6 February / March 2016
ATD/GC/FID. The tubes are placed on the autosampler and the operator starts the instrument, which initiates the process of moving the tube from the carousel into the primary desorption flow path. This process is shown schematically in Figure 2.
Figure 3: The secondary desorption step Sample Tube Desorb for 18 min @ 375o
Concentrator Trap Trap Low 5o Pneumatics Purge
Transfer Line Valve Temp
GC Cycle Time Column C C @ 30mL/min C; Trap high 380o C; Trap Hold 14min
inlet split 50mL/min; Outlet split 30mL/min; Column flow 0.8mL/mim Purge for 3min @ ambient temp @ 50mL/min 290°C 260o
34 min Table 1: Thermal Desorber Parameters
non-polar stationary phase 100% dimethyl polysiloxane: dimensions 20 m × 0.18 mm × 0.2 µm was used in this research
Carrier Flow Rate 0.4mL/min Oven
GC Run Time Detector temp
35°C for 4min, ramp 15°C/min to 230°; ramp 10o hold for 3min 30 min 340o
C Table 2: Gas Chromatographic Parameters C/min to 330o C and
The residue is desorbed from the sorbent tube using heat, inert gas flow and time. The effluent from the tube is focused onto a secondary (cold) trap. After residue recovery from the tube to the focusing trap is complete, the trap heats very rapidly to volatilise the components from the trap and the inert gas flow brings the effluent onto the analytical column of the gas chromatograph for separation and onto the FID for detection. This secondary desorption step is exemplified in Figure 3.
This acquired (raw) data is stored in the data handling system for processing. The processing method, which contains the response factor (RF) and integration parameters from standards previously analysed, is applied to the sample, and the mass of residue in the sample is calculated.
Operating Conditions
Parameters for the thermal desorption process are shown in Table 1, while the GC operating conditions are shown in Table 2.
Recovery Validation
The performance comparison between direct injection into a split/splitless (S/SL) injector port and an injection into the ATD was investigated to ensure recovery of the residue boiling point range by the ATD and to validate the sorbent tube injection technique. The inlet injection method was carried out by injecting a standard directly into the split/splitless injector of the GC, while with the thermal desorption technique, this standard was spiked onto a tube, and the analytes were desorbed onto the GC column. The results of the two injection techniques were compared.
To ensure recovery with no discrimination, a hydrocarbon standard was prepared representing the residue range. This is considered a recognised and valid test, since these targets have approximately the same response factor in an FID at the same component concentration. Percent (%) recoveries of the conventional liquid injection technique compared to thermal desorption are shown in Table 3, using the response factor for C22
as a reference. It Figure 4: A chromataogram of the residue standard, showing the low end in the red box
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