VOC 524.2: My calibration curves fail too soon...

[Chroman]

I am running a Clarus 500 GC-MS coupled with Velocity XPT.

My calibration curve fail after 2 days or no more than 25-30 samples. All peak areas decrease, including both the internal standard and the surrogate. "Normality" is restored if I cut column injector end (1 loop), but only for a short time.

Any experience with this kind of behavior?

[Bigbear]

More information would be helpful. Trap type, Velocity conditions, GC parameters, and column size and flow rate would be helpful. I had similar problems when transitioning to a Velocity. In my case it was the water "activating" my MS source.
My guess that anything you can do to reduce the water would help.
I use a Vocarb 3000 trap and use 6 min drypurge @ 40 cc/min. I purge a 25 ml sample so I need to split 75:1 ( 0.18mm column). There are other water limiting tricks I use with an Agilent GC/MS that may not be applicable to your system.

What happens to the compound responses if you let the system sit idle for a few days and then run a few injections. I would suspect that if water is the problem the response would be increased for a while.

[Chroman]

Hi Bigbear:

My 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

Split ratio: 12.3

Flow: 20 mL/min

Column: RTX-VMS 60 mts, ID 0.25 mm

After a couple of days of sitting idle, response (peak area) decreases

[Bigbear]

How old is your trap? How old is the column? Do you have another column you can try?
Do you have a useable injection port? It would be interesting to direct inject an equivalent methanolic standard when the response is low and see what you get.
If you clean your source does the response tail off after 20-30 samples, or can you run longer?
Lucky that you can desorb for <4 min. 524.2 says "about" 4 min. The lowest my auditors will allow is 3 min!

Is your column flow realy 20 cc/min.? When I was using a 60M X 0.25 column I was flowing 1.7 cc/min.

[Chroman]

How old is your trap? How old is the column? Do you have another column you can try?

Lucky that you can desorb for <4 min. 524.2 says "about" 4 min. The lowest my auditors will allow is 3 min!

.

Is your column flow realy 20 cc/min.? When I was using a 60M X 0.25 column I was flowing 1.7 cc/min.

Both column and trap are three months old.

We are just starting our certification process so no auditor has seen it yet..

I already installed a new column because I noticed in the last standard I ran that all peaks that elute before the beginning of the final ramp are tailing big time.

20 mL/min is the total flow, column flow is 1.5 mL/min

[Bigbear]

You may be overloading your column. Try increasing your split ratio to 50:1 or more. BTW what volume of water do you purge. If 25ml you may be well served to set the split ratio between 60-80:1.

If you can I would recommend going to a 0.18 column. I use a 20M X 0.18 X 1.0. All my compounds are out by 15 min , I have the trichlorobenzenes in my mix. My column flow is 0.6 ml/min.

[Chroman]

I´ll try 50-60 split since I am purging 25

What brand is your column?

[Bigbear]

I use Agilent DB-624.

[Chroman]

I installed a new column, increased split flow up to 30 and the problem is still there: peak tailing and poor chromatography for peaks eluting before chlorobenzene...

[Bigbear]

I still think you should split more. When I was using a 0.25 column I split 50:1. Also I think you would be well served by increasing your dry purge time.

I just remembered something a tech from Supelco told me about the Vocarb 3000 trap. DO not go higher than 250 cc/min flow. Tekmar claims 400 is ok but, I never go higher than 250 cc/min.

[Chroman]

With the new column installed I made a direct injection and chromatogram came out perfect, by the book.

Then I replace the trap, ran a CC and there is still some tailing. I think that there could be some activity in the transfer line...

[Bigbear]

The direct injection working well makes a strong case for water being your problem.
Have you done a top of the trap injection? The "thingy" Tekmar gives you that attaches to the sample mount is ok, but I prefer the one supplied with the Velocity (I attach mine between the 6 port and the condenser and inject durring dry purge).

I chased the same problem that you are, ane in my case it was water.
Please for my sake try a run or two with a 6 min dry purge @ 40 cc/min. If you could try increasing your split rate to 50-60:1.

[Chroman]

No, I haven't done a trap injection. As a matter of fact, I have never done it before.

I set 6 min dry purge time (it was set at 1 minute), flow was the same. I am running a CC, I'll try higher split ratio after that

[Chroman]

I must say that 6 minutes dry purge seems to have helped my cause: the water "hump" at the beginning of the run is notably smaller.

Additionally, I increased split ratio to 40 but nothing happened, tailing is still there. But I notice that responses for some brominated compounds are not consistent. Still think that transfer line is my culprit

[Bigbear]

Does your GC have a gas saver option? If so do the following ( works well for me)
You desorb for 1.2 min, so set your gas saver to 100 - 150 cc/min to come on @ about 0.7 min. I know it sounds crazy but most all of your compounds are on the column in the first 30 sec or so. By using the gas saver this way you will dry the transfer line between runs. I also hold my gc at the final temp for about 10 min ( to get rid of more water). I established this time by noting how long it takes for my oven to cool and get to ready. I set the time so the GC is ready when the P&T is in the last stages of the dry purge. It is best if the GC is ready before the P&T is at the desorb step.