Chromatography Forum

You haven't had a problem with the sample "sticking" to the glass beads?

July,

Compare the surface area of a few glass beads to the surface area of the inside of the glass sampling bulb. No, I don't think they should be an issue.

Best regards

Hi again.

In the six valve sampling port discussed above, how is the sample delivered to the sampling port from the glass sampling bulb or canister? Are samples usually pressurized in the canister so that the sample with flow down the pressure gradient and fill the metal tubing? Also, how do you know how much sample is delivered to the metal tubing. I saw online that the sample loops come in different volumes, but how do you know that the entire volume is filled?

Bruce: The thermoses you spoke of - are holes cut in each end to allow the packed column to enter and exit into and from, or another way?

Thanks!

I'm not sure how to transfer sample from glass sampling bulb to sample loop, but for canisters, you have to pressureize it first to at least 8 psig(22.7 psia). The flushing time depends on the system volume which includes the loop and the tubings. You can measure the flushing flow rate using a flow meter. For example, if you have 5 ml loop, flushing flow rate of 50 ml/min, then 0.2 min flush should be enough. You can also try different flushing time to find out which gives you the optimum response.

For cannisters, you pressurize the cannister with a balance gas like air or nitrogen and then allow the cannister gas to flow through the sampling loop continuously with an upstream flow controller. As noted earlier, you let the sample flow long enough to ensure that the loop has been completely filled. For a gas sampling bulb, I use a syringe adapter that is connected directly to the sampling valve. I pull a volume of sample in a gas tight syringe, thread the syringe on the adapter, push out the contents of the syringe, wait a few seconds, and inject. Just make sure the syringe volume exceeds the loop volume by a fair amount.

Hi again.
In the six valve sampling port discussed above, how is the sample delivered to the sampling port from the glass sampling bulb or canister?
Thanks!

OK, there's another active thread about measuring pressure in gas sampling systems. If you use a cannister that is at ambient pressure, you need some way of sucking out the sample, and knowing the pressure in the sample loop.

On a six port valve, I have the sample loop ( 2 ports ) and the carrier in and out, and you have the sample loop fill lines ( in and out ). What I normally do is have a tee with a shutoff valve between the sample and the loop, along with a absolute pressure transducer ( means that I can measure vacuum and pressure up to about 5 pisg ).

I connect the sample cannister, to one arm of the tee, and the fill port of sample valve to the other. The other fill port can be blanked off, but
I prefer to connect a small volume of about 2 -5 mls there to remove tramp gases from the loop as it is evacuated. The bottom arm of the tee is connected to the transducer and then a shutoff valve going to a vacuum pump.

With the loop in the fill position, I open the vacuum valve and pull a high vacuum down to zero. Then I close the shutoff valve ( very important - otherwise you will suck your sample out ), and open the cannister valve slowly to fill the loop to the pressure or vacuum I want, and then firmly close the cannister valve. I then record the pressure and switch the valve into the inject position.

I suppose you could utilise the MS vacuum system, as the potential for contaimination from your samples will be small, but I prefer a separate two stage oil pump.

Bruce: The thermoses you spoke of - are holes cut in each end to allow the packed column to enter and exit into and from, or another way?

The stainless steel column is twisted into a coil with both the inlet and outlet at the same end, just like a dreshel bottle. It's usually formed first, then filled with packing, but probably doesn't have to be.

Remember that the column/trap connections to the sampling valve have to be sufficiently robust and leakproof to hold the column for both filling and emptying. We used to hold it suspended in a retort clamp and lift the bath up to it on a lab jack.

The coiled column/trap fits inside any suitable standard thermos flask. If you have two flasks, you have one with coolant and one hot water, but you can use a small plastic bucket/pail for the hot water. I'm sure there are better ways, but that worked for us.

Please keep having fun.

Bruce Hamilton