MALDI Explained For Beginners (Matrix Assisted Laser Desorption Ionization)

Matrix assisted laser desorption ionization, commonly known as MALDI is an ionization technique used in mass spectrometry. MALDI uses a matrix in order to absorb the energy from a pulsating laser to create ions from large molecules with minimal fragmentation. It has a high sensitivity as well as a wide application range and simple operation.

Let us start by a closer examination of the matrix used in MALDI. The matrix is a substance that coexists with the sample to be tested and absorbs the incident laser light to prevent direct irradiation to destroy the sample. In other words it acts as a sort of shield, protecting the sample. The sample gets dispersed in the matrix at a high dilution rate (as much as 10 000 parts matrix for every 1 part sample). This allows the matrix to efficiently absorb the energy of the pulsed laser of a certain wavelength and then uniformly transmit this energy to the sample causing instantaneous vaporization and ionization. In addition, this large amount of matrix effectively disperses the sample, thereby reducing the interactions between the sample molecules further promoting ionization. The choice of matrix is one of the most important steps in MALDI analysis.

The ideal matrix generally has the following properties:

- strong electron absorption at the laser wavelength used
- better vacuum stability
- lower vapor pressure
- good miscibility with the analyte in the solid state

The choice of matrix depends primarily on the wavelength of the laser used, followed by the nature of of the object being analyzed.

Now let us examine how MALDI ionization is actually carried out. The MALDI method is divided into 3 steps:

1. First, the sample is mixed with the appropriate substrate material and applied to the metal plate
2. Second, the pulse laser irradiates samples, triggering the ablation and desorption of samples and matrix materials
3. Finally, the analyte molecules are ionized by protonation or deprotonation in the hot plume (i.e. an upwelling of hotter stuff) of the ablation gas, and then accelerated to the mass analyzer

So why is MALDI so useful? Well:

- It can ionize some samples that are difficult to ionize by other ionization techniques and obtain complete ionization products without obvious fragments
- It has a dominant single-charged molecular ion peak and simple mass spectrum which is suitable for analysis of multi-component samples;
- The requirements for sample processing are not strict and even untreated can be directly analyzed;
- Finally MALDI can be coupled to Fourier transform ion cyclotron resonance mass spectrometry and its resolution is hence further improved