Extractive electrospray ionization EESI

Several other ionization methods have been developed based on DESI, including desorption atmospheric pressure chemical ionization (DAPCI), desorption atmospheric pressure photo-ionization (DAPPI), laser ablation electrospray ionization (LAESI), and extractive electrospray ionization (EESI). Each technique uses variations of the solvent, how the charged beam is formed, and how the beam is nsed to facilitate the prodnction of analyte ions. Because these are surface methods (except EESI), they are incompatible with LC. 

Desorption electrospray ionization (DESI) [1] was introduced at the end of 2004, and direct analysis in real time (DART) [2] soon after in 2005. The apparent potential of both DESI and DART in high-throughput applications soon led to the development of some derivatives with the intention to broaden the field of applications or to adapt the underlying methodology to specific analytical needs. Now, the repertoire of methods includes variations of the DESI theme such as desorption sonic spray ionization (DeSSI) [3], later renamed easy sonic spray ionization (EASI) [4] or extractive electrospray ionization (EESI) [5,6]. Then, there are the DESI analogs of APCI and APPI, i.e., desorption atmospheric-pressure chemical ionization (DAPCI) [7,8] and desorption atmospheric pressure photoionization (DAPPI) [9]. 

Analyte ions can also be efficiently generated when sample vapor or finely dispersed sample droplets transported by a carrier gas stream are admixed to the expanding electrospray plume. This technique, simple yet effective, has been introduced as extractive electrospray ionization (EESI) [37]. It utilizes two separate sprayers, one conventional ESI sprayer to provide the electrostatically charged mist and another to supply the sample vapor or mist (Fig. 13.13). While this approach is suggested for API interfaces with the heated transfer capillary design, the sample carrier stream may alternatively be passed into the desolvation gas of interfaces employing the heated curtain gas design (Fig. 13.14) [6,38]. 

In more recent work, Gu et al. have reported a geometry-independent neutral desorption device [57]. The instrument uses a neutral desorber gas which is directed onto the surface to remove the adsorbed analytes. A multistage extractive electrospray ionization mass spectrometer (EESI-MS) was used to successfully obtain spectra of traces of volatile explosives (i.e., TATP) and nonvolatile explosives, (i.e., 2,4,6-trinitrotoluene [TNT], 1,3,5-trinitroperhydro-l, 3,5-triazine [RDX], and octahydro-13,5,7-tetranitro-1,3,5,7-tetrazocine [HMX] from a series of surfaces, including wet and oily skin). Samples in the subpico-gram level were detected. 

Regarding the sample pretreatment step, traditional extraction techniques that involve the use of high amounts of toxic solvents should be avoided. In this sense, Chingin et al. [95] presented a novel procedure based oti extractive electrospray ionization mass spectrometry (EESI-MS) for the detection of DEP in perfumes with no need of sample pretreatment. Su et al. [96] determined DMP, DBP, dicyclohexyl phthalate (DCHP), and DNOP in perfumes and lacquer removers based on polymer monolith microextraction (PMME) and LC-UV. The conditions for extraction were optimized leading to a sensitive method. 

Chingin K, Gamez G, Chen H, Zhu L, Zenobi R (2008) Rapid classification of perfumes by extractive electrospray ionization mass spectrometry (EESI-MS). Rapid Commun Mass Spectrom 22 2009-2014... 

EESI Extractive electrospray ionization Sample vapor or mist admixed to electrospray plume [6,37]... 

Chingin, K. Gamez, G. Chen, H. Zhn, L. Zenobi, R. Rapid Classification of Perfumes by Extractive Electrospray Ionization Mass Spectrometry (EESI-MS). Rapid Commun. Mass Spectrom. 2008, 22, 2009-2014. 

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