Hello all!

I am planning to use an EZ:faast kit (https://www.phenomenex.com/Products/AminoAcidDetail/EZfaast) to analyze, among other compounds, 5-hydroxyindoleacetic acid (5HIAA) from whole blood on a single quadrupole ESI LC/MS (Shimadzu LCMS-2020). The EZ:faast kit utilizes a propyl chloroformate derivatization.

I expect to get a molecular ion with 319 m/z for the derivatization product of 5HIAA with propyl chloroformate. When I derivatize and run 5HIAA standards of increasing concentration, I do get a linearly increasing peak at 319 m/z, but I also get a larger peak with a linear increase at 337 m/z at the same retention time. Given the size of the molecule that I expect to arise from the derivatization of 5HIAA with propyl chloroformate, I cannot figure out how this peak at 337 m/z came to be.

Does anyone have any ideas for how derivatizing 5HIAA with propyl chloroformate could yield a MS peak at 337 m/z? Is this even a reasonable way to analyze 5HIAA?

Background:

I am basing my expectations for a 319 m/z for the derivatized 5HIAA molecular ion on Naccarato et al 2014, where they derivatized 5HT with propyl chloroformate to yield a derivative with both the alcohol and amine groups altered, to yield a derivative product molecular ion with 348 m/z:

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Because I know that propyl chloroformate will affect carboxyl groups as well, I am therefore expecting that only the carboxyl and alcohol groups will be affected by the propyl chloroformate derivatization, which would mean a molecule with MW of 319 for the derivitized product.

At this point, I have run standards of increasing concentration for 5HIAA, 5HTP, and 5HT (which are two of the other compounds I am analyzing). For 5HTP and 5HT, I get linear increases for the expected 434 m/z and 348 m/z, respectively, both of which are the base peaks.

Note: Please forgive any poor usage of chemistry terminology. I'm not what anyone would call an organic chemist!

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