Electrochemical detection is a sensitive detection technique characterized by extremely low detection limits. A typical detection limit of 100 pmol/L or lower for catecholamines is no exception. Improving detection limits will always be limited by the weakest link in an LC-EC system. In daily practice a couple of ‘rules’ must be obeyed to fully exploit the incredible linear dynamic range and low detection limits of an EC detector. These are not only hardware related, but also refer to mobile phase composition, degassing, temperature and pH stability and several other issues.
Mobile phase requirements:
- Electrochemically clean, HPLC grade or better
- Ion strength 20 - 200 mmol/L
- Buffer pH near pKa
- In-line 0.2 µm filter & degassing of mobile phase
- EDTA for trapping of metal ions
Mobile phase must be clean as it is in direct contact with the working electrode in EC detection. High purity chemicals including water is a pre requisite.
Electrolytes assure contact between 3 electrodes in an electrochemical flow cell. Low ion strength destabilises an EC system and noise will increase. Extremely high buffer concentrations cause problems of salt formation. Therefore concentrations between 20 and 200 mmol/L are recommended. Also constant pH is important for baseline stability and reproducible results. Stability of pH is best when close to pKa of a buffer ion. Often used buffers are phosphate, carbonate, acetate and citrate. Modifiers such as methanol, propanol and acetonitril can be used without problems in DC amperometry, but not in pulsed amperometric detection as peaks are strongly attenuated. In our experience the quality and expiration date of organic modifiers can be a problem resulting in increased noise levels.
Metal inlet frits in mobile phase bottles are not advised as metal ions are introduced and baseline noise is increased.