Agriculture Reference
In-Depth Information
substrate also affect both ionization ef
ciency and homogeneity of the sample (when it
is prepared by deposition from a solvent). Materials with high surface roughness, for
which low af
nity is provided by analytes, should be used [97].
In high-throughput DESI measurements, it is important to achieve suf
cient
spatial resolution, as sample-to-sample cross-contamination can occur during
analysis of sample series deposited close to one another. The spatial resolution
can be improved by tuning of
and
d
1
parameters and/or by decreasing both the
internal diameter of the spray capillary and the
α
flow rate. Additional attention has to
be paid to sample preparation prior to DESI of powder or dust samples due to
potential MS system contamination problems. The use of double-sided adhesive
tape or rinsing with methanol followed by analysis of dried droplets has been
proposed to handle such sample types. Regardless of the type of sample, the mass
spectra should always be background corrected with the use of records obtained
from
“
blank
”
surfaces [95,97].
DART Source Parameters
The following parameters should be considered when
optimizing DART analyses:
DART source geometry (sample position and ion source exit-to-MS inlet
distance).
Ionization gas parameters (type, temperature, and
ow).
DART source voltages (discharge needle voltage and perforated and grid
electrode voltages).
Dopants (type and method of introduction into the ionization region).
SettingslikelytobeusedinatypicalDART-MSapplicationareprovidedin
Table 2.4.
The position of the sample in the DART ionization region is a critical factor that
in
uences sensitivity. Liquid samples spread on the surface of a glass capillary
should always be placed slightly off the axis between the source exit and the MS
inlet so that the gas stream is not blocked. Alternatively, the sample can be moved
through the gas stream in the perpendicular direction. Harris et al. studied the
impact of solid sample (tablet) position on the signal intensity and found that the
highest ion transmission was observed when placing the tablet in an upright
position close to the DART gun exit [119]. Because there are no comprehensive
data from which some generally applicable settings for positioning of samples
can be derived, a case-to-case optimization of this parameter should always be
performed [96,115].
The most frequent ionization gas employed in DART is helium. However, the use
of nitrogen, neon, and argon has also been considered. Because the metastables
derived from various gases have different energies, their ability to directly ionize
atoms and molecules present in the ionization region differs greatly. Only species with
lower ionization energies can be directly ionized. The helium 2
3
S excited-state
metastable species has an energy of 19.8 eV, which is high enough to induce
