Acetic Acid in Peptides

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

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1 INTRODUCTION 

This chapter provides procedures to be used to determine the amount of acetic acid in peptides. Acetic acid/acetate is a common counter ion in peptide preparations. 

1.1 Method 1 

Strong sodium hydroxide solution: Dissolve 42 g of sodium hydroxide in water, and dilute with water to 100 mL. 

Solution A: Add 0.7 mL of phosphoric acid to 1000 mL of water, and adjust with Strong sodium hydroxide solution to a pH of 3.0. Solution B: Methanol 

Mobile phase: See Table 1. 

Table 1 

Diluent: Prepare a mixture of Solution A and Solution B (95:5). 

Standard solution: Dissolve an accurately weighed quantity of USP Glacial Acetic Acid RS in Diluent to obtain a solution having a known concentration of about 0.1 mg/mL. [Note—The concentration can be adjusted, depending on the amount of acetate or acetic acid expected to be present in the test material.] 

Sample solution: Prepare as directed in the individual monograph. If no direction is given in the individual monograph, the sample concentration can be adjusted so that the mid-range of the amount of acetic acid stated in the specification for the test material corresponds to that of the Standard solution. 

Chromatographic system 

(See Chromatography 〈621〉, System Suitability.) 

Mode: LC 

Detector: UV 210 nm 

Column: 4.6-mm × 25-cm; 5-µm packing L1 

Flow rate: 1.2 mL/min 

Injection volume: 10 µL 

System suitability 

Sample: Standard solution 

Suitability requirements 

Relative standard deviation: NMT 5% 

Retention time of acetic acid: 3–4 min 

Analysis 

Samples: Standard solution and Sample solution 

Calculate the percentage of acetic acid in the portion of test material taken: 

Result = (r_U/r_S) × (C_S/C_U) × 100 

r_U = peak response from the Sample solution 

r_S = peak response from the Standard solution 

C_S = concentration of USP Glacial Acetic Acid RS in the Standard solution (mg/mL) 

C_U = concentration of the Sample solution (mg/mL) 

1.2 Method 2 

Solution A, Solution B, Mobile phase, Diluent, and Chromatographic system: See Triuoroacetic Acid (TFA) in Peptides 〈503.1〉. Standard solution: 0.7 mg/mL of USP Sodium Acetate Trihydrate RS in Diluent 

Calculate the concentration of acetic acid in the Standard solution (C_S), in mg/mL, taken:  

C_S = 0.441 × C  

0.441 = molecular weight conversion factor (60.05/136.08) 

C = concentration of USP Sodium Acetate Trihydrate RS in the Standard solution (mg/mL) 

Sample solution: About 4 mg/mL of the test sample in Diluent. [Note—The sample concentration can be adjusted so that the mid-range of the amount of acetic acid stated in the specification for the test material corresponds to that of the Standard solution.] [Note—Alternatively, the Standard solution and Sample solution can be prepared as described in Method 1.] 

System suitability 

Sample: Standard solution 

Suitability requirements 

Relative standard deviation: NMT 5% 

Analysis: Proceed as described in Method 1. 

2 ADDITIONAL REQUIREMENTS 

USP Reference Standards 〈11〉 

USP Glacial Acetic Acid RS 

USP Sodium Acetate Trihydrate RS