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© 2015 AOAC INTERNATIONAL
C
= [(
m
×
P
×
Mo
)/
V
] × [
MW
/
MW
(salt)
]
(1)
where
C
= concentration of standard solution in mg/L;
m
= exact
mass of primary standard weighed in mg;
P
= purity of standard
expressed as a decimal equal to % purity/100;
Mo
= additional
purity correction for moisture/water (if applicable). Expressed as a
decimal equal to (100 – % moisture)/100;
V
= volume of solution in
L; MW = molecular weight of target analyte; MW
(salt)
= molecular
weight of analyte as salt.
(
2
)
Internal standard
.—
13
C
2
D
2
fluoroacetic acid (500 mg/L)
.—
Weigh approximately 7.2 mg (corrected for chemical and isotopic
purity) of
13
C
2
D
2
labeled sodium fluoroacetate into a calibrated
10 mL volumetric flask. Add deionized water, make to volume,
and mix until solid is completely dissolved. Transfer to a 15 mL
polypropylene screw cap test tube and cap tightly. Store in the
freezer at –10°C or below. Calculate exact concentration correcting
for purity, moisture (if applicable), and salt using Equation 1.
(
c
)
Intermediate
standards.—
(
1
)
Analyte
.—
Fluoroacetatic
acid intermediate standard (50 mg/L)
.—Dilute 2.5 mL (calculate
exact volume based on concentration of secondary standard using
Equation 2) of fluoroacetic acid secondary standard (1000 mg/L) to
50 mL with deionized water in a calibrated volumetric flask. Use
a calibrated positive displacement pipet. Dispense aliquots of the
standard into 15 mL polypropylene tubes for frozen storage. Store
in the freezer at –10°C or below.
C
2
= (
C
1
×
V
1
)/
V
2
(2)
where
C
2
= concentration of required diluted solution in mg/L;
C
1
= concentration of high standard in mg/L;
V
1
= volume of high
standard required in mL;
V
2
= total volume of diluted solution in
mL.
(
2
)
Internal standard
.—
13
C
2
D
2
fluoroacetic acid standard
(50 mg/L)
.—Dilute 2.5 mL (calculate exact volume based on
concentration of secondary standard using Equation 2) of
13
C
2
D
2
fluoroacetic acid secondary standard (500 mg/L) to 25 mL with
water in a calibrated volumetric flask. Use a calibrated positive
displacement pipet. Dispense aliquots of the standard into 15 mL
polypropylene tubes. Store in the freezer at –10°C or below.
(
d
)
Working
solutions
(WS)
.—(
1
)
Analyte
.—(
a
)
WS1
(1.0 mg/L)
.—Dilute 1000 µL fluoroacetic acid intermediate
standard (50 mg/L) to 50 mL with deionized water in a calibrated
volumetric flask. Dispense aliquots of the WS1 standard into 15 mL
polypropylene tubes. Store in the freezer at –10°C or below. A
thawed and opened tube WS1 can be stored for up to 2 months in a
refrigerator between 2–8°C provided it is resealed and immediately
refrigerated after each use.
(
b
)
WS2 (0.1 mg/L)
.—Dilute 10 mL of WS1 (1.0 mg/L) to
100 mL with deionized water in a calibrated volumetric flask.
Dispense aliquots of the WS2 standard into 15 mL polypropylene
tubes. Store in the freezer at –10°C or below. A thawed and opened
tube WS2 can be stored for up to 2 months in a refrigerator between
2–8°C provided it is resealed and immediately refrigerated after
each use.
Figure 2015.02B. Chromatograms of confirmation ion for recovery 1 (matrix blank; top), recovery 2 (0.1 µg/kg, ≈LOQ; middle),
and recovery 4 (1.0 µg/kg, LOR; bottom).
Candidates for 2016 Method of the Year
22