Evaluation - Voltammetric Methods of Analysis

Evaluation

Scale of Operation 

Voltammetry is routinely used to analyze samples at the parts-per-million level and, in some cases, can be used to detect analytes at the parts-per-billion or parts-per-trillion level. Most analyses are carried out in con- ventional electrochemical cells using macro samples; however, microcells are available that require as little as 50 μL of sample. Microelectrodes, with diame- ters as small as 2 μm, allow voltammetric measurements to be made on even smaller samples. For example, the concentration of glucose in 200-μm pond snail neurons has been successfully monitored using a 2-μm amperometric glu- cose electrode.

Accuracy 

The accuracy of a voltammetric analysis often is limited by the ability to correct for residual currents, particularly those due to charging. For analytes at the parts-per-million level, accuracies of ±1–3% are easily obtained. As expected, a de- crease in accuracy is experienced when analyzing samples with significantly smaller concentrations of analyte.

Precision 

Precision is generally limited by the uncertainty in measuring the limit- ing or peak current. Under most experimental conditions, precisions of ±1–3% can be reasonably expected. One exception is the analysis of ultratrace analytes in com- plex matrices by stripping voltammetry, for which precisions as poor as ±25% are possible.

Sensitivity 

In many voltammetric experiments, sensitivity can be improved by ad- justing the experimental conditions. For example, in stripping voltammetry, sensi- tivity is improved by increasing the deposition time, by increasing the rate of the linear potential scan, or by using a differential-pulse technique. One reason for the popularity of potential pulse techniques is an increase in current relative to that ob- tained with a linear potential scan.

Selectivity 

Selectivity in voltammetry is determined by the difference between half-wave potentials or peak potentials, with minimum differences of ±0.2–0.3 V re- quired for a linear potential scan, and ±0.04–0.05 V for differential pulse voltamme- try. Selectivity can be improved by adjusting solution conditions. As we have seen, the presence of a complexing ligand can substantially shift the potential at which an analyte is oxidized or reduced. Other solution parameters, such as pH, also can be used to improve selectivity.

Time, Cost, and Equipment 

Commercial instrumentation for voltammetry ranges from less than $1000 for simple instruments to as much as $20,000 for more sophisticated instruments. In general, less expensive instrumentation is limited to linear potential scans, and the more expensive instruments allow for more complex potential-excitation signals using potential pulses. Except for stripping voltammetry, which uses long deposition times, voltammetric analyses are relatively rapid.