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Easy Maintenance and Changing of Columns in MS and vacuum GC using a 4-port Valve

22 Mar 2012

2011-jaap-pasfoto4

To set up a GC-MS detection system, it always takes considerable time to stabilize the system once the vacuum is applied. This pumping down and stabilization can take from several hours to some days to a night, before a decent calibration is possible and the instrument is in good shape to run the methods. If maintenance is done at the injection side, the vacuum usually is kept on the system. The power on the MS ion source might be turned off as well as the heating of the transfer line. Any air that is “sucked” into the system will do minimal damage. If a column is to be replaced, the vacuum of the system is turned off, and with the installation of a new column, the whole system has to go through the stabilization cycle again.

Products have been proposed, like Restek's EZ No-Vent. where a restriction is positioned inside the coupling device. This restriction limits the flow, which allows only a limited amount of air into the MS during column changes.

Challenges of such systems are:

There is still a certain amount of air allowed into the MS every time a column is changed. If the transfer line is not cooled down, it can activate the surface.

A higher inlet pressure is required because of the restriction of the EZ No-Vent.

The system cannot be used for operating vacuum (or low pressure) GC using 0.53mm ID columns.

diagram, schematic

Fig.1 Position 1: Column 1 connected to MS with Injector 1;

One can also consider using a 4-port valve which is connected like shown in figures 1 and 2.

A system is set up using 2 injection systems and 2 columns. In position 1, column 1 (injector 1) is connected with the transfer line to the MS. The effluent of column 2 (injector 2) is going to a second detector or to vent.

diagram, schematic

Fig. 2 Position 2: Column 2 connected with MS; Column 1 is ready for maintenance

When the valve is changed in position 2, column 2 is directly coupled with the MS, without any introduction of air. In this situation, column 1 can be cut or replaced. The MS in such systems will be kept under high vacuum; it does not have to be shutdown and also the transfer line is kept hot.

The valve will have a higher heat capacity, which can cause peak broadening due to thermal lag. A heated valve would be the best choice. Example of typical valve is the VICI 4N4WT, Heater assembly’s type HA1, and controllers, type ITC10399, are only at 110V available.

Especially when implementing vacuum GC, where a restriction is used in front of a 0.53mm column, such a valve system would be of interest. Maintenance here is usually done by replacing the entire restriction on the front of the column, meaning the MS vacuum and power has to be turned off. In the case of vacuum GC, the transfer line has to be a 0.53mm to minimize the pressure drop and have lowest absolute pressure inside the 0.53mm column.