exchanged for copper to form a neutral nonconducting chelate. In a second scheme, ethylenediammonium carbonate reacts with resin-bound Cu 2+ to form the copper-ethylenediamine complex and carbonic acid. The third scheme involves postcolumn reagent introduction in stoichiometric amounts to reduce the background conductance. The superiority of any of these approaches over existing practices is unclear. Gjerde and Benson (16b) have described a method in which suppression is achieved by reacting the column effluent with a colloidal solution of ion-exchange resin in the suitable ionic form. The application of this method for trace analysis has been demonstrated by Robles (16c). It has long been claimed that application of a voltage of appropriate polarity across a suppressor membrane should aid the efficiency of chemical ion exchange. Tian et al. (16d) described an acid-regenerated suppressor to which voltage was applied. However, they made no performance comparisons in the presence and absence of applied voltage. Strong and Dasgupta (16e) have since shown that the application of
voltage in an acid-regenerated suppressor actually deteriorates suppressor performance. They report the successful use of water as régénérant in an electrodialytically operated suppressor, using two membranes to isolate the eluent channel from electrolytically evolved gas. Not only does such a suppressor obviate the necessity of preparing a régénérant; the penetration of the régénérant counterion can be completely eliminated. In one mode, the detector cell effluent can be used as the régénérant. (For practical purposes, this eff l u e n t is p u r e w a t e r c o n t a i n i n g traces of acids.) A simpler form of this suppressor based on sheet membranes has also been described (16e, 16/). A similar suppressor is expected to be commercially available soon. Columns of 2-mm i.d. and suitable hardware are available for IC. One of the advantages claimed for this "microbore" format is t h a t significantly larger eluent concentrations can be exchanged by the suppressors.
erating high-purity acids. They described how very high purity eluents (suppressed conductance of 0 - 1 7 5 mM NaOH is 340 ± 40 nS/cm) can be generated on line, either without gas in the eluent channel or with removal of the electrolytic gas through a porous hydrophobic m e m b r a n e . The latter method provides greater current efficiency and permits higher eluent concentrations. Such ultrapure eluents may be beneficial for performing trace analysis of common ions in concentrated acids, bases, and salts. Additionally, the eluent concentration generated is controlled by the current. Thus it has been shown that gradient elution can be performed by programming the eluent generator current (16f, 17) rather than by using mechanical valves. Figure 5 shows a typical gradient anion chromatogram generated by this technique. On-line electrodialytic generation of pure substances is still in its infancy and may hold great promise for many areas.
Electrochemical eluent generators
Gradient IC
Strong et al. (17) demonstrated the possibility of electrodialytically gen-
Significant new advances have been made in gradient IC for single-
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, 1992 · 779 A
