Are My Shut-off Valves Contaminated?

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.

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.

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.

 

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6 Responses to “Are My Shut-off Valves Contaminated?”

  1. Rebecca Stevens says:

    Wow! Great work Linx,

    This is a new one to me! I shudder to think what might happen using one of these valves inline with something like the FID makeup gas.

    I know many people like to use these switching/shutoff (myself included) valves to enable easy instrument maintenance and carrier gas switching.
    Just confirms that high quality components and some careful planning are critical for long term peak performance of GC systems.

  2. Thanks Becca! Hope all has been well with you!

    This was a surprise to me as well. The plug valves were especially bad, as you can see. Having now taken one apart, it’s not surprising to me, as they are covered in lubrication that can easily get in the flow path.

    -Linx

  3. Renee Gallik says:

    Very nice

  4. Deer Linx,

    Very nice work! Makes GC people think … And worry! :-)

    A valve for shutting off gases between GC and filters is indeed very nice for GC maintenance purposes… Could it be an idéa to simply remove the lubricant from the shut-off valves? Will plug/ball valvs leak without lubricant inside?

    Kind regards –
    Lars Kürstein, Copenhagen

  5. Hi Lars,

    Thanks for reading! As far as I understand, the lubricant is mostly there for ease of turning and extending the lifetime of the valve. I think the plug valve in particular would be very difficult to turn without lubricant, due to the size of the plug itself. I am not sure if this would make them more likely to leak.

    An alternative to the more traditional ball/plug shut-off valves is a sealed diaphragm valve. These valves do not have any lubricant in the actual sample flow path. While Restek does not sell a dedicated “shut-off” style diaphragm valve, we do have the RAVE valve, a quarter turn diaphragm valve designed for use with air canisters. The valve is only available with hook-ups for 1/4″ fittings; however, you can use tube end reducers to allow for use on standard 1/8″ instrument lines. Note that these valves are heavier than a typical shut-off valve, so placement should be planned accordingly (i.e. I wouldn’t suspend them in the middle of a long unsupported gas line, as they’d put stress on the line).

    Thank you,
    Linx

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