laser desorbtion/ionisation mass spectrometry abbr., LDI; MALDI (rus. масс-спектрометрия с лазерной десорбцией и ионизацией abbr., ЛДИ; МАЛДИ) — a method of mass spectrometry based on the desorption and ionisation of atoms or molecules of the sample by pulsed laser.

Description

This method is better known for its modification, called MALDI (matrix assisted laser desorption/ ionisation). There are different types of pulsed lasers used in this method for ionisation (gas, solid-state, dye lasers). The wavelengths of the laser irradiation belong either to the near UV region or IR region. The vast majority of commercial MALDI mass spectrometers use nitrogen lasers with a wavelength of 337 nm and pulse duration of ~ 10-9 s.

The sample for MALDI analysis is prepared by mixing the substance under study (analyte) with a specially selected chemical compound (matrix). Properties of the matrix should mitigate the destructive properties of laser radiation and provide the so-called "soft" ionisation without significant fragmentation of the analyte.

The laser breaks the desorbed sample into particles ranging in size from several tens to hundreds of microns that form the so-called plume. An area of high local pressure, which mainly consists of neutral particles, occurs above the sample surface. However, it also contains charged particles, whose fraction, by various estimates, ranges from 10-5 to  10-3 of the total number of particles. At the initial stage of plume formation its density is close to the density of the substance in the condensed state.

While the plume is expanding the micron particles break down to individual molecules or their fragments, and ions (mainly matrix ions). The ionisation of molecules, which occurs during the plume formation, is usually considered as primary ionisation.

Continuous collisions between the particles are observed in the expanding plume, including ion-molecule reactions between charged particles of the matrix and analyte, which lead to ionisation of the latter. Such ionisation is considered as secondary ionisation.

The method is widely used in the analysis of high molecular weight bioorganic molecules, synthetic polymers, dendrimers, fullerenes, as well as different clusters and nanoparticles. Modern mass-spectrometric equipment with LDI (MALDI) ionisation make it possible to determine the molecular weight and empirical formulas of compounds, and some of their structural features.

Illustrations

Schematic representation of substance ionization using the 
Schematic representation of substance ionization using the MALDI technique.

Author

  • Streletskiy Alexey V.

Sources

  1. Pasch H., Schrepp W. MALDI–TOF mass spectrometry of synthetic polymers. — Springer, 2003. — 298 p.
  2. Knochenmuss R., Zenobi R. MALDI Ionization: The Role of In-Plume Processes // Chem. Rev. 2003. V. 103. P. 441–452.

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