Post-source decay technique

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. 

The post-source decay technique suffers from some limitations, such as the limited selectivity obtainable on the parent ion, modest mass accuracy of the daughter ions (compared to the accuracy achieved for parent ions), and the frequently incomplete information obtained from the metastable decay spectrum. These factors have stimulated the development of various tandem instruments, as discussed below. 

Many classes of polar lipids have been characterized by MALDFMS. " MALDFMS has also been employed for characterization of nonpolar lipid classes such as cholesterol and TG. Positive-ion MALDl mass spectra of TG exclusively display the sodium adduct as its quasi-molecular ion. In the fragment region, MALDl mass spectra of TG molecular species are featured by the presence of ions corresponding to the loss of sodium fatty acyl carboxylate(s). A post-source decay technique has been commonly employed for characterization of polar lipids as previously described. Since MALDl QqTOF or TOF/TOF mass spectrometers have been commercially available, true tandem MS analyses of lipids through MALDl have been reported. ... 

Several original papers must be mentioned that deal with mass spectrometric techniques which the numerous reviews do not comprise. Kaufmann and coworkers268,288 studied the mass spectrometric analysis of carotenoids and some of their fatty acid esters using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry and its post-source-decay (PSD) variant. Some advantages concerning the thermal instability and limited solubility were discussed, but the fragmentation paths of the carotenoid cations were found to be essentially the same as those observed with conventional techniques. 

Because the soft ionization methods used for oligosaccharides produce few fragments, collision-induced dissociation (CID) or post-source decay (PSD) must be used for structural study. These two techniques have been applied to deprotonated, protonated or alkaliated... 

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). 

Recently, fragmentations by post source decay (PSD) was introduced as a technique to obtain structural information in MALDI MS [6,7]. We describe here the combined use of automated Edman sequencing and MALDI sequencing for the determination of proteolytic peptide fragments in the low picomole range. 

A certain amonnt of structural information, snch as sequencing of peptides, can be obtained from single-analyzer MALDl-TOF systems used in the reflectron mode becanse some ions leaving the ion source are metastable (Section 3.3.1.2) and fragment as they move down the flight tube the process is called post-source decay (PSD). The technique is limited because it is not possible to preselect ions for fragmentation and becanse stepwise changes in the parameters of the reflectron are needed to obtain prodnct ion spectra. 

An older book that describes the basic principles of TOE, and several innovative techniques, including orthogonal extraction, post source decay, and delayed extraction. 

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 ... 

The use of the fragmentation of oligosaccharides by using post-source decay (PSD) MALDI-TOF MS was shown by Van Alebeek et al. (31). This technique was reported to permit the determination of the positions of methyl esters or other substituents in the sequencing of methyl-esterified oligogalacturonides (31). Nevertheless, also by using PSD MALDI-TOF MS no fragmentation of O-acetylated XOS was obtained. 

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... 

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