Page 135 - Analytical method for food addtives

E220–8: Sulphites 85

scale. It may be necessary to adjust recorder span to achieve desired peak
height. Following each injection of either test solution or standard into FIA
system, wash out sample valve and first reaction coil by making injection of
1M H SO to remove any mercuric oxide that can precipitate inside reaction
2 4
coil. Following is typical timing pattern that can be used to ensure adequate
washing of valve and coil with no overlap of sample and 1 M H SO :
2 4
Repeated injections of 10 µg/mL standard should yield series of peaks with
CV for peak height ≥5 % (n = 5). If this precision is not achieved, determine
and correct source of problem.
Once system has stabilised, inject series of sulphite standard solutions once
each. Inject prepared test solutions, taking care to inject only clear (or slightly
turbid) supernate. Inject all test solutions in duplicate and be sure to include
1M H SO washing procedure between each injection. After c. 10 test
2 4
solution injections (5 test solutions in duplicate), repeat series of standards.
For any test solution with peak height that exceeds peak height of 10 µg/mL
SO standard, dilute test solution with TCM solution into analytical range (1–
2
10 µg/mL).
After all test solutions and standards 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 of each test and standard solution
injection to ±0.5 mm. If blank injection produced peak, subtract this peak
height from all test and standard solutions. Average peak heights of each
injection for individual standard concentrations and plot µg/mL vs peak
height.
Determine SO concentration in test extracts from standard curve and
2
calculate concentration in test portion as follows:
SO µg/g = C × (F/S) [8.5]
2
where
C = concentration in test extract
F = final total weight of test portion slurry
S = initial weight of test portion taken.

(For further details see AOAC Official Method 990.29.) 12

Analysis of wine for determination of sulphite (free) using flow injection
analysis method 14
Scope
Flow injection analysis method for determination of free sulphite and bound
sulphite that is labile at pH 2.2.
Results of the interlaboratory study supporting the acceptance of the method:
S = 0.55; S = 1.18; RSD = 8.77 %; RSD = 19.27 %
r R r R