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I use to run a method to analyse all the amino-acids in a food sample. For that I have to hydrololyse the sample and in the last stage of the method I read the amino acids with a ion exchange chromatography. However, when using this method, due to the hydrolysis, the glutamine transforms into glutamic acid, thus in my result I get a big peak for glutamic acid and I cannot read glutamine.

My question is if there's a method to quantify glutamine in a food sample.

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A Google search for glutamine assay reveals a number of commercial kits. Some of these refer to food assays. I don't know what the basis of these assays is however. –  Alan Boyd Jan 15 '13 at 21:17
    
Maybe there's an alternate hydrolysis method that doesn't convert glutamine to glutamate? –  user137 Dec 8 '14 at 6:33

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I am not so familiar with enzymatic kits for quantifying glutamine, but most enzyme-based analysis kits come with a number of fairly strong assumptions on the enzymology involved, and in my experience they are difficult to use reliably. Commercial kits are hopeless to troubleshoot since their components are not disclosed --- you don't know what you're working with.

Are you at a clinical chemistry facility? If so you may have access to analyzers used to measure blood biochemistry parameters, and some models measure glutamine, for example this one. These instruments are also based on enzymatic assays, but they are carefully tested since they are used in human medicine, and so I would consider them more reliable than the average Sigma kit.

The best alternative I think is mass spectrometry, or NMR. Glutamine is easy to measure on any LC-MS or GC-MS setup (an old triple-quadrupole will do nicely), it ionizes well in positive mode electrospray, is well retained on a typical HILIC column, and isotope-labeled standards are readily available. I'm no expert in NMR but I know glutamine is measured routinely.

Another caveat with analyzing glutamine is that it decomposes spontaneously in water to form pyroglutamate, with a half life of about two weeks at room temperature, as shown in this paper. But I'm sure you're aware of that.

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