Thank you Anne and Bill for another great tips on Amino Acids Analysis.
I hope to see more tips and trick on sample preparation next time.
Working with animals feed myself required me to homogenize the samples with a blender and digest them in 6M HCl for 24 hours, and finally re-constituted the dry sample in 0.1M HCl prior to injection.
However, since I worked with Cysteine, I have been using DTDPA myself. As pointed out in application notes (both Zorbax Eclipse Plus and HPH C18), borate buffer (0.4N, pH 10.2) should be used to prevent solubility problem. I tried with both HCl and borate buffer, no crystallization/particulate formed after filtration with 0.2 um RC filter. However, letting samples stand at room temperature for a few hours, or thawing samples from -20'c freezer always see formation of precipitate. I have to filter them again before injection. I wonder what goes wrong.