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ion is SO2"; (8) I
20 30 40
Separation of inorganic analyte anions using chelation for separation. Analyte: each 1(T4 M, lOO-yuL sample; (1) F", (2) Cl", (3) N02-; (4) Br; (5) NOj; (6) ÍÐÎ^“; (7) (9) S20^~. Eluent: 2mM K2EDDA at 1.0 mL min1. Column: Tosoh IC anion PW.
(From Ref. 72.)
and the enhancement in detection owing to the suppression. The peak heights are higher by more than two times those obtained in the absence of suppression.
In a third suppression scheme involving coordination , a solution of a borate or carbonate salt of ethylenediamine or other polyamine is used as the eluent for the separation of analyte anions on an anion exchanger. The suppression column is packed with a Cu2+ charged cation exchanger. In the suppression column the cations replace the Cu2+, which forms the Cu-amine complex and the poorly dissociated H2C03 or H3B03. Thus, the background conductance is reduced and the analyte anions are detected on this background. When the eluent composition is optimized from both the standpoint of the separation and the sup-pression, background conductance is reduced by a factor of more than 20 by the suppression. If an ethylenediamine solution is used as the eluent, OH“ is the eluent counteranion owing
to the ionization of the amine, and reduction in conductance by the Cu21-charged cation exchanger suppression column is almost 80-fold. But. OH~ is a weak eluent counteranion. and this limits which anions can be conveniently separated on the analytical column. One of the useful features of the Cu2+-charged suppressor column is that one can visually determine when the column is exhausted and recharging is required. As the Cu2+ is removed from the cation exchanger, the exchanger loses its bluish coloration.
Gjerde and Benson , similarly to Small et al. (6), used a cation exchanger for suppression in the separation of analyte anions. The difference is that Gjerde and Benson introduced a slurry of a strong acid cation exchanger continuously postcolumn into the column effluent and before the conductivity detector to obtain suppression. This kind of suppression has been termed solid-phase suppression. The strong acid-type cation exchanger is insoluble, and it stays suspended during the chromatographic run because it is used as small, colloidal (< 7-/ø³) microparticles charged in the H+ form. The suspended particles do not interfere in detection by conductance. The reactions describing the suppression are the same as if an H+-charged cation exchanger suppressor column was used [see Eqs. (3a) and (3b)]. That is, the eluent stream is converted into a low-conducting background, and the analyte conductivity signal is enhanced. However, because a column is not used for the suppression, peak dispersion and broadening effects are reduced and dispersion and broadening are primarily due to the postcolumn mixing chamber. Also, this ion-exchange slurry suppression system can be used continuously, unlike the cation-exchange suppressor column strategy, and is compatible with
gradient elution . The effectiveness of the slurry suppression system is indicated by considering a typical ÍÑÎç~ÑÎç“ eluent. For a 2.8 mM/2.2 mM ratio at 1 mL min-1 eluent, background conductance was 624 /us cm'1 and was reduced to 18 /in³ cm-1 when the slurry suspension was used . Other practical applications of solid-phase suppression are described elsewhere .
A postsuppressor device can be used to enhance the detection or increase the number of detection options in suppressed IC by placing the device between the suppressor and the column (see Fig. 2). Consider the HCO3-CO2' eluent, which is widely used in suppressed analyte anion separations. As indicated in Eqs. (5a) and (5b), H2C03 is formed in the suppressor and partial dissociation of the H2C03 affects background conductance. Furthermore, H2C03 dissociation is also influenced by the acid form of the analyte anion, which subsequently, affects conductance detection of the analyte anion. Formation of C02 can cause degasing problems and changes in the eluent stream, for the C02 can partially diffuse through the suppressor membrane.
The presence of C02 in basic eluents, particularly strong base eluents, provides a low conductivity background after suppression and affects the detection limit . The CO? can permeate a basic eluent if not protected, or can permeate the eluent through porous tubing used to transport the eluent. Figure 15 illustrates the extent that C02 can permeate Tefzel tubing. The C02 permeation of water was determined by IC as a function of residence time of the water within the tubing .
Removal of the H2C03 before conductivity detection or its removal from the strong base eluents will, therefore, improve both the chromatographic performance and the detection limit. Postsuppression devices have been designed to remove C02 before conductivity detection; strategies for the removal of C02 from strong basic eluents are described later. The basic idea