Anal. Chem. 1995, 67,4536-4541
Volatile Organic Compound Analysis by an inertial Spray Extraction interface Coupled to an ion Trap Mass Spectrometer Franqois St=Gennain,tOrval Mamer,* Julie Bcunet,t Benoit VachOn,t Robert Tardif,, Thierry Abribat,t Christine Des Rosiers,t and Jane Montgomery*lt
Depatfements de Nutrition et de Medecine du Travail et dHyg&ne du Milieu, Universite de Montreal, Quebec, Canada H2L 4M1, and Biomedical Mass Spectrometry Unit, McGill Univecsiiy, Montreal, Quebec, Canada H3G 1Y6
A new interface has been developed for direct analysis of volatile organic compounds (VOCs) from aqueous solutions without prior sample treatment. The device consists of a nozzle directed vertically downward in a spray chamber, which is in turn coupled through a helium jet separator to an ion trap mass spectrometer. The nozzle reduces the liquid to an aerosol with the help of a helium carrier gas and rapid sample injection. The large surface area of the aerosol permits a rapid equilibration of the VOCs between the gas and liquid phases. The aerosol droplets impinge by their inertia on the floor of the spray chamber, where they coalesce for subsequent removal from the interface. The VOCs are carried from the interface by the helium flowing countercurrent to the spray. The combined VOC-helium mixture passes through a jet separator, which removes most of the carrier gas, and the concentrated VOCs are then analyzed by an ion trap mass spectrometer. This inertial spray extraction mass spectrometry method is very sensitive and has detection limits of < 1 ppb for most compounds analyzed (e.g., benzene is detected at 90 ppt). Using this system, we have analyzed halothane in blood from an anesthetized pig. Excellent correlations are obtained between this interface and conventional headspace gas chromatography for the analysis of toluene and xylene in blood from rats exposed to these compounds and for spiked human blood.
review of this field has been published2). A practical limitation of MIMS is that the membrane discriminates against compounds on the basis of their solubilities in the membrane material. Sparging of 10 mL of whole blood and trapping of VOCs on Tenax resin for later desorption and GC/MS analysis is reported to have improved sensitivity over headspace analy~is.~ A hollow fiber membrane device has been described for the extraction of VOCs from fluid samples into a carrier gas, which is conducted directly to a GC, where the volatiles are trapped on the cool head of the column and then flash heated for chromatographic analysis and detection by flame ionization dete~tion.~ Other approaches to the analysis of VOCs use spray extraction techniques. Large volumes (250-900 mL) of aqueous solutions containing VOCs may be sprayed into enclosures and swept by a carrier gas into Tenax cartridges for subsequent thermal desorp tion and G U M S analysis.jI6 However, these approaches do not readily lend themselves to the introduction of the small volumes required for blood analysis. The inertial spray extraction mass spectrometry interface (ISEMS)’ described in this report, coupled through a jet separator to an ion trap mass spectrometer, provides a quick, simple, and reproducible way to analyze VOCs in a small volume of aqueous solutions, including such complex liquids as blood. Unlike MIMS, this method avoids the requirement that the analyte be soluble in a membrane, and it is not subject to memory effects. Several examples of VOC analysis in water and blood are presented which demonstrate the versatility and sensitivity of this system. EXPERIMENTAL SECTION
Quantitation of volatile organic compounds (VOCs) in aqueous solution is of great interest in environmental, medical, and many other fields. VOC analysis by headspace gas chromatography has been recently reviewed by Seto,’ but it can be time consuming and difficult because of losses of VOCs or contamination from external sources. Because of these limitations, methods utilizing minimal sample handling are preferable. Membrane introduction mass spectrometry (MIMS) allows direct analysis of VOCs in liquids, and a large number of designs have been described (a * Address correspondence to this author: Laboratoire du metabolisme intermediaire, M.8208, HBpital Notre-Dame, Centre de recherche Louis-Charles Simard, 1560 Sherbrooke St. E., Montreal, Quebec, Canada H2L 4M1. + Departement de Nutrition, Universite de Montreal. Biomedical Mass Spectrometry Unit, McGill University. 9 Departement de Medecine du Travail et #Hygiene du Milieu, Universite de Montreal. (1) Seto, Y. J. Chromatogr. A 1994,674, 25-62.
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Analytical Chemistry, Vol. 67, No. 24, December 75, 7995
Spray Interface and Experimental Setup. Figure 1 shows the overall scheme of the apparatus used. The interface is composed of two parts: a spray nozzle and an extraction chamber. The nozzle consists of an outer stainless steel and a coaxial inner brass cylinder designed to maximize the efficiency of aerosol formation and to permit fine control of the carrier gas flow. The space between the two coaxial cylinders is pressurized with helium carrier gas, which flows through and mixes turbulently at the exit of the nozzle with the aqueous solution injected through a silicone (2) Kotiaho, T.; Lauritsen, F. R; Chaudhury, T. IC; Cooks, R. G.; Tsao, G. T. Anal. Chem. 1991,63,875A-883A (3) Ashley, D. L.; Bonin, M. A ; Cardinali, F. L.; McCraw, J. M.; Holler. J. S.; Needham, L. L.; Patterson, D. G. Anal. Chem. 1992,64,1021-1029. (4) Yang, M. J.; Pawliszyn, J. Anal. Chem. 1993,65, 1758-1763. (5) Matz, G.; Kesners, P. Anal. Chem. 1993,65, 2366-2371. (6) Baykut, G.; Voigt, A. Anal. Chem. 1992,64, 677-681. (7) Montgomery, J.; St-Germain, F.; Mamer, 0.; Brunet, J.; Des Rosiers, C., U S . Pat. Appl. 231,202, 1994.
0003-2700/95/0367-453659.00/0 0 1995 American Chemical Society
5
-6
Figure 1. Flow diagram and schematc lor t h e interface and its connection to tne ion trap mass spectrometer The numbered pads are identifieo as loll ow^: 1, nehm supp y: 2. gas flowmeter and regL ator; 3. ISEMS spray cnamoer assemoly;4, glass I ber 11lei; 5. jet separator; 6. vacuum pump: 7. ion trap mass spectrometer; 8. rJboer septum; 9, adjustable nozzle assembly: 10, he Jm inlet poll; I t . spray chamoer cap; 12. spray chamber cup; 13. exit poll lor helium-VOC mixture: 14. adysrab e nozzle constriction; and 15, drain lor oepleted sample. Valves are ioent tie0 by eners A-E. Operation is as descriDed in text.
rubber sepNm into the inner cylinder. A pressure gradient between the nnzzle and the e m c t i o n chamber creates an aerosol. permitting an efficientand rapid equilibration of the VOCs between the aqueous solution and the carrier ms. The aerosol droplets are driven by their inertia and by gravity to the bonom of the chamber against the carrier gas flow as it exits the interface. The nozzle assembly is sealed to the cap with an O-ring. which in Nrn is sealed to the stainless stwl rxbaction chamber, which has an exit at the bonom. The cap has an OK-axis exit port. through which the VOCs and the carrier gas leave the chamber. A jet separator (Finnigan type,Allen Scientific Glassblowers Inc.). which removes more than 90% of the helium and some of the water vapor. is installed between the spray extractor exit port and the ion trap (Varian Saturn 11. Software Version Revision C). A compacted glass fiber filter and a 25 em filter are placed in the entrance of the separator to remnve particulate maner. The interface (nozzle and extraction chamber) is kept at ambient rnom temperaNre. The helium now rate is 300 mL/ min during experiments and 40-50 mL/min in standby mode. The transfer line between the separator and the ion trap is a short length of 0.25 mm i.d. deactivated fused silica tubing. The remaining Nbing is 1/16 in. stainless steel connected to stainless steel valves Rlupro SS2P49 with Swagelok fittings. The flowmeter and integral meterinEvaive were from Matheson (E14C301. E910). The ion trap manifold temperamre was 130 "C. and the transfer line and the jet separator were maintained at 100 "C. Operating Conditions of the Ion Trap. Daily. the ion trap was calibrated following the automatic SeNp procedure with the
interface placed in the standby mode, closing valves B and D and opening valve A (Figure l), and with a helium flow rate of 300 mL/min. Better sensitivity was obtained when the trap was calibrated at the operating pressure. For sample injection,valves B, D, and E and the flowmeter were opened, valve A was closed, and the ion trap was operated in the E1 or CI mode with the automatic gain control (AGC) function htmed on, with filament emission current varying between 10 and 1 5 4 In CI, the target total ion current was set at 5MN) or 10 OOO. S i a l strength was optimal when the trap was operated at lOpscan/s, corresponding to about 1.3 s/scan. Depending on the compound analyzed, the low mass cut-off was set manually at 2 mass units below the lowest mass scanned. Once the acquisition routine was started, a 30 s background was recorded before injection of up to 1 mL of an aqueous solution. When the acquisitionwas finished, valve D was closed and valve C opened to permit the waste solution and rinse water to be drained. finally, the interface was placed in the standby mode and the helium flow rate reduced to 40-50 mL/ min. Chemicals. Toluene, benzene, chlorofom. and acetone were obtained from local suppliers. Propanal, hexanal, acetaldehyde, halothane, m-xylene, and dimethyl sulfide are from Aldrich Chemical Co. The carrier gas is ultra-high-purity (UHP) helium from Liquid Air Canada, and the reagent gas for chemical ionization is UHP methane from Matheson. Analysis of Standard Solutions. Dilutions of chlorofom, halothane, acetone, acetaldehyde, hexanal, propanal, benzene, toluene, and dimethyl sulfide were prepared in water puriiied on a Milli-Q system (Millipore). Chlorofom and halothane were analyzed in electron impact mode (ED,and the remainder were analyzed in methane positive ion chemical ionization mode (CO. Analysis ofHalo!bne in Pig Blood. A pig was exposed to increasing concentrations of halothane of 0, 0.5, 1, 2, 3, and 5% (v/v) in the oxygen used for ventilation for 5 min at each concentration. Peripheral blood samples were taken at the end of each period. Following the 5 min exposure to 5% halothane, anesthetic adminstration was stopped, and blood samples were taken at 1, 2, 4, 8, and 16 min thereafter to monitor the rate of disappearance of halothane. Aliquots (1 mL) of blood samples were analyzed by ISEMS in EI. Aqueous solutions of hown concentrations of halothane were analyzed under the same conditions for the purpose of establishing a standard curve to be used to quantitate halothane in the pig blood samples. Comparison of ISEMS with Headspace Analysis of Blood Samples. Human blood samples spiked with toluene and xylene to correspond to a range of 0-1.5 ppm and blood taken fromrats exposed to increasing amounts of toluene and xylene vapor in air in an inhalation chambe6 were analyzed by ISEMS in methane CI and by headspace gas chromatography as described by Tardif et al.9 Blood samples of either 1 mL or 0.2 mL diluted to 1mL with water were analyzed to evaluate analyses of VOCs in smaller volumes. RESULTS AND DISCUSSION Analysis of VOC Standards in Aqueous Solution. Figure 2 illustrates the total ion profile (llP) response and the mass spectrum obtained for an aqueous solution of halothane (90ppm), (8) Tardii, R Plaa. G. L: Brodeur. J. Con. I. phw'ol. Phonnomf. 1992.70.
385-393. (9)Tardii. R Lap+. S.: Krishnan, K: Brodeur. I. Torirol. Appf. p h o m c o f . 1993,IZO. 266-273.
Analytical Chemistry, Vol. 67, No. 24, December 15, 1995 4537
\
BrCHCI C F,
t
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MW %1
I
93
r
TIME (min)
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59=Acetone 63= Dimethylsulfide 79= Benzene 83 = Hexanal 93 = Toi uene
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M/Z Figure 2. Analysis of a halothane standard solution in water. Upper panel is the El total ion profile obtained for the injection of 90 ppm of halothane in distilled water. The lower panel is the backgroundsubtracted mass spectrum of halothane (CHCIBrCF3) taken from the profile. The spectrum for halothane is modified by the operating conditions and pressures in the ion trap (see text).
l2
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20
I
I
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40
60
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Halothane (ppm) Figure 3. Calibrating plot for halothane in water. Areas are determined for the sum of extracted ion profiles for m / . 176 and 178. Table 1. Detection Limits (DL) for ISEMS Analysis of Various VOCs in Water compound (MW)
acetone (58) acetaldehyde (44) propanal (58) hexanal (100) benzene (78) toluene (92) dimethyl sulfide (62) chloroform (118) halothane (196)
WZ
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ions selected: m/z 59 45 59 83 79 93 63 83 176
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ionization
DL @pb)
CI CI CI CI CI CI CI E1 E1
0.82 0.79 0.81 0.83 0.09 0.44 0.51 7.5 9.4
=Areas measured for ions extracted from full scans obtained
following injection of 1 mL of serial aqueous dilutions.
a commonly used volatile anesthetic. The sample produces a transient pulse lasting -4 s from baseline to baseline, as shown in Figure 2. The profile resembles a sharp chromatographic peak due to the rapid equilibration of halothane between the liquid and gas phases, followed by efficient transport by the carrier gas to the ion trap. The mass spectrum produced is different from the normal E1 spectrum for halothane due to the high source pressures of helium and water under which the ion trap operates in this experiment. Under normal E1 conditions, the molecular 4538 Analytical Chemistry, Vol. 67, No. 24, December 75, 1995
Figure 4. Methane CI mass spectrum obtained for a mixture of VOCs in water in the concentration range 80-100 ppb. The mass spectrum has been averaged over the profile peak and has been background subtracted.
ion cluster is very abundant; here, the most intense ion cluster is due to the loss of HF, and the spectrum more closely resembles While the prothat for l-bromo-l-chlorc-2,2-dffluoroethylene. cesses producing this type of spectrum are not clearly understood, they are highly reproducible and probably involve interactions of halothane molecular cations with high partial pressures of water prior to ion ejection. In fact, injection of halothane vapor into a dry chamber resulted in a typical E1 spectrum (not shown). Subsequent injection of water while halothane was still in the ion trap resulted in reversion to the mass spectrum exhibiting a loss of HF. Chemical ionization of halogenated compounds produces spectra with a characteristic loss of HX, where X is the halogen atom.IOBauer has recently reported similar losses of HX in VOC analysis using his MIMS system, where transfer of water vapor is also unavoidable.I1 Signal response is proportional to the amount of analyte present, as shown in Figure 3, which relates the areas obtained for a series of known concentrations of halothane in water over the concentration range in blood necessary for anesthesia induction. Each point is the average of five injections, with error bars visible only at one concentration (CV = 4%). Analysis of Milli-Q purified water used to prepare the solutions demonstrated very little background signal. Typical ions found in E1 include m/z 41, 55, 69, and 81, in order of decreasing intensity. These ions are typical of hydrocarbons and probably are due to mechanical pump oil vapor. These ions were also detected when air was injected into a dry chamber. They were not present when the trap was operated in the CI mode, and this may explain why the background noise was much lower and the sensitivity better in CI. This background spectrum was always subtracted from the actual sample spectrum. The detection limits determined in water for several VOCs with different chemical characteristics (Table 1)were in the range 100800 ppt, except for chloroform and halothane. These limits compare very favorably with those of Brodbelt et a1.,12 who describe a silicone tubing membrane probe submerged in aqueous solutions of dichloromethane (5 ppb), methoxyflurane (100 ppb), ~
~
(10) Harrison, A. G. Chemical Ionization Mass Spectromety. 2nd ed.; CRC Press: Boca Raton, FL, 1992; p 141. (11) Bauer, S.; Solyom, D. Anal. Chem. 1994,66, 4422-4431. (12) Brodbelt, J. S.; Cooks, R G.; Tau,J. C.: Kallos, G. J.; Dryzga, M. D.Anal. Chem. 1987,59, 454-458.
1
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Figure 6. Halothane concentrations measured in 1 mL aliquots of blood taken from a pig after exposure for 5 min intervals to a sequence of increasing halothane/oxygen concentration percentages (lower abscissa, W). Following 5 min of exposure at 5%, halothane was measured at time intervals after administrationwas withdrawn (upper abscissa, 0).
and styrene (500 ppb). For a 10 mL sample, Ashley reports 72 ppb for acetone, 0.025 ppb for benzene, and 0.020 ppb for t ~ l u e n e . ~ More recent work by Bauer demonstrates 3 ppt sensitivity for toluene using MIMS. Figure 4 represents the averaged and background-subtracted CI mass spectrum obtained for a mixture of VOCs in water. It is significant that the different chemical and physical characteristics of these compounds do not a€fect the ability to analyze them in the same sample simultaneously. Analysis of VOCs in Blood. Figure 5 presents the results of an in vivo experiment conducted in a pig, which demonstrates
the ability to monitor halothane use in anesthesia. Two consecutive injections of pig blood were made in the same computer acquisition file before (A) and during (E3) anesthesia. The mass spectra are background subtracted, and spectrum B is dominated by ions characteristic of halothane (see Figure 2 for comparison). Figure 6 summarizes the increase in halothane concentration observed in blood samples taken from the pig during exposure to increasing concentrations of halothane (lower abscissa). A small amount of halothane is present in the blood taken before the pig was anesthetized, probably due to exposure to residual halothane present in the air in the operating room, a demonstration of the sensitivity of the interface. The halothane signal is proportional to the amount of halothane administered in the 0-2% (v/v) concentration range. The therapeutic range is 1-2%, and 3-5% represents a toxic dose. A plateau observed at these high concentrations is characteristic of blood saturation effects. Figure 6 also demonstratesthe disappearance of halothane from pig blood on cessation of administration of 5%halothane (upper abscissa), The last sample was taken 16 min after exposure to halothane was stopped, corresponding to the time at which the pig started to regain consciousness, preventing further blood sampling. When human blood samples spiked with known amounts of toluene and xylene were analyzed (Figure 7), very good correlations were found between the amounts added and the amounts detected. Smaller volumes of blood such as 0.2 mL when diluted to 1mL produce results that are similar to those obtained from 1 mL of undiluted blood. The smaller volume would be more compatible with sampling by finger-prick rather than venopuncture. The injected volume for optimal response is between 0.4 and 1mL. Smaller volumes are not expelled completely from the injection channel, while larger volumes are poorly handled by the small volume of the spray chamber and cause excessive water to be entrained in the exiting carrier stream. Analytical Chemistry, Vol. 67, No. 24, December 15, 1995
4539
TOLUENE
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QUANTITY ADDED (mg/L) Figure 7. Analysis of human blood spiked with increasing quantities of toluene and xylene. A 1 mL aliquot of blood (0) or 0.2 mL diluted to 1 mL (A)was used in each analysis. Dotted lines represent regression lines with 95% confidence limits. g,
TOLUENE
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Figure 8. Comparison of ISEMS versus headspace gas chromatography. Samples of blood were taken from rats exposed to increasing or 0.2 mL diluted to 1 mL (A)was analyzed by ISEMS and concentrations of toluene and xylene in their environment. Either 1 mL of blood (0) by headspace GC analysis of 1 mL of blood. Dotted lines represent regression lines with 95% confidence limits.
The accuracy of ISEMS quantitation of toluene and xylene in blood from rats exposed to air containing increasing amounts of these solvents was evaluated by comparing ISEMS measurements with analyses of aliquots of the same blood samples by headspace GC. These analyses are well correlated (Figure 8). The present interface is the result of a series of optimizations of the size of the spray chamber, helium flow rates, and temperatures. The helium flow rate is constrained to one low enough for jet separator best operation, while efficient nebulization has its own requirement. The ability to vary the nozzle constriction permits the interface to be operated at a flow rate optimum for both. Compared to the spray interface described by Baykut and Voigt,6which operates with volumes of 250 mL, our interface is optimized for the analysis of small sample volumes, suitable for blood analysis. We expect that detection limits can be improved substantially by optimization of certain parameters. The temperature of the interface and its transfer lines may be increased to improve the 4540 Analytical Chemistty, Vol. 67, No. 24, December 15, 1995
volatility of more polar VOCs. However, increasing the temperature has the disadvantage of increasing the water load on the ion trap. While the mass spectrometer used here was a conventional ion trap unmodsed except as described, upgrading to permit selected ion storage would allow greater accumulation of ions of interest. MS/MS capability would permit positive identification of individual compounds and positional isomers in mixtures. The separator's jet dimensions and spacings could be adjusted so that more VOCs of lower molecular weight may be directed into the MS without being lost to the vacuum pump. Among the principal advantages of the ISEMS interface are the short times and small sample sizes required for analysis, as well as the ability to analyze blood and other samples directly without prior treatment. The peak produced by the volatiles appears virtually instantaneously on sample injection. No time is required for the sample volatiles to equilibrate with the headspace volume, and transfers of samples to the headspace sampling containers with possible loss of volatiles are not needed. While
this brief description of blood analyses cannot be regarded as a validation of a new method for blood volatiles, it is qualitatively illustrative of the value of the technique. The interface is easy to use, robust, very reproducible, and easy to clean and maintain, and the volume required for analysis is as low as 0.4 mL. No memory effect is observed after rinsing with water, and there does not appear to be discrimination effects similar to those shown by membrane interfaces. The detection limits are in the parts per trillion to low parts per billion range, permitting a broad range of utility. Potential applications include medical diagnosis and monitoring, acute intoxication detection, occupational health and safety monitoring, pharmacologicalanalyses, and environmental and industrial process analyses, to name but a few.
ACKNOWLEDGMENT We acknowledge the financial support of Lomedic and Theratechnologies, Inc., as well as the Medical Research Council of Canada (MT-11726 to J.M.; MT-10920 to C.D.R) and the Heart and Stroke Foundation of Canada (T.M.).
Received for review May 9, 1995. Accepted September 21, 1995.@
AC950450N
@Abstractpublished in Advance ACS Abstracts, November 1. 1995.
Analytical Chemisfry, Vol. 67,No. 24,December 75,1995
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