Matrix-free Laser Desorption/Ionization on Vertically Aligned Carbon Nanotube Arrays

Abstract

The matrix-assisted laser desorption/ionization (MALDI) has established its position as one of the most useful ionization techniques in mass spectrometry and its contribution to the scientific communities related to mass spectrometry was appreciated with the 2002 Nobel Prize in Chemistry given to the first developer, Tanaka. It is really an effective and sensitive ion formation technique which commonly employs an organic acid as the matrix. The use of such organic acids as the matrices has provided the high efficiency of desorption and ionization of large molecules including peptides, proteins, synthetic polymers, etc. for the MALDI technique. Interestingly, the matrix, which made the MALDI technique practical, poses an intrinsic constraint to the analyses of relatively small molecules below 1000 Da because the low mass regions in MALDI mass spectra are dominated by the strong background signals of the matrix molecule, its dimer, and their clusters with solvent molecules. Several approaches have been tried to find the methods that give the high desorption/ionization efficiency of MALDI without using the organic matrices. Probably the most successful matrix-free laser desorption/ ionization technique must be the use of porous silicon as a replacement of the matrix. The desorption/ionization on porous silicon(abbreviated as DIOS) was found to be highly sensitive for the sample below 3,000 Da and showed no interference at all in this low mass region, and is now commercially available. Although the DIOS has been very successful, it has a non-negligible shortcoming that its performance deteriorates with time after fabrication. The degradation may be slowed down by derivatization of the surface but cannot be completely eliminated. In this Note, we report that the vertically grown carbon nanotube arrays may be used as a substrate for matrix-free laser desorption/ionization of relatively small molecules. It turned out to be highly sensitive for small peptides near and below 2,000 Da and its ionization efficiency does not change after a long-time storage in the room atmosphere.