We have had an occurrence where the 13C delta mean for our usgs40 has moved to ~ -25.5 permil which it should be ~ -26.39 permil or something like that.  All of our other in-house references seem fine.  The only recent difference is that we have been using the usgs40 more often in the last two weeks.  So we dug out an older sample of usgs40 that has not been touched in a long time and it reads correctly.  Any ideas why it would change? Moisture for having it open a lot more?  If it were contamination we thought we would see it vary a lot more since everything else we have is of higher delta (standards that range from -11.2 up to +245).