A simple MALDI target plate with channel design to improve detection sensitivity and reproducibility for quantitative analysis of biomolecules

Overcoming the detrimental effects of sweet spots during crystallization is an important step to improve the quantitative abilities of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. In this study, we introduce MALDI targets, which exhibit a channel design to reduce sweet spot phenomena and improve reproducibility. The size of the channels was 3.0 mm in length, 0.35 mm in depth, and 0.40 mm in width, adjusted to the width of the implemented laser beam. For sample deposition, the matrix/sample mixture was homogenously deposited into the channels using capillary action. To demonstrate the proof-of-principle, the novel plates were used for the quantification of acetyl-L-carnitine in human blood plasma using a combined standard addition and isotope dilution method. The results showed that the reproducibility of acetyl-L-carnitine detection was highly improved over a conventional MALDI-MS assay, with RSD values of less than 5.9% in comparison with 15.6% using the regular MALDI method. The limits of quantification using the new plates were lowered approximately two-fold in comparison with a standard rastering approach on a smooth stainless-steel plate. Matrix effects were also assessed and shown to be negligible. The new assay was subsequently applied to the quantification of acetyl-L-carnitine in human plasma samples.