Anal. Chem. 1990, 62, 1043-1050 Pivonka, D. E.; Fateley, W. G.; Fry, R. C. Appl. Spectrosc. 1986, 4 0 , 291-297. Pivonka, D. E.; Schleisman. A. J. J.; Fateley, W. G.; Fry, R. C. Appl. Spectrosc. 1986, 4 0 , 766-772. Cerbus. C. S.; Gluck, S. J. Spectrochim. Acta 1983, 388, 387-397. Goode, S. R.; Kimbrough, L. K. Spectrochim. Acta 1987. 428. 309-322. Eckhoff, M. A.; Ridgway. T. H.; Caruso, J. A. Anal. Chem. 1983, 55. 1004- 1009. Holcombe, J. A.; Harnly, J. M. Anal. Chem. 1986, 58, 2606-2611. de Galan, L.; Kornblum, G. R.; de Loos-Vollebregt, M. T. C. Recent Advances in Analytical Spectroscopy;Fuwa, K., Ed.; Pergamon: New York. 1982; pp 33-50. Olesik. J. W. Inductively Coupled Plasma Emission Spectroscopy; Boumans. P. W. J. M., Ed.; Wiley-Interscience: New York. 1987; Part 1. pp 466-535. Haas, D. L.; Caruso. J. A. Anal. Chem. 1985. 5 7 , 846-851. Talmi, Y.; Simpson, R . W. Appl. Opt. 1980, 19, 1401-1414. Jones, D. G. Anal. Chem. 1985. 5 7 , 1057A-1073A and 1207A1214A. Borman, S . A. Anal. Chem. 1983, 55, 836A-842A. Blades, M. W.; Horlick, G. Appl. Spectrosc. 1980, 3 4 , 696-699. Choot. E. H.; Horlick, G. Spectrochim. Acta 1986. 4 7 8 , 935-945. Levy, G. M.; Quaglia. A,; Lazure, R. E.; McGeorge, S. W. Spectrochim. Acta 1987. 428, 341-351. Karanassios. V.; Horlick. G. Appl. Spectrosc. 1986, 4 0 , 813-821. Freeman. J. E.; Hieftje, G. M. Spectrochim. Acta 1985, 4 0 8 , 475-492. Keane. J. M.; Brown, D. C.; Fry, R. C. Anal. Chem. 1985, 57, 2526-2533. Keane, J. M.: Fry, R. C. Anal. Chem. 1986, 58, 790-797. Cham S . ; Montaser, A. Appl. Spectrosc. 1987, 4 1 , 545-552. Takigawa. Y.; Hanai, T.; Hubert, J. HRC CC. J. High Resolut. Chromatogr. Chromatogr. Commun. 1986, 9 . 698-702. Kamins, T. I.; Fong, G. T. IEEE J. Solid-state Circuits 1978, SC-73,
Grabau. F.; Talmi, Y. Multichannel Image Detectors ; ACS Monograph Series No. 236; Talmi, Y., Ed.; American Chemical Society: Washlngton, DC. 1983; Volume 2, pp 75-116. McGeorge, S. W.; Salin, E. D. Anal. Chem. 1985, 57, 2740-2743. Kempster, P. L.; Strasheim, A.; Bohmer. R . G. Spectrochim. Acta 1987, 428, 1139-1143. McGeorge, S. W.; Salin, E. D. Spectrochim. Acta 1986, 4 7 8 , 327-333. Taylor, P.; Schutyser, P. Spectrochim. Acta 1986, 478, 81-103. Stewart, J. A.; Scheeline, A. Anal. Chem. 1984, 5 6 , 2995-2997. van der Plas. P. S. C.; Uitbeijerse, E.; de Loos-Vollebregt, M. T. C.; de Galan, L. Spectrochim. Acta 1987. 428. 1027-1038. Hirschfeld, T. Appl. Spectrosc. 1976, 30, 67-68. Bialkowski, S.E. Appl. Spectrosc. 1988, 42, 807-811. Cordelle, J.; Flamand, J.; Pieuchard, G.; Labeyria, A. Optical fnstruments and Techniques 7969; Dickson, J. Home, Ed.; Oriel Press: Newcastle, 1970; pp 117-124. Noda, H.; Namioka. T.; Seya, M. J. Opt. SOC. Am. 1974, 6 4 , 1031-1048. Braun, W.; Peterson, N. C.; Bass, A. M.; Kurylo, M. J. J. Chromatogr. 1971, 55, 237-248. Uehiro. T.; Morita, M.; Fuwa, K. Anal Chem. 1984, 5 6 , 2020-2024. LaFreniere, B. R.; Houk, R. S.; Wiederin, D. R.; Fassel. V. A. Anal. Chem. 1988, 6 0 , 23-26. Suilivan, J. J.; Ouimby, B. D. HRC CC,J. High Resolut. Chromatogr. Chromatogr. Commun, 1989, 72. 282-286. Zaidel'. A. N.; Prokof'ev, V. K.; Raiskii, S. M.; Slavnyi, V. A,; Shreider, E. Ya. Tables of SDectralLines, IFI/Plenum: New York. 1970. Pearse. R. W. B.; daydon, A. G. The Identification of Molecular Spectra; Chapman & Hall: London, 1963. Krupenie. P. H. The Band Spectrum of Carbon Monoxide; National Standard Reference Data Series, National Bureau of Standards NSRDS-NBS 5; U.S. Government Printing Office: Washington, DC, 1966.
80-85. Brett, L.; Stahi, R. G.; Timmins, K. J. J. Anal. At. Spectrom. 1989, 4 , 333-336. (28) Burton, L. L.; Blades, M. W. Spectrochim. Act8 1987, 428, 513-519. (29) Winge, R. K.; Fassel, v. A,; Eckels, D. E. Appl. Spectrosc. 1986, 40, 461-464.
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RECEIVEDfor review October 6, 1989. Accepted February 5, 1990.
Multielement Trace Metal Determination by Electrodeposition, Scanning Electron Microscopic X-ray Fluorescence, and Inductively Coupled Plasma Mass Spectrometry Ngee-Sing Chong, Michael L. Norton,* a n d James L. Anderson*
Department of Chemistry, University of Georgia, Athens, Georgia 30602
Multielement analysis of multicomponent metallic eiectrodeposits is described, based on scanning electron microscopy with energy dispersive X-ray fluorescence [EDXRF] detection, followed by dissolution and Inductively coupled plasma mass spectrometry [ICP-MS] detection. Application of the method is described for determination of trace elements in seawater, including Zn, Mn, Co, Cu, Cr, Ni, Fe, Cd, Pb, and Hg. These elements are simultaneously electrodeposited onto a niobium wire working electrode at -1.40 V vs an Ag/AgCi reference and subjected to EDXRF analysis. Internal standardization is practical for quantitative calibration at the 1 ppm anaiyte concentration level In an ana1yte:internal standard concentration ratio range of 0.02-50. Detection limits for EDXRF range from 1.9 ppb for Fe to 50 ppb for Cd. The deposit is dlssolved for subsequent ICP-MS determination. Significant reduction in ICP-MS matrix interferences by Na, Ca, Mg, K, and CI ions is achieved by deposition at potentials more positive than their very negative reduction potentials. Measurement of elemental isotope ratios is achieved with 0-8 % relative error. ICP-MS detection limlts for all elements except Zn and Fe are superior to those of EDXRF. Mn, Ni, Cd, Pb, and Hg can easily be determined In the range of 13-86 parts per trillion with ICP-MS.
INTRODUCTION Determination of trace metals in seawater represents one of the most challenging tasks in chemical analysis because the parts-per-billion (ppb) or sub-ppb levels of analyte are very susceptible to matrix interference from the alkali or alkaline-earth metals and their associated counterions. For instance, the alkali metals tend to affect the atomization and the ionization equilibrium processes in atomic spectroscopy, and the associated counterions such as the chloride ions might be preferentially adsorbed onto the electrode surface to give some undesirable electrochemical side reactions in voltammetric analysis. Thus, most current methods for seawater analysis employ some kind of analyte preconcentratioin along with matrix rejection techniques. These preconcentration techniques include coprecipitation (1,2), solvent extraction (3-5), column adsorption (4-6), electrodeposition (7-10), and Donnan dialysis (11, 12). Measurement techniques that can be employed for the determination of trace metals include atomic absorption spectrometry [AAS] (5, 7,8), anodic stripping voltammetry ( I 1, 13, 14), differential pulse cathodic stripping voltammetry (15, 16), inductively coupled plasma atomic emission spectrometry [ICP-AES] (9, 12, 13,and liquid chromatography of the metal chelates with ultraviolet-visible absorption
0003-2700/90/0362-1043$02.50/0 1990 American Chemical Society
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[LC-UV-vis] (5, 18) or other detectors. AAS, generally a single-element technique, and voltammetric techniques usually have a low sample throughput and are quite susceptible to matrix interferences. ICP-AES and LC-UV are both laborsaving multielement techniques with the latter being somewhat slower due to the elution time. AAS with a graphite furnace and voltammetric methods both have impressive detection limits that ICP-AES and LC-UV have yet to emulate. Voltammetric techniques offer the unique advantage of being capable of in situ determination since commercial portable units can easily be carried into the field (10, 19). This is very attractive because trace determination frequently involves preconcentration of a large volume of sample, which might be too cumbersome for transporting back to the laboratory. ICP-AES is also quite attractive owing to its large linear dynamic range, its relatively low matrix interference, and its ability to accommodate both on-line (e.g. Donnan dialysis) and off-line (e.g. electrodeposition and solvent extraction) preconcentration. Electrochemical deposition has been used in conjunction with many techniques to achieve impressive preconcentration factors. However, the difficulty of quantitative calibration has plagued this technique because the deposition is not quantitative for all elements at a given potential and the fraction deposited for a given element is not always reproducible between analyses at different concentrations (9). It has also been shown that the electrolytic preconcentration ratio frequently decreases as sample concentration decreases, due to oxidation by solution constituents (20,211. EDXRF and ICP-MS, with their multielement capability, are amenable to the use of an internal standard to solve these problems. Furthermore, ICP-MS can perform quantitation by isotope dilution, the ultimate calibration technique that can account for differences among elements and between analyses. These differences include factors such as different reduction potentials, different kinetic rates of deposition, ease of precipitation as pH changes, dislodgement of poorly adhering deposit, and unforeseen changes in analysis conditions during replicate analyses. From the perspective of ICP-MS determifiation, electrodeposition offers the benefits of reducing matrix-derived interferences and avoiding the clogging of both the pneumatic nebulizer and sampler orifice by rejecting the salt matrix. The interference can be spectral overlaps, e.g. 35C1160H+and 52Cr+[83.76% abundance] (22), and 48Ca160+ with @Zn+[48.9% abundance] (23),or the matrix suppression of the trace transition metals by virtue of the low ionization potentials of Na, K, Ca, and Mg (24, 25). An advantage of using EDXRF and ICP-MS in tandem is that the nondestructive EDXRF technique can be used to screen samples for ICP-MS determination. This ensures that the sample salt content is below the tolerable limit for introduction into the ICP and can help diagnose any potential isobaric interferences. The use of EDXRF in conjunction with scanning electron microscopy (SEM) can potentially allow the ultimate sensitivity to be achieved by using a very fine electron beam (e.g. a diameter of less than 100 nm) to probe the microelectrode surface onto which trace elements from a large sample volume have been electrodeposited. The microscope also allows the detection of undesirable properties that could lead to serious errors of elemental determination. These include poor adhesion, nonuniform coverage, formation of heterogeneous phases, and codeposition of hydroxides and chlorides. Prior studies involving the elemental determination of electrodeposited trace metals using AAS and ICP-AES had not mentioned these factors with regard to the accuracy of their analyses (1-5, 7-9, 12, 17). The present study supplements such information by using EDXRF and SEM.
In the present study, a portable and compact electrochemical cell incorporating a rotating niobium wire electrode is used to electrodeposit the trace elements in seawater. This Sample-coated electrode is then mounted in the scanning electron microscope equipped with an EDXRF microprobe for elemental determination. Sensitivity of the technique is achieved by raster scanning the 100 nm diameter electron beam over the surface of the 0.25 mm diameter niobium wire. Scanning electron microscopy is used to correlate the quality of the deposit with the deposition conditions and to define the spot for nondestructive EDXRF analysis. The deposit is then dissolved in nitric acid and introduced into an inductively coupled plasma mass spectrometer for elemental and isotope ratio determination. EXPERIMENTAL SECTION Chemicals and Equipment. Chemicals used were of reagent grade and were used as received. Milli-Q deionized and distilled water of 18 mQ cm resistivity was used for sample preparation and rinsing of electrodes and cell. Synthetic seawater was prepared according to a modified method of Kester et al. (26),with the following chemical composition (g/kg of solution): NaCl, 23.93; Na2S04,4.01; KCl, 0.68; NaHCO,, 0.20; KBr, 0.098 H3B03,0.026; MgCl2.6HZO,10.83; CaClZ,1.03; SrC12.6H20,0.024. Synthetic seawater samples were prepared by adding the proper amount of multielement stock solution to give a series of six standard solutions from 1 ppb to 100 ppm at decade intervals. Standard solution A contained Ni, Zn, Fe, Cr, and Cd whereas standard solution B contained Mn, Co, Cu, Pb, Sn, and Hg. A 3.0 M sodium acetate buffer solution of pH = 4.6 was also prepared. For the dissolution of electrodeposit,redistilled nitric acid and deionized distilled water were used. A Phillips 505 scanning electron microscope was used for the examination of the electrodeposit and for definition of the small probe area on the electrodeposit for analysis by the X-ray detector (EDAX International, Inc.). A Tracor Northern-5500 data acquisition system was used for spectral collection, quantitative X-ray determination, and line-scan generation. The inductively coupled plasma mass spectrometer was a Sciex Elan Model 250 (with upgraded ion optics) that used a Fassel-type torch and ultrasonic nebulization. The locally constructed electrochemical cell used a rotating working electrode which consisted of either a piece of niobium wire (Johnson Matthey) or a niobium wire ensheathed in melted quartz with a polished disk surface. Other electrode materials tested include polished graphite and mercury-film-coated graphite electrodes, which were found to be inferior to the niobium electrode. A Bioanalytical Systems SP-2 POtentiostat was used for controlling the working electrode potential. The auxiliary electrode was a Pt tube in a compartment isolated from the bulk sample by a fritted disk. The reference electrode consisted of a Ag/AgCl wire dipped in a 3 M KCl filling solution. The rotation rate of the niobium working electrode was maintained at 750 rpm, fast enough to prevent the hydrogen bubbles from adhering and slow enough to avoid a turbulent vortex that would dislodge the deposit. The detailed conditions of the electrodeposition, X-ray microanalysis, and plasma mass spectrometry are listed in Table I. Procedures. The niobium electrode of 0.25 mm diameter was prepared by polishing it with emery paper followed by alumina powder until it was shiny and smooth under 5-1OX magnification. The electrode was then mounted onto the rotating shaft and cleaned by spinning it for 5 min in 1:1 HNO, to strip off any trace impurities. The electrochemical cell containing 40.0 mL of seawater sample and 4 mL of pH 4.6 acetate buffer was mounted and the sample bubbled for 20 min with nitrogen gas before the potential was applied so as to reduce the extent of oxidative dissolution of the deposit by dissolved oxygen (21). The electrolysis was allowed to proceed with simultaneous nitrogen bubbling for the times specified in Table I. The electrodeposit obtained was rotated and washed in deionized water for 20 min to remove any occluded salt before being allowed to dry. The niobium wire bearing the deposit was then mounted on a carbon stub (EM Sciences, Inc.) with carbon paint (Bio-Rad, Inc.) for microscopic examination and X-ray microanalysis. The analyzed sample was then dissolved in 1 cm3 1:l nitric acid for 2 h before
ANALYTICAL CHEMISTRY, VOL. 62, NO. 10. MAY 15. 1990
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Table I. Experimental Conditions
(vi9
(A) Electrodeposition pH = 4.6 reduction potential: -1.40 V vs Ag/AgCI deposition times: 40 min for 100/10 ppm level 60 min for I ppm/lM) ppb level 90 min for 1011 ppb level working electrode: 0.25 mm diameter niobium wire electrode area: 0.982 mmz for l O O / l O / l ppm-level 0.049 mm2 for lCil/lO/l ppb-level electrode rotation rate: 750 rpm volume of sample in the cell: 40 mL
(i) (ii) (iii) (iv)
electron beam energy: 30 kV beam spot: 100 nm tilt angle: Oo acquisition time: 120 s
(i) (ii) (iii) (iv)
(4 (vi)
(B) Energy Dispersive X-ray Analysis
(C) Inductively Coupled Plasma Mass Spectrometry (i) (ii) Mi) (iv) lv)
rf power: 1.25 kW aerosol carrier gas flow: 1.6 L/min Ar auxiliary flow: 0.4 L/min Ar sampling position: 25 m m above load coil acouisition time: 10 min
Figure 1 Scanning electron micrograph of the electrodeposit formed horn soiution A at 100 ppb level on a niobium wire electrode at -1.40 V.
being finallydiluted to the 5% acid level. The acid totally stripped the metal deposit into the solution and only minimally dissolved the niobium substrate, as indicated hy the low niobium ion intensity of less than loo0 counts/s (i.e. l o pm). Response for each element is normalized to the maximum response for that element in the time series. Each time represents an independent experiment. Hg (A),Pb (X), Cd (O), Fe (B),Cr (0). Key: Cu
(o),
positive reduction potential of +0.34 V. The pH at which the sample is buffered is also very important. In general, a compromise pH a t 4.6 gave very good deposition characteristics for most elements. For certain elements, such as Mn, a buffered pH a t 7.5 will be more suitable because this tends to shift the hydrogen reduction wave far enough to unmask the manganese wave. However, one has to be wary of the levels of Cr, Cu, and Hg in the sample that may precipitate out as hydroxides in the effort to determine Mn. The dependence of the apparent electrodeposit composition determined by EDXRF upon the time of electrodeposition for various metals is strongly dictated by the area of the working electrode and the quantities of metallic cations that can be electrodeposited. This is because the 30-kV electron beam can only probe the topmost ca. 2 pm of a thick deposit and hence the X-ray profile is indicative of the temporal segment prior to the halting of electrodeposition. Therefore, even though the composition of the bulk electrodeposit of a given sample for replicate analyses is the same, the X-ray profile or apparent composition are not necessarily the same if the deposition time or electrode area are changed between analyses. For deposit thickness in excess of 10 pm, a great variability of the apparent deposit composition is evident, as seen in Figure 6, in which the signal for each element at any time is normalized to the maximum signal for that element during the experiment. The variability is accentuated by the use of a solution containing Hg, Cu, Pb, Cd, Fe, and Cr a t 1 ppm level for electrodeposition onto the niobium electrode with an area of 0.049 mm*. This yielded a deposit that ranged from 20 to 200 pm thick assuming that 5-5070 of the metal species was electrodeposited in the deposition period of 5-40 min. Figure 6 reveals that the self-normalized signals for Cu, Hg, Cd, and P b grow from zero approximately linearly with time for the first 10 min of deposition, with essentially identical slopes, as expected for mass-transfer-controlled deposition at a rotating disk electrode. After deposition periods of 10-20 min, these signals fall off again, in the order Cu, Hg, Pb, and Cd, due to significant depletion of bulk concentration at longer times. The order of falloff approximately follows reduction potentials for chloride media (Hg > Cu > P b > Cd > Fe > Cr), although Cu and Hg are reversed. The signal for each element is normalized to the maximum signal recorded for that element during the run. Maxima occur at 10 min for Cu (45476 counts), Hg (22623 counts), Cd (9632 counts), and P b (1570 counts). Total EDXRF signal counts follow the same trend as the individual elements, with count values of 38666, 81 852,89 298,67 372, and 41 893 at times of 5,10,20,30, and 40 min, respectively. The normalized signal for Fe also shows approximately linear growth for the first 10 min, with a much lower slope.
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The signal for Cr approximately parallels that for Fe, after a delay of 5 min. The signals for Fe and Cr continue to grow throughout the deposition, dominating near the end of the deposition. The maximum for Fe occurs at 40 min (26427 counts), and the maximum for Cr occurs at 30 min (9973 counts). This behavior can be understood qualitatively in terms of the relatively negative reduction potentials of these elements and a significant contribution of electron transfer kinetics to the rate-limiting step, such that these elements are still being deposited a t a slower rate even after the various elements whose deposition rates are limited by mass transfer control have become significantly depleted. The depletion and enrichment of the elements with deposition time and deposit thickness must therefore be considered carefully for thick deposits. Thus, it is very important to make sure that the deposit thickness is compatible with the range of electron excitation (i.e. ca. 1-2 pm) (27). Therefore, the time and mass transfer conditions of electrodeposition and the working electrode area must be controlled exactly to ensure good precision. The use of X-ray excited EDXRF spectrometry can alleviate this problem of the time-dependent enrichment effect because of the greater penetration range of X-rays as an excitation source. A plausible alternative explanation for the observation of response variation with deposit thickness is that, for a thick deposit with uniform composition, the X-ray photons emitted by Cu and Hg are more severely absorbed by other elements whose absorption edges are just below the energies of Cu and Hg X-rays. Generally, the extent of self-absorption is inversely proportional to the difference between the X-ray energy of the fluorescing elements and the absorption edge of the absorbing elements. Hence, the effect of self-absorption in the deposit should be minor because the absorption edges of Cr, Cd, and Fe are significantly below the X-ray energies of Cu and Hg. Furthermore, the attenuation of signal due to self-absorption in a uniform, multielement specimen is only typically about 20%, which is much lower than that observed in Figure 6. For example, the absorption factors for the three constituents in a specimen of 45% Fe-45% Cr-10% Ni at various tilt angles range from 0.973 to 1.344 (27). This strongly supports the argument that the major source of variability in the apparent composition of the deposit at longer times is due to the time-dependent elemental depletion and enrichment in the electrodeposit. The consequent effect of secondary fluorescence should also be quite small or completely absent. This is manifested by the fact that P b has an absorption edge that is greater than the energies of corresponding Cu and Hg peaks and yet it shows an increase in signal as a function of time. The effects of absorption and fluorescence in quantitative EDXRF analysis are discussed in greater detail elsewhere (27). It should be noted that many of the problems evidenced for thick deposits can be avoided by use of thinner deposits, since all of the elements shown in Figure 6 exhibit approximately linear dependence of signal intensity on deposition time for the first 10 min of deposition. Longer deposition times should afford similar linearity for more dilute samples. Characterization of Energy-Dispersive X-ray Spectrum. The X-ray spectrum of the electrodeposit from a synthetic seawater sample containing trace transition metals and heavy metals of environmental interest is shown in Figure 7. It is quite obvious that a large spectral window between 3 and 16 keV is obtained when niobium is used as the electrode. This allows the analysis of third through fifth period elements from C1 to Zr and fifth through seventh period elements from Rh to Ra readily. For Hg, even though its M peak overlaps with the L peak of Nb, its L peak is free from spectral overlap and is available for determination. Spectral
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 10, MAY 15, 1990
a
Nb 0 al \ VI
I
1 1
I
VI
c
Fe
h
3 0 0 v
>
c .-
ul C
0 +
-C =.. e X
Energy of X-ray Peak ( k e V ) Energy dispersive X-ray spectrum of the electrodeposit of trace metals in seawater at 10 ppm level. Figure 7.
-0
-2,870 -2.00 -1.699
-100
-0699
000
EDXRF calibration plot for Cr using Ni (O), Zn (R), and Cd (A) as internal standards (IS). Figure 8.
overlap also occurs for the Kj3 peak of a given transition element with the K a peak of the adjacent element of next higher atomic number. However, the overlap can be mathematically deconvolved for quantitative determination since the mass absorption coefficients of the elements are wellknown. The spectrum also indicates a higher intensity of the niobium L peak relative to that of its K peak expected on the basis of their relative absorption coefficients as depicted by the heights of the two lines under the L and K peaks. This is due to the secondary fluorescence effect in which the X-ray photons of the deposited transition elements are absorbed by the niobium substrate to produce the fluorescent niobium L peak at lower energy. This effect that complicates quantitative accuracy can be attenuated by increasing the deposit thickness (27). The X-ray spectrum even allows the observation of Na and Mg, and hence EDXRF can serve as a diagnostic tool for the subsequent ICP-MS determination. For instance, the absence of Na, K, Mg, Ca, C1, and S signals in Figure 8 implies successful electrochemical rejection of these elements so that the matrix interference from the first four elements and the isobaric oxide interference from the last three elements can be precluded. The rejection factors (RF) of some of these elements are estimated by ICP-MS determination according to the equation
[MI (before electrolysis) RF = log [MI (after electrolysis)
as the analyte with Zn, Ni, and Cd as possible internal standards. The plot of intensity ratios versus concentration ratios of the analyte and internal standards is shown in Figure 8. A directly proportional relationship between analyte concentration and X-ray signal intensity is observed for all three cases from 1:l (ppm) to 150 (ppm) (Le. 0 to -1.699 on the logarithmic scale) ratios of [analyte]:[internal standard]. However, beyond the 1:50 (pprn) ratio, the plot begins to rise in the opposite direction in the cases of Zn and Ni as internal standards. This can be attributed to the self-absorption effect in which the more energetic X-ray photons of Ni (7.5 keV) and Zn (8.6 keV) are absorbed by the chromium atoms which have a Ka absorption edge of 6.0 keV. It is interesting to note that in the case of Cd as the internal standard, the slope does not rise drastically in the opposite direction because its less energetic photons (3.4 keV) did not exceed the chromium absorption edge. Another effect that could explain the similar rise in slope is the preferential deposition of Cr above Ni, Zn, and Cd due to its relatively negative reduction potential of -0.74 V vs NHE along with the ease of Cr(OH)3formation. I t is very likely that this surface enrichment of Cr is superimposed upon the self-absorption effect because one can still discern the gradual rise in the slope beyond the 1:50 (ppm) ratio for Cd. The minimum detectable concentration ( c m d ) of EDXRF using an electron microprobe is described by Wittry as (28)
(1)
The RF values for Na, K, Mg, Ca, and C1 are 4.61, 4.18, 4.63, 4.36, and 5.25 respectively. Thus, the concentrations of these interfering matrix elements are all reduced by at least 104-fold and the adverse effects of signal suppression, spectral overlap, and plugging of the ICP-MS sampling orifice are mitigated. Quantitative Aspects of X-ray Microanalysis for Multielement Trace Analysis. An internal standardization technique was used to evaluate the feasibility of quantitative X-ray microanalytical determination. Chromium was chosen
where f is the product of the ZAF correction factors due to the difference in atomic number (Z), self-absorption ( A ) ,and secondary fluorescence (F),Q is the efficiency of signal generation (events/incident particle per unit area), e is the efficiency of collection (events detected/events produced), b is the brightness of the source (incident particles/unit time per unit area), t is the data acquisition time, and S J B , is the signal-to-background ratio on a pure standard. Thus, it will be helpful to ensure that the following conditions are met when performing trace analysis: (a) minimize absorption by niobium substrate or matrix by selecting electrodeposition conditions that yield a d.eposit thickness compatible with the depth of electron excitation; (b) choose an optimal tilt angle that favors the generation of X-rays over that of backscattered electrons; (c) adjust specimen height or detector position to obtain a maximum solid angle for X-ray collection; (d) increase the filament current beyond the saturation level for SEM; (e) increase the detector counting time; (f) cool the X-ray detector adequately with liquid nitrogen to lower the thermal source of background noise or use the maximum accelerating voltage allowed because S,/ B, is directly proportional to the difference between the initial electron energy and the critical excitation energy. Typically, EDXRF can achieve a Cmdof only about 0.1-1%. However, owing to the microprobing capability of a fine electron beam, a very small sample weight (M,) can be excited according to the equation (28)
M , = 7~(d'/4)pz (3) where d is the beam diameter (cm), p is the density (g/cm3), and z is the thickness (cm) of the deposit. Usually, lo-'* g can be achieved for M,. Therefore, the minimum detectable mass is on the order of g assuming a c m d of 1% . For the analysis of 100-mL seawater samples containing trace metals at 1 ppb level and assuming an arbitrary deposition efficiency of lo%, a deposit of g can be obtained which exceeds the minimum detectable mass by 6 orders of magnitude. This implies that detection limits below parts per trillion can potentially be obtained. Unfortunately, the present experiments have to sacrifice the superior detection capability of EDXRF using the electron microprobe because of the unduly long times involved for electrodeposition at trace concentrations and the
ANALYTICAL CHEMISTRY, VOL. 62, NO. 10, MAY 15, 1990
Table 11. Comparative Detection Limits Achieved with Electrochemical Preconcentration M
ICP-MS," ppb
EDXRF," ppb
Zn Mn co cu Cr
17 0.022
7.0 21 8.6 3.4 5.3 2.9 1.9 50 36 16
Ni Fe Cd Pb
Hg
3.0 0.2
1.1 0.086 2.3 0.013
0.063 0.030
Detection limits of both methods are computed with respect to the original seawater samples before manipulation.
difficulty of preparing microelectrodes with areas of less than 1 pm2. Comparison of Detection Limits between ICP-MS and EDXRF. Table I1 shows that the detection limits of ICP-MS are generally superior to those of EDXRF. However, the detection limits of the microprobe EDXRF of electrodeposited sample are still about 100-1000 times lower than those obtained by direct analysis of liquid samples with conventional EDXRF (29). Relatively poor detection limits were observed for Mn, Cd, Pb, and Hg. These could be ascribed to the lowest fraction of element being deposited for Mn, the absorption of Cd signal by the niobium substrate, and the relatively high E, values for the most intense L peaks of Hg (12.28 keV) and P b (13.04 keV) and hence the smaller fractions of their atoms being excited by the incident electron beam in the deposit. The detection limits for ICP-MS after the preconcentration are comparable to those reported for directly introduced standard solutions because the dissolved deposit was diluted to the same volume of seawater originally used for the electrodeposition. Better detection limits would be achievable by using a smaller dissolution volume. However, one will readily note that direct determination of seawater without initial electrodeposition will generally require a dilution to reduce the salt content, yielding significantly degraded detection limits for the trace elements of interest. According to Olivares and Houk, a dilution factor of nearly a hundredfold is necessary to reduce the undesirable matrix effects of ionization suppression, decrease in aerosol transport efficiency for analyte, and the plugging of sampler orifice to an acceptable level (24). The ICP-MS detection limits reported here are 1-2 orders of magnitude higher than those reported for seawater using ICP-MS after preconcentration on a silica-immobilized 8-hydroxyquinoline column (30),chiefly due to the difference in the volume of sample used for preconcentration and the final volume used for determination. The use of a flow cell for electrodeposition can conveniently increase the volume of seawater for preconcentration and hence offer better detection limits. In general, the ICP-MS detection limits are excellent for elements that have low background signals and less negative reduction potentials. Cu, Hg, and P b fall into this category whereas Zn, Cr, and Fe have the worst detection limits for the opposite reasons. The fact that the detection limits of Hg and P b are among the worst for EDXRF but are among the best for ICP-MS can be partially explained by the high E , values and hence the lower fractions of Hg and P b atoms being excited for X-ray emission. Various factors account for the relatively poor ICP-MS detection limits that are above the parts-per-billion level for Zn, Co, Cr, and Fe. The various isotopic analogues of CaO+, e.g. 40Ca160+,43Ca160+,and 48Ca160+, seem to be the chief culprits that contribute to the background noise of =Fe+, 59C0+,and &Zn+,respectively. It
1049
Table 111. Significance of Matrix Interferences on Isotope Ratio Determination elemental theoretical observed 70 isotopes ratio ratio error 58Ni/60Ni
2.59
2.38
52Cr/53Cr
8.81
9.07
64Zn/66Zn 56Fe/54Fe
1.76 15.75
1.89 1.46
63Cu/65Cu 56Fe/57Fe
2.24 41.85
2.24 38.53
a
sources of interferences
"Ar180, "ArI70H BONi: uCa160" 3.0 W r : =Ar"N, "ArI5NH, 4oAr12C,36Ar160 53Cr: 38Ar15N,%Ar"NH, 36Ar17036Ar160H 7.4 WZn: 4 8 ~ i 1 6 0 MZn: "Cr16O 90.7 %Fe: @ArL6NH, 38Ar180, 4oAr180,=ArI70H 4oAr14N, "Fe: 38Ar160.36Ar170H 8.1 =Ni:
0 7.9
67Fe: 4oAr160H, "Ar170, 38Ar180H
Boldfaced sDecies are the maior sources of error
is important to note that even though the Ca level in the synthetic seawater has been reduced by about 23 000-fold by electrochemical pretreatment, the ease of CaO+ formation still plagues the analysis of Zn at trace level (i.e. 1-10 ppb). 56Fe+ and 52Cr+are further susceptible to interference by nitric acid and sodium acetate buffer. The acid for dissolution of deposit gives rise to 40Ar160+,36Ar160+,38Ar14N+,and the buffer produces 40Ar12C+,all of which cause spectral overlap. The presence of trace impurities of metals like Fe, Cr, and Cu in the reagent grade chemicals used for preparation of the seawater is indicated by the data for a seawater blank. These impurities are responsible for degrading the detection limits somewhat because the overall noise is equal to the square root of the sum of the noise components due to background and signal. It is conceivable that some of these matrix-derived interferences can be avoided by applying a stripping potential to the niobium electrode. It remains to be seen how the reversibility of the redox process for the metals affects the sensitivity and accuracy of the determination. Accuracy of Isotope Ratio Determination. As mentioned earlier, the extremely ideal isotope dilution technique can be used to account for the different enrichment factors of electrodeposition for the various metals by using an isotopic standard that behaves identically as its analyte counterpart during deposition and dissolution. Table I11 shows the accuracy of isotope ratio determination and the likely sources of interferences for five elements at the 1 ppm level. CaO+ is again suspected of causing errors of 8.1% and 7.4% in the isotope ratio determinations of Ni and Zn, respectively. Furthermore, the presence of ArO+ and ArN+ perturbs the measured isotope ratios of Cr and Fe. The egregiously large error of 56Fe+/"Fe+ determination (90%)is due in large part to ArN+ rather than ArO+ because the error of the corresponding 56Fe+/57Fe+determination is much smaller (7.9%). For Cu isotope ratio determination, which is not susceptible to any matrix-derived interference, the relative error is indeed zero. CONCLUSION The preceding discussion has demonstrated the feasibility of multielement determination of trace metals uiing EDXRF and ICP-MS after electrochemical preconcentration. For EDXRF, careful consideration must be given to the compatibility of the electron excitation range and the deposit thickness. Furthermore, the variables of electrodeposition must be carefully controlled to ensure reliable determinations by both techniques. It is worth noting that the approach of electrochemical matrix rejection can be similarly employed for other complex samples such as marine sediment extracts, industrial wastewaters, and physiological fluids. The conse-
1050
Anal. Chem. 1990. 62. 1050-1055
quent removal of organic constituents and easily ionizable elements like the alkali and alkaline-earth metals should allow a more accurate measurement to be made. The preconcentration achieved with the electrodeposition improves the detection limits of both techniques significantly relative to levels that might have been obtained for direct determination. Much work remains to be done to further establish the accuracy and precision of the techniques. ACKNOWLEDGMENT We gratefully acknowledge the use of electron microscopy facilities at the Ultrastructure Center at the University of Georgia and the ICP-MS results provided by S. J. Jiang and R. S. Houk of Iowa State University. Registry No. Zn (element),7440-66-6;Mn (element), 743996-5; Co (element), 7440-48-4;Cu (element), 7440-50-8; Cr (element), 7440-47-3;Ni (element),7440-02-0;Fe (element),7439-89-6; Cd (element), 7440-43-9; Pb (element),7439-92-1; Hg (element), 7439-97-6; H20,7732-18-5; Nb, 7440-03-1. LITERATURE CITED Akagi, T.; Fuwa, K.; Haraguchi, H. Anal. Chim. Acta 1985, 777, 139. Hudnlk, V.; Gomiscek, S.; Gorenc. Anal. Chim. Acta 1978, 98, 39-46. Danielsson. L. G.: Magnusson, 6.; Westerlund, S. Anal. Chkn. Acta 1978. 98, 47-57. Bruland, K. W.; Franks, R . P.; Knauer, G. A,; Martin, J. H. Anal. Chim. Acta 1979, 105, 233-245. Bond, A. M.; Wallace, G. G. Anal. Chim. Acta 1984, 164, 223-232. Isshiki, K.: Tsujl, F.; Kuwamoto, T.; Nakayama, E. Anal. Chem. 1987, 59, 2491-2495.
(7) Batley, G. E.; Matousek, J. P. Anal. Chem. 1977, 49, 2031-2034. (8) Abdullah, M.: Fuwa, K.; Haraguchi, H. Appl. Spectrosc. 1987, 41, 715-720. (9) Matusiewicz, H.; Fish, J.; Malinski, T. Anal. Chem. 1987, 5 9 , 2264-2269. (10) Manthey, M.; Riley, P. J.; Wallace, G. G. Am. Lab. 1988, July, 21-29. (11) Cox, J. A.; Twardowski, 2 . Anal. Chlm. Acta 1980, 779, 39-45. (12) Koropchak, J. A.: Zlotorzynska, E. D. Anal. Chem. 1988, 6 0 , 328-331. (13) O'Halloran, R . J. Anal. Chim. Acta 1982, 740, 51-58. (14) Hoyer, 6.: Florence, T. M.; Batley, G. E. Anal. Chem. 1987, 59, 1608- 1614. (15) Van den Berg, C. M. G. Anal. Chim. Acta 1984, 764, 195-207. (16) Donat, J. R.; Bruiand, K. W. Anal. Chem. 1988, 60, 240-244. (17) Long, S.E.; Snook, R. D. Ana/yst 1983, 708, 1331-1338. (18) King, J. N.; Fritz, J. S . Anal. Chem. 1987. 59, 703-708. (19) Bond, A. M. 26th Eastern Analytical Symposium Official Program, N.Y., 1987; Paper NO. 202, p 179. (20) Sioda. R . E. Anal. Chem. 1988, 6 0 , 1177-1179. (21) Ciszewski. A.; Fish, J. R.; Malinski, T.; Sioda, R . E. Anal. Chem. 1989, 67, 856. (22) Tan, S. H.; Horlick, G. Appl. Spetrosc. 1988, 40, 446-464. (23) Vaughan, M. A.; Horlick, G. Appl. Spectrosc. 1986, 40, 434-445. (24) Olivares, J. A.; Houk, R. S . Anal. Chem. 1988, 58, 20. (25) Gregoire, C. Spectochim. Acta, Part B 1987, 42, 895. (26) Kester, D. R.; Duedall, I.W.; Connors, D. N.; Ptykowicz, R . M. Limnol. Oceanogr. 1967, 72, 176. (27) Chong, N. S.:Norton, M. L.; Anderson, J. L., unpublished work. (28) Wittry, D. B. European Congress on Electron Microscopy, 3rd Leiden; Elsevier: New York, 1980; p 14. (29) Driscoll, J. N.; Jacobus, N. Am. Lab. 1988, August, 68-75. (30) McLaren, J. W.; Mykytluk, A. P.; Willie, S . N.; Berman, S. S. Anal. Chem 1985, 57, 2907-2911.
RECEIVED for review August 1,1989. Accepted February 15, 1990. We gratefully acknowledge the financial support of NSF Grant CHE-8600224.
Photoinitiation of Peroxyoxalate Chemiluminescence: Application to Flow Injection Analysis of Chemilumophores Robert E. Milofsky and John W. Birks*
Department of Chemistry and Biochemistry and Cooperative Institute for Research i n Environmental Sciences (CIRES), Campus Box 216, University of Colorado, Boulder, Colorado 80309
Photolnltlatlon of peroxyoxalate chemllumlnescence Is reported for the first tlme and applied to the detectlon of amino-substituted polycycllc aromatic hydrocarbons in flow injection analysis. Thls novel detection system Is slmpwfled by ellmlnatlon of the reagent hydrogen peroxide used In most IIqukCphase chemllwnlnescence detection schemes. Like the H,O,-Inltlated reaction, the sensltlvlty of detection in photolnltiated chemilumlnescence (PICL) Is enhanced by use of a base catalyst such as Imidazole. Lhnlts of detection are in the low- to mld-picogram range and are comparable to those obtalned by fluorescence detection using the same apparatus. I t is proposed that the PICL reaction begins wlth hydrogen abstraction by the triplet-excited state of the oxalate ester, followed by addition of 02. Subsequent steps In the mechanlsm proceed to form high-energy Intermediates that transfer energy to the chemllumophore.
INTRODUCTION Since the discovery of peroxyoxalate chemiluminescence in the 1960s and its characterization in subsequent years (1-4), the phenomenon has been applied to the detection of a variety
of analytes in both static (5-9) and flowing systems (10-38). The chemical reactions involved in these detection schemes can be summarized by three basic steps, written as follows: oxalate ester chemilumophore
-
+ H202
intermediates
+ intermediates
chemilumophore*
-
-
chemilumophore*
chemilumophore
+ hu
These reactions are extremely complicated and have numerous side reactions, making them very sensitive to factors such as solvent composition, pH, temperature, and base catalyst (38). Rauhut postulated dioxetanedione as the intermediate (2). McCapra suggested that dioxetanedione transfers its energy to a fluorophore via the chemically initiated electron exchange luminescence (CIEEL) mechanism (39,40). The role of dioxetanedione in peroxyoxalate chemiluminescence recently has been questioned and other high-energy intermediates proposed (41, 42). In this paper we report photoinitiation of peroxyoxalate chemiluminescence for the first time and demonstrate its application to the sensitive detection of amino-PAH, which serve as model chemilumophores. This unique method of analysis, although not as sensitive as hydrogen peroxide
0003-2700/90/0362-1050$02.50/0 8 1990 American Chemical Society
