Aromatic Sulfonates

The earliest demonstration of the ability of electro-chemically modulated liquid chromatography to separation complex mixtures at performance levels that rivaled conventional liquid chromatographic separations used porous graphitic carbon as the column packing and the aromatic sulfonates in Table 1 as analytes. Examples are presented in Figure 3. These results show that: (1) the retention for all of the analytes increases as the applied potential moves positive, (2) the retention of all the analytes (Figure 4) exhibit a linear dependence on applied potential, (3) the magnitude of the change in retention differs between analytes, and (4) all six analytes in the mixtue are baseline resolved at + 0.5 V. These results confirm that the analytical figures of merit (e.g. resolution and retention time) can be readily manipulated by changes in applied potential, and that the change in the surface charge of the stationary phase plays a key role in altering the extent of retention.

It is also possible to affect such separations by manipulating the applied potential during analyte elution, a strategy that parallels temperature pro-

Table 1 Chemical structures and labelling schemes of the aromatic sulfonate derivatives

Compound

Acronym

A

1

ABS

nh2

2

HBS

OH

3

BS

H

4

TBS

CH3

5

CBS

CI

6

BAS

COOH

Figure 3 Separations of a mixture of aromatic sulfonates (see Table 1) using a porous graphitic carbon stationary phase at # 0.50 V (A), open circuit (B), and -0.60 V (C). The mobile phase waswater(0.1 M LiCIO4, 0.1 %TFA) and acetonitrile(0.1 M LiCIO4) (88 : 12, v/v). The flow rate was 0.90 mL min~1. (Reproduced with permission from Ting EY and Porter MD (1998) Analytical Chemistry 70: 94-99. Copyright ACS Publications.)

Figure 3 Separations of a mixture of aromatic sulfonates (see Table 1) using a porous graphitic carbon stationary phase at # 0.50 V (A), open circuit (B), and -0.60 V (C). The mobile phase waswater(0.1 M LiCIO4, 0.1 %TFA) and acetonitrile(0.1 M LiCIO4) (88 : 12, v/v). The flow rate was 0.90 mL min~1. (Reproduced with permission from Ting EY and Porter MD (1998) Analytical Chemistry 70: 94-99. Copyright ACS Publications.)

gramming in gas chromatography. Figure 5 demonstrates this possibility whereby a linear voltage ramp is imposed upon the stationary phase from + 0.3 V to -1.0V immediately upon sample injection. This strategy increases the resolution of the compounds that eluted early at the open circuit potential (i.e. the condition used in conventional liquid chromatogra-phy) by increasing their retention. At the same time, this waveform decreases the retention and narrows the bands for those compounds eluting much later in the open circuit separation. There is also a notable decrease in the time required to complete the separation (&4min). Thus, a stationary phase gradient offers a pathway for the optimization of separations.

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