Linear, Pulsed Extraction Instrument

Pulsed discharges may be utilized to analyze nonconductive samples. Steiner et al. described the use of a pin-type pulsed rf-GD in conjunction with a linear orthogonal extraction TOF-MS [58]. Figure 12.24 depicts differences in the spectra obtained 4.0 msec after the power plateau and 7.0 msec after the plateau. As expected, the sputtered species (Si) in the discharge are found in the afterpeak at 7.0 msec (see Fig. 12.24). The resolution and sensitivity of this early system were limited and improvements were planned in the input optical system of the instrument. 

Two methods are used to sequence MALDI-produced ions by TOFMS One approach uses ion-source decay, sequence ions thus produced are characterized by operating the TOF instrument in the delayed-extraction mode. Fast dissociation in the source-accelerating region creates a wealth of sequence-specific fragment ions, which otherwise would be missed in the normal mode of operation of TOF (i.e., in which the laser pulse and ion-extraction pulse are both applied concurrently). By applying a delayed-extraction pulse, fragments can, however, be accumulated between pulses and mass-analyzed with an ordinary linear TOFMS instrument [98,99]. This process preferentially forms c -type N-terminus ions along with y and C-terminus ions. Similar to FAB and ESI in-source CID, this approach requires a pure sample and is not applicable to a mixture of peptides. 

Applications presented in this chapter have employed a Voyager DE-STR MALDl TOP mass spectrometer and a QStar Pulsar i QqTOF mass spectrometer (Applied Biosystems, Foster City, CA). The Voyager instrument utilizes a 337 mn nitrogen laser with a 2.5 ns pulse and a repetition rate of 2 Hz. The instrument was operated in linear mode under delayed extraction conditions. The laser spot size on target is —50 xm in diameter. The QStar Pulsar instrument is equipped with a MALDl source and a 337 mn nitrogen laser operating at a 20 Hz repetition rate. The laser spot size is —200 p,m in diameter on target. 

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