volatiles by gcms

[mjh32586]

I was wondering if anyone can help a newbie with some volatiles questions...I last calibrated on march 22. I have been having high recoveries for my last eluter (1,2,4-trichlorobenzene). On my 10 ppb, it has been coming out around 135-140% recovery. I tried new septa, new liner, new trap, rinse of purge and trap sample pathway and rinse of transfer line. None of the above have helped. I even tried opening a new stock standard. My internal standard response is very close to what it was when I initially calibrated. Since I have a long desorb time, I bake my trap for 15 minutes at 210 (OI #10 trap) in between samples. When I run a blank after the CCV, there is no carryover of 1,2,4-trichlorobenzene. All other analytes look great for ccv's and even though the acceptance criteria is +/-30% recovery, everything has been coming out around +/-20% recovery except the 1,2,4-trichlorobenzene. Does anyone have any idea why I would be having high recoveries of this high molecular weight analyte? Do I need to re-cal? If I re-cal, can I just cal using my previous PFTBA tune or do I need to perform another one prior to re-calibrating?

[tlahren]

If you're running EPA methods don't you have to calibrate/tune every month regardless of performance? We had too when running 8270 and 8260 methods. Even if it's not required it's good practice.

I would definitely re-tune and re-calibrate and compare the slopes of the calibration curves for your trichlorobenzene. My guess is that they are different now.

You said it's around 135-140% recovery in the 10 ppb standard; what does it look like in the other standards and at what concentrations are they?

Do you run a Matrix Spike (MS) sample? If so what are the recoveries of the trichlorobenzene in this sample?

There are a lot of variables in PT-GC-MS systems that can cause variations in response. I don't have a ton of experience with these systems. Maybe we will get some more input from some PT experts if you give a little more information.

[mjh32586]

I am running EPA 524.2. I will try a new PFTBA tune and new calibration to see if my ccv's are better. My other CCV is at 20 ppb and it's also recovering around 130-135%. Instead of MS, I run LFB's at 5 ppb in which 1,2,4-trichlorobenzene also recovers 128-135% recovery.

[Yama001]

Its not unusual to need to recalibrate often, especially for those heavy analytes. We have a lot of old equipment; sometimes we recalibrate several times a week (this is excessive, but the equipment is old and takes a beating - if you only do drinking water samples it may not be as bad). As Tlahren says, lots of things will affect it.

[01310040231]

What kind of calibration model are you using. If you use second order, it may cause you problem.