Agriculture Reference
In-Depth Information
Figure 2.7.
A scheme of DESI ion source and ionization process.
In standard DESI, a geometric setup similar to that depicted in Figure 2.7 is used.
The sampling surface is mounted on a stage that can be freely moved in
x
-,
y
-, and
z
-
directions and allows rapid examination of a number of samples. The sampling
surface can be represented by either the sample itself (solid sample) or, in the case of
liquid sample or sample extract, deposition onto a suitable nonconductive material
followed by solvent evaporation. DESI can also be used for ionization of analytes
after their chromatographic separation directly from silica TLC plates [100]. Com-
pared with ESI, DESI has a greater number of critical experimental parameters that
dramatically in
uence the outcome of analysis. The main DESI parameters relate to
solvent (composition and pH), electrospray probe (e.g., solvent
ow rate or spray
voltage), source geometry (e.g., the incident and collection angle), and sampling
surface [97]. Their optimization for food-related DESI applications is discussed in
more detail in the following section.
In addition to the standard geometric con
guration, several modi
cations have
been developed to enable simpli
cation and to improve sensitivity of DESI measure-
ments. Chipuk and Brodbelt described the transmission mode (TM) DESI of liquid
samples and solid residues from evaporated solvents [101]. Instead of de
ecting the
spray from a surface, in TM-DESI, the electrospray is directed through a mesh screen
placed between the electrospray tip and the ion transfer line. Both incident and
collection angles are 0
. Such geometry enables reduction in experimental variables
while producing high-quality mass spectra. Another approach was introduced by
Venter and Cooks who enclosed the DESI source in a pressure-tight enclosure
with
°
fixed spatial relationship between the sprayer, surface, and the ion transfer line.
Of the geometries tested, the combination of 90
incident and collection angles was
found to be most favorable in terms of ease of operation and signal stability [102].
°
