Anal. Chem. 1995, 67, 1409- 1412
Direct Determination of Organic Compounds in Water at Parts-per-QuadrillionLevels by Membrane Introduction Mass Spectrometry Manish Soni,t Scott Bauer,* Jon W. Amy,t Philip Wong,t and R. Graham Cooks*J
Department of Chemistty, Purdue University, West Lafayette, Indiana 47907, and MIMS Technology Development Inc., Palm Bay, Florida 32906
Parts-per-quadrillion level detection of toluene and tram1,2-dichloroethene in water has been achieved on-line, without preconcentration, by employing selective ionization in conjunction with membrane introductionion trap mass spectrometry. The stored wave form inverse Fourier transform technique is used to create broad-band wave forms, notched at the resonance frequencies of analyte ions of interest. A series of such pulses is applied during ionization to eject unwanted ions and store only analyte ions. This capability is used over long ionization times to obtain extraordinarily low detection limits for aqueous solutions of volatile organic compounds, introduced into the ion trap using a silicone membrane located within in a commercially available capillary membrane inlet system. A continuing objective in analytical chemistry is the detection of ever smaller numbers of atoms and This objective has been pursued for its own sake, although practical applications of enhanced detection limits have followed. Recently, additional constraints have been introduced into this type of problem, typiiied by the attempts of Yeung? Jorgen~on,~ and others to determine trace levels of biological compounds in ultrasmall sample volumes. This paper describes progress in on-line detection of ultralow levels of organic compounds in aqueous solution. The experiments are performed by membrane introduction mass spectrometry (MIMS), a method of on-line analysis which has been in use for several decadesa and which has recently seen rapid de~elopment.~-'~ * Purdue University.
* MIMS Technology Development, Inc. (1) Dovichi, N. J.; Martin, J. C.; Jett, J. H.; Trkula, M.; Keller, R A A n a l . Chem. 1 9 8 4 , 56, 348. (2) Nguyen, D. C.; Keller, R A; Jett, J. H.; Martin, J. C. Anal. Chem. 1 9 8 7 , 5 9 , 2158. (3) Soper, S. A; Shera, E. B.; Martin, J. C.;Jett, J. H.; Hahn, J. H.; Nutter, H. L.; Keller, R A Anal. Chem. 1 9 9 1 , 63, 432. (4) Huang, X.;Zare, R N.; Sloss, S.; Ewing, A. G. Anal. Chem. 1991, 63, 189. (5) Sweedler, J. V.; Shear, J. B.; Fishman, H. A.; a r e , R N.; Scheller, R. H. Anal. Chem. 1 9 9 1 , 63, 496. (6) Yeung, E. S. Acc. Chem. Res. 1 9 9 4 , 27, 409. (7) Cooper, R B.; Janowski, J. A; Leczczyszyn, D. J.; Wightman, M. R; Jorgenson, J. W. Anal. Chem. 1 9 9 2 , 64, 691. (8) Hoch, G.; Kok, B. Arch. Biochem. Biophys. 1963, 101,160. (9) LaPack, M. A; Tou, J. C.; Enke, C. G. Anal. Chem. 1 9 9 0 , 62, 1265. (10) Kotiaho, T.; Lauritsen, F. R; Choudhury, T. K; Cooks, R. G. Anal. Chem. 1 9 9 1 , 63, 875k (11) Lauritsen, F.R; Kotiaho, T.; Choudhury, T. IC; Cooks, R G. Anal. Chem. 1 9 9 2 , 64, 1205. (12) Brodbelt, J. S.; Cooks, R. G.; Tou, J. C.; Kallos, G. J.; Dryzga, M. D. Anal. Chem. 1987,59,454. Gilbert, J. R; Langvardt, P. W.; Dryzga, M. D.;Stott, W. T. Proceedings of the 42nd ASMS Conference on Mass Spectrometry and Allied Topics, Chicago IL, May 29-June 4, 1994, WP-7. 0003-2700/95/0367-1409$9.00/0 0 1995 American Chemical Society
The MIMS experiment is simple in concept and in the equipment needed for its execution. Typically, a sample, often an aqueous solution, is passed across the surface of a membrane such as a silicone polymer film which is impermeable to water but permeable to organic compounds. These compounds selectively diffuse through the membrane and into the vacuum system of the mass spectrometer, where they are ionized, mass analyzed, and detected. On-line monitoring of sample streams is facilitated by using flow injection analysis PIA)methods of sample handling and by the rapid diffusion rates typical of volatile compounds through silicone membranes. Principal applications are to online environmental analysis and to monitoring of biological and chemical r e a ~ t o r s . ' ~AJ ~more common method for the analysis of organic compounds in aqueous solution is GC/MS,I5 but in spite of its many advantages, this method does not allow on-line monitoring, nor does it achieve the detection limits of MIMS.I4 The utility of MIMS has already been demonstrated in various fields where on-line monitoring and detection of low levels of volatile organic compounds (VOCs) in water and air are required.+"J6 For example; MIMS has been shown to be a valuable tool for analyzing chlorinated drinking water samples for trihalomethane~.'~These compounds are hazardous, and their concentration is regulated at sub-parts-per-billion(ppb) levels by the US. EPA. In this application, on-line MIMS displays lower quantitation levels and has greater speed than the conventional purge and trap GC/MS method. This study, and a set of associated papers in this i s s ~ e , ~ ~ J ~ J ~ describe a number of advances in MIMS. They are (i) the use of tailored wave form methods to achieve, in favorable cases, partsper-quadrillion (ppq) detection limits for organic compounds in aqueous solution, (ii) the use of a silicone membrane/jet separator membrane introduction system for the direct analysis of partsper-trillion (pptr) levels of organic compounds in air,I6 (iii) the on-line monitoring of biological reactions at extremely low levels,'* and (iv) in situ monitoring of the constituents of the extremely complex mixtures of organic compounds encountered in toxic waste dumps.lg A variety of MIMS interfaces are employed in (13) Heinzle, E.; Reuss M. Mass spectrometry in biotechnological analysis and control; Plenum: New York, 1987. (14) Bauer, S. J.; Cooks, R G. Am. Lab. 1993, 8, 36.
(15) Watson, J. T. introduction to Mass Spectromety; Raven Press: New York, 1985. (16) Cisper, M. E.; Gill, C. G.;Townsend, L. E.; Hemberger, P. H. Anal. Chem. 1 9 9 5 , 67, 1413. (17) Bauer, S. J.; Solyom, D. Anal. Chem. 1994, 66, 4422. (18) Lauritsen, F. R; Gylling, S. Anal. Chem. 1 9 9 5 , 67, 1418. (19) Krkki, V. T.; Ketola, R. A; Ojala, M.; Kotiaho, T.; Komppa, V.; Grove, A; Facchetti, S. Anal. Chem. 1 9 9 5 , 67, 1421.
Analytical Chemistry, Vol. 67, No. 8, April 15, 1995 1409
these experiments, which use either quadrupole or ion trap mass spectrometers. An attractive feature of MIMS is the capability to perform analysis of aqueous solutions for VOCs without any sample pretreatment. The capability for ultratrace analysis is enhanced by employing the highly sensitive ion trap mass spectrometer. The recent development of tailored wave form resonance methods of mass-selectiveion storage allows the limited ion storage capacity of the trap to be fully utilized for analyte ions. We apply the stored wave form inverse Fourier transform (SWIFT) method,Z0introduced by Marshall and co-workers2*for ion cyclotron resonance instruments, to selectively eject all but the analyte ions during ionization. Other authors have used the related methods of selective ion injection such as the Varian selective ion storage (SIS) technique,22the selected window ion monitoring (SWIM) technique,23and the Teledyne filtered noise field (FNF) techniqueSz4 In this article, we demonstrate how the MIMS/ion trap/SwIFT combination allows the achievement of extraordinarily low levels of detection for VOCs in aqueous solution. Analysis at the partsper-quadrillion level was set as a goal of this work, not because there are currently practical problems which require this level of performance but in the spirit of the tradition in analytical chemistry in which ultratrace analysis is pursued, at least initially, for its own sake. EXPERIMENTAL SECTION
These experiments were performed using the Magnum ion trap mass spectrometer (Finnigan MAT, San Jose, CA) fitted with a direct insertion membrane probe system (MIMS Technology Inc., Palm Bay, FL) operated at 35 "C. The probe is fitted with a 2 cm long, 0.635 mm id., 1.19 mm 0.d. silicone membrane @ow Corning, Midland, MI). Sample plugs were introduced into a continuous stream of water using a peristaltic pump (Masterflex, Barnant Co., Barrington, IL) operating at a flow rate of 2 mL/ min. The size of the sample plug, typically 20 mL, was chosen to allow a steady state response to be reached, although such large samples are not needed to achieve low detection limits. For each trial, multiple sample plugs were introduced into the carrier stream to investigate the reproducibility of results. Highly purified (doubly distilled and deionized) cold water (4 "C), free of volatile organics, was used to prepare all single compound solutions. Solutions were used within a few hours of preparation. Certhied standards of truns-1,2-dichloroethene(t-DCE) and toluene (each lo00 pg/mL in methanol) were commercially obtained (Protocol Analytical Supplies Inc., Middlesex, NJ>and diluted using the purified water to obtain solutions of the desired concentrations. A mixture containing 1 ppb each of isopropyl benzene, chlorobenzene, mxylene, and 1,3-dichloropropenewas prepared in tap water which typically contains ppb levels of trihalomethanes. This mixture was spiked with the t-DCE solution as needed for the experiment. (20) Soni, M. H.; Cooks, R. G. Anal. Chem. 1994, 66, 2488. (21) Marshall, A G.; Wang, T.-C.; Ricca, T. L. J. Am. Chem. Soc. 1985, 107, 7893. (22) Buttrill, S. E.; Shaffer, B.; Kamicky, J.; Amold, J. T. Proceedings of the 40 th ASMS Conference on Mass Spectrometry and Allied Topics, Washington, DC, May 1992, p 1015. (23) Mordehai, A. V.; Henion, J. D. Rapid Commun. Mass Spectrom. 1993, 7, 1131. (24) Weber-Graubau, M.; Kelley, P. E.; Bradshwaw, S. C.; Hoekman, D. J. Proceedings of the 35th ASMS Conference on Mass Spectrometry and Allied Topics, Denver, CO, 1987, p 263. Kelley, P.; Hoekman, D.; Bradshaw, S.; Stiller, S.; Proceedings of the 41st ASMS Conference on Mass Spectrometry and Allied Topics, San Francisco, CA, May 31-June 3, 1993, paper TP160.
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Analytical Chemistry, Vol. 67, No. 8, April 15, 1995
Ion chromatograms were recorded using the Magnum ion trap data system (version 2.4). A peak threshold of 5 counts was used for all acquisitions. The ion trap was operated at 50 "C and 60 pA E1 ionization. Prior to analysis, the instrument was calibrated (using the fluorocarbon FC-43) and the electron multiplier voltage set to provide 105 gain using the autoset routines of the data system. Data points were acquired every 5 s so as to obtain the maximum degree of signal averaging possible with the data system. Postacquisition smoothing was performed using a five point moving average routine using Microsoft Excel. The desired ion trap scan function, created using a special scan editor program, incorporated a 150 ms ionization time (a value chosen through experimental optimization) followed by a 25 ms cooling period. A radio frequency voltage level corresponding to trapping of m/z 40 (and all ions of greater mass) was used during ionization to exclude ionized water and its monohydrate. The mass analysis scan (m/z 40-650) was performed with axial modulation (485 kHz, 3.5 VO-))for improved r e s o l u t i ~ n . ~ ~ Notched pulses were created using the SWIFT method described previouslyz0with the notches positioned at resonance frequencies corresponding to the mass(es) of the analyte ion($. The notch width was optimized at 1.7 kHz for achieving single mass resolution in experiments where ions of a single mass/ charge ratio were monitored and at 3.7 kHz for the t-DCE experiment, where two ions (m/z 96 and 98) were simultaneously monitored the notch performance was confirmed from mass spectra obtained during each experiment. Each SWIFT pulse was 4.2 ms long (600 kHz), and a series of 37 pulses was applied over the 150 ms ionization period. The hardware is very similar to that reported elsewhere.ZOSWIFT wave forms were clocked out at a preset rate (1.95 MHz) and amplitude (4.5 V&) from an arbitrary wave form generator (ARB, Wavetek, San Diego, CA), passed through a balun/ampWier (gain 3.5) and applied to the endcap electrodes of the trap in a dipolar fashion. Since the axial modulation signal was also used, an analog switch (AD 7592DI) was used to alternate the input to the endcaps between the SWIFT pulses and axial modulation signal. RESULTS AND DISCUSSION
Quadrupole ion traps can hold a relatively small number of ions before space charge effects cause a degeneration in performance?'Q7 Low-level experiments therefore require selective storage of analyte ions. In addition, because of the small number of analyte molecules present in small volumes of ppq solutions, ionization must be carried out for a relatively long period compared with standard ion trap operating conditions. When mass-selective ionization is performed, the ion carrying power of the trap is utilized exclusively for analyte ions, and the signal-tonoise (S/N ratio is expected to increase with the length of the ionization period. Space charging of the trap is also avoided this way, in spite of the long ionization period. Even though analyte ions are not ejected from the trap during mass-selective ionization using SWIFT, the mass-selective storage process leaves the target ions kinetically excited. This is a result of the frequencies of the analyte ions lying near the frequenciesused to eject adjacent ions. Hence, the trapped ions must be collisionally cooled to the center of the trap before mass analysis to achieve optimum resolution. (25) Kelley, P. E.; Stafford, G. C.; Syka, J. E. US.patent 4749860, June 1988. (26) Vedel, F.; Andre, J. Phys. Rev. A 1984,29, 2098. (27) Guan, S.; Marshall, A. G. J. Am. Soc. Mass Spectrom. 1994, 5, 64.
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Figure 3. ion chromatogram showing the smoothed signal obtained from monitoring t-DCE (m/z 96) in a mixture of VOCs in tap water using MIMS without SWIFT. Signal averaging was performed as described for Figure 1 b. The absence of notched SWIFT excitation during ionization results in nonselective trapping of all ions of the mixture, thereby reducing the detection sensitivity to 250 pptr. The inset shows the mass spectrum for this sample.
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Figure 1. (a) Ion chromatogram showing the signal (m/z 91) recorded for 1 pptr and 500 ppq samples of toluene in pure water using MIMS. S/N ratios of 16.0 and 3.4 (average for both replicates in each case) are calculated from these data. (b) Ion chromatogram from (a) after postacquisition signal averaging performed using a five point moving average routine using Microsoft Excel. The S/N ratios increased to 41.6 and 11.0 for the 1 pptr and 500 ppq samples, respectively.
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Figure 4. Ion chromatogram showing the smoothed signal (m/z 96 and 98) recorded for 1 pptr and 500 ppq samples of t-DCE in pure water using MIMS. Signal averaging was performed as described for Figure 1 b. The background-subtracted mass spectrum obtained during the analysis of the 1 pptr sample is shown in the inset. The ion abundances are 77 counts for m/z 96 and 23 counts for m/z 98.
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Figure 2. Ion chromatogram showing the smoothed signal obtained from monitoring m/z96 (CDCE, 500 ppq) in a mixture of VOCs (each 1 ppb) in tap water using MIMS with SWIFT. Signal averaging was performed as described for Figure 1b. Detection limits in the ppq range are also obtained for this compound.
Based on these considerations, various scan functions were tested to optimize the sensitive, mass-selective trapping of ions from lowconcentration solutions, and optimal conditions were found. These conditions included 150 ms ionization time at high and 25 ms cooling time at low qr value.
Figure 1 shows data recorded by MIMS for toluene in pure water. Notched SWIFT pulses were continuously applied during the 150 ms ionization time to selectively accumulate the @ - I H)+ ions having m/z 91. The raw data Figure la) show a S/N ratio of 16.0 for the 1pptr sample and 3.4 for the 500 ppq sample, while the smoothed data (Figure lb) show S/N ratios of 41.6 and 11.0, respectively. Note that the S/N ratio was calculated as the ratio of the average signal height to the standard deviation of the noise using the actual numerical data. The detection limit is defhed as S/N 3 and is estimated to correspond to approximately 300 ppq. The standard deviation of this measurement is 4~5%. Figure 2 shows smoothed data, obtained with SWIFT, for PDCE solutions at the 1pptr and 500 ppq levels in the tap water mixture described in the Experimental Section. For comparison, Figure 3 shows smoothed data for the same experiment but without SWIFT. There is a difference of several-hundred fold in the detection limits for the experiments in the presence and absence of the notched SWIET excitation pulses during ionization. This clearly indicates that interference from other ions of the Analytical Chemistry, Vol. 67, No. 8, April 15, 1995
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complex mixture, seen in the corresponding mass spectrum (inset), is the principal limitation in performance in the case where selective ionization is not used. Low-level (1pptr and 500 ppq) samples of t-DCE prepared in pure water were also examined using a wider notch so as to retain both isotopic forms of the molecular ion (m/z 96 [35C11and 98 k3’Cll) in the trap. The detection limit was again 500 ppq for an experiment performed without preconcentrationor other sample workup, as shown in Figure 4. The mass spectra obtained during this experiment confirmed their presence in the characteristic 3:l 35C1:37C1ratio (Figure 4,inset), although between individual scans this ratio varied between 2.5:l and 5:l due to varying signal intensity during the analysis. Data for more concentrated samples, e.g., 5 and 10 pptr, showed correspondingly greater signals and more accurate isotope ratios in the individual scans. The response was linear over the range considered for all three cases. Although samples above 10 pptr were not examined in this study, a linear dynamic range of more than 3 orders of magnitude is typically obtained using this in~trumentation.~~
1412 Analytical Chemistry, Vol. 67, No. 8, April 75, 7995
These experiments show that it is possible to detect ppq levels of organic compounds in aqueous solution, both as pure solutions and in mixtures with excellent reproducibility, as is evident from data shown above. The improved performance over previous experiments is clearly due to the use of notched SWIFT excitation to achieve mass-selective storage over long ionization times and the use of signal averaging. ACKNOWLEDGMENT
This work was supported by the Office of Naval Research. We thank Mark Bier, George Stafford, Karl Trier, and Eric Johnson, all of Finnigan Corp., for providing valuable suggestions. Received for review August 29, 1994. Accepted January 23, 1995.@ AC9408639 Abstract published in Adounce ACS Abstracts, March 1, 1995.
