No method wins on every count. Ion-exchange with ninhydrin is the classical reference style; LC-MS/MS reaches the lowest levels and unusual metabolites; AQC and automated OPA/FMOC give fast full profiles on a standard HPLC. We use AQC with UV detection for free amino acids at micromolar to millimolar levels.
The right amino acid method is the one that fits your analytes, your concentrations and your matrix. No single technique wins everywhere. The harmonized pharmacopeial chapter on amino acid analysis describes eight separation-and-detection schemes. It also observes that many labs run more than one to get the benefits of each 1. Below are the five you are most likely to meet.
Five approaches in brief
Ion-exchange with post-column ninhydrin
Amino acids are separated on a cation-exchange column, then mixed with ninhydrin in a heated reactor. Primary amino acids turn purple and are read at 570 nm; proline and other imino acids turn yellow and are read at 440 nm 1. The chapter calls this one of the most common methods for quantitative amino acid analysis 1. EU official control of animal feed still prescribes ion-exchange with post-column derivatization 2. Because the reaction happens after separation, it is generally less affected by buffer components than pre-column methods 1. The cost is time: published comparisons put ion-exchange at one to two hours per plasma sample 3,4.
Pre-column AQC
AQC reacts with primary and secondary amines in one step, forming stable urea derivatives 1,5,6. Excess reagent hydrolyzes within seconds to 6-aminoquinoline, N-hydroxysuccinimide and carbon dioxide 1, and these by-products do not interfere with quantitation 5. Peak areas stay essentially unchanged for at least a week at room temperature 1, which permits batch processing and repeat injections 5. The original report found that buffers and detergents did not reduce derivatization yield 6. Vendor methods for cell culture medium and for beverages both used UV detection at 260 nm, with separations of about 18 and 10 minutes 7,8.
Pre-column OPA with FMOC
OPA plus a thiol gives strongly fluorescent products, but OPA does not react with secondary amines such as proline 1. OPA derivatives are also unstable, so separation must follow immediately 1. Automated methods handle both problems. The autosampler mixes sample, borate buffer, OPA and then FMOC, which tags proline, hydroxyproline and sarcosine, right before injection 9. Agilent reports runs under 20 minutes including re-equilibration, and recommends fluorescence detection below about 100 pmol 9. Thermo describes the same in-needle approach with fluorescence detection 10.
Pre-column PITC
PITC forms phenylthiocarbamyl derivatives with all amino acids, including proline, detected by UV at 254 nm 11. It was described in 1988 as the first realistic alternative to ion-exchange 12. The workflow involves vacuum drying: the volatile excess reagent is removed in vacuo, and dried derivatives keep for several weeks frozen 1,11.
Underivatized LC-MS/MS
Hydrophilic-interaction (HILIC) or mixed-mode columns coupled to tandem mass spectrometry measure amino acids with no derivatization. One HILIC method separated 36 amino acids in plasma (24 quantitative) in 18 minutes and showed good agreement with ion-exchange across 41 paired samples 13. Its limits of quantitation ran from 0.001 to 0.36 µM 13. A mixed-mode method quantified 38 amino acids from 50 µL of plasma or urine in a 15-minute run 3. The HILIC method could not separate isoleucine from allo-isoleucine, and matrix effects ranged from 16.9 % to 358.6 % 13. Both methods lean on isotope-labeled internal standards, and the mixed-mode authors saw bias where a matched standard was missing 3,13. Ion-pairing LC-MS/MS is another route; the HILIC authors cite unstable retention times and reduced MS sensitivity in negative mode as its drawbacks 13.
A 2020 plasma study went further, concluding that ion-exchange should no longer be called the gold standard for plasma amino acids 4. Agreement with ion-exchange varied by analyte and method. Two underivatized LC-MS/MS methods met the authors' concordance threshold for 10 of 21 analytes each. An AQC-derivatized method with a single-quadrupole mass detector met it for 6 4.
Side by side
Detection figures are as stated in the cited sources. Read them as method classes, not any one lab's performance.
| Ion-exchange + ninhydrin | AQC (pre-column) | OPA/FMOC (automated) | PITC (pre-column) | Underivatized LC-MS/MS | |
|---|---|---|---|---|---|
| Proline and other secondary amines | Yes, read at 440 nm 1 | Yes, same reagent 1 | Only through FMOC 1,9 | Yes 11 | Yes, measured directly 3,13 |
| Derivative stability | Formed in-line after separation 1 | At least 1 week, room temperature 1 | OPA unstable; derivatized on the autosampler's clock 1,9 | Several weeks, dry and frozen 1 | No derivative |
| Detection limit class | ~10 pmol; 50 pmol for proline 1 | 40–320 fmol by fluorescence 1; picomole range by UV 14 | ~1 pmol practical for OPA 1 | ~1 pmol 1 | LOQ 0.001–0.36 µM in one plasma method 13 |
| Run time per sample | About 1–2 h 3,4 | ~10–18 min 7,8 | Under 20 min 9 | As short as 10 min, after drying steps 11,12 | 15–18 min 3,13 |
| Matrix tolerance | Tolerates small amounts of salts and urea; least affected by buffers 1,15 | Buffers and detergents did not cut yield in the original study 6; pre-column methods can be salt-sensitive 15 | Pre-column; can be salt-sensitive 15 | Pre-column; can be salt-sensitive 15 | Variable matrix effects; relies on isotope-labeled standards 3,13 |
| Instrument | Dedicated ion-exchange amino acid analyzer 4 | Standard HPLC or UHPLC, UV or fluorescence 7,8 | HPLC with programmable autosampler 9 | HPLC with UV, plus vacuum drying 11 | LC with tandem MS; one vendor rates AQC with UV/fluorescence as simpler and cheaper 7 |
When each method is the right choice
- Ion-exchange with ninhydrin: when you need the reference-style method. It is listed first in the harmonized pharmacopeial chapter and prescribed for EU official feed control 1,2. It also tolerates samples with small amounts of salts, urea or other buffer components 15. Expect long runs.
- LC-MS/MS: when concentrations are very low, volumes are small, or you need metabolites outside a standard panel 3,13. Expect the lab to rely on isotope-labeled internal standards 3,13.
- Automated OPA/FMOC: when you run many samples on an HPLC whose autosampler can execute injector programs, and want fluorescence sensitivity 9.
- AQC: when samples may queue before injection, you want primary and secondary amino acids from one reagent, and a standard HPLC is available 1,5.
- PITC: when a lab already has a validated PTC method and the drying steps fit its workflow 11.
Why we use AQC with UV detection
We measure free amino acids by pre-column AQC derivatization, reversed-phase HPLC on a C18 column, and UV detection at 260 nm. Three properties drove that choice.
One reagent for primary and secondary amines. AQC tags secondary amines such as proline along with the primary amino acids 1,5. Our AA-1 panel covers 19 amino acids, proline included, plus ammonia. The AA-2 extension adds GABA, ornithine, citrulline, taurine and hydroxyproline.
Stable derivatives. Week-long stability at room temperature 1 puts standards, QC samples and your samples on the same footing, and allows re-injection 5.
Detection matched to the working range. UV quantitates most AQC derivatives down to about the picomole level; fluorescence and MRM reach tens of femtomoles 14. Our sample requirement assumes 5 µM to 5 mM per analyte, and we dilute in-house. If you need lower, LC-MS/MS or fluorescence is the better tool, and we will tell you so on a free feasibility call.
The limits are worth stating. We report free amino acids, not total amino acids after hydrolysis; see free versus total amino acids. Sung and colleagues found that cysteine behaves anomalously with AQC across detection modes 14; our cysteine/cystine analysis (AA-3) uses a separate reduction–alkylation workup, on request. Pre-column chemistry can be influenced by salts 15. That is why every sample carries an internal standard, and why we add spiked amino acids where a matrix calls for a recovery check. Every run also includes a QC standard of known concentration, detector sensitivity checks and reagent blanks. Results below the quantitation limit are flagged, not reported as numbers. Our quality control page gives the details.
We are not a GLP, GMP or ISO 17025 laboratory; our work supports research, development and process monitoring. If you need a pharmacopeial or regulated method, say so on the feasibility call and we will tell you whether method development can help. See also how AQC derivatization works, what amino acid analysis costs and our amino acid analysis service.
References
- 〈1052〉 Biotechnology-Derived Articles—Amino Acid Analysis (harmonized text) — United States Pharmacopeia, harmonized with the European and Japanese Pharmacopoeias, 2017 (official 2018).
- Method validation for determination of amino acids in feed by UPLC — Szkudzińska K, Smutniak I, Rubaj J, Korol W, Bielecka G, Accreditation and Quality Assurance 22:247–252, 2017.
- Quantitation of non-derivatized free amino acids for detecting inborn errors of metabolism by incorporating mixed-mode chromatography with tandem mass spectrometry — DeArmond PD, Bunch DR, Journal of Mass Spectrometry and Advances in the Clinical Lab, 2022.
- Challenging the status quo: A comparison of ion exchange chromatography with liquid chromatography–mass spectrometry and liquid chromatography–tandem mass spectrometry methods for the measurement of amino acids in human plasma — Carling RS, McDonald BAC, Austin D, et al., Annals of Clinical Biochemistry, 2020.
- Derivatization of Amino Acids Using Waters AccQ•Tag Chemistry — Waters Corporation, Comprehensive Guide to Hydrolysis and Analysis of Amino Acids (primer), accessed 2026.
- Synthesis of a fluorescent derivatizing reagent, 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate, and its application for the analysis of hydrolysate amino acids via high-performance liquid chromatography — Cohen SA, Michaud DP, Analytical Biochemistry 211:279–287, 1993.
- Amino acid analysis of mammalian cell culture medium by liquid chromatography with UV and fluorescence detection and derivatization with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AN 73057) — Lovejoy K, Fabel S, Pietsch M, Park SH, Steiner F, Thermo Fisher Scientific application note, 2019.
- Determination of Free Amino Acids Content in Alcoholic and Non-Alcoholic Beverages Using the AccQ•Tag Ultra Derivatization Kit — Barknowitz, Henry, Gould, Foddy, Waters Corporation application note, 2022.
- Agilent Biocolumns Amino Acid Analysis "How-To" Guide (5991-7694EN) — Agilent Technologies, 2020.
- Automated in-needle OPA/FMOC derivatization of amino acids analysis with the Thermo Scientific Vanquish Core HPLC system (AB-004093) — Thermo Fisher Scientific application brief, 2025.
- PITC (Edman's Reagent) product instructions — Thermo Fisher Scientific, 2012.
- Amino acid analysis utilizing phenylisothiocyanate derivatives — Cohen SA, Strydom DJ, Analytical Biochemistry 174:1–16, 1988.
- Rapid quantification of underivatized amino acids in plasma by hydrophilic interaction liquid chromatography (HILIC) coupled with tandem mass-spectrometry — Prinsen HCMT, Schiebergen-Bronkhorst BGM, Roeleveld MW, et al., Journal of Inherited Metabolic Disease, 2016.
- A Closer Examination of 6-Aminoquinolyl-N-Hydroxysuccinimidyl Carbamate Amino Acid Derivatization in HPLC with Multiple Detection Modes — Sung Y-S, Berthod A, Roy D, Armstrong DW, Chromatographia, 2021.
- Analysis of Amino Acids — International Council on Amino Acid Science, accessed 2026.
OpenChemWorks Laboratory · Reviewed by the laboratory director, PhD chemist · Published September 23, 2026