Cephalexin for Oral Suspension
Cephalexin for Oral Suspension is a dry mixture of Cephalexin and one or more suitable buffers, colors, diluents, and flavors. It contains the equivalent of NLT 90.0% and NMT 120.0% of the labeled amount of C16H17N3O4S per mL when constituted as directed in the labeling.
•  The retention time of the major peak of the Sample solution corresponds to that of the Standard solution, as obtained in the Assay.
•  Procedure
Mobile phase:  0.985 g/L of sodium 1-pentanesulfonate in a mixture of acetonitrile, methanol, triethylamine, and water (20:10:3:170), adjusted with phosphoric acid to a pH of 3.0 ± 0.1
Standard stock solution:  1 mg/mL of USP Cephalexin RS in water
Standard solution:  Mix 10.0 mL of Standard stock solution with 15.0 mL of Mobile phase.
Sample stock solution:  Nominally equivalent to 1 mg/mL of cephalexin from Oral Suspension, constituted as directed in the labeling, freshly mixed and free from air bubbles. Sonicate, if necessary, to assure complete dissolution of the cephalexin. Filter, if necessary, to obtain a clear solution.
Sample solution:  Mix 10.0 mL of Sample stock solution and 15.0 mL of Mobile phase.
Chromatographic system 
Mode:  LC
Detector:  UV 254 nm
Column:  4.6-mm × 25-cm; packing L1 of low acidity
Flow rate:  1.5 mL/min
Injection size:  20 µL
System suitability 
Sample:  Standard solution
Suitability requirements 
Relative standard deviation:  NMT 2.0%
Samples:  Standard solution and Sample solution
Calculate the percentage of cephalexin (C16H17N3O4S) in each mL of the constituted Suspension taken:
Result = (rU/rS) × (CS/CU) × P × F × 100
rU== peak response from the Sample solution
rS== peak response from the Standard solution
CS== concentration of USP Cephalexin RS in the Standard stock solution (mg/mL)
CU== nominal concentration of cephalexin from the Sample stock solution (mg/mL)
P== potency of USP Cephalexin RS (µg/mg)
F== conversion factor, 0.001 mg/µg
Acceptance criteria:  90.0%–120.0%
•  Uniformity of Dosage Units 905 For solid packaged in single-unit containers: meets the requirements
•  Deliverable Volume 698: Meets the requirements
•  pH 791: 3.0–6.0, constituted as directed in the labeling
•  Packaging and Storage: Preserve in tight containers.
•  USP Reference Standards 11
USP Cephalexin RS Click to View Structure
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Ahalya Wise, M.S.
Senior Scientific Liaison
(SM12010) Monographs - Small Molecules 1
Reference Standards RS Technical Services
USP35–NF30 Page 2587
Pharmacopeial Forum: Volume No. 36(1) Page 45