Anal. Chem. 1985, 57,2907-2911
Cut, 17493-86-6;Ag, 7440-22-4;Agt, 14701-21-4;Sn, 7440-31-5; Sn', 26288-30-2;Al,7429-90-5;Alt, 14903-36-7;Zn, 7440-66-6;Zn', 15176-26-8.
LITERATURE CITED Grimm, W. Naturwissenschaften 1967, 54, 588. Grlmm, W. Spectrochim. Acta, Part B 1988, 238,433. Berneron, R.; Charbonnier, J. C. SIA, Surf. Interface Anal. 1981, 3, 134. Waitievertch, M. E.; Hurwitz, J. D. Appl. Spectrosc. 1976, 30, 510. Belle, C. J.; Johnson, J. D. Appl. Spectrosc. 1973, 27, 118. Takadoum, J.; Pivin, J. C.; Pons-Corbeau, J.; Berneron, R.; Charbonnier, J. C. S I A , Surf. Interface Anal. 1984, 6, 175. Oechsner. H. Appl. Phys. 1975, 8, 185. McDonald, D. C. Spectrochlm. Acta, Part B 1982, 378,747. Moore. C. E. Natl. Bur. Stand. Circ. ( U S . ) 1949. No. 467. Herzberg, G. "Molecular Spectra and Molecular Structure"; Van-Nostrand Reinhold: New York, 1950. Dogan, M.; Laqua, K.; Massmann, H. Spectrochim. Acta, Part 1971, 268,631. Laegreld, N.; Wehner, G. K. J. Appl. Phys. 1981, 32,365. Rosenberg, D.; Wehner, G. K. J . Appl. Phys. 1962, 33, 1842.
2907
(14) Wagatsuma, K.; Hirokawa, K. SIA, Surf. Interface Anal. 1984, 6, 167. (15) Wagatsuma, K.; Hirokawa, K. Anal. Chem. 1984, 56, 2024. (16) . . Pearse, R. W. 6 . ;Gaydon, A. G. "Identification of Molecular Spectra"; Chapman and Hall: London, 1965. (17) Herzbera, G. "Atomic Spectra and Atomic Structure"; Dover Pubiications: N>w York, 1944.' Bochkova, 0. P.; Shreyder, E. Y. a. "Spectroscopic Analysis of Gas Mixtures"; Academic Press: New York, 1965. Mitchell, A. C. G.; Zemansky, N. W. "Resonance Radiation and Excited Atoms"; Cambridge: New York, 1971. Copley, G. H.; Lee, C. S. Can. J . Phys. 1975, 53,1705. Ellis, E.; Twiddy:,N. D. J. Phys. B 1989, 2, 1366. von Engei, A. Ionized Gases"; Oxford University Press: Oxford, 1965. Strauss, J. A.; Ferreira, H. P.; Human, H. G. C. Spectrochim. Acta, Part B 1982. 378,947.
RECEIVED for review May 20, 1985. Accepted July 17, 1985. We are grateful to Nissan Science Fundation for the financial support of our work.
Determination of Trace Metals in Seawater by Inductively Coupled Plasma Mass Spectrometry with Preconcentration on Silica-Immobilized 8-Hydroxyquinoline J. W. McLaren,* A. P. Mykytiuk, S. N. Willie, and S. S. Berman Analytical Chemistry Section, Chemistry Division, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR9
The application of Inductively coupled plasma mass spectrometry (ICP-MS) to the determlnation of eight trace metals In a coastal seawater reference material is described. Accurate calibration has been achleved for Mn, Co, NI, Cu, Zn, Cd, and Pb by standard additions techniques, while stable Isotope dilution has been applied for Cr, Ni, Cu, Zn, Cd, and Pb. I n both cases the trace metals were separated from the seawater and concentrated 50-fold by adsorption on sliicalmmobllized 8-hydroxyqulnoline prior to Instrumental determinatlon. Detection limits are in the range from 0.2 to 2 ng L-', low enough to permit the analysis of even open ocean samples.
Perhaps the single feature of inductively coupled plasma mass spectrometry (ICP-MS) most responsible for the tremendous interest in this new technique, especially among present users of inductively coupled plasma atomic emission spectrometry (ICP-AES), is its remarkable detection power. Current ICP-MS detection limits in dilute aqueous acid solution are in the range from 0.01 to 0.1 Mg L-' for most elements (1, 2), often 1-3 orders of magnitude lower than for ICP-AES. Whether such impressive detection limits can be achieved in solutions of higher dissolved salts concentrations is the subject of much current ICP-MS research. Already, though, ICP-MS is making an impact in applications to samples of low dissolved salts concentrations for which ICP-AES detection limits have previously been found to be inadequate. The example provided in this report concerns the determination of trace metals (in a 1 M HC1/0.1 M "OB acid mixture) separated from seawater by adsorption on silicaimmobilized 8-hydroxyquinoline (I-8-HOQ). Previously reported determinations of trace metals in seawater by ICP-AES have almost invariably involved a prior
separation by solvent extraction (3-6), coprecipitation (7),ion exchange (8), or adsorption (9-11), not only to avoid the difficulties of introducing a 3.5% salt solution to the ICP but, more importantly, to concentrate the trace metals sufficiently to permit their determination. Concentration factors of at least 100, and preferably 200-500, are required for analysis 7, of unspiked and uncontaminated seawater samples (4,6, 11). Such large concentrations are often rather time-consuming, and difficulties in controlling the blank can be severe. Determinations of trace metals in seawater by ICP-AES have been accomplished in our laboratory by two techniques. The first approach involved separation and 25-fold concentration of the trace metals by means of a chelating ion exchange resin, Chelex-100,and introduction of the concentrates to the ICP by means of an ultrasonic nebulizer with an aerosol desolvation system (8). This method permitted the determination of Fe, Mn, Cu, Ni, and Zn in coastal seawater samples but was inadequate for determination of Cd and Pb. Subsequently, we abandoned ultrasonic nebulization and instead used a more efficient separation procedure, developed by Sturgeon et al. ( l o ) ,which involves adsorption of the metals on silica-immobilized8-hydroxyquinoline (I-8-HOQ) followed by elution with a 1 M HC1/0.1 M HNOBacid mixture. By this means, Cu, Cd, Mn, Ni, V, and Zn were determined in an open ocean seawater sample after a 225-fold preconcentration, but lead could not be determined and the concentrations of several of the other metals were uncomfortably close to the detection limits ( 2 1 ) . The recent development of a coastal seawater reference material, with the acronym CASS-1, provided an ideal opportunity to assess the performance of ICP-MS in an application which remains difficult by ICP-AES because of inadequate detection power, and also to assess its capability for stable isotope dilution analyses, previously accomplished in our laboratory by spark source mass spectrography (22).
0003-2700/85/0357-2907$01.50/00 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
Table I. ICP-MS Hardware and Operating Conditions ICP plasma Ar auxiliary Ar nebulizer Ar rf power
12 L min-’
2.0 L min-’ 1.2 L min-’ 1.3 kW
“Long” torch (SCIEX standard) Meinhard “C” concentric glass nebulizer Scott-type spray chamber sampler skimmer
Mass Spectrometer nickel, 0.94 mm orifice nickel, 0.89 mm orifice
operating pressures
interface region, -1 torr mass mectrometer chamber, -4
X
low5torr
EXPERIMENTAL SECTION Instrumentation. The inductively coupled plasma mass spectrometer used fQr this work was the ELAN 250 from SCIEX Division of MDS Health Group, Ltd. (Thornhill, Ontario, Canada). The only modification made by us was the addition of a mass flow controller (Model 5850, Brooks Instrument Division, Emerson Electric, Hatfield, PA) to the aerosol carrier gas line and a peristaltic pump (Minipuls 11, Gilson Medical Electronics, Inc., Middleton, WI) to the sample delivery tube. Other details of the hardware and operating conditions are listed in Table I. The ICP torch provided with the instrument was approximately 15 mm longer than a conventional ICP-AES torch. It was positioned such that the sampler orifice was 30-35 mm above the load coil. Reagents. All reagents for synthesis of the silica-immobilized 8-hydroxyquinoline (I-8-HOQ) and the subsequent seawater processing were purified prior to use as described in ref 10. Enriched isotopes purchased from the Oak Ridge National Laboratory included 63Cr,61Ni,@Cu,67Zn,lllCd, and mPb. These stable isotopes were received as metals or oxides (lead as the nitrate) with an isotopic enrichment greater than 95%. Stock solutions of approximately 100 mg L-’ of each were prepared by dissolution of an accurately weighed quantity of the material in nitric acid and dilution to volume. Cr203was dissolved by prolonged digestion with several milliliters of perchloric acid. The concentrations of the spike solutions were checked both by atomic absorption spectrometry and by reverse spiking and found to be within 3% of the calculated values. The nearshore seawater reference material CASS-1 was collected in the outer part of Halifax harbor at between 10 and 20 m in water 25-40 m deep, 40C-800 m offshore, about 10 km south of the inner harbor. The salinity is 31.8%0,and total dissolved organic carbon content is 0.5 mg L-l. The seawater was filtered immediately after collection through 0.45-pm Millipore filters and acidified to pH 1.6 with high purity nitric acid. Preconcentration Procedures. All sample preparations were carried out in a clean laboratory equipped with laminar flow benches and fume cupboards providing a class 100 working environment. I-8-HOQ was prepared as described in ref 10. The preconcentration procedure has been described in detail in ref 10. Briefly, it involves the passage of an aliquot of seawater (500 mL in this work), after pH adjustment to 8.0 k 0.2 with high-purity ammonium hydroxide, through a column of 600 mg of the I-8-HOQ at a flow rate of about 10 mL min-l. The column is then washed free of seawater with two 10-mL aliquots of deionized distilled water at pH 7.0, after which the trace metals are eluted with 10 mL of a 1 M HC1/0.1 M HNOBacid mixture. This general procedure was used for both standard additions ICP-MS analyses and isotope dilution ICP-MS. In the former case, each analysis involved processing of three 500-mL aliquots of seawater: an unspiked sample, a sample spiked with 500 ng of Mn and Zn, 100 ng of Ni, Cu, and Pb, and 19 ng of Cd and Co, and a third sample spiked at twice these quantities. In the case of the isotope dilution analyses, the stable isotope spikes were added to the 500-mL seawater sample, which was heated overnight at 80 OC to ensure equilibration before passage through the column. (This step was found to be essential for accurate chro-
mium results.) Masses of the spikes were adjusted to be approximately equivalent to the masses of the analytes in the seawater aliquot. Reagent blanks for the method were evaluated by a slight modification of the isotope dilution technique used for the seawater samples. An aliquot of deionized distilled water (-50 mL) containing the same amount of high-purity nitric acid as a 500-mL seawater sample was processed as described above, except that the masses of the stable isotope spikes were appropriately smaller. Mass Spectra Acquisition. Virtually all aspects of mass spectral data acquisition with the ELAN can be controlled through the software. To obtain spectra such as those presented in Figures 1-4, the operator specifies the desired mass range, measurement time and/or precision, and the desired resolution. This latter parameter can be set to “high”, “low”, or “manual”. The high and low settings are factory preset to give nominal peak widths of 0.6 amu and 1.1 amu, respectively, at 10% of maximum intensity. In manual mode, the resolution can be adjusted to any setting between (or even somewhat beyond) these values. In low resolution mode, intensity measurements are made at 0.1 amu intervals across the specified mass range; in manual or high resolution mode, the measurement interval is 0.05 amu. The spectra presented in Figure 1 were recorded in high-resolution mode, while those in Figures 2-4 are in low resolution. As the resolution is increased, sensitivity is decreased; sensitivity in the high-resolution mode is about half that in low resolution. The user must therefore make a compromise between resolution and sensitivity appropriate to the complexity of the spectra. Data Acquisition for Elemental Analysis. Intensity data for elemental analysis can be acquired with the ELAN in either of two scanning modes, named “sequential” and “multichannel”. In both of these, the mass spectrometer scans through the specified list of masses in a discontinuous fashion referred to as “peak hopping”. In sequential mode, the time spent at each mass is the total measurement time specified by the operator; i.e., the mass spectrometer scans through the list of elements only once. In multichannel mode, more rapid repetitive scans are enabled by specifying a “dwell time” as well as a total measurement time; the spectrometer continues to scan until the accumulated dwell times at each mass reach the total measurement time. This mode of scanning is intended primarily for applications in which a transient signal must be recorded, for example with an electrothermal atomizer or flow injection device. The maximum scanning rate attainable is ultimately limited by the software overhead. For this work, with conventional pneumatic nebulization, there seemed to be no advantage in using multichannel mode, so the somewhat simpler sequential scanning mode was used. Another parameter that can be controlled through software is the number of measurements to be made per mass. For manual or high resolution, the allowable range is from 1, corresponding to a single measurement at the assumed peak center (based on the mass calibration), to 20, corresponding to a complete profile of the peak. When multiple measurements per peak are requested, the reported intensity is simply the arithmetic mean of these measurements. If the mass calibration is perfectly stable, this will result in a loss of sensitivity. On the other hand, use of multiple measurements per peak can help to compensate for small amounts of drift in the mass calibration. Standard Additions Analyses. For these analyses the trace metal concentrates in the 1 M HC1/0.1 M HNO, acid mixture were introduced directly to the ELAN, and ion intensities were measured for the following isotopes: %Mn,“Ni, 69C0,63Cu,64Zn, lI4Cd,and ‘O’Pb. Blank intensities, observed while running the 1 M HCl/O.l M HNO, acid mixture, were subtracted to obtain net intensities. The instrument was operated in sequential scanning mode at an intermediate resolution set manually to ensure adequate resolution of the &Mn peak from the much larger peak at mass 56 due largely to the ArO+ molecular ion. Each determination involved six measurements per peak at 1 s per measurement and was performed 3 times for each of the three samples. Linearity of the standard additions plots for all seven elements WQS excellent. The concentrations of the analytes in the unspiked seawater were calculated by first-order linear regression.
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
2909
70000d
52
53
54
MASS
55
56
57
Figure 1. ICP mass spectra in the mass range 52-57 in high-reso-
lution mode: -, blank; 100 pg L-' Mn; -, trace metal concentrate from 500-mL seawater sample; - - -, trace metal concentrate from 500-mL seawater sample spiked with 1 pg L-' Mn. -e,
Isotope D i l u t i o n Analyses. Trace metal concentrates for isotope dilution ICP-MS were evaporated to dryness to remove HCl, and the residue redissolved in 7-10 mL of 0.1 M HNOBprior to introduction to the ELAN. This was necessary to eliminate a number of spectroscopicinterferencesdue to chlorine-containing molecular species, as described in the next section. The ELAN was operated in sequential scanning mode and at low resolution to achieve maximum sensitivity. Five measurements (at 0.1 amu spacing) of 0.3 s each were made for each of the six reference and six spike isotopes, which were as follows: 52Cr,53Cr;60Ni,"Ni; @Cu,'Wu; @Zn,67Zn;l14Cd,'llCd; mPb, *Pb. The measurement process was repeated ten times for each solution; total analysis time was approximately 5 min. Blank subtraction was performed as described for the standard addition analyses. The analyte concentrations in the unspiked seawater were calculated by means of the following formula: M,K(A, - B,R) C= V(BR - A) where C is the analyte concentration in the sample, in micrograms per liter, M , is the mass of the stable isotope spike, in micrograms, Vis the volume of the sample, in liters, A is the natural abundance of the reference isotope, B is the natural abundance of the spike isotope, A, is the abundance of the reference isotope in the spike, B, is the abundance of the spike isotope in the spike, K is the ratio of the atomic weight of the element to the spike isotope mass, and R is the measured isotope ratio after spike addition.
57
58
61
,
62
59 MASS 6o Flgure 2. ICP mass spectra in the mass range 57-62 in low resolution mode: -, blank; 10 p g L-' Ni, 1 pg L-' Co; -, trace metal concentrate from 500-mL seawater sample; - - -, trace metal concentrate from 500-mL seawater sample spiked with 0.2 pg L-' Ni and 0.02 p g L-1 co.
.-,
6oool
p! 5000
2000Ly "..I
1000
;,,, I
0
:
""
62
63
64
65 MASS
66
67
68
Figure 3. ICP mass spectra in the mass range 62-68 in low-resolution mode: -, blank; --, 10 pg L-' Cu, 10 pg L-' Zn; -, trace metal concentrate from 500-mL seawater sample; - - -, trace metal concentrate from 500-mL seawater sample spiked with 0.2 pg L-' Cu and 1 pg L-' Zn.
'2001
IO00
800'
RESULTS AND DISCUSSION I n t e r p r e t a t i o n of I C P M a s s Spectra. Prior to the
attempt of any analyses by either standard additions or isotope dilution methods, the ICP mass spectra of trace metal concentrates from both spiked and unspiked seawater samples were examined and compared to spectra for blank and reference solutions in the 1 M HC1/0.1 M HN03 eluant acid mixture. These spectra, presented in Figures 1-4, yielded much useful information on possible spectroscopic interferences as well as an indication of the percentage recovery of the various metals of interest. Spectra in the mass range 52-57 are shown in Figure 1. In addition to the peak a t mass 55 for the monoisotopic element Mn, observed for the trace metal concentrates from spiked and unspiked seawater as well as the 100 pg L-l Mn reference solution, peaks for a number of molecular ions are observed. The peak a t mass 53 is almost entirely due t o 37C11sO+,one of several species observed when solutions containing hydrochloric acid or perchloric acid are run. The most intense of these peaks, a t mass 51, is due to 35C1160+and is approximately 3 times as intense as the peak at mass 53, as would be expected from the natural abundances of the two chlorine isotopes. Peaks a t mass 52 and 54, at 10-20 times lower
0
204
205
206
MASS
207
208
209
Figure 4. ICP mass spectra in the mass range 204-209 in low-resolution mode: -, blank; - e , 10 pg L-' Pb; -, trace metal concentrate from 500-mL seawater sample; - - -, trace metal concentrate from 500-mL seawater sample spiked with 0.2 pug L-' Pb.
intensity, are believed to be due to the protonated analogues of the two C10+ parent ions. A much less intense quartet of peaks from mass 67 t o 70 is attributed to the corresponding C102+ and C102H+species. The peak a t mass 56 is due partly to 56Fe,the most abundant iron isotope, but also to 40Ar160+,a molecular ion always observed when aqueous solutions are introduced to the ELAN. The high and somewhat variable signal due to ArO+ precludes the use of 56Fe for trace iron determinations. To further complicate matters, the next most abundant isotope, a t mass 54, is overlapped by a peak attributed to 4oAr14N+.
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
Table 11. ICP-MS Analysis of the Coastal Seawater Reference Material CASS-1“ std additions
Mn co
Ni
cu Zn Cd Pb Cr
1.80 f 0.06 0.039 f 0.003 0.302 f 0.005 0.263 f 0.007 1.05 f 0.004 0.027 f 0.001 0.22 f 0.02
aResults in pg
isotope dilution
0.319. f 0.004 0.311 f 0.018 1.00 f 0.04 0.026 f 0.002 0.24 f 0.02 0.13 f 0.01
Table 111. Estimated Detection and Determination Limits by this Method (in pg L-I)
ICP-MS
accepted value
2.27 f 0.17 0.023 f 0.004 0.290 f 0.031 0.291 f 0.027 0.98 f 0.10 0.026 f 0.005 0.25 f 0.03 0.12 f 0.02
L-l.
No attempt was made to determine iron in this work. The determination of chromium (by the isotope dilution technique) was possible only after evaporation of the original eluate to dryness, to volatilize the HCl, followed by redissolution of the residue in 0.1 M nitric acid prior to the instrumental determination. In contrast, manganese could be directly determined in the 1 M HCl/O.l M “OB eluate by the method of standard additions. Spectra in the mass range 57-62 are shown in Figure 2. This range includes four of the five nickel isotopes (at 58, 60, 61, and 62) and the single stable isotope of cobalt at mass 59. It is noteworthy that the blank intensities for nickel are very low despite the fact that the sampler and skimmer of the ELAN are made of nickel (part of the intensity at mass 58 is due to 40Ar1sO+). Spectra in the mass range 62-68 are shown in Figure 3. This range includes the two copper isotopes a t masses 63 and 65 and four of the five zinc isotopes (at 64,66,67, and 68). Peaks in the blank spectrum at masses 63-66 are due to measurable Cu and Zn blanks, but the peak at mass 67 is due primarily
to 35c11602+. Spectra in the mass range 108-115, which includes six of the eight cadmium isotopes, were rather noisy because of the very low cadmium concentrations and are not shown here. They did nonetheless indicate that spike recovery was essentially quantitative, and that neither l W d nor ‘14Cd were subject to any molecular ion interferences. A small correction to the l14Cd intensities, to correct for the lI4Sn isobaric interference, was made automatically by the ELAN software. Spectra in the mass range 204-209, showing peaks for the three major lead isotopes, are presented in Figure 4. Seawater Analysis. In Table 11, results of ICP-MS analysis of the coastal seawater reference material CASS-1 by both standard additions and isotope dilution techniques are compared with the accepted values. The accepted values arise from a pooling of results of a number of techniques, including graphite furnace atomic absorption spectrometry, anodic stripping voltammetry, isotope dilution spark source mass spectrography, and ICP-MS. Good agreement of the results from at least two independent techniques is the criterion for publication of an accepted value; in fact the minimum number for the elements reported here was three. The standard additions ICP-MS results are blank-corrected means based on three separate analyses, with precision expressed as the standard deviations. They are in very good agreement with the accepted values, with the exception of the manganese result, which appears to be low, and the cobalt result, which is high. A possible explanation for the high cobalt result is that the 59C0peak may be overlapped by a peak due to 24Mg35C1.The trace metal concentrates prepared by the I-8-HOQ adsorption method generally contain 50-100 mg L-l of residual magnesium from the seawater. If this hypothesis is correct, the severity of the interference might be reduced by an evaporation of the concentrate to dryness to remove HCl, as was done in preparing solutions for isotope dilution
Mn co
Ni
cu
Zn Cd Pb
detection
seawater detection
seawater determination
limit
limit
limit
0.07 0.01 0.04 0.02
0.001 0.0002 0.001 0.0004 0.002 0.002 0.0004
0.01 0.002 0.008
0.10
0.09 0.02
0.004
0.02 0.02 0.004
ICP-MS. This additional step would also permit the determination of chromium by standard additions. The isotope dilution ICP-MS results are blank-corrected means based on eight to ten separate analyses, with precision again expressed as the standard deviations. All of these results are in good agreement with the accepted values. Detection Limits and Seawater Determination Limits.
In the left-hand column of Table 111,detection limits for the seven elements determined by standard additions ICP-MS are reported. They are based on the usual definition of the detection limit as that concentration of analyte yielding a signal equivalent to 3 times the standard deviation of the blank signal. In the middle column of the table are seawater detection limits based on the 50-fold preconcentration achieved (for most elements) in the method described here. The seawater “determination limits” in the right-hand column are 10 times higher than these values and represent a realistic estimate of the minimum concentrations determinable in seawater, taking into account that recoveries of some of the metals (e.g., Co, Cr) are not quantitative and that reagent blanks for others are not negligible. It is interesting to note that the values for a number of these elements are only about a factor of 2 lower than their typical concentrations in an open ocean water sample; values for Cd, Co, and Mn in the open ocean seawater reference material NASS-1 are 0.029 f 0.004, 0.004 f 0.001, and 0.022 f 0.007 pg L-l, respectively. We have made no attempt as yet to directly determine any trace metals in seawater by ICP-MS. At present it appears that introduction of solutions of high dissolved salts concentrations to an inductively coupled plasma mass spectrometer results in a severe suppression of sensitivity, the mechanism of which is not understood. But even if this suppression were to be overcome somehow, the determination of many of the metals of interest in uncontaminated offshore samples would be difficult because of inadequate detection limits. Thus, determination of trace metals in seawater remains an interesting challenge for even the most sensitive instrumental techniques currently available. Procurement of CASS-1. Complete information on the procurement of the coastal seawater reference material CASS-1 and other marine reference materials can be obtained from S. Berman, Marine Analytical Chemistry Standards Program, Chemistry Division, M-12, National Research Council of Canada, Ottawa, Canada K1A OR9. Registry No. Mn, 7439-96-5; Co, 7440-48-4; Ni, 7440-02-0; Cu, 7440-50-8; Zn, 7440-66-6; Cd, 7440-43-9; Pb, 7439-92-1; Cr, 7440-47-3; water, 7732-18-5.
LITERATURE CITED (1) Date, A. R.; Gray, A. L. Spectrochirn. Acta, Part B 1985, 4 0 8 ,
115-122. (2) Douglas, D. J.; Houk, R. S. Prog. Anal. A t . Specfrosc. 1985, 8 ,
1-18. (3) Sugimae, Akiyoshi Anal Chim. Acta 1980, 121, 331-336. (4) McLeod, C. W.; Otsuki, A,; Okamoto, K.; Haraguchl, H.; Fuwa, K. Analyst (London) 1981, 106, 419-428. (5) Miyazaki, Akira; Kirnura, Akira; Bansho, Kenji; Urnezaki. Yoshimi Anal. Chim. Acta 1982, 744, 213-221
Anal. Chem. 1985, 57, 2911-2917 (6) Tao, Hiroaki; Mlyazaki, Akira; Bansho, Kenji; Umezaki, Yoshimi Anal. Chlm. Acta 1984, 156, 159-168. (7) Hiraide, Masataka; Ito, Tetsumasa; Baba, Masafurnl; Kawaguchi, Hlrcshi; Mizuike, Atsushi Anal. Chem. 1980, 52, 804-807.
(8) Berman, S. S.;McLaren, J. W.; Wlllie, S. N. Anal. Chem. 1980, 52, dRR-492. .- - . - - . (9) Watanabe, Hiroto; Goto, Katsumi; Taguchi, Shigeru; McLaren, J. W.; Berman, S. S.; Russell, D. S. Anal. Chem. 1981, 53, 738-739.
2911
(10) Sturgeon, R. E.; Berman, S. S.;Willie, S. N.; Desaulniers, J. A. H. Anal. Chem. 1981, 53, 2337-2340. (11) McLaren, J. W.; Willie, S. N., unpublished results, Ottawa, 1981. (12) Mykytiuk, A. P.; Russell, D. S.;Sturgeon, R. E. Anal. Chem. 1980, 52, 1281-1 283.
RECEIVED for review June 11,1985. Accepted July 22,1985.
Resonance Enhanced Multiphoton Ionization Spectroscopy for Detection of Azabenzenes in Supersonic Beam Mass Spectrometry Roger Tembreull, Chung Hang Sin, Ho Ming Pang, and David M. Lubman* Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109
The analytlcai lonlzatlon spectroscopy of azabenzenes is investigated in supersonic jets for selectlve detectlon In mass spectrometry. The detection of arabenzenes represents an lnterestlng problem since the lonlzatlon potentials are often too hlgh for direct resonant two-photon lonlzatlon. For this work, we develop an efficient ionlzatlon scheme based on a three-photon process where the first photon excites a molecule to a resonant state, a second photon then excltes the molecule to an upper hlgh lylng “state”, and a third photon Ionizes the molecule. The spectra obtalned reflect the absorption of the n-p* transitlon since the second photon Is resonant with a dense reglon of valence and Rydberg states where there is no sharp structure evldent. Thls method has been applled to aza- and dlarabenzenes with resultlng sharp spectral ilnes for unique detectlon and production of only the molecular ion for mass spectrometry.
In this work, we have studied the use of laser photoionization methods for selective detection of azabenzenes in mass spectrometry. This class of molecules was chosen because of its importance in biological chemistry and their detection presents an interesting challenge in molecular spectroscopy. In previous work, the analytical spectroscopy of various aromatic hydrocarbons was studied ( I , 2). In these systems the transitions studied were a-a* where an electron was excited from a a molecular orbital in the ground state to a a* orbital in an excited electronic state. The azabenzenes bear a close resemblance to benzene and other hydrocarbon analogues since in every case one or more of the benzene -CH functions are replaced by the isoelectronic nitrogen atoms. However, the addition of the N2pu lone pair orbitals adds the possibility of n-a* transitions occurring by excitation of the lone pair electrons in addition to a-a* transitions. The detection of azabenzenes through the n--R* transitions may afford several advantages. The n-r* transitions generally occur in the near-UV and visible and the addition of more N’s to the aromatic ring often serves to shift the absorption to longer wavelength. Thus, this transition can generally be detected in a region of the spectrum easily accessible with current tunable dye lasers. Further, the n-a* transitions are generally sharp, whereas the first a-r* transition may often be diffuse and may not exhibit any sharp bands. The main problem is that study of the n-a* transitions is often limited due to small absorption cross sections and rapid radiationless transitions which provide low quantum yields in fluorescence.
In the case of quinoline, isoquinoline, and many diazanaphthalenes, n-a* bands have not been detectable in the past. Indeed, as the size of the conjugated system increases, one needs more aza substitution to move the n-a* band to longer wavelength than the first a-a* band so that the stronger a-r* transition does not totally overlap it. Thus, in many cases a detection scheme based on the a-a* transition may be preferable to the n-a* and an important aim of this study will be to determine the conditions under which each is appropriate. Herein we will demonstrate the detection of azabenzenes and several derivatives in a supersonic expansion by laserinduced multiphoton ionization. In principle resonant twophoton ionization (R2PI) is the most desirable process for ionization of molecules due to its high efficiency and the ability to efficiently produce soft ionization, i.e., no fragmentation in a mass spectrometer for identification (1-10). In RBPI two photons are used to produce ionization where the first photon excites a molecule to a resonant intermediate state followed by a second photon that ionizes the molecule. The necessary condition for ionization is that the sum of the two photons must be greater than the ionization potential of the molecule. However, this is not possible for study of the n-a* transitions of many azabenzenes since twice the one-photon energy is often less than the IP. Of course, a two-color ionization scheme could in theory be used. However, for molecules with very short (
