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Anal. Chem. 1987, 59, 774-778
Micelle-Stabilized Room-Temperature Liquid Phosphorimetry of Metal Chelates and Its Application to Niobium Determination Alfred0 Sanz-Medel,* P e d r o Luis Martinez Garcia, a n d M a r t a Elena Diaz Garcia Department of Analytical Chemistry, Faculty of Chemistry, University of Oviedo, Oviedo, Spain
A detalled examination of 8-hydroxyquinollne and some of its derlvatlves as potential compiexlng reagents for the roomtemperature phosphorescence determlnatlon of Nb(V) In micelles of dlfferent charge-type surfactants has been conducted. Enhanced Huoresceme was observed In most cases. To observe analytical room-temperature phosphorescence (RTP), however, several essential requirements were Identifled: Flrst, charged mlceIJe8 aibwtng electrostatk interaction with the oppositely charged binary complex have to be present. Second, external heavy atoms, ldealy bromoform, should be added. Thlrd, oxygen has to be expelled out of mlcelles, whlch Is accornflshed by Na,SO, addition. Experlmental condltlons, lncludlng SunUght exposwe, are optlmired for the formatlon of the phosphorescent complex nloblum(V)-ferron In micelles of cetyltrlmethylmonlum bromide. Two complexes of metal-ferron with stoichiometries of 1:1 and 1:3 are formed. The last complex constltutes the basis of a RTP method for the determlnatlon of Nb (detection limit, 4 ppb; f2% relative standard devlatlon at 500 ppb of Nb level). Therefore, the flrst example of the determlnatlon of a metal Ion by micelle-stabillred RTP Is reported along with an Insight Into possible mechanisms of these micellar reactions.
The long lifetimes of the phosphorescence phenomenon determine the high probability of excited organic molecules in solution losing their energy by radiationless processes, mainly by collisional deactivation. In order to prevent this nonradiative decay of the triplet state, phosphorescence measurements are usually performed at 77 K (liquid nitrogen). The conventional low-temperature phosphorescence technique has been mainly applied in the areas of biology and medicine. Examples of analytical determinations of metal ions via formation of phosphorescent complexes are scarce. So far, only Be (1,2),boron (3), and N b (4) with dibenzoylmethane and 2-(2’-hydroxyphenyl)benzoxazole,benzoylacetone, and 8-hydroxyquinoline, respectively, have been determined by low-temperature phosphorescence. In spite of the major advantages of conventional low-temperature phosphorimetry (low detection limits, wide range analytical calibration curves, and great selectivity ( 5 ) )the reluctance of analytical chemists to use this technique arises from the need to cool samples to liquid-nitrogen temperatures, from tedious, nonreproducible procedures of sample introduction, and from limitations concerning the solvent choice. An alternative to frozen glassy samples emerged based on stabilizing the triplet state by adsorbing the organic compound on particular solid substrates like filter paper and silica gel (6).
Only recently have advances been made that allow the measurement of room-temperature phosphorescence in normal liquids (RTPL) from many organic molecules, which permits continuous flow operation as required in high-performance liquid chromatography (HPLC) or flow injection analysis (FIA). The simplest method is based on the use of peculiar molecules (7) such as biacetyl, considered a pathological ex-
ception (8) because it emits strong phosphorescence in fluid solution; biacetyl phosphorescence can be used for “sensitized and “quenched” RTPL from potential analytes of organic or inorganic nature (9, 10). Direct RTPL coming from the analyte can be achieved today by other techniques including colloidal dispersions (11), cyclodextrin R T P (12), and micelle-stabilized (MS) R T P (13), the three of them having in common the use of an “organized chemical assembly” able to include the lumiphor minimizing quenching pathways that normally deactivate the triplet state at room temperature (14). Very recently we have shown (15) that micelle-stabilized R T P is facilitated by chemical deoxygenation of the solution using sodium sulfite. The simplicity and convenience of such a deoxygenation technique allow easy MS-RTP measurements. Here, a method for MS-RTP Nb(V) determination, using 8-hydroxy-7-iodo-5-quinolinesulfonic acid (ferron) as phosphor agent, is presented. As far as we know, this is the first paper dealing with application of MS-RTP to a metal ion determination, and thus, a new avenue to the use of phosphorimetry in the trace and ultratrace analysis of metals is opened by this approach. EXPERIMENTAL SECTION Chemicals. Niobium stock solution (200pg/mL, 2.14 X M) was prepared as described elsewhere (16). Standard Nb (V) solutions were prepared fresh by appropriate dilution of the stock solution with 2% tartaric acid solution. Cetyltrimethylammonium bromide solution (CTAB),0.2 M, was prepared by dissolving the surfactant in water by gently warming. Ferron solution, 2.85 X M, was made by dissolving 0.1 g of the reagent in 100 mL of the 0.2 M CTAB solution. Sodium sulfite solution, lo-’ M, was prepared daily by dissolving 2.623 g of Na2S0, in 100 mL of water. A pH 5.7 buffer solution was prepared with acetic acid adjusted to pH 5.7 with 1 M sodium acetate (pH meter control). Bromoform (Merck) was used as received. All reagents were of analytical reagent grade and were used as received. Distilleddemineralized water was used throughout. Apparatus. Phosphorescence emission measurements were made on a Perkin-Elmer LS-5 fluorescence spectrometer,equipped with a Perkin-Elmer 3600 data station, which employs a xenon pulsed excitation source (10-ws half-width, 50 Hz). The delay time used was typically 0.04 ms and the gate time was 2 ms; instrument slits were set at 10 nm throughout this study. A temperature of 15 f 0.1 “C was maintained via a circulating water bath setup. General Procedure for Phosphorescence Measurements. An aliquot of the Nb(V) solution containing up to 10 pg of niobium was transferred into a 10-mL standard flask. Then 2.6 mL of 0.2 M CTAB solution, 2 mL of HAc/NaAc 1 M buffer pH 5.7, 0.25 mL of 2.85 X M ferron solution, 0.02 mL of Br,CH, and 0.9 mL of 0.1 M Na2S03solution were added. After dilution to the mark and mixing thoroughly, argon was flowed through the top of the flask. The solution was then allowed to stand about 30 min. Finally, the solution was transferred to a stoppered sample cell and exposed to visible light for 15-20 min. Phosphorescence intensity was measured at 593 nm with excitation at 383 nm. Reagents blanks lacking Nb(V) were prepared and measured following the same procedure. RESULTS AND DISCUSSION Experimental Background. Preliminary experience on the use of sulfite as oxygen scavenger in micellar media showed that it was possible to observe room-temperature phos-
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Table I. Luminescence Properties of Niobium(V)-Hydroxyquinoline Complexes in Micellar Media
Nb(V)-complex oxine
surfactant CTAB
5,7-dichlorohydroxyquinoline 5,7-dibromohydroxyquinoline
8-hydroxy-5-quinolinesulfonicacid 8-hydroxy-7-quinolinesulfonicacid ferron
re1 fluorescence intens
A,,
hem,
nm
nm
PH
360
500
6.2
100
382
505
300"
386 383 383 357
510 593 593
6.2 6.2
474
8.0
317b
350
234' -b
-b
5,7-dichlorohydroxyquinoline 5,7-dibromohydroxyquinoline
350, 385 350, 397
520 532 532
6.2 6.2 6.2
ferron
347
473
7.3
117 44 40 67
347
473
8.3
150
8-hydroxyquinoline
ferron
Brij-35
SDS
The system also displays delayed fluorescence. The complex displays phosphorescence at lower pH. phorescence from naphthalene and other different aromatic polycyclic hydrocarbons when dissolved in sodium lauryl sulfate micelles using T1+ as heavy atom (15). In our search for improved sensitive and selective determinations for niobium (16, 17), the above approach was assayed for several niobium chelate complexes. Considering that the niobium(V)-8-hydroxyquinoline complex displays low-temperature phosphorescence (4),we started by selecting 8-hydroxyquinoline and some of its derivatives as lumophoric reagents for MS-RTP experiments. The influence of micellar media of different charge types (i.e., anionic, sodium lauryl sulfate; nonionic, Brij-35; and cationic, CTAB) on the fluorescent and phosphorescent emission characteristics of the different niobium complexes was studied by employing surfactant concentrations well above their respective critical micelle concentration. The effects of different heavy atoms were studied in the search for phosphorescence signals: T1+ and Ag+ (usually with anionic micelles), Br- and I- (with cationic micelles), and Br3CH and 13CH (with anionic, cationic, and nonionic micelles). It was observed that the presence of an external heavy atom is of paramount importance for observation of MS-RTP. A summary of the photoluminescent results observed in all these exploratory experiments is given in Table I. Some interesting features are inferred from Table I data concerning the observed fluorescent behavior: (a) Nonionic micelles of Brij-35 seem to display only its solubilization power: niobium(V)-oxine complex is more fluorescent than the other halogen derivatives in such micelles as expected considering the heavy atom effect due to the halogens. (b) It seems apparent that for halogen derivatives of oxine, cationic micelles exhibit an enhancement or "sensitization" effect. In such media intensity signals are three times those of the oxine complex. (c) On the other hand, anionic micelles of SDS solubilizate all complexes, but only that with ferron showed measurable fluorescence. At the same time all the complexes in solution were tested for phosphorescence signals (following the analytical procedure). Curiously enough, it was noticed that only those complexes with a reagent bearing the sulfonic group displayed micelle-stabilized room-temperature phosphorescence in a cationic micellar media and using Br3CH as external heavy atom. The order of phosphorescence emission intensity was Nb(V)-ferron > Nb(V)-8-hydroxy-5-quinolinesulfonicacid > Nb(V)-8-hydroxy-7-quinolinesulfonicacid. In view of these preliminary results we studied in detail the features of the phosphorescent analytical signals of the Nb(V)-ferron complex.
460
520
580
640
A q nm Figure 1. Photoluminescence spectra of niobium(\/)-ferron complex at pH 5.9 In CTAB micelles for different "delay" times: (-) 0.01 ms, (---) 0.02 ms, (---) 0.03 ms, 0.04 ms; t , = 2 ms; scale, X1; M, [ferron] = 3.25 X [Nb(V)] = 1.07 X M, [CTAB] = 5 X lo-' M, [Na,S03] = 1 X lo-' M, 0.2% (vlv) Br,CH; A, 363 nm. (..e)
Phosphorescence Spectral Characteristics. The total luminescence spectrum of the niobium complex in CTAB micelles, with Br,CH as heavy atom and Na2S03as oxygen scavenger, is shown in Figure 1 (solid curve). The time-resolved spectra are also depicted in Figure 1;the three curves in the lower portion of the figure correspond to different delay times and show that as the delay time is increased, the broad fluorescence band at 480 nm decreases while the peak at 572 nm, which is due to the complex phosphorescence emission, remains. Even after a delay time as long as 0.1 ms, such phosphorescence band still remains. In subsequent studies all phosphorescence measurements were carried out with a delay time of 0.04 ms, in order to secure negligible background emission of blank reagents with a minimum sacrifice of the signal intensity. The optimum pH for the phosphorescent complex formation in micelles lies in the range 5-6.2 as shown in Figure 2. For further experiments we selected a pH 5.7 buffer (1 M HAc/NaAc). Cationic Micellar Media Influence. Detailed information on the interaction of the phosphorescent complex with the surfactant was obtained by examining the influence on the phosphorescence intensity of the CTAB concentration, both below and above the surfactant's critical micelle concentration (9.55 X lo-' M (18)).At CTAB concentration levels below its critical micelle concentration the surfactant essentially does not influence the complex luminescence. (CTAB
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987
0
?c I
0.9
1.8
2.1
B r c n , . % v/v
Figure 2. Optimum pH for niobium(\/)-ferron phosphorescent complex formation in CTAB micelles (scale, X2.5; experimental conditions as in Figure 1): t , = 0.04 ms; t , = 2 ms; A,, 363 nm; A,, 567 nm.
concentrations well below its cmc could not be thoroughly studied due to Br3CH solubility limitations.) Further addition of CTAB above the cmc resulted in a continuous increase in phosphorescence intensity, until a plateau is reached at a CTAB concentration about 20 times the cmc value. A surM was considered adequate for factant concentration 5 X subsequent experiments. As observed in fluorescence studies (17), the nature and geometry of the cationic polar head of the surfactant play a paramount role. Although we observed MS-RTP by using cetylpyridinium bromide micelles, the phosphorescence signal is higher in CTAB micelles due, quite probably, to the fact that the positive charge in the head group of cetylpyridinium is more delocalized causing the electrostatic interactions to be weaker (17). Quenching properties of the pyridinium ring should be taken into account as well. No phosphorescence was observed in either anionic and nonionic micelles. Heavy Atom Effect. After close experimental examination of oxine and its derivatives as possible organic phosphors for Nb(V) in micelles a t room temperature, it was observed that the specific presence of a sulfonic group and external heavy atom addition seem to be decisive in obtaining RTP. Only the sulfonic derivatives tested produce R T P and even the reagent ferron with its stronger internal heavy atom (most intrinsically favored intersystem crossing) hardly produced RTP in the absence of added heavy atoms. Considering the cationic character of the micelles of CTAB, Br- and I- were initially tested as heavy atoms residing near the cationic micellar surface (where the niobium complex would be bound). Unfortunately, we observed that the niobium(V)-ferron complex was not sensitive to that particular external heavy-atom perturbation. Cations as Ag+ or T1+were also unsuitable since precipitate formation is observed with the micelle counterior (Br-) and these ions also tend to displace Nb(V) from its complex. Small haloalkanes, 13CH and Br,CH, can also be used as external heavy-atom sources. Direct comparison of 13CHand Br3CH particularly reveals the specificity of the heavy atom effect. Whereas Br3CH strongly enhances the complex phosphorescence, I,CH did not appreciably cause any enhanced phosphorescent signal. It is important to note at this point that we are dealing with a ternary system (water + surfactant + Br,CH) and, depending on the relative amount of every component, it is
Flgure 3. Influence of the heavyatom content on the metal-chelate phosphorescence at different surfactant concentratins (scale X2.5; rest of experimental conditions as in Figure 2): pH 5.6; [CTAB] (0) 2.6 X M, (0)5.2 X lo'* M, and ( X ) 7.2 X M.
possible to observe the appearance of a liquid crystalline phase (anisotropic phase), two liquids, a solid phase, or a liquid phase (19). The amount of Br&H solubilized in the micellar media (without phase changes) depends mainly on the surfactant concentration. Figure 3 shows the effect of Br3CH concentration on the niobium(V)-ferron complex phosphorescence for three different CTAB concentrations (all a t which an isotropic liquid of normal micelles is the stable phase). It is M apparent that a surfactant concentration about 5 x (which coincides with that previously selected) and a 0.2% v/v Br3CH are adequate for analytical purposes. Chemical Deoxygenation: Effect of Sample Irradiation. The influence of increasing Na2S03concentration on the luminescence properties of the niobium(V)-ferron complex was determined and the results demonstrated that 6 X M Na2S03is enough to obtain maximum phosphorescence intensity. A slight decrease in phosphorescence intensity was observed a t higher sulfite concentration (above M) and this effect may be due to static quenching or disturbance of the micellar surface by the high ionic content of the solution SO,2-). (Na+, Several interesting phenomena were observed in connection with the presence of Na2S03in the micellar solution. Early experiences showed the important effect of sunlight sample preirradiation in order to obtain a strong and stable phosphorescence signal. Therefore we compared the phosphorescence obtained for a sample exposed to sunlight and that of another kept out from natural light. Results showed that higher signals were always attained after irradiation. Several experiences (sample irradiation with a tungsten lamp at different wavelengths) only revealed that the complex was not being photodecomposed and that visible light was responsible for this effect. A sample kept inside the sample compartment and irradiated continuously a t the excitation wavelength of the complex produced only half of the normal signal obtained by illumination with sunlight, and illumination in glass cuvettes produced the same effects observed in quartz cuvettes. The need of sample sunlight preirradiation seems to be related with the oxygen consumption in the special ternary system used in this work (water + surfactant + Br3CH). In fact, for the complex of ferron with A13+,currently under study, this effect has not been observed at all. Composition of the Complex. The nature of the complex was investigated by the continuous variation and molar ratio methods. As shown in Figure 4,depending on the excitation and emission wavelengths, two complexes, the 1:l and the 1:3
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987
0
2
4
6
0
10
iitnnoiii I O - ~ M
Figure 4. Influence of dye concentration on the phosphorescence intensity (scale, X2.5: [Nb(V)] = 5.35 X lo-' M, [Na,SO,] = 1 X M, 0.2% (v/v) Br,CH, [CTAB] = 5.2 X lo-' M; pH 5.6; ( X ) A,, 363 nm, A,, 567 nm; (0) A,, 383 nm, A,, 593 nm; t , = 0.04 ms, t , = 2 ms.
Scheme I Nb(V1 + forron
pH 6 .
[NblVl-ferronl binary soluble complex weakly fluorescent
CT*B 8 [Nb(Vl-Ierronl micelle micollar complex non-fluorescent
, 4
5
8
7
6
9
PH
Figure 5. Comparison of the pH effect on the fluorescence (A,, 362 nm, A,, 491 nm; scale, X1) and on the phosphorescence (A,, 363 nm; A, 567 nm; scale, X2.5) of the niobium chelate: (0) fluorescence measurements; ( X ) phosphorescence measurements (td = 0.04 ms; t , = 2 ms).
// \i""
0%
(Nb(V1-ferronl
ELECTROSTATIC FORCES
micellar complex PHOSPHORESCENT
metal:dye ratio, seem to build up concurrently. The former seems to be more "phosphorescent" than the latter, although it is the 1:3 complex, formed at the relatively high dye concentration used in the analytical procedure, which is the analytically useful complex. The ferron concentration must and 7 X M be carefully controlled between 4 X because the inner-filter effect (as Figure 4 demonstrates) is observed a t higher concentrations. Temperature Influence. Temperature is an important parameter that highly influences luminescence emissions. An anomalous temperature effect was observed for the niobium complex phosphorescence: Between 15 and 5 "C no significative change in phosphorescence was noticed. This behavior is analytically beneficial since rigorous temperature control is not necessarily required. Temperatures lower than 5 "C caused surfactant crystallization and higher than 17 "C caused a linear decreasing of the signal. An optimum temperature of 15 O C was selected for further experiments. Analytical Performance Characteristics. Standard calibration graphs prepared according to the recommended procedure were linear, passing through the origin for niobium concentrations up to 1pg/mL in the solution. The detection limit, using the 2uB criterium (uB being the standard deviation of the blank) was found to be 4 ppb. The standard deviation, determined by measurement of the phosphorescent intensity of 10 replicates each containing 5 pg of Nb(V), was &2%. Preliminary experiments on foreign metallic element effects have shown two types of interferences, namely, metal ions that form phosphorescent complexes, such as aluminum, which interfere positively, and the more numerous metal ions forming nonphosphorescent complexes, which interfere negatively by combining with ferron and thus reducing the amount available for Nb(V) complex formation. Reaction Mechanism. In view of the previously mentioned experimental facts, Nb(V) should react in the micellar
( w
d
H Y D R O P H O B I C INTERACTIONS
Flgure 6. Schematic representation of the phosphorescent niobium(V)-ferron complex in CTAB micelles.
media according to Scheme I. At pH ca. 6, Nb(V)reacts with ferron to form a binary soluble complex that is weakly fluorescent. If CTAB solution is added to the binary complex solution, the complex became nonfluorescent at pH ca. 6 (although it is highly fluorescent at pH around 8 as shown by Figure 5). External heavy atom (such as Br3CH) addition and O2elimination at pH 6 lead to the development of RTP. It seems that in this latter situation the complex finds a new microenvironment in which hydrophobic and electrostatic forces acting simultaneously ( 1 7) are able to shelter the triplet state at room temperature. Electrostatic interaction between the metal complex and the micellar surface is essential to observe phosphorescence: Those oxine derivatives lacking the sulfonic group gave no rise to RTP signals, but ferron, 8-hydroxy-5-quinolinesulfonic acid, and 8-hydroxy-7-quinolinesulfonicacid did. Then, only electrostatic and hydrophobic forces acting concurrently seem necessary to secure MS-RTP. In the modern Dill conception of micelles (20),the complex does not need to be buried inside micelles; bending of surfactant's tails toward the aqueous phase would provide the necessary hydrophobic contact with the complex within the micellar surface as visualized in Figure 6. Niobium was proved to show a peculiar complexation behavior in micellar media (17,21). Most metal ions, including Ti(IV) (22)tends to coordinate more dye molecules in micellar media than in aqueous media (23);we have failed to find any fluorescent complex of Ti(1V) in micelles. Niobium, on the
778
Anal. Chem. 1987, 59, 778-783
contrary, tends to react with the same number, and even with fewer dye molecules producing fluorescent reactions. As shown above, Nb(V) reacts with ferron in micellar media forming simultaneously the 1:l and the 1:3 complexes, the former being more phosphorescent than the latter. These facts tend to indicate that the more dye molecules around the metal ion, the higher is the probability of nonradiative deactivation (perhaps related to steric hindrance of the complex to fit on the micellar surface in a more planar and immobilized situation). Of course, this topic is rather speculative and awaits much more experimental work. Registry No. CTAB, 57-09-0;SDS, 151-21-3;Nb, 7440-03-1; Brij-35, 9002-92-0; ferron, 547-91-1. LITERATURE C I T E D (1) Solovev, E. A,; Lebedeva, N. A,; Sidenko, Z. S. Zh. Anal. Khim. 1974, 29, 1531-1534. (2) Holzbecher, 2 . ; Hafrnanek, M.; Sobalik, Z. Collect. Czech. Chem. Common. 1918, 4 3 , 3325-3338. (3) Marcantonatos, G. F.: Garnba, G.; Monnier, D. Anal. Chim. Acta 1973, 67,220-224. (4) Kirkbright. G.F.; Thompson, J. V.; West, T. S. Anal. Chem. 1970, 42, 782-784. (5) Aaron, J. J.; Winefordner, J. D. Talanfa 1975, 22, 707-715. (6) Hurtubise. R. J. Anal. Chem. 1983, 5 5 , 669A-678A. (7) Parker, C. A,; Hatchard, C . G. J . Phys. Chem. 1962, 66, 2506-2511. (8) Turro, N. J.; Liu, K-Ch.; Chow, M. F.; Lee, P. Photochem. Phofobiol. 1978, 27, 523-529. (9) Frei, R. W.; Birks, J. W. Eur. Spectrosc. News 1984, 5 7 , 15-20.
(10) Gooijer, G.; Velthorst, N. H.; Frei, R. W. Trends Anal. Chem. 1984, 3 , 259-265. (11) Weinberger, R.; Cline Love, L. J. Appl. Spectrosc. 1985, 39, 5 16-5 19. (12) Scypinski, S.;Cline Love, L. J. Anal. Chem. 1984, 56, 322-327. (13) Cline Love, L. J.; Skrilec, M; Habarta, J. G. Anal. Chem. 1980, 52, 754-759. (14) Cline Love, L. J.; Weinberger. R. Specfrochim. Acta., Parf 6 1983, 386,1421;1433. (15) Diaz Garcia, M. E.; Sanz-Medel, A. Anal. Chem. 1986, 58, 1436- 1440. (16) Sanz-Medei, A.; Cdrnara Rica, C.; Perez-Bustamante. J. A. Anal. Chem. 1980, 52, 1035-1039. (17) Sanz-Medel, A.: Garcia Alonso, J. I.; Blanco Gonzilez, E. Anal. Chem. 1985, 5 7 , 1681-1687. (18) Fennell-Evans, D.;Allen, M.; Ninham, B. W.; Fouda, A. J . Solution Chem. 1984, 13, 87-101. (19) Shinoda, K. Solvent Properties of Surfactant Solutions; Marcel Dekker: New York, 1967; pp 16-20. (20) Dill, K. A.; Koppel, D. E.; Cantor, R. S.;Dill, J. D.; Bendedouch, D.; Chen, S. H. Nature (London) 1984, 309, 42-45. (21) Dlaz Garcja, M. E.; Sanz-Medel, A. Talanfa 1985, 32, 189-193. (22) Diaz Garcia, M. E.: Blanco Gonzllez, E.; Sanz-Medel, A. Microchem. J . 1984, 30, 211-220. (23) Savvin. S. B. CRC Crit. Rev. Anal. Chem. 1979, 55-109.
RECEIVED for review March 26, 1986. Accepted September 22, 1986. Support from the Comision Asesora para la Investigacih Cientifica y TBcnica (CAICYT), Proyecto No. 2837183, is gratefully acknowledged. This work was presented at the European Congress on Molecular Spectroscopy, Madrid (Spain), Sept. 1985 (Abstract No. 260).
Determination of Trace Metals in a River Water Reference Material by Inductively Coupled Plasma Mass Spectrometry Diane Beauchemin,* J. W. McLaren, A. P. Mykytiuk, a n d S . S . Berman
Analytical Chemistry Section, Chemistry Division, National Research Council of Canada, Ottawa, Ontario, Canada K 1 A OR9
The detectlon power of inductlvely coupled plasma mass spectrometry (ICP-MS) and Its capaclty for rapM multlelement analysis were demonsh.ated by the analysis of a rlverlne water reference materlal. Fifteen elements (Na, Mg, K, Ca, AI, V, Cr, Mn, Cu, Zn, Sr, Mo, Sb,Ba, and U) were determined dlrectly while five (As, Co, Ni, Cd, and Pb) requlred a preconcentration prior to analysis, elther by evaporatlon (As) or by chelation by slWcaimmaMUzed 8-hydroxyqulnoflne (Co, NI, Cd, and Pb). Accurate results were obtalned by external calbratlon, standard addnlons, or Isotope dllutlon techniques. However, stable isotope dUutlon generally glves the most accurate and precise results.
Inductively coupled plasma mass spectrometry (ICP-MS) is a powerful technique many features of which have been summarized in three recent review articles (1-3). Essentially, it combines the high detection power of mass spectrometry with the capability of simultaneous elemental analysis of solutions. Furthermore, it enables very rapid isotope ratio determinations which in turn makes possible stable isotope dilution techniques. However, the application of ICP-MS to routine analysis has been somewhat hampered by its greater susceptibility to ionization interferences than inductively coupled plasma atomic emission spectrometry (ICP-AES) (3, 4) and by the problem of isobaric interferences from molecular species arising from either the solvent used in sample preparation (5,6)or from the sample itself (7-9). This is why most 0003-2700/87/0359-0778$0 1.50/0
of the analyses performed to date have required some pretreatment of the sample, either a special dissolution (e.g., ref 10) for solid samples and/or a separation with preconcentration (6, 11). A few reports describe the direct analysis of water samples (12-14). Taylor and Garbarino (12) reported the determination of metals in a standard reference water sample by using stable isotope dilution. Date and Gray (13) illustrated their system performance with the determination of 13 trace elements in three international standard reference water samples. Boomer and Powell (14) discussed a method for estimating the concentration of Al, Mn, Fe and Zn in acid precipitation. In all these cases, the water analyzed did not contain high salt concentrations; thus, no great problems of ionization interferences were encountered. Recently, the development of a river water reference material in this laboratory, with the acronym SLRS-l, provided an opportunity to assess the performance of ICP-MS when determining many trace metals directly in the presence of a complex matrix. The certificate for SLRS-1 gives the total concentrations of 21 elements. ICP-MS was used for the determination of 20 of them: 15 directly in the water itself, and 5 after a preconcentration. EXPERIMENTAL SECTION Instrumentation. The inductively coupled plasma mass spectrometer used for this work was the ELAN 250 from SCIEX Division of MDS Health Group, Ltd. (Thornhill, ON, Canada). Three modifications were made to the originally supplied in0 1987 American Chemical Society
