Page 99 - Analytical method for food addtives

E210–13: Benzoic acid 57

23 ‘Quantitative high-performance thin-layer chromatographic determination of organic-
acid preservatives in beverages’, Khan S H, Murawski M P, Sherma J. Journal of Liquid
Chromatography (1994) 17(4), 855–865.
24 ‘Benzoate ion determination in beverages by using a potentiometric sensor immobilized
in a graphic matrix’, Pezza L, Santini A O, Pezza H R, Melios C B, Ferreira V J F, Nasser
A L M. (2001) 433(2), 281–288.

7.5 Appendix: method procedure summaries

Gas chromatographic method – NMKL–AOAC method 1

Preparation of test sample
Homogenise test sample in mechanical mixer. If consistency of laboratory sample
makes mixing difficult, use any technique to ensure that the material will be
homogeneous.

Extraction
(a) General method – Accurately weigh 5.0 g homogenised test portion into
30 mL centrifuge tube with Teflon-lined screw cap. Add 3.00 mL internal
standard solution, 1.5 mL H SO (1 + 5), 5 g sand, and 15 mL ether. Screw cap
2 4
on tightly to avoid leakage. Mechanically shake 5 min and centrifuge 10 min
at 1500 g. Transfer ether layer with disposable pipette to 250 mL separator.
Repeat extraction twice with 15 mL ether each time.
Extract combined ether phases twice with 15 mL 0.5M NaOH and 10 mL
saturated NaC1 solution each time. Collect aqueous layers in 250 mL separator,
add 2 drops of methyl orange, and acidify to pH 1 with HC1 (1 + 1). Extract
with CH C1 , using successive portions of 75, 50, and 50 mL. If emulsion
2 2
forms, add 10 mL saturated NaC1 solution. Drain CH C1 extracts through
2 2
filter containing 15 g anhydrous Na SO into 250 mL round-bottom flask.
2 4
Evaporate CH C1 solution in rotary evaporator at 40 ºC just to dryness.
2 2
(b) Cheese and food products with paste-like consistency – Accurately weigh
5.0 g homogenised test portion into 200 mL centrifuge flask. Add 15 mL H O
2
and stir with glass rod until test portion is suspended into aqueous phase. Add
3.00 mL internal standard solution, 1.5 mL H SO (1 + 5), and 25 mL ether.
2 4
Stopper flask carefully and check for leakage. Mechanically shake 5 min and
centrifuge 10 min at 2000 g. Transfer ether layer with disposable pipette to
250 mL separator. Repeat extraction twice with 25 mL ether each time.
Continue as in (a), beginning ‘Extract combined ether phases. . .’

Derivatisation and gas chromatography
Add 10.0 mL CHC1 to residue in 250 mL round-bottom flask. Stopper and shake
3
manually 2 min. Transfer 1.00 mL CHC1 solution to 8 mL test tube with Teflon-
3
lined screw cap and add 0.20 mL silylating agent. Cap and let stand 15 min in oven
or H O bath at 60 ºC. Inject duplicate 1 µl portions of residue solution into gas
2