Desorption electrospray ionization advantages

Currently, there are at least three ionization techniques routinely used in IMS (1) secondary ion mass spectrometry (SIMS), (2) matrix-assisted laser desorption ionization (MALDI), and (3) desorption electrospray ionization (DESI). Each of these ion sources has their own advantages and drawbacks thus, they are usually selected in the context of the imaged sample. In the following text, we provide a concise description of these devices. 

Recently, it was been shown that an electrospray emitter can be used to provide transla-tionally excited projectiles (charged microdroplets) which serve as projectiles for desorption and ionization of condensed-phase analytes present on surfaces. This hybrid technique, DESI (desorption electrospray ionization), is applicable to analysis of samples in the ambient environment. The practical advantage is that the sample can be examined directly, without any preparation hence the experiment is extremely fast (typically <5 s) and can be conducted in a high-throughput fashion and tandem mass spectrometry can be used for identification of components of complex mixtures. These features provide the speed of analysis and high chemical specificity that are needed in applications such as public safety monitoring. 

An absolute method for molecular weight determination is matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF) (Kona et al., 2005 Creel, 1993 Nielsen, 1999 Cho et al., 2001). The sample is dispersed in a UV-absorbing matrix (e.g., trans -cinnamic acid or 2,5-dihydroxybennzoic acid). Irradiation with a UV laser induces evaporation of ionized polymer chains, which are then detected using TOF. The technique requires relatively narrow MWD samples. Alternative ionization methods have been employed, such as electrospray ionization mass spectrometry (ESI-MS), which may have advantages for certain polymer end groups (Vana et al., 2002). IFFF and MALDI-TOF can be coupled to analyze polydisperse samples and polymer mixtures (Kassalainen and Williams, 2003). 

Soft ionization MS techniques [9] like electrospray ionization (ESI) and soft laser desorption, often known as matrix-assisted laser desorption/ioniza-tion (MALDI), facilitated the polymer analyses over the last years. The advantage of the soft ionization techniques is the transformation of dissolved liquid or solid sample into the gas phase, where no change in the molecular composition/structure of the sample will be induced, while hard ionization in mass spectrometry (e.g., electron ionization (El) or fast atom bombardment (FAB)) preferentially destroys the chemical and molecular structure into fragments prior to the detection of the molar mass fragments of the sample by mass spectrometry. 

El and Cl methods can be used if the compound to be studied is sufficiently volatile and stable to be vaporized intact. However, only 20% of the organics found in surface water are volatile enough to be amenable to GC-EI-MS or GC-CI-MS. Today, there are a variety of other ionization techniques available electrospray ionization (ESI), atmospheric pressure chemical ionization, matrix-assisted laser desorption ionization, and fast atom bombardment. Each of these has its advantages and disadvantages. A simple guideline to the most likely optimum ionization technique for a given class of substance is given in Table 1. 

In Situ Digestion and Desorption from Membranes. While the electrospray ionization technique enjoys a considerable advantage as an online chromatographic (HPLC) detector, the direct desorption of proteins and peptides from electroblotted membranes using MALDI provides an equally important link to one-dimensional and two-dimensional gel electrophoresis. In such a scheme the laser beam would... 

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