Liquid chromatographic determination of barbiturates using a hollow-fibre membrane for postcolumn pH modification

A sensitive, high-performance liquid chromatographic method is described for the determination of barbiturates by postcolumn pH modification. The barbiturates (barbital, phenobarbital, hexobarbital and amobarbital) were separated on a C18 column using a mixture of methanol and water as an eluent. Then the pH of the eluent was raised to 10 by introducing ammonia or ammonium ion through a sulphonated hollow-fibre membrane inserted between the column and the detector. The detection was based on the primary ionized barbiturates at 240 nm. At barbiturate concentrations of 2.0 micrograms/ml, the within- and between-experiment precision (relative standard deviation) was 0.65-3.28 and 0.76-1.90%, respectively. The limits of detection were about 0.5-2.5 ng at a signal-to-noise ratio of 3. The method was applied to the determination of amobarbital in saliva.