Peak heights of VMA and i-VMA decrease in time. Typically, about 0.5% decrease in height per analysis is ‘normal’.
A small but continuous decrease in peak height is confirmed and unavoidable. Remedies are:
- The decrease is less when low concentrations VMA are analyzed (< 0.15 µmol/L).
- The internal standard (iVMA) shows the same decrease as VMA, and should be used for quantification.
- Regular cleaning of the electrode (f.e. at the end of a sequence) will restore the signal to the original height.
Cleaning of the working electrode after a series of analyzes results in a recovery of the initial signal of VMA (peak area equal to that obtained at the first run). After cleaning, it takes some time to stabilize the baseline. We therefore advise to do it well in advance of running a sequence, for example right after a sequence is finished. Cleaning can be done by opening the cell and manually wiping the working electrode firmly with a tissue with acetone. If the signal is not restored completely, repeat the cleaning by polishing the WE. Also, cleaning can be automated by an electrochemical cleaning pulse without opening the cell. For example, a method that contains a cleaning pulse can be programmed as last entry in sequence. A pulse of E1=+1.20 and E2=-0.20 Volt for 5 min will help (t1 and t2 = 20s, t3 .. t5 = 0s).
Event files for cleaning the glassy carbon electrode are available in Dialogue software, for DC and pulse mode. Note that in the below example, the 2 “Ecell DC” steps are repeated 9 times (at time t=0.6 min). Such steps are also programmable in our OpenLab or Chromeleon drivers.