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No signal

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Applies to

Electrochemical detector

Summary

After injection of the electrochemically active standard no signal is obtained.

Cause

Lack of detector response usually points at:

  1. Incorrect detector settings. For example the cell is off, cables are not connected, an insensitive range setting is selected or working potential not correct.
  2. Poor quality of standard or sample solution. There is no signal simply because there is no analyte in the sample. Many electrochemically active analytes are not stable at alkaline pH. The sample buffer must therefore have a well-defined (often acidic) pH. Also, dilution errors are mentioned at this point.
  3. Incorrect chromatographic system parameter. Detection in LCEC is highly dependent on the performance in chromatography. Preparing a fresh mobile phase using incorrect amount of an additive may result in a shift in retention times and chromatographic peaks may even disappear. Injection volume can be too small (setting of autoinjector) or column performance has deteriorated. Also, other factors such as absorption to tubing or valve may play a role.
  4. Incorrect data acquisition parameter. Cables are not properly connected, or a software setting is incorrect. Certain data systems can not measure below 0 Volt (or for example below -100 mV). In that case the baseline is completely flat, or only the top of peaks are measured.
  5. Malfunctioning detector. The cell connector cable is internally broken or heavily oxidized. Also, if a dummy cell is connected the signal is zero.
  6. Autosampler related problem. Syringe speed autosampler too high for micro LC or viscous solution.
  7. Another cause not related to the detector, such as a problem with injection or see decreased response.
  8. Specific for SenCell: wrong position of spacing (put the spacing to 1 or 2, not 0).

Diagnosis

  • Incorrect detector settings: baseline is flat, cell current (I-Cell) is fixed or an unusual value, system is ‘dead’.
  • Quality of sample: peaks became smaller and smaller before they finally disappeared.
  • Chromatography: problems started after changing a chromatographic parameter such as a new mobile phase or a new column.
  • Data acquisition: flat baseline, no response at all. Even changing the percentage offset results in no response.
  • Malfunctioning detector: dummy cell test is not successful.

Remedy

  • Detector settings: cell is off, or the working potential is not correct Standard or sample solution
  • Chromatography: take out the column from the LC-EC system and analyze a standard in flow injection mode. The standard (10 – 100 µmol/L) must be dissolved in mobile phase to reduce system peaks. Always compare with blank sample buffer.
  • Data acquisition: change the percentage offset and monitor detector output. If no signal is obtained measure the output using a voltmeter.
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