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(
b
)
Native stock solutions.--
(
1
)
Choline hydroxide (approximately 3000 µg/mL).--
Store at 2-8°C.
Expiration: 1 month (currently trying to extend to 1 year). (
a
) Dry approximately 200 mg choline
chloride USP Reference Standard in a vacuum oven at 65°C (±3°C) for 4 h. (
b
) Weigh 170 (±17) mg into
an appropriate weighing boat. (
c
) Quantitatively transfer into a 50 mL volumetric flask. (
d
) Dissolve and
bring to volume with laboratory water.
(
2
)
L-Carnitine (approximately 750 µg/mL).--
(
a
) Weigh 150.0 ± 8 mg into an appropriate weighing boat
as quickly as possible. (
b
) Transfer into a 200 mL volumetric flask. (
c
) Dissolve and bring to volume with
laboratory water. (
d
) Correct weight for moisture content. (
e
) Aliquot the stock solution into 2 mL
plastic vials with each vial receiving at least 1 mL and store ≤-15°C. Expiration: 1 year.
(
c
)
Internal standard (IS) stock solutions.--(1) Choline chloride-d
9
(approximately 20,000 µg/mL).--
(
a
)
Transfer 1 g choline chloride-d
9
into a 50 mL volumetric flask. (
b
) Dissolve and bring to volume with
laboratory water. (
c
) Aliquot the stock solution into 2 mL plastic vials with each vial receiving about 1
mL and store at ≤-15°C. Expiration: 1 year.
(2) L-Carnitine chloride-d
3
HCl (approximately 10,000 µg/mL).--
(
a
) Transfer 0.5 g L-carnitine chloride-d
3
to a 50 mL flask. (
b
) Dissolve and bring to volume with laboratory water. (
c
) Aliquot the stock solution
into 2 mL plastic vials with each vial receiving about 1 mL and store at ≤-15°C. Expiration: 1 year.
(
d
)
Mixed intermediate internal standard (MIX-IS) solution
.--Prepare fresh on day of analysis. Must use
same MIX-IS solution for both calibration working standards (WS) and samples.
(
1
) Transfer 400 µL of each choline chloride-d
9
and 400 µL carnitine chloride-d
3
HCl stock solution by a
calibrated pipet into a 10 mL volumetric flask.
(
2
) Bring to volume with laboratory water and mix well.
(
e
)
Intermediate working standards (IWS).
–Prepare fresh on day of analysis.
(
1
) Pipet 100 µL MIX-IS into three individual 50 mL volumetric flasks.
(
2
) Using Table
2014.04A
, add the required volumes of the stock choline and carnitine solution by
proper sized calibrated pipets.
(
3
) Bring to volume with laboratory water and mix well.
(
f
)
WS solution.
–Prepare fresh on day of analysis. Use 90% acetonitrile in final working solution.
(
1
) Pipet 200 µL of each IWS (IWS 1-3) into three 10 mL volumetric flasks.
(
2
) Add 800 µL laboratory water to each volumetric flask.
(
3
) Bring to volume with acetonitrile and mix well.
(
4
) Transfer to autosampler vials for analysis.
Candidates for 2016 Method of the Year
313