a) Explain the difference between
- laser desorption/ionization
- ultrafine-metal-plus-liquid-matrix technique
- matrix-assisted laser desorption/ionization.
Laser desorption/ionization (LDI) brings the laser light directly into interaction with the bulk analyte which is usually presented as a thin solid layer on a metal target. The other methods offer a matrix for absorption of the primary energy, i.e., an ultrafine metal powder in glycerol or an organic compound in case of MALDI. Both procedures make the process much softer for the analyte molecules and allow to generate quasimolecular ions of very large molecules, e.g., proteins of more than 100.000 u molecular weight (Chap. 11).
b) Why has MALDI become so successful as compared to the ultrafine-metal-plus-liquid-matrix technique?
Organic matrices can be adapted to the analyte of interest, whereas the metal powder in glycerol does not offer much variability. Thus, MALDI can be optimized to yield superior softness and sensitivity.
c) What is the reason for writing a slash between desorption and ionization instead of a simple space?
The slash is employed to indicate that the laser may either effect desorption of ionic analytes or may cause desorption plus ionization of neutrals. As there is no precise borderline in case of many analytes, this “either … or … and … ” writing convention has established.
d) Why did MALDI cause a rapid evolution of time-of-flight analyzers?
A laser generates ions only during a very short pulse in the range of of some to a few hundred nanoseconds. The time-of-flight analyzer (TOF) is best suited to deal with such packets, because it disperses ions of any m/z simultaneously in time. However, until the advent of MALDI, TOFs offered poor resolving power, and thus their development was driven forth immediately as a better use of MALDI strongly demanded for powerful TOFs.