3. FOOD ChEMISTRy & bIOTEChNOLOGy 3.1. Lectures

Chem. Listy, 102, s265–s1311 (2008) Food Chemistry & Biotechnology
L10 APPLICATION POTENTIAL OF NOVEL GAS
ChROMATOGRAPhy hIGh ThROuGhPuT
TIME-OF-FLIGhT MASS SPECTROMETERy
SySTEM (TRu TOF) IN FOOD AND
ENVIRONMENTAL ANALySIS
JAKUB SCHůREK, JAnA PULKRABOVá and JAnA
HAJŠLOVá
Department of Food Chemistry and Analysis, Faculty of Food
and Biochemical Technology, Institute of Chemical Technology,
Technická 5, Praha 166 28, Czech Republic,
jakub.schurek@vscht.cz
Introduction
In last years, analytical approaches employing gas chromatography
coupled to time-of-flight mass spectrometry
(GC-TOF MS) proved to be a useful tool in assessment of
quality and safety of food 1–4 and also environmental matrices
5 . But only recently, at the end of 2007, new time-of-flight
mass spectrometric (TOF MS) detector specialy designed for
high-throughput of samples, has been introduced. High throughput
is the key to increased profitability of the analyses,
while obtaining faster results and optimization. The need for
selected ion monitoring (SIM) operation and low dynamic
range associated with traditional quadrupoles and ion traps
may take time and money away from laboratory’s bottomline.
The assessed instrument (GC-HT TOF MS) is combining
fast acquisition mass spectrometer (80 Hz) with specific
data-mining algorithms. The aim of this benchtop instrumental
set-up is to achieve the speed and resolution necessary
to accomplish Time-Compressed Chromatography. Using
such detector, sufficient data density is obtained to accurately
characterize even the narrowest GC peaks produced under
conditions of fast chromatography separation. The acquisition
of the full mass spectral information of the sample with
comparable sensitivity as obtained by selected ion monitoring
(SIM) mode with quadrupole or ion trap instruments makes
feasible the application of a deconvolution algorithm obtaining
pure mass spectra even for coeluting compounds and
achieving reliable confirmation. Consequently, trace level
analysis of unknown sample components can be performed.
The schematic view of GC-HT TOF MS is shown in
Figure 1. Within the mass spectrometer source, the filament
continuously generates an electron beam. The GC effluent
is introduced into the source through a heated transfer line.
Electron ionization (EI) occurs as a result of interactions
between an electron beam with an analyte molecule from the
GC effluent. Chemical ionization (CI) occurs as a result of EI
interactions between the electron beam with the CI reagent
gas which creates charged reagent ions that ionize the analyte
molecules from the GC effluent. Ions are pulsed from the orthogonal
accelerator at a nominal frequency of 20 kHz. Each
pulse of ions into the flight tube results in a mass spectrum
which is referred to as a transient. The transients are then
summed to provide mass spectral acquisition rates up to
80 spectra second –1 . The focusing optics are used to direct
s564
ions through the system and ensure a high recovery of signal
at the detector. Deflection optics are used to prevent unwanted
signals, such as ions from a solvent front or unwanted background
ions generated by carrier gas or residual gas, and
extend the life of the detector. The ions are pulsed into the
flight tube with equal kinetic energies (K.E. = 1/2 mv 2 ).
Therefore, ions of varying mass-to-charge ratios will have
different velocities as they move through the flight tube.
Fragment ions with different velocities traveling over the
same fixed distance will have different arrival times at the
end of that distance (velocity = distance/time). Masses are
resolved in time-of-flight mass spectrometers by the time
each mass takes to reach the detector at the end of the flight
path (time = constant x m 1/2 ). For example, a mass of 100 mu
will take approximately 15 microseconds to reach the detector
while a mass of 1,000 mu will take 50 microseconds to
travel the same distance.
In this work we aimed to evaluate new GC-HT TOF
MS instrumentation in analysis of pesticides, pharmaceuticals,
poly-chlorinated biphenyls (PCBs) and poly-brominated
dibenzo ethers (PBDEs). The best GC-MS settings has
been optimized in order to obtaine fast and reliable analytical
methods for routine control of purified extracts of different
food-stuffs, or sediment, and water. Appraisal of mentioned
technique with respect to the cost and time demands was also
done.
Fig. 1. Schematic diagram of GC-hT TOF MS
Experimental
R e a g e n t s a n d M a t e r i a l
Tested compounds (listed in Table II) with purity ranging
from 95 to 99 % were purchased from Dr. Ehrenstorfer
(Augsburg, Germany) in case of pesticides and PCBs.
PBDEs and estrogenic pharmaceuticals were obtained from
Cambridge Isotope Laboratories (CIL, UK). All solvents
used within sample preparation (see Table I) were of analytical
grade (Scharlau, Barcelona, Spain). Working solutions
(concentration 1.25–250 µg dm –3 ) were prepared by series of
dilutions of the stock solutions (10 mg dm –3 ) with appropriate
solvent.