Ambient desorption techniques

Another interesting application of microwave energy was reported by Mueller et al The authors studied the amount of benzene and alkylated benzenes (BTX) in ambient and exhaled air by microwave desorption coupled to GC-MS. Microwave desorption was proved as an effective sample preparation technique for the analysis of BTX in air samples. A similar desorption technique was applied for the GC analysis of nicotine in indoor air as well. "... 

Desorption electrospray ionization (DESI), an ambient MS technique, was used for trace detection of the explosive RDX, directly from a wide variety of surfaces (metal, plastic, paper, polymer) without sample preparation or pretreatment. Increased selectivity was obtained both by MS/MS and by performing additional experiments in which additives were included in the spray solvent. Pure water could be used as the spray solution for DESI, and it showed ionization efficiencies for RDX in the negative ion mode similar to those given by methanol/water <2005ANC6755>. 

Perhaps, due to a high number of samples, no pre-separation will be done. But then, it should be ensured that suitable so-called ambient desorption ionization techniques such as DESI, DART, ASAP, and DIP-APCI can be coupled to the MS. 

By nature, thermal desorption techniques are limited to lower mass species amenable to vaporization and thermally stable compounds. Several approaches are not commercially available so far. New ion sources working under ambient conditions are not yet status quo in the most MS laboratories and must be bought specifically for TLC/HPTLC-MS, which might be hindered by an expensive price. These circumstances lessen the impact of the new development. On the other hand, having a forced-flow technique in the laboratory or the TLC-MS interface, coupling is readily feasible with any HPLC-MS system. 

Due to the open planar surface of UTLC layers, especially ambient desorption- or elution-based techniques were used to transfer analytes from the layer to the ionization region (Table 9.2). In addition, MALDI applications under vacuum were reported and the introduction of the whole ultrathin layer plate was possible without any special mounting devices, due to the small plate dimension. As UTLC-MS is a very new hyphenation, only a few applications of MS detection were reported after separation of different substances on UTLC layers. In addition, desorption- or elution-based approaches for detecting analytes without a separation directly from the UTLC layer are mentioned to show the capabilities of this hyphenation technique. However, as there is no chromatography (separation), it may not be termed UTLC-MS. 

To determine the distribution of pores with diameters smaller than 20 nm, a nitrogen desorption technique is employed which utilizes the Kelvin equation to relate the pore radius to the ambient pressure. The porous material is exposed to high pressures of N2 such that P/Po 1 and the void space is assumed to be filled with condensed N2, then the pressure is lowered in increments to obtain a desorption isotherm. The vapor pressure of a liquid in a capillary depends on the radius of curvature, but in pores larger than 20 nm in diameter the radius of curvature has little effect on the vapor pressure however, this is of little importance because this region is overlapped by the Hg penetration method. 

A new family of ionization techniques allows ions to be created under ambient conditions and then collected and analyzed by MS. They can be divided into two major classes desorption electrospray ionization (DESI) and direct analysis in real time (DART). 

Electrospray (ESI) is an atmospheric pressure ionization source in which the sample is ionized at an ambient pressure and then transferred into the MS. It was first developed by John Fenn in the late 1980s [1] and rapidly became one of the most widely used ionization techniques in mass spectrometry due to its high sensitivity and versatility. It is a soft ionization technique for analytes present in solution therefore, it can easily be coupled with separation methods such as LC and capillary electrophoresis (CE). The development of ESI has a wide field of applications, from small polar molecules to high molecular weight compounds such as protein and nucleotides. In 2002, the Nobel Prize was awarded to John Fenn following his studies on electrospray, for the development of soft desorption ionization methods for mass spectrometric analyses of biological macromolecules. ... 

In the first step, the material located in the tube is heated in the stream of carrier gas and VOCs are released. Simultaneously, the volatile components are focused on a cold trap and later thermally desorbed with carrier into the GC. Thermal desorption is a useful technique, not only for analysis of air pollutants, but also in the case of ambient particulate matter [61]. 

Two new independently developed techniques called Dart ° (direct analysis in real time) and Desi (desorption electrospray ionisation) are making a huge impact on mass spectrometry. Together they remove the need for sample preparation and vacuum, speed up analysis time and can work in the open air. The sample is held in a gas or liquid stream at room temperature and the impact induces the surface desorption of ions. The ions then continue into the vacuum interface of the MS for analysis. Samples can be hard, soft or even liquid in nature. Ifa et al. have used Desi to image biological samples in two dimensions, recording images of tissue sections and the relative concentrations of molecules therein. Jeol have launched a commercial Dart ion source for non-contact analysis of materials in open air under ambient conditions. 

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