ortho-Hydroxyphenylacetic acid (2-hydroxybenzeneacetic acid) can be found in a number of food items such as natal plum lemon verbena half-highbush blueberry and parsley which makes ortho-hydroxyphenylacetic acid a potential biomarker for the consumption of these food products. ortho-Hydroxyphenylacetic acid can be found primarily in blood feces and urine.

Reference standards.

Guanine is a purine base that is a constituent of nucleotides occurring in nucleic acids.