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Anal. Chem. 1900, 60,323-328
the homogeneity of the 200-lb batches of each that were prepared and the precision with which the measurements were being reproduced. The resulta given below are the means for the number of replicate measurements made. The uncertainties shown are the experimental standard deviations of the individual results about their own mean. Uncertainties given in parentheses are the average standard deviations with which the individual measurements were made, with all random uncertainties incurred anywhere in the entire measurement process being propagated to the final result. The sample shea taken for analysis were 5 g for the soil, 2 g for the tailings, and 1g for the ore. Counting times were 6 h for the soil and 2 h for the other samples. At least eight samples can be prepared for counting per day. On 16 measurements each, the counting efficiency obtained through 6.04 mg/cm2 of aluminum foil was 0.348 f 0.002 (f0.002) and background was 1.87 f 0.12 (f0.07) cpm. The means and standard deviations of the 10 analyses made on each sample are 1.00 f 0.13 (fO.09) pCi/g for soil, 420 f 7 (f3) pCi/g for tailings A, 548 f 9 (f4) pCi/g for tailings B, and 754 f 16 (f6) pCi/g for the ore. Except for the lowactivity soil, the experimental standard deviations among the replicates are 1.7-2.1% on 10 determinations each, about twice the uncertainties of about 0.8% of the individual measurements. Chemical yields were generally higher than 98% overall and were determined to better than 0.5%. Because of the high and reproducible chemical yields obtained and the
small size of sample taken for analysis compared to the large batches and high activities of the standards prepared, it seems more likely that the larger experimental uncertainties are due to a slight residual inhomogeneity in the samples than to the analytical procedure. Three-gram samples of a standard pitchblende containing 254 f 2 pCi/g of lead-210 gave values of 249 (f2) and 250 (f2) pCi/g. Thus, it seems likely that both precision and accuracy are better than 3% at the 95% confidence level. Detection limits are about 0.1 pCi/g for 5-g samples and 6-h counting times.
ACKNOWLEDGMENT Special thanks are extended to J. R. Duray and R. B. Chessmore of U.N.C. Technical Services, Inc., Grand Junction, CO, for permission to cite analyses of the standard samples prepared by them. Registry No. 210Pb,14255-04-0; pitch blende, 12143-25-8. LITERATURE CITED (1) Sill, C. W.; Wlllls, C. P. Anal. Chem. 1965, 3 7 , 1661-1671. (2) Sill, C. W.; Willis, C. P. Anal. Chern. 1977, 4 9 , 302-306. (3) Smithson, G. L.; Fahri, M.; Petrow, M. CANMET Report 78-22; CANMET, Energy, Mines and Resources Canada, Ottawa, Canada, 1979. (4) Wiegand, C. J. W.; Lann, 0. H.; Kalich, F. V. Ind. Eng. Chem., Anal. Ed. 1941, 13, 912.
RECEIVED for review July 31,1987. Accepted October 20,1987. Work performed under the auspices of the U.S. Department of Energy, DOE Contract No. DE-AC07-76ID01570.
Spectrofluorometric Determination of Pesticide Residue Mixtures by Isodifferential Derivative Spectroscopy F. Garcia SQnchez*and C. Cruces Blanco Department of Analytical Chemistry, Faculty of Sciences, The University, 29071 Mhlaga, Spain
A synchronous derlvatlve spectrofluorometrlc method has been developed for quantltatlve residue determination of thiabendazole (TBZ) and 2-benrlmldarol-2-yicarbamlc acid methyl ester (MBC) In varlous crops. The compounds are extracted Into ethyl acetate, and fluorescence Intensity Is compared to standards slmliarly treated with either TBZ or MBC. A detailed study of solvent effects and wavelength scanning Intervals on UV absorption and fluorescence spectra of both compounds was carrled out. The method Is sensltlve to 24 ng/mL TBZ and 17 ng/mL MBC. Recoveries for slmultaneous determlnatlon of TBZ and MBC In fortified vegetable samples at the 5 pg/g level average 91.5%. This paper reports the testing of a graphkal model to measure derivative amplitudes which Is based on the interference-free character of the lsodifferential points In the derlvatlve callbratlon graphs.
The family of benzimidazole derivatives such as benomyl, thiabendazole, and carbendazime is widely used in Spain (I). They act as systemic fungicides controlling a wide range of fungi affecting field crops. They are also used as pre- and postharvest sprays for the control of storage diseases of fruit and vegetables (2). Because benomyl rapidly decomposes during ordinary analytical procedures in organic solvents and water (3), gen-
erally it is measured by determining its main fungitoxic metabolite carbendazime (MBC). Several papers have been published regarding analysis of thiabendazole (TBZ), MBC, or analogues. Most of the procedures involve rigorous cleanup procedures of sample extracts, and the final measurement is usually by spectrofluorometry ( 4 , 5 ) ,spectrophotometry (6,7),and high-performance liquid chromatography (HPLC) (8-11). Due to their similar structure, these two compounds present overlapping fluorescence spectra which cause a problem for their simultaneous determination. Also, when fluorescence spectroscopy is used as a detection technique in residue analysis, many interfering fluorescence substances from coextractives affect the sensitivity and reproducibility of the methods. The improved sensitivity and selectivity obtained in different fields of analytical chemistry using the synchronous derivative technique have been demonstrated in the last years (12-1 7). In the present study, the natural fluorescence of TBZ and MBC was used to test the feasibility of determining both compounds in binary mixtures by taking advantages of synchronous derivative spectroscopy. This work offers an analytical method which overcomes some difficulties encountered in the previously proposed analytical procedures for determining residues of TBZ, MBC,
0003-2700/88/0360-0323$01.50/00 1988 American Chemical Society
324
ANALYTICAL CHEMISTRY, VOL. 60,
NO.4,
FEBRUARY
15, 1988
I
I I
I I I I I I I I
I I I
I I
1: i
'0
I
I I I I I
I
Flgure 1. (a) Excitation and emission spectra of TBZ (-) = 20 nm (-), 40 nm (---), 60 nm (-.-), 80 nm (---), 120 nm/min; response, 1 s; slits, 2.5 nm.
and MBC (---) in ethanol. (b) Synchronous spectra for TBZ iMBC mixture at AX and 120 nm In (a) and (b): [TBZ] = 5 pg/mL; [MBC] = 10 pg/mL; speed,
and their mixtures without prior cleanup procedures. EXPERIMENTAL SECTION Apparatus. Emission measurements were done with a luminescence spectrometer, Perkin-Elmer LS-5, equipped with a xenon discharge lamp pulsed at line frequency (9.9 W), monochromators, F/3 Monk-Gilleon type, and 1 X 1 cm quartz cells. The spectrofluorometer was operated in the computer-controlled mode via the RS232C serial interface by a Perkin-Elmer Model 3600 data station microcomputer. Instrumental control and data collection were achieved by using the commercially available Perkin-Elmer computerized luminescence software (PECLS-11). The system responds to spectral derivatives. To compare all measurements and make feasible reproducible experiences, the LS-5 fluorometer was checked daily. A polymer fluorescence sample of p-terphenyl (lo-' M) give a relative fluorescence intensity (RFI) of 90% at A,, = 340 nm with A,, = 295 nm with slits of 2.5 nm and sensitivity factor of 0.5973. The conditions were scan speed of 120 nm/min, response of 3 s, and slits of 5 nm with a sensitivity factor of 0.2. For graphical recording, a Epson FX-85 printer-plotter was connected to the spectrofluorometer. All fluorescence spectra are uncorrected because no significative wavelength shifts are observed when comparing with corrected spectra. UV absorption spectra were recorded with a Shimadzu UV-240 Graphicord recording spectrophotometer. A rotary vacuum evaporator (W. Buchi Scientific Apparatus, Flawil, Switzerland) and a roller mill blender with porcelain cup were used for extraction procedures. Reagents. Stock solutions of the pesticides thiabendazole and carbendazime (>99% pure, Riedel-de Haen, Seelze, Hannover), quality PESTANAL, were prepared in ethanol at concentrations of 1.0 and 0.15 mg/mL, respectively. Working solutions at 100 pg/mL of both compounds were prepared in ethanol. All solvents used were from Merck and Co., Rahway, NJ. Quantitative Experiments. To prepare solutions containing 0.05-2.0 pg of TBZ or MBC/mL, the standard solutions were diluted to final volume with ethanol. The fluorescence intensity
(.e.).
is measured by using the following conditions: TBZ (excitation 297 nm, emission 341 nm), MBC (excitation 241 nm, emission 309 nm), synchronous scanning interval of 20 nm, integrer factor of 20. Extraction of Crop Samples. Each of the three different kinds of crops analyzed (peppers, tomatoes, and potatoes) was sliced and homogenized. Ten grams of each sample is weighed and placed in a roller mill cup and 40 mL of ethyl acetate is added. The extract was homogenized for 3 min and filtered through fritted disk funnel (Schott no. 3) and reextracted twice before all extracts are combined and evaporated to dryness on a rotary evaporator at 45 "C. The concentrate is diluted to 25 mL with ethanol. Vegetables were fortified with the compounds at the 5 pg/g level by addition of 50 pL of TBZ and 333.3 p L of MBC of the ethanol stock solutions to 10 g of crop in the homogenizer. A crop control and triplicate fortification analyses for each kind of crop were performed. RESULTS AND DISCUSSION Chemical and Physical Optimization. In order to increase the sensitivity and selectivity for the simultaneous analytical procedure of TBZ and MBC, a selection of the appropriate solvent and wavelength interval for the synchronous technique was carried out. The studies of each compound in the solvents indicated in Table I show that ethanol gave the maximum RFI in both compounds, the minimum half band width of the synchronous spectra (B), and a good separation between maxima of these spectra (C = 56 nm). Therefore, 100% ethanol was used as solvent for experimental work. A detailed study of the synchronous spectra of both compounds in the solvent selected containing 5 pg/mL TBZ and 10 pg/mL MBC from the wavelength scanning interval (AX) 20 to 120 nm was performed to avoid mutual interference. As is seen in Figure la, the emission maxima of both compounds are separated by 32 nm, but the excitation maxima clearly overlap.
ANALYTICAL CHEMISTRY, VOL. 60, NO. 4, FEBRUARY 15, 1988
325
Table I. Solvent Effect on UV Absorption and Fluorescence Spectra of TBZ and MBC dielectric const (at 25 solvent water
log
c
ER
X
lo4"
A,,
nm
A,
nm
RFIb
B,'nm
C,d nm
"C)
50
78.52
22
36.7
56
32.7
TBZ 4.34 MBC 4.15 TBZ 4.34 MBC 4.10 TBZ 4.37 MBC 4.18
1.9 0.2 1.9 2.0 3.4 1.2
292 241 305 266 297 241
356 309 357 309 342 310
48.7 5.4 47.6 53.5 85.4 30.5
20.5 11.5 17.2 14.5 17.2 9.0
ethanol
TBZ 4.36 MBC 4.13
3.7 1.6
297 241
341 309
92.3 41.5
17.0 90.0
56
20.5
ethyl acetate dioxane
TBZ 4.35 MBC 4.06 TBZ 4.39 MBC 4.07
1.8 1.9 2.5 1.6
299 276 301 258
354 310 356 308
45.2 49.9 58.6 41.4
19.7 16.5 16.2 5.5
18
6.0
44
2.2
DMF
methanol
a Efficiency ratio (ER) = RFI/t. maxima of both compounds.
* Relative fluorescence intensity.
After a careful observation of the marked modification of the spectral shape of the synchronous spectra of Figure lb, we selected AA = 20 nm as the most appropriate wavelength interval for the analytical quantitation for both fungicides. The variables to be optimized when applying the derivative technique to this synchronous spectra are the number of data points and the measurement wavelength for the correct determination of both compounds. The maximum integrer fador is 20, giving the better signal-to-noise ratio, both in fiist and second derivative, which was chosen for the experimental work. The model used in the paper to discriminate the overlapping bands needs Beer's law to be obeyed in the full concentration range studied. This fact was proved with two solutions containing each analyte and a mixture of both. The resolution of the two equations resolve satisfactorily the mixture indicating that the law is satisfied in the concentration range studied. In such a case and assuming that the derivative of a spectral band is equivalent to the sum of the derivatives of its individual bands, when dXl/dA1 = 0 (zero crossing at this A, corresponding to a maximum in the zero-order spectrum), the contribution of component 1 on the overall derivative amplitude is zero and, consequently, component 2 may be measured free of component 1 interference. The same applies to component 2 when dZi/dAz = 0. Similar considerations apply in quantitative derivative spectrometry measurements in those areas of spectroscopy where a function of spectral intensity is linearly related to analyte concentration. Maximum sensitivity and precision are obtained in the isodifferential derivative method when the first derivative amplitude of a component is zero at the same wavelength in which the second derivative of component 2 is zero. The solutions for d2I2(x)/dx2= 0 and for dIl(x)/dx = 0 are satisfied when C1 = x and ( x - C 2 ) / B 2= 1, Le., when C1 - C, = Bz. This implies that the best conditions for evaluation if amplitude values of component 2 from the zero crossing of component 1are obtained when separation between maxima (Cl - Cz = c ) is equal to the half band width at half maximum height of component 2 (&). Inversely, component 1presents optimal conditions when (C, - C2 = C) is equal to Bl. These considerations are of analytical interest because it may be predicted that two overlapping bands can be measured by using the isodifferential derivative approach. Quantitative Analysis. Studies with solutions of TBZ and MBC alone have established that the concentration of TBZ and MBC correlates well with the RFI at a wavelength
Half band width at half maximum intensity.
240
260
200
300
320
24
Separation between
MO
X
(nm)
I -
240
20 0
tM
1w
azo
XOHII)
Figure 2. Synchronous (a) first and (b)second derivative of TBZ and MBC at AA = 20 nm: (1) 0.70,(2) 0.50, and (3)0.20 bg/mL MBC and (1') 0.45, (2') 0.35,(3') 0.25,and (4') 0.10 pg/mL TBZ; speed, 120 nmlmin; response, 3 s; slits, 5 nm; integer factor 20.
of 312 and 285 nm, respectively. The measurement wavelength for both compounds using PA = 20 nm is indicated in Figure 2. These amplitudes in the first and second derivative spectra show a proportional relationship with concentration with correlation coefficients in the range of 0.9993-0.9997. By use of AA = 20 nm, the interference from the other compound is avoided by measuring at 257 and 335 nm for MBC and TBZ, respectively. This causes a diminution of the sensitivity and detection limit but an increase in selectivity of the analytical determination. In order to evaluate the accuracy and reproducibility of the individual methods, a series of seven solutions were prepared with concentrations of 250 ng/mL TBZ and 500 ng/mL MBC. The results of this study are given in Table 11. The lower
326
ANALYTICAL CHEMISTRY, VOL. 60,
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FEBRUARY 15, 1988
Table 11. Analytical Data Obtained from TBZ and MBC Solutions compound
method"
S A t b ng/mL
CLICng/mL
linear dynamic range, ng/mL
RSD, %
TBZ
A B
8.73 8.09 17.57 15.00 5.73 15.38
26.20 24.27 52.73 45.00 17.18 46.15
87.33-450.00 80.91-450.00 175.77-450.00 150.00-900.00 57.27-900.00 153.84-900.00
3.10 2.99 3.07 2.91 1.08 1.38
C A B
MBC
C
A, synchronous spectrofluorometry; B, synchronous first derivative spectrofluorometry; C, synchronous second derivative spectrofluorometry. *SA= Ss/m analytical sensitivity, ss standard deviation of analytical signal, m slope of calibration curve. CL = 3sB/m detection limit, sR standard deviation of the blank signal.
Table IV. Recoveries of TBZ and MBC from Fortified (5 p g / g ) and Untreated Samples
Table 111. Recoveries (7~)" of the Fungicides from Synthetic Mixtures
untreated samples,
molar ratio mixtureb 200
+ 200
TBZ:MBC 1:l.O
400
+ 400
1:l.O
200
+ 400
1:2.1
100
+ 500
1:5.2
200 + 1000
1:5.2
100 + 1000
1:10.5
method'
TBZ
MBC
A B C A B C A B C A B C A B
99.1 101.1 100.8 114.2 115.7 101.1 107.1 110.1 112.9 126.7
94.7 114.3 101.9 87.1 104.1 102.3 107.9 113.5 112.1 102.0 102.5 103.1 124.8 97.8 98.4 105.5 101.9 99.5 119.9 92.3 95.4 96.4 116.5 114.3
C
100 + 2000
1:21.2
200 + 100
1:9.1
600 + 300
1:9.1
500 + 100
4.7:l
A B C A B C A B C A B C A B C
121.3 105.7 111.2 105.4 107.7 118.2 120.2 196.3 109.7 111.4 107.2 98.8 99.5 98.5 102.9 103.6 99.9
101.2 102.2 125.7 95.3
a Duplicate determinations. Concentrations in ng/mL. A, B, and C, as indicated in Table 11.
detection limit, cL ( k = 3), and the lower limit of the dynamic range, cQ ( k = lo), as defined by IUPAC (18),as well as the sensitivity sA (15) for each compound are also indicated. Studies with mixtures of TBZ and MBC at different concentration ratios (Table 111)revealed that the amplitude selected for the individual quantitation of the compounds is appropiated. It is concluded that with the application of the synchronous derivative technique, good recoveries of TBZ are obtained when MBC is in a 20-fold excess, and MBC with a &fold excess of TBZ if present in the same solution. The additional resolution obtained by using the derivative spectroscopy is observed by comparing recoveries of TBZ and MBC for the same mixture, measured by zero synchronous spectra and first and second derivatives. The concentration ratio is increased from 1:10.5 (TBZ:MBC) with the normal synchronous spectra to 1:21 using the second derivative synchronous determination. Assay Results. Quantitative recovery is only one aspect to be considered in evaluating the applicability of t8hemethod to these pesticides. The other aspect is selectivity. Selectivity
vegetable samples
rglg
method
f
RSD"
fortified samples, Lcg/g
f
RSD
recovery, %
3.5 f 1.4 3.9 f 0.4 3.8 i 1.2 4.1 f 1.7 3.9 f 1.8 4.0 f 1.5 4.9 f 0.4 6.1 f 0.3 6.1 f 0.2
70 78 76 82 78 80 98 122 128
4.8 f 0.4 5.6 f 0.3 4.9 f 0.3 4.1 f 0.6 4.9 f 0.5 5.0 f 0.3
96 112 98 82 98 100
TBZ green pepper
A B C
tomato
A B C
potato
A B C
1.0 A 0.05 0.6 f 0.02 1.4 f 0.07 0.8 f 0.15 0.7 f 0.07 1.4 f 0.30 1.4 f 0.30 0.8 f 0.20 2.2 f 0.10 MBC
green pepper
A B
tomato
C A B C
1.2 f 0.10 1.4 f 0.02 0.4 f 0.02 0.2 f 0.10 1.1 f 0.07 0.2 f 0.02
Mean value of triplicate analysis of three redications. is a function of the ability to eliminate components which would be detected with the determination procedure. To test the efficiency and selectivity of the proposed analytical procedure, recovery experiments on a number of vegetables were performed. These crops (peppers, tomatoes, and potatoes) were purchased in markets and their spray histories were unknown. These samples were fortified with both compounds at the 2 Fg/mL level to see the effect of the presence of each compound in the recovery of the other. For quantitative analysis, the vegetable extracts were conveniently diluted to obtain a final concentration of TBZ and MBC of 0.2 or 0.4 Hg/mL. The vegetable samples analyzed in this study contained a quite considerable amount of fluorescent materials which were present as coextractives. To avoid the possible interference caused by these substances preliminary experiences using the full view of three-dimensional image plot (3-D) of the extract with and without the fungicides show that the matrix effect on the luminescence spectra is negligible in all cases (see Figure 3). Because the synchronous approach used simplifies the overall spectral shape, we use a sequential recovery of synchronous spectra data based on a new modification software (19) that permits a three-dimensional synchronous spectra (Figure 3). From these figures it can be seen that matrix effects due to vegetable extracts in pepper, tomato, and potato are controlled using the selected quantitative determination of TBZ and MBC. Despite this selection, some interferences are observed from these fluorescent materials, which have been taken into account by subtrating, in each case, the blank
ANALYTICAL CHEMISTRY. VOL. 60, NO. 4, FEBRUARY 15, 1988
327
(h')
fLi
( C '
,
1
Ii il
1
Y l jl
r"r/.Pl,nY
lllrllll
Figure 3. Sequential scanning synchronous spectra of vegetable extracts 01 peppers. tomatos, and potatoes with (a. b, c) and without (a', b', c') TBZ MBC at the 0.4 pglmL Ievek speed, 240 nmlmin; sins. 5 nm; monochromator difference. 10 nm; excitation inaement, 3 nm.
+
readings from the recovery data. These interferences gave a RFI corresponding to about 1.3 pg/g TBZ and 0.75 pg/g MBC which limit the sensitivity of the analytical procedure t o 0.2 pg/mL TBZ and 0.4 pg/mL MBC. T o improve the sensitivity, a preliminary cleanup of the vegetable extracts would be required. Recovery results for the whole procedure together with that obtained from the analysis of unfortified crops are shown in Table IV. The results are obtained as mean values of triplicate analysis. As previously reported by Kitada et al. ( I I ) , ethyl acetate gives the best efficiency for extracting TBZ, MBC, and related compounds and it was used in the present work. Despite the fact that vegetable blanks were extracted from the recovery data, the interference caused by these blanks was
more serious in the quantitative determination of MBC, as previously indicated (4). For a correct determination of pesticidee as residues in different matrices, recoveries should be within 7&110%, with a mean value of greater than 80% after removal of the blanks (20). From the results in Table IV,it is otxervd that this is accomplished for both compunds in the different matrices showing an average recovery of 90.2% (TBZ) and 97.7% (MBC). In conclusion, we have established a method for the simultaneous determination of two compounds of very similar structure in vegetable samples without cleanup procedures, using the synchronous derivative approach with the isodifferential graphical model described. Registry No. TBZ,148-79-8;MBC, 10605-21-7.
Anal. Chem. 1988, 60,328-331
328
LITERATURE CITED (1) de Lifian, C.; Vicente. C. Vademecum deproductos fitosanilarios 8485; de Llfian, C.; Vicente, C.. Ed., EmbaJadores: MadrM, Spain, 1985. (2) Pesticide Manual, British Crop Protection Councll, 7th ed.;Worthing, C. R., Ed.; BrMsh Crop Protection Council: Croydon, England, 1983, pp 89 and 523. (3) Chlba, M.;Cherniak, E. A. J . Agric. FoodChem. 1078, 26, 573. (4) Aharon, N.; Ben-Aziz, A. J . Assoc. Off. Anal. Chem. 1073, 56. 1330. (5) On, D. E. J . Assoc. Off. Anal. Chem. 1075, 58, 160. (6) Chlba, M. J . Agdc. food Chem. 1077, 2 5 , 368. (7) Chlba. M. J . Agrc. FoodChem. 1078, 2 6 , 573. (8) Farrow, J. E.; Hoodless, R. A.; Sargent, M.; Sldwell, J. A. J . Agrlc. Food Chem. 1977, 2 5 , 102, 752. (9) Tafuri, F.; Marucchlnl, C.; Patuml, M.;Buslnelll, M. J . Agrlc. Food Chem. 1080, 28, 1150. (10) Chiba, M.;Veres, D. F. J . Assoc. Off. Anal. Chem. 1080, 63, 1291. (11) Kbda, Y.: Sasaki, M.; Tanigawa. K. J . Assoc. Off. Anal. Chem. 1082, 65, 1302. (12) Vo-Dinh, T . Anal. Chem. 1070, 5 0 , 396. (13) Mlller. J. N.; Ahrnad, T. A.; Fell, A. F. Proc. Anal. Div. Chem. SOC. 1082, 19, 37.
(14) Cruces Blanco, C. Doctoral Thesis, Unlversity of MBlaga, Spain, Feb 1987: (15) Garcia Slnchez, F.; Cruces Blanco, C. Anal. Chem. 1086, 58, 73. (16) Rubio, S.; Mmez-Hens, A.; Valclrcel, M. Anal. Chem. 1085, 5 7 , 1101. (17) Cruces BlanOO. C.; Garcia Slnchez, F. J . Assoc. Off. Anal. Chem. 1086, 69, 105. (18) Guidelines for Data Adquisitlon and Data Quality Evaluation In Environmen91 Chemistry Anal. Chem. 1080, 52, 2242. (19) Garcia Snchez, F.; Ramos Rublo, A. L.; MHrquez G m e z , J. C.; Herrdndez Lbpez, M.; Carnero, C.; Cruces Blanco, C. Presented at the Scientlflc and Cornputlng Automation Congress (SCA), Amsterdam, 13-15 May 1987. (20) Telling, 0. M. P r w . Anal. Dlv. Chem. SOC. 1070, 20, 38.
RECEIVED for review June 30, 1986. Resubmitted May 11, 1987. Resubmitted August 12, 1987. Accepted October 14, 1987.
Tubular Donnan Dialysis-I nductively Coupled Plasma Atomic Emission Spectrometry John A. Koropchak* and Ewa Dabek-Zlotorzynska' Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901-4409
The use of tubular Donnan dialysis as an on-line means of cation preconcentration can provide rapid enhancement of inductively coupled plasma atomk emission spectrometry ( ICP-AES) signals and corresponding limit of detection improvements. Fiow rate requirements of the ICP-AES sample Introduction system are compatible with those required to provide high cation enrichment factors for Donnan dialysis, especialry uslng a glass-frll nebuWzer. Intraaikai interferences are alleviated in an on-line fashion within certain mole ratio limits, which are cation dependent. Easily ionizable components in the receiver appear to normalize the matrix with regard to slgnai enhancements in the initial radiation zone of the ICP.
Donnan dialysis is a process by which ions are transported across an ion-exchange membrane as a result of an ionic strength gradient (1). If the volume of a high ionic strength receiver solution is smaller than that of a low ionic strength sample solution, enrichment of sample ions in the receiver results (2). The ion-exchange membrane employed may be flat (3) or tubular ( 4 ) . Donnan dialysis has been shown to provide essentially matrix-independent ion enrichment for samples of moderate to low ionic strength (2, 5 ) . Modest success has been reported for the application of tubular ion-exchange membranes to on-line preconcentration of cations prior to flame atomic absorption (FAA) analysis (6). More recently, detailed characterization of this approach was described, with signal enhancement factors exceeding 20 achievable within 5 min (7). Enrichment factors were shown to increase with tubing length, lower receiver pHs, and temperature (7). Easily ionizable elements (EIE) could be in-
* Author t o whom correspondence should be sent.
On leave from Department of Chemistry, Warsaw University, Warsaw. Poland.
cluded in the receiver solution for ionization suppression, further enhancing signals (7). Interferences due to counterions, such as PO4*, were also alleviated in this on-line fashion within the ionic strength limitations of Donnan dialysis (7). In order to select flow rates for the on-line tubular Donnan dialysis experiment, compromise between the low flows optimal for Donnan dialysis (
