F

DIOS chip

Fig. 8.7 Electrospray deposition (ESD). During the ESD process, an electrospray capillary is used to generate small positively charged droplets containing the analytes. The droplets are sprayed onto the silicon surface, where a uniform and thin layer is formed.

DIOS chip

Fig. 8.7 Electrospray deposition (ESD). During the ESD process, an electrospray capillary is used to generate small positively charged droplets containing the analytes. The droplets are sprayed onto the silicon surface, where a uniform and thin layer is formed.

the application of electrospray deposition (ESD) [15]. During the ESD process, small positively charged droplets are sprayed onto the porous silicon chip and form a uniform thin layer of the analyte (Fig. 8.7). Subsequent to spotting, the chip is evaporated to dryness. Analysis can then be performed by using standard MALDI-MS equipment, i.e. pulsed laser set-up and TOF mass spectrometer.

Regarding the ionization process, it is believed that - similar to MALDI - the porous structure of silicon serves as a scaffold for both solvent and analyte molecules while, simultaneously, the high UV absorptivity of the material enables an efficient transfer of the laser pulse energy towards the analyte molecules [14]. A major advantage of DIOS is the fact that little to no fragmentation is induced by the desorption/ionization process. However, the mass range of the methodology is limited to analytes of a molecular weight below 18 000 Da, with the highest efficiency for analytes below 3000 Da [16].

Although DIOS-MS is mainly a tool for qualitative analysis, many examples have shown that quantitative analysis is possible when internal standards are used. These may either be isotope-labelled - mostly deuterated - compounds or structurally related analogues. For example, subsequent to electrospray deposition, amino acids such as phenylalanine and tyrosine have been successfully quantified by means of DIOS-MS using their deuterated analogues as internal standards.

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