374 / 471
(
q
)
Pipets.—
5-25, 50-250, and 100-1000 μL adjustable.
(
r
)
Pipet tips.
(
s
)
pH meter.
(
t
)
Volumetric flasks
.--10, 50, 100, and 250 mL.
C. Reagents (Solvents and Chemicals)
(
a
)
Acetonitrile.
—Fisher Optimal (Fisher Scientific, Fair Lawn, NJ)
LC-MS grade, or equivalent.
(
b
)
Nitric acid.
—69-70% reagent grade (J.T. Baker, Avantor Center Valley, PA, or equivalent).
(
c
)
Ammonium acetate.
—Fluka (Sigma-Aldrich, St. Louis, MO)
LC-MS additive grade or equivalent.
(
d
)
Ammonium hydroxide.
—28–30% reagent grade (for adjusting pH in mobile phase also; check before
use; MACRON, Avantor Center Valley, PA, or equivalent).
(
e
)
Deionized laboratory water.
—≥18 Mohm/cm (EMD Millipore, Merck, Darmstadt, Germany, or
equivalent).
(
f
)
pH buffer solutions
.--pH 4.0, 7.0, and 10.0.
D. Standards
(
a
)
L-Carnitine
.--USP Reference Standard Cat. No. 1359903; store desiccated.
(
b
)
L-Carnitine-d
3
HCl (methyl-d
3
)
.--Cambridge Isotope Laboratories Inc. (Andover, MA) DLM-1871-0.1;
store refrigerated.
(
c
)
Choline chloride
.--USP Reference Standard Cat. No. 1133547.
(
d
)
Choline chloride-d
9
(trimethyl-d
9
98%)
.--DLM-549-1 (Cambridge Isotope Laboratories Inc.).
E. Solution Preparation
(
a
)
Mobile phase.—
(
1
)
Mobile phase A.
—10 mM ammonium acetate [water-acetonitrile (95 + 5, v/v)].
Quantitatively transfer 770 (±15) mg ammonium acetate into a 1 L glass bottle and dissolve with 950
mL laboratory water. Adjust pH up to 8.2-8.6 by ammonium hydroxide,
C
(
d
). Record the amount of
ammonium hydroxide added. Add 50 mL acetonitrile to the bottle and mix well. Store at room
temperature. Expiration: 1 week.
(
2
)
Mobile phase B.
—10 mM ammonium acetate [water-acetonitrile (5 + 95, v/v)]. Weigh 770 (±15) mg
ammonium acetate into a 100 mL beaker. Dissolve with 50 mL laboratory water. Add same amount of
ammonium hydroxide as added into mobile phase A and then mix well. Quantitatively transfer the 50
mL solution into a 1 L glass bottle. Add 950 mL acetonitrile into the bottle and mix well. Store at room
temperature. Expiration: 2 weeks.
Candidates for 2016 Method of the Year
312