你对句子结构理解错误。

“ Dr Priego-Capote is able to find in dried sweat substances that are undetectable at the moment in fresh perspiration ”

划线部分是to find的地点状语，substances为宾语，that引导定语从句，修饰substances。由于宾语带了很长的定语从句，所以宾语后移到状语的后面了，这在语法上称为宾语后移。