378 / 471
where C
x
is the concentration of the analyte in the product (µg/g), C
s
is the concentration in the
analytical solution measured by the instrument (ng/mL), and DF is the general dilution factor.
(
d
) For this analysis (for either choline or carnitine), given the mass of IS added to the samples and the
concentration of IS in the standards, the DF can be shown to be:
DF = 3.125/sample weight (g) (Equation 2)
(
e
) Thus, the final result (C
x
) can be calculated by combining Equations 1 and 2.
Notes
: (
1
) Free choline or carnitine can be determined by weighing same sample size (as for microwave
digest procedure) into a disposable tube, skipping the microwave digestion and filtering the sample at
an equivalent dilution (neither ammonia nor nitric acid to be added).
(
2
) Unless the sample contains milk proteins, free carnitine = total carnitine (through microwave
digestion). Thus, the method is able to determine free and total carnitine and total choline
simultaneously through microwave digestion. The nonmicrowave preparation is typically used for free
choline in products (rare) or to determine free choline/carnitine in premixes. Note that the typical
control sample, SRM 1849a, has substantially different values for carnitine depending on whether the
SRM is microwave digested.
References:
J. AOAC Int
. (future issue)
AOAC SMPR 2012.010
J. AOAC Int
.
96
, 488(2013); DOI: 10.5740/jaoac.int.SMPR2012.010
AOAC SMPR 2012.013
J. AOAC Int
.
96
, 492(2013); DOI: 10.5740/jaoac.int.SMPR2012.013
Posted: December 2014
Candidates for 2016 Method of the Year
316