Anal. Chem. 1983, 55,1669-1673
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Spectrophotometric Determination of Metals at Trace Levels by Flow Injection and Series Differential Detection R. A. Leach, J. Ruzicka,' and J. M. Harris* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
Flow lnjectlon analysls has several attrlbutes whlch would appear to be useful In trace level determlnatlons, Including small volume sampilng, sample processlng wlthout contact with the laboratory envlronment, and minlmal sample carryover for al glven expenditure of tlme and reagents. While modern spectrophotometrlc detectors exhlbit adequate photometrlc sensitlvlty for trace level determlnatlons, the llmlt of detection Is most often controlled by the reproduclbllityof the reagent blank. In this work, Injecting samples into a matched carrler followed by merging wlth a reagent stream was comblned with series differential detection to overcome optical perturbations from sample lntroductlon and to provide base ilne stablllty In splte of a varlabie blank. The technique was evaluated with respect to the spectrophotometrlc determlnatlon of Iron, chromlum, and copper, for which the concentratlon detection ilmlts were 0.7 ppb, 0.3 ppb, and 1.0 ppb, respectlveiy. The effect of sample refractlve Index on the measurement was consldered where the absorbance and refractlve Index responses are distinguishable by their respectlve peak shapes.
Flow injection analysis (FIA), an automated technique based on the injection of a liquid sample into a continuous stream of reagent or carrier (1-9), appears to hold considerable promise for trace level determinations. The technique is capable of manipulating small sample volumes (10-100 &) which avoids loss of sensitivity by unnecessary dilution when the original sample size is restricted. In addition, the sample can be carried through various stages of physical and chemical processing without contact with the laboratory environment thus reducing the possibilities for contamination. Finally, the rinsing efficiency of the flow injection system is high, which eliminates sample carry-over while conserving high-purity solvents and reagents required. The use of flow injection analysis for trace level spectrophotometry is presently limited to a few examples. Kelly and Christian (10, 11) and Harris (12) have demonstrated the combination of flow injection and laser-induced fluorescence detection. Betteridge et al. (13) introduced to FIA a stable, miniature absorption detector having no optical elements other than a light-emitting diode and phototransistor. Although somewhat limited in applicability by the restricted spectral output of the light emitting diode, this detector was capable of 10 ppb determinations for samples reacted on-line. Due to refractive index difference between the sample and the reagent stream and their effect on the detection response, determination of samples below 10 ppb required off-line colorimetric reactions and reagent-free carrier. Despite the excellent photometric stability (RMS absorbance noise 2 X relative to the carrier. This indicates that only a 5% residual change in stream composition appears at the detector following a valve-throw, indicating efficient mixing in the reaction coil. The limits of detection for each determination are estimated as the sample concentration which would produce a peakto-peak absorbance equivalent to four times the base line noise (28),A- = 1.0 X lo4. These results are summarized in Table 11. As an example, the minimum detectable concentration of iron a t the point of injection is predicted to be 0.7 ppb or about 20% of the concentration of the sample determined in Figure 4. In all cases, the detection limits are part-per-billion
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ANALYTICAL CHEMISTRY, VOL.
55, NO. 11, SEPTEMBER 1983
Table 11. Spectrophotometric Determination Results
a
analyte
concn range determined
of linear plot
corr coeff
slope of log-log plot
iron chromium copper
3.6 ppb-0.71 ppm 2.0 ppb-2.0 ppm 7.4 ppb-3.7 ppm
0.999 84 0.999 97 0.999 65
0.979 1.033 0.995
limit of detectiona analyte analy te concn, ppb mass, pg 0.7 0.3 1.o
56 25 82
In the 80 p L injected sample volume.
, 2 rnin I rnin
Figure 4. Low level absorbance response of flow InJectlon-serles differential detection system. Three "blank" inlectlons (left) followed by four 3.5 ppb iron samples.
or lower, corresponding to less than 100 pg of analyte in the 8 0 - ~ Linjected sample volume. As a final concern, the sensitivity of the series differential detector to refractive index differences between the sample and carrier was examined. Since the parabolic flow profile establishes a radial gradient in sample concentration, refractive index differences between sample and carrier can create a lenslike optical element within the detector flow cell (13). This gradient index lens can either focus more light onto the detector or defocus the radiation off of the detector depending on whether the sample has a refractive index which is greater or less than the carrier, respectively. This is illustrated in Figure 5, where the concentration gradient of a higher refractive index sample (An = 4 X enterir,g the flow cell gives a negative apparent absorbance response due to the focusing of additional radiation on the detector. Although the variation in refractive index perturbs the shape of an absorbance peak, the effect disappears where the concentration gradient is zero. Fortunately, this point corresponds to the peak concentration where the absorbance response maximizes. As shown in Figure 5, a t the two points where the refractive index response goes through the base line, the composite sample response intersects the peaks of the "pure absorbance" curve. Precisely timed measurements of the absorbance a t these two points would be free of refractive index contributions. Similarly, when the concentration gradient is a maximum as the absorbance response goes through the base line, the composite sample curve intersects the "pure refractive index" peak allowing an absorbance-free refractive index measurement. Although this approach to simultaneous absorbance and refractive index measurement could be convenient, the refractive index sensitivity is considerably lower than that obtainable with state-of-the-art differential refractometers, presently capable of detecting refractive index changes as small as (29). In this work, much larger detection limits (An 2X were observed, corresponding to injections of 2
-
-
Figure 5. Effect of sample refractlve Index on series differential detection. Line marked with circles is the response to an injection of 36 ppb Iron in delonlzed water into a deionized water carrier. Line marked wlth triangles is the response to a 0.039 M NaCl solution injection. Heavy solid line is an injection of 36 ppb iron In 0.039 M NaCi.
mM NaCl. The sensitivity of a particular detector to refractive index gradients depends strongly on the optics and flow cell (13,30),which are generally designed to minimize refractive index sensitivity. Some reversal of recent technological progress might improve this application of absorbance detectors. To summarize, the combination of flow injection and series differential detection provides a simple, inexpensive means of determining trace level constituents spectrophotometrically. The concentration detection limits achieved in this study were equivalent or superior to flame atomic absorption which requires considerably larger sample volume, and the absolute detection limits (based on mass of analyte) were about 30 times larger than graphite furnace AA results (31). While the present work has been limited to elemental determinations, there should be no impedement to applying the technique, with the appropriate chromogenic reagents, to the spectrophotometric determination of trace level molecular species as well. The detection capabilities of the method could be quite beneficial, for example, in clinical assays where sample size is restricted and a significant fraction of important analytes are present a t trace levels.
ACKNOWLEDGMENT The authors are grateful to Jim Row of Air-Row Instruments, Arvada, CO, for assistance with the Kratos detector. Registry No. Iron, 7439-89-6; chromium, 7440-47-3; copper, 7440-50-8. LITERATURE CITED (1) Ruzicka, J.; Hansen, E. H. Anal. Cbim. Acta 1975, 78, 145-157. (2) Ruzicka, J.; Hansen, E. H. Anal. Cbim. Acta 1978, 99, 37-76. (3) Betteridge, D. Anal. Cbem. 1978, 50, 832A-846A. (4) Ruzicka, J.; Hansen, E. H. I n "Trace Organic Analysis: A New Frontier in Analytical Chemistry"; National Bureau of Standards: Washing-
Anal. Chem. 1983, 55,1673-1676 ton, DC, 1979; NBS Special Publication 519, pp 501-507. (5) Wolf, W. R.; Stewart, K. K. Anal. Chem. 1070, 5 1 , 1201-1205. (6) Ruzicka, J.; Hansen, E. H. Anal. Chlm. Acta 1080, 1 14, 19-44. (7) Ranger, C. B. Anal. Chem. 1081, 5 3 , 20A-32A. (8) Ruzicka, J.; Hansen, E. H. "Flow Injection Analysis"; Wlley: New York, 1981. (9) Stewart, K. K. Talanta 1081, 28, 789-797. (10) Kelly, T. A.; Christlan, G. D. Anal. Chem. 1081, 5 3 , 2110-2114. (11) Kelly, 7.A.; Christlan, G. D. Anal. Chem. 1082, 5 4 , 1444-1445. (12) Harris, J. M. Ana/. Chem. 1082, 5 4 , 2337-2340. (13) Betterldge, J. D.; Dagiess, E. L.; Fields, B.; Graves, N. F. Ana/yst(London) 1078, 103, 897-908. (14) Pardue, H. L.; Deming S. N. Anal. Chem. 1080, 4 1 , 988-989. (15) Mitchell, D. N.; Wayne, R. P. J. fhys. E 1080, 13, 494-495. (16) Kaye, W. Anal Chem. 1981, 5 3 , 369-374. (17) Harrls, T. D. Anal. Chem. 1082, 5 4 , 741A-750A. (18) Banerjse, S.; Pack, E. J. Anal. Chem. 1082, 5 4 , 324-326. (19) Ruzicka, J.; Hnnsen, E. H.; Ramsing, A. U. Anal. Chim. Acta 1082, 134, 55-71. (20) Blau, F. Monatsh. Chem. 1808, 19, 847-883.
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(21) Willis, R. B.; Sangster, D. Anal. Chem. 1976, 48, 59-82. (22) Dovlchl, N. J.; Harris, J. M. presented at the 1979 ACS Analytical Summer SympOSlum, Purdue University, West Lafayette, IN, June 1979. (23) Harris, T. D. Anal. Chem. 1081, 5 3 , 1727-1728. (24) Blalr, D.; Dlehl, H. Talanta 1081, 17, 183-174. (25) Cazeneuve, P. Bull. SOC.Chlm. Fr. 1900, 23, 701. (26) Tljssen, R. Anal. Chlm. Acta 1980, 174, 71-89. (27) Sano, H. Anal. Chlm. Acta 1062, 27, 398-399. (28) Currie, L. A. Anal. Chem. 1068, 40, 588-593. (29) Colin, H.; Jaulmes, A.; Guiochon, G.; Corno, T.; Simon, J. Chromafogr. Sci. 1070, 17, 485-491. (30) Stewart, J. E. Anal. Chem. 1081, 5 3 , 1125-1128. (31) Slavin, W. Anal. Chem. 1082, 5 4 , 685A-894A.
RECEIVED for review March 3,1983. Accepted June 1,1983.
This work was supported, in part, with funds from the National Science Foundation, under Grant CHE82-06898.
Characterization of Metal Complexes Formed by Interaction of Allyl Alcohol and Copper(I)in Aqueous Solution Naohisa Yanagihara, Gerard0 Ulibarri
D.,and Tetsuya O g u r a
Departamento de Qiiimica, Universidad Aut6noma de Guadalajara, A.P. 1-440 Guadalajara, Jalisco, Mexico Nelson Scott a n d Quintus Fernando*
Department of Chemistry, University of Arizona, Tucson, Arizona 85721
The equlllbrla that are establlshed In aqueous solutions containing copper( I I), copper metal, and oleflnlc ligands can be studled by the determlnatlon of the concentratlon of copper( I ) In solution by a slpectrophotometrlc method and the concentration of the uncomplexed oleflnlc llgand by a gas chromatographic method. These methods have been used for the determlnatlon of Ithe equlllbrlum constants that correspond to the formation of copper( I)-allyl alcohol complexes that are obtalned when copper( I I ) Is reduced by copper metal In the presence of allyl alcohol. The rate of the reductlon reactlon Is Influenced by hydrogen Ions and Inorganic anlons In the aqueous solution. A solid complex Cu,(L)(SO,) has been Isolated from the reaction mlxture. Both the allyl alcohol molecule and the sulfate Ion act as multldentate llgands In thls complex.
The stabilizatialn of copper(1) in the presence of allyl alcohol proved to be useful for the determination of mixtures containing copper(1) and copper(I1) in aqueous solutions and was the subject of a previous article ( 1 ) . In a continuation of this work we have studied the equilibria that have been established in aqueous solutions containing copper(I1) sulfate, allyl alcohol, and copper metal. and we have identified the various factors that affect the rate of reduction of copper(I1) to copper(1) in these systems. Reactions of this type, in which copper(1) complexes of olefinic ligands are formed, separated from the reaction miixture, and then allowed to disproportionate, have been found to yield high-purity copper metal. Such reactions are potentidly useful in the production of printed circuits and in the large scale purification of copper. The successful application of these reactions will depend on the optimization of all solution parameters and the criteria used in the choice of an olefinic ligand. In this work we have demonstrated that
the measurement of the concentrations of copper(I), copper(I1) and uncomplexed olefinic ligand (allyl alcohol) in aqueous solutions can provide useful information about the manner in which copper(1) is stabilized by the olefinic ligand. EXPERIMENTAL SECTION Determination of Copper(I), Copper(II), and Allyl Alcohol in a Reaction Mixture. An aqueous solution of copper(I1) sulfate (0.5 M), allyl alcohol, and sulfuric acid (0.1 M) was freed from dissolved oxygen by passing nitrogen gas through the solution. This mixture together with copper metal was introduced into a thermostated reaction vessel in an atmosphere of nitrogen. The reaction vessel was evacuated and the reaction was allowed to proceed with continuous stirring for 25-50 h. The progress of the reaction was monitored by the measurement of the vapor pressure in the reaction vessel with a mercury manometer. The concentration of allyl alcohol in the vapor phase was determined by trapping a small amount of the vapor in a liquid nitrogen cooled capillary tube and analyzing the condensate by a gas chromatographic technique. The separation was carried out at 90° on a 6 ft column of silylated diatomaceous earth containing 25% (w/w) triethanolamine with helium as a carrier gas at a flow rate of 30 mL/min. The concentrations of copper(1)and copper(I1)in solution were determined titrimetrically as described previously (I). Kinetic Experiments. A measured volume of a copper(I1) solution was introduced into a three-neck round-bottom flask maintained a 25 "C. Oxygen-free nitrogen gas was passed through the solution to remove dissolved oxygen and the required volume of allyl alcohol added to the solution from a buret. The mixture M) and allyl alcohol (0.5 M) was containing copper(I1) (1.0 X stirred vigorously in a reproducible manner and a weighed amount of acid-washed copper granules added to the reaction mixture. The reaction was allowed to proceed in a nitrogen atmosphere and 1-mL aliquots of the reaction mixture were withdrawn periodically and analyzed for copper(1) and copper(I1) as described previously (2). The variation of the rate of formation of copper(1) in solution with varying initial concentrations of copper(II),allyl alcohol, and hydrogen ion was investigated. Additional factors
0003-2700/83/0355-1673$01.50/00 1983 American Chemical Society
