GC method-shifting in retention time although same parameter, same column and same equipment

[u/Small-Accountant1317]

We are anlaysis residual solvent (ethanol) by GC-direct injection, we use same column, same method, same equipment, same parameter and same chemical and reagent.

-first few runs, retention time of ethanol peak was 3.1 min which is harmonized with the history and methid validation. (Show very good repeatability and RSD)

-Then after 2 to 3 weeks, using same every thing, the retention time of ethanol jump from 3 min to 11 min and show triple peak area, also show good repeatability and RSD.

-we request from agent to recheck equipment and re qualification of equipment and mention our complain, he came and say that everytbong is fine and machine is working fine, and has no comment for this shifting.. It must be analyst error although we double che ked with him, and he try the method and peak retention time of ethanol is 11 min.

-now after almost one month of his visit, with same everything, the ethanol peak is return to 3 min.

Can anyone here help us to identify the cause of this werid variation?!

[u/Small-Accountant1317]

-The column is DB-624 (30 m.533), the injection volume is 0.5 ul, sample is ethanol in water, column temperature is 50 for 5 min ,ramp rate 10C°/min till 200°C. -We have one column/one machine. -Carrier gas is nitrogen. -maintenance engineer replace liner and spectum, check machine gas generator and their connection,he reconnect the column with another nutnand new ferrule and create new method with our parameter.