Soft laser desorption

Soft laser desorption is laser desorption of large molecules that results in ionization without fragmentation. "Soft" in the context of ion formation means forming ions without breaking chemical bonds. "Hard" ionization is the formation of ions with the breaking of bonds and the formation of fragment ions.

Background
The term "soft laser desorption" has not been widely used by the mass spectrometry community, which in most cases uses matrix-assisted laser desorption/ionization (MALDI) to indicate soft laser desorption ionization that is aided by a separate matrix compound. The term soft laser desorption was used most notably by the Nobel Foundation in public information released in conjunction with the 2002 Nobel Prize in Chemistry. Koichi Tanaka was awarded 1/4 of the prize for his use of a mixture of cobalt nanoparticles and glycerol in what he called the “ultra fine metal plus liquid matrix method” of laser desorption ionization. With this approach, he was able to demonstrate the soft ionization of proteins. The MALDI technique was demonstrated (and the name coined) in 1985 by Michael Karas, Doris Bachmann, and Franz Hillenkamp, but ionization of proteins by MALDI was not reported until 1988, immediately after Tanaka's results were reported.

Some have argued that Karas and Hillenkamp were more deserving of the Nobel Prize than Tanaka because their crystalline matrix method is much more widely used than Tanaka's liquid matrix. Countering this argument is the fact that Tanaka was the first to use a 337 nm nitrogen laser while Karas and Hillenkamp were using a 266 nm YAG laser. The "modern" MALDI approach came into being several years after the first soft laser desorption of proteins was demonstrated.

Examples in mass spectrometry
The matrix-assisted laser desorption/ionization approach typically uses a small organic acid solid matrix, although liquids have been used. The light source is a pulsed IR or UV laser. The surface-enhanced laser desorption/ionization (SELDI) variand is similar to  MALDI, but uses a biochemical affinity target. The technique known as surface-enhanced neat desorption (SEND) is a variant of MALDI with the matrix is covalently linked to the target surface.The surface-assisted laser desorption/ionization (SALDI) approach can be described as MALDI using a liquid plus particulate matrix. The desorption ionization on silicon (DIOS) approach is laser desorption/ionization of a sample deposited on a porous silicon surface. The technique known as laser induced acoustic desorption (LIAD ) is transmission geometry LDI with a metal film target.