Mobile mass spectrometry A new technique for rapid environmental
Douglas A. Lane SCIEX, Inc. Ontario, Canada L3T 1P2 Environmental matrices can be, and usually are, exceedingly complex. Since the concentrations of the contaminants to be analyzed are often in the sub-ppm range, it is necessary to collect a large sample of the matrix, separate the components of interest from the matrix, and then perform an analysis for the specific compound desired back at the laboratory. Frequently, highly sophisticated analytical procedures such as GC/MS are required. The entire process is subject to contamination and alteration of the sample; in addition, the investigation may have to be delayed several weeks or more pending the results of the analysis. Over the past few years, increased demand has been placed upon the analyst to transport the analytical laboratory into the field to obtain the results as quickly as possible. The detection and analysis of chemical dumpsites, the detection of toxic compounds in both fugitive and stack emissions, and the tracking of chemical emissions from sites such as train derailments and explosions have all demanded the ability to perform complex analyses in a short period of time from a mobile platform. In the case of emergency response actions, the analytical instrumentation must provide real-time results to adequately warn local inhabitants of hazardous concentrations of toxic compounds. Delays of even a few minutes in the return of data could be fatal. The scientist and his laboratory must, therefore, become mobile and be able to provide the necessary analytical services whenever and wherever required. A mobile, mass spectrometer-based system has been operating in the field during the past five years and has been exposed to many environmental problems—ranging from the acquisi38A
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tion of chemical "fingerprints" of fugitive emissions to tracking a chlorine plume from a derailed railway tank car through a major urban area. This article explores some of the applications of the system and attempts to demonstrate the utility of mobile mass spectrometry. In 1971, the University of Toronto Institute for Aerospace Studies was contracted by the Canadian Department of Energy, Mines, and Resources to design and build a mass spectrometer-based instrument that could be mounted in a small aircraft and that would be used for the detection of gases such as iodine during geological surveys. The basic system design criteria demanded a real-time response to trace contaminant concentrations down to the sub-parts per billion level, that the system run entirely upon electrical power and be free from any water cooling requirements, and that the system be rugged enough to operate at maximum sensitivity while in motion. In order to achieve these objectives, many radical departures from standard mass spectrometric design techniques were necessary. These techniques included reconstructing the quadrupole rod assembly so it would withstand the high- and low-frequency vibration of the mobile platform, converting to cryogenic pumping for the mass spectrometer to eliminate the contamination and cooling problems inherent in oil diffusion pumping systems, and developing a direct air sampling atmospheric pressure chemical ionization (APC1) source to permit real-time analysis of trace contaminants at sub-ppb levels. The instrument that evolved out of this program has become known as the TAGA system. Over the past several years, mobile TAGAs have been involved in both routine and emergency monitoring programs ranging from the detection of PCBs in the stack gas from a cement kiln (4) to the tracking of the chlorine plume from the train derailment in
analysis
Mississauga in November 1979 (5, 6). Many new approaches to environmental problems have evolved as a result of these and other programs and the benefits of a real-time monitoring instrument have been clearly demonstrated. One very intriguing potential application for the mobile TAGA is that of plume model validation. Since most plume models generally assume constancy of wind direction and velocity, the shorter the time to collect the necessary data, the closer the model may be compared to the actual results. The mobile TAGA unit would seem to be ideally suited to such studies. The instrument In the direct air sampling mode, a high capacity air pump draws ambient air directly into the APCI source at flow rates from 1 -2 L/s. Ions formed in the corona, point-to-plane discharge (see Figure 1 ) are guided by electrostatic fields through the orifice in the atmospheric pressure-to-vacuum interface assembly, while the unionized gas molecules and particulate matter experience a counter-current flow of ultrapure nitrogen and are, consequently, diverted away from the orifice region. Thus, only ions from the air and nitrogen molecules from the counter-current flow gas membrane are able to pass into the vacuum chamber. As the ion-molecule mixture is admitted to the vacuum chamber, it expands in a free jet expansion as shown in Figure 1. To eliminate the problem of clustering of the water molecules with the ions in the free jet region, the expanding ion-molecule mixture is subjected to an electrostatic field. This imparts to the clustered ions, through collisional processes, sufficient vibrational energy to decluster the ions. The field of this "cluster-breaker" region also helps to focus the declustered ions towards the next stage of vacuum ion optics. The vacuum system consists of a
0013-936X/82/0916-038A$01.25/0 © 1981 American Chemical Society
Monitoring. A mobile unit is being used for the detection o/PCBs in the stack gas from a cement kiln. Instrument is in the inset. Environ. Sci. Technol., Vol. 16, No. 1, 1982
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FIGURE 1
Cross-sectional schematic of the TAGA 3000 system
CI reagent gas
Gas membrane
Cryogenic
Corona discharge Air sample inlet
Quadrupole MS
• Orifice Cryogenic vacuum pump
two-stage, closed-loop, cryogenic re frigerator that is coupled to a baffled cryo-array in the vacuum chamber. The configuration of the system per mits an equivalent pumping capacity of 40 000 atm L/s, which in turn per mits the use of a large (.010 cm) orifice in the atmospheric pressure-to-vacuum interface. The neutral gas molecules follow their expansion trajectories and are collected by the baffled cryo-array while the ions are collimated by the ion optics into the quadrupole mass filter for mass separation. For mobile operations, the TAGA has been mounted in a 26-ft General Motors Corporation Transmode. The vehicle is ideally suited to mobile op erations since it has an excellent air suspension system. Electrical power to operate the instrument and all ancil lary equipment (air conditioners, heaters, lighting, etc.) is supplied by two 6-kW Onan generators. APCI Atmospheric pressure chemical ionization is particularly suitable for real-time mobile operations since air is drawn directly into the ion source without preseparation (gas-particle, or trace from air) being required. The nature of the chemical ionization processes is such that one can operate quite efficiently with water chemical ionization (CI) in the positive mode (since water naturally exists in the air in sufficient quantity to act as a CI reagent) or oxygen CI in the negative 40A
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mode to effect an instantaneous class separation of trace contaminants. Positive mode mechanism The ambient air APCI reaction mechanism is complex yet well docu mented since the reactions taking place are similar to those in the ionosphere (/). The initial ionization is electron bombardment of the major air com ponent nitrogen (see Figure 2 for the reaction sequence). Although minor components would also undergo direct electron impact ionization, the reaction mechanism has been simplified to in clude only the most probable reac tions. The N2 + ion then undergoes a charge transfer reaction with oxygen (since the ionization potential of O2 is lower than that of N 2 ). The 0 2 + then forms a cluster with the water vapor present in air, and by a series of reac tions, forms the proton hydrate, Η3θ + . This ion then clusters with other water molecules to form a series of proton hydrates HaO + • (Η.2θ)„. These are the reactant ions in normal ambient air APCI. In ultra-pure air with absolutely no impurities, these would be the only ions present. The lower proton hydrates are the major ions since declustering of the higher hydrates occurs in the high vacuum region of the TAGA system. This declustering technique is used to simplify the spectrum and to enhance the detection of trace compounds in the air.
Trace constituents then react with the proton hydrates with either a pro ton, or a proton and a few water mol ecules, being transferred to the trace. The only criterion is that the effective proton affinity of the trace be higher than that of water. In general, this is true for any organic compound con taining a heteroatom such as N, O, S, or P. Negative mode mechanism The reaction scheme for the for mation of reactant ions in the negative mode is shown in Figure 3. The initial ionization involves electron capture and dissociative electron capture by O2. The mechanism then becomes more complex due to the formation of a series of radicals and neutrals in the corona discharge region. These lead to the formation of a series of very stable ions: N 0 2 _ , 0 3 _ , CO3-, HCO3-, and NO3 - . However, it is 0 2 _ and its clusters (0 4 ~ and 0 2 _ · (H 2 0)„) that are the main reactant ions in trace analysis. For a trace to react, it must have either a higher electron affinity than O2 to allow charge transfer, or it must be more acidic than HO2 (hydrogen superoxide) to allow proton transfer. For example: Charge transfer:
o 2 - + so2 — so 2 - + o 2 Proton transfer: 0 2 - + HC1 -* CI- + H 0 2
Fewer compounds meet these criteria than meet those imposed in the positive mode so that the negative mode spectra are less prone to interference by impurities than are positive mode spectra. Chemical ionization (CI) reagent It is often advantageous to add a CI reagent gas to the ambient air to modify the ion-molecule chemistry in order to highlight a particular trace or to suppress the appearance of a particular compound or class of compounds. For example, the addition of ppm levels of ammonia to the inlet line will result in a depletion of the water ion clusters. The ion chemistry will now be dominated by the ammonium ion, and only those traces such as the aliphatic and aromatic amines and nitrosamines that have proton affinities greater than that of ammonia will appear in the mass spectrum. Similarly, if an aromatic hydrocarbon is added to the air sampling line, the positive mode ionization process will be interrupted and the parent molecular cation of the aromatic hydrocarbon will be formed. Charge transfer (CT) chemistry will dominate in the ion source and only those species with ionization potentials below that of the reagent cation will be detected. For example, if benzene is added to the air sampling line as the CT reagent, FIGURE 2
Simplified positive ion reaction mechanism to form APCI reactant ions
High energy electrons
traces such as the PAH, PCBs, the dioxins, and many of the chlorobenzene isomers can be detected. Calibration Calibration of the mobile TAGA for targeted compounds is normally performed at the beginning and at the completion of each day's fieldwork. The calibration method selected depends primarily upon the vapor pressure and physical state of the compound. NBS traceable standard gas mixtures are used whenever possible. The gas mixture (typically about 100 ppm) is admitted to the TAGA air inlet line at various flow rates (measured by a mass flowmeter). The TAGA response is then plotted against mass concentration. For most compounds, the range of linearity extends from the parts per trillion up to the parts per million level. For liquid samples, a few microliters of the pure standard are deposited in the barrel of a glass syringe, which is fitted with a glass capillary needle. The plunger is depressed and retracted to the extent that the standard coats the inside wall but does not enter the needle. After a few minutes, an equilibrium is established between the vapors of the standard and the air in the barrel. The syringe is then placed in an automated syringe drive unit and the needle is inserted into the injector port, which is heated slightly to overcome the cooling effect of the air passing over the needle. By injecting the gas mixture in the syringe at various selected rates, the concentration of standard in the sampling line can be varied. The concentration in the air
line is, therefore, defined by the vapor pressure of the standard, the temperature, syringe drive rate, and the air flow rate into the TAGA ion source. By this technique, a simple, direct, in situ calibration may be performed in a few minutes, and can produce accurate and reproducible standard concentrations from the ppt to ppm level. Solids can be handled in the same manner as liquids or, alternatively, a solution containing the standard can be flash-evaporated in a hot injector. The integrated signal response is then plotted against the mass of compound injected. This form of calibration is particularly advantageous if small discrete liquid or solid samples are to be analyzed. Modes of operation Field analysis with a mass spectrometer-based system may be considered to be either mobile or portable. During mobile operation, data is collected while the laboratory is in motion, and during portable operation, the data is obtained while the laboratory is stationary. In addition, the mass spectrometer may be operated either in the selected ion mode in which the signal from one ion is monitored as a function of time, or in the scan mode in which a particular mass range is observed. The multiple ion detection mode, although a very useful mode, is considered to be a subset of the scan mode. Four basic operating modes, which combine the mobility of the laboratory and the mass spectrometer operating modes, are thus available to the mobile laboratory operator. They include: • mass scan while stationary • mass scan while mobile
FIGURE 3
Negative ion reaction mechanism to form APCI reactant ions
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FIGURE 4
Air "fingerprints" taken upwind (top) and downwind (bottom) of a chemical manufacturer 3
*The peak assignments were as follows: 1, CHZ ; 2, protonated ammonia; 3, protonated methylamine; 4, protonated methylenimine; 5, protonated dimethylamine and/or formamide; 6, a water cluster of protonated methylamine; 7, a protonated C5 H 9 N alkylamine and/or methylformamide; 8, protonated
dimethylformamide; 9, protonated dimethylnitrosamine; 10, protonated dimethylacetamide; 1 1 , a protonated C 2 H e N 2 0, alkylnitramine; 12, protonated dimethylnitramine; 13, protonated tetramethyldiazoethane.
FIGURE 5
T h e r m a l d e s o r p t i o n s y s t e m s h o w i n g soil h e a t e r a n d a d s o r b e r '
Carrier gas Sample stream Soil heater
CI gas
Discharge needle
To mass spectrometer Telfon plug Ionization chamber
Adsorber
Valve Exit
"The adsorber is moved into the inlet tube, leading to the ionization chamber, by sliding the teflon plug forward in the line until it contacts the end surface of the valve.
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Sliding handle
• selected ion monitoring while stationary • selected ion monitoring while mobile. Each mode of operation gives rise to highly unique results and provides the operator with a new information channel by which he may view the environmental situation being studied. Mass scan while stationary If the mobile laboratory is driven downwind of an emission source and a mass scan of that emission is obtained, that mass spectral scan is, in reality, a chemical fingerprint of the source giving rise to that emission. If the chemical nature of the emission is known, then the appropriate chemical ionization techniques can highlight the contaminants of interest. Otherwise, a general scan of the air
using water CI chemistry may be used. If the nature of the emission from the source changes, for example, as a function of chemical process, then the emissions from each stage in the process may be identified. The mass scans shown in Figure 4 demonstrate the nature of a fingerprint of the fugitive emissions from a chemical manufacturer. The upper scan shows the upwind background spectrum principally containing the water cluster ions, while the lower scan shows a chemical fingerprint obtained about 50 miles downwind of the industry on a public roadway. Many of the compounds observed in this scan were either starting products or end products of the manufacturing process taking place at the time of monitoring. Of particular interest was the detection of «-nitrosodimethyl-
FIGURE 6
Ground level cross-sectional profile of the plume from a coal-fired power plant using multiple ion detection"
(SOr) (S0 3 ) (SO, ·0 2 ) (background)
m/z=64 m/z = 80 m/z = 96 m/z = 40
"The ions monitored included SO,- ( m / z = 64), S 0 3 ( m / z = 8 0 ) , S O , -O, ( m / z = 96) and background represented by m / z = 40 The maximum SO, concentration as measured by the SO," ion was 110 ppb.
amine among the alkylamines. In another study, the rapid analysis of PCBs in soil samples was required. An underground pipe carrying a PCB-chlorobenzene mixture from a storage tank to a factory building ruptured, permitting leakage of from 6800-21 000 L of the fluid into the soil. Some of the PCBs were removed, but a large portion remained in the ground. Public concern over the spread of the PCBs through the soil and the potential contamination of the underground streams resulted in a major study of the area by the National Research Council of Canada (2, 3). The field technique developed for the T A G A permitted the complete analysis of PCBs in wet soil at concentrations ranging from 1-20 000 Mg/g in a time interval of no greater than 3 min. A mini-core of soil was obtained by pushing a 10 cm-long piece of 6-mm pyrex tubing into a fresh core sample to a depth of 1.5 cm. The soil plug was extruded from the glass tube with a glass rod and the mini-core was directed into a coil of heater wire (see Figure 5). An electrical current applied to the heater coil for 60 s raised the soil temperature to several hundred degrees centigrade. The desorbed PCBs were swept to a region containing a helical wire probe which served as a sample integrator. After the soil desorption had been completed (1 min), the valve was opened and the integrator was moved forward into the ionization region, through which flowed a stream of zero air carrier gas containing a few ppm of C 6 F 6 C T reagent gas. The integrator was also electrically heated to drive off the PCBs that were ionized and subsequently analyzed. Using multiple ion detection techniques to check for the appropriate chlorine isotope ratios for the various PCB congeners, one sample could be processed quite conveniently every five minutes. At the site of the spill, investigators performed more than 100 PCB analyses each day. The rapid analysis in the field allowed the investigators to build up a three-dimensional profile of PCB concentration in the soil in a very short span of time. In addition, since the results were almost immediately available, the location of the next coring operation could be directed from an ever-increasing data base of accumulated results. Mass scan while mobile The acquisition of mass spectra while in motion appears to be of limited value unless rapid scans (greater than 100 amu/s) are obtained and the Environ. Sci. Technol., Vol. 16, No. 1, 1982
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FIGURE 7
The trace shown here, records the rapid fluctuations in the concentration of nitric acid, when the plume impinged on the mobile laboratory3
'Full scale sensitivity is 1 200 000 ion counts/s. The concentrations recorded above far exceeded the upper range for which the TAGA was calibrated. The dotted line indicates 400 ppb—the upper limit for calibration.
FIGURE 8
Single ion scans across the emissions from a chemical manufacturer
vehicle is traveling at a relatively slow speed. A particular subset of this mode of operation, namely multiple ion monitoring, is an extremely useful tool. With multiple ion monitoring it is a simple matter to monitor rapidly and sequentially the intensities (and hence concentrations) of several species at once. Figure 6 shows the rise and fall of sulfur dioxide as the plume from a coal-fired power station was traversed by the mobile laboratory. Sulfur dioxide reacts, in the negative mode, with the oxygen anion forming ions at m/z = 64 ( S 0 2 - ) , m/z = 80 (SO3-), and m/z = 96 ( S 0 2 - • 0 2 ). All of these ions were observed to rise and fall as the plume was traversed while the background (represented by m/z = 40) remained constant. Definite plume structure was evident and was consistent from ion to ion. In the illustration, the highest concentration as measured by the S0 2 ~ anion was 110 ppb. Ion monitoring while stationary While the laboratory is stationary, the concentration of a particular contaminant may conveniently be followed as a function of time simply by tuning the mass spectrometer to monitor a single ion. Since a packet of air requires approximately 0.3 s to travel through the sampling manifold to the TAGA ion source, the TAGA virtually instantaneously sees the concentration of the air immediately outside the mobile laboratory. Timeweighted average concentrations may be determined by integrating the ion signal over the desired period of time.
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The advantage of the real-time monitoring capability is that short term excursions to high concentration levels are readily detected. The detection of nitric acid downwind of a chemical manufacturer is shown in Figure 7. To detect nitric acid, bromoform is added to the air stream as a CI reagent. In the ion source, the bromide anion clusters with the HNO3 to form the ( B r H N 0 3 ) _ cluster ion. In this case, clustering is promoted. The isotopic information resulting from the presence of the two isotopes of bromide aids in the identification of nitric acid. Bromide will cluster with HNO3 since the gas phase basicities of NO3"" and B r - are very similar. However, a switching reaction has been observed between B r - and NC>3~ to generate the proton-held dimer of nitric acid, ( N O 3 - H - N O 3 ) - , an entity more stable than the bromide cluster. As a result, the proton-held dimer actually provides a more sensitive indication of the concentration of the nitric acid present in the air. Single ion monitoring while mobile One of the most useful monitoring techniques available to the mobile mass spectrometer operator is that of monitoring a single ion while the laboratory is in motion. Through this type of operation, a detailed picture of concentration as a function of location can be developed. Under typical operating conditions, one would manually collect full mass scans at fixed locations downwind of a complex source and, after selecting ions of interest, would then focus interest on those particular ions, one at a time. Through a single traverse of the emissions from a complex source, the location of the source of the contaminant within the complex may be discerned. Figure 8 demonstrates the power of this method as it indicates very clearly the origin of each fugitive emission within the industrial complex. An extension of this technique is to carry out numerous transits of the emissions at successively greater distances from the complex while obtaining accurate positional and concentration information. When all the data is combined and plotted on a map of the area, a series of equal concentration isopleths such as those shown in Figure 9 and Figure 10 may be generated. Concentration maps such as these are useful in assessing the extent to which emissions penetrate an inhabited area. Figure 10 is particularly interesting since it clearly shows the impingement point of the plume.
FIGURE 9
Aniline concentration isopleths. The concentrations are in the parts per trillion (ppt)
FIGURE 10
Benzothiazole concentration isopleths. The concentrations shown are in parts per billion (ppb)
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Environ. Sci. Technol., Vol. 16, No. 1, 1982
Benzothiazole was, in fact, being emitted from a low-level stack, whereas the aniline emissions shown in Figure 9 were generally fugitive emissions and, consequently, no im pingement point would be anticipated. Another interesting feature of the plots in these two figures is the slight jog in the path of each plume. The industrial buildings are situated in the bottom of a river valley and the jog in the plume path is caused by the valley wall. There is, however, a potential problem with this pollutant mapping procedure. The wind direction must remain relatively constant over the course of the transits. If the transits can be completed in a relatively short period of time, the problem of wind direction fluctuation is minimized. For example, the plume maps shown in Figures 9 and 10 included transits along seven roads to a distance of sev eral kilometers, yet required only 20 min to complete. Prognosis What does the future hold for mo bile mass spectrometry? In a phrase: "Mobile Triple Quadrupole Mass Spectrometry." Within the past few years the TAGA family has grown to include a triple quadrupole mass spectrometer system that also uses APCI techniques. This system can analyze components in mixtures without having to separate them for analysis. The first quadrupole selects up to five compounds; quadrupole two breaks up the fragments; quadrupole three detects the individual frag ments. The triple quadrupole instrument resembles closely the instrument il lustrated in Figure l with the excep tion that between the quadrupole and the detector, there are two additional quadrupoles in tandem. The first and third quadrupoles are active (they can be made mass-selective), whereas the second quadrupole is an RF-only quadrupole that acts as a total ion fil ter. When the second quadrupole is pressurized with a neutral gas such as Argon, ions passing through the first quadrupole are fragmented through a process termed collision-induced dis sociation. The fragments may then be mass-scanned by the third quadrupole. The combination of the real time class separation inherent in the APCI ion source and the fragmentation capa bilities of the triple quadrupole system will greatly help to identify unknown species in complex environmental matrices. One of these systems is slated for installation on a mobile platform early in 1982. Both low pressure CI and el
emental sources (which also permit instantaneous analysis of samples) are being developed and have begun to expand the capabilities of the system. We have just begun to explore the real-time monitoring capabilities of the mobile mass spectrometer system to solve environmental problems, and we anticipate that the technique will re ceive increased interest and wider ap plication over the next few years. Acknowledgment
This article was read for technical accuracy by A. P. Altschuller, U.S. Environmental Protection Agency, Research Triangle Park, N.C. 27711, and William H. Glaze, University of Texas-Dallas, Richardson, Tex. 75080. References ( 1 ) Huertas, M. L.; Fontan, S. Atmos. Environ. 1975,9, 1018. (2) "A Case Study of a Spill of Industrial Chemicals—Polychlorinated Biphenyls and Chlorinated Benzenes," National Research Council of Canada, NRCC No. 17586, Ot tawa; 1980. (3) Thomson, Β. Α.; Roberts, J. R. intern. J. Environ. Anal. Chem., in press. (4) Thomson, Β. Α.; Sakuma, T.; Fulford, J.; Lane, D. Α.; Reid, Ν. Μ. Adv. Mass Spectrom. 1980, SB, 1422-1428. (5) Sakuma, T.; Fulford, J.; French, J. B.; Reid, N. M.; Thomson, Β. Α., presented at the 28th Ann. Conf. Mass Spectrom. Allied Top., New York, May 1980. (6) SCI EX, Summary Report prepared for the Ontario Ministry of the Environment, Air Resources Branch; 1980. (7) SCIEX, Report prepared for the Ontario Ministry of the Environment; ARB-TDA Report 05-80. (8) Lane, D. Α.; Thomson, B. A. J. Air Pollul. Control Assoc. 1980, 31(2), 122.
Douglas A. Lane is a senior research sci entist at SCIEX, Inc. His research at SCIEX has involved the development and application of atmospheric pressure chemical ionization mass spectrometric techniques to environmental problems. He was part of the design team that produced the mobile mass spectrometer system de scribed in this article; he subsequently field tested the unit for two and one-half years. He received his B.Sc. in chemistry from the University of Toronto in 1970 and his Ph.D. in analytical chemistry from York University in 1975.
