Page 133 - Analytical method for food addtives

E220–8: Sulphites 83

Analysis of foods and beverages with sulphite content ≥≥ ≥≥ ≥10 mg/kg using ion
exclusion chromatographic method 10

Scope
Ion exclusion chromatographic method for determination of SO at ≥10 µg/g. Not
2
applicable to dark-coloured foods or to ingredients where SO is strongly bound,
2
e.g. caramel colour. Method does not detect naturally occurring suphfite. Results
of the interlaboratory study supporting the acceptance of the method:
S = 4.7; S = 8.9; RSD = 12.3 %; RSD = 21.2 %
r R r R

Principle
SO is released by direct alkali extraction. Diluted test portions of liquids or diluted
2
filtrates of solid test portions are injected into LC or anion exclusion chromato-
graphic system equipped with anion exclusion column and electrochemical detector.

Procedure
Determination
Dilute liquid test portion with pH 9 buffer so that height of sulphite peak from test
portion is similar to that of 0.60 µg/mL standard within 50 %. For solid test
portions, homogenise 0.2–1.0 g test portion in 10–100-fold excess pH 9 buffer for
1 min with homogeniser and filter (0.2–0.45 µm). Dilute filtrate as necessary,
comparing signal intensity with that of 0.60 µg/mL working standard solution. For
acidic test portions such as lemon juice, if pH of diluted test portion is <8, adjust
to pH between 8 and 9 with dilute NaOH solution or perform extraction with
100 mM Na HPO , 10 mM D-mannitol solution. Inject 0.60 µg/mL standard
2 4
solution, and then inject prepared, diluted test portion.
Extraction, filtration, dilution and injection should be done within 10 min
because sulphite concentration in extract tends to decrease gradually. Errors due to
gradual decrease in detector sensitivity during multiple test portion injections can
be minimised by injecting standard solution alternatively with test portion injec-
tions. Cleaning electrode at beginning of each chromatographic run may alleviate
decrease in sensitivity. To clean electrode, apply –1.0 V for several min followed
by +1.8 V for several more min and then equilibrate at +0.6 V. Alternatively, short
train of electrode cleaning voltages can be applied automatically after each
injection.
Calculations
Calculate µg/g SO in test portion as follows:
2
SO µg/g = 0.60 × (PH/PH′) × dilution factor [8.4]
2,
Where PH and PH′ = peak height from test portion and standard, respectively, and
dilution factor takes into account initial dilution for extraction and any subsequent
dilution. (For further details see AOAC Official Method 990.31.) 10