Anal. Chem. 1983, 55, 713-710
The variation in total TcO; levels from day to day over the lifetiine of a given generator has been investigated; however, these results will be published elsewhere.
CONCLUSIONS SMDE detection is applicable to most wMo/Bg”Tcgenerator eluents, i.e., other electroactive components are readily separated by the “,-bonded column, and the majority of generator eluents contain TcO; within the established working concentration range. For those generator eluents drawn late in the generator life or resulting from second elutions, solid electrode detection (i.e., glassy carbon electrode) may have to be employed. With these two detection schemes, this method should be applicable as well to determining TcO; in other sample matrices such as environmental samples, biological samples, and actual radiopharmaceutical preparations. This developed procedure constitutes an example of the capabilities of reductive LCEC for determining ionic inorganic species. More generally, however, it illustrates the versatility that can be achieved in an LCEC system; i.e., the capability of performing analyses in either the oxidative or reductive mode is realized with this system (even though the oxidative mode was not required for the TcO; determination), as well as the capability of obtaining qualitative information on low-level components by constructing hydrodynamic voltammograms.
ACKNOWLEDGMENT The authors gratefully acknowledge T. William Gilbert, Peter T. Kissinger, Karl Bratin, and Kenneth R. Wehmeyer for helpful discussions and assistance. Registry No. Technetate, 23288-61-1.
713
(2) Deutsch, E. I n “Radiopharmaceuticals 11”; Proceedings of the 2nd
International Symposlum on Radiopharmaceuticals; Society of Nuclear Medicine: New York, 1979;p 129. (3) Deutsch. E.; Helneman, W. R.; Zodda, J. P.; Gilbert, T. W.; Williams, C. C. Inf. J. Appl. Radiat. h o t . 1982,33,843-848. (4) Zodda, J. P.; Helneman, W. R.; Gllbert, T. W.; Deutsch, E. J. Chroma-
fwr. 1982,227,249-255. (5) Klssinger, P. T. Anal. Chem. 1977,4 9 , 447A-456A. (6) Kemula, W. Rocz. Chem. 1952, 26, 281. (7) Blaedel, W. J.; Todd, J. W. Anal. Chem. 1958, 30, 1821-1825.
(8) Rebertus, R. L.; Cappell, R. J.; Bond, G. W. Anal. Chem. 1958, 30,
1825-1827. (9) Tustanowski, S.J. Chromafogr. 1967,37, 266-268. (IO) Funk, M. 0.;Keller, M. B.; Levison, B. Anal. Chem. 1980, 52, 771-773. (11) Bratin, K.; Klssinger, P. T. J. Llq. Chromafogr. 1981, 4 (Suppi. 2), 321-357. (12) Samuelsson, R.; Osteryoung, J. Anal. Chlm. Acta 1981, 723, 97-105. (13) Vohra, S. K.; Harrlngton, 0. W. J. Chromafogr. Scl. 1980, 78, 379-383. (14) Lyle, S.J.; Saleh, M. I.Talanfa 1981, 28, 251-254. (15) Wlghtman, R. M.; Paik, E. C.; Borman. S.;Dayton, M. A. Anal. Chem. 1978,50, 1410-1414. (18) MacCrehan, W. A.; Durst, R. A,; Bellama, J. M. I n “Trace Organic Analysis: A New Frontier In Analytlcal Chemistry”; Proceedings of the 9th Materials Research Symposium; NBS: Galthersburg, MD, 1978; pp 57-63. (17) Maltoza, P.; Johnson, D. C. Anal. Chlm. Acta 1980, 778, 233-241. (18) Buchanan, E. B.: Jr.; Bacon, J. R. Anal. Chem. 1967,39, 615-620. (19) gtutk, K.; PacHkovrI, V. J. Necfroanal. Chem. 1981, 729, 1-24. (20) Magee. R. J.; Cardwell, T. J. I n “Encyclopedia of Electrochemistry of the Elements”; Bard, A. J., Ed.; Marcel Dekker: New York, 1974; Vol. 11, pp 161-162. (21) Van Rooijen, H. W.; Poppe, H. Anal. Chlm. Acta 1981, 730. 9-22. (22) Johnson, D. C. “Applications of Pulse Techniques to EC Detection”; presented at the LCEC Symposium on Environmental and Industrial Applications of LCEC and Voltammetry, May 17-19,1981,Indianapolis, IN, Abstract No. 2. (23) Miner, D. J.; Bopp, R. J. “The Reproducibility of Measurements by LCEC”; presented at the LCEC Symposium on Environmental and Industrial Appllcations of LCEC and Voltammetry, May 17-19,1981,Indlanapolls, IN, Abstract No. 25. (24) Hanekamp, H. B.; Voogt, W. H.; Bos, P.; Frei, R. W. Anal. Chlm. Acta 1980, 778, 81-86. (25) Larochelle. J. H.; Johnson, D. C. Anal. Chem. 1978, 50,240-243. (26) EG&G Princeton Applied Research Model 310 Polarographic Detector Operating and Servlce Manual, p IO.
LITERATURE CITED (1) Saha, 0. P. “Fundamentals of Nuclear Pharmacy”; Sprlnger-Verlag: New York, 1979;pp 33-94.
RECEIVED for review September 28,1982. Accepted December 20, 1982.
Linearization of Electron Capture Detector Response to Strongly Responding Compounds W. 6. Knlghton and E. P. Grlmsrud” Department of Chemlstv, Montana State lJnIvers&, Bozeman, Montana 597 17
An extended means of processlng the signal of a constantcurrent electron capture detector (CCECD) is proposed. This new response function provides a slgnificant improvement In quantitative analysis by the ECD because It provldes h e a r calibration curves for strongly electron attaching molecules over the entire dynamic range of the Instrument. Thls refinement in the ECD response occurs because ti accounts for the alteration of the analyte concentration in the detector by the electron capture process, ttsetf. The theoretical basls and experlmental support for the proposed EC response functlon are presented where CCI,, CFCI,, and CHCl, are used as test compounds.
The electron capture detector (ECD) is widely used in gas chromatography (GC) for the analysis of numerous environ0003-2700/83/0355-0713$01.50/0
mentally important compounds because of its unsurpassed sensitivity often permitting detection at the femtogram level (1). Unfortunately quantitation with this detector has often been difficult because of nonlinear and unpredictable relationships between the. measured response and analyte concentration. During its early use the ECD was operated in the direct current (DC) or the fixed frequency (FF)pulsed modes of operation where the change in current ( A I ) was taken as the response. Detectors operated in either of these two modes yielded linear calibration curves only over the initial 10% of the entire response range. In 1967 Wentworth and Chen (2) showed that if the analytical response in the fixed frequency mode was taken as (Io - o/I, where 19 is the measured standing current in the absence of sample and I is the cell current measured continuously during the chromatogram, that linear calibration curves could be obtained up to and exceeding 90% of detector saturation. A practical limitation of this mode 0 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
of signal processing is its requirement for unusually clean chromatographic conditions so that a high standing current with long pulse periods can be achieved. In 1971 Maggs et al. (3) proposed operating the ECD in the frequency modulated or constant current (CC) mode of operation. In this case a feedback network maintains the magnitude of the measured current constant and equal to a preselected reference value by control of the pulsing frequency. As an electron capturing compound passes through the cell the frequency of pulsing increases by an amount required to keep the current constant The analytical response is taken as the increase in frequency of pulsing. The constant current ECD has been shown to yield responses to many compounds which are linear with sample concentration up to 99% of detector saturation. The present acceptance of this mode of ECD operation is evidenced by the fact that it is now the standard offering of commercial vendors of gas chromatographs. Ironically, with all the above advances in the operation of the ECD, this detector is still not expected to provide linear responses to those compounds (such as highly halogenated hydrocarbons) for which it is most sensitive. The lack of linear responses to this important group of compounds is not due to instrumental deficiencies but is attributed to the alteration of the analyte concentration within the detector by the electron attachment process, itself. Nonlinear responses of this type have been previously described and explained (4-6). Briefly, this behavior of strongly responding compounds is explained as follows. At low sample concentrations a significant fraction of the analyte is destroyed by the electron capture reaction because the EC rate constant and the average electron density are both large. Under this low sample concentration condition an approximately linear response is observed as long as the electron population remains high and relatively constant, causing the same fraction of analyte molecules to be reacted over this limited sample concentration range. However, as the concentration of the sample increases, a smaller population of electrons consumes a successively smaller fraction of the analyte entering the cell. The monitored responses for ECDs operated in either the (Z"- I)/Z or CC mode are directly proportional to the instantaneous concentration of the analyte within the detector. Therefore, the response cannot also be linearly related to the analyte concentration entering the cell over the entire dynamic range of the detector. One approach to minimizing this problem is to make the detector volume very small or make the carrier flow rate very fast. The residence time of the analyte in the cell is then very short and little destruction of the sample by the EC reaction is allowed. The small detector approach has been demonstrated by Patterson (7), where the active volume of a small cell was made effectively smaller by use of a displaced coaxial anode. Alternatively, high flow rates are easily obtained through the addition of makeup gases. The compromise inherent in these approaches is that while only partially improving the quantitative response to strongly responding compounds the sensitivity to weakly and moderately responding compounds will be proportionately reduced. In this paper a new means of ECD signal processing is described by which a linear response over the entire dynamic range is achieved for strongly responding compounds, using a detector of any size or design with conventional flow rates. This is made possible by combining the usual constant current mode of control with an extended definition of the processed response. This improvement occurs because any alteration of analyte concentration by the attachment of electrons is accounted for and incorporated into this response function. Following its theoretical description, experimental support for this refinement of the ECD response is provided by using
the strongly electron attaching compounds, CFCIBand CC14, as test compounds. This choice of test compounds is made because the electron capture reactions of these will occur with a one-to-one electron-to-analytestoichiometry (8). Complications which migbt be expected from more complex EC reactions as well as those caused by physical effects of the ion-electron dynamics of an ECD are briefly discussed at the conclusion of this article.
THEORY It is appropriate to fmt describe how the measured response is related to sample concentration in the usual CC-ECD. The reactions occurring within the ECD which are of primary significance include the following: neutrals
kA
+ eB + eA
'
S
P+ + e-
+ neutrals kB negative ions + neutrals R P+ + eneutrals P+ + negative ions neutrals
(la)
negative ions
(lb)
-
(IC)
-
(14 (le)
Where reaction l a represents the production of positive ions (P') and electrons (e-) by the ionizing radiation (usually provided by 63Ni)with 5 ' being the first-order ion-electron pair production rate constant. The electron loss pathways are given by reactions lb-ld. Electron capture by the electron attaching sample (A) is given by reaction l b where k A is the second-order rate constant for electron capture. Loss of electrons by attachment to carrier gas impurities and column bleed (B) is combined into a single reaction ICwith a rate constant, k g . The final electron loss mechanism is recombination of positive ions (P+) with electrons (e-). Reaction l e is the recombination of positive ions with negative ions to form neutrals. For the above reactions the change in electron density with time can be expressed as a simple differential equation of the form cW,/dt
+
S - LN,
(2)
+
where L = (Rn+ kBnB kAnA) is a first-order rate constant which represents the sum of all the electron loss rates and Ne is the instantaneous electron population. Positive ion concentration, n+,is assumed to be relatively constant throughout a 63Ni ECD (9) (the validity and implications of this assumption are further discussed at the conclusion of this paper). Since nAand nB can be assumed constant over the short time of a period between pulses, the entire term, L, can be treated as a constant independent of the variable, Ne. Integration of eq 2 and evaluation at time t = T,the pulse period, yields the following expression for NeT,the population of electrons at the instant prior to a pulse.
= (S/L)(1(3) Given that the level of demanded current Z = K 'NeT/T,where K'is a proportionalityconstant somewhat less than unity that accounts for the perturbation in the observed current by the arrival of positive ions at the anode between pulses (91, eq 4 is obtained. N,T
I = (K'S/LT)(l -
(4)
Inspection of the above equation reveals that the magnitude of the current can remain constant as the input of analyte changes only if the values of L and T vary in a definite manner such that the product of L and T remains constant at all times. = 2,must also be a Therefore the entire term, (1 constant. Realizing that frequency of pulsing, f, is equal to
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
715
the reciprical of T, eq 5 is obtained. In the constant current
f = (I/K'SZ)L
(5)
ECD the response is taken as the change in frequency f - f " where f " and f correspond to the frequency of pulsing in the absence ( L = Rn, + kBnB)and presence ( L = Rn+ + kBnB + kAnA) of sample, respectively. The usual response of the CC-ECD defined as the frequency increase is then given by eq 6, where G = IkA/KsZ. This response function is seen
f - f"
(I/K'SZ)kAnA= GnA
(6) to be linearly related to the instantaneous concentration of analyte within the detector. This response will also be linearly related to the amount of material entering the detector from the GC provided the concentration of the analyte is not significantly perturbed by the EC process itself. This condition is met only for compounds which have low to moderate electron capture rates. For molecules which have fast electron attachment rates the concentration of analyte entering the cell is not the same as the instantaneous average concentrationwithin it. For these cases it is necessary to relate the analyte concentration which is entering the cell (the quantity sought) to the instantaneous concentration in the cell (the quantity sensed by the normal CC-ECD). In eq 7 three terms account for the change of anal@ concentration with time. The first term is ventilation dnA/dt = FnAo/ - kAnANe/ - FnA/ (7)
v
v
v
into the cell where F is carrier gas flow rate, V is the detector volume, and nA" is the concentration of the analyte entering the cell. The second term is the electron capture term where nAis the instantaneous concentration of sample and Ne is the average electron population over several pulse periods. The final contribution is ventilation out of the cell. Because gaseous diffusion is fast relative to the flow rates used here, it is reasonable to m u m e that the ECD is a well-mixed reactor with respect to the distribution of analyte within it. A steady-state approximation is then valid and eq 7 can be set equal to zero. Solving for nA and substituting into eq 6 yields eq 8. The time-averaged electron population,Ne,is equivalent
to NeTonly if the period between pulses is unusually long so that an equilibrium between the production and loss of the electrons is established. In the CC-ECD considerably faster pulsing is normally used and then, Ne = K"NeT where K'' is a proportionality constant between 0.5 and 1.0 (the proportionality constant can be evaluated from the general expression K" = (1- e-LT)-l- ( ~ 5 v - l which ) accounts for the reduced value of the average electron population compared with the maximum instantaneous electron population just prior to a pulse. Combined with the affect of the positive ion contribution on the measured current, the measured current can be redefined as I = K"Ve/K'l'. Substituting Ne obtained from this expression and T = l / f into eq 8 results in eq 9, where
f - f" = G ~ A " / ( ( H+/ ~1))
(9)
again, G = IkA/K'SZ and H = IkAK"/FK'. Equation 9 is of the same form as eq 6 except for the denominator, (Hlf) + 1,which accounts for the alteration of analyte concentration by the EC reaction. Most importantly, nAo, the quantity desired in the analytical measurement, rather than nA occurs as the independent variable in eq 9. In the case where the electron eapture rate constant is small the term H/f becomes negligible and eq 9 reduces to eq 6. A slight rearrangement of eq 9 yields our final result, eq 10. This expression more
(f-f")(H
f ) / f = GnAo
(10)
J 01
01
10
10
01
01
10
'0
CONCENTRATION (relative)
Flgure 1. Synthesized molar response curves predicted from (A) the conventional response functlon calculated from eq 9 and (B) the extended response function calculated from eq 10, for three analytes of differing electron capture rate constants.
conveniently describesthe response of the CC-ECD than does eq 6, because the response is linearly related to nA",the analyte concentration eluting from the GC into the detector, rather than the instantaneous concentration within the cell. Simulated responses derived from eq 9 and 10 and from reasonable approximations of G and H a r e shown in Figure 1for two relatively strongly responding compounds and one moderately responding compound. The following are the values used to determine H: K' = 0.75 (for detectors of concentriccoaxial design about 25% of the positive ions arrive a t the anode between pulses (9));I = 7 X A (the value for the reference current used in experiments to be reported here); F = 0.5 mL/s (a typical volume flow rate); and K" = 0.5 (the time averaged electron density is about half of the maximum instantaneous electron population when relatively fast pulsing is used). With the above estimates, H = 6000, 3000, and 60 s-l for EC rate constants of 1 X lo-', 5 X and 1 X mL/s, respectively. The shape of the response curves is dependent only on the values of H and the base frequency of pulsing in the absence of sample. A value off" = lo00 s-l is chosen here to mimic our experimentalconditions. The constant G contains many of the terms previously determined for the constant H. However, it is also dependent on values of the constants S and Z, both of which are more difficult to evaluate. Fortunately, the magnitude of G determines only the relative amplitude of the response curves and has no effect on their shape. Because the shape of the response curves is of primary concern here values for G have been selected rather arbitrarily. For the rate constants of 1 X lo-', 5 X and 1 X lo* mL/s the corresponding values of G assigned were 24 000, 12 000, and 240 concentrations-' s-l, respectively. Figure 1is divided into two parts. In part A response curves are predicted for the usual response, f f " , and in part B for the newly developed response function, (f - f")(H + f)/f. In part A only the response associated with the small EC rate constant is linear with concentration. This is expected because H is then small compared with f for all concentrations. The response curves corresponding to the values of k = lo-' and 5 X lo-* are S-shaped. This behavior is expected for any case in which H / f > 1 in the low concentration region. It is interesting to note that when the analytical signal is processed as (f - f " ) , halving the EC rate constant such that G and H are halved results in a halving of the molar response only in the high concentration region and not over the entire response range. In the low concentration region the molar responses of the imagined compounds
716
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
I
i A II
//
CC?,
I
/
f
TIME ---ie
@@-o---E-c-o. I
Figure 2. Typical chromatograms of standards containing CFCI,, CHCI,, and CC14. Concentratlonsare (A) 3.8 ppb CFCi,, 48 ppb CHCI,, and 9.5 ppb CCI, and (6) 0.38 ppb CFCI,, 4.8 ppb CHCI,, and 0.95 ppb
cci,.
tend to converge to a similar value. When the response function as defined in part B is used, the predicted response curves are linear with concentration over the entire response range regardless of the rate of the electron attachment reaction.
EXPERIMENTAL SECTION The gas chromatograph and constant current ECD used in this study were both home built. The ECD is of concentric coaxial design with an internal volume of 1.5 mL where the cylindrical walls are formed by a 15 mCi 63Niplated on Pt foil. The electronics which operate the ECD in the constant current mode have been previously described (IO). This electronics package allows for the selection of any desired reference current and provides a dynamic range of up to 100 kHz. For all the experiments performed here a reference current of 7.08 nA was used. The value of the current was selected so as to give a base line frequency of lo00 Hz. Each pulse was of 1ps duration and 50 V in amplitude. The output signal, a voltage proportional to frequency, was monitored on a strip chart recorder. Samples were introduced via a 2-mL gas sampling loop (Carle 8030) onto a 10 ft X l/* in. packed column containing 10% SF-96 on Chromosorb W. The oven was operated at ambient temperature. The detector temperature was 150 OC. A carrier gas flow of 30 mL/min of 10% methane in Ar which was first passed through oxygen and water traps was used. Sampleswere prepared by dilution of the neat liquid or gas into nitrogen by use of airtight containers which were held at above ambient pressure. Final dilutions and transfer of the sample to the gas sampling valve was done with a 100-mLgas tight syringe. Typical chromatograms thus obtained are shown in Figure 2. In order to compare the molar responses of different compounds introduced by gas chromatography,it is necessary to account for analyte broadening within the column. Use of the term relative flux,which is proportional to the rate of entry of the analyte into the detector at a point corresponding to the center of the peak, accounts for this analyte broadening and is obtained by dividing the known concentration of analyte by its corresponding chromatographic peak width at half height. A low concentration chromatogram is chosen for the half-widthmeasurement to ensure that the molar ECD sensitivity for the given compound will be constant over the entire chromatographic peak. The relative molar responses of all compounds is then determined by the peak height response divided by relative flux. To process data in accordance with the proposed response function, (f - f“)(H+ f i / f , the value of H must first be determined. While various methods for this can be envisioned,the following procedure was used here. For both CFCl, and CC4,H was determined from two selected points on their response curves. Two
i
ai
io
io FLUX (relotive)
Flgure 3. Measurements of relative molar responses vs. relative input rate (flux) for halogenatedmethane standards, where the response has been measured by uslng (A) the conventional response function and (6) the extended response function.
points are required because the response function (f - f”)(H + = GnAo contains two unknowns, G and H. From two simultaneous equations representing points on a calibration curve, H can be determined according to
fi/f
The superscripts of one prime and two primes designate the appropriate frequency and concentration terms for the different points. The points chosen for the determination of H are represented by the crosses on the response curves of CFC13and CC14 in part A of Figure 3. The portion of the response curves from which the values are taken to determineH is somewhat arbitrary. The best values of H, i.e., those which give the most linear response curves in part B, were obtained by chosing points from within the steeply rising section of the molar response curves. By use of the marks designated on the response curves of CFC13and CC14 in part A of Figure 3, values of H = 4600 and 7200 5-l for these two compounds, respectively, are obtained. For weakly responding compounds such as CHC13for which linear response curves are obtained when the response is taken (f - f ” ) the value of H is assigned to be zero. The response curves in part B of Figure 3 were then determined by using these values of H in conjunction with the normal responses for the three compounds.
RESULTS AND DISCUSSION The analyses of samples containing CC14,CFCl,, and CHC13 over a wide range of concentrations are shown in Figure 3. This figure is constructed in the same format as Figure 1. On the left side, part A, the molar responses for the three cornpounds are plotted as (f - fo)/flux, and on the right side, part B, the response is taken as ((f - P ) ( H + f)/f)/flux. The abscissa, relative flux, is a measure of analyte input rate to the detector and has been defined (see Experimental Section) so that the relative molar responses for the different compounds can be directly compared. In Figure 3 CC14 is seen to be the most strongly responding compound followed by CFC1, and CHCl,, respectively. Comparison of the theoretical curves shown in Figures 1with those in Figure 3 indicates considerable similarity. The response curves for CFC13 and CCll in part A of Figure 3 are seen to be nonlinear with concentration of analyte entering the cell and are similar in shape to the synthesized response curves in part A of Figure 1 fork = lo-’ and 5 X With the extended definition of
ANALYTICAL CHEMISTRY, VOL.
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APRIL 1983
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I , the flow rate, F, and the second-orderelectron capture rate constant, KA, all of which are either constant or easily measured and incorporated into the value of H. CFCI3
The use of the proposed response function will be further facilitated by an associated data system. The analytical signal can then be determined in real time, in terms of the new function (f f ) / f as well as the conventional (f - f " ) method. This would allow chromatograms to be processed in either or both modes. The new response function will allow peak areas as well as peak heights to be used as a measure of concentration of strongly responding compounds (peak areas obtained from any responses which are nonlinear are complicated and probably meaningless because the molar response is not constant over the concentration variation occurring during a single chromatographic peak). While the above describes and accounts for a major cause of the nonlinearity in the EC response of strongly responding compounds, additional complicating influences on the total quantitative response of the B3NiECD have not yet been fully characterized. For certain compounds, the mechanism of electron capture will be more complex than the relatively well-behaved cases of CFC13 and CC14 studied here. In our laboratory, alone, we have observed several such cases in which the EC responses to alkyl chlorides (11),anthracene (121,and methyl iodide (6) cannot be explained by a single reaction between an electron and the analyte molecule but require the addition of other chemical steps to the total response mechanism. Also, in the cases of large highly halogenated molecules, for which the ECD is often used, further electron capture by products generated from the original EC reaction seems probable. In addition to such chemical effects another complexity is presented by the possible failure of our earlier assumption made in eq 2 that positive ion density is constant throughout the ECD under all conditions of sample concentration and pulsing frequency. This question has been previously addressed by Wentworth and Chen (13),but for the quite dissimilar tritium source of plane parallel design. In several studies in our own laboratory (9,14,15) positive ion concentrationwithin @Nisources of cylindrical geometry have been directly measured with the assistance of atmospheric pressure ionization mass spectrometry. In these studies it was found that positive ions are formed relatively evenly throughout a typical cylindrical 63NiECD (15) and that the positive ion concentration at a given location is relatively independent of pulsing frequency (9). Perhaps the most significant variation in positive ion density was observed when the ionization cell was saturated with an electron capturing compound. The positive ion density was then found to increase 50% relative to the sample free condition (16). This observation can be explained if the rate constant for the recombination of positive ions with negative ions is somewhat smaller than the corresponding rate constant for positive ion-electron recombination. The effects of these additional contributions to the quantitative response of the 63NiECD are presently being individually investigated in our laboratory. Preliminary indications are that, although measurable, these latter considerations are not nearly as significant to the response of strongly responding molecules as the effect characterized in this paper.
r)(H+
Flgure 4. Calibration curves for ECD responses to CFCI,, the conventional and extended definitlons of response.
uslng both
total EC response given by eq 10, the relative molar responses shown in part B of Figure 3 are obtained for the three test compounds. The improvement in linearity of the response to CC14in part B relative to that in part A is quite dramatic. When the CC14response is taken as (f - f " ) the relative molar response changea by about a fador of 6 over the dynamic range whereas, with the extended definition of response, only a 10% variation in molar response is observed over the same concentration range. The overall molar response for CFC13 appears to be about half of that of CClk This rate of attachment to CFC13 is, however, still large large enough so that its (f f " ) response in part A varies by a factor of 4 over the sample concentrationsexamined. Again, the molar response for CFC13 in part B is seen to remain relatively constant with concentration. The response curves for CHC13are seen to be linearly related to concentration in both parts A snd B of Figure 3. This result occurs because the rate of electron capture for CHC13is only moderately fast such that H / f is small for all concentrations. In this case the response defined in part B becomes equivalent to the usual response in part A. The superior characteristics of the newly defined response function are further illustrated in Figure 4 where the same data for CFC13 have been plotted as response vs. sample concentration. This figure shows two calibration curves in which the response is taken as either (f - f " ) or (f - p)(H+ f ) / f . The calibration curve obtained by using the conventional response shows considerable curvature and requires the use of a t least several data points to be adequately defined. However, the calibration curve using the proposed response function is a straight line passing through the origin. With this extended response function it is possible to define a useful calibration curve with as little as a single data point, provided the value of H is known for the compound of interest. In use of the proposed response function for quantitative analysis, it is necessary to know the value of H associated with each compound of interest. Determination of H can be accomplished from as little as two measurements, of high and low concentration samples. However, if greater precision is required a larger number of standards can be analyzed. Once values of H have been determined for a given detector temperature, these values should be valid in subsequent analyses even if different chromatographic conditions exist. This follows because H i s only dependent on the reference current,
LITERATURE CITED (1) Corklll. J. A.; Kuttab, S. H.; Glese, R. W. Anal. Chem. 1982, 54, 481. (2) Wentworth, W. E.; Chen, E. J. J . Qas Chromafogr. 1967, 5 , 170. (3) Maggs, R. J.; Joynes. P. L.; Davles, A. J.; Lovelock, J. E. Anal. Chem. 1971, 43, 1966. (4) Sullivan, J. J.; Burgett, C. A. Chromatographia 1975, 8 , 176. (5) Lovelock. J. E.; Watson, A. J. J . Chromatogr. 1978, 158, 123. (6) Grirnsrud, E. P.; Knighton, W. B. Anal. Chem. 1962, 54, 565. (7) Patterson, P. L. J . Chromafogr. 1977, 134, 25. (8) Grirnsrud, E. P.; Klrn, S. H. Anal. Chem. 1979, 5 1 , 537. (9) Gobby, P. L.; Grlrnsrud, E. P.; Warden, S. W. Anal. Chem. 1960, 5 2 , 473.
Anal. Chem. 1983, 55, 718-723
718
(10) Knighton, W. B.; Qflmsrud, E. P. Anal. Chern. 1982, 54, 1892. (11) Grimsrud, E. P.; Miller, D. A. J . Chromatcgr. 1980, 102, 117. (12) Grimsrud, E. P.; Miller, D. A.; Stebbins, R. Q.; Kim, S. H. J . Chromatogr. 1980, 107, 51. (13) Wentworth, W. E.; Chen, E. C. M. J . Chromafogr. 1979, 186, 99. (14) Qrimsrud, E. P.; Kim, s. H.; Gobby, P. L. Ana/. chern. 1970, 51, 223. (15) Grimsrud, E. P.; Connoily, M. J. J . Chromatogr. 1082, 239, 397.
(16) Miller, D. A,; Grlmsrud, E. P. Anal. Chern. 1979, 51, 851.
RECEIVED for review October 25,1982. Accepted December 27, 1982. This work is supported by the National Science Foundation under Grant No. CHE-8119857.
Automated Determination of Nickel and Copper by Liquid Chromatography with Electrochemical and Spectrophotometric Detection A. M. Bond’ and G. G. Wallace Division of Chemical and Physical Sciences, Deakln University, Waurn Ponds 32 17, Victoria, Australia
A microprocessor-based instrumental method for automated monltorlng of nickel and copper in a wlde varlety of matrlces found in industrlal plant Snuatbns has been developed. I n sltu formation of dlthiocarbarnate complexes, separatlon of complexes by high-performance reversed-phase liquid chromatography, and dual eiectrochemlcal and UV/vlslbie spectrophotometric detectlon are Included as integral components of the monttorlng system. The unusual versatiitly associated with the separation and detection system provides the analyst with methodology to determine nlckei and copper down to trace levels and with extremely large varlation In concentratlon ratio. Data obtalned during the trial perlod of operation suggest that matrix and other Interferences are likely to be minimal after appropriate choke of operatlng parameters.
Continuous determination of concentrations of chemical species is often required in quality control or effluent monitoring programs associated with industrial plants. If it is necessary to collect a sample from the plant, transport it to the laboratory, and finally undertake the determination, the time lag induced between collection and any appropriate action being taken may lead to a costly and inefficient operation. Furthermore, staff may not be available in the analytical laboratory 24 h a day. The absence of data at these times could lead to delays in detecting a safety or environmental hazard. Many on-line automated analytical systems have been devised. These have been based on various chemical principles as perusal of ref 1 to 8 would indicate. Recently, many refinements have been made to electrochemically based flow through systems, particularly in the area of cell design (9-11). These developments and the use of microprocessor-based equipment to apply potential wave forms and collect and process the relevant data (12-15) have substantially aided the ease of automation. Despite these advances, the application of automated “on-line” electrochemical methods is still limited. This is because of the need, in many cases, for extensive sample pretreatment prior to a determination (16-19). Often removal of metals, organics, or even oxygen (20,21) is necessary to avoid interference effects. Overlapping electrochemical or spectral parameters have frequently restricted the use of electrochemicalor spectrophotometric detectors respectively 0003-2700/83/0355-07 18$01.50/0
and variable matrix effects also cause difficulties. In this work, a system is described which is applicable to the automated “on line” determination of nickel and copper. A chemical reaction step and a separation stage are included prior to the determinationsin the flow through electrochemical and/or UV-visible absorption spectrophotometric cells to provide specificity and minimize problems associated with a variable matrix (see later). A microprocessor-based system controls most of the experimental variables, generates the electrochemicalwave form, and collects, stores, and analyzes the data. The system for monitoring copper and nickel has been tested on a wide range of industrial effluents and industrial plant liquors to demonstrate the reliability of the system. The chemical step, and separation stage, included in this new automated system is based on the kinetically rapid formation of a metd-dithiocarbamate complex. Both ammonium pyrrolidine dithiocarbamate (I) [pydtcl- and sodium diethyldithiocarbamate(11) [dedtcl- are considered. The ability
-s\
C-N
/1
S
-S\c-”c2H5
S’
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I or I1 = [dtclof these ligands to stabilize high oxidation states allows monitoring of the oxidation rather than the reduction process of the metal dithiocarbamate complex formed in situ in the liquid chromatographic system (22,23)and the difficulty in removing oxygen, as required with reduction processes is therefore eliminated. The amenability of metal-dithiocarbamates to chromatographicseparation (24-28) is the basis of the separation stage. The UV-visible absorption characteristics of metal-dithiocarbamates has led most workers to develop separations using normal-phasechromatographywith UV/visible spectrophotometric detection (29-32). In the automated system developed, both spectrophotometric and electrochemical detectors are considered with high-performance reversed-phase rather than normal-phase liquid chromatography. The new feature enabling continuous on-line monitoring is the inclusion of the ligand, [dtcl-, in the solvent (22) and complex formation occurs in situ rather than externally to the system. All stages of the experiment are 0 l9S3 American Chemical Society
