High pressure when system turns on

[Hooman]

I'm using an Agilent 1100 HPLC with a binary pump. When I use a C18 column with 250 mm x 2.1 mm dimensions and 5u particle size, and corresponding column guard, I usually get a pressure arround 91 bar (the mobile phase is acetonitrile 60% in water, flow rate 0.2 ml/min). However, every morning when I turn the system on, pressure shoots up to arround 124 bar and decreases back to the expected level in about 15 minutes. I wonder if this pressure increase at start time is a common and natural issue, or there's a cause to be fixed.

Thank you.

[bisnettrj2]

Do you use the column heater on the instrument? If so, what temperature do you have it set at?

[Kwet80]

Do you modify the composition of mobile phase before shutdown and at start up? For exemple going to 100% CH3CN at shutdown?

If you do so, a higher pressure during conditionning is normal. Maximum backpressure of CH3CN / water mixures arises around 30% CH3CN.

As bisnettrj2 said, the column temperature is also important. If you warm up your column, at start, the column needs some minutes to attains the corrcect temperature. During this time, the backpressure is higher than usual.

[Hooman]

Do you use the column heater on the instrument? If so, what temperature do you have it set at?

Yes, I use a column thermostate set on 25 degree. Also Lab temperature is arround 25 degree, then usually just 2 or 3 second after turning the column thermostate on, it reaches to desired temperature. I turn the thermostate on, immidiately after starting the system and pressure stays high for several minutes. Since there is not a very significant difference between working and ambient temperature, I think this high pressure can't be attributed to temperature.

[Hooman]

Do you modify the composition of mobile phase before shutdown and at start up? For exemple going to 100% CH3CN at shutdown?

If you do so, a higher pressure during conditionning is normal. Maximum backpressure of CH3CN / water mixures arises around 30% CH3CN.

As bisnettrj2 said, the column temperature is also important. If you warm up your column, at start, the column needs some minutes to attains the corrcect temperature. During this time, the backpressure is higher than usual.

I do not change the mobile phase composition before shut down. I just purge the pump with pre-mixed ACN 60% from one pump channel (instead of taking pure ACN and water from separate reservoirs). The purge valve also is open, then there is no chance for the new solvent to reach to the column, although the final composition of both mobile phases are the same.
As I mentioned in my previous post, working temperature is not significantly different with the lab temperature.