Chromatography Forum

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?

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.

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.

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.

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