gas chromatography

Dear geltlemen, 

I am facing the following issue during measurements of N2 with GC - TCD

I am using a standard gas of 99.50 % N2 (rest He) for calibration, 

I am doing three measurements in the morning and i calibrate the instrument 

after 5-6 hours the same gas is giving a value of 94,00 % 

Before the second measurement of the standard gas, which shows a value of 94,00% i have measured my LIN tank and another standard gas 96,00 % N2 in Helium. The measurement of the tank was 95% and of the 96,00% was 90%, These measurements were according to the initial calibration 

I am not experienced in GC, so i would like to answer the following questions 

1. Is this normal  having a difference of the value of 5% during the day for the same gases 

2. If it is not normal, can you reccomend any issues 

thank you very much 

Petros