Mass spectrometry : Schematic and Theory of MALDI

MALDI is a very sensitive technique for determining the mass of proteins, peptides, or polymers. Protein masses are identity of proteins and thus help in proteomics. Thus MALDI allows protein identification. MALDI sample preparation is relatively fast and easy. It is a first choice when it comes to protein study. Proteins, peptides, and polymers are fragile and tend to fragment when ionized by other ionization techniques.

MALDI is attached to a time of flight (TOF) analyzer which measures time it takes for the molecules to travel a fixed distance. MALDI is a soft ionization technique in which a short laser pulse, instead of continuous laser, of nitrogen gas usually around 237 nm is used to ionize molecules. A protein or peptide sample is placed on a target plate and mixed with an appropriate matrix on the target plate. The mixture of sample and matrix crystallizes due to the vacuum environment and then is irradiated with a short laser pulse. The sample molecules and the matrix now enter gas phase. This leads to release of matrix, samples molecules, and ions from the target plate. The ions then accelerate in TOF analyzer because they are subject to equal electric field. TOF is a field-free flight tube. The ions travel in a strait and linear direction to the detector. The mass to charge ration (m/z) of the sample ions can be calculated using the equation T= C₁(m/z)^0.5 +C₂. C1 and C2 are instrumental constants which can be determined with compounds of known mass. This equation is derived from the fact that potential energy equals kinetic energy.

Since velocity is distance/time v=d/t substitute it in equation 2 and solve for t to get eq. 3

The distance the molecules travel and their kinetic energy is constant. So it is replaced by C1. Furthermore since the relationship between the t and m^1/2 is linear an intercept of C2 is added to get equation of a line.