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Anal. Chem. 1984, 56,2971-2973
Capillary Supercritical Fluid Chromatography/Mass Spectrometry with Electron Impact Ionization Sir: The combination of capillary column supercritical fluid chromatography and mass spectrometry (SFC-MS) has recently been demonstrated to be a powerful new technique for the analysis of labile and less volatile materials not amenable to gas chromatography (GC) and GC-MS (1-4). The development of small-diameter capillary columns (- 50 pm) has provided the basis for high-resolution separations of complex mixtures ( 5 , 6 ) and extremely rapid separations at lower efficiencies (6). The selective and sensitive detection demonstrated with mass spectrometry has spurred these developmenb and provided significant potential advantages compared to HPLC-MS. Nearly all capillary SFC-MS has utilized chemical ionization (CI), and attempts using electron impact ionization (EI) have generally resulted in greatly decreased sensitivity (3) or spectra which contain both CI and E1 components (5). However, the low flow rates with 50-pm columns, which typically correspond to 0.2-0.5 mL/min of gas a t standard conditions, should be compatible with direct fluid injection ( 4 ) using E1 ionization. Most E1 sources are designed to operate at lower flow rates or, if a higher flow rate is used (as in GC-MS), with gases which will produce spectra quite similar to E1 (i.e., charge transfer from helium). However, this "tighter" ion source design can result in significant CI contributions and would benefit from a more open structure if sufficient ionization efficiency can be obtained. In addition to providing sufficient sensitivity the interface should also minimize contributions of solvent clusters to the mass spectra. In this communication we report the development of a simple interface for SFC-MS with EI. While the sensitivity provided by this approach is usually less than achievable by CI, the E1 interface eliminates solvent clusters and contributions from CI (ion/molecule) reactions. This approach provides a clear advantage over direct introduction HPLC-MS combinations since the extensive libraries of E1 spectra can be used for compound identification. EXPERIMENTAL SECTION The instrumentation used for capillary SFC-MS is similar to that described previously ( 1 , 3 , 4 ) . The SFC apparatus utilized a modified syringe pump to generate the high-pressure fluid flow and a chromatograph oven for temperature control. Pressure programs were generated by using a microcomputer which allowed a variety of density gradients (1-4, 6). Purified carbon dioxide (critical pressure of 72.9 atm and a critical temperature of 31.2 "C) was used as the supercriticalmobile phase in most experiments with pressures ranging between 75 and 300 atm at an operating temperature of 100 "C. Sample introduction was accomplished by using a Valco C14W 0.1-pL injection valve with a flow splitter providing a 1 O : l split. The chromatographic column consisted of various lengths of 50-pm-i.d. fused silica tubing coated with approximately a 0.25-pm film thickness of 5% phenylpoly(methylphenylsiloxane) (SE-54) stationary phase which had been stabilized and rendered nonextractable by extensive cross-linking (7).
Figure 1gives a schematic illustration of the SFC-MS interface and modified E1 ion source developed in this work. The SFC column was coupled to the mass spectrometer through an ovenair-heated probe. The short fused silica capillary restrictor (-4 pm i.d.) allowed rapid injection of the fluid into a heated expansion region (1cm X 0.14 cm i.d. with a 0.1-cm orifice to the ionization volume) which provided the higher pressure necessary for cluster breakup prior to the ionization region (4). The temperature of this region was typically 50-150 "C higher than the mobile-phase
temperature. The expansion region efficiently directed the SFC effluent into the ionization volume of an Extranuclear Laboratories (Pittsbugh, PA) high-efficiency ionizer. This ionizer was designed for molecular beam studies and has an extremely open source which serves to minimize pressure in the ionization volume. Ion source chamber pressure was in the range of (1-5) X torr, and the mass spectrometer chamber pressure was 50 for aminobiphenyl, and a detection limit of approximately 5 pg was determined for this compound. The mass spectra for these compounds, given in Figure 3, are of reasonable quality and demonstrate the absence of CI contributions. Spectra obtained using pentane and mixed carbon dioxide-methanol supercritical mobile phases also show the absence of these contributions. The application to the labile pesticide aldicarb (Mw. = 190) was investigated to examine the thermal effects of the chromatography and heating in the expansion region (ER). Figure 4 gives E1 mass spectra for aldicarb obtained at various column and ER temperatures. (The scans for spectra A and B started at m / z 100, and all spectra have been normalized to peaks due to aldicarb a t m / z 2100.) Spectra A and B are of high quality and are consistent with previously reported E1 spectra ( 4 9 ) . These results indicate that decomposition due to the higher SFC temperature (spectrum A) is small for the 5-min retention time. Decreasing the ER temperature to 115 "C (spectrum C) resulted in a minor contribution of the molecular ion ( m / z 190) and increased the m / z 144 abundance (due to loss of CH2S). Spectrum D was obtained a t extremely low ER temperatures relative to the SFC temperature, where operation can become eratic, and shows an even greater contribution of m/z 144 but a lower molecular ion intensity. This may be due to the higher SFC temperature, and comparison with other spectra (8,lO) suggests decomposition or isomerization of aldicarb under these conditions. Chemical ionization spectra obtained with various reagent gases and reported elsewhere (IO)clearly indicate a species with MW 190, as well as contributions due to decomposition a t higher temperatures including formation of aldicarb nitrile with MW 115. Regardless, these results show that electron impact spectra can be obtained for this highly labile compound which are comparable to those obtained by direct probe introduction (8, 9).
0003-2700/84/0358-2971$01.50/00 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984 1100
ALOICARB SFC-MS
Column * 100'C ER = 195%
-
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lonizO'
-
Column 4O'C ER l95'C
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Flgure 1. Schematic illustration of the capillary SFC-MS interface developed in this work for ebctron impact ionlzation. 100, io,.,
IO" ChromalilSrlm
mi2 0
4
7,m. iminuwr,
Figure 2. SFC-MS total and selected ion chromatograms for a 100-pg component injectlon of a three component mixture. Mass spectra from this separation are given in Figure 3.
Flgure 4. Electron impact mass spectra obtained for SFC-MS separatlons of aldicarb at various SFC and expansion region (ER) temperatures.
,'B4-Amino Blphmyl
!
I
100
I
150
200
250
200
260
3 3 Olohlorobenlidins
Flgure 3. Mass spectra (70-eV EI) obtained for 4-aminobiphenyl, benzidine, and 3,3'dichlorobenzidine for the separation shown in Flgure 2.
This work has also demonstrated that the ER serves to effectively breakup solvent clusters formed in the injection process. However, as indicated in Figure 4, lower ER temperatures will facilitate cluster formation. The cluster formation was independent of column temperature and depended only upon ER temperature and pressure (estimated to be 0.01-0.1 torr). For example, an SFC linear velocity of 2 cm/s at 100 "C and 275 bar resulted in an ion source chamber pressure of 4 X torr and yields a [(CO,),]/[CO,] value (assuming similar ionization and detection efficiencies) which
increased from 1 X 10"" at ER = 170 "C to 3 X at ER = 140 "C, to 6 X lo4 a t ER = 100 "C, and to 7 x low3at ER = 55 "C. For similar linear velocities at 100-bar SFC pressure and an ion source pressure of 2 X 10" torr, this relative dimer at ER = 140 "C to 6 X abundance increased from 2 X at ER = 55 "C. The low ER temperature and higher pressure also facilitated formation of larger clusters, as indicated in Figure 4D, with [(COJ3]/[C02] = 1.4 X low3and [(C0J4]/ [CO,]= 6 X lo4 at 55 "C and an ion source chamber pressure of 4 X torr. These results indicate that ER provided for rapid equilibration of the SFC effluent and the observed clusters are indicative of the higher ER pressure rather than the injection process. Additionally, the contribution of clusters larger than the dimer was negligible for all fluids studied to date for an ER temperature of more than 50 "C above the critical temperature of the fluid. The capillary SFC-MS interface for E1 ionization provided for efficient breakup of clusters formed during expansion and transfer of the SFC effluent to the ionization volume. While E1 sensitivity was somewhat less than obtained by CI, good mass spectra were obtained with 100-pg injections, and the spectra show no CI contributions. The flexibility in selection of the ionization method and the ability to use the existing E1 spectral libraries provide an additional advantage for SFC-MS relative to LC-MS combinations which involve direct liquid introduction. Registry No. 4-Aminobiphenyl,92-67-1;benzidine, 92-87-5; 3,3'-dichlorobenzidine, 91-94-1; aldicarb, 116-06-3.
LITERATURE CITED (1) Yonker, C. R.; Wright, B. W.; Udseth, H. R.; Smlth, R. D. Ber. Bunsenges. Phys . Chim ,, In press. (2) Smith, R. D.; Felix, W. D.: Fjeldsted, J. C.: Lee, M. L. Anal. Chem. 1902, 54, 1663-1665.
Anal. Chem. 1084, 56, 2973-2974 (3) Smith, R. D.; FJeldsted.J. C.; Lee, M. L. J . Chromatogr. 1982, 2 4 7 , 23 1-243. (4) Smith, R. D.; Udseth, H. R. Anal. Chem. 1983, 55, 2266-2272. (5) Fjeidsted, J. C.; Kang, R. C.; Richter, B. E.; Fields, S. M.; Jackson, W. P.; Lee, M. L., paper presented at the Pittsburgh Conference on Ana-’ lytlcal Chemistry and Applied Spectroscopy, Atlantlc City, NJ, March 5-9, 1984, No. 596. (6) Smith, R. D.; Kalinoski, H. T.; Udseth, H. R.; Wright, 6 . W. Anal. Chem. 1984, 56, 2476-2480. (7) Wright, B. W.; Peaden, P. A.; Lee, M. L.; Stark, T. J. J . Chromatogr. 1982, 2 4 8 , 77-64. (6) Benson, W. R.; Damico, J. N. J . Assoc. Off. Anal. Chem. 1988, 57, 347-365. (9) Slivon, L. E.; DeRoos, F. L., Development of a General Purpose LC/ MS Method for Compounds of Environmental Interest, 1963 Final Report U.S. Environmental Protection Agency, Contract 68-03-2960, March 1963.
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B. W.; Smith, R. D. J . Chromatogr., submitted. R i c h a r d D. Smith* Harold R. Udseth H e n r y T. Kalinoski Chemical Methods and Kinetics Section Pacific Northwest Laboratory (Operated by Battelle Memorial Institute) Richland, Washington 99352 RECEIVED for review June 8, 1984. Accepted July 27, 1984. This work has been supported by the U.S. Department of Energy, Office Of Basic Energy Sciences, U d ? r Contract DE-AC06-76-RLO 1830. (10) Wright,
Simple, Compact Visible Absorption Spectrophotometer Sir: Ultraviolet/visible (UV/Vis) spectrophotometry continues to offer a versatile analytical method for a variety of samples. While mechanically scanned double-beam instruments comprise the state of the art for making highly precise and sensitive spectroscopic measurements, alternative systems utilizing optoelectronic imaging devices (OIDs) have recently been introduced (1,2). With an OID, mechanical scanning is eliminated and a rapid readout rate is possible. Since measurements are taken over all of the wavelengths of interest simultaneously, drift problems are reduced and the multichannel advantage in signal-to-noise ratio or time is achieved. Rapid-scan systems incorporating linear photodiode arrays (PDAs) or charge-coupled devices (CCDs) have been described ( 3 , 4 ) ,and the former is now commercially available. In all cases, grating-based polychromators are used as dispersive elements to select the distribution of wavelengths reaching the channels of the OID. The purpose of this paper is to describe how an even further reduction in size, simplicity, and mechanical stability in an OID-based spectrophotometer is possible using a novel concept as follows: (a) Light from a lamp is collected and collimated by a lens and passed through a cuvette containing the sample. (b) The light transmitted by the sample is then analyzed by a “wedge” interference filter. The wavelength of the maximum transmission of this device varies linearly and continuously over its length. It is mounted so that the wavelength variation occurs along the vertical (long) axis of the cuvette. (c) Immediately behind the interference filter is placed a linear PDA. The long axis of this device is parallel to the wavelength variable axis of the wedge filter so that each photoelement observes a different portion of the spectrum. Thus, the PDA acts as an “electronic photographic plate” to capture the entire spectrum at once. This design retains all of the aforementioned advantages of the use of an OID as a detector. Additionally, it provides the benefit of compactness, due to elimination of the dispersive device, and the benefits of ruggedness and ease of servicing, due to elimination of critical optical alignment. EXPERIMENTAL SECTION Apparatus. A diagram of the spectrophotometer is given in Figure 1. The instrument consisted of a quartz halogen bulb, type S4A from Pelican Products, Inc., as a light source, powered by a Lambda Model LL-902 DC power supply operated at approximately 3.5 V; a 1-in.focal length collimating lens; an infrared blocking filter, type KG-1, manufactured by Schott Optical Glass Inc.; a 10-mm path length fused silica cuvetk, a 25 X 60 mm wedge filter obtained from Oriel Corp., no. 5748; a 25-mm focal length Canon TV lens, Model 6367, f no. 0.78; and a Reticon Corp. series
“S” 512 element diode array operated from an RC 1024SA evaluation board as detector. The infrared blocking filter served to eliminate the infrared sidebands which are passed by the wedge filter at twice the peak transmission wavelengths. The wedge filter incorporated blockage of the short wavelength sidebands that occur at two-thirds the peak transmission wavelength. The band-pass of the wedge filter increases linearly from 8 nm at 400 nm to 14 nm at 700 nm over a 42.9-mm active length. Since the diodes of the PDA extend over 12.8 mm, detection of the spectrum transmitted directly through the filter to the PDA would have afforded a wavelength range of only 89.5 nm. Accordingly, the video lens was used to image a slightly greater portion of the wedge onto the array to give a 130-nm range. The readout rate of the PDA was set to 50 kHz. The entire array was read out at this rate once every 50 ms. The diode signals were digitized by a 12-bit A/D converter and processed by a Digital Equipment Corp. PDP 11/04 computer. Procedure. Chromatographically pure tetraphenylporphine (H,TPP) was the kind gift of Professor Martin Gouterman. Absorbance spectra of HzTPP in dichloromethane at concentrations from 0.41 to 35 ppm were measured. Dark signal and dichloromethaneblank data were taken and then serial additions of HzTPP to the blank were made without disturbing the cuvette. Data were stored after each addition. The light source power was adjusted so that the signal from each of the array elements did not exceed nine-tenths of the saturation level in any part of the array during a blank reading. All data acquisition times were 10 s, representing 200 readout cycles summed. Absorbance at each array element was calculated as A = -log [(S- D ) / ( B - D)] (1) where S, D, and B are the sample, dark, and blank sums respectively. In order to improve the signal-to-noise ratio and in keeping with the resolution of the wedge filter, the signals from seven adjacent diodes were averaged together before displaying. For comparison, spectra over the range of 0.41-35 ppm HzTPP were obtained by using a Hewlett-Packard (HP) 8450A UV/Vis spectrophotometer, using a 10-sacquisition time for sample and blank measurements. R E S U L T S AND DISCUSSION Figure 2A gives a spectrum of HzTPP dissolved in dichloromethane a t a concentration of 2.5 ppm obtained with the prototype spectrophotometer described in the Experimental Section. The positions of the three absorbance maxima in this region are in good agreement with literature values (5). For comparison, in Figure 2B we give the spectrum of the same sample as recorded with the H P 8450A; the two are qualitatively quite similar. However, the lower resolution of the prototype when compared to the H P instrument is apparent. This arises from the wider band-pass of the present wedge filter (12 nm) when compared to the polychromator (2 nm).
0003-2700/84/0356-2973$01.50/00 1984 American Chemical Society
