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Maldi technique

When performing a mass spectrometry analysis, there are a variety of ionization methods available, however the most commonly used methods are Electrospray Ionization (ESI) and Matrix-assisted Laser Desorption Ionization (MALDI). In this lab, we will use MALDI.

There are two major steps involved in the MALDI technique, which uses a UV-laser to ionize a sample. First, the molecule to be analyzed is dissolved in a matrix, which is a solvent containing small organic molecules. These molecules present high absorption at the laser’s wavelength to shield the sample from direct exposure to the laser and help ionize it. The mixture is then left to dry prior to analysis. This results in the formation of a crystal matrix that contains the molecule of interest. It is important to keep in mind that this technique is very sensitive to interferences, therefore the plate should be kept in a dust-free environment and only filtered water should be used to clean it.

Second, the matrix molecule is excited to a higher energy state when it encounters the UV laser. This eventually leads to a fragmentation process, forming the ion of interest. The ionized molecule enters the mass analyzer and yields the mass spectrum.

This image consists of a MALDI scheme. In the left part of the image there is a target plate, where analyte and matrix spots are present and can be differentiated it. A laser beam is pointing at this plate, inducing desorption, which is indicated with an arrow from the plate and pointing to an analyte spot surrounded few matrix spots. After that, a process of desolvation and ionization takes place, where the matrix spots transfer protons to the analyte spots to make them positively charged. Here, an arrow points o an analyte spot still surrounded by matrix spots but with a bigger gap between them. A final arrow is pointing these charged and surrounded particles towards the mass analyzer.

Figure 1. MALDI Technique Overview