GC-IRMS problem

[leo1352]

I am using a delta xp GC-IRMS , the initial 20 samples ran well with consistant results. I am now getting -10 per mil increase in value from the same samples. I have checked for any leakage with Argon and checking for any peaks coming off on mass of 40 after a minute or more. there look to be no problem, except water background increased by 6 volts and peaks that were prviously showing at 3 volts are now appearing to be 1 volts. I wonder if any one could help me throughthis.

[trozen]

6V water is too much (should be below 2V), you probably know that it protonates CO2 to give HCO2 ion species at cup 45, resulting is positive shift of your delta values. is your nafion trap OK?