Ninhydrin Test: How Ruhemann's Purple Detects Amino Acids and Free Amines in Peptide Work

Short answer: The ninhydrin test is a colour reaction in which ninhydrin reacts with primary amino groups to form a purple dye now called Ruhemann's purple; the reaction was discovered by Siegfried Ruhemann in 1910 [1]. Because the dye contains only the primary amino nitrogen of the reacting compound, essentially every amino acid with a free primary amine gives the same product, which can be measured by spectrophotometry at 570 nm [2]. In peptide chemistry the same reaction underlies the Kaiser test, a colour check for free N-terminal amines on resin during solid-phase synthesis [6][8].

This article is educational analytical chemistry for laboratory readers. It does not describe effects in people or animals and is not guidance for human or animal use.

What the ninhydrin reaction detects

Ninhydrin's best-known partner is the primary amino group, and Friedman's review notes that the reaction is unusual among colour reactions because, at pH 5.5, all reacting primary amines produce the same soluble chromophore, whether they are simple amines, amino acids, peptides, proteins or even ammonia [1]. Stauß and co-workers explain why: the final reaction product contains only the primary amino nitrogen of the amino acid, so in theory all primary amino compounds should give the same response [2].

The reaction is not limited to primary amines. Friedman lists other groups that also react to form chromophores of analytical interest, including imines such as proline and pipecolic acid, the guanidino group of arginine, the amide group of asparagine, the indole ring of tryptophan, the sulfhydryl group of cysteine, and cyanide ions [1]. (For one-letter and three-letter residue names, see our amino acid codes reference.)

Two practical features follow from the chemistry, according to Friedman [1]:

The dye is soluble and not bound to the sample. Because the chromophore is not chemically attached to the protein or other insoluble material, it stays in solution when solids are removed by centrifugation or filtration after the reaction.
The colour is distinctive. The purple colour is generally not affected by the yellow colours found in many food, plant and tissue extracts.

What happens in the tube

A ninhydrin assay is not just "add reagent and look." Published methods vary in buffer, organic solvent, temperature and timing, and Stauß and co-workers describe the main ingredients as follows [2]:

An organic co-solvent. Ninhydrin is poorly water-soluble, so reagents mix an aqueous buffer with an organic solvent; 2-methoxyethanol, phenol, sulfolane and dimethyl sulfoxide (DMSO) have all been used.
An acetate buffer. Earlier publications used acetic acid with lithium or sodium acetate.
A reducing component. A small amount of ninhydrin must be reduced to hydrindantin for an efficient reaction. Tin(II) chloride, cyanide, sodium borohydride and ascorbic acid have been used to do this, each with drawbacks, or pure hydrindantin can be added directly.
Heat. The mixture must be heated for Ruhemann's purple to form. The most frequently used condition is 100 °C for 10 to 30 minutes, although lower temperatures, down to 60 °C for 30 minutes, have also been reported.

In their own systematic optimisation, using asparagine as a model amino acid, Stauß and co-workers settled on 0.8 mol/L potassium acetate, 1.6 mol/L acetic acid, 20 mg/mL ninhydrin and 0.8 mg/mL hydrindantin in DMSO/acetate buffer 40/60 (v/v), heated at 90 °C for 45 minutes, then diluted with 2-propanol/water and read at 570 nm [2]. They chose 90 °C because at 100 °C the colour peaked at 30 minutes and then declined, making timing critical, while at 90 °C there was a broad optimum from about 30 to at least 60 minutes [2]. They also found that an acetic acid to acetate ratio of about 2:1 worked best, higher than the ratios used in most earlier protocols [2].

Recorded spectra of the product showed two absorbance maxima, at 404 nm and 570 nm; the positions did not move as buffer acidity changed, which is why 570 nm is suitable for measurement [2].

Do all amino acids respond equally?

Mostly, but not entirely. When Stauß and co-workers compared the proteinogenic amino acids under their optimised conditions [2][3]:

Most amino acids gave a response highly similar to asparagine [2].
Proline gave only a very slight response, because its amino group is secondary [2][3].
Lysine gave roughly twice the response, because it carries two primary amino groups [2][3].
Cysteine gave a significantly lower response; the ninhydrin reaction proceeds differently for this amino acid [2].
Ammonia reacted about as strongly as most amino acids, so ammonium in a sample also contributes colour [2].

The same group's later work on protein hydrolysates adds that asparagine and glutamine, which are hydrolysed to ammonia plus aspartic and glutamic acid, also show roughly double the response after hydrolysis [3].

These deviations matter whenever the test is used for quantification. A ninhydrin result is a measure of total reactive amino groups, not of any single compound.

How sensitive and reproducible is it?

In the optimised method, Stauß and co-workers report a highly linear response to an absorbance above 2, a limit of detection of 0.03 mmol/L and a limit of quantification of 0.1 mmol/L, determined following the ICH Q2(R2) guideline [2]. Intra-day relative standard deviations were 0.5% to 2.4% and inter-day values 1.7% to 2.1% across tomato juice, potato extract and soy sauce, and spike recoveries were 100% to 105% [2].

From colour test to amino acid analyser

Friedman's review notes that both manual and automated ninhydrin reactions have been developed for analysis [1]. Two early papers by Moore, Stein and colleagues:

1954: Moore and Stein published a modified ninhydrin reagent for the photometric determination of amino acids and related compounds [4].
1958: Moore, Spackman and Stein described an automatic recording apparatus for use in the chromatography of amino acids [5].

Stauß and co-workers note that amino acid analysers are still frequently used for free amino acids; they are reliable, but sensitivity is low and run times are typically more than 30 minutes per sample [2]. Common alternatives use reversed-phase or HILIC HPLC after pre-column derivatisation with reagents such as phenyl isothiocyanate, o-phthaldialdehyde, AQC, dansyl chloride or Sanger's reagent [2].

For total protein, the ninhydrin assay works on hydrolysed samples: the protein is broken down with hydrochloric acid or sodium hydroxide to release free amino acids, which are then reacted with ninhydrin [3]. Alcock and co-workers describe it as more selective than the Kjeldahl method and elemental analysis, which measure total nitrogen, because ninhydrin reacts with primary amines including α-amino acids but not with nitrate and nucleic acids [3]. They established conversion formulas from ninhydrin results to protein content and report that the ninhydrin approach gave more accurate results than the Kjeldahl assay did when the latter used the default conversion factor of 6.25 [3]. (Why harsh conditions are needed to break the chain is covered in our peptide hydrolysis guide.)

The Kaiser test in solid-phase peptide synthesis

In solid-phase peptide synthesis, each new amino acid is coupled to the deprotected N-terminal amine of the resin-bound chain, and unreacted N-terminal amine left after a coupling step signals that the coupling is incomplete [8]. Kaiser and co-workers published a colour test for detecting free terminal amino groups in solid-phase peptide synthesis in 1970 [6], and Sarin, Kent, Tam and Merrifield later described quantitative monitoring of solid-phase synthesis by the ninhydrin reaction [7].

AAPPTec's technical guide describes the Kaiser test as a very sensitive test for primary amines that is commonly used to decide whether coupling reactions are complete: ninhydrin reacts with the deprotected N-terminal amine on the peptide-resin to produce an intense blue colour [8]. In their procedure, 10 to 15 resin beads and an empty reference tube each receive a few drops of ninhydrin, phenol and potassium cyanide/pyridine reagents and are heated at 110 °C for 5 minutes [8]. The guide reads the result like this [8]:

Colourless or faint blue: coupling complete; proceed.
Dark blue solution but colourless beads: nearly complete; extend coupling or cap unreacted chains.
Light blue solution but dark blue beads: coupling incomplete; recouple.
Intense blue solution and all beads blue: coupling failed; check the amino acid and reagents, then recouple.

If free amine remains after a second coupling, AAPPTec recommends capping it with acetic anhydride, which blocks further reaction at those sites and prevents deletion peptide impurities from forming [8]. A significant amount of unreacted amine also suggests the peptide is likely aggregating [8]. (For what the N-terminal amine is, see N-terminus vs C-terminus.)

Known limitations of the Kaiser test

Secondary amines. The Kaiser test is not reliable for secondary amines. When the N-terminal residue is proline, pipecolic acid or tetrahydroisoquinoline-3-carboxylic acid, AAPPTec recommends the isatin test or the chloranil test instead [8]. On resin, N-terminal proline gives a less intense red-brown colour rather than blue [8].
False positives from Fmoc loss. The Fmoc protecting group is not completely stable under Kaiser test conditions; excess heating can remove Fmoc through reaction with the pyridine in the reagents, giving a false positive [8].
Alternatives that also detect proline. The bromophenol blue and 2,4,6-trinitrobenzenesulfonic acid tests rely on an acid-base reaction with the basic amino group and detect secondary amines such as proline or N-methyl amino acids as well as primary amines [8].

Vojkovsky re-examined the chloranil test for secondary amines on solid phase and reported a procedure about ten times more sensitive than the original, detecting secondary amino groups reliably down to 2.8 µeq/g substitution (with N-(p-tolyl)glycine as the exception) [9].

Beyond the lab bench: fingerprints and other fields

Friedman's review lists applications of ninhydrin reactions across agricultural, biochemical, clinical, environmental, food, forensic, histochemical, microbiological, medical, nutritional, plant and protein sciences [1]. The forensic use dates to 1954, when Odén and von Hofsten reported the detection of fingerprints by the ninhydrin reaction in Nature [10].

Ninhydrin test vs other amine and protein methods

Ninhydrin: same chromophore from all reacting primary amines; read at 570 nm; proline, cysteine and lysine deviate [1][2].
o-Phthaldialdehyde (OPA) fluorescence: highly sensitive and simple, but the isoindole products are unstable and responses across amino acids are only moderately uniform [2].
TNBS: forms an orange product read at 420 nm, or a derivative at 340 nm that overlaps with the by-product picric acid [2].
Kjeldahl and elemental analysis: measure total nitrogen and need conversion factors to estimate protein [3].
UV absorbance: for a purified peptide, concentration can instead be calculated from absorbance and the molar extinction coefficient, without a colour reaction.

Frequently asked questions

What colour is a positive ninhydrin test?
In solution, reacting primary amines form the purple dye Ruhemann's purple [1][2]. On peptide-resin in the Kaiser test, a free N-terminal amine gives an intense blue [8].

Why does proline not turn purple?
Proline has a secondary amino group, so it gives only a very slight response in the standard reaction [2][3]; on resin, N-terminal proline gives a weaker red-brown colour, and the isatin or chloranil test is recommended instead [8].

What wavelength is used to measure Ruhemann's purple?
570 nm. The product also has a second maximum at 404 nm [2].

Does ammonia give a positive ninhydrin test?
Yes. Ammonia reacts to form Ruhemann's purple with a response similar to most amino acids [1][2].

Can the ninhydrin test tell which amino acid is present?
No. Because all primary amino compounds give the same dye, the colour reports total reactive amino groups; identifying individual amino acids requires separation first, as in chromatographic amino acid analysis [2][5].

Bottom line

The ninhydrin test turns free amino groups into a single purple dye, Ruhemann's purple, which is why it has served for more than a century as a general detector of amino acids, peptides and amines [1][2]. Its strengths are a uniform chromophore and a distinctive colour; its known exceptions are proline, cysteine, lysine and ammonia [2][3]. In peptide synthesis it lives on as the Kaiser test, a fast on-resin check for unreacted N-terminal amines, best paired with a proline-appropriate test and careful heating [6][8].

All products are for research use only — not for human or animal consumption.