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Anal. Chem. 1981, 53, 344-345
Bipolar-Pulse Conductivity Detector for Ion Chromatography Sir: A feasibility study on employing a bipolar-pulse conductivity detector (BPCD) as an alternate detector for ion chromatography (IC) (I)was initiated by this laboratory. The bipolar-pulse concept for conductance measurement was originally demonstrated by Johnson and Enke (2). Bipolar-pulse conductance measurements involve the application of two successive constant-voltage pulses of equal but opposite polarity to the conductance cell and measurement of the current passing through the cell a t the end of the second pulse. The bipolar-pulse technique is fast, is accurate, has an extended conductance range over conventional ac bridge methods, and is independent of series and parallel capacitance. In addition, this technique readily lends itself to electronic nulling of the background conductance due to the eluant. In conventional IC, a suppressor column in series with the analytical column removes the background conductivity due to the eluant. This arrangement introduces a number of disadvantages and complications for various determinations. There is a decrease in column efficiency due to the void volume of the suppressor. One is limited in the choice of mobile phase, since many eluants are not compatible with the suppressor. There are analysis complications due to ion exclusion of weak acids on the suppressor. Oxidation of anions, such as nitrite and sulfite, which are easily oxidized in an acid environment, results in peak splitting (3) and poor quantitation. The analysis of cations, in which an anion-exchange resin is employed in the hydroxide form, is limited by the low solubility of many metal hydroxides. The need for regeneration complicates system design and working procedures. Use of the bipolar-pulse technique for measurement of conductance overcomes many of the limitations of traditional bridge techniques. Therefore, by coupling of the analytical column directly to a BPCD, problems associated with the suppressor can be eliminated. Electronics for the BPCD were constructed in a breadboard fashion. A Wavetek signal generator (Wavetek Indiana, Inc.) was used to provide a base frequency for the bipolar pulse. The conductivity cell, Wescon 219-900, which was employged for the BPCD technique, was placed between the separator and the suppressor column. The Dionex Model 10 conductivity detector was located in its normal position after the suppressor column. A standard solution of the sodium salts of dibutyl- and monobutylphosphoric acid (DBP and MBP) was injected onto the column, and chromatographs were obtained. The chromatographs obtained from the BPCD and the Model 10 detector are shown in Figures 1 and 2, respectively. Chart speed and full-scale voltage setting for each recorder were the same, so that visual comparison of retention time and relative peak height is possible. The conditions employed on the IC unit are given in Table I. The first, narrow-band peak observed in Figure 1 is an injection peak which is proportional to the total ionic content of the sample. The anions of the sample displace the anion of the mobile phase at the top of the column. Then the eluant anion, together with the sample cations, move with the solvent front and are eluted a t the void volume. Another observation obtained by comparing Figures 1 and 2 is that the response of DBP is enhanced with the BPCD. This increase in response for the DBP can be explained by the low dissociation constant of DBP. Since the response observed in Figure 2 is after the sample has passed through the suppressor, the D B P will be in its acid form and in an acidic environment determined by the carbonic acid concen0003-2700/8 1 /0353-0344$01 .OO/O
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Figure 1. Separation of 1 FrnoVmL sodium dibutylphosphate and 1 pmol/mL disodium monobutylphosphate. Detection was by bipolarpulse technique; pulse frequency was 10 kHz, pulse height was f0.5 V, and current amplification was 10 mV/FA. The first peak is the injection peak (see text). Other conditions employed on the IC are given in Table I. DBP
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Separation of same sample observed in Figure 1. Chromatogram was obtained by a second detector on-line after the suppressor column. CondAions employed on the IC are given in Table 1. Flgure 2.
tration. However, in Figure 1,the DBP is dissociated because of the alkaline environment. Since DBP is a much weaker 0 1981 American Chemical Society
Anal. Chem. 1981, 53,345-350
Table I. Ion Chromatographic Conditions eluant: 1.5 mM Na,C0,/0.5 mM NaOH flow rate: 138 mL/h (30%) separator column: 3 x 150 mm anion precolumn plus 3 x 500 mm anion separator column suppressor column: 6 x 250 mm anion suppressor column meter full scale: 100 p n - ’ / c m (Model 1 0 detector only) injection volume: 100 p L acid than MBP, this dissociation effect is more pronounced for the DBP. In summary, the employment of a bipolar-pulse conductivity detector for ion chromatography, without the complications of a suppressor column, has produced encouraging data. Refinement of the electronics for thermal stability, noise immunity, and current output is in the development stage. Another application being considered for the bipolar-pulse detection technique is reverse-phase liquid chromatography, where the solution resistance may be too high for conventional bridge techniques. Also, the bipolar-pulse technique is being considered as a replacement for the conventional Wheatstone
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bridge electronics of a thermistor-based thermal conductivity gas chromatograph detector.
LITERATURE CITED (1) Small, H.; Stevens, T. S.;Bauman. W. C. Anal. Chem. 1875, 4 7 , 1801- 1809. (2) Johnson, D. E.; Enke. C. G. Anel. Chern. 1870, 42, 329-335. (3) Koch, W. F. Anal Chem. 1878, 57, 1571-1573.
John M. Keller Analytical Chemistry Division Consolidated Fuel Reprocessing Program Oak Ridge National Laboratory Oak Ridge, Tennessee 37830
RECEIVED for review July 30,1980. Accepted October 15,1980. Research sponsored by the Division of Nuclear Power Development, U.S. Department of Energy under Contract W7405-eng-26 with Union Carbide Corp.
AIDS FOR ANALYTICAL CHEMISTS Cleaning Methods for Polythene Containers Prior to the Determination of Trace Metals in Freshwater Samples Duncan P. H. Laxen‘ and Roy M. Harrison‘ Department of Environmental Sciences, University of Lancaster, Lancaster, LA 1 4Y0, England
Heavy metals are generally present at very low concentrations in water and hence their determination is extremely susceptible t o problems of contamination. The first sources of potential error are associated with the sampling and subsequent storage of the sample prior to analysis. T h e sample container must be carefully selected to avoid contamination due to leaching of metals into the sample and also to minimize losses of metal from the solution by adsorption onto the walls of the container. In their review of the literature Batley and Gardner ( I ) concluded that polythene and Teflon containers are suitable for sample collection and storage. Indeed, polythene sample containers have now gained widespread acceptance for routine use. Selection of a cleaning method for the sample container is, nevertheless, still somewhat arbitrary. A vast array of methods has been reported (Table I), but no attempt appears t o have been made to compare the different methods. T h e analyst therefore has either to hope that the method chosen is adequate or to perform a series of tests to establish the adequacy. Furthermore, some of the methods currently recommended require cleaning periods of several weeks and may be consuming a n undue amount of the analyst’s time, as well as tying up large numbers of sample containers “under preparation”. The method chosen to clean or prepare the sample container must fulfill two requirements. It must reduce contamination to a n acceptable level and it must minimize or prevent adsorption losses to the container wall. I t is widely accepted
’
Present address: Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JW, Scotland. 0003-2700/81/0353-0345$01 .OO/O
Table I. Methods Reported for the Cleaning of Sample Containers basis of method hot concd HNO, 50% HNO, 50% HC1 then 50% HNO, ca. 40% HCI 25% HNO, 20% HNO, ca. 15%HCI 10%HNO, 2.5% HCIO, 2% HNO, 1%HNO, detergent only sample rinse onlya a
ref 2 3 4
5 6 7,8 1
Y-13 14
15 16 17 1
After initial cleaning with 15% HCI.
that adsorption losses may be reduced by acidifying the sample, usually t o 0.5% ”03. This acidification step is, nevertheless, more likely to leach metals from an improperly cleaned container. In addition there is a growing requirement for samples to be maintained a t their natural p H in order to perform analyses which help elucidate the chemical form of the metal (9,18,19). These analytical schemes for speciation studies can be time consuming and hence necessitate the storage of samples at their natural pH for several days prior to analysis (19). Such storage’conditions favor adsorption losses from the sample. Indeed, Subramanian et al. (15) report significant losses of metals from river water samples (pH 6-8) during the first 10 days of storage in polythene containers. 0 1981 American Chemical Society
