The chromatographic conditions were
optimized so as to obtain a good separation
between benzoic acid, sorbic acid and matrix
interferences in noodles. Acetate and
phosphate aqueous solutions with different
pH and organic solvent (methanol and
acetonitrile) were investigated and system
suitability test (SST) parameters were
performed during the development and
optimization of method. Firstly, the separation
was attempted and compared on a C18
column (150 mm ×4.6 mm, 5 μm) and a
C18 column (50 mm ×4.6 mm, 5 μm) with a
wavelength at 224 nm (for sorbic acid),
234 nm (for benzoic acid) and 254 nm using
acetonitrile and 5 mM phosphate buffer
(pH 7) in the ratio range from 5:95 - 30:70.
Both acids were eluted quickly and could not
separate from matrix interferences. Methanol
was then replaced in the same ratio range but
the peaks of both acids were co-eluted.
Increasing of phosphate buffer concentration
(5-30 mM) led to increase in analytes’ retention,
decrease in tailing factor and resolution value.
Consequently, ammonium acetate aqueous
solution was employed instead of phosphate
to achieve the best resolution. Poor resolution
and peak tailing was obtained when acetonitrile
was used. Separation of benzoic acid and
sorbic acid was accomplished by varying the
concentration of ammonium acetate (5-30
mM) and the ratio of methanol. An increase
in retention and peak broadening of benzoic
acid and sorbic acid was observed when higher
concentration of ammonium acetate at higher
pH was utilized. Hence, the aqueous
component of the mobile phase was selected
to be ammonium acetate 5 mM, pH 4.4.
Methanol in varying ratio was studied and the
resolution (R) between two acids and
interferences, and the run time of the
chromatogram was taken into consideration.
The optimization condition was performed
using an isocratic elution at the proportion of
60:40 (v/v) 5 mM ammonium acetate aqueous
solution adjusted to pH 4.4: methanol at a
flow rate of 1 mL/min. The C18 column
with 5 cm length was preferred as the
complete separation was occurred within 3
min and the mobile phase operated was also
3 times decreased.