A new method to determine biological sample volume by short end multiple injection capillary electrophoresis: Application in determination of nitrate and thiocyanate in human saliva

The aim of this study was to develop a simple and rapid method of capillary electrophoresis using a short end multiple injection in free solution to determine simultaneously the biological sample volume and analytes concentration. The method consists of a sequence of injection steps with an internal standard as the reference for correction of the volume of sample collected. The procedure was applies in the determination of NO3− and SCN in saliva samples. The background electrolyte was composed of 12 mM tris(hydroxymethyl)aminomethane and 8.5 mM sulfuric acid, at pH 2.5. The internal standard used was BrO3−. A fused silica capillary (48.5 cm total length, 8.5 cm effective length and 75 μm i.d.) coated with chitosan was used in a short-end injection configuration. Modification of the electroosmotic flow (EOF) using dynamic coating resulted in a controlled and stable EOF, contributing to the rapid separation of anions (0.36 min) in co-electroosmotic mode. The validation of the method for correcting the volume of saliva collected with a swab showed a difference of less than 3.5% compared with the predicted value and a correlation of 0.999. The limits of detection for NO3− and SCN− were 0.13 and 0.23 mg L−1, respectively. The inter-day precision of the method determined for both analytes was less than 5% and the recovery ranged between 97 and 102%.