can your Integrator do this? Multiple Tailing Peak Analysis
flie SUPERGRATOR can! Does your programmable computing integrator provide multiple tailing peak analysis? The Supergrator can automatically determine when one tailing peak ends and another tailing peak begins, and skim the smaller peaks off of both. Multiple tailing peak analysis is only one of the dozens of reasons the Supergrators should be in your lab. Why not cail CSI today for a demonstration?
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discarding this very desirable feature. Furthermore, faradic reorganization of electrode surface groups (particularly on carbon) is often a slow process which wastes charge and leads to very significant background currents when trace analysis is attempted with a ramp excitation. If "cyclic voltammetry" is possible in "an electrochemical detector, then the concentration of analyte is so great that a sample could just as easily be collected and studied under static conditions using various microcells (29). How about potential pulse experi ments? These are far more practical than ramp excitations due to the pos sibility of discriminating against back ground effects as is done in pulse voltammetry (30). Repetitive waveforms such as those shown in Figure 5A and Β can be used with the usual sample and hold circuitry to provide "simul taneous" multipotential chromatograms analogous in some respects (general practicality is not one of them) to multiple-ion monitoring in GCMS. Differential pulse ideas can provide an advantage in some LCEC experi ments. Consider Figure 6. It is possi ble to selectively detect Β in the pres ence of C using a constant potential; however, we cannot ordinarily detect Β alone in the company of species A. Suppose that a pulse train is applied (Figure 5C) with the potential stepped between Ei and E2 and the current sampled at points marked S. If the difference in sampled current is plot ted, the response to molecule A will subtract out, and that to molecule Β
will be enhanced. An example of this approach has been recently presented by Swartzfager (31 ) . Figure 5D indicates another wave form which may have utility for "cleaning" an electrode in LCEC. For example, if a mercury film is used (32), E2 can be chosen positive of Εχ, Deposition of trace metals in the mer cury will lower the hydrogen overvoltage of a film electrode. Occasional pulses to E2 can extend the useful electrode life by anodically stripping the deposited metals. For the reasons described above, all pulse experiments tend to have significantly reduced sen sitivity when compared to the con stant-potential experiment. In most cases, simple DC amperometry will provide the best performance. Another approach to obtaining se lectivity is to operate two (or more) electrodes at different potentials in the same manner as for rotating ring disk electrodes (33) or twin electrode thin-layer cells (34). One can use the electrodes independently, or down stream electrodes can be used to de tect the products of upstream elec trodes. This can be fun to play with (35, 36); however, I doubt that it will ever see widespread application to practical chromatographic problems. Reaction Detectors
The limitations of amperometric and coulometric detection can be overcome in some cases by the use of post-column reactions (37). A reagent may be mixed with the column ef fluent to convert the sample compo nents into electroactive molecules.
Figure 4. Coulometric detection at planar glassy carbon electrode (a) UV detection at 265 nm: 1. 3-aminotoluenesulfonic acid, 2. 5-aminosalicylic acid, 3. 3-hydroxymandelic acid, 4. 3,5-dihydroxybenzoic acid, 5. 4-methylaminobenzoic acid. Conditions: UV detection at 265 nm; 10-cm, 2.8-mm i.d. stainless column packed with modified silica (4-7 μνη); eluent, 0.05 M per chloric acid, 0.05 M potassium chloride, 1.7% butanol in water, (b) Coulometric detection of same mix ture at 0.75 V. (c) Coulometric detection at 0.60 V Reproduced from ref. 12 by permission of Elsevier Scientific Publishing
CIRCLE 36 O N READER SERVICE CARD 4 5 4 A · ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL
1977
