Dual Column single MS setup, VOA/SVOA on one instrument

[James_Ball]

Ok, so I have a new Agilent 7000QQQ to play with and I am trying to maximize what can be done with it. Currently I have it set up with a purge and trap connected to the rear injector and an injection tower on the front injector. Rear column is 60m x 0.25mm x 1um Rxi-624SilMS and front column is Rxi-5SilMS 60m x 0.18mm x 0.36um. The columns go into a Restek MXT metal Y connector then into a 0.10mm transfer line into the MS. So far the setup works pretty good.

I have run EPA 525.3 and can calibrate for the PAH, Adipates an Phthalates down to a low calibration of 0.01ppm with a 1ul injection full scan. Can also go down to 0.25ppb on the EPA524 Volatiles with a 5ml purge.

The volatile column allows me to take the oven temperature up to 300C so I can elute the last three PAH compounds, but it does seem to have more bleed than I am used to seeing. I was using the Rtx-502.2 but the upper limit of 270C just would not allow elution of the last PAH compounds with reasonable peak shapes or retention times.

Has anyone else used the Rxi-624SilMS column and have you noticed higher bleed on MS applications?

I also seem to have extremely high bleed from this column if I use Hydrogen carrier in it.

I was playing with the concepts in the paper from Agilent: http://hpst.cz/sites/default/files/uplo ... alyzer.pdf

My idea was to use the second column as the source for Hydrogen instead of extra flow controllers. It works really well when you do the volatile analysis and bleed in about 0.2ml/min H2 through the SVOA column. I actually found it stabilized the responses for the 1,4-Dichlorobenzene and other late eluters so that they do not increase with increasing standard concentration, which has been a problem for me for ages. But when I try it with the SVOA I get a ton of bleed from the VOA column, if I put He through the VOA column it goes away. This didn't happen when I was using the Rtx-502.2 column in the VOA side.

I am still doing some tweaking, and have found some things that you have to watch out for when doing this, but overall the setup is working great. One side benefit is you can actually use the second column and Y connector to work as a No-Vent setup when you need to change columns. Just bump up the flow of the column you are not changing and it keeps air from being sucked into the MS so you can make a column change without venting. This is the connector I am using: http://www.restek.com/catalog/view/3355 .

One thing to watch out for, you need to keep the flow of the inactive column low, or you will begin to see waves in the baseline. I think this comes from the fact that the flow control is based on there being a vacuum at the end of the column, but with the two columns flowing into a restriction, there is not a true vacuum there and the competing pressures cause fluctuations in the actual column flows. If I use greater than 0.7 ml/min on the active and less than 0.3ml/min on the inactive column the fluctuations go away. I wish I could get the 624silms column in a 40m x 0.18 configuration as it would make setting the low flow with H2 much easier, but that is something to look into in the future.

Next test will be adding pesticides to the SVOA mix and seeing how low I can go with those, so that maybe we can replace the ECD methods for drinking waters. 525.3 allows for using SIM so hopefully that will allow me to move down even further. I only wish I could use MRM for those, that would be awesome. But that will be a future project for trying to do Quechers work.

Anyone have any comments or suggestions on ways to improve this setup? I am sure there is room for improvement and I will try anything that will make it better

[Yama001]

Looks like you are having some fun here, good luck .

I am a little surprised Restek is claiming a 300 limit on a 624 type phase - 250 was the last I was familier with.

[James_Ball]

Looks like you are having some fun here, good luck .

I am a little surprised Restek is claiming a 300 limit on a 624 type phase - 250 was the last I was familiar with.

It is a new phase that is a modified 624, the box lists 320 max but I found quite a bit of bleed at that temp. Online though they list 300 for the column I have so I backed it down and it is better, but still makes the baseline high for the 525 run. I was most surprised when I found much higher bleed when using H2, since with 5 phase columns like the rxi5 and rtx502 I don't see more bleed from just switching to H2 as carrier. I even baked the 624 out over night at 300c with H2 and it didn't seem to help.

I do like though that I can bake out the source at 350c with a flow of H2 which really cleans out the background. I plumbed in a manual switching valve at the back of the instrument so I can switch either inlet between He and H2 which makes experimenting much easier.

[James_Ball]

A little update.

So far I have had good results with the 525.3 and am now working on 524.2 for volatiles. I am now seeing very low response for 2,2-dichloropropane and carbon tetrachloride. No response for these at 0.5ppb and 1.0ppb and small response for carbon tetrachloride at 5ppb. This system has run nothing but blanks and standards, and I have replaced the transfer line with a 0.53mm silcosteel line and have tried both a regular 0.75mm straight inlet liner and one of the Restek Sky deactivated liners of the same size. Also I replaced the gold seal with a silcosteel coated one just to see what would happen, and still have the same problem. cis and trans 1,3-dichloropropene are also giving low response but not so low as the other two compounds. 1,1,2,2-tetrachloroethane has great response, which is odd since that is one that usually breaks down first.

Any ideas on what to try next?

[Bigbear]

How old is your trap? Vocarb 3000?
You can always do a direct injection at the GC and compare to a purged std of the same concentration.

[James_Ball]

How old is your trap? Vocarb 3000?
You can always do a direct injection at the GC and compare to a purged std of the same concentration.

I was using a Vocarb 3000 then switched to one I packed myself that is very similar and same results. I am going to try the direct injection next. It is an Encon Evolution, just got it in May and the only thing run on it is standards, but I do think I had contamination in the Helium line. When we added the new bench the plumbers used pipe dope on the connection between the valve and the line so now I am running it directly off the tank. I think I will also order a new MORT moisture trap tube and see if that is where the problem is, those can be rather picky when it comes to contamination problems.

[Yama001]

Strange to see the problem with those two analytes.

I supposed you try the old change out the split line and flush the inlet lines - I am sure you know that this trick works a lot (not sure if anyone can tell us why though )

[01310040231]

James:

I would try Agilent liner, gold seal, etc. We had installed similar Restek products and we have had problems. You have a new system and they usually give you these parts.

[James_Ball]

James:

I would try Agilent liner, gold seal, etc. We had installed similar Restek products and we have had problems. You have a new system and they usually give you these parts.

I don't have the small diameter liner from Agilent but I do have an extra Agilent gold seal I can try. I have the same setup now in this one I have in all my other single quad instruments, just not sure where the breakdown is coming from.

In the past if I lost Pentachloroethane I would see it show up as Tetrachloroethene(losing a Chlorine and forming a double bond) and that meant a contaminated transfer line. I am just not sure what Carbon Tetrachloride would break down to, so maybe it is an adsorption problem somewhere.