Chromatography Forum

Dear all!

We have a Stratum Purge and Trap with VOCARB trap. We are using with it a 7890GC-7000C MSD with volatile interface (20:1 split). There is also a water trap (condenstator) inside the instrument.

I make calibration standards in methanol. Then i add it to 40ml vials. If i add too much standard into the vial (like 500 ul or more), my retention times are not stable. For example fluorobenzene go to 6,8-6,9 -depends on methanol amount- instead of 6,6 min. The higher the retention time of my compound i see less different in retention times (like 1,2-dichlorobenzene). Also i get much smaller peaks.

I am using SIM/Scan with 25 to 250 mass, but i dont see any 31 ion in my chromatogram so the methanol shouldnt enter into the column.

I am just corious why it is happening if anybody could explain me, i would be grateful.

Erdno

the methanol is there even though you don't see it. You should increase the concentration of your stock standard so you need less MEOH/ standard.
For my 524 method,for my high standard I'm adding 1ul/ml of water.

Water will be a problem with your set up. The following will help:

6mm draw out plate in the source
1mm ID injector liner
150:1 split
and a 0.18 dia. column.

The problem could also be on the purge and trap side, with the excess methanol saturating the trapping material and allowing the lighter components to move farther through the trap, which will cause it to take a little longer to desorb from the trap onto the column.

I prefer to have at least a 40ml/minute desorb flow on the traps, which if the column flow is 1ml/min would be 40:1 split minimum.

On another note, are you running SIM/Scan at the same time on the 7000? I have been wanting to do that but have not figured out how to do it in Mass Hunter like I can on my 5975 with MSDChemstation?

I have occasionally had a trap where the methanol issue is worse than usual. It may be worth changing to a new one.

Dear Bear!

I am using Volatile interface, so i dont have any liner inside my injector. This interface made by agilent is only good for concentrators (P&T, HS, TD). With 150:1 split i would loose too much analyte and for epichlorohydrin my LOD have to be 0,025 ug/l.


This configuration has a water condensator trap inside the P&T then i have the analitycal trap so methanol and water should condensate at first trap.

I am using a RTX-VMS 20*0,18*1 column at the moment and since i have a backflush in my GC, i have a 5 meter guard column too.

Dear James!

I am using SIM/Scan with my 7000QQQ. In masshunter first u have to choose MS1 SIM on left side, then on right side SIM method parameteres, and finally at bottom right u have to use full scan paramteres (here u can give Scan parameters mass, dwell etc.)

If the problem is in the P&T as u said lighter compounds can go farther any method exist to check it out?

Dear Yama!

I am using this configuration since januar and i saw same thing at the begining just like now. So i hope it is not the trap but it is worth a try perhaps.

Thank u all for helping!

Erdno

Does the Stratum use back pressure on the trap? I know that increasing back pressure at the vent of the trap will increase the efficiency of the trap. The old Encon P&T had and adjustable back pressure regulator, but the Evolution no longer has it. I upgraded my old Tekmar 2000 by adding the back pressure regulator from a 5890 split/splitless inlet to the vent port and it worked wonders on that old unit. One simple way to add back pressure is to add a length of narrow diameter tubing to the vent port, the longer the tubing the more back pressure. This could possibly help if the methanol is carrying the lighter molecules further into the trap.

20 to 1 split with a 0.18 column sounds like a bit too much loading to me. What is your starting temperature? At 40 degrees you could be condensing methanol on the front of your column and changing your retention.

I agree , I do 524 with a Stratum. 25 ml purge and a 150:1 split and calibrate from 0.5-50 ppb with most compounds calibration %RSD <10 ( lots <5).

I still feel you are adding way too much MeOH while prepping your standards. You can't forget you are most likely adding more MeOH spiking internals and surrogates.
Please try making your working standard stock at a higher concentration so that you use less MeOH/ standard. For my 0.5 standard I spike 2ul of my stock into 200 ml of water.

Dear James!

I dont have any backpressure regulator on the trap. I heard it can be usefull with loop headspaces but does it work with P&T too? I think i will try it out somehow this is really interesting for me.

Dear Steve and Bear!

My oven starting temperature is 35 °C. I thought too methanol should modify my retention times but i dont see any methanol in scans (25 to250) not even if i extract 31 ions. How can it be there if MS dont see any?

I will try out bigger split and also i will not add so much methanol to my calibration standards.

Erdno

I am under the impression that the Stratum puts some back pressure on the trap.