Post-source decay instruments

In instruments without a reflectron (see Figure 3.7 above), both the precursor and product ions reach the detector at the same time and are not separated. The reflectron, however, is an energy analyser and product ions with different energies, after passage through the reflectron, will have different flight times to the detector and may be separated and their m/z ratios determined. This is known as post-source decay (PSD) [11],... 

With the advent of very sensitive ionization techniques such as matrix assisted laser desorption (MALDI) coupled with time-of-flight (TOF) mass analysis, measurement of the intact mass of peptides at sub-pmol levels has become a reality (2) of which we have taken advantage for the systematic screening of HPLC fractions. Partial sequence information can be obtained by carrying out enzymatic hydrolysis with exoproteinases (e.g. carboxypeptidases and aminopeptidases) (3, 4). More recently, MALDI has been used to measure metastable decomposition occurring in the first field free region of a reflectron TOF instrument (referred to as post source decay (PSD)) with only marginally more sample (5-7). 

Post-source Decay Tandem Mass Spectrometry The potential of PSD has also been exploited to identify phosphopeptides [61,62]. This technique is used for MALDI-generated ions in a reflectron TOP instrument. PSD can also distinguish any serine-threonine phosphorylation from tyrosine phosphorylation. Phosphotyrosine-containing peptides mostly yield the [MH — HP03J+ (loss of 80 Da) ion, whereas phosphoserine- and threonine-containing peptides produce [MH - H3P041+ (loss of 98 Da) and [M - HP03]+ ions. 

Several gas-phase fragmentation techniques have found a niche as a possible means to sequence oligonucleotides. These include ESI in-source CID (nozzle-skimmer voltage to induce fragmentation), IRMPD in an FT-ICR MS instrument, MALDI in-source decay and post-source decay, and CID-MS/MS of the ESI-produced ions. CID generates complementary (a — B )- and in-type ions, which can provide bidirectional sequencing from the 5 3 direction and 3 5 ... 

A linear TOF analyzer can not provide any MS/MS information but a reflection instrument can provide fragment ion spectra of metastable ions that fragment (either spontaneously or via laser photodissodation) while in flight between the final acceleration grid and the reflectron entrance (Weickhardt 1996). However, this post-source decay technique has never been used for quantitative trace analysis. 

Stiibiger and colleagues [36] used a similar approach (in addition to the MS analysis of lipid species subsequent to reelution from the TLC plate) to screen the compositions of various neutral (e.g., triacylglycerols) and polar (e.g., GPL and glyc-erosphingolipids [GSL]) lipid classes derived from crude lipid extracts of human plasma as well as soybean lecithin. These authors [36] have also provided evidence that combining TLC/MS with post source decay (PSD) MALDl MS helps to address structural problems such as the detailed fatty acyl composition and the differentiation of positional isomers sn- vs. sn- I). PSD may be considered an MS/MS-Uke experiment that can be performed on all TOP instruments equipped with a reflectron [20] but without the need of a dedicated collision ceU. About 70 different lipid species could be detected in just 50 jL of human blood plasma. 

In-Source Decay (ISD). In the next chapter we will describe a method for amino acid sequencing of peptides on reflectron TOF instruments known as post-source decay (PSD). This method exploits the considerable metastable fragmentation that occurs in the drift region after the ions are extracted from the ion source. The resulting product ions spend a shorter time in the reflectron, and therefore arrive at the detector sooner than their precursors. 

Post-Source Decay Spectra Using a Dual-Stage Reflectron. The discussion on coincidence techniques above suggests that the opportunities for recording product ion spectra on a reflectron instrument and the necessity for obtaining spectra at different reflectron voltages have been appreciated for some time. Nevertheless, the currendy popular technique known as post-source decay (PSD) is based upon an... 

FIGURE 8.8 The post-source decay scheme for a dual-stage reflectron instrument. (Reprinted with permission from reference 8). 

Mass calibration for instruments using dual-stage reflectrons is, of course, similar if one utilizes only those portions of the post-source decay spectra that are linear. 

TOF-RTOF instruments. An alternative approach to reacceleration uses a reflec-tron in MS2 to disperse the product-ion arrival times in the same manner as used for the post-source decay method. Additionally, the reflectron can be used in combination with reacceleration after the collision chamber. In the TOF-RTOF instrument reported by Cooks et al. (Figure 9.6), ion activation is provided by surface-induced dissociation (SID), and product ions are dispersed and focused by a reflectron. The target surface is raised to a potential above the drift region, so that product ions are dispersed by reacceleration as well. The effect of this combination is to... 

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