Knauer Azura ECD 2.1, Инструкция по эксплуатации

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19 Installation
AZURA® Detector ECD 2.1 Instructions, V6685
Mobile Phase
Electrochemical detection is a sensitive detection technique characte-
rized by extremely low detection limits. A typical detection limit of 100
pmol/L or lower for catecholamines is no exception. Improving detec-
tion 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 detec-
tor. 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:
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Electrochemically clean, HPLC grade or better
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Ion strength 20 - 200 mmol/L
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Buffer pH near pKa
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In-line 0.2 µm filter & degassing of mobile phase
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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. In some applications EDTA is added to the mobile phase
to traps electrochemically metals such as Fe2+ by forming an inactive
complex.
However at higher working potentials (typically > 1.2 V vs. salt bridge
AgCl ref) also EDTA can become electrochemically active and is not
recommended. In that case only a passivation step with 15% HNO3 is
recommended (see page 20 and following).
Electrolytes assure contact between 3 electrodes in an electrochemi-
cal 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 re-
sults. 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. Instead 0.2 µm nylon inlet
filters are recommended.
Vacuum degassing: Considerable amounts of the gases N2, O2 and CO2
may be dissolved in HPLC mobile phases. Whenever the temperature
changes, solvents are mixed or a pressure reduction occurs, these gases
may show up as very small air bubbles. To avoid noisy baselines an in-line
vacuum degasser with a low dead volume (less than 0.5 mL) and high
degassing efficiency should be added to the system. The advantage over
helium degassing is that the degasser does not change the mobile phase
composition.
Helium degassing: Degassing using helium is an effective and universally
applicable method but only recommended when working in reductive
electrochemical detection and pulsed electrochemical detection (analysis