THM's by 524.2

[mjh32586]

Can anyone offer me some help on this method? I have currently ran several calibration curves (0.5-50 ppb) which always end up with my 50 ppb point to be higher than it should be when I run it under avg rf curve fitting. I've heard that bromoform tends to be quadratic but for my curves it seems that dibromochloromethane also is running more quadratic. I am seeing some tailing on my higher cal points which I'm assuming is causing them to look quadratic. Someone told me that I may possibly need to increase my split ratio as long as I have the sensitivity to do so. Currently I have the split set to 20.0. I'm wondering if it could be something else also. I am running a 2 min desorb, 11 min purge, and 10 min bake. My oven program is 35 hold for 0.5 and then ramp 20 degress per min to 220 hold 2.75min. I am using an RXI 624-SIL MS column (30m x 0.25 IDx1.4 thickness). I am running it under linear velocity at 35.0 cm/sec.

[ChemSam]

I would definitely change your split ratio to a 1:50 split or a 1:100 split. Do you have a 1 uL liner in place? This is often a problematic area for THMs as well.

In addition, try lowering your desorb time to 1 min or 0.5 min. This often helps with these compounds as well.

[Bigbear]

If you can swing it change your column to a 20M X 0.18 column. You can run column flow between 0.5 and 0.7 cc/min and further reduce the water getting into the source.
It sounds as if your source is active. Clean it and see if Br2ClCh "straightens" out.

[mjh32586]

I can't change the desorb time because the methods mandates a 4 min desorb.

[Bigbear]

Talk with your auditor. I do 524.2 which states " about four min. desorb time". My guy allows us to desorb for 3 min (tried to lobby for 2 but no dice).

[Bigbear]

Just re read your post. Are you using a Vocarb 3000 type trap? If so use a 6 min dry purge @ 40cc/min.

Has this method ever worked for you? If yes, when was the last time you cleaned the source? How old is the trap?

[cc3135]

What kind of GC/MS and P&T concentrator are you running? If Agilent GC/MS are you running the 6mm drawout plate? It greatly improves the linearity of brominated compounds. Also depending on the condition of the concentrator you might need to refurb the concentrator. Have you had this method running previously. Want purge volume are you running. Are you saturating the detector with your low split ratio, you mentioned tailing of peaks. Is it linear if you drop some of the upper levels. Normally THMS will not tail unless you have instrumentation issues. THMS is one of the easier P&T methods out there.

Good luck.

[mjh32586]

I am using a Shimadzu GC/MS and an OI 4660 concentrator with the 4551 autosampler and SAM. We just purchased the unit in March brand new but have been having this problem since we got it. We are running a 5 mL purge volume. It doesn't say I am saturating the detector with this split ratio and most of the tailing peaks are my early eluters which makes me think 2 things...water is coming over and intefering and my oven program is 35C hold 0.5 min then 10C/min. I think if I hold the oven program at 35C for 4 min then ramp, it will improve the tailing problem I'm having. My calibration is linear though once I drop my high standard. I am running 0.5,1,2,5,10,20,50ppb range and that 50 ppb is coming out above the linear line where it should be.

[Bigbear]

First off I feel you are using a column that is too large and as a result letting in too much water ( broad early peaks). I also feel you should be splitting at 40-50:1 ( I was using 50:1 with the column size you're using). Chech your substituted benzene's extracted ion profiles on your high standard for overloading ( peak tops will be flat if overloaded).

[mjh32586]

I will try splitting 50:1 but I already have a low response (could be due to the water though). I checked my substituted benzene's on my high standard and they were nowhere close to flat topping. I also forgot to add earlier that I am using a 0.75 ID liner.

[Bigbear]

What GC are you using and how is your transfer line plumbed? I use an Agilent 6890 and I cut my supply line to the inlet and plumb the P&T in series. I use a 4mm single goosneck liner with the wide end up. The low volume liners didn't work for me ( I tried 1 and 2 mm ones).

[mjh32586]

I am using a Shimadzu GC 2010. I'm not sure I understand what you mean by how is my transfer line plumbed in? Maybe I will try a different volume liner...

[Bigbear]

I was asking how the transfer line is attached to the gc. Do you have it going through the septa, or directly connected to the inlet plumbing. I still feel your issue is caused by water buildup. Do you use a teap that can be dry purged? If so try a 6 min dry purge @ 40cc/min. Does your GC have a gas saver option? If yes try this. At 1/2 your desorb time set the gas saver to 100-150 cc/min. This will "dry" the transfer line between runs. If time is not critical for you also hold the oven at your final temp. longer. I hold mine for almost 10 min. The oven is still ready before the P&T.
I also found that over time the water "activates" the source so I have to clean it after 200 samples or so.

[mjh32586]

The transfer line is attached directly to the inlet plumbing. I did a 20 ppb run of BTEX yesterday using a 50:1 split and still saw the gradual decrease in response throughout the run. I am using a #11 trap. I tried dry purging for 6 min and even extending my final temp for 10 min. I even tried using the gas saver option of 100:1 split at 0.5 min, 1 min and 1.5 min (using a 4 min desorb) and I still have the continous dropping off of all analytes. I just cleaned the source about 2 weeks ago and even tried switching filaments. Nothing has helped. My next step is to try a new liner since I am using the 0.75 mm ID. What liner do you currently use? I think you said it is 4 mm but does it have wool, etc?

[Bigbear]

I use a 4mm single gooseneck liner with the open end up. No wool.
you may need to start 1/2 splitting the system to see where the losses are. The easiest is to make up a solution so that injectiong 1ml in MeOH gives the same concentration as your CCS. Inject into the injection port and check the areas of the bromonated compounds. If they are higher than that in a purged sample your injector and ms are fine. then you will need to test different parts of your P&t. Call OI they will have ideas of how. I have a sort of injection port that I can install in my P&T so I can inject durring desorb,or onto the top of the trap. These tests can isolate where the losses happen.