364
Anal. Chem. 1981, 53, 364-366
Differential Determination of Chromium(V1)-Chromlum(II1) with Poly(dithiocarbamate) Chelating Resin and Inductively Coupled Plasma-Atomic Emission Spectrometry Akira Miyazaki‘ and Ramon M. Barnes’ Department of Chemistiy, GRC Towers, University of Massachusetts, Amherst, Massachusetts 0 1003
The differential determination of trace amounts of Cr(VI) and Cr(III) is an important problem especially in clinical and environmental analysis because of the toxicity of Cr(V1) species (I). Many methods to solve this problem have been reported including solvent extraction and ion exchange (2-4). However, few reports have been published on the differential determination of Cr(V1)-Cr(II1) using a chelating resin. Recent testa with a novel poly(dithiocarbamate) chelating resin indicated that Cr(V1) complexed with the resin while Cr(II1) did not (5). Similar properties were described previously for a poly(acry1amidoxime) resin (6). These resins potentially could be used for the differential determination of Cr(V1) and Cr(II1). For the determination of chromium, colorimetric or atomic absorption spectrometric methods have been used (4, 7). However, in the colorimetric method, several conditions such as temperature or amount of reagent must be kept strictly constant to achieve good reproducibility. In flame AAS a solvent extraction step for chromium and special flame conditions (4,7) are required, and interelement interferencm exist for chromium species (8). Inductively coupled plasma-atomic emission spectrometry (ICP-AES), in contrast, exhibits a good detection limit and freedom from interferences for chromium determinations (9). Our purpose is to demonstrate that the poly(dithiocarbamate) resin is useful for differential determination of Cr(V1) and Cr(II1). Results are described for synthetic samples by using the chelating resin to separate and concentrate the chromium species and ICP-AES to determine the chromium concentration. EXPERIMENTAL SECTION Determination of Chromium. The determination of chromium was performed by ICP-AES using the Cr I1 267.72 nm wavelength. The ICP system was described previously (5). Typical plasma conditions for chromium determination are as follows: power, 0.9 kW; coolant argon flow rate, 16 L/min; auxiliary argon flow rate, none; sample aerosol argon flow rate, 1.3 L/min; sample uptake rate, 2.2 mL/min (for HN03-digested samples);observation height, 18 mm above the load coil; entrance slit, 40 pm X 5 mm, exit slit 40 pm. A calibration function for chromium was made by using a standard solution prepared as follows: a small volume (less than 1mL) of stock solution was taken in a 25-mL volumetric flask. Fifteen milliliters of concentrated nitric acid was added and made up to volume with (3 + 1) HN03 (hereafter referred to as the ‘“NO3 matrix“). The concentration of chromium was determined by using this calibration function, which was linear with correlation coefficient more than 0.994. No difference of signal intensity between a Cr(V1)-spiked (10 pg/mL) sample to the resin-digested matrix and a Cr(V1) (10 pg/mL) standard solution was observed. Reagent and Standard Solution. Cr(V1) and Cr(II1) stock and standard solutions were prepared from (NH4)2Cr04and Cr(N03)3.9H20,respectively, in 0.1 N “OB. Since the Cr(NOd3.9H20was slightly wet, the concentration of chromium in the Cr(II1) standard solution was calibrated with Cr(VI)stock solution by using ICP-AES. The Cr(V1) standard solution was prepared by diluting 2000 pg/mL of stock solution to the appropriate concentrationjust prior to use. ACS reagent grade chemicals and ‘On leave from National Research Institute for Pollution and Resources, Yatabe, Ibaraki, 305,Japan. 0003-2700/81/0353-0364$01.00/0
Table I. Recovery of Total Chromiuma amt of amt of Cr(III), pg recovery, % Cr(VI), pg 20 20 98,95 20 60 99,96 a For 125 mL. p H 3.5. and 7 0 mg of resin. distilled, deionized water were used. Resin Column and Sample Container. The resin column employed was the same as that described by Barnes and Genna (IO). After the resin was sieved, 70 or 100 mg of 65/80 mesh-size resin provided resin bed lengths of 20 mm (70 mg) and 30 mm (100 mg). Three types of containers were tested for the sample in these column studies: glass separatory funnels with a glass or a Teflon cock, polypropylene separatory funnels (Nalgene 4300-0500) with a Teflon cock, and glass separatory funnel silanized with methylchlorosilane with a glass or a Teflon cock. Cr(1V) Recovery Test. A solution (125 mL or 500 mL) which contained 20-200 pg Cr(V1) and various amounts of Cr(II1) was placed in a 200- or 600-mL glass beaker. The pH of the acid solution waa adjusted with dilute NH40H to 2.5-3.5, and the solution was transferred to the container connected to the resin column. The sample passed through the column at the flow rate of 3.0-4.0 mL/min (15.3-20.4 mL m i d cm-*). After the solution eluted through the column, the container and column were washed twice with 100 mL of water. The resin was transferred to a 10-mL glass beaker by use of the method described by Barnes and Genna (IO). Six milliliters of HN03was added, and the beaker covered with a watch glass was gently heated on a hot plate. After the resin was digested (usually 15-20 min), the solution was transferred to a 10-mL volumetric flask and made to volume with (3 + 1) HNOp Total Chromium Recovery Test. Cr(II1) was oxidized to Cr(VI) with KMn04 in acid media as follows: Solution (125 mL) which contained Cr(III) and Cr(VI)was placed in a 200-mL beaker and 5 mL of HN03was added. The solution was boiled for 10 min. After the solution was cooled, 0.6 mL of 2.000 mg/L (as Mn) KMn04 solution was added to the solution, which was covered with a watch glass, and heated without boiling on a hot plate for 30 min. After the solution was cooled, sodium azide solution (2.5%)was added dropwise until the color of MnO; disappeared. The pH of the solution was adjusted to 3.5 with NH40H, and the solution was paased through the column. After the solution passed the column, the reservoir and column were washed twice with 50 mL of water. The resin was transferred to a 10-mL beaker and digested in the manner described. RESULTS AND DISCUSSION Detection Limits a n d Relative S t a n d a r d Deviation of Chromium. The detection limits (at 30) of chromium in the 0.1 N H N 0 3 matrix and the ‘“NO3 matrix” were 12 and 36 ng/mL, respectively. The detection limit in 0.1 N “03 matrix was almost the same as the literature value (9). The relative standard deviations of an 8 pg/mL chromium standard solution (‘“NO3 matrix”) and resin digested samples were the same (2.5%). In an earlier report by Barnes and Genna (IO), who digested the resin in a 60% (v/v) 1:l nitric acid-ulfuric acid mixture, the precision of the digested resin samples was poorer than that of the standard solution. The improved analysis precision for the digested resin samples in this study results from the type and volume of acid employed in the digestive step. 0 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53,
Table 11. Differential Analysis of Cr(V1) and Cr(II1)“ Cr(VI) initial, found, recovery, initial,
rg
rg
%
clg
Cr(II1) found,b
recovery,
N?
%
96.5 57.9 104 20.8 101 10.1 102 30.5 96.7 58.0 29.5 98.3 By difference. Synthetic sample, 125 mL, pH 3.5, and 70 mg of resin. 20 20 20 30 20 10
18.3 19.0 18.9 28.5 20.0 9.5
91.5 95.0 94.5 95.0 100 95.0
60 20 10 30 60 30
Effect of Sample Container. Hackett and Siggia (11) demonstrated the loss of some metal when a glass container without silanization was used. However, since the possibility existed that Cr(V1) would be reduced to Cr(II1) in the presence of the silanization reagent, methylchlorosilane, column studies were made by using glass containers with and without silanization and a polypropylene container. The glass container, washed with an HN03-H2S04 mixture (1:l)and rinsed well with water, provided a recovery of 8043%. The recovery of Cr(V1) above 200 pg from a polypropylene container was 89-92%, but at the 40-pg level, recovery decreased to 7 4 4 % . Recovery from a silanized glass container (11) was 8 0 4 1 % at pH 2.5 and 91-92% at pH 3.5. In previous batch testa, the recovery was maximum at pH 2 and decreased slightly at pH 3.5 (5). Further experiments were performed at pH 3.5 by using silanized glass containers. At pH higher than 5, Cr(II1) may precipitate. Effects of Cr(II1) on the Recovery of Cr(V1). The effect of Cr(II1) was evaluated by adding 0.8-8 mg/L Cr(II1) standard solutions to a 16O-pglL Cr(V1) solution. The Cr(VI) recovery with 70 mg of resin averaged 92% for six determinations. In the solvent extraction experiments using sodium diethyldithiocarbamate (DDTC), Fukamachi et al. (12)reported that Cr(V1) was immediately reduced to an activated Cr(II1) which formed N complex with an excess of DDTC. Hence, large concentrations of Cr(II1) coexisting with Cr(V1) affect the complexation of Cr(V1) with DDTC. Although the chelating mechanism of the poly(dithi0carbamate) resin is presently not well-known, if the mechanism were similar to that of DDTC, an effect of Cr(II1) might be observed. However, no effect of Cr(II1) on Cr(V1) recovery was observed for a t least up to !SO times excess. Since 16 mg/L of Cr(II1) showed no resin uptake in the absence of Cr(VI), the Cr(III)/Cr(VI) might be extended to at least 100. However, in environmental samples like seawater, in which the determination of Cr(V1) is desired, Cr(III)/Cr(VI) is usually not this high (13). Recovery of Total Chromium. The recovery of total chromium was almost 100% (Table I) In the presence of KMn04 and Cr(VI), Cr(II1) was completely oxidized to Cr(VI). The manganese content of the digested resin solution was also determined by ICP-AES using the Mn I1 257.61 nm wavelength. The detection limit for manganese in the ‘“NO3 matrix” was 3 ng/mL. In two samples (Table I), the manganese concentrations were 8.1 and 3.9 pg. Since the manganese added was 1.2 mg, the percentages of separated and detected Mn in the final solution were 0.68% and 0.33%, respectively. Hackett and Siggia (11)reported no affinity by the resin was observed for Mn(I1). The detected manganese in these samples is considered to be precipitated hydrous MnOz, which is sometimes generated on the reduction of Mn04- by NaN3. In fact, MnOz was observed at the top of one column, and the flow rate of this column decreased to 2.0 from 3.5 mL/min. However, no effect of MnOz on the recovery of totalchromium was observed. For the blank solution, which
NO.2, FEBRUARY 1981 365
total Cr found,
recovery,
rg
rg
80 40 30 60 80 40
76.2 39.8 29.0 59.0 78.0 39.0
95.3 99.5 96.7 98.3 97.5 97.5
initial,
%
was prepared in the same manner as the chromium samples, the emission signal at 267.7 nm was indistinguishable from the background. The reduction of KMn04 with NaN3 was necessary, because KMn04 reacted with the resin and a large amount of hydrous MnOz was generated. Differential Determination of Cr(VI)-Cr(III). A differential determination was made by using synthetic samples. Total chromium was obtained following the KMn04 oxidation method. The same sample was passed through the resin column without oxidation to obtain the Cr(V1) content. The Cr(II1) concentration was calculated from the differences between total chromium and the Cr(V1) content. The results are summarized in Table I1
.
CONCLUSION This investigation has demonstrated that the poly(dithi0carbamate) resin is usable for the differential determination of Cr(VI)-Cr(III) mixtures. In this example, a concentration factor of 12.5 was obtained. However, on the basis of previous experience with the poly(dithi0carbamate) resin (5,10,11) both large sample volumes and smaller final solution volumes can be employed in order to increase the concentration factor. The present technique is suitable for chromium determination by ICP-AES, AAS, and other spectrochemical methods. However, the low limit of detection for chromium achieved by electrothermal atomization AAS (14) combined with a suitable small final digested resin solution volume may allow extension of the differential analysis to low initial Cr(V1) and Cr(1II) concentration levels. Although high chloride concentration does not interfere with the Cr(V1) determination in this technique, in the total chromium procedure a chloride concentration of greater than 250 mg of Cl/L prevents complete recovery of chromium possibly through the loss of chromyl chloride. In this situation, removal of chloride by precipitation with silver nitrate may be required (7). This limitation is not unique to the resin approach and is common in methods requiring oxidation of Cr(II1) to Cr(1V). The present technique is not affected by large concentrations of alkali and alkaline earth elements or manganese (10, II), because these elements do not complex with the resin. In addition, the technique is simple and provides acceptable precision. The application of the poly(dithi0carbamate) resin for the determination of chromium species in waste water, seawater, blood, and urine is under development.
ACKNOWLEDGMENT The assistance of G. Dabkowski in preparing the resin and H. Mahanti in performing some analyses is greatly appreciated.
LITERATURE CITED (1) Udey, M. J., Ed. ACS Monogr. 1956, No. 132, 78. (2) Matsuo, T.; Shida, J.; Abiko, M.; Konno, K. BunseklKegaku 1975. 24, 723-725. (3) Pankow, J. F.; Janauer, 0. E. Anal. Chim. Acta 1974, 69, 97-104.
Anal. Chem. 1981, 53.366-369 Naranjk, D.; Thomassen, Y.; Van Loon, J. C. Anal. Chim. Acta 1979, 770, 307. Miyazaki, A.; Barnes, R. M. Anal. Chem., companion paper in this issue. Colella, M. B.; Siggia, S.; Barnes, R. M. Anal. Chem. 1980, 52, 967-972. Annu. Book ASTM Stand. 1978, Part 37 (Water), 315. Kitagawa, K.; Yanagisawa. M.; Takeguchi, T. Anal. Chim. Acta 1980, 775,121. Winge, R. K.; Peterson, V. J.; Fassel, V. A. Appl. Spectrosc. 1979, 33, 206-219. Barnes, R. M.; Genna, J. S. Anal. Chem., 1978, 51, 1065-1070. Hackett, D. S.; Siggia, S. I n “Environmental Analysis”; Ewing, G. W., Ed.; Academic Press: New York, 1977; p 253. Fukamachi, K.; Morimoto, M.; Yanagisawa, M. Bunseki Kagaku 1972, 27, 26-31.
(13) Fukai, R. Nature (London) 1967, 273, 901. (14) Thompson, D. A,, Ann. Clln. Blochem. 1980, 77, 44.
RECEIVED for review July 28,1980. Accepted November 14, 1980. This work was supported by the Department of Energy (Office of Health and Environmental Research) Contract DE-AC02-77EV-04320. Results were presented in part at the 6th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies and at the 29th Annual Meeting of the Japan Society for Analytical Chemistry. A. M. acknowledges the Science and Technology Agency of Japan for travel support.
Noise and Digital Resolution in a Microprocessor-Controlled Spectrophotometer Wilbur Kaye‘ and Duane Barber Beckman Instruments, Inc., Imine, California 927 13
Until recently most UV-VIS spectrophotometers portrayed spectra in the analogue domain. The trend is now to digitize the signal prior to display. This facilitates data storage and signal manipulations such as integration, differentiation, scale expansion, conversion to concentration, spectral comparisions, etc. Digitization also facilitates, but is not essential, for microprocessor control of spectrophotometer functions. However, it is possible to lose information, i.e., increase the uncertainty or reduce signal-to-noise (S/N) in the digitization process, and an understanding of the problem is desirable to obtain optimum performance from a digital instrument. Of fundamental importance is the digital resolution or reciprocal of the pertinent digitizing counts. Digitization of transmittance in spectrophotometers is usually accomplished by means of an analogue-to-digital converter (ADC). In principle it is possible to operate in the digital domain by photon counting, but the light levels (and consequent S/N) in most absorption instruments is so high that this method is undesirable (1). Digital resolution is here defined as the reciprocal of the ADC counts corresponding to one unit of the recorded signal. In double-beam instruments digitization may occur either before or after taking the ratio I/Z@ In single-beam instruments each signal is digitized. Similarly the log conversion required for absorbance readout may be located before or after the ADC. The influence of digitization on noise is strongly influenced by these design details. Analyses of the digital problems are here illustrated with a Beckman DU-8 prototype spectrophotometer. The analogue noise characteristics of this instrument have been reported elsewhere (2).
EXPERIMENTAL SECTION Apparatus. The DU-8 is a single-beam, digital, microprocessor-controlled spectrophotometer. The prototype used here employed a conventional Littrow grating monochromator with stepped slits having spectral slit widths (SSW) of 0.1, 0.2, 0.4, 1, 2, and 4 nm. A 50-W tungsten-halogen source and R928HA and R375 photomultiplier detectors were employed. For purposes of this study the dynode power supply was removed from its normal microprocessor control and varied manually. Voltage at the preamp output was monitored with a Systron-Donner Model 7110 digital voltmeter (DVM). Ordinate operation is illustrated by the simplified schematic shown in Figure 1. When operating in the transmittance mode the shutter, Sh, may be considered open and the power supply, PS, providing a minimum voltage to the detector. Presumably wavelength has been set to the desired value and a reference or blank has been inserted into the beam. A “gain-set’’ command institues the following sequence. The anode signal is amplified by the operational amplifier, OA, and a capacitor, C, across the 0003-2700/81/0353-0366$01 .OO/O
feedback resistor, R, integrates the signal with a 0.5-5 period. Switches S1, S6, and S7 are closed by the microprocessor (pP). This applies a signal to the ADC resulting in a finite count proceeding to the pP. The ADC can deliver up to 20000 counts. If the counts do not fall between 7500 and 9000, the pP signals the PS to change voltage by small steps and thereby iteratively changes gain of the photomultiplier. When the appropriate count is reached, Sh is closed, S6 opens, and S5 closes. This effectively amplifies the anode dark signal by 5X. If this new signal differs from zero, the pP influences the digital-to-analogue converter (DAC) to change the bias applied to OA in iterated steps and thereby compensates for detector dark current. The pP then commands the shutter and switch S5 to open and switch S6 to close. A “run” command now causes the ADC count, proportional to the blank or reference signal Io,to be stored. A second “run” command causes the pP to place a sample in the beam changing the light level on the detector and the counts out of the ADC. The pP takes the ratio of this count and displays the resulting transmittance on LED, tape, or plotter readouts. The pP cycles every half-second called an “update” and all the above iterative processes occur at half-second intervals. S/N is improved by a “boxcar” or running average method. The number of updates averaged is called a “read average” (RA) and this value is entered via the keyboard. Both Io and I values are averaged. In the absorbance mode the “gain-set’’command establishes dynode voltage and dark current compensations in the same manner described above. The switch S8 closes (S7 open) applying to the ADC a signal proportional to the log of the anode current. Switches Sl-S4 are sequentially closed, and the pP is calibrated for absorbances of 0, 1, 2, and 3. Typically each signal decade produces 4300 counts. The first “run” command replaces the blank with a sample and the difference between the new ADC count and the stored count is read out as sample absorbance. When performing a wavelength scan, the pP is capable of storing 280 descrete Io or log Io counts along with the wavelength correspondingto each Io value. From the programmed wavelength interval and the scan speed, the pP distributes the wavelengths at which data are to be stored. A “background” scan (Io versus A) is made with a blank or reference cell in the beam and this is followed by a sample (I vs. A) scan. Only the background data are stored while the T or A values are calculated and plotted during the sample scan. Sample and background scans can be run at different speeds. If the sample scan takes longer than 140 s, a linear interpolation between stored Io values is taken to provide an Io value at the wavelength of each Z update.
RESULTS AND DISCUSSION Signal Stability. Any analysis of noise in a single-betun instrument requires a knowledge of the signal stability or drift. The drift characteristics of the instrument used here were studied by using a series of 24-h scans. Ambient temperature was monitored during these scans. Proper equilibration of 0 1961 American Chemical Society
