In the QuEChERS multi-pesticide method, some of the extracts, which are contaminated by matrix, are injected into a temperature-programmable injector (PTV/MMI).

Since up to 5 μL of the extract is injected, the solvent (acetonitrile) must first be evaporated at a temperature of 70 °C (solvent vent mode) before the injector is heated to 250°C - 280 °C with the split valve closed.

In this case, the vaporizable components should be completely transferred to the analytical separation column. In the glass liner remain the non-evaporable constituents of the extract from the sample matrix which gradually, increasingly with the number of sample injections, contaminate the inner surface of the liner.

This in turn leads to problems in the transfer of various components to the separation column. As a consequence the injector liner requires to be exchanged at short intervals, which can only happen automatically with longer sample sequences (overnight, weekend) with high and expensive technical effort (LINEX, ALEX etc.).

The ExLin injection method allows for having each sample its own liner, which can be disposed to waste after the injection.

Only vaporizable components are introduced into the measuring system (PTV/MMI injector), resulting in less contamination of the GC/MS system thus increasing the service life.

Furthermore significantly larger sample volumes than before can be introduced, since prior to the injection step the solvent is automatically evaporated.

System Description: Agilent GC 6890N/ UNIS PTV/ CTC Combi Pal and CHROMTECH TDAS2000

Methods Description:

Using the CTC Combi PAL autosampler, 10 μL each of sample extract is taken from a 2.5 mL vial and placed into a TDAS thermal desorption tube filled with glass wool.

The solvent is then evaporated to dryness (N2) for 10 minutes.

Thereafter the TDAS desorption tube is picked up by the CTC Combi PAL and placed into the Chromtech TDAS 2000 oven.

At n oven temperature of 300°C the vaporizable components are transferred by means of a helium flow (50 mL /min) onto the PTV/MMI inlet which itself is cooled down to -10 ° C for cryofocussing .

After about 5 minutes, the PTV/MMI inlet is heated up to 250 °C rapidly, transferring the focussed analytes onto the analytical GC column.

For a 3-point calibration, 10 μL each of a 0.1, 1 and 10 ppb standard solution is used.

Measured samples according to the QuEChERS method:

Pear, organic orange, organic carrot, grape and parsley

Below you will find the results and calibration curves. Click to enlarge the pictures: