What length and diameter column are you using? A 30m .32mmID column will have an inlet pressure below atmospheric at your flow rate. A 25m .25mmID column will have a head pressure of only about 3psi if you have shortened it to 20m to remove contamination. At these low inlet pressures air can diffuse into the injector when the split valve is closed. if you have a septum leak it can let air in too. Your flow rate is a little low, you could try increasing it to 1.5mL/minute which would put the inlet head pressure up, and should help.

Cheers, Tim

Tim Strutt Thanks Tim, it is a 30m 0.25mm column, I trimmed the column and did calculate the length according to the RT of butane, it is 29m. During the process I did not get any pressure warning and drop. It passed the inlet trap check, about 3.8 psi at 400ml/min for splitless mode. I did check the MSD leak check, inlet leak as well as gas line. Then this leak makes me so confused and have no any idea about it.

Thanks again!

When problems like this arise it is useful to consider the immediate past events. For example, did this problem arise after routine maintenance such as changing the inlet liner or doing a source clean or perhaps following the installation of the current column. Information like this is key to solving the problem.

I believe Tim Strutt's answer is correct, increase the flow rate and see what happens. Also tighten the nut at the MS, this can leak and not be seen by a leak detector. Spray Dust Off at the connections and monitor the difluoroethane to see where the leak originates.