Desorption electrospray ionization features

Direct analysis of solid samples or analytes present on solid or liquid surfaces without any sample preparation has recently gained much interest. Desorption electrospray ionization (DESI) is an atmospheric pressure desorption ionization method introduced by Cooks et al. (2006) where ions are produced directly from the surface to be analyzed. The DESI technique features the use of charged liquid droplets that are directed by a high velocity as jet (on the order of 300 m/s) to the surface to be analyzed. Analytes are desorbed from the surface and analyzed by the mass spectrometer. 

For ambient mass spectrometric approaches, techniques such as electrospray-assisted pyrolysis ionization (ESA-Py) (Hsu et al., 2005), desorption electrospray ionization (DESl) (Takats et al., 2004), easy ambient sonic-spray ionization (EASl) (Haddad et al., 2008), and atmospheric pressure laser-induced acoustic desorption chemical ionization (AP/LIAD-CI) (Nyadong et al., 2011) have been used for the direct analysis of crude oil with minimal sample pretreatment. Such approaches prevent unexpected effects on the composition of crude oil samples during preparation. Another attractive feature of performing analyses imder ambient conditions is the capacity for rapid sampling, thereby enabhng opportunities for high-throughpnit analysis. 

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. 

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

Mass spectrometry has been revolutionalized by the advent of electrospray ionization, but, before we concentrate on this relatively recent addition to the array of ionization methods, we will first discuss two other techniques which are routinely used for the ionization of biomolecules matrix assisted laser desorption ionization (MALDI) and fast atom bombardment (FAB). These techniques share common features in that ... 

Currently, high-performance liquid chromatography (HPLC) combined with atmospheric pressure ionization (API) triple-quadrupole mass spectrometry (MS) is the predominate quantitative technique used in modem pharmaceutical bioanalysis. The key technological achievement in API-MS was the efficient ionization in a liquid stream and transference of ions from atmosphere to vacuum. Of the API approaches developed, electrospray ionization (ESI) is the most commonly used. ESI provides an efficient means of soft ionization amenable to most molecules encountered in a dmg discovery setting. An alternative soft ionization approach is the use of desorption ionization (DI) techniques. The major distinguishing feature of DI techniques is that ions are typically produced from dried samples. 

The only mass spectrometric methods available during the era of the first cascade synthesis in 1978 [30] were electron impact (El) and field desorption (FD) mass spectrometry [31]. Fast atom bombardment (FAB) mass spectrometry is limited to fairly low mass ranges and not very suitable for compounds of low polarity. It was not until the development of new and gentle ionisation methods such as MALDI (matrix-assisted laser desorption ionization) [32] and ESI (electrospray ionization) [33] that the conditions were fulfilled for the start of intense research in the field of dendrimer chemistry. The following section will present the special features of these mass-spectrometric methods and their importance in dendrimer analysis. 

The dynamic development of mass spectrometry has had a huge impact on lipid analysis. Currently, a variety of suitable mass spectrometers is available. In principal, a mass spectrometer consists of an ion source, a mass analyzer, and an ion detector. The typical features of each instrument (Fig. 2) result mostly from the types of ion source and mass analyzer. To date, the ionization techniques apphed to lipid analysis include Electrospray Ionization (ESI or nano-ESI), Atmospheric Pressure Chemical Ionization (APCI), Matrix-Assisted Laser Desorption/Ionization... 

A variety of MS formats are widely accepted and applied in the pharmaceutical industry. The specific MS application is often defined by the sample introduction technique. The pharmaceutical applications highlighted in this article feature two types of sample introduction techniques dynamic and static. Dynamic sample introduction involves the use of high-performance liquid chromatography (HPLC) on-line with MS. The resulting liquid chromatography/mass spectrometry (LC/MS) format provides unique and enabling capabilities for pharmaceutical analysis. The electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) modes are the most widely used. Static sample introduction techniques primarily use matrix-assisted laser desorption/ionization (MALDI). ... 

This review provides an up-to-date overview of the application of analytical procedures based on MS for the characterization of organic natural materials in archaeological and historical objects. Applications that feature the use of gas chromatography/mass spectrometry (GC/MS), Py-GC/MS, high performance liquid chromatography combined with mass spectrometry (HPLC/MS), and direct MS analysis such as matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), electrospray ionization mass spectrometry (ESI-MS), and direct exposure mass spectrometry (DE-MS) are sununarized to highlight the different information provided by each of the various analytical approaches. Case studies and examples are also presented and include a description of the molecular markers and of the molecular profiles that are used to identify the original materials. 

Mass spectrometer equipped with desorption electrospray (DESI) ion source is capable of detection and identification of different substances from the surface. Moreover, the information about spatial distribution of those substances is retained. Connection of this technique with TLC not only allows for the measurement of retention times for separated chemicals (spatial distribution), but also for their unambiguous identification, based on the molecular weight of certain substances and their fragmentation spectra. Additionally, because DESI works under ambient conditions, there is no need to apply a high vacuum system for the sample introduction. Moreover, samples analyzed by DESI practically do not require any kind of preparation (e.g., covering with matrix prior to MALDI analysis), thus connection of those two techniques is relatively easy. Certainly, not all the substances, due to their chemical features, may be detected with this technique. Only compounds, which are able to ionize in this type of ion source, may be analyzed. 

With the introduction of fast-atom bombardment (FAB) in 1982, and matrix-assisted laser desorption/ ionization (MALDI) and electrospray ionization (ESI), most of the biomedical applications have been directed towards these methods. The 52( f.pD method has been found to have wide applicability, including the study of refractory materials, catalysts, semiconductors and frozen gases. Electronics capable of measuring the timing of events with subnanosecond resolution (the time it takes for a single photon to travel 1 cm) is used by this method as well as event-by-event data acquisition using the computer to make decisions at the molecular level, the basis of correlation mass spectrometry, a unique feature of 252Cf-PD. 

The technique of electrospray-assisted laser desorption/ionization (ELDI) combines two well-matured techniques of ionization for the benefit of inproved analysis of samples under ambient conditions. The development of ELDI emanates from the fact that in (MA)LDI by far more neutrals an ions are released from the sample layer (Chap. 11) [45]. Consequently, post-ionization of laser-desorbed neutrals is promising and such methods have indeed been developed (cf. Refs, in [46]). The unique feature of ELDI is to laser-irradiate the sanple in the ambient close to the ESI plume, wherein the neutrals are then ionized by ion-molecule reactions [46]. 

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