Matrix-assisted laser desorption ionization (Maldi) is a widely used method for ionizing molecules before they go through the mass spectrometer. However, the use of a matrix limits the method in several ways. Because of these limitations, laser desorption ionization techniques that do not require a matrix have been developed. Dominic S. Peterson of Los Alamos National Laboratory in New Mexico has reviewed these matrix-free methods.

Matrix-assisted methods use a laser to energize the matrix, which then transfers its energy to ionize the molecules in the sample. In contrast, matrix-free methods use the surface of materials to energize the molecules of interest.

The first matrix-free method developed — desorption ionization on silicon — uses silicon to transfer energy to molecules. Porous silicon and silicon thin films have been employed in experiments. Because this technique depends on surface characteristics, researchers have made surface modifications such as scratching silica with a graphite pencil or sandpaper.

Other matrix-free techniques use sol-gels and nanostructures as well as polymers and microstructures made of carbon. Researchers have used sol-gels made of silica, titanium and zirconium. Carbon-based polymers and microstructures can have various arrangements, forms and added chemicals. For nanostructures, chemists have used carbon nanotubes, silicon nanowires, mesoporous tungsten and titanium oxides.

Matrix-free laser desorption ionization can be applied to forensics, solid-phase synthesis, and to drug, protein and polymer analysis. In combination with other methods, matrix-free laser desorption ionization also can be used for analyte capture.

Although they have several advantages, matrix-free methods are limited to low-molecular-weight molecules with a mass-to-charge ratio of less than 3000. Peterson stated that improving this range probably will require modifying the surfaces of materials with light-absorbing and acidic chemicals to enhance energy transfer and ionization efficiency.

Recent research has focused on using liquid separations or atmospheric pressure ionization with matrix-free methods. The author wrote that atmospheric pressure ionization and matrix-free methods are easier to use than many ionization techniques, and that they might entice more casual users to employ mass spectrometry.