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Alkylation is one of the methods used to prepare amines. Direct alkylation of ammonia or a primary amine with an alkyl halide gives polyalkylated amines along with a quaternary ammonium salt through successive SN2 reactions. This process of making the quaternary salt through the direct alkylation method is called exhaustive alkylation.
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Back to basic: Using ammonium hydroxide to improve peptide epimer/isomer liquid chromatography separations.

Umang Dhaubhadel1, Arzoo Patel1, Daniel W Armstrong1

  • 1Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, TX, 76019, USA.

Analytica Chimica Acta
|October 24, 2025
PubMed
Summary
This summary is machine-generated.

Ammonium hydroxide (AH) improves the separation and detection of peptide epimers/isomers in liquid chromatography-mass spectrometry. This volatile basic additive enhances resolution and ionization, offering an alternative to traditional acidic mobile phases for peptide analysis.

Keywords:
Ammonium hydroxideCharge stateLiquid chromatography mass spectrometryPeptide epimersPeptide isomersRetention order change

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Area of Science:

  • Analytical Chemistry
  • Biochemistry
  • Chromatography

Background:

  • Peptide epimers and isomers, including d-amino acid and iso modifications, are crucial for disease research and diagnostics.
  • Separating these structurally similar peptides with identical masses presents analytical challenges.
  • Traditional methods use acidic mobile phases (formic acid, trifluoroacetic acid) for chromatography, which may not optimize mass spectrometry ionization.

Purpose of the Study:

  • To investigate the efficacy of ammonium hydroxide (AH) as a mobile phase additive for liquid chromatography (LC) separation of peptide epimers/isomers.
  • To evaluate AH's impact on peptide ionization and chromatographic resolution compared to conventional acidic additives for mass spectrometry (MS) detection.

Main Methods:

  • Exploration of ammonium hydroxide (AH) as an alternative additive in liquid chromatography (LC) for peptide epimer/isomer separation.
  • Testing various peptide epimers/isomers with different chain lengths and side chain properties.
  • Utilizing mass spectrometry (MS) for detection and analysis of chromatographic separation.

Main Results:

  • Ammonium hydroxide significantly increased base peak intensities (1.06–6.56 fold) in MS1 compared to acidic additives.
  • Enhanced resolution of peptide epimers/isomers was observed using AH.
  • Altered retention orders and improved resolution were noted with AH, sometimes leading to shorter run times.
  • Differences in charge state distribution were observed for certain peptide sets.

Conclusions:

  • This study is the first to demonstrate ammonium hydroxide's utility in enhancing peptide epimer/isomer separation.
  • Mobile phase additive choice, such as AH, can alter isomer retention order, facilitating purification and impurity separation.
  • The findings offer a new approach applicable to a broader range of peptide epimers and isomers.