Application of a Macrocycle Immobilized Silica Gel Sorbent to Flow

A simple and highly selective flow injection on-line pre- concentration and separation-flame atomic absorption spectrometric method was developed for ...
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Anal. Chem. 1999, 71, 4216-4222

Application of a Macrocycle Immobilized Silica Gel Sorbent to Flow Injection On-Line Microcolumn Preconcentration and Separation Coupled with Flame Atomic Absorption Spectrometry for Interference-Free Determination of Trace Lead in Biological and Environmental Samples Xiu-Ping Yan,*,† Michael Sperling, and Bernhard Welz‡

Department of Applied Research, Bodenseewerk Perkin-Elmer GmbH, D-88647 U ¨ berlingen, Germany

A simple and highly selective flow injection on-line preconcentration and separation-flame atomic absorption spectrometric method was developed for routine analysis of trace amounts of lead in biological and environmental samples. The selective preconcentration of lead was achieved in a wide range of sample acidity (0.075 to g3 mol L-1 HNO3) on a microcolumn (145 µL) packed with a macrocycle immobilized on silica gel. The lead retained on the column was effectively eluted with an EDTA solution (0.03 mol L-1, pH 10.5). Three kinds of potential interferences, i.e., preconcentration interferences from metal ions with an ionic radius similar to that of Pb(II) due to their competition for the cavity of the macrocycle, elution kinetic interferences from ions which form stable complexes with EDTA due to their competition for EDTA, and interferences in the atomizer from residual matrix, were evaluated and compared in view of the read-out mode of the analyte response (peak area vs peak height), column wash step (with vs without), column capacity (50 vs 145 µL), and column shape (conical vs cylindrical). The results showed that a combination of increase in column capacity, quantitation based on peak area, and use of dilute nitric acid for column wash before elution efficiently avoid the above-mentioned potential interferences. With the use of a 145 µL column the present system tolerated up to 0.1 g L-1 Ba(II), 1 g L-1 Sr(II), and at least 10 g L-1 Fe(III), Cu(II), Ni(II), Zn(II), Cd(II), Al(III), K(I), Na(I), Ca(II), and Mg(II) in the sample digest. Further improvement of the interference tolerance can be achieved by increasing column capacity if more complicated samples need to be analyzed. At a sample loading rate of 3.9 mL min-1 with 30-s preconcentration, an enrichment factor of 52, a detection limit (3s) of 5 µg L-1 Pb in the digest and a sampling frequency of 63 h-1 were obtained. The precision (RSD, n ) 11) at the 200 µg L-1 level was 1.9%. The enrichment factor and the detection limit can be further improved by increasing sample loading rate without degradation in the efficiency due to the favorable kinetics and low hydrodynamic 4216 Analytical Chemistry, Vol. 71, No. 19, October 1, 1999

impedance of the present system. The analytical results obtained by the proposed method for a number of biological and environmental standard reference materials were in good agreement with the certified and recommended values. The determination of lead in biological and environmental samples plays an important role in the diagnosis of clinical disorders and of intoxication and in the monitoring of environmental pollution. The lead concentrations in many such samples are usually below the detection limit of flame atomic absorption spectrometry (FAAS) after samples have been brought into solution by acid digestion. More sensitive techniques are therefore required for the determination of lead in such samples, and electrothermal atomic absorption spectrometry (ETAAS) is often employed for that purpose.1,2 However, ETAAS is more timeconsuming and requires more operator’s skill when “real” samples with complex and varying matrix have to be analyzed. Flow injection (FI) on-line preconcentration and separation coupled with FAAS has been shown to be a promising alternative to conventional ETAAS for such determinations in view of the enhanced sensitivity, the efficient removal of matrix, the lower cost of the FAAS equipment, and the substantially higher sample throughput achievable.3-5 One of the prerequisites for a robust method for preconcentration and determination of lead in a wide variety of biological and environmental samples is that the method should tolerate complex matrixes and high concentrations of some commonly occurring elements. With the appropriate complexing system and separation column, it is usually not difficult to † Present address: Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada. E-mail: [email protected]. ‡ Present address: Departamento de Quimica, Universidade Federal de Santa Catarina, 88040-900 Florianopolis, S.C., Brazil. E-mail: [email protected]. (1) Subramanian, K. S. Prog. Anal. Spectrosc. 1986, 9, 237-334. (2) Navarro, J. A.; Granadillo, V. A.; Parra, O. E.; Romero, R. A. J. Anal. At. Spectrom. 1989, 4, 401-406. (3) Fang, Z.-L. Flow Injection Separation and Preconcentration; VCH: Weinheim, 1993. (4) Welz, B. Microchem. J. 1992, 45, 163-177. (5) Fang, Z.-L. Flow Injection Atomic Absorption Spectrometry; John Wiley & Sons Inc.: Chichester, 1995.

10.1021/ac990341z CCC: $18.00

© 1999 American Chemical Society Published on Web 08/28/1999

overcome interferences from alkali metal and alkaline earth metal elements. The on-line preconcentration of lead for FAAS using a column packed with the controlled pore glass-8-hydroxyquinoline (CPG-8-Q) chelating ion exchanger6 or by extraction of the lead diethyldithiocarbamate complex in a column packed with C18bonded silica gel sorbent7 was reported to be tolerant of a seawater matrix. However, these methods suffered severe interferences from high concentrations of other heavy metals in the sample matrix, such as copper and iron, due to their competition with the analyte for complex formation and/or sorption of the complex on the column. The preconcentration and determination of lead in water samples were also made possible by on-line precipitation of the hydroxide with ammonia, followed by dissolution with nitric acid and detection by FAAS.8 Although the selectivity of this method was in general fairly good, the tolerance of iron was rather low with a maximum ratio of 10 even in the presence of tartrate as a masking agent.8 Recently, C60 fullerene and activated carbon were compared in terms of sorbent extraction in the preconcentration of lead for FAAS using ammonium pyrrolidinedithiocarbamate as the complexing agent.9 It was reported that the tolerated ratios were 2 for Cd(II), 1000 for Fe(III) and Ni(II) in the C60 fullerene system compared to 1 for Cd(II), 40 for Fe(III), and 4 for Ni(II) in the activated carbon system.9 A method using FI online coprecipitation with the hexamethylene ammonium hexamethylene dithiocarbamate (HMA-HMDTC) iron(II) complex was proposed for the determination of trace amounts of lead in biological samples.10 The method could deal with samples containing up to 2500 mg L-1 iron, assuming a 1 + 9 dilution of the sample in the digest.10 Although concentrations of iron higher than such a level are unlikely to occur in biological materials, this may be the case in other samples. Moreover, the coprecipitation procedure suffers great risk of contamination from the impurities in the large excess of coprecipitation reagent. The purpose of this work was to develop a simple, sensitive, and selective FI on-line preconcentration and separation FAAS method for routine determination of trace amounts of lead in biological and environmental samples. To reach this goal, a macrocycle immobilized silica gel (Pb-02), specified to be selective for Pb(II) ion, was used as the column packing material. A wide range of potentially interfering concomitant ions was investigated, as well as the conditions that could have influence on the interferences, such as the mode of signal evaluation, the use of a wash step, sample acidity, pH and concentration of the eluent, column shape, and column capacity. The accuracy of the proposed method was demonstrated by analyzing a number of biological and environmental standard reference materials. EXPERIMENTAL SECTION Instrumentation. A Perkin-Elmer Model 2100 atomic absorption spectrometer with deuterium arc background correction was used throughout. A Perkin-Elmer “System 2” lead electrodeless discharge lamp was used as radiation source at 283.3 nm with a (6) Fang, Z.-L.; Welz, B. J. Anal. At. Spectrom. 1989, 4, 543-546. (7) Fang, Z.-L.; Guo, T.-Z.; Welz, B. Talanta 1991, 38, 613-619. (8) Martinez-Jimenez, P.; Gallego, M.; Valca´rcel, M. Analyst 1987, 112, 12331236. (9) Gallego, M.; Petit de Pena, Y.; Valca´rcel, M. Anal. Chem. 1994, 66, 40744078. (10) Fang, Z.-L.; Sperling, M.; Welz, B. J. Anal. At. Spectrom. 1991, 6, 301306.

current of 360 mA and a 0.7-nm slit width (high). The recommended flame conditions were used (2.5 L min-1 acetylene and 8.0 L min-1 air). A flow spoiler was used in the spray chamber for all measurements. The spectrometer was operated in the flameFIAS mode with a time constant of 0.2 s for peak evaluation. The time-resolved absorbance signals were displayed on the monitor screen and were printed out together with peak height, peak area, and statistical data on a Model EX-85 printer. A Perkin-Elmer Model FIAS-200 flow injection system was connected to the flame atomic absorption spectrometer with the shortest possible length of 0.35-mm-i.d. PTFE tubing (ca. 20 cm). Ismaprene pump tubes were used to deliver the sample and reagents. Small-bore (0.35-mm i.d.) PTFE tubing was used for all connections, which were kept as short as possible to minimize the dead volumes. The FIAS-200 consists of two peristaltic pumps and a standard rotary injection valve (four ports on the rotor, and five ports on the stator). The rotation speed of the two peristaltic pumps, their stop and go intervals, and the actuation of the injection valve were programmed and controlled automatically by using the FIFU version of the Model 2100 instrument software running on an Epson Model Q201A PC. A cylindrically shaped microcolumn (Upchurch Scientific, 4.3mm i.d. × 1-cm length, ca. 145 µL), packed with the Pb-02 macrocycle immobilized on silica gel (100 µm, IBC Advanced Technologies, American Fork, UT), was used for most of the experiments, such as parameter optimization and sample analysis, because of its good capability for interference reduction (see Interferences and Their Elimination or Minimization). A laboratory-made conically shaped microcolumn (ca. 145 µL) from plastic Eppendorf pipet tip, as described by Fang et al.,6 and a PerkinElmer conically shaped microcolumn (ca. 50 µL), packed with the same sorbent material as for the cylindrically shaped microcolumn, were used to investigate the effects of column shape and column capacity on the interferences only. Reagents. All reagents used were of the highest available purity and at least of analytical reagent grade. Nitric acid (Suprapur, 65%, Merck) was used to digest the samples, to acidify the standards, and to prepare the wash liquid. A 0.03 mol L-1 EDTA solution was prepared from ethylenediaminetetraacetic acid disodium salt dihydrate (Pro analysi, Merck), and adjusted to pH 10.5 with ammonia solution (Pro analysi, 25%, Merck). Doubly deionized water (DDW; 18 MΩ cm-1) was used throughout. Standard solutions of lead were prepared by appropriate dilution of a 1000 mg L-1 stock solution (Titrisol, Merck) with 0.15 mol L-1 HNO3 just before use. Samples. The following standard reference materials were analyzed to check the accuracy of the developed method: MESS-1 (marine sediment; NRCC), BCSS-1 (marine sediment; NRCC), CRM-142 (light sandy soil; BCR), TORT-1 (lobster; NRCC), and Seronorm whole blood II and III (Nycomed Pharma AS, Oslo, Norway). Sample Treatment. Acid digestion of biological and environmental materials was carried out in sealed PTFE vessels using a model MDS-81D microwave digestion system (Kuerner Analysentechnik, Rosenheim, Germany). All instrumental parameters for the digestion were chosen according to the recommendations of the manufacturer. The clear digest was transferred into a 25mL calibrated flask and diluted to volume with DDW. The Analytical Chemistry, Vol. 71, No. 19, October 1, 1999

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in Figure 1 and Table 1, respectively. In step 1 (Figure 1a), the injection valve was in the loading position and pump 1 activated so that the sample was loaded onto the column; the effluent from the column was flowing to waste. In step 2 (Figure 1b), pump 2 started to work while pump 1 was stopped and the injection valve was still in the loading position to introduce the dilute nitric acid for rinsing the column; the effluent from the column was flowing to waste. Meanwhile, the EDTA solution was pumped into the nebulizer of the spectrometer to define the baseline signal. In step 3 (Figure 1c), pump 2 was still active while the injection valve turned to the elution position to introduce the EDTA solution for eluting the analyte retained on the column. A complete cycle of preconcentration and elution required 57 s with a sample loading time of 30 s.

Figure 1. FI manifold and its operation sequence. P1 and P2, peristaltic pumps; MC, microcolumn packed with Pb-02 (145 µL); W, waste; FAAS, flame atomic absorption spectrometer; dotted lines, active.

precipitate in the resultant solution from the sediment and soil was allowed to settle and the supernatant was used for analysis. In detail, the following amounts of sample and reagents were used. Blood: 2 mL of whole blood was mixed with 5 mL of 65% m/v nitric acid. Lobster: 0.1 g of lobster sample was mixed with 5 mL of 65% m/v nitric acid. Sediment: 0.1 g of sediment sample was mixed with 10 mL of 33% m/v nitric acid. Soil: 0.1 g of soil sample was mixed with 5 mL of 65% m/v nitric acid and digested. Then 5 mL of 30% m/v hydrogen peroxide (Pro analysi, Merck) was added to the digest dropwise. The digest was left until the effervescence stopped. Procedures. The FI manifold and its operation sequence for on-line microcolumn preconcentration and separation are shown

RESULTS AND DISCUSSION Factors Affecting Sample Loading. The potential factors affecting the preconcentration of lead during sample loading include sample acidity, sample loading rate, and loading time. The influence of sample acidity on the preconcentration of lead was examined at a sample loading rate of 3.9 mL min-1 with 30-s preconcentration for 200 µg L-1 Pb. It was found that the optimum acidity was almost identical for peak height and peak area, ranging from 0.075 to at least 3 mol L-1 HNO3. For further experiments, an acidity of 0.15 mol L-1 HNO3 was used for all standard solutions, the acidity of sample digests, however, was usually significantly higher, and needed not to be adjusted (see Analytical Performance). The effect of sample loading rate was tested using a standard solution of 200 µg L-1 Pb with 30-s preconcentration. Both peak height and peak area were found to increase linearly with the sample loading rate up to at least 10.6 mL min-1. This indicates that the kinetic property of the Pb-02 macrocycle immobilized on silica gel is sufficiently favorable to allow easy retaining of the analyte. In addition, a linear increase in both peak height and peak area for 200 µg L-1 Pb at a loading rate of 3.9 mL min-1 was observed with increasing sample loading time up to at least 3 min. It is worthwhile to note that the present column preconcentration system permits the use of high sample loading rates to achieve high enrichment factors within a defined time period due to its favorable kinetics and low hydrodynamic impedance. Factors Affecting Column Wash. The purpose of the column wash step is to remove residual matrix solution from the column since it could produce unfavorable baseline changes, affecting the accurate measurement of peak area (the importance of the use of peak area for quantitation will be highlighted later), and could cause interferences during elution and/or atomization. The influence of the wash liquid acidity on the signal of 200 µg L-1 Pb was investigated at a wash flow rate of 3.6 mL min-1

Table 1. FIAS Program and Sequence of Operation for On-Line Microcolumn Preconcentration and Separation for FAAS flow rate/mL min-1 step

function

time/s

pumped medium

1 (Figure 1a) 2 (Figure 1b)

sample loading column wash

30 7

3 (Figure 1c)

elution

20

sample 0.03 mol L-1 EDTA (pH 10.5) 0.15 mol L-1 HNO3 0.03 mol L-1 EDTA (pH 10.5) 0.15 mol L-1 HNO3

4218 Analytical Chemistry, Vol. 71, No. 19, October 1, 1999

pump 1 3.9 off off off off

pump 2 off 3.6 3.6 3.3 3.3

valve position load load elute

Figure 2. Influence of wash liquid acidity on the signal intensity of 200 µg L-1 Pb. Read-out mode: (a) peak area; (b) peak height. Other conditions as in Figure 1 and Table 1.

with a 7-s column wash time. As illustrated in Figure 2, the optimum acidity range of wash liquid for peak area was quite wide, i.e., from 0.075 to at least 2.1 mol L-1 HNO3, in good agreement with that of sample acidity. However, the maximum peak height was achieved only in a narrower range of 0.075-0.45 mol L-1 HNO3. Below 0.075 mol L-1 HNO3 both peak area and peak height deviated from the maximum probably because of partial loss of the retained analyte resulting from unfavorable acidity for the retention of the analyte. Above 0.45 mol L-1 HNO3 although no retained analyte loss (no change in peak area) occurred during column wash, peak height gradually decreased as wash liquid acidity increased, likely due to the consumption of free EDTA ions by the residual acid in the column and connecting tubing during elution. It could be expected that the more acidic the wash liquid, the lower the actual concentration of free ETDA ions passing through the column, resulting in a slower elution of the retained analyte from the column. This was supported by the fact that the absorption signal was delayed and broadened with increasing in wash liquid acidity. To obtain best sensitivity and a narrow peak shape, a solution of 0.15 mol L-1 HNO3 was chosen for column wash. Comparing the absorption signals of 200 µg L-1 Pb without column wash (but with removal of residual solution from the column and connecting tubing by an air flow) (Figure 3a) and with column wash with 0.15 mol L-1 HNO3 (Figure 3b), a much sharper signal is obtained in the former case, suggesting a faster elution and less dispersion in the absence of a column wash step. Nevertheless, no significant difference in peak area was observed between both cases. The advantage of the column wash step, however, is quite obvious in the presence of matrix. Figure 3c,d shows the influence of the column wash step on the signal of 200 µg L-1 Pb in the presence of 10 g L-1 of Fe(III). As can be seen, without column wash the analyte peak is broadened, indicating a slower elution (Figure 3c vs Figure 3a). A 7-s column wash with 0.15 mol L-1 HNO3 effectively eliminated this effect and quantitatively recovered the signal (Figure 3d vs Figure 3b). Another example is demonstrated in Figure 3e,f. Without column wash the presence of 10 g L-1 Mg(II) not only caused a significant signal broadening, but also created a background which could not be corrected properly with deuterium arc background correction due to overcorrection (Figure 3e). However, after the column was washed with 0.15 mol L-1 HNO3 for 7 s, both the analyte and background signals (Figure

Figure 3. Influence of a column wash step on the atomic absorption (solid line) and background (broken line) signals of 200 µg L-1 Pb. (a, c, e) Removal of residual solution from the column with an air flow of 3.6 mL min-1 for 7 s; (b, d, f) column wash with 0.15 mol L-1 HNO3 at 3.6 mL min-1 for 7 s. (a, b) No matrix; (c, d) in the presence of 10 g L-1 Fe(III); (e, f) in the presence of 10 g L-1 Mg(II). Other conditions as in Figure 1 and Table 1.

3f) were comparable with those without matrix (Figure 3b). Detailed discussion on interferences and their elimination will be given later. Factors Affecting Analyte Elution. EDTA solution was used as the eluent due to its efficient elution of the analyte retained on the column. The influence of EDTA concentration was examined in the range of 0.0075-0.15 mol L-1 at pH 10.5. It was found that with increasing EDTA concentration the analyte signal became narrower and the time needed for complete elution decreased, but the peak area remained nearly constant. This effect could be attributed to a faster elution of the retained analyte from the column in the presence of more concentrated free EDTA ions in the eluent. The influence of the pH of the EDTA solution on the elution kinetics was found to be similar to the effect of EDTA concentration since the concentration of free EDTA ion was controlled by the solution pH. Peak height increased with increasing pH as a result of a faster elution due to the increase of the concentration of free EDTA ion, but peak area did not change. The effect of elution flow rate was investigated at an EDTA concentration of 0.03 mol L-1 at pH 10.5. It was found that for optimum sensitivity and precision flow rates should be 1 g/L. Serious interferences occurred in the presence of g100 mg L-1 Sr(II) and Ba(II), and 1 g L-1 K(I) and Mg(II). In contrast, for peak area mode no such interferences were found except in the case of g400 mg L-1 Ba(II) and 10 g L-1 Mg(II). Also shown in Table 2 are the recoveries of lead obtained using dilute nitric acid for column wash. The interferences in peak height from high concentrations of Al(III), Ni(II), Cu(II), Fe(III), Mg(II), Ca(II), Cd(II), and Zn(II), and the interference in peak area from high concentration of Mg(II) were eliminated after a 7-s column wash with 0.15 mol L-1 HNO3 before elution. However, the wash step brought no reduction of the interferences due to Ba(II), Sr(II), and K(I) in peak height and that due to Ba(II) in peak area measurement, respectively. These interferences could be attributed to their competition with Pb(II) for the cavity of the macrocycle due to their similar ionic radius. Such interferences occur during sample loading, and the extent of interference depends on the relative kinetics and thermodynamics for the complex formation of coexisting ions against lead ion with the macrocycle, as well as column capacity. Obviously, this kind of interference cannot be eliminated or reduced by a column wash step. 4220 Analytical Chemistry, Vol. 71, No. 19, October 1, 1999

Figure 4. Absorption signals of 200 µg L-1 Pb in the presence of various concentrations of Ba(II) (mg L-1): (a) 0; (b) 20; (c) 100; (d) 400; (e) 1000. Other conditions as in Figure 1 and Table 1.

Figure 5. Absorption signals of 200 µg L-1 Pb in the presence of various concentrations of K(I) (g L-1): (a) 0; (b) 0.2; (c) 1; (d) 10. Other conditions as in Table 1 and Figure 1.

Figure 4 shows the influence of Ba(II) concentration on the absorption signal of 200 µg L-1 Pb under the conditions in Figure 1 and Table 1 (cylindrical column, 145 µL). The absorption signal was broadened in the presence of Ba(II), and the peak height decreased with increasing Ba(II) concentration. The peak area was not affected by Ba(II) concentrations lower than 100 mg L-1, but decreased in the presence of higher contents of Ba(II), indicating kind of a breakthrough due to a competition of Ba(II) for the cavity of the macrocycle. The effect of K(I) concentration on the absorption signal of 200 µg L-1 Pb under the conditions in Figure 1 and Table 1 (cylindrical column, 145 µL) is illustrated in Figure 5. Compared with the effect of Ba(II), the absorption signal was broadened to a much lesser extent even in the presence of very high concentration of K(I) (10 g L-1). Although the absorption signal broadened and peak height decreased with increasing in the concentration of K(I), no change in peak area was observed under the conditions used. This means that the competition of K(I) with Pb(II) for the cavity of the macrocycle was relatively weak, resulting in a less pronounced change of the analyte distribution and no analyte loss due to such competition on a 145 µL column. The effect of Sr(II) was found in between those of Ba(II) and K(I). However, no analyte loss due to the competition from up to 1 g L-1 Sr(II) was observed when a 145 µL column was used. The interference from high concentrations of Fe(III), Zn(II), Cd(II), Al(III), Ca(II), Cu(II), and Ni(II) in the absence of a column

wash step cannot be ascribed to their competition for the cavity of the macrocycle since the ionic radii are much smaller than that of the lead ion.11 This is also supported by the fact that these interferences can be eliminated by washing the column with dilute nitric acid before elution. Accordingly, the residual matrixes on the column should be responsible for such interferences. As these metal ions form stable complexes with EDTA, the concentration of free EDTA ions available for analyte elution decreased, resulting in a slower elution. The observed absorption signal was therefore broadened, while peak area did not change as long as the time set was sufficient for complete elution (Figure 3a,c). A similar explanation could be applied to the peak broadening in the presence of 10 g L-1 Mg(II) without a column wash step (Figure 3e). However, the signal distortion, the baseline change, and the decrease of peak area in Figure 3e cannot be explained in the same way. The overcorrection of the background produced by high content of Mg(II) matrix might be responsible for this effect. Therefore, the analyte signal was completely recovered only after the remaining matrix on the column was removed by a wash step prior to elution (Figure 3f). According to the foregoing results and discussion, the potential interferences in the present FI on-line preconcentration system for FAAS can be divided into three classes: (a) interferences with preconcentration from the metal ions (e.g., Ba(II), Sr(II), and K(I)) with an ionic radius similar to Pb(II) due to their competition for the cavity of the macrocycle; (b) interferences with elution kinetics from the ions which can form stable complexes with EDTA due to their competition for EDTA (e.g., Fe(III), Ni(II), and Cu(II)); (c) matrix effects in the flame such as overcorrection of background absorption. As described above, the last two kinds of interferences can efficiently be eliminated with a proper column wash step before elution, whereas the first one cannot. In a search for efficient ways of eliminating or minimizing the interferences from Ba(II), Sr(II), and K(I), a preliminary attempt was made by changing sample acidity. However, this was not successful as the optimum ranges of sample acidity for the retention of these ions were found to be even wider than that for the retention of Pb(II). Further experiments were conducted to investigate the effects of column capacity and column shape on the recovery 200 µg L-1 Pb in the presence of various concentrations of Ba(II), Sr(II), and K(I). All three columns described under the Experimental Section were used for this purpose. No significant effect of column shape on the recovery was observed. However, the column capacity significantly affected the recovery of the analyte. As shown in Figure 6, the maximum concentration tolerated was improved at least 100 times for K(I), Sr(II), and Ba(II) as the column capacity increased from 50 to 145 µL. This implies that the increase of column capacity can efficiently reduce interferences from coexisting ions due to competition with Pb(II) for the cavity of the macrocycle. Analytical Performance. Characteristic data for the performance of the FI on-line microcolumn preconcentration system for FAAS are shown in Table 3. For a sample loading flow rate of 3.9 mL min-1 with a preconcentration time of 30 s, a sampling frequency of 63 h-1 and an enrichment factor (EF) of 52 were (11) Lide, D. R. CRC Handbook of Chemistry and Physics, 74 ed.; CRC Press: Boca Raton, FL, 1993-1994; pp 12-8-12-9.

Figure 6. Influences of column capacity on the recovery of lead in the presence of K(I) (a, b), Ba(II) (c, d), and Sr(II) (e, f) based on the measurement of peak area. Column capacity: (a, c, e) 50 µL; (b, d, f) 145 µL. Other conditions as in Table 1. Table 3. Analytical Performance of On-Line Microcolumn Preconcentration and Separation for FAAS Determination of Lead Under Conditions Given in Table 1 preconcentration time/s enrichment factor sampling frequency/h-1 sample consumption/mL reagent consumption/mL 0.03 mol L-1 EDTA (pH 10.5) 0.15 mol L-1 HNO3 concentration efficiency/min-1 precision (RSD, n ) 11)/% detection limit in digest (3s)/µg L-1 range of calibration graph/µg L-1 regression equation (8 standards, n ) 3; Cpb in µg L-1) correlation coefficient

30 52 63 2 1.4 1.4 55 1.9 (200 µg L-1) 5 30-2000 Aint ) 0.00096 + 0.00053Cpb 0.9999

obtained. This gave a concentration efficiency of 55 EF min-1. The detection limit (3s) was found to be 5 µg L-1 in the digest. The precision at the 200 µg L-1 level (RSD, n ) 11) was 1.9%. The enrichment factor and the relative detection limit can be improved further by increasing sample loading rate without degradation in the efficiency due to the favorable kinetics and low hydrodynamic impedance of the present system. To demonstrate the accuracy of the present method, a number of biological and environmental standard reference materials were analyzed. It was found unnecessary to remove the remaining silicate in the digests of sediment and soil samples by filtration or by evaporating the digests with hydrofluoric acid to dryness. Although the acidity of the dilute digests could be as high as 3 mol L-1 HNO3, there was no need to readjust to the same acidity as the standard solutions since no significant influence of sample acidity on the sensitivity was observed in the range of 0.075 to at least 3 mol L-1 HNO3. This not only greatly reduced the analysis time but also avoided a potential contamination and/or analyte loss. Therefore, the samples after microwave digestion were analyzed directly. The analytical results based on peak area measurement using simple aqueous standards for calibration are given in Table 4. For the standard reference materials of sediment, soil, and lobster, the concentrations of lead determined by the present method are in good agreement with the certified values despite high contents of potentially interfering elements in these Analytical Chemistry, Vol. 71, No. 19, October 1, 1999

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Table 4. Analytical Results (Mean ( s, n ) 5) for Determination of Lead in Standard Reference Materials sample

unit

certified

determined

Seronorm whole blood II Seronorm whole blood III TORT-1 lobster MESS-1 marine sediment BCSS-1 marine sediment CRM-142 soil

mg L-1 mg L-1 µg g-1 µg g-1 µg g-1 µg g-1

0.394a 0.663a 10.4 ( 2.0 34.0 ( 6.1 22.7 ( 3.4 37.8 ( 1.9

0.42 ( 0.03 0.69 ( 0.05 10.5 ( 0.3 35.3 ( 1.3 22.9 ( 0.6 36.8 ( 1.0

Table 5. Contents of Some Coexisting Elements in the Standard Reference Materials concentration/µg g-1 sample blooda

whole TORT-1 lobster MESS-1 marine sediment BCSS-1 marine sediment CRM-142 soil a

a

Information values.

samples (Table 5). For the blood samples the recommended values were stated to be only preliminary, and no control ranges were given. Therefore, the results can only be used to show the general capability of the procedure. CONCLUSIONS The FI on-line microcolumn preconcentration and separation system using a macrocycle immobilized on silica gel for FAAS has been demonstrated to be promising for routine determination of trace amounts of lead in samples with complicated and variable matrixes due to its simplicity, high efficiency, and good selectivity (12) Venkatesh Iyengar, G. Elemental Analysis of Biological System; CRC Press: Boca Raton, FL, 1989; Vol. I, pp 3, 177.

4222 Analytical Chemistry, Vol. 71, No. 19, October 1, 1999

Fe

Cu

Al

K

500 186 30 500 32 900 19 600

1.4 439 25.1 18.5 27.5

0.01

2 000 10 410 18 600 18 000 20 000

58 300 62 600 50 100

Sr 113

Maximum contents in mg L-1 in whole blood from ref 12.

and accuracy. With an increase of the microcolumn capacity from 50 to 145 µL, the use of a column wash step prior to elution, and peak area evaluation, most concomitant ions, particularly transition metal ions, could be tolerated at concentration levels >10 g/L. Only Ba(II) started to interfere with the preconcentration of Pb(II) at concentrations >100 mg/L and Sr(II) at concentrations >1 g/L, i.e., at levels that should be no problem in biological and environmental samples. For the analysis of a large variety of samples with much more complicated matrixes, however, a further increase of the column capacity is essential for interference-free determination of trace amounts of lead. Received for review April 5, 1999. Accepted June 25, 1999. AC990341Z