Agriculture Reference
In-Depth Information
Figure 2.8.
A scheme of EESI ion source and ionization process.
This so-called geometry-independent DESI enabled simpler and more ef
cient
sampling compared with the conventional DESI setup.
Extractive Electrospray Ionization
Extractive electrospray ionization (EESI) was
primarily developed for ionization of (semi)volatile analytes in complex matrices and
represents a variant of DESI [103]. The EESI source uses two separate sprayers: one is
an auxiliary electrospray, which generates charged droplets of solvent, while the
second acts as a nebulizer and is used to deliver the sample into the ionization region
(Figure 2.8). The ionization process itself takes place in the EESI plume and involves
liquid- and gas-phase interactions between the neutral analyte droplets/molecules and
solvent-derived charged droplets/ions. The predominant ionization mechanism is
probably based on condensed phase extraction of analyte molecules into the charged
droplets and ESI processes leading to analyte ion formation [104]. Because the sample
nebulization and ionization are separated in both space and time, EESI exhibits
signi
cantly higher tolerance to sample matrix and lowers the adverse impact of
ion suppression effects on analyte intensities.
Liquids can be sampled with EESI by direct infusion and subsequent nebulization
in the sampling sprayer. Alternatively, noninvasive neutral desorption (ND) of
analytes can be performed by directing the neutral nitrogen gas stream onto the
sample surface. As a consequence, sample droplets are formed through a micro-
ejection mechanism: microdroplets are transported with carrier gas into the electro-
spray plume for ionization [105].
