51 / 63
1446
K
oshy
et al
.:
J
ournal of
AOAC I
nternational
V
ol
.
99, N
o
.
6, 2016
rotary evaporator until the volume is less than 40 mL. Cool the
solution, transfer quantitatively to a 50 mL volumetric flask and
bring to 50 mL with methanol. Filter through a 0.45 micron
membrane filter.
(b)
Standardized (common) extract
.—Accurately weigh
a sample quantity of
W. somnifera
extract equivalent to 5 mg
(~0.5 g is sufficient) of withanoside IV, withanosideV, withaferin
A, 12-deoxywithastromonolide, withanolide A, and withanolide
B in a 250 mL beaker. Extract the standardized extract with
100 mL methanol, boil in a water bath for 10–15 min, and
repeat this procedure three to four times until the raw material
has been completely extracted or the extracts become colorless.
Combine all extracts and evaporate the methanol on a water
bath or by using a vacuum rotary evaporator until the volume
is less than 40 mL. Cool the solution, transfer quantitatively to
a 50 mL volumetric flask and bring to 50 mL with methanol.
Filter through a 0.45 micron membrane filter.
F. Analysis
(a)
Chromatographic conditions
.—(
1
)
Column
.—Phenomenex
Luna C18(2), 250×4.6 mm, 5 μm particle size (Part No. 00G-
4252- E0).
(
2
)
Temperature
.—Maintained constant between 20 and
30°C (preferably 27°C).
(
3
)
Detector
.—SPD-M 10Avp PDA or UV detector.
(
4
)
Wavelength
.—227 nm.
(
5
)
Flow rate
.—1.5 mL/min.
(
6
)
Run time
.—45 min.
(
7
)
Injection volume
.—20 μL.
(
8
)
Peak integration
.—Base-to-base.
(
9
)
Gradient
.—
See
Table
2015.17
.
(b)
Procedure
.—(
1
) Inject 20 μL mixed standard
preparations in triplicate at three different concentrations:
50, 100, and 150 μg/mL.
(
2
) Inject 20 μL of each test solution in duplicate.
(c)
System suitability
.—Verify that the following system
suitability requirements are met with each run. If the system
suitability requirements are not met, adjust the composition
of the mobile phase or use a new LC column to meet system
suitability before analyzing samples.—(
1
)
Repeatability
.—
The RSD of the peak areas from the triplicate injections of the
50 μg/mL mixed standard preparation must be ≤2.0% for each
withanolide.
(
2
)
Retention times
.—The relative retention times of the
standards should be 0.70 for withanoside IV, 0.89 for withanoside
V, 0.92 for withaferin A, 0.96 for 12-deoxywithastramonolide,
1.0 for withanolide A, and 1.15 for withanolide B.
(
3
)
Resolution
.—Calculate
the
resolution
between
withanoside V and withaferin A peaks in the 50 μg/mL mixed
standard preparation as follows:
= × −
+
R 2 T2 T1
W1 W2
where T1 and T2 are the retention times of withanoside V
and withaferin A, respectively; and W1 and W2 are their peak
widths measured at the baseline between tangents drawn to the
sides of the peak. The resolution between withanoside V and
withaferin A should be ≥3.0.
(
4
)
Tailing
.—Calculate the tailing factor (F) for each
withanolide in the 50 μg/mL mixed standard preparation as
follows:
L R
L
= +
F
2
where
L
is the width (measured at 5% maximum peak height)
from the front slope of the peak to the center line and
R
is the
width (measured at 5% maximum peak height) from the center
line to the back slope of the peak. The tailing factor must be
≤1.5 for all individual withanolides.
(
5
)
Coefficient of determination
.—Plot peak area versus
concentration for the mixed standard preparations in the
range of 50 to 150 μg/mL. The r
2
for the regression line of peak
area versus concentration for each withanolide must be ≥0.998.
(d)
Calculation of withanolide content.
—(
1
) Calculate the
percentage of withanoside IV, withanoside V, withaferin A,
12-deoxywithastramonolide, withanolide A, and withanolide
B content from the mean peak areas of the duplicate test
solution injections and the triplicate mixed standard preparation
injections producing the most similar peak areas using the
formula:
(
)
( )
( )
( )
( )
( )
=
×
×
×
Individual withanolide %w w Mean peak area of sample
Mean peak area of standard
Weight of standard mg
Final standard preparation volume mL
Final test solution volume mL
Sample weight mg
Purity of standard %
(
2
) Alternatively, calculate the percentage of eachwithanolide
from the mean peak areas of the duplicate test solution injections
and the plots of peak area versus concentration from Section
F(c)(
5
)
using the formula:
b
m
(
)
( )
( )
( )
=
−
×
×
Individual withanolide %w w Mean peak area of sample
Final test solution volume mL
Sample weight mg
Purity of standard %
where
m
is the slope of the plot and
b
is the
y
-intercept.
Specificity
Specificity was assessed by spectral similarity, by determining
the resolution between each peak, and by checking peak purity
using a PDAdetector. Reference standard solutions of withanoside
IV, withanoside V, withaferin A, 12-deoxywithastramonolide,
Table 2015.17. Gradient
Time, min
Solvent A %
Solvent B %
0.01
95.0
5.0
18.0
55.0
45.0
25.0
20.0
80.0
28.0
20.0
80.0
35.0
55.0
45.0
40.0
95.0
5.0
45.0
95.0
5.0
51