On-line, In Situ Analysis with Membrane Introduction MS A new analytical technique promises rapid environmental analyses for volatile organics in air and water. P H I L I P S . H . WONG, M A R Y E . CISPER,
R . G R A H A M COOKS
P H I L I P H . HEMBERGER
significant trend in analytical chemistry today is away from the use of laboratory methods and instrumentation toward online, in situ methods. The latter methods provide diagnostic information that can help solve critical problems faster, less expensively, and more reliably than current analytical methods. Mass spectrometry (MS) already plays a prominent role in environmental monitoring because of its high chemical specificity and exquisite sensitivity; molecular weights and structural information are both available through the appropriate choice of ionization methodology. However, online in situ methods are not prominent for MS, although progress is expected. One approach to the development of such a capability is membrane introduction mass spectrometry (MIMS) (1-3). MIMS was introduced in 1963 for the study of photosynthetic reaction kinetics in water. The experiment employs a semipermeable membrane, which acts as an interface between the sample solution and the vacuum of the mass spectrometer.The membrane selectively allows the dissolved gas to enter the mass spectrometer. MIMS is developing rapidly with an increasingnumber of applicationsin bioreactor monitoring as well as environmental analysis (1-5). On-line monitoring of volatile organic compounds (VOCs)in water and air, in vivo analysis of blood, biological wastewater treatment, bioreactors, and chemical reactions are all topics currently being explored using MIMS. Environmental scientists commonly analyzeVOCs in water by using a purge-and-trap procedure (6), which concentrates the analyte. VOCs in water are purged by an inert gas and trapped on a sorbent. To release the compounds, the trap is heated and backflushed with the inert gas. The desorbed VOCs are separated using a gas chromatographic (GC) column and then detected by MS. This method provides reproducible, quantitative data but is time0013-936X/95/0929-215A509.00/0 0 1995 American Chemical Society
consuming and labor intensive, particularly when many samples are involved. Also, because this method provides data only at well-spaced time intervals, short-term fluctuations of environmental significance may be excluded,especially when the samples are volatile. Thus there is a need for a complementary method that provides more rapid and, preferably, on-line data. MIMS provides capabilities that fit these requirements. The method employs flow injection analysis procedures for sample handling to provide an online capability, detects organic compounds in aqueous solution or in air, and offers relatively rapid response times (in the range of 0.5-5 min). Thus, there is the capability for continuous, on-line operation. In addition, internal or external standard solutions can provide quantitation. MIMS requires minimal operator intervention. Most strikingly, detection limits forVOCs are in the low part-per-trillion range (Le., a few nanograms per liter). Similar detection limits can be achieved using preconcentration with GUMS methods, but MIMS experiments do not require preconcentration or other sample workup. One consequence of this is that, compared with GC/MS, chemical speciation for MIMS is not as good, and the dynamic range, although several orders of magnitude, is not as great. On the other hand, the procedure is surprisinglyfree of matrix effects, and a single instrument is used for simultaneous, multicomponent analysis. MIMS performs best with relatively nonpolar and low molecular weight compounds, which can pass through silicone membranes. Much effort is going into expanding the range of analytes. MIMS has been used in various configurations with a variety of types of mass spectrometers. Data for ion trap mass spectrometers are summarized in Table 1. It can be seen that MIMS has limits of detection in the part-per-trillionto part-to-billionrange. Although MIMS is not likely to achieve the isomer VOL. 29, NO. 5, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Detection limns of some volatile ana semivolatile organic compounds' Oetectlon limits are for aqueous solutions measured by the ion trap -athod (References4 and 9).
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specificity and quantitative accuracy of the G U M S method, it can rapidly screen for VOCs in water.
Implementing MIMS in different ways
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In the MlMS experiment, a polymer membrane on the order of 100 pm thick serves as the interface between the sample (often water or air) and the vacuum of the mass spectrometer. The membrane can take the form of a sheet across which the sample passes or of a capillary tube through which the sample flows.Transport of the analyte through the nonporous membrane occurs by pervaporation. This involves three stages: adsorption to the outer surface of the membrane, diffusion through the membrane, and evaporation from the inner membrane surface into vacuum. Selective pervaporation of the analyte results in its enrichment. The permeation rate
VOL. 29. NO. 5.1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY
r,, =ADS( P. / I ) (1) where I,. is the steady-state flow of analyte through the membrane (mol/s),A is the membrane surface area (cm'), D is the diffusion constant (cm*/s) and S is the solubility constant (mol/torrcm3) of the analyte in the membrane matrix, P, is the vapor pressure of the analyte on the sample side of the membrane (torr), and I is the membrane thickness (cm). This equation shows the dependence of permeation rate on the solubility of the analyte in the membrane, the membrane thickness, and the temperature. For appropriate analyte-membrane combinations, enrichment in favor of analyte can result in low detection limits. The MIMS experiment has been implemented in a number of ways (7,s)(Figure 1).In one common method, a membrane probe, similar to a direct insertion probe, is placed in the ion source of the mass spectrometer, to which it delivers the analyte (Figure la).The flowingliquid or gaseous sample stream can be handled by flow injection analysis methods. External standards, which can be prepared to mimic the analyte, provide quantitation. (Although exceptionally valuable in MIMS, the use of external standards for quantitation is unusual in MS.) Figure l b shows a recently developed interface. The membrane is located some distance from the ion source, and the permeate is transported to the source in a gas stream. In this design a jet separator is used to remove excess water (or air dependingon the type of sample); hence this design has the advantage of two stages of analyte enrichment. Various polymeric materials have been employed as membranes. The most commonly used are hydrophobic, nonporous polymers such as the silicones. These membranes have excellent permeabilities for VOCs present in water or air and low permeabilitiesfor the sample matrix Mimpmus membranes (9)have also seen some use in MIMS. In spite of the lack of selectivity of such membranes, their fast response times allow specialized applications where solvent removal is not essential-for example, in the determination of polar organic compoundsin hydrfcarbon matrices, where the hydrocarbon was used as the chemical ionization reagent gas in the subsequent m a s spectrometric analysis. Like other sample introduction systems, MlMS can readily be adapted to mass spectrometers of all types. A great deal of work has been done with quadrupole mass spectrometers. However, ion traps are especially weu-suited, considering the simplicity and ruggedness of the interface, which matches these same qualities in the ion trap mass spectrometer itself and facilitates the use of the overall system for in situ, on-line operation. Newer methods of operating ion traps allow the trapping of selected ions (lo). These instruments can be used to suppress ions arising from the sample matrix (e.g., water ions) and hence to employ all the charge-storing capacity of the ion trap so that selected analyte ions may be studied at maximum efficiency. Rapid screens for VOCs in water Among theVOCs in water, trihalomethanes (THMs) are a major concern because of the suspected car-
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cinogenicity of some species. The THM compounds formed during water chlorinationtypicallyoccur in this order of concentration: chloroform > bromodichloromethane > dibromochloromethane > bromoform, with chloroformcomprising about 75%of the total concentration. MIMS can directly determine THMs in tap water (11,12).Figure 2 shows ion chromatograms, which sum the signals for several THMs. The average analysis time is less than 6 min per sample with a detection limit of below l ppb (1 pg/L). Individual compounds can be monitored just as easily by examining individual ion chromatograms. MIMS is also applicable to the analysis of complex aqueous solutions, including polluted seawater. The nonporous silicone membrane excludes all particulate matter and ionic compounds, including the salts present in seawater. Only the desiredVOCs and a small amount of water pass into the mass spectrometer, offering trace-level measwement of volatile organics in a complex aqueous matrix without sample preparation. Selective ion trap ionization techniques (10)allow only the ions of interest to be detected. As an example, ions of chlorobenzene (mlz 771, carbon tetrachloride ( d z 117& 119). trans 12-dichloroethylene (mlz 611,and toluene (mlz 91) were simultaneously monitored during each of four injections of a sea water solution containing 250 ppb of each of the above analytes (Figure 3).The ions monitored are the only ions present in the trap during data acquisition, and the selective ionization method provides additional selectivity and sensitivity to MIMS.
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