A better way to configure your EZ No-Vent GC-MS Connector

If you are using the EZ No-Vent for Agilent mass spectrometers (cat.# 21323), you are probably tricking your instrument into working properly by inflating the length of your column in your acquisition software. If you have MassHunter, or a recent version of MSD ChemStation  (G1701EA), you don't have to do this. Instead of configuring your 30 m x 0.25 mm ID column as a 112 m column, you should configure a composite column (see Figure 1). Even if you aren't using the EZ No-Vent with your mass spectrometer, you should be configuring your column as a composite column because the 17 cm transfer line is heated by the transfer line heater, not the oven.

Figure 1 - 30 m column configured as a composite column in MassHunter (without the EZ No-Vent)

Figure 2 shows a composite column configured with the 17 cm x 0.10 mm ID vacuum restrictor, which is the part of the EZ No-Vent that allows you to disconnect your analytical column without turning off your MSD. You'll notice the average linear velocity is significantly reduced when the EZ No-Vent is used (39 cm/sec drops to 26.8 cm/sec) even though the volumetric flow stays the same. This has some negative chromatographic effects.

Figure 2 - 30 m column configured as a composite column in MassHunter (with the EZ No-Vent)

Figure 3 is an example chromatogram collected on a 30 m x 0.25 mm x 0.25 µm column without using the EZ No-Vent. Figure 4 shows the chromatographic effect of installing the EZ No-Vent while keeping all acquisition parameters the same (aside from configuring the restrictor in the composite column).

Figure 3 - Example Chromatogram collected without the EZ No-Vent

Figure 4 - Same acquisition parameters as Figure 3, but collected with the EZ No-Vent installed

The good resolution of the first eluting compound (1,4-Dioxane) and the dichloromethane solvent peak has collapsed, in Figure 4, and Aniline and bis(2-Chloroethyl)ether are now coeluting. The resolution for the two PAH pairs has also degraded. Lowering starting oven temperature 20°C improves the separation of 1,4-dioxane and solvent peak, though 1,4-dioxane is still on the solvent peak's tail (see Figure 5). The PAH resolutions have also improved, almost to the level of performance when the EZ No-Vent isn't installed.

Figure 5 - Chromatography is improved by reducing the starting oven temperature.