Desorption electrospray ionization solvents

Desorption Electrospray Ionization (DESI). DESI is a novel gentle ionization method for surface analysis (Figure 2.6).[19,20] Like classical ESI, it operates at atmospheric pressure. No sample preparation is required. A solvent passes through the capillary of the electrospray source charged droplets are produced (primary ions) and they are directed to a solid sample. Their impact with the surface causes sample molecules to be ionized and... 

DESI Desorption electrospray ionization (DESI) is a recently developed technique that permits formation of gas-phase ions at atmospheric pressure without requiring prior sample extraction or preparation. A solvent is electrosprayed at the surface of a condensed-phase target substance. Volatilized ions containing the electrosprayed droplets and the surface composition of the target are formed from the surface and subjected to mass analysis (Takats et al., 2005 Wiseman et al., 2005 Kauppila et al., 2006). 

A new ionization method called desorption electrospray ionization (DESI) was described by Cooks and his co-workers in 2004 [86]. This direct probe exposure method based on ESI can be used on samples under ambient conditions with no preparation. The principle is illustrated in Figure 1.36. An ionized stream of solvent that is produced by an ESI source is sprayed on the surface of the analysed sample. The exact mechanism is not yet established, but it seems that the charged droplets and ions of solvent desorb and extract some sample material and bounce to the inlet capillary of an atmospheric pressure interface of a mass spectrometer. The fact is that samples of peptides or proteins produce multiply charged ions, strongly suggesting dissolution of the analyte in the charged droplet. Furthermore, the solution that is sprayed can be selected to optimize the signal or selectively to ionize particular compounds. 

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>. 

Methods Ambient ionization methods, of which there are now over 20, e.g., desorption electrospray ionization (DESI), desorption atmospheric pressure chemical ionization (DAPC), desorption atmospheric pressme photo-ionization (DAPPI), and direct analysis in real time (DART), are now joined by paper spray, a method where ESI is initiated at the pointed tip of a piece of filter paper. A drop of blood ( 15 pi) is dried on the paper, and then the paper is moistened with 25 pi of a solvent suited to both the extraction of the analytes from the blood and the ESI process (e.g., 90% methanol 10% water with either 100 ppm acetic acid or 200 ppm sodium acetate). When the paper is exposed to high voltage (3-5 kV) while held close ( 5 mm) to the entrance of the mass analyzer, a spray (similar to electrospray) is induced at the tip of the paper as capillary action carries extracted compounds through the paper (Figure 4.5). The spray is maintained for 30-90 s at a flow rate comparable to that used in nano-electrospray. 

Ionization techniques derived from ESI, such as electrosonic spray ionization (ESSI), are also suitable for studies of fragile molecules. The ESSI-MS approach enables observation of non-covalent complexes of myoglobin, protein kinase A/ATP complex, and other proteins [47]. It can be used to study on-line deprotonation reactions on peptides and proteins [48,49]. Such analyses can be done by introducing volatile bases between the ion source and mass analyzer. This method is fast one reference base can be scanned in a time interval of 1 min. The desorption electrospray ionization (DESI) technique was also implemented in the study of protein conformation in solution [50]. The interaction time between the spray solvent and the protein was estimated to be 1 ms, and it was suggested that this timescale would be too short for the studied proteins to unfold [50]. 

Desorption electrospray ionization (DESI) is a variant of ESI [33]. Electrosprayed aqueous solvent, with charged droplets and solvent ions, is directed at the sample of interest at ambient conditions. The sample may be a native sample or an analyte placed on an insulating surface. The analyte is desorbed by the result of either sputtering or impact desolvation. Desorbed ions are transferred into the vacuum system via an ion transfer line and mass analyzed. Jackson et al. [33] used DESI to analyze PMMA, poly (alpha-methyl styrene), and other polymers. 

Monolithic silica UTLC plates were used for desorption electrospray ionization-mass spectrometry (DESI-MS) after partial separation of acetylcholine (m/z 146 M+), diazepam (m/z 285 [M-i-H]+), testosterone (m/z 289 [M-i-H]9, midazolam (m/z 326 [M-i-H]9, dobutamin (m/z 302 [M-i-H]+), and verapamil (m/z 455 [M-i-H]9 with LODs of 1-100 pmol. An electrospray solvent consisting of water-methanol-formic acid (50/50/0.1%) was used at a flow rate of 7 pL min [10]. 

The first publication on desorption electrospray ionization mass spectrometry (DESI-MS) appeared in 2004 (1). That paper has been followed by a stream of publications on a variety of atmospheric pressure surface sampling/ionization techniques for mass spectrometry (2). The popularity of DESI can be owed to its ease of use. In DESI, charged solvent droplets from a pneumatically assisted electrospray ionization source impact the surface to be analyzed, desorbing and ionizing analytes. The gas-phase analyte ions are then collected and transferred into the inlet of the mass spectrometer either directly or by using a transfer tube. 

The third focus is the use of IL as supports for MS. The application of the IL enabling electrospray ionization (ESI) using nonpolar solvents will be demonstrated. A major emphasis will be put on the use of ILs as matrices for matrix-assisted laser desorption/ionization (MALDI) MS, a field gaining emerging interest in the last few years. 

Electrospray Ionization/Mass Spectrometry (ES/MS), a soft-ionization desorption technique using polar solvents such as water, methanol, or acetonitrile, was used for direct measurement of cations in solution. The first measurements carried out with mono or bis(crown-6) calix[4]arenes from an equimolar cation-extractant solution confirm that the calixarenes mono(crown-6) extract only one cesium cation. On the contrary, in the same conditions, bis(crown-6) calix[4]arenes can extract two cesium cations for a ratio Cs/BC6 equal to 2.5. The binuclear complex (composed of two cesium cations) is the major species. Cesium/sodium selectivity measurements implementing various mono or bis(crown-6) calix[4]arenes were in agreement with liquid/liquid results.42... 

With electrospray ionization tandem mass spectrometry (ESI-MS/MS), the liquid sample is introduced into the ion source through a capillary tube and exposed to a strong electric field and a counter flow of nitrogen gas, which in combination produce the electrospray (see Chapter 7). The solvent evaporates and eventually desorption of charged ions occurs into the mass analyzer. ESI occurs under atmospheric... 

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