Paul, We run alot of N-15 here and have noticed the same thing, especially in the summer when things are humid. We have tried different manufacturers, sometimes you get a bad bottle right "out of the box". I know of no way to "rejuvenate" the magnesium perchlorate, in fact that might be more dangerous than just buying a new bottle. We keep our stock in an evacuated bell jar and only vent the jar and take out the bottles in a climate controlled room that has a dehumidifier. In our experience, if you leave the top off a bottle of magnesium perchlorate off for too long in a humid room while changing the water trap, the whole bottle picks up moisture and is not of much use in the future. By taking care during the water trap change, you can keep you bottles dry until they are empty. We also have found problems with the mass 30 background while running N-15 from a CE NC2500. This was related to small leaks in the system, the reference standard tank, and regulator. We found minute leaks in the N2 regulator and connections by putting the regulator on a He tank and charging the system with He (you do not need an adaptor for N2-He). That way you can detect small leaks with a He sniffer that you could not see with N2 in the system. After elimination of the leaks in the Conflow hookup and buying a new regulator we still had problems with mass 30 backgrounds. These backgrounds were reduced by going to the purest N2 we could buy (6.0 Research Grade), but are still too high for our purposes since we run CO as well. However, mass 18 is low so we do not think this is a water contamination problem, but something in our reference std gas at mass 30. Has anyone tried a N2 gas purifier trap in the reference gas line to the Conflow? Are there isotope effects as the trap "ages"? We don't know what to try next to reduce our mass 30 backgrounds with N2. When we go over to our pyrolysis system and run CO, we have to wait for a 24 hour period for the mass 30 backgrounds to drop, and then we are OK. If anyone has some advice, thanks in advance. Bill At 05:34 PM 05/04/2000 -0700, you wrote: >When using an Elemental Anlayzer (Carlo Erba) to measure natural abundance >15N I seem to find that the age of the magnesium perchlorate can affect the >results. If I get poor results, replacing the magnesium perchlorate seems >to solve the problem, but eventually this doesn't seem to work. Then I >switch to a new previously unopened bottle of magnesium perchorate, and >this again seem to solve the problem. It is as though the magnesium >perchlorate gradually deteriorates in the bottle after it has been opened a >few times. > >Has anyone else experienced this? > >I recall someone telling me that they used a small amount of P2O5 after the >Magnesium perchlorate in the water trap, and that this would take out >additional water. Has anyone tried this and does it affect 15N or 13C >analysis? > >Thanks in advance, > >Paul Brooks. > > > >Paul D. Brooks. > >Dept. ESPM-Ecosystem Sci., >151 Hilgard Hall, MC 3110, >UC Berkeley, Ca. 94720-3110. > >phone (510)642-3155 >FAX (510)643-5098 (to Att. Paul Brooks) > ---------------------------------------------------------- William J. Showers Dept. of Marine, Earth, & Atm Sciences 1125 Jordan Hall North Carolina State University Raleigh, NC 27695 (919) 515-7143 - voice (919) 515-7802 - fax [log in to unmask] - email "Never be afraid to try something new. Remember: Amateurs built the ark. Professionals built the Titanic."