Abstract

During the past several years, there has been an increasing use of lasers with mass spectrometry for the structural analysis of biological molecules. The techniques used may be characterized by the method of ion generation: (1 ) desorption of preformed ions from a surface (2) matrix-assisted ionization, and (3) laser photoionization of volatilized samples. For the desorption of large, involatile molecular species, a wide range of lasers have been used, including Nd:YAG, excimer and CO₂ lasers, over a wavelength range of 160Å to 11 um. Laser power densities have ranged from 10⁶ to 10¹⁰ W/cm² with laser spot sizes of 1.1 to 10 µm. IR laser desorption generally results in direct formation of gaseous molecular ions, usually observed as protonated or cationized (M+Na⁺) species. In the matrix-assisted technique, biological molecules are first recrystal- ized from a solution containing nicotinic acid, or other suitable UV absorber. This technique has been successful in desorbing intact molecular ions of proteins weighing in excess of 100 kDa. Photoionization of nonvolatile biomolecules can be carried out in the gas phase when a short wavelength laser is synchronized with the pulsed desorption laser. Laser power densities required for photoionization range from 10³ to 10⁹ W/cm².

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