423 / 471
© 2013 AOAC INTERNATIONAL
ܫ
ܷ/݉
ܮ
=
ܣ
ܾ
ݏ
43.6 × 100 3 × 10
where
Abs
= average absorbance reading, determined above;
43.6 = extinction coefficient of tocopheryl acetate at 284 nm; and
10 = conversion of percent to mg/mL. (
Note
: For dl-α-tocopherol
acetate, 1 mg = 1 IU.)
(
d
) dl
-α-Tocopherol
.—(
1
) Pipet 3 mL stock solution E2 into a
l00 mL volumetric flask and make up to volume with iso-octane.
(
2
) Determine the absorption at 292 nm, zeroed against
iso-octane in a 1 cm quartz cell.
(
3
) Repeat the reading twice, rinsing the sample cuvet with the
solution before each reading. Calculate the average absorbance
reading. The potency of the α-tocopherol stock solution is then
calculated as follows:
ܫ
ܷ/݉
ܮ
=
ܣ
ܾ
ݏ
75.8 × 100 3 × 10 × 1.1
where
Abs
= average absorbance reading, determined above; 75.8
= extinction coefficient of α-tocopherol at 292 nm; 10 = conversion
of percent to mg/mL; and 1.1 = conversion from mg to IU for dl-
α-tocopherol.
(
e
)
Calculation of working standard concentrations
.—The
concentration of each vitamin in the working standards is calculated
from the stock concentration using the appropriate dilution factor,
as shown (IU/mL), Section
F
. Refer to the example calculations
below.
(
1
)
Level 1 calibration standard
.
10
2
50
5.0
50
4
50
4
/
) /
(
Palmitate
A Vitamin
u u u u
mL IU Stock
mL IU
10
2
50
5.0
50
4
50
4
/
) /
(
Acetate
A Vitamin
u u u u
mL IU Stock
mL IU
10
2
50
5.0
50
4
50
7
/
) / (
u u u u
mL IU Stock
mL IU Acetate
Tocopherol
DL
D
10
2
50
5.0
50
4
50
20
/
) / (
u u u u
mL IU Stock
mL IU
Tocopherol
DL
D
Figure 2012.10B. VitaminApalmitate sample chromatograms.
Figure 2012.10C. VitaminA acetate sample chromatograms.
Candidates for 2016 Method of the Year
359