Page 132 - Analytical method for food addtives

82 Analytical methods for food additives

Determination
Shake prepared test sample and immediately weigh aliquot of suspension (≤10 g)
containing ≤60 µg SO into 25 × 200 mm tube and, if necessary, add 5 % alcohol
2
so total weight of mixture is c. 10 g. Add 10.0 mL electrolyte-trapping solution to
dry polarographic cell and assemble apparatus as in Fig. 987.04. 8

(a) Total SO – Add to test sample tube 0.5 mL 2M ammonium acetate buffer and
2
0.2 mL H SO (1 + 1) to adjust pH to c. 1.5. Purge with O -free N for 10 min
2 4 2 2
at 1 L/min. Stop N flow, add 2 mL H SO (1 + 1) to test sample tube and place
2 2 4
tube in block heater preheated to 100 ºC. Purge with N for 10 min at 1 L/min.
2
Stop N flow and lift exit tubing from electrolyte-trapping solution in polaro-
2
graphic cell (to prevent solution from backing up). To verify complete transfer
of SO from test sample, reinsert exit tubing into electrolyte-trapping solution,
2
purge again for 5 min, and obtain polarogram. Repeat if necessary.
(b) Reagent blank – Prepare reagent blank in same manner as for test sample, and
carry through analysis.

Calibration curve
Prepare calibration curve at time of test sample analysis as follows: Pipette
10.0 mL electrolyte-trapping solution into dry polarographic cell and add 50 µL
200 µg/mL standard solution. Bubble N through solution for 4 min and obtain
2
polarogram as for test sample. Repeat 5 times with additional 50 µL aliquots of
standard, bubbling N for 30 s after each addition. Construct calibration curve,
2
representing 10, 20, 30, 40, 50, 60 µg SO in cell.
2

Calculation
Obtain amount total SO , µg, in cell from calibration curve by using highest peak
2
currents produced by test sample. Calculate SO in test sample, µg/g, correcting for
2
reagent blank if necessary.

Alternative trapping technique
Add 10.0 mL electrolyte-trapping solution to 25 mL graduated cylinder and insert
exit tubing (Fig. 987.04, items 2 and 7). Add to test sample tube 0.5 mL 2 M
ammonium acetate buffer and 0.2 mL H SO (1 + 1) to adjust pH to c 1.5. Purge
2 4
with O -free N for 10 min at 1 L/min. Stop N flow, add 2 mL H SO (1 + 1) to test
2 2 2 2 4
sample tube and place tube in block heater preheated to 100 ºC. Purge with N for
2
15 min at 1 L/min. Stop N flow and transfer solution to polarographic cell,
2
deaerate, and obtain polarogram. Dilute aliquot with electrolyte-trapping solution
if too concentrated. Calculate total SO by using calibration curve. Purging time
2
given should be sufficient to completely transfer SO from most test portions.
2
Verify complete transfer by additional purging, e.g., 5 min, using 10.0 mL fresh
electrolyte-trapping solution. Allow longer purging time if required. (For further
details see AOAC Official Method 987.04.) 8