Water determination by GC

[JAROCZAJKA]

Dear All,

I'm looking for any support for analysis of water in permanent gases. I have one mixture of 500ppm H2O in He. I use GC with 6 port sample GSV valve with 250ul sample loop and TCD detector. I have packed column inlet optimized for 0,53 colum. I use HP Plot Q 30m column.
In case of injection by sample valve i recived very bad results. It is really difficult to find out any peak that comes from water.
in case of injection by syringe 500ul i can see very good peak with quite good shape.
I'm not really trined in GC technics so pleas ehelp me.
I need to determine water in different gases like O2, Ae, N2, CO2.

JCZajka

[aldehyde]

It is possible that the valve is not actuating either because the valve is physically not moving, or the rotor is clogged with residue. Since you have tried manual injection and it appears to be working correctly the problem is most likely the valve and not anything to do with the inlet, column, or detector (phew).

When did this start? Is this a new method or something that was working fine for a long time and just suddenly stopped working?

Do you have any chromatograms so we can see what is going on? Preferably 1 good and 1 bad?

[JAROCZAJKA]

We are just starting with new methode and it seems that the problem is due to w ehave only one valve. we are purging by standard mixes of 500ppw H2O in He several seconds and that we start injection with injection time of the valve 1min.
The GC system is new brand and there is VALCO valve installed.

best regards,
JCzajka

[Johnny Rod]

It is possible that you see a peak with manual injection because you are injecting much more water. Atmospheric moisture varies but is more like 20-30x what you have in your standard.

Moisture is difficult to measure compared to other materials. You'll need to purge the sample valve etc. for several minutes at least to make sure you have it all equilibrated. Typically the less careful you are, the bigger the peak, as there is some air left in your sample system.

Can you increase the TCD sensitivity? Or try a larger sample loop e.g. 1ml?

[JAROCZAJKA]

It is possible that you see a peak with manual injection because you are injecting much more water. Atmospheric moisture varies but is more like 20-30x what you have in your standard.

Moisture is difficult to measure compared to other materials. You'll need to purge the sample valve etc. for several minutes at least to make sure you have it all equilibrated. Typically the less careful you are, the bigger the peak, as there is some air left in your sample system.

Can you increase the TCD sensitivity? Or try a larger sample loop e.g. 1ml?

We use max sensityvity of TCD and 1ml of the sample. we heat ovent at 100C and tried to heat valve over 100c - no results. We are experimenting with gradient flows and different temperatures.
I hope the syringe was purged well with our standard mixture - 500ppm H2O in He. We used special bags for gas sample and purged it several seconds without syringe plunger. After purging we have place the plunger into the syringe and took out from the sampel bag. We have made the injection. maybe some of air moistures came into syringe but it seems we have purged it well. If we inject from valve we do not observed anything looking like one peak. It looks like double hill curve so long.

Best regards,
JCzajka

[Johnny Rod]

First of all I would not try to analyse for any atmospheric gas by using a syringe. Pipe it straight from the cylinder (via a regulator) into the GSV.

What flow etc. are you using for your column?

Have you run any standards? Do you know the retention times of some common peaks e.g. O2/N2, CO2, methane, ethane? According to Agilent Tech Note 228-387, water should elute between ethane and propane, closer to propane. It doesn't say what concentration of water they have in the sample though.

How often do you have to do this work? It might be easier to use something like a Shaw meter? Although for those you need a large amount of sample.

[JAROCZAJKA]

First of all I would not try to analyse for any atmospheric gas by using a syringe. Pipe it straight from the cylinder (via a regulator) into the GSV.

What flow etc. are you using for your column?

Have you run any standards? Do you know the retention times of some common peaks e.g. O2/N2, CO2, methane, ethane? According to Agilent Tech Note 228-387, water should elute between ethane and propane, closer to propane. It doesn't say what concentration of water they have in the sample though.

How often do you have to do this work? It might be easier to use something like a Shaw meter? Although for those you need a large amount of sample.

Hello,

We are preparing methods for determination several different gas mixes and to confirm +/-2% their contents. Also we need to determine purity of each gas used for preparing mixes accroding H2O presents. due to our customer ship gase mixes for lasers system they have control the level of h20 as impurity in each gas.
We use HP-Plot Q but it separate rather poor N2 and H2 and the separation of Ar and O2 is bad as well. We will start with Molsieve capillary column next week. We are using TCD but I realize it must be PD-HID for 1-2ppm of waters as LOD. Customer informed us with wrong LOD at 200ppm so we have chosen TCD.
We have some standard gas mixes ordered from AirProducts. The water standard is 500ppm in He.
We are purging directly GSV from cylider of our standard mixes by regulator connected with GSV sample inlet by teflon 1/8" tube (similiar like in hplc systems). The flow of filling sample is around 40-60ml/min. We have measured it. The CO2 determination in that way gave us very good linearity and precision. r2=0,999761 with crossing 0,0 point.

best regards,
JCzajka

Best regards,
JCzajka

[Johnny Rod]

For laser cutting yes your customers will need very low moisture content. You won't get far with a molsieve column though - the water will adsorb and will not elute. Plot Q is not great for separating permanent gases, the molsieve is better but CO2 and water will not elute. A few things to think about:

If you use PDID you'll need an arrangement to heartcut otherwise the detector could be damaged by the matrix peak (I'm no PDID expert but this is as I understand it and is how ours is set up).

Do you definitely need to measure moisture by GC? It sounds like you should have plenty of sample (i.e. a cylinder full) in which case a moisture meter may be a better bet.

Do you need the GC to separate all gases or is it good enough that you can separate the water from the matrix? If you do then it could be quite a job.

[AICMM]

JAROCZAJKA,

Johhny Rod's advice is great. First, if you can afford it, moisture in gases is really easy to do with a moisture meter. Problem quickly solved. Second, if you have to use GC (which is a puzzle in this application) then I would recommend a really thick film DB-5 type column. 30 or 60 m X 0.53 with 5 u film for example. The water will elute fairly symetrically and after the gases. This is how a friend of mine did it with his application. (He started out with moisture in helium and then moved to moisture in ethylene oxide.)

Third, a mol-sieve will not work (as advised.) Fourth, you don't say what carrier you are using with your TCD but it should be helium for this application for you to have a chance of seeing 500 ppm with a TCD. You also don't discuss flow rates. Fifth, an helium ionization detector is essential to get to low level water (DBD, PDD, or HID, standard disclaimer for me is applicable here.) If shooting inerts like oxygen, argon, etc, then the detector should be unaffected by the bulk gas (at least mine is.) Stick with the valve injection (no syringes if you can avoid them) since you can control the system much more.

Best regards,

AICMM