Development of C60-based labeling reagents for the determination of low-molecular-weight compounds by matrix assisted laser desorption ionization mass (I): Determination of amino acids in microliter biofluids. 
Quantification of low molecular weight compounds (<800 Da) using matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI MS) is challenging due to the matrix signal interference at low m/z region and poor reproducibility of MS responses.  In this study, a C60 labeling-MALDI MS strategy was proposed for the fast, sensitive and reliable determination of amino acids (AAs) in biofluids.  An N-hydroxysuccinimide functionalized C60 was synthesized as the labeling reagent and added as an 880 Da tag to AAs; a carboxyl acid containing C60 was employed as the internal standards to normalize MS variations.  This solved the inherent problems of MALDI MS for small molecule analysis.  The entire analytical procedure-which consisted of simple protein precipitation and 10 min of derivatization in a microwave prior to the MALDI MS analysis-could be accomplished within 20 min with high throughput and great sample matrix tolerance.  AA quantification showed good linearity from 0.7 to 70.0 μM with correlation coefficients (R) larger than 0.9954.  The limits of detection were 70.0-300.0 fmol.  Good reproducibility and reliability of the method were demonstrated by intra-day and inter-day precision with relative standard deviations less than 13.8%, and the recovery in biofluid ranged from 80.4% to 106.8%.  This approach could be used in 1 μL of urine, serum, plasma, whole blood, and cerebrospinal fluid.  Most importantly, the C60 labeling strategy is a universal approach for MALDI MS analysis of various LMW compounds because functionalized C60 is now available on demand.