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Are My Shut-off Valves Contaminated?

24 Jul 2018

a machine with wires
Figure 1: Experimental set-up for testing valve contamination (plug valve shown). Gas flow was set at 20mL/min for 1 hour (1200mL total extraction volume) with oven held at 35⁰C and the split vent closed. After a one hour hold, oven was ramped to 330⁰C @ 20⁰C/min. An Agilent 7890A was used with an Rxi-5MS (30m x 0.53mm x 0.25µm) (cat. #: 13425) to allow for high flows. Inlet liner was a Topaz 4mm single taper liner with wool (cat #: 23303).

Shut-off valves provide a convenient means to control gas flows and are often used in conjunction with gas chromatography systems.  There are several types of shut-off valves including ball valves, plug valves, and diaphragm-sealed valves.  Some of these valves, ball and plug valves included, use a silicone-based lubricant to make opening and closing the valve easier and to extend lifetime by reducing wear on moving parts.  Unfortunately, this lubricant can off-gas into the flow path and lead to system contamination, causing baseline interferences.

Use a gas filter containing a hydrocarbon trap between lubricated valves and the GC to prevent any off-gassed components from entering your system.

I performed an experiment where I attached ball valves (cat. #: 23144) and plug valves (cat. #: 23146) directly behind the GC EPC and set a high flow of carrier gas for one hour (1200mL of Helium total volume), while leaving the split vent closed (Figure 1).  The GC oven was left at 35⁰C to trap off-gassed components on the head of the column.  Following this, I ran a series of “no-injection” blanks, with a temperature program, to qualitatively see if anything was off-gassing from the valves.  This experiment was also performed with a Super Clean Triple Gas filter (cat. #: 22019) installed between each valve and the instrument.

I found that the plug valves introduced a significant amount of contamination to the system (Figure 2).  Ball valves produced some contamination, as well, but at a much lower level than the plug valves (Figure 3).  This is likely due to the different opening and closing mechanisms of each valve and the much larger surface area of the lubricated plug.  Based on this, I would never recommend using a plug valve without a filter for carrier gas lines and would encourage the use of filters even with a ball valve.  Testing of multiple valves of each type showed similar results.

When these experiments were repeated using a filter in between the valve and the instrument, this contamination was effectively captured, as can be seen in Figure 2 and Figure 3, for each respective valve.  Note that I used a triple filter (traps hydrocarbons, moisture, and oxygen); however, a hydrocarbon filter is most critical for trapping this specific type of contamination.  Nonetheless, gas lines should have moisture and oxygen traps prior to the instrument, as well, hence my choice of the triple filter.


diagram

Figure 2: Contamination introduced from plug valve (red trace), measured with a GC-FID. By contrast, using a filter between the valve and the instrument (blue trace) produced a clean baseline.


chart, histogram

Figure 3: Contamination introduced from ball valve (red trace), measured with a GC-FID. Note that this is at a much lower level than the plug valve, but could still interfere with trace analyses. Using a filter between the valve and the instrument (blue trace) produced a clean baseline.

 

I was surprised at the amount of contamination witnessed, especially from the plug valves.  Many GC gas plumbing guides show shut-off valves downstream from the filters, placed directly before the instrument.  Keep in mind that with the large volume of gas passed over the filter with the split vent closed, this was designed to be somewhat of a worst-case scenario; however, similar results could be witnessed simply from letting an instrument sit idle for a period of time.

For these experiments I used an oven temperature of 35⁰C to trap off-gassed components on the column.  There are undoubtedly additional volatile components that are not effectively captured using this method.  Off-gassing of these volatile components could potentially create major contamination issues with sampling techniques where valves are placed before analytical traps for volatiles, such as thermal desorption units, purge and trap, etc.