Delayed ion extraction

E. Sugiyama, A. Hara, and K. Uemura, A quantitative analysis of serum sulfatide by matrix-assisted laser desorption ionization time-of-flight mass spectrometry with delayed ion extraction, Anal. Biochem., 274 (1999) 90-97. 

Advances in TOF/MS technology, such as reflectron ion optics, delayed ion extraction, and orthogonal ion acceleration, have given rise to bench-top instrumentation with resolution capabilities in excess of that obtained by quadra-pole MS systems. Each of these devices limits or corrects the kinetic energy spread of ions with identical m/ ratios. 

Even slower dissociation rates can be measured by storing ions in an ion trap such as a pulsed ion cyclotron resonance (ICR) cavity (So and Dunbar, 1988) or a quadru-pole ion trap (March et al., 1992), both of which can trap the ions up to several seconds. In the ICR, ions are trapped by a combination of DC electric and magnetic fields, while in the quadrupole trap, they are stored by a combination of RF and DC electric fields. Analysis of either the depleted parent ions or the newly formed product ions is carried out by pulsed extraction of mass selected ions. Thus, the timing with respect to the photodissociation pulse is achieved by delayed ion extraction. The long time limit in this experiment is determined not by the trapping time of the instrument, but by the IR fluorescence rate of ions which is typically 10 to 10 sec > (Dunbar, 1990 Dunbar et al., 1987). 

Figure 5.24 The relative abundance of C(,H5+ PEPICO signal from iodobenzene as a function of the ion internal energy at four delayed ion extraction times. The four different curves (from right to left) were taken with ion residence times of 1.0, 5.9, 21, and 57 jjLsec, respectively. Taken with permission from Dannacher et al. (1983).

Mass spectrometer There are several commercially available MALDI-MS apparatus. These have been recently reviewed m the section product review of Analytical Chemistry (497 A, 1995). The analysis of the relatively complex peptide profiles of nervous tissue requires a high resolution mass spectrometer, i.e., an apparatus equipped with a reflectron (or m the linear mode with a capacity for delayed ion extraction). 

Mowat, I. A., Donovan, R. J., and Maier, R. R. J., Enhanced Gationization of Polymers Using Delayed Ion Extraction with Matrix-Assisted Laser Desorption/ Ionization, Rapid Comm. Mass. Spectrom., 11, 89, 1997. 

Menzel, C., Berkenkamp, S., and Hillenkamp, F. (1999) Infrared matrix-assisted laser desorption/ ionization mass spectrometry with a transversely excited atmospheric pressure carbon dioxide laser at 10.6 pm wavelength with static and delayed ion extraction. Rapid Commun. Mass Spectrom., 13, 25-32. 

The introduction of delayed ion extraction in MALDI-TOF instruments in 1995 resulted in a tremendous improvement in analytical performance, mass resolution and accuracy. Using this technique, peptide mass mapping gained high specificity for protein identification and an ability to identify individual components among simple protein mixtures [60, 61]. Improved sample preparation methods also provided an enhanced sensitivity to enable analysis of femtomolar levels of protein [5, 19], Today, the automation of MALDI-MS data acquisition enables the analysis of hundreds of samples each day in large-scale protein identification experiments [62]. 

Samples of soot collected above fuel-rich flames [38,39] and extracted with dichloromethane showed a contamination with ofigomeric dimethyl-siloxanes with a mass spacing of 74 u, from a silicone oil used as lubricant in the burner equipment, when sinapinic acid matrix was used. A series of oligomeric peaks with a spacing of 24 u was also seen and attributed to the large polycyclic aromatics liable to be formed in soots. LPs up to 90% of available were used in this work. Subsequent work involving more samples and the application of the delayed ion extraction (DIE) method is detailed in Sections 33.3-33.5. 

The important instrumental development of delayed (ion) extraction (DE) or time-lag focusing (TLF) [279] has had a great impact on MALDI development [280,281]. The major limitation to the resolution provided by MALDI-MS is the initial velocity distribution of the ions. Ions with the same mass/charge ratio but different energy distributions yield a broad peak with a decrease in resolution. Correction for this by the use of pulsed extraction for time-lag focusing [282] has greatly improved the quality of the mass spectra from low masses (<100 Da) up to high masses. The term time-lag... 

FIGURE 7.28 MALDI in-source decay (ISD) mass spectrum of the oxidized B chain of bovine insulin obtained with delayed ion extraction.The extraction pulse was 1.5 kV applied 350 ns after the laser pulse. (Reprinted with permission from reference 45). 

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