to dramatic differences in the peak shapes a t the two elution pressures. However, the results obtained with and without cryopumping a t each pressure are directly comparable. The nature of the ammonia CI spectra of the n = 8 AEC standard obtained a t an elution pressure of 55.7 MPa (550 atm) may be examined in Figure 4. The spectrum produced without cryopumping (shown in Figure 4A) and the spectrum produced with cryopumping (shown in Figure 4B) are very similar. The ammonium adduct ion is the base peak in both spectra The protonated molecule is the only other prominent ion in the spectra. The only significant difference between the two spectra is their intensity (as discussed above).
ANALYTICAL CHEMISTRY, VOL. 65, NO. 23,DECEMBER 1, 1993 3537 1601
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Fburo 4. Comparison of the SFC/MS NHs CI spectra of the TBDMS AEC standard obtained at an elution pressure of 55.7 MPa (550 atm). The TSQ7O was scanned from mlz 90 to 790 at 1 s/scan. Spectrum A was obtained wlthout cryopumplng,while spectrum B was obtained with cryopumping.
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Percent change due to cryopumping In the Intensity of the ammonium adduct bn of TMS-PEW dlgomers relative to the ammonium adduct ion of the ollgomer with an ethoxyiate chain length of 3. Flgur. 5.
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Flguro 7. Percent change due to cryopumping in the Intensity of the ammonium adduct ion of the oligomers of the TBDMS derivative of a commercialalkylethoxycarboxylate surfactant,Miranate LEC (structure shown in the ExperlmentaiSectlon),relathre to the ammonium adduct ion of TBDMSglycoiate, a component of the mixture.
performed with and without cryopumping. Figure 5 is derived from analysesof the TMS derivative of a poly(ethy1eneglycol) mixture, PEG-M. Figure 6 is derived from analyses of the TMS derivative of ethoxylated allyl alcohol. Figure 7 is derived from analyses of the TBDMS derivative of a commercial alkylethoxycarboxylate surfactant, Miranate LEC. In all three cases, the abundances of the ammonium adduct ions are normalized to the abundance of the ammonium adduct ion of a particular low molecular weight species eluting early in the analysis (i.e., at an SFC pressure below 20.3 MPa (200atm)). (Cryopumping has little effect on the relative abundances of ions in spectra acquired at low SFC pressure.) For the first two samples, the low molecular weight species chosen was the oligomer containing three ethoxylate groups. For the last sample, TBDMS-glycolate was chosen. The normalized abundances are compared using the following formula: C = 100(A, - A,)/A,
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Flguro 6. Percent change due to cryopumping in the intensity of the amkmadductbnof TMSethoxylatedaHylaicoholoiigomersrelathre to the ammonium adduct ion of the ollgomer with an ethoxylate chain length of 3.
Effects of Cryopumping on Oligomeric Samples and Complex Mixtures. Figures 5-7 illustrate the effect of the cryopumping on the molecular ion region of spectra of compounds eluting at higher SFC pressures during the characterization of real samples. These figures display data derived from NHs CI SFC/MS analyses of three samples
where C represents the percent change due to cryopumping, A, represents the normalized abundance of the ammonium adduct ion of the species of interest with cryopumping, and A, represents the normalized abundance of the ammonium adduct ion of the species of interest without cryopumping. The percent change due to cryopumping is plotted vs ethoxylate number in Figures 5-7. In all three examples, cryopumping helps to increase the relative abundance of the ammonium adduct ions when going from lower to higher ethoxylate numbers (corresponding to lower to higher SFC pressures). The improvement is relatively modest in the middle portions of the separations, but increasesto over 1007% near the end of all three separations. These differences may be due to collision-induceddissociation occurring in the highpressure regions of the mass spectrometer. Figure 8 comparestwo SFC/MSseparations of the TBDMS derivative of a fraction isolated from a commercial personalcare product. The two separations were performed under identical conditions with the exception of cryopumping. The upper chromatogram (A) was obtained without the use of cryopumping,while the lower chromatogram (B)was obtained with cryopumping. The mixture is quite complex, and the peaks eluting beyond 30.4 MPa (300 atm) in mobile-phase pressure are broad due to the coelution of many components. Note the improved response obtained above 30.4 MPa (300 atm) with cryopumping. The effect is especially apparent for the broad peaks eluting beyond 40.5 MPa (400atm) in mobile-phase pressure. Effect of CryopumpConfiguration. Subsequent to the comparisons described above, a series of combinations and
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Mobile Phase Pressure (aim) Flguro 8. Comparison of two SFC/MS separations of the TBDMS derlvatlve of a fractlon isolated from a commercial personalcare product. The separations were performed under Identical conditions, except that chromatogramA was obtained wlthout cryopumping,while chromatogram B was obtained wlth cryopumping.
Table I. Cryopump Configurations Investigated config no.
cryopump 1
not used not used not used not used
side of ion source side of ion source side of ion source side of ion source side of ion source
cryopump 2
cryopump 3
not used not used over ion source over ion source not wed not used over ion source over ion source over Q1
over Q1
over Q3 not used over Q1 not used over Q1 not used over Q1 over Q3
permutations of the cryopumps were investigated in order to ascertain whether all three pumps were needed and to determinewhich configuration offered the greatest advantage. Table I shows the various configurations examined. The configurationshown in Figure 1,and used in the comparisons described in the previous paragraphs, corresponds to configuration 8. The utility of the various configurations was comparedby simply measuring the analyzermanifold pressure at 10.1-MPa (100-atm) intervals between SFC pressures of 10.1 (100 atm) and 50.7 MPa (500atm) and again at 55.7 MPa (550 atm). As expected, a combination incorporatingall three cryopumps provided the lowest analyzer manifold pressures. Not surprisingly, a cryopump provided a greater drop in analyzer pressure when located in the analyzer manifold rather than in the ion source manifold region. Configuration 9 provided slightly lower analyzer manifold pressures than configuration 8. Configuration 9 has therefore been used in all subsequent work involving cryopumping. Effect of Flow Restrictor Type. Various types of flow restrictors are used in open-tubular SFC. The three most common are the Yapered”,the “integral”, and the “frit”. The
tapered restrictor is described in the Experimental Section. The integral typically tapers from a 25- or a 50-pm4.d. to an aperture a few micrometers in diameter over a distance of 0.3-1 mm.19 The frit restrictor consists of a 1-3-cm porous ceramic frit formed in the end of a 50-pm-i.d. fused silica tube.20 Subsequent to the investigations described above, we found that the rate of flow increase vs pressure differs dramatically for tapered and integral restrictors.21 This type of behavior had been predicted and observed by other investigators.22-26 Specifically, as the pressure of the mobile phase increases, the flow rate of the tapered restridor rises much more rapidly than that of the integral restrictor. (This difference is due to the flow characteristics of the restrictors-the tapered has more laminar flow charaderiatics while the integral has more turbulent flow characteristics.upB) This effect translates to lower pressures in the mass spectrometer when an integral restrictor is used for SFC/MS rather than a tapered restrictor. We have therefore used an integral restrictor for much of our SFC/MS work since these fiidinga. Under typical open-tubular SFC conditions, the analyzer pressure rarely rises above 3 X 10-9 Pa (2 X 106 Torr) when an integral restrictor is used, and cryopumping does not improve the results obtained. Therefore, we do not usually use cryopumping when an integral restrictor is installed. We occasionally use a faster flowing integral restrictor during investigations involving thermally labile analytes. On these occasions, the analyzer pressure typically rises above 3 X 10-9 Pa (2 X 106Torr) withoutcryopumping,and the supplemental pumping provided by cryopumping has proved beneficial. We have not determined the rate of increase of mobile-phase flow vs pressure for the frit restrictor. Since decompression takes place through a multitude of narrow pathways in the ceramic frit, we anticipate that its flow vs pressure characteristics would be closer to those of the tapered restrictor than to those of the integral. Despite the fact that we now typically use an integral restrictor for SFC/MS, and therefore do not typically use cryopumping, we report these investigations of cryopumping because they will provide guidance for others interested in performing open-tubular SFC/MS. There are a number of instances where cryopumping will be beneficial. Some manufactuters have advocated using nondifferentidy pumped mass spectrometers for SFC/MS (for example, the FinniganMAT Incos 50). Cryopumping will certainly be beneficial for SFC/MS using this type of instrumentation, regardless of the type of restrictor employed. Second, the upper pressure limit of SFC instruments will most probably rise in coming years14@. Even with an integral restrictor, cryopumpingmay be required for proper SFC/MS operation with these new, higher pressure SFC instruments. Third, other investigators prefer the frit or the tapered restrictor over the integral for a number of reasons (e.g., resistance to plugging, ease of fabrication, or commercial availability). Cryopumping will be beneficial when SFC/MS is performed with a tapered or frit restrictor. (19)Guthrie, E. J.; Schwartz, H.E. J. Chromatogr. Sci. 1986,24,236. (20)Cortes, H.J.; Pfeiffer, C. D.; Richter, B. E.; Stevens, T. S. U.S.
Patent 4793920,1988. (21)Pinkaton, J. D.;Hentschel, R. T. J. High Reeolut. Chromatogr. 1993,16,26%2’74. (22)Bally, R. W.;Cramers, C. A. HRC CC, J. High Resolut. Chromatogr., Chromatogr. Commun. 1986,9,626. (23)Oleeik, S.V.; Pekay, L. A. Chromatographia 1990,29,69. (24)Berger, T.A. Anal. Chem. 1989,61,366. (26)Berger, T.A.; Toney, C. J. Chmmotogr. 1989,466,157. (26)Chester, T.L.;Innis, D.P. Presented at the 16th International Symposium on Capillary Chromatography (pp 1678-1588) and the 2nd EuropeanSymposiumon Analytical SupercriticalFluidChromatography and Extraction (pp 16-26), May 24-27,1993,Riva Del Garda, Italy (joint
meeting).
ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
CONCLUSION The resulta presented here clearly demonstrate that cryopumping improves the performance of a differentially pumped, commercial triple-quadrupole mass spectrometer when SFC/MS is performed using the direct-fluid-introduction interface. Other forms of supplemental pumping would also improve performance,but few are as simple, inexpensive, and efficient (for C o d as liquid nitrogen-cooled cryopumps. Generally, such pumps are easily adapted to commercial mass spectrometers and do not impede the operation of the instrument in other applications. Performance improvements will be most apparent when SFCIMS is performed using tapered or frit flow restrictors at higher SFC pressures (>30.4
9590
MPa (300 atm)) or using a nondifferentially pumped mass spectrometer.
ACKNOWLEDGMENT We thank Jim Hurst of Finnigan-MAT, San Jose, CA, for his assistance and for many helpful discussions. Thanks also to Thomas L. Chester for his help in reviewing the manuscript. We thank Lisa Burkes, Thomas A. Cripe, Larry H. Sickman, and James E. Thompson for providing samples.
RECEIVEDfor review June 30, 1993. Accepted September 9,1993.
