376 / 471
(
5
) Table
2014.04B
summarizes the concentrations of choline and carnitine and their associated ISs in
the three IWS solutions and in the three WS solutions.
F. Procedure
(
a
) For liquids and ready-to-feed products, select sample weights between 1 and 5 g based on expected
concentration of the two nutrients in each sample.
(
b
) Powder products are reconstituted with water prior to analysis, typically 11.1% (w/w; 25 g powder
added to 200 g water). Typically a 5 g sample aliquot is taken from the reconstituted material for
analysis, but adjust as needed.
(
c
)
Sample analysis.--
(
1
) Weigh sample into a tarred microwave reaction vessel.
(
2
) Add 125 µL MIX-IS.
(
3
) Add water to sample to achieve a total sample volume of about 8 mL.
(
4
) Add 2.2 mL of about 70% nitric acid and seal vessels. The final volume should be close to 10 mL
(sample + acid + water + MIX-IS = 10 mL) prior to microwave digestion, which makes acid concentration
around 3.5 N.
(
5
) Vortex vessels for 30 s, and then place vessels into turntable and insert into microwave.
(
6
) Complete microwave digestion using the conditions defined in Table
2014.04C
. Operate with
microwave venting to a fume hood.
(
7
) After cool down (by air), uncap each vessel, and add 3.5 mL concentrated ammonium hydroxide to
each vessel. Operate in a fume hood.
(
8
) Vortex each sample for 30 s.
(
9
) Allow samples to stand in a hood for 30 min to react any acylcarnitines under basic conditions.
(
10
) Vortex samples for 30 s.
(
11
) Transfer 1 mL digested sample into a disposable centrifuge tube containing 25 mL laboratory water.
(
12
) Vortex for 30 s.
(
13
) Filter about 1 mL diluted sample using a 0.45 µm nylon syringe filter.
(
14
) Transfer 0.1 mL filtered solution into autosampler vial containing 0.9 mL acetonitrile. Mix well.
Sample is ready for analysis.
(
15
) Samples diluted into acetonitrile are stable for 24 h when stored in a refrigerated (8°C) autosampler.
Filtered samples (aqueous) and WS are stable for 7 days stored at 2-8°C.
Candidates for 2016 Method of the Year
314