Anal. Chem. 1980. 52, 2215-2216
the comparative effects of other acids, most notably Lewis acids such as strongly complexing metal ions. Does the back-extraction proceed via acid attack of the chelate or does it first undergo dissociation? Further, can Lewis bases, Le., other ligands, catalyze back-extraction, particularly if the chelate is coordinatively unsaturated? An interesting series of chelates to use for such a study is the nickel(I1) dithizonates. Not only have they long been known to exhibit slow rates of extraction ( I ) but the pH range required for their back-extraction is 3-4 units below that needed for extraction, exhibiting a kind of “hysteresis”. Back-extraction kinetics can be followed readily by removing samples a t predetermined intervals from a highly rapidly stirred mixture of the chelate in chloroform and an appropriate aqueous phase. The procedure used closely followed that used in this laboratory for extraction kinetics ( 2 , 3 ) . The rates of back-extraction from chloroform were readily followed by monitoring the concentration of the nickel chelate in the chloroform phase spectrophotometrically inasmuch as its spectrum is significantly different from those of other dithizonates. In every case, the back-extractions were found to be first order in chelate. Elaboration of experimental rate expressions was accomplished by obtaining the dependencies of the pseudo-first-order constants upon the relevant concentration variables. Thus, the back-extraction rate was found to exhibit first-order dependence on [H30+],[Hg2+],and [EDTA] and second-order dependence of [Ag+] and [CN-I. Among the acids, Ag+ and Hg2+enhance back-extraction rates much more than does H30+. Interestingly, CU*+,a strong Lewis acid, does not affect back-extraction. Among the bases, cyanide is much more effective than ethylenediaminetetraacetate (EDTA). Because of the great variation in stripping rates observed among the various reagents, none of which are soluble in CHC13, it appears that a common point in the reactions is the prior, rapid transfer of the nickel chelate to the aqueous phase. This is further corroborated by the qualitative observation that acetic acid, a weak but chloroform-soluble acid, was a better stripping catalyst than H30+. Once in the aqueous phase, the chelate might either form an intermediate complex with reagents like Ag+, Hg2+,or CN- or undergo an inherently slow dissociation with reagents like H,O+ or EDTA. Inasmuch as reactions in the intermediate complex forms do not require the nickel dithizonate to dissociate first, these are more rapid.
2215
For example, when phenanthroline or dipyridyl form mixed ligand chelates with nickel dithizonate (4),both the extraction and the back-extraction rates of nickel are considerably enhanced (5). In the systems studied here a suitable intermediate complex can be formed by a reagent which either bonds to the central metal ion, provided its coordination number is capable of increasing, or to one of the bonding atoms of the ligand, provided it has unused bonding capability. Cyanide ion, a softer ligand than EDTA, probably adds to the nickel ion in much the same fashion as do the neutral heterocyclic nitrogen ligands (4), whereas Ag+ and Hg2+,which can form complexes with bound sulfur (as in thioethers), probably bond to the sulfur atoms in the chelated dithizone. Thus, nickel dithizonate may be viewed as an “amphoteric Lewis salt”, capable of interacting with both Lewis bases and acids. These findings suggest a novel approach to the rapid decomposition, and possibly rapid formation as well, of complexes of metal ions such as Co, Ir, Rh, and Cr, whose usual substitution reactions are inherently slow. This approach would provide the bases for improved analytical procedures for such metal ions. Further, because of the restricted list of back-extraction catalysts (relatively), inert complexes, such as nickel dithizonate, can be used in the development of highly selective analytical methods for catalytically active species (e.g., for Hg2+in the presence of C:u2+). Studies along the various lines suggested by this work are under way.
LITERATURE CITED (1) Sandell, E. 8 . “Colwmetric Determination of Traces of Metals”,3rd ed.; Interscience: New York, 1959; p 147. (2) Honaker, C.; Freiser, H. J. Phys. Chem. 1962, 66, 127-130. (3) Carter, S.; Freiser, H. Anal. Chem. 1979, 51, 1100-1101. (4) Math, K. S.; Freiser, H. Anal. Chem. 1969, 4 7 , 1682-1685. (5) Freiser, B. S.; Freiser, H. Taknta 1970, 77. 540-543.
Kousaburo Ohashi Henry Freiser* Department of Chemistry University of Arizona Tucson, Arizona 85721 RECEIVED for review May 23,1980. Accepted August 19,1980. This work was supported by research grant from National Science Foundation.
Rapid Scan Square Wave Voltammetric Detector for High-Performance Liquid Chromatography Sir: Voltammetric detectors for high-pressure liquid chromatography (HPLC) are widely used because of their good sensitivity for many electroactive compounds (1). In the most commonly employed constant potential mode, this technique has only modest selectivity. Although electroanalytical techniques are inherently unselective, the selectivity should be improved dramatically by using a scanning technique which provides potential as well as time resolution. As pointed out by Johnson ( 2 ) ,despite pessimistic attitudes toward pulse techniques in this application (11, square wave voltammetry ( 3 , 4 )should provide the full resolution obtainable with respect to potential without degradation of sensitivity. In principle square voltammetry should have the following attributes: (1) discrimination against charging currents and against “background” currents because of the current measurement scheme, (2) little electrode fouling because of the small amount 0003-2700/80/0352-22 15$01.OO/O
of material converted, and (3) good potential resolution on a time scale short in comparison with that required for chromatographic resolution because high scan rates can be employed. We describe here the use of rapid scan square wave voltammetry for HPLC detection. The model compounds chosen are N-nitrosodiethanolaniine (NDELA) and N nitrosoproline (NPRO), and the chromatographic separation is based on work reported elsewhere (5,6). The advantages and possibilities of the technique are discussed, and the selectivity and sensitivity of the detector are compared with others. The chromatographic system, which is described in detail elsewhere ( 5 ) ,employed a detector based on the PARC 310 polarographic detector (Princeton Applied Research, Princeton, NJ). The excitation wave form was generated by D/A converters 0 1980 American Chemical Society
2216
ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980 J
10
I
I
I
,
100
I
....
-1 2
- 101
-700
I
-BOO
I
1
I
-900
-1000
-1100
PUTENTIRL V 5 .
-1200
I
RG/RGCL
Figure 1. Square wave polarogram of NPRO in 1 % phosphate buffer (pH 3.5): ( 7 ) forward current; (2) reverse current; (3) net current. Conditions: [NPRO] = 6.03 X M, step height = 5 mV, square wave amplitude = 25 mV, square wave frequency = 10 Hz. Background currents have been subtracted.
controlled by Digital Equipment Corp. PDP 8 / e computer equipped with 32K words of main memory, floating point processor, cathode ray tube display, 12 bit D/As, programmable real time clock, 12 bit A/D, and 2.5M byte hard disk for mass storage. This system supports Digital Equipment Corp. OS/8 operating system and Fortran IV, with assembly language subroutine calls to handle the clock and converters. The real time clock was used to initiate A/D conversions immediately before the next potential step of the wave form was applied (3,4 ) . The individual forward and reverse currents or their difference was displayed in real time for the benefit of the experimenter and was also stored on the disk for analysis at a later time. The potentiostat, which was fast settling and immune to voltage saturation, was homemade. The theory for square wave voltammetry for irreversible systems is discussed by O’Dea and Osteryoung (7)and by O’Dea (8), and experimental studies on such systems have been reported by O’Dea (8). NPRO exhibits one 4e- totally irreversible reduction wave in acidic solution (9). Figure 1, which shows a square wave polarogram of NPRO, displays the expected behavior. Sensitivity studies indicated that concentrations of the order M are easily determined. Buchanan and Bacon have reported the use of square wave polarography at constant potential for detection in ion exchange chromatography (IO). They measured the square wave current at four different potentials applied in sequence to the detector cell. In our case the potential was scanned over a potential range of 500 mV every 2 s. A total number of 102 forward and reverse current samples were collected and stored. The step height and frequency were 10 mV and 100 Hz, respectively. A predetermined number of scans were recorded and stored for each chromatogram. The data were analyzed with a program that was able to subtract the background, filter the noise, and find the peak position and the peak height for each N-nitrosamine. Figure 2 shows a three-dimensional chromatopolarogram of NDELA and NPRO. The nitrosamines are completely resolved in time (5) and the peak potentials are -1.07 and -1.04 V for NDELA and NPRO, respectively. A peak is also observed at towhich corresponds to an unknown impurity in the standard solution. A calibration plot for NPRO over the range 1-15 mM was linear with intercept -3.4 nA, slope 15.2 nA/pM,
Figure 2. Three-dimensional chromatopohrogram of NDELA and NPRO: (1) NPRO; (2) NDELA; (3) unknown impurity. Conditions: [NPRO] = 1.34 X M, [NDELA] = 1.2 X M, step height = 10 mV, square wave amplitude = 25 mV, square wave frequency = 100 Hz, mobile phase = 1 YO phosphate buffer (pH 3.5). Background currents have been subtracted.
and pooled standard deviation 3.23 nA, which gives an estimate of the detection limit of 0.6 MMa t the 95% confidence level. The values of retention time and peak potential and their standard deviations for six concentrations in the same range were 266 f 2 and 308 f 3 s and -1.07 f 0.01 and -1.04 0.01 V for NDELA and NPRO, respectively. Thus, peak potential provides a reliable additional parameter for compound identification. In addition, the resolving power of the chromatopolarographic system is increased because separation in both time and potential can be made. As reported earlier (51, detection is limited by a high noise level caused by pulsations in flow rate. By improvement of the pumping system, a decrease in the detection limit by 1 order of magnitude may be possible. Optimization of the square wave parameters and the geometry of the flow cell should give even better performance. Work on this and various applications to organic and inorganic systems is proceeding in our laboratory. LITERATURE CITED (1) Kissinger, Peter T. Anal. Chem. 1977, 49, 447A-456A. (2) Johnson, Dennis C. Anal. Chem. 1980, 52, 131R-188R. (3) Christie, J. H.; Turner, John A.; Osteryoung, R. A. Anal. Chem. 1977. 4 9 , 1899-1903. (4) Turner, John A.: Christie, J. H.; Vukovic M.: Osteryoung, R. A. Anal. Chem. 1977, 4 9 , 1904-1908. (5) Samuelsson, Robert; Osteryoung, Janet Anal. C h h . Acta, in press. (6) Iwacka, W.; Tannenbaum, S. R. J. Chromatogr. 1976, 724, 105-110. (7) O’Dea, John; Osteryoung, R. A , , unpublished work.
(8) O’Dea, John Ph.D. Dissertation, Colorado State Universky, Ft. Collins, CO, 1979. (9) Hasebe, Kjoshi; Osteryoung, Janet Anal. Chem. 1975, 47, 2412-2418. (10) Buchanan, E. B., Jr.; Bacon, J. R. Anal. Chem. 1967. 39, 615-620.
Robert Samuelsson Department of Analytical Chemistry University of Umeb S-90187 Umeb, Sweden
John O’Dea J a n e t Osteryoung* Department of Chemistry State University of New York a t Buffalo Buffalo, New York 14214
RECEIVED for review June 24,1980. Accepted August 7,1980. This work was supported by the National Science Foundation under Grant No. CHE-7917543. R.S. wishes to thank the Swedish Work Environment Fund for financial support.
