and retention time for analysing each analyte, 30 C was chosen
as the optimal column temperature for the analysis.
Moreover, the effect of flow rate of mobile phase on the retention
behaviour was examined by adjusting the flow rate from 0.8 to
1.2 mL/min. The optimum flow rate was determined to be 1.0 mL/
min by evaluating the analytical resolution and retention time for
each analyte.
3.4. Temporal stability of the sulphite
Because of the instability of sulphite and easy oxidation to sulphate,
1.0 (w/v)% triethanolamine (TEA) was added to a standard
sulphite solution as a stabilizer (Tokuda, Hirai, Fukui, & Kanno,
1978). The solution was preserved in a light-resistant container
under refrigerated conditions. To confirm the effect of the stabilizer,
the refrigerator-preserved solutions with or without the
addition of TEA were subject to periodic analysis for evaluating
the effect of standing time on sulphite. The relationship between
the standing time and the residual rate of the sulphite is shown
in Fig. 3, where the residual rate was calculated by using the following
equation:
Residual rate ½%¼
Peak height of the sulphite
Total peak height of the sulphite and sulphate
100
ð1Þ
As shown in Fig. 3, the residual rate of sulphite in the solution
with the addition of TEA was approximately constant for four
weeks. In contrast, the sulphite residual rate decreased to around
40% in the standard solution without TEA. Therefore, the standard
solution with added TEA demonstrated good stability and usability
over one month under refrigerated conditions.
3.5. Chromatogram of sulphite and several inorganic anions
A typical chromatogram of sulphite and several inorganic
anions analysed under the optimum operating conditions
described in Section 2.3 is shown in Fig. 4. Each species was
detected within a period of 25 min. The separation of sulphite
and other inorganic anions present in food samples was