Experimental Conditions
All samples were prepared using the QuEChERS technique,
and calibration was performed using a blank QuEChERS
extract from cucumber. All target compounds were
measured using at least two SRM transitions for each
compound to a level of 0.001 mg/kg, which is ten times
lower than the current maximum concentration limit.
All sample analyses were carried out using the TSQ
Quantum XLS Ultra GC-MS/MS system, equipped with
a Thermo Scientific TRACE GC Ultra gas chromatograph.
The TRACE GC Ultra
™
was configured with a B.E.S.T.
PTV injector equipped with a backflush device. Sample
introduction was performed using the Thermo Scientific
TriPlus RSH autosampler. The capillary column was a
Thermo Scientific TraceGOLD TG-5MS column (5%
phenyl film) of 30 m length, 0.25 mm inner diameter
and 0.25 μm film thickness (Table 1).
The pre-column used was a 1.2 m TG-5HT, 0.15 μm
film thickness and 0.53 mm inner diameter (see Table 1).
Maximizing Robustness
High boiling compounds in sample matrix have a negative
effect on the analytical column’s quality and lifetime,
requiring a bake out process at high temperatures, thus
limiting sample throughput. A backflush process was
used to protect the column, allowing more samples to be
injected before the phase attachment on the surface of
the column becomes weak. Being able to inject more
samples before necessary column replacement improves
throughput and reduces costs per analyses.
During backflushing of the pre-column, the injector was
set to a higher temperature and increased flow. This also
allowed the injector liner to be swept of residual matrix
contaminants during analysis time. This concurrent
backflush operation results in the complete system staying
clean and inert for a high number of injections, resulting
in less maintenance frequencies.
3
Method Setup
The method parameters for the PTV concurrent backflush
operation, GC separation and TSQ Quantum XLS Ultra
mass spectrometer setup are given in Table 1.
Each compound SRM transition was only monitored for
a narrow time window around the established retention
time (timed SRM). This led to a fully optimized instrument
duty cycle for maximum analytical performance being
handled automatically by the system. The complete list
can be copied into the instrument method, thus saving
time and avoiding entry errors.
4
For data acquisition, the two most selective transitions were
chosen after reviewing data from spiked matrix samples.
Selection criteria were based on the absence of interferences
from the matrix, along with signal generation of the
transition.
Results and Discussion
Advanced GC-MS/MS Experiments – U-SRM
The patented Thermo Scientific HyperQuad technology
in the TSQ Quantum XLS Ultra system offered high
sensitivity by high ion transmission already found at
the standard nominal mass resolution settings (0.7 Da
FWHM). In addition, the HyperQuad
™
technology allows
the possibility to enhance the applied mass resolution for
increased selectivity during analysis. The significantly
increased selectivity further reduces the background caused
by matrix components, thus giving a cleaner peak detection
and high signal-to-noise results.
Some compound transitions are more susceptible to
matrix interference than others. Standard SRM resolution
(0.7 Da) can often provide enough selectivity to overcome
most matrix interference challenges. In complex matrices,
however, even with the structure-selective SRM acquisitions,
removal of the isobaric matrix interference is insufficient.
2
TRACE GC Ultra
Injection Volume
2 μL injection
Liner
Siltec
®
baffled liner
(part number 453T2120)
Carrier Gas
He, constant flow, 1.3 mL/min
Column Type
TraceGOLD
™
TG-5MS column (5% phenyl film) of
30 m length, 0.25 mm inner diameter and 0.25 μm
film thickness (part number 26098-1420)
Precolumn
1.2 m of TraceGOLD TG-5HT column of 30 m
length, 0.53 mm inner diameter and 0.15 µm film
thickness (part number 26095-0620)
GC Method
Initial 65 °C, Hold 1.5 min,
Ramp 30.0 °C/min–150 °C,
Ramp 5.0 °C/min–290 °C,
Ramp 30.0 °C/min–320 °C, Hold 5.0 min
Transfer Line
300 °C
TRACE GC Ultra PTV Program
Injector Temperature
70 °C, splitless injection 1.5 min
PTV Inject
70 °C, 0.2 min, 8 °C/sec to transfer step
PTV Transfer
280 °C, 21 min, 10 °C/sec to clean step
PTV Clean
350 °C, 33 min, clean flow 30 mL/min
Transfer Time
21 min
TSQ Quantum XLS Ultra Mass Spectrometer
Source Temperature
240 °C, CEI volume
Ionization
EI, 70 eV
Emission Current
50 μA
Resolution
0.7 Da Q1, Q3; 0.1 Da on Q1, 0.7 on Q3 for the
wheat examples
Collision Gas
Argon, 1.5 mTorr
Table 1: Selected instrument conditions for the employed TRACE GC
Ultra and TSQ Quantum XLS Ultra mass spectrometer
