Background Current and Noise

Background current usually results from the oxidation or reduction of the mobile phase. Since noise increases with background current, EC detection becomes limited at very high potentials. In aqueous solutions, the positive potential limit is essentially restricted by the oxidation of water  

Since this reaction generates hydronium ions, the oxidation potential decreases with increasing pH. The Nernst equation predicts a 59mV decrease per pH unit, when the reaction generates equal amounts of electrons and hydronium ions. At pH 4.5 the positive potential limit is about +1.2V vs. Ag/AgCl. 

One must keep in mind that impurities in the mobile phase contribute to background current. Impurities may also adsorb to the electrode surface. In order to minimize these problems, one should use reagents and solvents of highest purity and keep the buffer concentration low. In addition, the mobile phase additives should be chosen carefully. For example, tertiary aliphatic amines should be avoided as competing bases since they may be oxidized at low potential (Sect. 3.2). 

In reductive EC detection, dissolved oxygen must be carefully removed from the mobile phase, as it is reduced to hydrogen peroxide at low negative potential (Kissinger 1989). This makes reductive detection more laborious than oxidative detection. After removal of dissolved oxygen from the mobile phase, the negative potential limit is ultimately restricted by the reduction of hydronium ions to hydrogen. 

Care is needed in selecting the mobile phase pH, since it affects separation of the components, EC properties of the analytes, and background current. 

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Based on these electrochemical studies we developed a method for the quantitation of ajmalicine and catharanthine in cell cultures. These alkaloids were extracted from freeze-dried cells and purified by the solid-phase procedure described by Morris et al. (1985), except that ethanol was used as the extracting solvent instead of methanol. A dual-electrode coulometric cell was used in the screen mode. The potential of the first electode was set at +0.2 V (vs. Pd), which was at the base of catharanthine s voltammogram. The alkaloids were detected by the second electrode at +0.8 V, as this offered the best S/N ratio. Higher potentials led to lower S/N ratio, since the background current and noise started to increase exponentially above +0.85 V, due to the oxidation of water. The mobile phase was purified by a guard cell between the pump and injector. The guard cell operated at +0.8V. 

Boron-doped diamond presents another attractive material with low and stable background current and noise over a wide potential range, corrosion resistance, high thermal conductivity, and high current densities. Usually no mechanical or electrochemical pretreatment of BDD film electrode is needed. Therefore, BDD film electrodes find use also in the area of environmental analysis for organic explosive determinations. BDD-based electrochemical detector allowed, e.g., amperometric detection of 2,4,6-trinitrotoluene, 1,3-dinitrobenzene, and 2,4-dinitrotoluene over the 200-1,400 ppb range, with detection limits at the 100 ppb level. ... 

The working electrode surface in contact with the mobile phase contributes to background current and thereby to noise. For optimum signal-to-noise ratio all wetted working electrode surfaces should therefore contribute to analyte electrolysis as well. 

Since these iron compounds are electrochemically active, they will increase the background current and cause extra drift and noise. 

Table 9.2 compares the backgrounds with a 293 K source and one at 300 K for detectors with different cutoff wavelengths. For a 3-pm cutoff detector, the background changes by about 7% per degree Celsius, and day-to-day variations in room temperature can cause observable changes in detector current and noise. 

However, the influence of working electrode potential on background current (electrolysis current of mobile phase constituents) i.e. on baseline offset, stability and noise should also be taken into account. 

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