Determination of Tryptophan Metabolites in Serum and Cerebrospinal Fluid Samples Using Microextraction by Packed Sorbent, Silylation and GC–MS Detection

Indole-containing acids—tryptophan metabolites—found in serum and cerebrospinal fluid (CSF) samples of patients with diseases of the central nervous system (CNS) were determined with the use of microextraction by packed sorbent (MEPS) followed by silylation and gas chromatography–mass spectrometry (GC–MS) analysis. MEPS with the following silylation led to the reproducible formation of derivatives with an unsubstituted hydrogen ion in the indole ring, the chromatographic peaks of which are symmetric and can be used for GC–MS analysis without additional derivatization. The recoveries of analytes at the limit of quantitation (LOQ) levels were 40–80% for pooled CSF and 40–60% for serum. The limit of detection (LOD) and LOQ values were 0.2–0.4 and 0.4–0.5 µM, respectively, for both CSF and serum. The precision (the reproducibility, RSD) value of less than 20% and the accuracy (the relative error, RE) value of less than ±20% at the LOQ concentrations meet the Food and Drug Administration (FDA) recommendations. Linear correlations for all analytes were determined over a potentially clinically significant range of concentrations (0.4–10 µM for serum, R(2) ≥ 0.9942, and 0.4–7 µM for CSF, R(2) ≥ 0.9949). Moreover, MEPS significantly reduced the matrix effect of serum compared to liquid–liquid extraction (LLE), which was revealed in the example of reducing the amount of cholesterol and its relative compounds.