A simple dilute-and-shoot LCMS method for the determination of free and modified amino acids in dietary supplements

Importance of topic

Dietary supplements frequently include free and chemically modified amino acids to enhance exercise performance, support recovery, and maintain overall health. Reliable quantitation of these compounds ensures product quality, consumer safety, and compliance with labeling regulations.

Goals and study overview

This work presents a streamlined liquid chromatography–mass spectrometry (LC-MS) method that allows direct determination of all 20 proteinogenic L-amino acids, selected modified amino acids, taurine, and caffeine in tablets, capsules, and powders within a single analytical run. The primary objective was to minimize sample preparation steps while maintaining high sensitivity and specificity.

Methodology and instrumentation

• Sample preparation: Dietary supplement powders, capsules, and tablets were extracted with 0.1 N HCl and then diluted with the initial mobile phase (dilute-and-shoot approach).
• Chromatography: Shimadzu Nexera-i system equipped with a Sigma Aldrich Discovery C18 column (150 × 2.1 mm, 5 µm). Mobile phase A consisted of 0.1 % formic acid in water and B was 0.1 % formic acid in acetonitrile. Gradient elution was performed at 0.2 mL/min, column oven at 40 °C, injection volume 0.5 µL.
• Mass spectrometry: Shimadzu LCMS-2020 single-quadrupole with electrospray ionization in both positive and negative polarities. Fast polarity switching in selected-ion monitoring (SIM) mode monitored predefined m/z values and retention times for each analyte.

Main results and discussion

• Twenty proteinogenic amino acids and six modified forms (including DL-α-hydroxymethionine, ketoisoleucine, ketoleucine, and α-ketovaline), plus taurine and caffeine, were successfully monitored in a single 12-minute run.
• Calibration curves exhibited excellent linearity (r > 0.98) across 0.1–25 µg/mL ranges for most analytes.
• Representative SIM chromatograms demonstrated clear separation of isobaric compounds such as leucine/isoleucine and hydroxyproline.
• Quantitative results for a commercial capsule closely matched label claims, with minor deviations within acceptable limits.

Benefits and practical applications

• The dilute-and-shoot workflow minimizes sample manipulation, preserving analyte integrity and reducing preparation time.
• SIM mode enables unambiguous identification of target analytes even in cases of partial chromatographic coelution.
• The method is easily transferable to routine quality-control laboratories equipped with single-quadrupole LC-MS instruments.

Future trends and opportunities

Further developments could include automation of sample handling for higher throughput, expansion to other biologically relevant small molecules, and integration with high-resolution mass spectrometry for non-targeted screening. Additionally, method adaptation for stability studies and metabolic profiling in complex matrices presents a promising avenue.

Conclusion

A robust, rapid, and user-friendly LC-MS method was established for comprehensive analysis of free and modified amino acids in dietary supplements. The approach combines simple sample preparation, fast chromatography, and selective SIM detection to deliver reliable quantitation and high laboratory productivity.

Reference

• Chitranshi P. and Gilles C.T. Direct determination of free and modified amino acids in dietary supplements using the LCMS-2020 single quadrupole mass spectrometer. Shimadzu Application News SSI-LCMS-111.