Question: In a Russian document I'm translating, an HPLC system is used to analyse the amino acid content of a substance. 

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The detector wavelength is set at 262 nm for "secondary amino acids" and 338 nm for "primary amino acids". But what are those? 

I googled and found a lot of mentions with exactly these wavelengths being used, but I haven't been able to quickly understand how a "primary" amino acid differs from a "secondary" one. Is it that the secondary amino acids are the products of the destruction of the primary amino acids (those that initially existed in the substance)? 

What is the difference between a "primary amino acid" and a "secondary amino acid"?

Answer: 
I would guess that 'secondary' refers to amino acids that have been derivatized in some way, possibly by reacting with [o-phthalaldehyde](https://www.agilent.com/cs/library/slidepresentation/Public/Amino%20Acid%20Analysis_062410_Rita%20Steed.pdf). (To measure the fluorescence of an OPA-derivatized amino acid, you excite at 340 nm, so I presume the derivative will absorb at this wavelength?).
Based on the reference I read for my biochem exam, secondary amino acids are those which are made of primary amino acids by adding some materials to them for ex : 

1. hydroxylysine is made by hydroxilation of lysine
2. cystine is made by integrating two cysteins so cysteine is primary but cystine is secondary
3. etc but in other references the explanation maybe different :)