We are running an Agilent 7500ce and have recently had problems with Se at 82. We run EPA 200.8 for most samples so we must use mass 82 and no collision/reaction gases. We have a two cylinder automatic switchover manifold and use grade 5 Ar as we are unable to get liquid Ar delivered to our facility. The problem first presented itself as calibration blanks run every 10 samples being quite positive or negative out of range (-idl<concentration<+idl) during a long run. I was able to determine this happened after a tank switch over. To try and accommodate this I warmed up and tuned on one tank and switched to a new tank just prior to calibration. This worked for some time. Now I am experiencing a drift in the 82 signal during a tank too. Yesterday, I began to monitor counts at 82 during warm up. I had 73 at mass 82 on the first tank. I manually switched to another tank and the counts went up to 1400. I put a third tank on and manually switched to it and the counts were about 800. The change in signal was instantaneous with the changeover each time. All 3 of these tanks were from different lots.
I have seen many discussions here about Kr being present and accumulating in dewars but I couldn't find any discussions about Kr in cylinders. I talked with the supplier and they felt Kr wouldnt be an issue because it wouldn't be liquefied in the tank at that pressure so it wouldn't accumulate in the tanks nor would the tank be stratified either. The supplier also felt since Kr is present at low levels in the atmosphere that the proportion in their gases would end up being relatively constant across lots. In looking around the only other instance of problems I have found at 82 was Br81H+. I am not sure if Br is checked in grading the gas or not and haven't taken it up with the supplier yet. Has anyone else experienced this sort of thing or have any suggestions? Thanks in advance.
Public Health-Madison & Dane County
210 MLK Jr Blvd, Room 507
Madison, WI 53703