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- Posts: 478
- Joined: Fri Jun 02, 2017 11:10 pm
- Location: Western Colorado
I'm back worrying this bone again because after getting a very nice calibration on Jan 18, 2018; I found out that my certified 54 cpd 8260 standard did not actually have styrene in it. Its not been an issue for BTEX but I just got in a new set of standards to do another calibration. I figured I would do another tune on the SISalloy+yttria coated #2 filament and run a calibration. I used the Atune followed by ramping the ion focus and adjusting the 69/219 ratio to 60% which got my 50/69 ration right at 0.8%. I adjusted abundances to a 600K (I normally use a 500K) target for mass 69 and put on my calibration. I'm using 5uL of a 70 ppm internal standard mix in every 5mL of sample water. Inlet split ratio is 40:1. So, I end up with about 16 million counts on the Fluorobenzene 96 peak. With the 500K target, I was getting around 12-13 million counts.
So, now the problem. As concentration goes up above 160 ppb, in the calibration; everything from ethylbenzene onwards gets radically non linear as in peaks actually dropping in intensity to half or less of they values they should have. This is happening to the BFB surrogate but not the toluene-d8. The chlorobenzene-d5 seems to be stable but not the 1,4-dichlorobenzene-d4 which is also varying widely in intensity. The highest TVH(GRO) standard seems to cause loss of fluorobenzene dropping it from 16 million to 6 million counts [Edit]but not affecting should be... also affecting to a lesser extent[/Edit] the other surrogates or internal standards. My calibration curves are typically 10% or less RSD for a 20ppb to 480ppb calibration.
I have recently had several samples that had very high branched pentanes or benzene preceeding the fluorobenzene peak and fluorobenzene drops to half its normal value, while all the remaining internal standards and surrogates are within limits.
This sort of thing normally does not phaze my fluorobenzene recovery and so I have to ask, is this a trap problem with sample analytes tying up trap capacity so I get this weird behaviour. Or, is this some sort of quenching behaviour at the filament?
I've got two replacement traps on order but I wonder if anyone has actually seen this sort of thing happen. In 17 years of doing 8260 work, I've never seen an internal standard get wacked by analyte concentrations when they were only a few thousand ppb.