416
Anal. Chem. lB88, 60,416-420
Paper Substrate Room-Temperature Phosphorimetry of Polyaromatic Hydrocarbons Enhanced by Surface-Active Agents G. Ramis Ramos,l M.C. Garcia Alvarez-Coque,’ A.
M.O’Reilly, I. M. Khasawneh? and J. D. Winefordner*
Department of Chemistry, University of Florida, Gainesville, Florida 32611
The effect of the presence of surfactants of different character (sodium dodecyl sulfate, sodium decyl sulfate, BrlJ35, and dodecyitrlmethylammonlm chloride) on the paper-subdrate room-temperature phosphorescence (PS-RTP) of 88vera1 polyaromatlc hydrocarbons (PAHs) and carbazole was studkd. Thallun nitrate was used as a heavy-atom pertwber. When an anlonk surfactant was added or when analytes were spotted from micellar solutlons, enhancements of the sendtlvky ranglng between factors of 2 and 9 were found. On the other hand, the PS-RTP signal was totally quenched In the presence of the cationic surfactant. Similar effects were found for the phosphorescence of the paper background. Absolute limits of detection In the range of 0.2-3 ng were obtained In the presence of sodium dodecyl sulfate. A discuuion on the pordble mechanisms producing the observed effects Is Included,
Room-temperature phosphorimetry on solid substrates and, particularly, on paper (PS-RTP) has been developed into a sensitive, selective, and simple method for the analysis of small amounta of many organic compounds. Applications have been described including drugs (1-3), pesticides (4-61, and other compounds of environmental and biomedical interest (7). Two major reviews of the field have been made by Hurtubise (8) and Vo-Dinh (9). Phosphorescence emission intensity depends strongly on the environment of the phosphors. Immobilization in a rigid medium is required to avoid collisional deactivation. The cellulose fibers of the paper seem to offer an excellent protection for a wide variety of molecules. The presence of moisture reduces the emission intensity which has been attributed to a “softening” effect of the medium, as well as to an increase in the diffusion rate of quenchers as oxygen. On the other hand, a heavy atom, such as T1+,Ag+, or I-, which increase the intersystem crossing rate, is necessary to obtain RTP of nonpolar compounds, such as polyaromatic hydrocarbons (PAHs), and also to enhance the signal of some polar compounds. Association of the analyte to the surface by hydrogen bonding has been shown to be very important to induce RTP from polar compounds (8,9).Weaker interactions probably also play an important role and seem to be necessary to explain the presence of RTP of nonpolar compounds. An entrapment mechanism has also been suggested as a possible means to achieve the required environmental rigidity for all types of molecules (10). Cellulose undergoes considerable swelling in the presence of strongly polar solvents, particularly water, due to solvation of the hydroxyl groups. Penetration of the solvent inside the fibers opens submicroscopic pores which are acOn leave from Department of Analytical Chemistry, University of Valencia, 46100 Burjasot, Valencia, Spain. On sabbatical leave from Department of Chemistry, Yarmouk University, Irbid, Jordan. *To whom all correspondence should be sent.
cessible to the phosphors. Subsequent entrapment occurs when the fibers collapse upon desiccation. This entrapment mechanism does not exclude the contribution of other phosphor-cellulose chemical interactions. X-ray photoelectron spectroscopy (XPS) has been used to study the distribution of the phosphors and heavy-atom enhancers through the substrate (11,12). The results indicated that both, heavy atoms and organic molecules penetrate into the paper, the former more extensively than the latter. de Lima et al. (12) suggested that the use of a surfactant could retain more heavy atoms near the surface, thus producing enhanced RTP signals. Enhancements of the PS-RTP emission of carbaryl up to 1OX were found when silver and thallium dodecyl sulfates were used instead of the respective nitrate salts. The XPS data indicated a higher concentration of the heavy atoms near the paper surface. However, the retention of heavy atoms near the surface could be due to a change in the solvent used to spot the heavy-atom salt rather than the presence of the surfactant. Thallium and silver nitrates were spotted from aqueous solutions, whereas ethanol and 2-propanol were used to spot the respective dodecyl sulfates for the XPS studies (12). It has been shown that organic compounds spotted from ethanol or propanol solutions give RTP signals up to 2 orders of magnitude lower than when spotted from water (10). The results were attributed to the lack of penetration and poor entrapment of the compounds into the fibers, which is promoted extensively by strong polar solvents as water. It seems to be likely that the large surfactant salts will behave similarly to the phosphors. Besides, two types of distribution of the species must be considered in the paper substrate. First, the species can undergo different degrees of penetration inside the individual fibers which depends upon swelling of the fibers, solubility in water, polarity, molecular size, and steric hindrance. Second, the distribution along randomly intertwined layered fibers can be also considered. In this case, factors such as volatility of the solvent used to spot the compounds and the chromatographic properties of the species are important. Although more work has to be done to establish the relative importance of the diverse mechanisms which possibly contribute to the RTP signal, the results of de Lima et al. (12) suggested that the use of surfactants as substrate modifiers in PS-RTP could be promising. In this work, the effect of the presence of anionic, nonionic, and cationic surfactants on the PS-RTP of several PAHs and carbazole is studied. Enhancing and depressing effects, spectral shifts, changes in phosphor lifetimes, and photostability of both the analytes and phosphor impurities of the paper are discussed. Analytical figures of merit are also given.
EXPERIMENTAL SECTION Reagents. Hexane, heptane, and methanol (high-puritysol-
vents, American Burdick and Jackson, Muskegon, MI), 1-pentanol and Brij 35 (Aldrich Chemical Co., Milwaukee, WI), dodecyltrimethylammonium chloride (DTAC) and sodium decyl sulfate (EastmanKodak Co., Rochester, NY), and sodium dodecyl sulfate
0003-2700/88/0360-0416$01.50/00 1988 Amerlcan Chemical Society
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417
Table I. Influence of the Surfactant Character
peaks found surfactant sodium dodecyl sulfate sodium decyl sulfate Brij 35
dodecyltrimethylammonium chloride
intensitp sample blank
L, nm
Xemtb nm
260 260 260 260d
450 (81),478 (100) 450 (81),476 (100) 451 (84), 475 (100) 474d
472 560 58 18
enhancement factore 4.4 5.3 0.5
100 135 17 14
OPaper spotted with 2 pL of 0.25 M T1N03 and 5 pL of 8 pg/mL o-terphenyl in a 2% surfactant solution. *Relative intensities in parentheses. cIntensity at the most intense peak of the analyte. dNo peak observed, but this wavelength was used for the intensity measurements. ‘Enhancement factors were calculated with respect to the intensity obtained by spotting 2 pL of 0.25 M TINOBand 5 pL of 8 pg/mL o-terphenyl in hexane.
(SDS)and reagent grade thallous nitrate (Fisher, Fair Lawn, NJ) were used as received. Aqueous solutions were made with Nanopure deionized water (Barnstead Sybron Corp., Boston, MA). Fluorene and 2,3-dimethylnaphthalenewere recrystallized three times from ethanol, and 0-terphenyl waa recrystallid three times from methanol. Other PAHs and carbazole from different sourcea were used as received. Stock solutions of the PAHs were prepared by dissolving 1-5 mg in 0.5 mL of heptane and 0.5 mL of 1-pentanol and diluting to 25 mL with aqueous 0.5 M SDS,Similar amounts of the PAHs were dissolved in hexane and in aqueous solutions of the other surfactants, in the latter case, making use of an ultrasonic bath (Cole Parmer, Chicago, IL). Carbazole was dissolved in pure methanol. Apparatus. Phosphorescence spectra were obtained with a Perkin-Elmer LS-5 luminescence spectrometer interfaced to a Model 3600 data station. Unless otherwise stated, excitation and emission slit widths were set at 10 and 5 nm and delay and gate times were 0.03 and 9 ms, respectively. A laboratory-builtsample compartment, previously described (13) was used. Strips of Whatman No. 1filter paper were held on the sample holder by means of a blackened metallic frame and two screws. A 6 X 6 mm2square window limited the exposed surface. The reagents and samples were spotted on the paper in the center of the window using 5-pL Hamilton syringes. The paper was then dried for 15 min in an oven at 110 O C and allowed to cool in the sample compartment for 10 min under a stream of N2which was previously dried over CaS04. The sample holder position was optimized for each sample (13). o-Terphenyl was chosen as a model compound for most of the studies. In these experiments, excitation and emission wavelengths were 265 and 474 nm, respectively. RESULTS AND DISCUSSION Order of Addition of the Reagents. When the paper substrate was spotted with 2 pL of 0.25 M TINOBand 5 pL of 16 pg/mL o-terphenyl solution in hexane, the average RTP intensities were 170 f 10 and 35 i 5 for the analyte and paper background, respectively (four trials). No significant differences were observed when the reverse order of addition was used. When 5 pL of a 0.5 M SDS solution was also added, enhancements of both, analyte and background signals, were observed. The enhancement of the net signal ranged between 60% and 80%, depending on the order of addition of the three reagents. However, the best results were obtained when the analyte and the surfactant were spotted together using an aqueous micellar solution of the former; 5 pL of 0.5 M SDS containing 60 pg/mL o-terphenyl was used. Sample and background intensities were 375 f 15 and 90 f 10, respectively (four trials). Surfactant and Thallium Concentrations. To study the influence of the SDS concentration, a solution of 160 pg/mL o-terphenyl in 0.5 M SDS was prepared. Dilutions containing 16 pg/mL o-terphenyl and different concentrations of SDS were made. Two microliters of 0.25 M TINOBand 5 pL of the micellar solutions were spotted on the paper. The results are shown in Figure 1. The three points with the highest amounts (equivalent to molar concentrations above 0.6 M) of SDS were obtained by spotting the analyte solution
0
01 0
0.1
SOS(Y) 0.4 0.6
0.6
9 .O
1 .I
2.0
2.6
3.
SDS (me)
Flgurr 1. Influence of SDS concentration: (a) paper spotted wlth 2 pL of 0.25 M TINO, and 5 pL of 16 pg/mL o-terphenyl In SDS at various concentratlons; (b) blanks; (c) net slgnals. See text for the three polnts wlth the largest amounts of SDS.
several times and drying the paper between applications. In this case, a 16 pg/mL solution of o-terphenyl in 0.5 M SDS was diluted 2,3, and 4 times with 0.5 M SDS and spotted the same number of times. The emission intensity decreased as the surfactant concentration increased. An enhancement factor of the net signal of 5X was obtained with 0.05 M SDS. Smaller concentrations of the surfactant were not tried due to the limited solubility of o-terphenyl in aqueous micellar solutions containing a low concentration of SDS. Surfactant concentrations of 2% (corresponding to 0.07 M SDS)were used below. As shown in Figure 2, the phosphorescence intensity increased rapidly with thallium concentration, but no further improvement was achieved beyond 0.1 M, where saturation of the substrate seemed to have been reached. A thallium concentration of 0.25 M was adopted for the experiments below. Influence of the Surfactant Character. Triplicate experiments were performed in the presence of TINOBusing surfactants with 12-carbon-atomlinear aliphatic tails but with different head character. The polar head groups were sulfate and trimethylammonium in SDS and DTAC, respectively, and a 23-unit polyoxyethylene chain in Brij 35. A shorter tailed anionic surfactant, sodium decyl sulfate, was also used. As shown in Table I, enhancement factors were similar for the two anionic surfactants, but drastic intensity reductions were observed in the other two cases. In the presence of the cationic surfactant, the spectrum of the analyte disappeared completely. On the other hand, the phosphorescence intensity of the blanks was similarly affected, increasing by a factor of 3-4 with anionic surfactants and decreasing by a factor of 2-3 with Brij 35 and DTAC. Phosphorescence Spectra of Several PAHs and Carbazole. A comparison of the spectral characteristics for nine PAHs and carbazole, spotted from hexane (methanol for carbazole) and aqueous 2 70SDS solutions, is given in Table 11. When micellar solutions were used, the excitation spectra of the PAHs showed small blue-shifts (6 nm) and somewhat
418
ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988
Table 11. Paper-Substrate R T P Spectral Characteristics of PAHs and Carbazole Spotted from Hexane and 2% SDS Aqueous Micellar Solutions and in the Presence of TINOJ paper spotted with hexane solutions
compound biphenyl
Xexe?
half widths,c nm
nm
444 (71), 471 (loo), -495 (72) sh 451 (73), 477 (loo), 31 (1) -500 (76) sh 416 (72), 437 (loo), 19 (21, -462 (65) sh 14 (1) 454 (71), 481 (loo), 31 (1) -505 (76) sh 43 (3-4) 495 (90),515 (100)
266
p-chlorobiphenyl 2,3-dimethylnaphthalene fluorene phenanthrene pyrene retene o-terphenyl
258 (loo), 291 (94) 268 277 271 (loo), 299 (48) 259 (loo), 294 (52) 324 (76), 337 (100) 268 (loo), -308 (15) sh 266
32 (1) 34 (2) 29 (2) 33 (1) 32 (1)
Aexwb
paper spotted with micellar solutions half half widths: widths: nm nm A,,,b nm nm
261
29 (1)
m,m '-bitoluene 267 carbazole"
nm
half widths,' nm
441 (67), 472 (1001, -500 (81) sh 452 (72), 478 (loo), -505 (74) sh 416 (74), 438 (loo), -462 (67) sh 453 (71), 482 (loo), -505 (76) sh 488 (881, 518 (100)
262 260 (loo), 292 (89) 264 276
430 (73), 459 (loo), -475 (78) sh 472 (94), 505 (loo), 544 (55) 594 (loo), -605 (70) sh 481 (99), 514 (100)
265 (loo), 297 (42) 257 (loo), 292 (38) 320 (78), 337 (100) 265 (loo), -300 (26) sh 260
450 (78), 478 (loo), -500 (76) sh
430 (72), 459 (loo), -485 (72) sh 472 (92), 505 (loo), 543 (52) 594 (1001, -605 (71) sh 482 (97), 514 (100) 450 (81), 478 (loo), -510 (72) sh
"Methanol instead of hexane was used for carbazole. bRelative intensities in parentheses; sh = shoulder. cIn parentheses, number of bands involved.
\
/J
b 0
0
t
/
01
a60
400
4SO
sso
so0
EO"
A (nm) Figure 2. Influence of TINO, concentration: (a) paper spotted with 2 pL of TINO, of various concentratlons and 5 pL of 16 pg/mL oterphenyl in 0.5 M SDS;(b) blanks.
Figure 3. Influence of TINO, and SDS on the background emission spectra: (a) paper spotted wlth 2 p l of water; (b) 2 p l of 0.25 M TINO, added: (c) 5 pL of 2 % SDS added to b. The maximum of the background excitation spectra at 270 nm was used.
broader bands. On the other hand, spectral shifts and changes in the shape of the emission spectra were negligible. Source of the Background. The influence of the presence of TlN03 and SDS on the emission spectra of the blanks is shown in Figure 3. Spectrum a, which corresponds to a paper spotted only with water, showed narrow peaks at 404,434, and 542 nm and a continuum with a broad maximum at about 450 nm. When TlN03and SDS were successively added (spectra b and c), the background emission increased, and new broad bands centered at about 475,505, and 520 nm appeared. Very likely these bands were due to phosphor impurities in the paper. This type of background is well documented in the literature (14-16). When the background spectrum, which appeared in the absence of thallium (Figure 3, spectrum a), was recorded by using smaller excitation and emission slits (3 nm) and shorter delay times (-0.01 ms), narrower peaks at the same wavelengths (404, 434, and 542 nm) along with new peaks were obtained. Because these peaks and continuum were observed when the paper was replaced with reflective surfaces such as
stainless and aluminum foil, it was concluded that this background arose from the scatter of stray radiation, probably from the afterglow of the Xe flash lamp in the LS-5 fluorometer. The wavelengths of most of the peaks agreed well with the most prominent emission lines of the Xe II emission spectrum (17). Influence of Exposure to the Excitation Radiation. Both analytes and phosphor impurities of the paper can undergo photodegradation when exposed to UV radiation; a decrease in the emission intensity of all the compounds studied here was observed. Measurements of the relative decomposition rates were carried out by using the following empirical procedure. After the initial period of 20-30 s, which was necessary to "optimize" the sample position, the emission intensity was measured for a 1.5-min period. A plot of logarithm of the intensity versus time gave curves that were approximately linear for most of the analytes. The points were fitted to a straight line and the relative decrease in the intensity during the first minute of exposure, -AI,was calculated. The results are given in Table III. In all cases, a higher
ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988
419
Table 111. Limits of Detection, Lifetimes, and Intensity Losses in the First Minute Produced by Exposure to the Excitation Radiation paper spotted with hexane solutions" LOD,ng lifetime, ms -AI,b %
compound biphenyl m,m'-bitoluene carbazole p-chlorobiphenyl 2,3-dimethylnaphthalene fluorene
phenanthrene pyrene
retene o-terphenyl
5.8 12 8.2 4.0 20
1.31 1.20 0.27 2.10 1.14
0.8
0.68
2.2 0.5 2.4 1.7
1.36 1.85 1.02 1.30
9.2
1.86 25 3.5
5.8 6.4 16 0.6
34 8.1
paper spotted with 2% SDS solution LOD,ng lifetime, ms -AI: % 3.0 3.0 1.0 0.7 2.0 0.3 0.9 0.7 0.3 0.2
1.12 1.17 0.28 1.49 1.16 1.01 0.88 1.62
0.90 0.88
7.5 4.7 20 3.0 3.0 4.2 12.0 -0
6.4 2.2
enhancement factof 1.3 6.1 9.0 2.9 3.1 7.2 2.6 3.7 4.8 4.4
" A methanol solution was used for carbazole. bRelativedecrease of the intensity during the first minute of exposure to the excitation radiation. Calculated from the slopes of the calibration curves.
0.1
I
280
210
I
210
200
I
280
280
Aerc inm)
Flgure 4. Influence of the excltatlon wavelength on the relatlve decrease of the RTP intensity durlng the first minute: circles, paper spotted with hexane solutions;triangles, paper spotted with 2 % SDS solutions. For a and b, values of -AI corresponding to the compounds of Table 111 were represented against thelr respective excitation maxima. For c and d, blanks were excited at different wavelengths. For e and f, o-terphenyl was excited at different wavelengths. stability of the compounds spotted from micellar solutions was observed. On the other hand, a plot of the values of -Al versus the corresponding excitation wavelengths (Figure 4a,b) showed an increased stability at longer wavelengths. A similar plot obtained with the blanks is given in Figure 4, curves c and d. In this case, the intensity decay was only a result of photodecomposition of the phosphor impurities; the different stabilities of the analytes, which probably caused most of the dispersion for curves a and b, were no longer present. The points indicated again a better stability at longer excitation wavelengths and in the presence of SDS. The same conclusionswere obtained from measurements performed with o-terphenyl (Figure 4e,f). Lifetimes. Accurate measurement of the lifetimes of the phosphors spotted on paper substrates is difficult to accomplish since the observed intensity decay can arise from several sources: phosphorescence of the analyte and impurities of the paper, photochemical degradation of both of them, and decay of the scattered stray source radiation. However, data obtained under the same experimental conditions can be useful for comparative purposes. The OBEY-DECAYprogram was used to obtain the values in Table 111. Six points were measured at evenly spaced delay times between 0.1 and 0.6 ms, using a gate time of 0.1 ms and an integration time of 5 s. Lifetimes showed a clear tendency to be shorter when SDS was present. Taking into account that, in the presence of the surfactant, the photochemical degradation was slower, it is reasonable to assume that after correction for this effect, even larger differences would have been found; the lifetimes with SDS present were always shorter than in its absence.
Analytical Figures of Merit. Limits of detection (LODs) for nine PAHs and carbazole in the presence and absence of SDS are given in Table 111. Background noise was measured by taking 16 measurements of the blanks; the sensitivity was established from triplicate measurements at two different concentrations in the range 5-30 pg/mL and the LODs were calculated from these values for an S I N = 3. Limits of detection were about a factor of 2-10 better when 2% SDS solutions were used. The enhancement factors, calculated from the ratio of the slopes of the calibration curves in the presence and absence of the surfactant, also ranged within these values. The relative standard deviation (RSD) was measured by using 10 independent pieces of paper spotted with T1N03 and 5 pL of a 8 pg/mL solution of o-terphenyl in 2% SDS. A value of 9% was found. Measurements performed for six independent blanks gave an RSD of 6%. All calibration curves for polycyclic aromatic hydrocarbons had correlation coefficients for the best-fit straight line of 0.999 or better. CONCLUSIONS It is interesting to observe that enhancing and depressing effects produced by surfactants of different character affect both analytes and substrate impurities. A possible explanation for these effects could rely on the association of the nonpolar phosphor molecules with the nonpolar surfactant tails. As the probability of finding a thallium ion increases in the proximity of a surfactant anionic head and decreases if the surfactant head is cationic, the phosphor molecules and thallium ions can be closer to each other in the presence of an anionic surfactant and farther apart if the surfactant head is a large nonionic chain or a cationic group. On the other hand, association of nonpolar analytes with the surfactant could aid the migration of such species inside the submicroscopic pores of the cellulose fibers, thus producing a better entrapment of the analytes. The spectral blue-shifts observed for the excitation spectra could be due to a more rigid environment, produced by this mechanism. Blue-shifts have been also observed in the excitation spectra when solvents of increasing polarity were used to spot the samples (IO). The higher stability against photodecomposition could also be explained by the higher probability of recombination of the photoproduced ion pairs in a more rigid environment. These suggested explanations do not exclude each other and the contribution of other mechanisms is also possible. ACKNOWLEDGMENT We thank Benjamin W. Smith of the University of Florida for technical assistance. Registry No. SDS, 151-21-3; Brij 35, 9002-92-0; DTAC, 112-00-5;T1N03, 10102-45-1;biphenyl, 92-52-4; m,m'-bitoluene, 612-75-9; carbazole, 86-74-8; p-chlorobiphenyl, 2051-62-9; 2,3-
Anal. Chem. 1988, 60,420-427
420
dimethylnaphthalene, 581-40-8; fluorene,86-73-7; phenanthrene, 85-01-8; py-rene, 129-00-0; retene, 483-65-8; o-terphenyl, 84-15-1; sodium decyl sulfate, 142-87-0.
LITERATURE CITED Lue Yen-Bower, E.; Winefordner, J. D. Anal. Chim. Acta 1978, 101, 319. Bateh, R. P.; Winefordner, J. D. Anal. Left. 1982, 15, 373. Bateh, R. P.; Winefordner, J. D. J . Pharm. Biomed. Anal. 1983, I , 113. Aaron, J. J.; Kaleei, E. M.; Winefordner, J. D. J . Agric. Food Chem. 1979, 27, 1233. Vaneiii, J. J.; Schulman, E. M. Anal. Chem. 1984, 56, 1030. Su, S. Y.; Asafu-Adbye, E.; Ocak, S. Analyst (London) 1984, 109, Ward, 1019. J. L.; Walden, G. L.; Winefordner, J. D. Taknta 1981, 28, 201. Hurtubise, R. J. SOM surface Luminescence Analysis; Guilbautt, F. G., Ed.; Marcel Dekker: New York, 1981. Vo-Dinh, T. Room-Temperature Phosphorimetty for Chemical Analysls; Eking, P. J., Winefordner, J. D., Eds.; Wiley: New York, 1984. McAleese, D. L.; Dunlap, R. B. Anal. Chem. 1984, 56, 2244.
(11) Andino, M. M.;Kosinski, M. A.; Winefordner, J. D. Anal. Chem. 1986, 58, 1730. (12) de Lima, C.G.; Andino, M. M.; Winefordner, J. D. Anal. Chem. 1986, 58. 2867. (13) Scharf, G.;Smith, B. W.; Winefordner, J. D.Anal. Chem. 1985, 57, 1230. (14) Ward, J. L.; Bateh, R. P.; Wlnefordner, J. D. Analyst (London) 1982, 107, 335. (15) Bateh, R. P.; Winefordner, J. D. Taknta 1982, 29,713. (16) McAleese, D. L.; Duniap, R. B. Anal. Chem. 1984, 56, 600. (17) Strlganov, A. R.; Sventitskii, N. S. Tables of Spectral Lines of Neutral and Ionized Atoms; IFI/Plenum: New York, 1968.
RECEIVED for review April 27, 1987. Accepted November 7, 1987. This research is supported by NIHiGM-11373-23. G. Ramis Ramos and M. c. Garcia Alvarez-Coque thank the “Conselleria de Cultura, Educacio i Ciencia de la Generalitat for the grants that made it possible for them to stay in Gainesville.
Matrix Effects in the Separation of Rare-Earth Elements, Scandium, and Yttrium and Their Determination by Inductively Coupled Plasma Optical Emission Spectrometry D. W.Zachmann Institut fur Geologie und Palaontologie, Technische Universitat Braunschweig, 0-3300 Braunschweig, Federal Republic of Germany Due to Its sensltivity and reproduclbillty, inductlvely coupled plasma optlcal emtsdon spectrometry (ICP-OES) has developed Into a very promiskrg technlque In the determlnatlon of the rare-earth elements (REE), Sc, and Y. However, the changlng composltlon of natural samples may cause severe problems durlng separation from concomltant elements (matrlx elements). I n thls study a slmplified verslon of REE separatlon from 8 major and 27 trace elements Is proposed. Some major matrlx elements, depending on their kind and quantity, cause a speclfk depression of recovery rates of light REE (LREE) durIng the ion exchange process. The separatlon characterls#cs of two sLnUar resins are described. Some trace m a t h elements are separated Inadequately. N s effect and the Interference of matrix element residues after separation on the REE detennlnatlon are quantified. Finally, some RE interelement interferences are described. The developed methods are verifled by the analyses of reference materlals.
That inductively coupled plasma optical emission spectroscopy (ICP-OES) has become increasingly important in the determination of rare-earth elements is due to the high sensitivity and reproducibility of REE measurements by ICP (1-3). As can be seen in Table I the REE concentrations in many natural substances exceed by far the detection limits. Problems in the determination of REE arise from their very low concentration in many common substances (rocks, ores, slags) as compared to the concentration of main elements (matrix elements, cf. Table I). Most of the matrix elements falsify the REE determination because of spectral overlaps and background continuum interferences. Also, elements which are reported as not interfering (e.g. Ba, ref 9) falsify the REE analyses if they are present in higher concentrations. As an example, Table I1 shows the interferences of some major and trace elements with the REE.
Table I. Rare-Earth Elements (REE including Sc, Y) Abundance8 and Mean Concentrations of Major Elements (ME) in Chondrites, in the Present Earth Crust and North American Shales (NASC), and Detection Limits by ICP-OES (All Values in ppm)
compos-
optimum present det limits chonchondrites Earth bY REE drites ( 4 ) (5, 6)” crust (7) NASC (8) ICP-OES ite of 9
sc
Y La Ce
Pr Nd Sm Eu Gd Tb DY
Ho Er
Tm Yb Lu
1.96 0.33 0.88 0.112 0.60 0.181 0.069 0.249 0.047 0.325 0.070 0.200 0.030 0.200 0.034
av of C 1
7.8 2.1 0.367 0.957 0.137 0.711 0.231 0.087 0.306 0.058 0.381 0.085 0.249 0.036 0.248 0.038
30 22
19 38 4.3 16 3.7 1.1 3.6 0.64 3.7 0.82 2.3 0.32 2.2 0.30
27 32 73 7.9 33 5.7 1.24 5.2 0.85
0.008 0.002 0.005 0.030
0.025 0.060 0.040
0.001 0.007 0.020 0.008
1.04
0.004
3.4
0.005 0.004 0.002 0.005
0.50
3.1 0.48
ME
Na K Mg Ca
Ba Ti Mn Fe
A1 Values
8100 900 143000 14100 3.7 670 2740 275900 13080
26000 12500 21100 53600 350 4800 1100
58300 95000
multiplied by 1.5 to allow for volatile loss.
An additional matrix effect is caused generally by high amounts of matrix elements. High salt loads change the
0003-2700/88/0360-0420$01.50/00 1988 American Chemical Society
