LC-MS Applications for Environmental Analysis

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5 Table 2. Operating conditions of desorption ionization probe
Instrument DART-SVP
Temperature 300 ˚C
Sample loading volume 5 µL
Carrier gas, pressure Helium, 75 psi
Mass Spectrometry
Due to the absence of separation in the DART source, the
whole sample is introduced into the mass spectrometer.
This unavoidably leads to a significant number of spectral
interferences. To correctly determine the masses of
relevant compounds and potential unknowns in the case
of fingerprinting analysis, it is essential to separate them
from the matrix ions. A mass spectrometer based on
chieves high mass resolving power
while maintaining excellent mass accuracy, without the
use of internal mass correction.9 These features make it an
ideal tool to complement DART ionization for the
analysis of complex samples.
A Thermo Scientific™ Exactive™ Orbitrap high-resolution,
accurate-mass mass spectrometer was used in full scan
mode. The resolving power was set to 50,000 (FWHM)
at m/z 200. The detailed conditions for the operation of
the mass spectrometer are summarized in Table 3.
Table 3. MS operating conditions
Parameter Setting
Scan range m/z 100–500
Resolving power 50,000 (FWHM at m/z 200)
Polarity Positive
Run time 0.5 min
Spray voltage 0 kV
Capillary temperature 250 °C
Capillary voltage 25 V
Tube lens voltage 170 V
Skimmer voltage 36 V
Results and Discussion
Mass Spectrum of Quinine and Mass Accuracy
Prior to analyzing the agricultural pesticides under review,
quinine (C20H24N 2O 2) was selected as a standard
compound for preliminary testing. A spectrum of quinine
was collected and analyzed using the DART-Exactive MS.
Five microliters of 1 ng/µL solution was applied to a metal
mesh using a micropipette. The mass spectrum for quinine
shown in Figure 2, was acquired under the operating
conditions outlined in Table 3. Comparison using the
simulated elemental composition feature in Thermo
Scientific™ Xcalibur™ software version 2.1 confirmed the
results and presence of carbon isotopes in the form of
[M+H]+. A mass accuracy 0.632 ppm was measured, so it
was possible to confirm the compound within an accuracy
of <1ppm.
Figure 2. Preliminary expanded ionization spectrum of quinine (C20H25O2N2)
Mass Spectrum Measurement of Target Compounds
A diluted solution of the 23 standard agricultural
pesticides was prepared at a concentration of 500 ng/mL
each and was measured three times under the
DART-Exactive MS conditions described in the previous
section. The mass spectra and corresponding mass
accuracies were recorded and confirmed by comparison to
the simulated elemental composition. The mass spectra
and accuracies of the target compounds are summarized
in Table 4 and Figure 3. All agricultural pesticides were
detected as [M+H]+, similar to quinine. There were no
Na+ or NH 4
+ adducts detected, confirming the ionization
as a Penning-type mechanism. The carbon isotopic
distribution was also used to confirm the compounds.
Those target compounds with a chlorine atom, such as
procymidone, acetochlor, propiconazole, dichlorovos,
tefluthrin, and prothiophos, showed isotopic ratios
typical of Cl-35 to Cl-37, with its natural abundance ratio
of 3:1. Bromacil, with bromine, showed the natural
abundance isotopic pattern of Br-79 to Br-81, which is
1:1. Mass accuracy was observed to be in the range of
0.053 to 0.870 ppm, which satisfied the condition of
being less than 1 ppm. Thus, DART combined with
HRAM mass spectrometry has substantial advantages as
an identification analysis method.