Determination of creatine and guanidinoacetate by GC-MS: study of their stability in urine at different temperatures.

Evaluation of a GC-MS method using N-tert-butyldimethylsilyl-N-methyltrifluoroacetamide (MTBSTFA) as the silylating agent for GC-MS. Study of the stability of creatine and guanidinoacetate in urine. 22 urines were kept at RT, 4 degrees C and -30 degrees C for 15 days. MTBSTFA produces a single chromatographic peak in contrast with other derivatizing agents. Creatine concentration increases at room temperature (326% on average), and at 4 degrees C (75%). However, detection decreases after freezing (-37%). Guanidinoacetate is stable, but decreases after freezing (-37%). Sonication before analysis is crucial to obtain repetitive results. A modified GC-MS method has been validated and the conditions for preservation of the urine have been established.