To approximately 1 g of food (3 g of fruit) 10 ml of
2 M sulfuric acid was added and the volume adjusted
to 25 ml with deionised water. For recovery tests, 1 ml
of a 100 mg/ml nicotinic acid solution was added. The
mixture was thoroughly mixed and heated in an autoclave
for 2 h at 121C (104 kPa). The cooled mixture
was diluted to 50 ml with deionised water, mixed thoroughly
and centrifuged at 2500 rpm for 15 min at 0C.
A 15 ml aliquot of the supernatant was adjusted to pH 7
with saturated barium hydroxide solution (®nal adjustment
with dilute solution) and made to 100 ml with
deionised water. The resultant suspension was centrifuged
at 2500 rpm for 10 min at 0C. A C18 Sep-Pak
Vac Cartridge (500 mg) was placed on top of a SCX
column (500 mg) and the columns were conditioned
with 10 ml methanol followed by 10 ml deionised water.
For the vegetable and fruit samples, a Mega Bond Elut
C18 cartridge (1 g) and an SCX column (1 g) were
employed. With the exception of the vegetable samples
(in which 40 ml was loaded), a 20 ml aliquot of the
supernatant was loaded onto the C18 column at a ¯ow
rate of 1±2 drops per s. Water was passed through the
columns at a ¯ow rate of 1±2 drops per s, the C18 SepPak
cartridge discarded, and the SCX column washed
with 5 ml of methanol at a ¯ow rate of 1±2 drops per s.
Nicotinic acid was removed from the SCX column with
5 ml of freshly prepared 2% solution of concentrated
ammonium hydroxide in methanol. The solvent was
evaporated to dryness under a stream of nitrogen at
room temperature. The residue was dissolved in 1 ml of
a 40 mg/ml aqueous saccharin solution for CE analysis
or 1 ml of deionised water for HPLC analysis. The
solutions were ®ltered through 0.8 mm cellulose acetate
®lter discs before analysis.