Chromatography Forum

Hi guys:

I am using GC-MS Clarus 600 T and Tekmar autosampler for VOC analysis (524.2). Column is RTX VMS 60 m, 0.25 ID.

There are issues with bromoform, styrene and 1,2,3 trichloropropane.

Bromoform and styrene coelutes and MS only “seesâ€

chroman,

I would first have suspected your purge flow conditions except that you say that you see bromoform when you are running THM mix. Do you mind posting your purge and trap conditions?

Is your GC pressure programmable?

Best regards.

Thank you for your prompt response AICMM.

I don't see any option for programmable pressure on my GC so I think it is not programmable.

P&T conditions:

Trap: Vocarb 3000
Purge: 11 min. @ 40mL/min.
Dry purge: 1 min.
Desorb preheat: 245°C
Desorb flow: 300 mL/min.
Desorb: 250°C for 1.2 min.
Bake: 270°C for 25 min.

I run the EPA 500 series tests all the time for purgeable VOCs.

This is my P&T method:

Setting Value Setting Value
Line 150 Bake 10 min
Valve 150 MSC Bake 300
Mount 40 Temp 225
MCS Line 40 Pressurize 0.2
Purge 11 Xfer Sample 0.5
Desorb Preheat 220 Rinse Lines 0.25
Time 4 min Purge Line 0.5
Temp 225 Bake Rinse 0.25
GC Start Start Bake Rinse 2 1
Bake xfer 0.5

I am not familiar with your GC/MS system, but I believe we use the same Teledyne Tekmar P&T.

TCP comes up very late, it comes with nathaplene and hexachlorobutadiene

Does the methanol concentration between the two prepped standards - the VOC and the THM - stay relatively the same?

Also, I was under the impression that a dry purge was not allowed in 524.2. I think it is allowed in the new update 524.3. Does anyone have any input on this?

JTM,

From a first look I would be suspicious of your desorb flow. Most traps are built for around 40-50 mL/min so I am not sure what your desorb flow entails. Also, 25 minute bake is really long and may in fact be hurting your trap rather then helping it. Unfortunately, I would suggest a new trap at this point with less bake and see if that changes the picture.

Best regards.

I do 524.2 and never been " called " on my dry purge. I DP for 6 min@ 40 cc/min.
What GC/MS are you using? If a 6890/5973 or 75 you may want to change the column to 20MX0.18. I started with the samp size column as you and the only result was to put too much water into my MS.