Sample Preconcentration Using Ion-Exchange Polymer Film for Laser

The sample probe removes only a small fraction of the sample solution and, therefore, in principle, does not disturb the original solution significant...
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Anal. Chem. 1997, 69, 4872-4877

Sample Preconcentration Using Ion-Exchange Polymer Film for Laser Ablation Sampling Inductively Coupled Plasma Atomic Emission Spectrometry Wing Tat Chan* and Henry H. C. Yip

Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong

An efficient sample pretreatment/introduction technique for the inductively coupled plasma atomic emission spectrometry (ICP-AES) using ion exchange for analyte preconcentration and matrix separation and laser ablation sampling for sample introduction has been developed. Ammonium pyrrolidine dithiocarbamate (APDC)-polystyrene films are coated on glass plates for analyte preconcentration. Repetitive laser ablation sampling of the polymer film removes the ion-exchanged metal ions from the polymer film as fine particles for sample introduction into the ICP. After immersing the sample probe in a sample solution for 5 min, the ICP emission intensity for laser ablation of the polymer film is a few times larger than that after solution nebulization. The sample probe removes only a small fraction of the sample solution and, therefore, in principle, does not disturb the original solution significantly. Single-pulse laser ablation of the polymer film shows that the ion-exchanged metal ion concentration in the film reduces exponentially with the depth of the polymer film. Ion exchange to the polymer film is probably limited by the rate of metal ion diffusion into the film. Calibration curves for Cu, Hg, Pb, and Zn show linear dynamic range of ∼1-2 orders of magnitude. The linear dynamic range for Cu increases to >3 orders of magnitude when using Pb as an internal standard. RSD of the ICP emission intensity is ∼8%.

monium pyrrolidine dithiocarbamate (APDC).10 The compound has been used extensively in solvent extraction11-14 and ionexchange resins.15-17 Direct elemental analysis using a Pt wire sample probe coated with an APDC-embedded polymer film has also been reported.18 The preconcentrated Pb ions were determined by direct inserting the sample probe into a flame for atomic absorption spectrometric measurements. In this report, glass sample probes coated with APDCpolystyrene film are used for metal ion preconcentration and the ion-exchanged metal ions are analyzed using laser ablation sampling inductively coupled plasma atomic emission spectroscopy (ICP-AES). Preparation of the sample probe is straightforward: microliter quantities of APDC and polystyrene in methyl isobutyl ketone solution are pipetted onto a glass plate, and the solution is dried to form a polymer film. The sample probe is then immersed in a sample solution for a few minutes for ionexchange preconcentration. Repetitive laser ablation of the polymer film removes the film completely at the laser spot, and the laser-ablated materials is transferred to the ICP for quantitative elemental analysis. When using the ion-exchange polymer film for preconcentration, sensitivity is improved versus solution nebulization. The use of polymer films may also simplify calibration procedures for samples of a complex matrix; the film isolates the analyte ions from the original sample matrix, and the same polymer matrix is used for laser ablation sampling and ICP-AES analysis.

Sample preconcentration is often used in trace elemental analysis to increase the concentration of an analyte to a level compatible with an analytical technique and/or to isolate the analyte from the original sample matrix.1 Ion exchange and solvent extraction are common techniques for trace metal preconcentration.1-9 A workhorse of these techniques is am-

EXPERIMENTAL SECTION Sample Probe Preparation. Microscopic cover glasses were used as the support of the ion-exchange sample probe (diameter, 1.8 cm; thickness, ∼0.15 mm; Smiec, Shanghai, China). The glass plates were cleaned by immersing the plates in 1% HNO3 overnight and rinsing with doubly distilled water and acetone to remove trace metals and grease. The plates were dried in an oven at 103 °C for 1 h and cooled to room temperature in a desiccator.

(1) Mizuike, A. Enrichment Techniques for Inorganic Trace Analysis; SpringerVerlag: New York, 1983; Chapters 1, 5, and 9. (2) Zolotov, Yu. A.; Bodnya, V. A.; Zagruzina, A. N. CRC Crit. Rev. Anal. Chem. 1984, 14, 93-174. (3) Cresser, M. S. Solvent Extraction in Flame Spectroscopic Analysis; Butterworths: London, 1978; Chapter 5. (4) Burba, P.; Willmer, P. G. Fresenius Z. Anal. Chem. 1987, 329, 539-545. (5) Wang, X.; Barnes, R. M. J. Anal. At. Spectrom. 1989, 4, 509-518. (6) Siriraks, A.; Kingston, H. M.; Rivello, J. M. Anal. Chem. 1990, 62, 11851193. (7) Thompson, M.; Ramsey, M. H.; Pahlavanpour, B. Analyst 1982, 107, 13301334. (8) Tao, H.; Miyazaki, A.; Bansho, K.; Umezaki, Y. Anal. Chim. Acta 1984, 156, 159-168.

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(9) McLeod, C. W.; Otsuki, A.; Okamoto, K.; Haraguchi, H.; Fuwa, K. Analyst 1981, 106, 419-428. (10) Malissa, H.; Scho ¨ffmann, E. Mikrochim. Acta 1955, 1, 187. (11) Brooks, R. R.; Presley, B. J.; Kaplan, I. R. Talanta 1967, 14, 809-816. (12) Takla, P. G.; Mohamed, H. A.; Fahmy, F. Analyst 1987, 112, 1697-1699. (13) Popova, S. A.; Bratinova, S. P.; Ivanova, C. R. Analyst 1991, 116, 525-528. (14) Lee, J. D.; Lo, J. M. Anal. Chim. Acta 1994, 287, 259-266. (15) Barnes, R. M.; Genna, J. S. Anal. Chem. 1979, 51, 1065-1069. (16) Yamagami, E.; Tateishi, S.; Hashimoto, A. Analyst 1980, 105, 491-496. (17) van Berkel, W. W.; Massen, F. J. M. J. Spectrochim. Acta 1988, 43B, 13371347. (18) Lu, G.; Wang, X.; Xu, H. Y. Talanta 1995, 42, 557-560. S0003-2700(97)00615-X CCC: $14.00

© 1997 American Chemical Society

Solutions of 1% APDC and 1% polystyrene in methyl isobutyl ketone (MIBK) were prepared daily (APDC, 97% pure, Aldrich; polystyrene, average molecular weight ∼45 000, Aldrich; MIBK, LR grade, Ajax Chemicals). Equal volumes of the solutions were mixed to produce a 0.5% APDC + 0.5% polystyrene in MIBK solution before film coating. APDC-polystyrene films were prepared by pipetting 30 µL of the APDC-polystyrene solution onto a glass plate. To ensure even coverage of the film over the entire glass plate, the solution was spread with the pipet tip. The film was dried by irradiating it with an IR lamp for 5 min. The films were about 5 µm thick. The APDC-polystyrene film-coated glass plates were used as a sample probe to selectively preconcentrate metal ions from sample solutions. Preconcentration of Metal Ions onto the Sample Probe. Sample and standard solutions were prepared in Britton-Robinson buffer (0.0286 M of citric acid, boric acid, potassium hydrogen phosphate, and diethylbarbituric acid),19 and the pH was adjusted to 3.0 with diluted NaOH. All solutions were prepared with doubly distilled water. Then, 50 mL of the sample solution was pipetted into a 100 mL beaker, and a microscope slide (2.5 cm × 7.5 cm) was placed into the beaker in an inclined position to support the sample probe. An APDC-polystyrene film-coated glass plate was immersed in the solution with the film surface up and resting on the inclined glass slide. The solution was stirred vigorously with a magnetic stirrer for 5 min to allow for the ion-exchange reaction. The sample probe was then retrieved with a pair of forceps, blotted with filter paper, dried under an IR lamp, and subjected to laser ablation sampling. Laser Ablation Sampling of the Sample Probe for ICPAES Elemental Analysis. The sample probes were mounted in a glass ablation chamber for laser ablation sampling. The ablation chamber window is a fused silica plate (28 mm in diameter, 3.2 mm thick; Heraeus Amersil, Duluth, GA) for high transmittance to the UV laser beam. The laser ablation setup is similar to the setup described in detail previously.20 The major difference is that a Nd:YAG laser (SureLite II-10 with frequency doubling and quadrupling crystals; Continuum, Santa Clara, CA) was used for laser ablation sampling. The laser wavelength was 266 nm, pulse duration 5 ns, and pulse energy 50 mJ. The laser beam was focused with a single fused silica plano-convex lens (focal length, 200 mm). The sample was placed before the focal point to avoid laser breakdown of the atmosphere (laser power density, 5 × 108 W/cm2) and to obtain a large spot size (∼2 mm). The probe was laser sampled with 10 laser pulses (pulse rate, 2 Hz) to remove the film completely at the laser spot. Each film was sampled six times at locations evenly distributed over the film. Four repetitions were used to establish the relative standard deviation (RSD) between films. The laser-sampled material was transported to the ICP (Plasmaquant 110, Carl Zeiss, Jena, Germany) with a stream of Ar carrier gas (flow rate, 1.0 L/min). Temporal ICP emission intensity of the analytes was recorded at a sampling rate of 0.03 s/data point. Peak areas are used as the integrated intensity. Emission lines used include Cu I, 324.7 nm; Pb II, 220.3 nm; Zn I, 213.8 nm; Hg I, 253.6 nm; and Ag I, 328.0 nm. Sample solutions were also analyzed using the conventional nebulization sample introduction method for comparison. (19) Perrin, D. D.; Dempsey, B. Buffer for pH and Metal Ion Control; Chapman & Hall Laboratory Manuals: London, 1974; Chapter 10. (20) Lam, K. K. K.; Chan, W. T. J. Anal. At. Spectrom. 1997, 12, 7-12.

Figure 1. Typical temporal profile of ICP emission intensity for laser ablation sampling of APDC-polystyrene film. The sample was 500 ppb Cu. ICP emission intensity for solution nebulization of Cu solution is shown for comparison.

RESULTS AND DISCUSSION Laser Ablation Sampling of the APDC-Polystyrene Films. A frequency-quadrupled Nd:YAG laser beam (λ ) 266 nm) was used for laser ablation sampling of the polymer film because of the higher ablation efficiency of the UV laser beam. The laser ablation threshold of polymer reduces with laser wavelength.21 UV lasers also produce clean ablation craters without charring of the polymer target,21-23 while IR lasers may melt or char the polymer target during laser ablation.24 The laser power density of the Nd:YAG laser pulse was ∼5 × 108 W/cm2, and the laser spot was 0.2 cm in diameter. The laser power density is well above the typical laser ablation threshold of 106 W/cm2 for polymers.21,22 In fact, laser power density is not critical in this study as long as it is above the ablation threshold, because multiple-pulse laser ablation of the polymer film removes the film at the laser spot completely, i.e., the total amount of laserablated materials depends on the spot size but not on the laser power density. Laser spot size is related to laser beam size, focal length of the focusing lens, and lens-to-sample surface distance. The lens-to-sample surface distance was kept constant throughout this study to obtain a fixed laser spot size on the polymer surface. A typical temporal profile of ICP emission intensity for laser ablation sampling of APDC-polystyrene film is shown in Figure 1. The APDC-polystyrene-coated sample probe was immersed in 500 ppb Cu solution for 5 min before laser sampling. The shape of the temporal profile is mainly related to the carrier gas flow rate and the geometry of the ablation chamber and sample transport tubes to the ICP.20,25 There is a significant enhancement in sensitivity when using the sample probe for preconcentration. The peak ICP emission intensity is ∼8 times higher than that with conventional solution nebulization sample introduction. The ICP emission intensities of the Cu solution before and after the ion-exchange reaction are not different statistically. Furthermore, calculation based on the Cu ICP emission intensities (21) Brannon, J. H.; Lankard, J. R.; Baise, A. I.; Burns, F.; Kaufman, J. J. Appl. Phys. 1985, 58, 2036-2043. (22) Dyer, P. E.; Oldershaw, G. A.; Schudel, D. Appl. Phys. 1990, 51B, 314316. (23) Hemmerlin, M.; Mermet, J. M. Spectrochim. Acta 1996, 51B, 579-589. (24) Sumpter, B. G.; Voth, G. A.; Noid, D. W.; Wunderlich, B. J. Chem. Phys. 1990, 93, 6081-6091. (25) Chan, W. T.; Russo, R. E. Spectrochim. Acta 1991, 46B, 1471-1486.

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Table 1. Enhancement Ratio of ICP Peak Emission Intensity for Laser Ablation of APDC-Film vs Solution Nebulization element

wavelength, nm

enhancement ratio

Cu Pb Hg Ag Zn

324.7 220.3 253.6 328.0 213.8

8.6 ( 0.8 6.3 ( 1.7 5.8 ( 1.5 2.2 ( 0.4 1.9 ( 0.2

Table 2. Composition of APDC-Polystyrene Films vs ICP Emission Intensity relative ICP thickness emission intensity of Cu of IR polystyrene APDC lamp-dried IR lamp-dried air-dried film concn % concn, % film, µm film film 1 2 3 4

2.0 2.0 1.0 1.0

0.5 1.0 1.0 0.5

7 7 5 5

0.14 ( 0.02 0.67 ( 0.06 1.00 ( 0.08 0.51 ( 0.04

0.033 ( 0.022 0.031 ( 0.016 0.056 ( 0.022 0.045 ( 0.002

a APDC and polystyrene were dissolved in MIBK. Volume of the solution used was 30 µL/film.

Figure 2. ICP emission intensity of Cu for laser ablation sampling of APDC-polystyrene film versus pH of sample solution.

for laser sampling and solution nebulization shows that ∼0.6% of Cu ions from the 50 mL 500 ppb Cu sample solution are attached to the APDC-polystyrene film. The original solution is not disturbed significantly by the ion-exchange process. The significant ICP emission intensity enhancement is, however, due to a much higher concentration of Cu in the APDC-polystyrene film. Using simple calculation based on film volume and the amount of Cu ions attached to the film, Cu concentration in the polymer film is found to be g500 times larger than that of the original solution. Introduction of the laser-ablated polymer film into the ICP as a pulse of fwhm ≈ 1 s, therefore, gives enhanced peak ICP emission intensity relative to that with solution nebulization. Enhancement ratios of ICP peak emission intensity for laser ablation sampling of the APDC film to that with solution nebulization for several elements are shown in Table 1. The pH of the sample solutions was maintained at pH 3.0 using Britton-Robinson buffer for maximum sensitivity (Figure 2). In highly acidic medium (pH < 3), APDC molecules in the polymer film are protonated and not readily available for ion exchange. At pH > 4, APDC is in its ionic form (pKa of APDC, 3.29)26 and is readily soluble in water. APDC may be lost from the film surface by dissolution into the sample solution, reducing the concentration of APDC in the film. At pH > 6, metal oxides form in the solution, and the concentration of free metal ions reduces. Dissolution of APDC and oxide formation reduce the sensitivity. Sensitivity Enhancement versus Film Composition. The relative preconcentration capability of an APDC-polystyrene film versus film composition is shown in Table 2. Sample probes of APDC:polystyrene ratios of 0.25, 0.5, and 1 were prepared and (26) Townshend, A., et.al., Eds. Dictionary of Analytical Reagents; Chapman & Hall Chemical Database: London, 1993; p 842.

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immersed in 500 ppb Cu solution for preconcentration. For IR lamp-dried films of the same amount of APDC (films 2 and 3), the relative ICP emission intensity is reduced by 33% if the amount of polystyrene is doubled. Ion-exchange capability is probably related to the concentration of APDC in the polymer film instead of the absolute amount of the ion-exchange molecules, because ion-exchange occurs mainly at the surface of the film (discussed in a later section). Using a micrometer, the average film thicknesses of films 2 and 3 are found to be 7 and 5 µm, respectively. Doubling the amount of polystyrene increases the film thickness and film volume by 40% and reduces the APDC concentration in the film by 28%. The mean distance between APDC molecules also increases by 12%. The amount of Cu ions attached to the film is reduced as the concentration of APDC decreases. It is also interesting to note that the amount of polystyrene mainly determines the volume of the film; increasing the APDC concentration does not increase the film thickness (film 1 vs film 2, and film 3 vs film 4). APDC molecules probably occupy the void between the polystyrene molecules. Similarly, ICP emission intensity reduces significantly if APDC concentration is reduced by half while the same amount of polystyrene is used (IR lamp-dried film 4 vs film 3 and film 1 vs film 2). The RSD of the ICP emission intensity increases as APDC concentration reduces, mainly because of the significant reduction in ICP emission intensity. The reduction in ICP emission intensity is not linearly proportional to the APDC concentration in the film, probably because more than one APDC molecules bond to one metal ion during complex formation in the film. In general, two APDC molecules bond to one metal ion in liquids.27,28 Even though the APDC molecules are immobile in the polymer film, divalent metal ions will still probably bond to two APDC molecules for charge balance. Reduction in APDC concentration increases the distance between APDC molecules and reduces the stability of the APDCmetal complex, thereby, reducing the ion-exchange capability of the film. The ion-exchange capability of a film also depends strongly on the manner of film preparation (Table 2). Films dried in air at room temperature instead of under an IR lamp were prepared. The films took approximately 15-30 min to dry versus 1-2 min using an IR lamp. The ion-exchange capability of air-dried films is more than 10 times less than that of the IR-dried films (Table 2). The drying rate of the APDC-polystyrene solution influences the ion exchange capability significantly. (27) Dingman, J. F., Jr.; Gloss, K. M.; Milano, E. A.; Siggia, S. Anal. Chem. 1974, 46, 774-777. (28) Hulanicki, A. Talanta 1967, 14, 1371-1392.

Figure 3. Micrograph of APDC-polystyrene film showing APDC crystalline. Magnification 50×. Scale, 1 cm ) 120 µm. Volume of the APDCpolystyrene solution, 40 µL.

The drying rate of a film is also related to the volume of the APDC-polystyrene solution used for film coating. Using the same concentration of 0.5% APDC and 0.5% polystyrene, ICP emission intensity increases slightly with solution volume from 15 to 30 µL and decreases by ∼20% when the solution volume increases above 30 µL. The reduction is probably related to the longer drying time, which allows crystallization of APDC, i.e., the drying rate of the APDC-polystyrene solution influences the film morphology. During the slow drying period of the films, treelike crystalline APDC forms (Figure 3). The number density of APDC molecules that are evenly distributed over the film is reduced, and the ionexchange capability reduces. The occurrence of treelike crystalline structure increases with the volume of the solution. The slight reduction in ICP emission intensity as volume of APDCpolystyrene solution reduces is probably due to uneven coverage of the film over the glass plate using a small volume of solution. Therefore, 30 µL of 0.5% APDC and 0.5% polystyrene in MIBK with IR lamp drying is used for film preparation for other experiments. Diffusion of Metal Ions in the APDC-Polystyrene Film. The distribution of metal ions in an APDC-polystyrene film can be measured directly using repetitive single-pulse laser sampling of the films at the same laser spot. The ICP emission intensity of Cu versus the laser pulse number for single-pulse laser ablation of APDC-polystyrene films is shown in Figure 4. The films were immersed in 500 ppb Cu solution for 0.5 and 5 min, respectively. Each laser pulse removes a layer of the polymer film, and the ICP emission intensity corresponds to the concentration of Cu ions in that layer. The ICP emission intensity reduces to baseline level after five or six laser pulses, and visual inspection shows that the polymer film is completely removed at the laser spot after 5-6 laser pulses. Therefore, each laser pulse removes approximately 1 µm of the polymer film. The concentration of Cu ions decreases exponentially with the depth of the film (Figure 4), which indicates that ion diffusion into the film is a major rate-limiting step for the ion-exchange

Figure 4. ICP emission intensity of Cu for single-pulse laser ablation sampling of APDC-polystyrene film versus laser pulse number. 9, 0.5 min film solution contact time; b, 5 min contact time.

process.29 The metal ions are mainly attached to the top layer of the film. Therefore, an increase in the thickness of the polymer film will not increase the preconcentration capability of the film for the same period of film-solution contact time. The ICP emission intensity for repetitive laser ablation (10 pulses) of the film is roughly linearly related to the square root of the film-solution contact time (Figure 5), which also follows the diffusion model.29 Reduction in ICP emission intensity for a long contact time (>25 min) is probably due to deterioration of the film after prolonged exposure to the acidic sample solution and vigorous stirring. A film-solution contact time of 5 min is used for other studies because it provides adequate precision (RSD ≈ 8%) and sensitivity. Compared to the short film-solution contact time, reproducibility is considerably improved for 5 min (29) Crank, J. The Mathematics of Diffusion, 2nd ed.; Oxford University Press: Oxford, 1975; Chapter 4.

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Figure 5. ICP emission intensity of Cu for laser ablation sampling of APDC-polystyrene film versus square root of film-solution contact time.

contact time versus 1 min because there is an uncertainty of ∼10 s in timing for the sample probe retrieval from the solution. The uncertainties are 3% and 16% for 5 and 1 min contact time, respectively. On the other hand, a longer immersion time increases the length of the overall experiment, which is undesirable if the number of samples is large. Matrix Effects on the Preconcentration Factors. Preconcentration of metal ions onto the APDC-polystyrene film relies on ion-exchange processes. Ion exchange primarily depends on the strength of the complexes formed between the APDC ligand and metal ions. However, the presence of complexing agents in the sample solution reduces the concentration of free metal ions and, therefore, the extent of ion exchange. Also, the presence of other metal ions capable of forming stable complexes with APDC may influence the preconcentration factors. The effect of complexing agents on the ion-exchange process is demonstrated by measurement of ion-exchanged copper ions versus chloride concentration. Chloride ions form complexes with Cu ions in the aqueous phase,30 and the free copper ion concentration decreases. The ICP emission intensity of Cu for laser ablation of APDC-polystyrene film versus NaCl concentration is shown in Figure 6. Cu emission intensity decreases exponentially with chloride concentration. Similar reduction is observed with KCl matrix. Addition of salts that do not form complexes with the analyte ions, e.g., NaNO3, has little effect on the ICP emission intensity. Therefore, the reduction is due to the chloride matrix. Calibration using a simple aqueous standard solution cannot be applied directly to the analysis of samples of a complex matrix. For samples containing more than one metal ion capable of ion-exchanging with APDC, there is competition between the metal ions for the ion-exchange sites in the polymer film. A metal ion may be replaced by another metal ion if the latter forms a stronger complex with the APDC ligand. The competition between Cu and Pb is shown in Figure 7. A constant concentration of 5 ppm Pb was used. Cu concentration ranges from 0 to 100 ppm. As the concentration of Cu increases, the Pb concentration in the APDC film decreases quickly. Cu forms stronger complexes with APDC than Pb,26,31,32 and the ion-exchange (30) Sillen, L. G.; Martell, A. E. Stability Constants of Metal-Ion Complexes, Supplement No. 1; The Chemical Society: London, 1971.

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Figure 6. ICP emission intensity of Cu for laser ablation sampling of APDC-polystyrene film versus chloride content of the sample solution.

Figure 7. ICP emission intensity of Pb for laser ablation sampling of APDC-polystyrene film versus Cu concentration. The inset shows Cu ICP emission intensity versus Cu concentration.

capacity of Cu is about an order of magnitude higher than that of Pb, depending on the matrix of the ion-exchange resin.26 Replacement of Pb by Cu in the ion-exchange film is, therefore, expected. Conversely, if the Cu concentration is kept constant at 500 ppb and the Pb concentration increases from 0 to 500 ppm, the Cu emission intensity does not decrease significantly until the Pb concentration increases to 300 ppm or above. Quantitative Analysis. Calibration curves of Cu, Hg, Pb, and Zn show a linear dynamic range of ∼1-2 orders of magnitude when APDC-polystyrene films are used for preconcentration. A typical calibration curve of Cu is shown in Figure 8. The calibration curve rolls off at Cu concentration above 2 ppm. With the addition of a fixed concentration of a metal ion (Pb), however, the linear dynamic range of the Cu calibration curve increases to 3 orders of magnitude if the intensity ratio of Cu/Pb is plotted against the concentration ratio. The dramatic increase in the linear dynamic range is related to the equilibrium between Cu, Pb, and APDC. (31) Myasoedova, G. V.; Savvin, S. B. CRC Crit. Rev. Anal. Chem. 1986, 17, 1-65. (32) Malissa, H.; Gomiscek, S. Anal. Chim. Acta 1962, 27, 402-404.

Figure 8. Calibration curve for Cu using APDC-polystyrene film for preconcentration. The inset shows expanded scale for low concentration.

The equilibrium between APDC-1 (the ionized form of ammonium pyrrolidine dithiocarbamate in the polymer film) and Cu ions can be represented by the following equation: K1

2APDC-1 + Cu2+ 798 Cu(APDC)2 Similarly, the equilibrium for APDC and Pb is K2

2APDC-1 + Pb2+ 798 Pb(APDC)2 where K1 and K2 are the equilibrium constants,

K1 )

K2 )

[Cu(APDC)2] [APDC-1]2[Cu2+] [Pb(APDC)2] [APDC-1]2[Pb2+]

[Cu(APDC)2] [Pb(APDC)2]

)

K1 [Cu2+] K2 [Pb2+]

Therefore, the ratio of the ion-exchanged Cu and Pb ions in the polymer film is proportional to the ratio of the ions in solution. Further investigation is underway in this laboratory to determine if the calibration scheme using an internal standard can be extended to other system. Calibration using an internal standard (Pb) for Cu in 3% NaCl matrix gives a linear calibration curve for a similar concentration range (Figure 9). The slopes for calibration of a simple Cu solution and for Cu in a NaCl matrix are statistically identical (paired t-test, P ) 0.05). Therefore, an internal standard may also

Figure 9. ICP emission intensity ratio of Cu/Pb versus concentration ratio of Cu/Pb with 3% (w/v) NaCl matrix. Concentration of Pb, 5 ppm. The inset shows expanded scale for low concentration.

be applied for complex matrix calibration using simple aqueous standards, if the analyte and the internal standard behave similarly in the sample solution. CONCLUSIONS A simple ion-exchange sample probe using APDC-polystyrene film for analyte preconcentration and laser ablation sampling for sample introduction into the ICP has been developed. The sample probe removes only a small fraction of the sample solution and, therefore, does not disturb the original solution significantly. The use of Britton-Robinson buffer, however, changes the matrix of the sample. Further experiments for pH control without using the universal buffer are underway in this laboratory. The ion-exchange process is probably limited mainly by the diffusion rate of the analyte ions into the polymer film. Diffusion may also contribute to the fact that the ion-exchange capacity of the APDC-polystyrene films (Table 1) does not follow previous reports exactly.27,31 Further investigation is needed to confirm this hypothesis. With internal standard, the linear dynamic range for quantitative analysis using the APDC-polystyrene film is more than 3 orders of magnitude. Using a simple standard solution and internal standard, elemental quantitative analysis in a complex matrix, e.g., chloride, may also be possible. ACKNOWLEDGMENT This research was supported by the Department of Chemistry and CRCG research grants of the University of Hong Kong. Received for review June 16, 1997. Accepted September 9, 1997.X AC970615N X

Abstract published in Advance ACS Abstracts, October 15, 1997.

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