The use of polarity switching for the sensitive determination of nitrate in human cerebrospinal fluid by capillary electrophoresis with contactless conductivity detection

• Electrophoretic stacking method for nitrate determination in cerebrospinal fluid.
• 60-fold higher sensitivity compared to classical electrophoretic separation.
• Forcing of chlorides out of capillary by polarity switching.
• Combination of ITP stacking with subsequent CZE separation in one capillary.
• Trace analysis of nitrate and sulphate.

A new electrophoretic stacking method has been developed for the sensitive determination of nitrates in cerebrospinal fluid. 2 M acetic acid was used as the BGE; inorganic anions were detected using a contactless conductivity detector and separation was carried out in an INST-coated capillary with inner diameter of 25 μm. The sample of cerebrospinal fluid was injected in a large volume into the short end of the separation capillary (15 cm) and separation first occurred in the isotachophoretic mode, where a long zone of the majority chloride migrates in the capillary and is followed by a concentrated zone of the unseparated nitrates. The sample zone passes to the end of the capillary where more than 99% of the chlorides are let out. Then the polarity of the voltage is switched and separation occurs in the zone electrophoresis mode, in which the nitrates are separated from the zone of chlorides. The time of switching the polarity is determined by the decrease in the electrophoretic current. Up to 99.95% of the original amount of chlorides present in the cerebrospinal fluid could be let out of the capillary by this technique, thus increasing the signal/noise ratio by up to 60-fold compared to classical electrophoretic separation.