Page 136 - Analytical method for food addtives

86 Analytical methods for food additives

Principle
Injected test portion aliquot is mixed with concentrated citric acid solution (pH
about 2), which forms SO gas from free sulphite and from portion of bound
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sulphite that is labile under these conditions. SO gas diffuses across Teflon
2
membrane in gas diffusion cell into flowing stream of malachite green solution.
Malachite green is discolourised in proportion to amount of SO gas that diffuses
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across membrane. Degree of discolouration is measured spectrophotometrically.
Procedure

Determination
(a) System start-up – Begin pumping citric acid donor reagent and 2 recipient
reagents, malachite green solutions and phosphate buffer reagent through
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pump tubes specified in Fig. 990.29 and carry out start-up procedure as in
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990.29E(a) (see ).
(b) Test sample analysis – Repeatedly inject 15 µL of 40 µg/mL sulphite working
condition solution until peak height is stable and recorder deflection is 60–
90 % full scale. It may be necessary to adjust recorder span to achieve desired
peak height.
Five injections of 40 µg/mL standard should yield series of 5 peaks with CV
for peak heights ≥5 %. If this precision is not achieved, determine and correct
source of problem. Once system has stabilised, inject 15 µL portions of
sulphite working standard solutions, injecting each standard once. No test
portion pretreatment is required. Thoroughly mix undiluted wine and inject
15 µL portion directly into FIA system. To prevent loss of free SO , protect
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test portion from atmosphere until just before injection. Inject all test portions
in duplicate. After about 10 test portion injections, repeat series of standards.
For wines with free SO levels <3 µg/mL, increase sensitivity either by
2
injecting >15 µL portion or by decreasing full scale response on detector.
Prepare standards in range of test samples (e.g. 2 µg/mL, 1 µg/g) and repeat
determination of low level test portions using standards of 0–3 µg/mL for
calibration.
After all test portions have been analysed, thoroughly rinse FIA manifold by
pumping water through all 4 lines for 15 min, followed by FIA rinsing reagent
for 15 min. Manifold can be stored with this reagent. Release tension on pump
rollers to prolong tubing life.
(c) Data reduction – Determine peak height for each test portion and standard
injection to ±0.5 mm. If blank injection produced peak, subtract this peak
height from all test portions and standards. Average peak heights of each
injection for individual standard concentrations and plot peak height versus
µg/mL SO .
2
Determine SO concentration in wine test portion directly from standard
2
curve.
(For further details see AOAC Official Method 990.30.) 14