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Cathode o n transfer detector measures the time for an ion to fly from the ion source to the detector (Fig. 26.07). The time-of-flight is proportional to the square root of m/z. Typically molecular ions up to 100,000 daltons may be measured by MALDI/TOF. However, advances in instrumentation will probably increase this limit substantially in the near future.

The amount of sample needed for MALDI/TOF is now less than a picomole (10-12 mole). Mass resolution is 1 in 10,000 or better. Thus proteins separated by 2D-PAGE may be routinely identified by MALDI/TOF often after preliminary digestion to give a characteristic set of peptides. Post-translational modification of proteins may also be detected by shifts in molecular weight. Phosphate or sugar residues yield characteristic ion fragments and analysis after protease digestion may reveal the location of such groups within the protein.

Electrospray ionization (ESI) refers to the generation of gas-phase ions from ions in solution. A narrow capillary tube allows droplets of liquid to emerge into a strong electrostatic field (Fig. 26.08). The solvent evaporates and the droplet breaks up. Repeated evaporation and splitting of droplets eventually releases separate ions (either with single or multiple charges) that are accelerated towards a mass analyzer by an electric field. Mass analyzers such as quadrupole or ion-trap detectors are normally used with ESI mass spectrometers. The typical range for a singly charged ion is up to 5,000 daltons, but multiple charges allow heavier ions to be analyzed.

An advantage of ESI is that it can be directly coupled to liquid separation techniques such as capillary electrophoresis or HPLC (high performance liquid chromatography). Also, a parent ion can be isolated and fragmented into daughter

Nitrocellulose membrane with attached proteins

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