Desorption-chemical ionization mass sample preparation

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. 

One of the most significant developments in mass spectrometry in the recent years is the introduction of a new class of ionization methods where samples in either solid or liquid state can be directly ionized in their native environment under ambient conditions (rather than inside a mass spectrometer) without any sample preparation. This new class of ionization methods is often referred to as ambient ionization methods [1,2], Because these methods generally ionize analytes on the surface or near the surface of the samples at atmospheric pressure, they have also been called atmospheric pressure surface sampling/ionization methods or direct/open air ionization methods [3], Since the first reports on ambient ionization with desorption electrospray ionization (DESI) [4] and direct analysis in real time (DART) [5], numerous reports have been published on the applications of these new ionization methods as well as the introduction of many related ambient ionization methods such as desorption atmospheric pressure chemical ionization (DAPCI) [6], atmospheric solid analysis probe (ASAP) [7], and electrospray-assisted laser desorption/ionization (ELDI) [8], Recently, two reviews of the various established and emerging ambient ionization methods have been published [2,3],... 

A new generation of mass spectrometer inlets allow for direct sampling of a substrate under ambient conditions. Theoretically, this eliminates the need for any sample preparation. Examples include direct analysis in real time (DART) and desorption electrospray ionization (DESI), as well as desorption atmospheric-pressure chemical ionization (DAPCI) and atmospheric solids analysis probe (ASAP). These techniques utilize a source of energy interacting directly with a sample surface at ambient pressure, causing molecules of interest to desorb, ionize, and be sampled by a mass spectrometer. 

The laser-based optical and chemical imager (LOCI) is a unique instrument that combines accurate isotope ratio analyses obtained both by laser desorption Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and by LIBS without any sample preparation (Scott and Mcjunkin 2009). A single photon ionization (SPI) process is implemented allowing near 100% ionization efficiency for elements and compounds with ionization energies less than 10.5 eV. The FTICR-MS and LIBS isotope capability coupled with LOCI s wide mass range, mapping capability, high resolution, and automated data collection as well as data interpretation offers an alternative to the labor-intensive bulk analysis of traditional methods. 

Several mass spectrometric techniques including fast atom bombardment (FAB), plasma desorption (PD), matrix-assisted laser desorption/ionization (MALDI), and electrospray (ES) mass spectrometry (MS) are presently available for the analysis of peptides and proteins (Roepstorff and Richter, 1992). Of these techniques, mainly PDMS has gained footing in protein laboratories because the instrumentation is relatively cheap and simple to operate and because, taking advantage of a nitrocellulose matrix, it is compatible with most procedures in protein chemistry (Cotter, 1988 Roepstorff, 1989). Provided that the proper care is taken in the sample preparation procedure most peptides and small proteins (up to 10 kDa) are on a routine basis amenable to analysis by PDMS. Molecular mass information can be obtained with an accuracy of 0.1% or better. Structural information can be gained by application of successive biochemical or chemical procedures to the sample. 

Ambient MS is another advance in the field. It allows the analysis of samples with little or no sample preparation. Following the introduction of desorption electrospray ionization (DESI) [108,109], direct analysis in real time (DART) [110], and desorption atmospheric pressure chemical ionization (DAPCI) [111, 112], a number of ambient ionization methods have been introduced. They include electrospray-assisted laser desorption/ionization (ELDI) [113], matrix-assisted laser desorption electrospray ionization (MALDESI) [114], atmospheric solids analysis probe (ASAP) [115], jet desorption ionization (JeDI) [116], desorption sonic spray ionization (DeSSI) [117], field-induced droplet ionization (FIDI) [118], desorption atmospheric pressure photoionization (DAPPI) [119], plasma-assisted desorption ionization (PADI) [120], dielectric barrier discharge ionization (DBDI) [121], and the liquid microjunction surface sampling probe method (LMJ-SSP) [122], etc. All these techniques have shown that ambient MS can be used as a rapid tool to provide efficient desorption and ionization and hence to allow mass spectrometric characterization of target compounds. 

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. 

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. 

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