Pulsed Amperometric Detection
ABOUT PAD
Pulsed Amperometric Detection (PAD) is a technique used to detect certain classes of compounds, notably sugars and polyalcohols, among others. These compounds tend to foul the surface of an electrode, making ordinary constant-potential amperometric detection difficult.
In PAD, cleaning potentials are applied to the electrode roughly once per second, interspersed with the detecting potential. The result is a waveform like the one pictured below.
The application of cleaning potentials increases the background current and noise compared to what we've come to expect with constant-potential amperometric detection. This is because the electrode is never allowed to equilibrate for more than a few hundred milliseconds before it is pulsed again. Consequently, PAD is inherently unsuited to high-sensitivity analyses. We recommend using a range of no less than 1µAfs for most PAD applications. This may result in extremely small peaks being visible during data collection. But there is enough resolution in ChromGraph that these peaks can be resolved and quantified in the data-analysis section, Report.
SETTING PAD OPTIONS
The PAD OPTIONS screen is reached through the PAD OPTIONS section of the Main Menu or by using the <
From the PAD OPTIONS screen you can interactively enter the PAD waveform, the range, and the filter:
STEP # is the sequential number of steps in the cycle, with each step having independent control of applied potential during its duration. Up to 16 steps are allowed. Enter a step number directly, or scroll to the desired step by clicking the up and down arrows with the mouse. Alternatively, clicking on any section of the graph brings its step into the edit boxes.
mSEC is the duration of a step, from 0-1000 milliseconds in 5 ms increments. A duration of 0-4 milliseconds will be interpreted as the end of a cycle.
SAMPLE, if checked, indicates that this is the step during which data will be collected.
mVOLTS is the potential to be applied during the step, from -2000 to +2000 millivolts in 10 mV increments.
RANGE is the gain range to be used during the SAMPLE step. We recommend a gain of no less than 1µAfs for obtaining a stable baseline.
FILTER is the noise-reduction filtering to be applied to the data. The optimum filter setting can be determined by measuring signal-to-noise ratio (peak height ÷ baseline noise) at various filter settings, and choosing the filter setting that provides the highest ratio. But for most uses a value of 0.1 Hz. is adequate. (Large numbers provide less filtering than small numbers.)
ZERO AFTER RUNS programs an automatic rezero at the end of each run. A rezero is needed if the peaks go offscale because of a high background. An alternative to using an automatic rezero is to use a sufficiently insensitive range (no less than 1µAfs) so that peaks remain onscale.
The PAD waveform can be modified directly on the graph, by clicking on any section of the line and dragging it, or by using the scroll bars. During the dragging operation the values in the edit boxes change, allowing you to locate the line more precisely.
PAD WAVEFORM FOR SUGARS
A typical three-step pulse must be configured in four steps with the BASi PAD. This is because we allow more flexibility in setting the sampling duration and potential, so an extra step is required. For example, a typical carbohydrate waveform for a gold electrode requires the following four steps:
|
STEP
|
POTENTIAL
|
DURATION
|
FUNCTION
|
|
(mV)
|
(mSec)
|
||
|
1
|
50
|
400
|
Equilibration |
|
2
|
50
|
200
|
Data |
|
3
|
800
|
200
|
Cleaning |
|
4
|
-600
|
200
|
Regeneration |
