Nanoparticle-assisted MALDI-TOF MS combined with seed-layer surface preparation for quantification of small molecules.

Despite the advantages of simplicity and high-throughput detection that matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) has over other methods, quantitative analysis of low-molecular-weight analyte is hampered by interference from matrix-derived background noise and signal fluctuation due to the inhomogeneous MALDI sample surface. Taking advantage of improved sample homogeneity through matrix-conjugated magnetic nanoparticles (matrix@MNP) and the seed-layer method, we report a new strategy for the rapid identification and quantification of drugs in urine samples, using morphine and 7-aminoflunitrazepam (7-aminoFM2) as model compounds. To our knowledge, this is the first attempt using the seed-layer method for small molecule analysis. By applying the proposed seed-layer method, which was specifically optimized for the 2,5-dihydroxybenzoic acid@MNP (DHB@MNP) matrix, homogeneous sample crystallization examined by microscopy analysis was obtained that generated reproducible MALDI signals (RSD<10.0%). For urine sample analysis, simple liquid–liquid extraction as a sample pretreatment step effectively reduced the ion suppression effect caused by the endogenous components in urine; good recoveries (82–90%) were obtained with a small ion suppression effect (<14% of signal decrease). This newly developed method demonstrated good quantitation linearity over a range of 50–2000ngmL−1 (R2>0.996) with reduced signal variation (RSD<10.0%). The detection limit is 30ngmL−1 with good precision (intra-day, 2.0–9.3%; inter-day, 5.0–10.0%) and accuracy (intra-day, 95.0–106.0%; inter-day, 103.0–115.5%). The nanoparticle-assisted MALDI-TOF MS combined with seed-layer surface preparation provides a rapid, efficient and accurate platform for the quantification of small molecules in urine samples.