(fen'' il al' a neen).
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C9H11NO2 165.19
l-Phenylalanine     [63-91-2].
Phenylalanine contains NLT 98.5% and NMT 101.5% of l-phenylalanine (C9H11NO2), calculated on the dried basis.
•  Procedure
Sample:  160 mg of Phenylalanine
Blank:  Mix 3 mL of formic acid and 50 mL of glacial acetic acid.
Titrimetric system 
Mode:  Direct titration
Titrant:  0.1 N perchloric acid VS
Endpoint detection:  Potentiometric
Analysis:  Dissolve the Sample in 3 mL of formic acid and 50 mL of glacial acetic acid. Titrate with the Titrant. Perform the Blank determination.
Calculate the percentage of phenylalanine (C9H11NO2) in the Sample taken:
Result = {[(VS VB) × N × F]/W} × 100
VS== Titrant volume consumed by the Sample (mL)
VB== Titrant volume consumed by the Blank (mL)
N== actual normality of the Titrant (mEq/mL)
F== equivalency factor, 165.2 mg/mEq
W== Sample weight (mg)
Acceptance criteria:  98.5%–101.5% on the dried basis
•  Residue on Ignition 281: NMT 0.4%
•  Chloride and Sulfate, Chloride 221
Standard solution:  0.50 mL of 0.020 N hydrochloric acid
Sample:  0.73 g of Phenylalanine
Acceptance criteria:  NMT 0.05%
•  Chloride and Sulfate, Sulfate 221
Standard solution:  0.10 mL of 0.020 N sulfuric acid
Sample:  0.33 g of Phenylalanine
Acceptance criteria:  NMT 0.03%
•  Iron 241: NMT 30 ppm
•  Heavy Metals, Method I 231: NMT 15 ppm
•  Related Compounds
Diluent:  Glacial acetic acid and water (1:1)
Standard solution:  0.05 mg/mL of USP l-Phenylalanine RS in Diluent. [Note—This solution has a concentration equivalent to 0.5% of that of the Sample solution. ]
System suitability solution:  0.4 mg/mL each of USP l-Phenylalanine RS and USP l-Tyrosine RS in Diluent
Sample solution:  10 mg/mL of Phenylalanine in Diluent
Chromatographic system 
Mode:  TLC
Adsorbent:  0.25-mm layer of chromatographic silica gel mixture
Application volume:  5 µL
Developing solvent system:  Butyl alcohol, glacial acetic acid, and water (3:1:1)
Spray reagent:  2 mg/mL of ninhydrin in a mixture of butyl alcohol and 2 N acetic acid (95:5)
System suitability 
Suitability requirements:  The chromatogram of the System suitability solution exhibits two clearly separated spots.
Samples:  Standard solution, System suitability solution, and Sample solution
After air-drying the plate, spray with Spray reagent, and heat between 100 and 105 for 15 min. Examine the plate under white light.
Acceptance criteria:  Any secondary spot of the Sample solution is not larger or more intense than the principal spot of the Standard solution.
Individual impurities:  NMT 0.5%
Total impurities:  NMT 2.0%
•  Optical Rotation, Specific Rotation 781S
Sample solution:  20 mg/mL in water
Acceptance criteria:  32.7 to 34.7
•  pH 791
Sample solution:  10 mg/mL solution
Acceptance criteria:  5.4–6.0
•  Loss on Drying 731: Dry a sample at 105 for 3 h: it loses NMT 0.3% of its weight.
•  Packaging and Storage: Preserve in well-closed containers.
•  USP Reference Standards 11
USP l-Phenylalanine RS Click to View Structure
USP l-Tyrosine RS Click to View Structure
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Huy T. Dinh, M.S.
Scientific Liaison
(DS2010) Monographs - Dietary Supplements
Reference Standards RS Technical Services
USP35–NF30 Page 4296