Anal. Chem. 1987, 59, 1664-1670
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molecular orientational order of lignin in cellulose exists over a macroscopic volume of about 0.9 cm3. Work is currently under way to enable one to quantitate the type and degree of order present in similar systems.
(10) Pines, A.; Gibby, M. G.;Waugh, J. S.J. Chem. Phys. 1973, 59, 569. (11) Schaefer. J.; Stejskal, E. 0. J. Am. Chem. SOC. 1978, 98, 1031. (12) Schaefer, J.; Stejskal, E. 0. Topics in Carbon-13 NMR Spectroscopy; Levy, G. C.. Ed.; Wiley: New York, 1979. (13) Veeman, W. S. Prog. Nucl. Magn. Reson. Spectrosc. 1984, 16, 193. (14) Browning, 6. L. The Chemistry of Wood: Robert E. Krieger Publishing Co.: New York, 1975. (15) Hatfieid, G. R.; Maciel, G. E.; Erbatur, 0.; Erbatur, G. Anal. Chem. 1987, 59, 172. (16) Vanderhart, D. L.; Atalia, R. H. Macromolecules 1984, 17, 1465. (17) Atalla. R. H.; Vanderhart, D. L. Science 1984, 223, 283. (18) Dudley, R. L. J. Am. Chem. Soc. 1983, 105, 2469. (19) Maciei, G. E.; Kolodziejski, W. L.; Bertran, M. S.;Dale, B. E. Macromolecules 1982, 15. 686. (20) Cote, W. A. Cellular Ultrastructure of Woody Plants: Syracuse University Press: Syracuse, NY, 1965. (21) Frey-Wyssling, A. The Plant Cell Wall; Borntraeger: Berlin, 1976. (22) Koilmann, F. F. P., Cote, W. A. Principles of Wood Science an Technology; Springer-Verlag: New York, 1968; Vol. 1. (23) Cote, W. A. Wood Ultrastructure; University of Washington Press: Seattle, WA, 1967.
ACKNOWLEDGMENT The authors thank Joseph Reibenspies for the crystallographic work and Cyril Heitner, Pulp and Paper Research Institute of Canada, for providing the oriented cellulose samples. Registry No. Lignin, 9005-53-2; cellulose, 9004-34-6;sucrose, 57-50-1;homovanillic alcohol, 2380-78-1. LITERATURE CITED (1) (2) (3) (4) (5)
(6) (7)
(8) (9)
Hentschel, R.; Sillescu, H.; Spiess, H. W. Polymer 1981, 22, 1516. Maciel, G. E. Science 1984, 226, 262. Herzfeld, J.; Berger, A. E. J. Chem. Phys. 1980, 7 3 , 602. Bax, A.; Szeverenyi, N. M.; Maciel, G.E. J. Magn. Reson. 1983, 5 5 , 494. Mehring. M. High Resolution NMR in Solids; Springer: New York, 1983. Maciel. G. E.; Szeverenyi, N. M.: Sardashti, M. J. Magn. Reson. 1985, 64,365. Preston, R. D. The Molecular Architecture of Plant Cell Walls; Wiley: New York, 1952. Fengel. D.; Wegener, G. Wood: Chemistry, Ultrastructure, Reactions; de Gruyter: Berlin, 1984. Atalla. R. H.; Agarwal, U. P. Science 1985, 227, 636.
RECEIVED for review December 8, 1986. Accepted March 3, 1987. The authors gratefully acknowledge partial support of this research by a grant from the Colorado State University Experimental Station. The Nicolet R3m/E diffractometer and computer system a t Colorado State University were purchased with funds provided by NSF Grant CHE-8103011.
Use of the Microwave-Induced Nitrogen Discharge at Atmospheric Pressure as an Ion Source for Elemental Mass Spectrometry Daniel A. Wilson,' George H. Vickers, and Gary M. Hieftje* Department of Chemistry, Indiana University, Bloomington, Indiana 47405
A microwave-induced nitrogen dlscharge at atmospheric pressure (MINDAP) is used as the Ion source for elemental mass spectrometry (MS)and compared to the use of the inductively coupled plasma ( ICP). Optimiratlon studies are presented to Illustrate the dependence of slgnais on various instrumental parameters. Detectlon llmits determhed for five elements range from 3 to 22 ng/mL, somewhat higher than those determlned with an ICP and the same mass spectrometer system. The background mass spectrum from the MINDAP ISdominated by NO'; oxide and hydroxide ion ratios are higher than for ICP-MS. The linear dynamic range is similar to that In ICP-MS, but Interferences caused by concomitant elements are much worse in MINDAP-MS.
Since the introduction of inductively coupled plasma mass spectrometry (ICP-MS) ( I ) , the technique has shown a great deal of potential for elemental analysis. Recent reviews ( 2 , 3 ) have described the advantages of ICP-MS, including a relatively simple mass-spectral background, the ability to perform isotope-ratio determinations, and sub-nanogramper-milliliter detection limits for many elements. 'Present address: Alcoa Technical Center, Bldg C, Alcoa Center,
PA 15069.
Although the ICP has been proven to be an attractive ion source for elemental mass spectrometry, it is important to investigate other types of plasmas as ion sources to assess whether further improvement in analytical characteristics is possible. Possible improvements that are desired include better sensitivity, greater freedom from interferences, ease of analytical use, lower operating and instrumental costs, and better precision. One type of discharge which has been used as an ion source is a microwave-induced plasma (MIP). In fact, the earliest work reported using continuum plasma sampling, the type most commonly used in ICP-MS at present, employed an argon MIP (4). This early work was later carried further and compared with ICP-MS (5). Detection limits in MIP-MS were about an order of magnitude lower than those from ICP-MS. However, matrix interferences were much more severe in the MIP than in the ICP, with the ICP being reported as matrix-free (5). More recent papers (6, 7) on ICPMS have revealed that matrix interferences are observed in that method also. Work with a microwave plasma as the ion source for mass spectrometry continues (8). These recent studies coupled both argon and helium MIP ion sources with a mass spectrometer adapted from a commercial gas chromatograph-mass spectrometer system. An alternative and attractive plasma which has been'used for atomic emission studies is the microwave-induced nitrogen discharge at atmospheric pressure (MINDAP) (9, IO). This
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plasma is similar to argon and helium MIPSbut is supported in nitrogen (at a relatively high flow rate compared to many MIPS) and at forward powers of 200-250 W. Atomic emission detection limits for the MINDAP were comparable to those obtained for the ICP or argon MIP. In general, atomic emission lines provided better sensitivity than ion lines and for most elements were significantly more sensitive than those from atom lines in the ICP; ion lines, however, were much less sensitive than in the ICP. Ionization and vaporization interferences were noted in the MINDAP but could be eliminated by the use of Cs as an ionization suppressant (for an ionization interference) or a releasing agent such as EDTA to suppress refractory-complex formation (to eliminate a vaporization interference). One of the most interesting features of the MINDAP from the standpoint of mass-spectral sampling is its generation of a long flamelike plume which extends several centimeters beyond the cavity and the torch. This tail flame permits sampling of gases and ions directly from a fairly hot portion of the plasma and thus provides less opportunity for ion recombination with ambient gases. Results obtained by sampling ions from the MINDAP and separating them in a quadrupole mass spectrometer are presented. Analytical characteristics that were assessed include mass-spectral background, precision, detection limits, the degree of formation of oxide and hydroxide ions, and interference effects. The linear working range is more than 4 orders of magnitude and detection limits range from 3 to 22 ng/mL for five elements. Interferences are a much more important problem in MINDAP-MS than in ICP-MS.
EXPERIMENTAL SECTION Instrument. Plasma System. The plasma instrumentation used here is similar to that described previously (9, 10). The power supply for the microwave plasma (Model 420, Micro-Now Instrument Co., Chicago, IL) operates at 2450 MHz, with a maximum power level of 500 W. Tuning stubs (Model DS109, Weinschel Engineering) were used to match the impedance of the plasma and cavity to that of the generator. The resonant structure which is used for coupling the power into the plasma is a modified Beenakker-type TMolo cavity (11). This cavity has a depth (thickness) of 1 cm and is not cooled. The torch used with the MINDAP, based on the ICP-torch design, consists of two concentric quartz tubes. The central tube has an inner diameter of 1 mm and an outer diameter of 2 mm and extends approximately halfway into the Beenakker cavity. The portion of the torch in the cavity is used as a secondary tuning device (9). The outer tube has a 4-mm i.d. and a 6-mm 0.d. and extends slightly past the end of the cavity. The central gas flow is approximately 300 mL/min while the outer gas flow is about 3 L/min. The gas flow into the outer tube is introduced from a side arm oriented tangentially to the tube and produces a gas-flow pattern similar to that in the ICP. Sample aerosol is introduced through the central tube of the torch and into the center of the plasma, which is in turn suspended in the middle of the torch by the tangential plasma gas flow. The tail flame of the plasma extends about 4 cm beyond the end of the cavity and can be unsymmetrical if the quartz tubes are not concentric. Even if the tail flame is not symmetrical, however, samples introduced into the plasma are observed both visually and mass spectrometrically to be concentrated in a small radial region of the discharge. Mass-spectral signals drop off by 90% within 0.25 rnm of the optimum sampling position. This sampling position must be optimized each time the plasma is ignited;&r a warmup period of approximately 30 min, the sample location within the plasma is stable. The plasma is then moved beneath the sampling orifice until the maximum analyte signal is obtained. Once the optimal position is determined, the same sampling location can be used continuously, until or unless the plasma has to be reignited. Sample-Introduction System. Because the central gas flow is relatively low in a stable MINDAP, a glass-frit nebulizer of the type described by Layman and Lichte (12)was used in this study.
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This nebulizer is reported to produce a high efficiency and small droplet-size distribution even at low nebulizing gas flows. The MINDAP is able to be sustained stably when aqueous aerqols are introduced into it. For the work performed here, however, a desolvation system was used to minimize the amount of water introduced into the plasma and to increase the stability of the plasma (9). The desolvation apparatus consists of a glass tube, 20 cm long by 2 cm diameter, wrapped with heating tape and followed by a 14 cm long condenser with a spiral condensing tube. The temperature of the heated tube was maintained at 120 "C by a Variac connected to the heating tape. The cooling water to the condenser was held at 5 "C by use of a recirculating cooler (Model CFT-33, Neslab, Newington, NH). This unit also provided cooling water for the mass spectrometer sampling-orifice plate and for the turbomolecular pumps. Vacuum System and Mass Spectrometer. The vacuum system and mass spectrometer are the same as those used previously for ICP-MS (13). The two-stage interface between the atmospheric-pressure plasma and the low-pressure mass-spectrometer region is similar to that used in commercial ICP-MS systems. Beyond this interface, ions are directed by ion lenses into the third region of the vacuum system, in which the quadrupole mass spectrometer (Model QMG 511, Bakers, Hudson, NH) is located. The discrete-dynode secondary electron multiplier used for detection of the ions is located off-axis from the mass spectropeter and is used in the ion-counting mode for this work. Procedures. Plasma Background Scans. Distilled, deionized water was used as the blank for mass-spectral scans of the plasma background. Plasma conditions were optimized for detection of an analyte (barium) before the scans were begun. The resolution of the mass spectrometer was increased slightly over the unit-mass setting routinely used for single-ion monitoring in order to provide better separation of adjacent mass peaks. The resolution setting on the QMG 511 was 52 for the scans compared to 46 usually used for single-ion monitoring studies. Scans were taken at the rate of 30 s/Da. Detection Limits. Detection limits were determined from signal-to-noise (S/N) ratios obtained for solutions over a concentration range within 1-2 orders of magnitude of the detection limit. For this purpose, S I N is defined as the background-corrected signal divided by the standard deviation of the backgfound noise, here approximated by one-fifth of the peak-to-peaknoise on the blank trace (14). Linear regression was used to obtain an equation for S/N w. concentration, from which the detection limit was obtained at SIN = 3. Examination of Ionization Effects. For determination of the effect of atomic ionization potential on elemental signals, 1 x 10-~ M of each element was introduced into the plasma, by means of a simple flow-injection system. Elements were chosen to provide a fairly wide range of ionization potentials. For each element, the plasma and mass spectrometer were optimized for analyte signal on its most abundant isotope. Distilled deionized water was then used as the flow-injection mobile phase while at least five replicate injections of the sample were made through a four-way valve with an 85-pL injection loop. Signals for the injections were a factor of about 3 lower than for continuous sample introduction, and relative standard deviations for the groups of injections were in the range 3-9%. Signals obtained for each element were corrected for the isotopic abundance of the isotope used, but no correction was made for mass bias. The mass bias curve for this instrument, published elsewhere (15),is fairly flat in the middle of the mass range, so elements from about 50 to 150 Da were used for the comparison. OnidelAtomic Ion Ratios. To measure oxide/singly charged ion ratios, the instrument was first optimized for determination of the major isotope of the singly charged analyte ion. Signals were determined for that isotope, adjacent lesser-abundance isotopes, and the oxide and hydroxide of the most abundant isotope. From these signals and literature values of the natural abundances, it wths possible to calculate dxide and hydroxide ratios corrected for interferences by oxides and hydroxides of the other isotopes. Interferences. The analyte species used for interference studies was calcium. Sodium, lead, and uranium at molar concentration ratios of 1ooO.1 Ca were studied for the effects of an easily ionized element and of a heavy concomitant element. The vaporization
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 13, JULY 1, 1987 I
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interference of phosphate on calcium was studied at a 5:l molar ratio. The solutions of elements being used as interferents were checked on the instrument for the presence of calcium, which was not found at levels which would be significant in this study. Because fairly high concentrations of interferents were present, the flow-injectionsystem was used in order to minimize washout time and solids deposition on the mass-spectrometer sampling orifice. The instrument was optimized for determination of calcium at 40 Da before the interferent solutions were run. Each injection of an interferent-containing solution was bracketed by two injections of a solution containing only calcium. The ratio of the signal for the interferent solution to the average of the calcium signals was calculated and four such replicates were averaged. This procedure corrects for instrumental drift.
RESULTS AND DISCUSSION Instrument Optimization. Studies were performed to determine the sensitivity of the instrument to four operating parameters: central (nebulizer) gas flow, forward plasma power, and two ion-lens potentials, termed extraction voltage and bias voltage. Other parameters such as sampling height, outer gas flow, and other ion-optic potentials were held constant. Sampling height was found not to have a large effect on the determined signal over a small distance range and changes in the nebulizer-gas flow could recover signal losses caused by height adjustments. Similar effects have been reported for ICP-MS (16). Outer gas flow cannot be altered a great deal in the MINDAP without causing the plasma to become asymmetric and producing damage to the torch; over the small permissible range of outer flows, ion signals are influenced only slightly. Small changes in ion-lens potentials (other than the bias or extraction voltages) were found also to have little effect on signal levels. Figure 1 shows the effect of nebulizer gas flow on the signal from a 5 pg/mL solution of barium. A convenient region where small changes in gas flow do not greatly affect the signal extends from about 0.45 to 0.52 L/min. The highest flow rate explored, 0.55 L/min, produced a 10% decrease in signal below the peak at 0.48 L/min; a t higher flow rates, the plasma became unstable and contacted the walls of the torch. The exact position of the optimum nebulizer flow rate varies with each plasma ignition and needs to be optimized somewhat for each element. However, the shape of the curve is consistent regardless of the exact position of the peak. The shape of the optimization curve for the MINDAP nebulizer gas is less sharply peaked than that seen in ICP-MS, both on the present instrument (17,B) and on a commercial system (16). This difference is probably due in large part to the fact that the efficiency of the glass-frit nebulizer is not as sensitive to gas flow as is the concentric pneumatic nebulizer often used for ICP-MS. In ICP-MS, the nebulizer gas flow can strongly affect both the nebulizer and the sampling conditions, possibly necessitating a compromise value for the flow.
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N E B U L I Z E R G A S FLOW lo%), but for the (M He)- ions the efficiencies were relatively low. Conversion efMendes were d#rerved to differ by as much as 2 orders of magnitude between anions. The major cause for these ditferences is atlrlbutaMe to dmerences in the propensity for collisional electron detachment vs. that for collision-activated dissociation.
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The performance characteristics of the ion trap or threedimensional quadrupole (1-4) have been dramatically improved in recent years with the development of the mass selective instability mode of mass analysis and the use of a light background gas (usually helium) at a pressure of =1 mtorr (4, 5). It has been found that collisions with the background gas tend to reduce large amplitude motions of the ions in the ion trap (6,7). The net effect of the background gas is to concentrate the ions in the center of the trap, thereby enhancing mass resolution and sensitivity. These developments have resulted in the recent commercial introduction of an ion trap as a detector for gas chromatography (8) and has generated interest in other possible uses of the ion trap as an analytical tool. Recently, for example, it has been demonstrated that two or more stages of mass spectrometry can be performed in a single ion trap (9) making available the technique of mass spectrometry/mass spectrometry (MS/MS) (10, 11). Most analytical applications of MS/MS use the technique of collision-activated dissociation (CAD) between stages of mass analysis in order to identify a mass-selected ion. CAD of positive ions has been demonstrated in an ion trap (9, 12) where an ac voltage (the frequency of which is tuned to excite only one m / z ratio) with an amplitude of a few hundred millivolts is applied to the end caps of the trap for a few tens of milliseconds. CAD can then result from 0003-2700/87/0359-1670$0 1S O / O
collisions between the translationally excited ions and the background gas. The two major objectives of this study were to form polyatomic anions in an ion trap and to determine the facility with which these ions undergo CAD using the conditions under which the ion trap is currently being operated for MS/MS. Although the widespread use of negative ions in analytical mass spectrometry is relatively recent, the high sensitivity and specificity obtained for the analysis of some compounds in the negative ion mode relative to the positive ion mode has been amply demonstrated (13). In an ion trap both positive and negative ions can be trapped simultaneously under conditions in which both ion polarities are formed (14). With trapping times on the order of tens to hundreds of milliseconds, it is conceivable that positive ion-negative ion recombination could reduce sensitivity for a compound ionized in an ion trap. The only negative ions reported to have been trapped in a three-dimensional quadrupole are I- (15) and CI(14). These ions were formed by photodissociation of thallium iodide and 50-eV electron impact of dichloromethane, respectively. I t has recently been demonstrated that positive ion chemical ionization could be readily performed in an ion trap using an appropriate pulse sequence (16, 17). We therefore chose to investigate the possibility of using negative ion chemical ionization (NICI) in an ion trap with molecules known to form very stable negative ions. We studied p-dinitrobenzene, 2,4-dinitrotoluene (DNT), 2,6-dinitrotoluene, 3,4-dinitrotoluene, 2,4,6-trinitrotoluene (TNT), and 1,3,5trinitro-1,3,5-triazacyclohexane(RDX). As a further test of the applicability of the ion trap to the study of negative ions we also studied the CAD of anions derived from these compounds.
EXPERIMENTAL SECTION All experiments were carried out with a Finnigan-MAT ITMS research ion trap (shown schematicallyin Figure 1)equipped with a DeTech Model 300 conversion dynode/electron multiplier detector (18). Helium was used as a background gas at a pressure of =1 mtorr. Samples were admitted via a solids probe and for most chemical ionization experiments water was admitted into the vacuum system to a pressure of =2 X lo" torr through a separate valve. NICI MS/MS spectra were obtained by using 0 1987 American Chemical Society
