Higher responses for later eluting compounds in THMs 524.2

[jspody]

Hi,

We are running the 524.2 method for THMs using the 8890/5977B GCMS with the Teledyne Tekmar Atomx XYZ Purge & Trap Concentrator and Autosampler.

We are having difficulty passing the QCS for the 4 THMs. Chloroform passes, bromodichloromethane sometimes barely passes, but dichloromethane is high, approx. 1.4X or 35% higher than the certified value and outside the acceptance range, and then bromoform is as much as 2X higher. The trend is the later the compound elutes, the higher it is relative to what the certified value should be. All compounds achieve a linear calibration for the range 0.5 to 50 µg/l, non-weighted and weighted 1/x.

What else can we look at to give us clues, what do you suggest is happening, and/or how do we improve the recovery for the last 2-3 eluting compounds?

Note: We also successfully run Taste & Odor (SPME) on the same inlet and column, so the column and liner are chosen in the hopes of optimizing for both methods. We have tried other liners and columns, with some success but we’ve also had trials and tribulations with getting the THM side of the instrument working properly, so here is what we currently have installed:

Column:
Agilent CP7412, CP Select 64 CP
30 m x 0.25 mm x 1.4 µm

Liner:
Splitless straight UI, 2 mm ID, 250 µl vol. (#5190-6168)

Oven ramp:
Initial: 35°C, 4 min. hold
Ramp: 15°C/min. to 250°C and hold for 1.666 min.

Split ratio: 100:1

Purge and Trap:
K trap
5 ml sparger
4 min. desorb
11 min. purge
All other parameters are also standard to the method…

Any insight is appreciated!

[James_Ball]

Does the same level of the initial calibration quant back accurately if you run it after the curve?

If so then you may need to validate if the initial calibration solution or the QCS solution could be faulty. If you have lost some of those analytes in the initial calibration solution it would make the QCS from a different source high. I have experienced times when I had to get a third source to determine if one of my standards was inaccurate when such things occur.

We normally calibrate from 0.5ppb to 200ppb and the THMs are normally a good linear fit or even good at average response factor. We have noticed though that the 5977 does not have quite the linear range that the 5973 does, it is much more sensitive but seems to lack the range of the older instruments. Also it seems a diffusion pump instrument handles volatiles better than the turbos, but not enough difference to really matter.