Anal. Chem. 1990, 62, 1679-1686
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Neutralization of Sample Charging in Secondary Ion Mass Spectrometry via a Pulsed Extraction Field Anthony D. Appelhans,* David A. Dahl, and James E. Delmore Idaho National Engineering Laboratory, EG&G Idaho, P.O. Box 1625, Idaho Falls, Idaho 83415
A technique has been developed for preventing surface charging of electrically nonconducting samples In secondary lon mass spectrometry (SIMS) when a negative Ion or neutral primary beam (but not a positive ion prlmary beam) is used. Posltlve and negative particles are alternately extracted from the sample by rapldly reversing the polarity of the secondary ion extraction voltage such that the emisslon of positive and negatlve charge is balanced and the sample remains neutral. The technique is nonlntrusive, is applicable to ail materials, and will permit the interlaced acquisitlon of positive and negative secondary ion spectra. The technique Is demonstrated wlth a statlc-SIMS Instrument utilizing a negative Ion and neutral molecular prlmary beam. Both positive and negatlve secondary Ion spectra of several highly nonconducting samples are shown.
INTRODUCTION The charge that can build up on the surface of nonconducting samples under ion (and even neutral particle) bombardment in secondary ion mass spectrometry (SIMS) often has significant detrimental effects on the quality of secondary ion spectra and can completely degrade the signal. A variety of methods and techniques have been applied to reduce or compensate for this problem, including placing conducting grids over the sample (1,2),flooding the sample with electrons (2-4), and using neutral or negative ion primary beams (2, 5-8). While all of these methods have been shown to work, some better than others, they all have negative aspects. Conducting grids contribute additional background signal to the spectra, may introduce contamination, and interfere with imaging. Electron flooding requires the additional equipment of the electron gun, the added complexity of balancing the electron flux and the primary ion flux, and high-energy (over a few hundred electronvolts) electrons contribute to sample damage (3), thus reducing the useful analysis time. Low-energy (few electronvolts), pulsed, electron beams are quite efficient at reducing charging in pulsed static-SIMS applications, but they still complicate the instrument and its operation. Neutral and negative ion primary beams can significantly reduce charging for many materials, but problems still exist (8, 9). In a previous short communication (IO), we reported the preliminary results of tests of a new technique for overcoming surface charging in a totally nonintrusive way. This technique involves alternately extracting negative and positive particles by rapidly reversing the polarity of the secondary ion extraction lens. In this paper, we present a complete discussion of the technique and present additional data that demonstrate the utility of the technique for a variety of sample materials commonly plagued by surface charging. Since all materials, when bombarded by kiloelectronvolt particles, emit both positively charged particles (ions) and negatively charged particles (ions and electrons), the net charge that accumulates on the surface of a sample will be a function of the resistivity of the bulk and surface materials 0003-2700/90/0362-1679$02.50/0
and the flux of the incoming and outgoing charged particles. For the energy range typical for static-SIMS primary beams (3-30 keV) and for a field-free sample region, the secondary electron flux is typically greater than the primary beam flux, which is greater than the secondary ion flux (11). It is this imbalance that results in sample charging. The outgoing flux of charged particles can be altered by imposing a potential gradient between the sample and the secondary ion extraction lens, and thus the sample can be charged either negatively or positively, depending upon the conditions (i.e., the instrument design). By taking advantage of the fact that both positive and negative particles are ejected, the sample surface charge can be regulated by controlling the amount of time that positively charged particles are extracted relative to the time negatively charged particles are extracted. This forms the basis for the new technique. The secondary ion extraction field is cycled from positive to negative, and the dwell time a t each polarity is varied so that the net flux of charge over one cycle is zero. In this way, the amount of charge that builds up on the sample is controlled and the corresponding potential kept within the range of the secondary ion optics and mass spectrometer. We show below that the technique is applicable in general when neutral or negative ion primary beams are used but will not work for positive ion primary beams unless the secondary positive ion yield is >1 (we know of no materials for which this is the case). Charge Flow Analysis. The buildup of charge on a sample surface is the result of an imbalance in the net incoming and outgoing charged particle flux as well as the inability of charge to move through the sample (poor conductivity) to correct the imbalance. The rate a t which charge accumulates on a sample surface (perfect insulator) can be expressed in differential form as where Q, is the surface charge, Pi,and J-in are the primary is the flux of negative ions and beam flux components, J-out electrons leaving the surface, Pout is the flux of positive ions leaving the surface, and there is no contribution from charges moving through or across the surface of the sample (perfect nonconductor). When dQ,/dt is equal to zero, there is no net charge accumulation and the surface charge (if any) remains unchanged. In the following, eq 1 is evaluated for each combination of primary beam polarity and secondary ion extraction field polarity to illustrate the various charging conditions that result. In all cases, ideal conditions are assumed (perfect nonconductor and total suppression of any particle whose polarity is the same as the extraction field). Positive Ion Primary Beam. In the most common mode of SIMS (positive ion primary beam and extraction of positive secondary ions), J+h > 0, J-i, = 0, and the negative potential extraction field inhibits ejection of electrons and negative ions (Le., J-out = 0), and since Pin > Pout for known materials (positive ion yield Pout * positive charging condition. For common 0 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1, 1990
polymers (Teflon, acrylic, Mylar, etc.), this can result in a high surface potential (hundreds of volts) after only a few seconds causing defocusing of the primary beam and of irradiation (9), gross distortion of the secondary ion energy distribution. When negative ions are extracted (positive potential extraction field) while a positive ion primary beam is used, this situation is exacerbated, since now the extraction of secondary electrons and negative ions drives the sample surface potential > 0 and Pout = 0). In both cases, even more positive (J-out the sample surface potential will be driven positive unless > 0), hence the common use negative charge is added (J-ln of electron flood guns when the primary beam is positive. If the extraction field is zero, that is, the region between the entrance to the secondary ion lens and the sample is "field free", then the degree and polarity to which the sample charges will be dependent upon the balance of the fluxes (eq 1). For a positive ion primary beam, the sample would still charge and Sou, > Pout). positive (typically J+,, > J+out Neutral Primary Beam. If a neutral primary beam is equal zero, and the resultant charging employed, Pi,and Sin rate will be determined by the relative positive secondary ion and negative ion and electron yields:
(3) dQS/dt = J-out - J+out Under positive secondary ion extraction conditions, the sample will tend to charge negative (secondary electrons and negative ions are repelled back onto the sample by the negative extraction field), while under negative secondary ion extraction conditions, the sample will charge positive. If the extraction region was field free, the charging rate and polarity would be dependent upon the relative secondary particle yields (eq 3) but would usually be driven positive (typically J-,,, > Pout due to secondary electron emission). Negative Ion Primary Beam. Using a negative ion pri= 0) mary beam and extracting positive secondary ions (Zout will drive the sample potential negative, while in the negative = 0), the potential will secondary ion extraction mode (Pout depend upon the relative secondary negative ion and electron yield: dQs/dt = (0 - J-iJ - (J+o,t - J-out) = J-out - (J-in + J+o,J (4) For a field-free extraction region, the sample would usually charge slowly positive (typically Sout > S i , + J+,,t). In summary, for positive ion bombardment, the sample potential is driven positive under both positive and negative secondary ion extraction conditions and the only means for inhibiting this is to add electrons. However, for negative ion or neutral primary beams, the sample potential is driven one direction (polarity) when positive ions are extracted and in the opposite direction (polarity) when negative ions are extracted, and thus the sample potential can be controlled by alternating the extraction field polarity. Constraining Conditions. Figure 1 summarizes the methodology, in which the voltage on the secondary ion extraction lens is alternated, resulting in periods when either positive ions or negative ions and electrons are extracted. The extraction field has the following characteristics: (1) the period of a cycle is scaled to one; (2) for the period k , negative field, positive ions are extracted; (3) for the period 1- k , positive field, negative ions and electrons are extracted; and (4) 0 < k < 1 denotes the usable range of k values. For an extraction field with the above characteristics, restating eq 1 to include k dQ,/dt = ((1- k ) S 0 , t - $in) - (kJ+,,t - P i , ) (5) and solving for the value of k for which dQ,/dt = 0 (noncharging condition)
k =
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from eq 6, in order to maintain sample neutrality with a neutral primary beam, it is clear that k = (rFOUJ / (J+,,t + J-out) (7) Since this ratio is always within the bounds 0 < k < 1, it is possible to maintain neutrality with any sample. When a negative ion primary beam is used, eq 6 reduces to For materials for which J-out> J-h (combined secondary electron and negative yield > l),k is within the bounds 0 < k < 1, indicating that this technique will work with negative ion primary beams. Note that there is little data on kiloelectronvolt ion-induced secondary electron yields for materials other than selected metals and metal oxides (used for conversion dynodes in electron multipliers). However, our experience with a variety of organic and inorganic polymers, powders, and glasses indicates that the yield must be 21 since the pulsed technique works. For a positive ion primary beam, eq 6 reduces to and for this technique to work (i.e., for k to fall within the bounds 0 < k < 11,the secondary positive ion yield must be >1 (i.e., Pout > Pin), for which no materials are known to us (cf. ref 11). Thus it is unlikely that this technique would be useful for positive ion primary beams. Since the secondary ions and electrons are emitted over a broad energy range ( I I ) , the strength of the extraction field also plays a major role. The fraction of emitted ions and electrons that are repelled back onto the sample, and where they return to on the sample, is a function of the magnitude of the extraction field, as well as its polarity. Thus the extraction field magnitude is important in controlling sample charging. Although the foregoing analysis assumed ideal conditions, the conclusions remain valid in practice, as will be demonstrated. Pulsing Frequency. Practical implementation of the proposed technique involves using an asymmetric, bipolar, square-wave-shaped secondary ion extraction voltage such that the extraction field varies between positive and negative in the correct proportion to produce an average charging rate of zero. The required frequency of the alternating extraction voltage is governed by the time constant of the surface charging. The time constant is controlled by the sample material properties (conductivity and ion and electron
ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1, 1990
emission) and eq 1. The effects of charging on secondary ion detection are a function of the characteristics of the secondary ion mass spectrometer and ion optics (width of the energy acceptance window); as the charge induced voltage (potential) at the sample surface changes so will the secondary ion energy distribution and thus the transmission efficiency of the ion optics and mass analyzer. As long as the change in sample surface potential does not move the secondary ion energy distribution out of the acceptance window, there will be no loss in signal if the potential changes slightly. Note that the energy distribution for organic molecular ions is not well characterized and can probably vary significantly from one molecule to the next; thus one would not expect a sharp cutoff (drop in signal) resulting from a slight change in sample potential. In summary, the practical time constant is dependent on the primary beam flux, the sample material (secondary ion and electron yields and energy distributions), and the energy window of the analyzer. If the practical time constant (for charging) is long with respect to the time necessary to alternate the extraction field from positive to negative, the sample can be kept at an effectively neutral potential with no effect on the quality of the signal. If the practical time constant is short with respect to the time necessary to alternate the extraction field (i.e., if the secondary ion energy distribution can shift beyond the bounds of the analyzer acceptance window during a measurement period), then the signal quality will be compromised. In theory, one would like to use the lowest frequency possible so that the dead time that occurs during the voltage switch is as small a fraction of the data acquisition time as possible. Our experience to date indicates that the practical time constant for static-SIMS conditions is long enough that an acceptable frequency (- 20 Hz) can be easily achieved. Interlaced Spectra. In addition to preventing excessive charge buildup, the pulsed extraction technique will permit the interlaced collection of positive and negative secondary ion spectra. By alternating the mass analyzer from positive to negative mode a t the same frequency that the extraction optics are being alternated, the positive ions can be measured during times of negative extraction voltage and negative ions measured during times of positive extraction voltage. We demonstrate later in this paper that this should be possible with a quadrupole mass spectrometer and feel that it could also be implemented on a time-of-flight system. The ability to collect near-simultaneous positive and negative ion spectra would enhance the ability to follow transient events, increase the specificity, and reduce the analysis time where both polarity spectra are needed. This has been demonstrated by using a chemical ionization source with a quadrupole mass spectrometer by Hunt et al. (12). Preliminary studies (IO)using this technique showed good results for positive secondary ion analysis using negative ion, neutral, and combined negative ion/neutral primary beams. In the following, we present results of more detailed measurements demonstrating (1) the production of high-quality negative secondary ion spectra of bulk plastics, (2) the analysis of residual chemicals on plant leaves, and (3) the negative and positive ion spectra of plastics taken under conditions representative of those necessary to produce interlaced positivefnegative ion spectra. In addition, the trajectories of secondary particles under various extraction field conditions have been calculated and are shown.
EXPERIMENTAL SECTION The SIMS apparatus employed in these experiments has been described in more detail elsewhere (8, 10, 13-15). A schematic diagram of the system is given in Figure 2, and a brief description follows. The ion source produces a beam of SF6-anions that are focused and accelerated into the flight tube. Within the flight tube -25% of the anions autoeject an electron, resulting in a beam
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Figure 2. Diagram of the experimental system. composed of SF,- and SF,' molecules. By using the deflection electrode at the end of the flight tube, either the anions can be deflected, allowing only neutrals to strike the sample, or the combined SF,'/- beam can strike the sample. Only the combined neutral/negative ion beam was used in this study (see ref 10 for results obtained with pure neutral and pure negative ion primary beams). The primary beam strikes the sample at 30' relative to normal to the sample surface. Beam currents were typically -20 PA; the beam was -2 mm in diameter and had an energy of 10 keV. In all cases, static-SIMS conditions were maintained (primary currents less than -20 PA, exposure times of minutes). The samples were mounted on a grounded stainless steel sample probe that permitted X,Y , 2,and rotational translation of the sample. The secondary ion extraction lens was oriented normal to the sample surface -4 cm from the sample. The primary beam was shielded from the extraction lens by an outer ground shield (on the lens), and the first lens element (extraction element) consisted of an 80% pass wire grid typically operated between 60 and 200 V. Thus the sample was immersed in a field of from 15 to 50 V/cm. The extraction lens was controlled with a pulser capable of applying an alternating, asymmetric square wave to the voltage-programming circuit of the lens power supply, independent of the data system. This permitted the lens to be operated with either a constant voltage (constant mode) or an alternating voltage (pulsed mode). The duration and magnitude of each phase (polarity) of the square wave were independently controllable and were monitored with an oscilloscope to confirm the amplitude, frequency, and positive/negative dwell time ratio of the pulse (duty cycle). The pressure in the sample chamber Torr during the measurements. was 50 V/FS). Indeed quadrupole systems have been pulsed at up to 10 kHz (12),and interlaced positive/negative ion spectra have been successfully acquired with a chemical ionization source. Thus, there appears to be no instrumental limitation for applying the technique to quadrupole-based imaging static SIMS (other than efficiency (4, 17)). The application of this technique to dynamic SIMS would require significantly higher pulsing frequencies. The higher primary beam flux commonly employed in dynamic SIMS would result in very fast charging rates, necessitating pulse rates faster than commercially available power supplies can presently accommodate. The problem would stem from the fact that for a significant portion of each pulse period the sample would have charged to a value outside the acceptance window of the analyzer. This is shown schematically in Figure 7, in which the shaded regions indicate the times when the secondary ion energies are within the range of the analyzer. T o simulate this, we collected spectra a t pulse rates of 1 Hz, where we knew that the sample was charging to a point
ANALYTICAL CHEMISTRY, VOL. 62. NO. 15. AUGUST 1, 1990
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Flgure 7. Hypothetical charging of a sample undergoing pulsed extraction in which the sample potential deviates beyond the energy acceptance window of the secondary ion optics. Voltage COnt0"iS
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(off-normal).
outside of the acceptance window of the quadrupole within each phase of the pulse cycle. This resulted in lower count rates and a reduced signal-to-noise ratio hut still produced usable spectra. Ions were transmitted into the mass spectrometer only during the fraction of each phase of the cycle for which the sample was within the energy window (shaded regions in Figure 7). Obviously the signal-to-noise ratio is going to decrease in proportion to the percent of time the sample is outside the limits of the energy window, hut it is clear that the technique can still he applied. Thus, for dynamic-SIMS applications, pulsed extraction could still be applied, and with appropriate electronics (gating), the signal-to-noise ratio could he maintained at an acceptable level. The second area of concern, microcharging, arises when the application of this technique to imaging static SIMS is considered. Charged particles that are initially ejected from the surface and repelled hack by the extraction field will obviously not return to their point of origin (except those few ejected exactly normal to the field lines or with very low energy). For a microfocused primary beam, this may lead to development of a microgradient (electric field) around the point where the primary beam is sputtering. This microgradient may cause microlensing effects that degrade the secondary ion signal. SIMION (18) was used to model the trajectories of charged particles ejected over a range of energies and angles. A three-dimensional, cylindrically symmetric model scaled to the experimental system was used. In the examples shown in Figures 8 and 9, the extraction lens was biased positive and the potential surfaces are shown as a positive ion would experience them (note that the field would look the same to an electron if the lens polarity was reversed). Figure 8 shows the
30-
SIMION PC/PS2 generated bajectwies for (lefl) I O e V ions ejected from the center of the sample at 5' increments; (middle) 10and 30eV ions ejected 1 mm off-center in 10' increments ranging from 40 to -60' (off-normal):and (right)ions ejected from the center of the Samole at 15' off-iumnal wilh eneraies ranaino from 0.1 to 55.1 eV in 5-eV increments. Figure 9.
SIMION model along with the calculated constant-potential lines (top) and a potential energy surface view of the sample region (bottom). In addition, a series of trajectories are shown (bottom) for ions ejected with 20 eV of energy at 5' increments (off-normal) from the center of the sample. Figure 9 shows an expanded view of the sample region for ions ejected with 10 eV a t 5O increments (left), two sets of trajectories for ions (or electrons) ejected off-center a t 10 and 30 eV in 10' increments (middle), and a potential energy surface with trajectories of ions ejected a t 15' (off-normal) in 5-eV energy increments (right). Most notable about all of these calculations is that ions ejected a t angles greater than 5O (off-normal), with energies above a few electonvolts, return to the sample a significant distance from where they were ejected, typically on the order of millimeters away. For a constant primary beam current, as the primary beam diameter is decreased, the ejected charged particles leave from a smaller area yet are distributed to essentially a constant (much larger) area. Thus, as the primary beam diameter is decreased, the local field gradient may increase and the possibility for disturbing the focusing of the ejected ions may increase. To reduce the field gradient, a higher pulsing frequency would be necessary. We have ohserved no significant changes in performance using primary beams over the range of 0.5-5 mm in diameter; however, further experimental studies are needed. While the results presented herein do not specifically address the quantitative mechanistic aspects of charge buildup, they do provide for some interesting observations. The fact that the surface charge of insulators can apparently stabilize (at near zero) under steady-state conditions (for example, when a neutral primary beam is used) implies that the outgoing fluxes of positive and negative particles are in balance. This is believed to he the reason why many nonconductors could he successfully analyzed by using the pure neutral primary beam (8,9) and steady-state extraction of positive ions. When the sample is immersed in an electric field for extracting positive ions, most negative ion and electron emission is suppressed (the degree dependent upon the potential gradient at the sample surface). If all negative particle emission were suppressed, the sample would charge negative; however, since the potential gradients typical of the extraction fields (15 Vjcm over 4 cm) used in these studies are on the order of the secondary ion and electron kinetic energies, a significant number of higher energy negative particles can escape the sample. Thus, if the extraction gradient can he appropriately matched to the secondary negative particle emission energy distribution so that enough negative particles escape to offset the emission of positive particles, the sample will remain neutral, or a t some constant potential. (This is not possible when negative ions are extracted (by using a neutral primary
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Anal. Chem. 1990. 62. 1686-1690
beam) since the negative particle emission (ions + electrons) is greater than the total positive ion emission.) Indeed our experience, using the SF,O beam and a constant extraction field, has been that the samples are more stable (charging is less of a problem) when the extraction fields are low (10-15 V/cm over 4 cm), allowing more leakage of high-energy negative particles as compared to when the extraction fields are high (30-50 V/cm). This implies that control of the extraction field potential is an important function for any system in which charging is a problem and that the extraction field potential can be used to optimize instrument performance. The basic pulsed extraction technique is simple to implement and easy to use, as only the front lens of the secondary ion optics needs to be pulsed and this can be accomplished with a simple pulser. For mass analyzers, such as quadrupoles and, perhaps, time-of-flights that can be quickly alternated between positive and negative ion analysis, the collection of near-simultaneous (interlaced) positive and negative ion spectra should be possible. We plan to implement the necessary capabilities to demonstrate this with a quadrupole system in the future. This capability could be important in areas such as polymer analysis since the different polarity spectra often contain different information and electron-induced damage can be serious. In addition, utilizing both phases of the pulsed extraction would keep primary ion fluence to a minimum, an important consideration when mapping of molecular distributions (imaging) is performed. An advanced version would lend itself to computer-based optimization schemes for adjusting the positive/negative dwell time ratio, making it essentially transparent to the user. We have filed a patent application based on this work (19).
LITERATURE CITED Werner, H. W.; Warmoltz, N. J. Vac. Sci. Technol., A 1984, 2 , 726. Benninghoven, A.; Rudenauer, F. G.; Werner, H. W. Secondary Ion Mass Spectrometry; John Wiley & Sons: New York, 1987; pp 88 1-899. Briggs, D.; Wootton, A. E. S I A , Surf. Interface Anal. 1982, 4 , 109. Eccles, A. J.; Vickerman, J. C. J. Vac. Sci. Technol., A 1989, 7 , 234-244. Brown, A.; van den Berg, J. A.: Vickerman, J. C. Spectrochim. Acta 1985. 408. 871. van den Berg, J. A. Vacuum 1988, 36, 981. Eccles, A. J.; van den Berg, J. A.; Brown, A,; Vickerman, J. C. J. Vac. Sci. Technol., A 1988, 4 , 1888. Appelhans, A. D.; Delmore, J. E. Anal. Chem. 1987, 59, 1685. Appelhans, A. D. Int. J. Mass Spectfom. Ion Processes 1989, 88, 161-1 73. Appelhans, A. D.; Dahl, D. A.; Delmore, J. E. Rapid Commun. Mass Spectrom. 1989, 3 , 356-359. Benninghoven, A.; Rudenauer, F. G.; Werner, H. W. Secondary Ion Mass Spectrometry; John Wiley & Sons: New York, 1987; pp 157-172, 726-727, and 1102-1106. Hunt, D. F.; Stafford, G. C.; Crow, F. W.; Russel, J. W. Anal. Chem. 1978, 4 8 , 2098-2104. Delrnore, J. E. Int. J. Mass Specfrom. Ion Processes 1988, 51, 191. Delmore, J. E.; Appelhans, A. D.; Dahl, D. A. Rev. Sci. Instrum. 1990, 6 7 , 633-635. Appelhans, A. D.; Delmore, J. E. Anal. Chem. 1989, 67,1087-1093. Briggs, D.; Brown, A.; Vickerman, J. C. Handbook of Static Secondary Ion Mass Spectrometry; John Wiley & Sons: New York, 1989. Briggs, D.; Hearn, M. J. S I A , Surf. Interface Anal. 1988, 73, 181. Dahl, D. A.; Delmore. J. E.; Appelhans. A. D. Rev. Sci. Instrum. I9gO. .- - -, 61 - . , 607-609 - - . - - -. Appelhans, A. D.; Dahl, D. A.; Delmore, J. E. US. Patent Application 375,442, DOE Case S-69,345, 1989.
RECEIVED for review December 8, 1989. Accepted March 12, 1990. Work was supported by the U.S. Department of Energy INEL internal research funds and by the Office of Health and Environmental Research, Office of Energy Research, DOE, under Contract 4AA901.
Liquid Chromatography/Particle BeamIMass Spectrometry of Polar Compounds of Environmental Interest Thomas D. Behymer, Thomas A. Bellar, and William L. Budde*
US.Environmental Protection Agency, Office of Research and Development, Environmental Monitoring Systems Laboratory, 26 W . Martin Luther King Drive, Cincinnati, Ohio 45268
High-performance llquld chromatography (HPLC)/particie beam (PB)/mass spectrometry (MS)was studied to determine Its suitablilty as a major component of a general purpose, broad-spectrun anatytlcal method for the determination of nonvolatile organic compounds In envlronmentai samples. A reverse-phase gradlent elution HPLC separation was developed tor a test mlxture of compounds that are hrsufflclentiy voiatlie for gas chromatography. Two commercial PB interfaces were used, and retention data were collected for these compounds of environmental interest. Other performance factors studied included tuning and signal optlmlratlon, instrument detection Hmits, linear caiibratlon range, and shortterm Ion intensity stablilties.
INTRODUCTION In research to develop an improved liquid chromatography/mass spectrometry interface, Willoughby and Browner ( I , 2) demonstrated promising performance characteristics for a system consisting of an aerosol generator, a desolvation This artlcle not subject to
chamber, and a particle beam momentum separator. This design was named MAGIC, but with the commercial development of several variations (3, 4 ) of MAGIC, it became known simply as a particle beam interface. This HPLC/MS interface introduced a new approach to the determination of environmental contaminants that are not amenable to capillary column gas chromatography (GC)/mass spectrometry (nonvolatiles). Previously, we reported results of a research program to explore approaches to the development a broad spectrum HPLC/MS method for the determination of nonvolatile organic environmental pollutants ( 5 ) . In that work, the thermospray (6) HPLC/MS interface was used with NH,+-OAcassisted ionization. This technique gave spectra and retention time data that were adequate for target compound analyses. Furthermore, method detection limits were generally in the range (1-25 gg/L) required for environmental analyses. But thermospray HPLC/MS, because of the relatively soft ionization process, did not provide the variety of structurally significant fragment ions characteristic of conventional electron ionization (EI) mass spectrometry. Therefore, thermospray HPLC/MS is not as useful as E1 for the identification
U S . Copyright. Published 1990 by the American Chemical Society
