Peak on isocratic mode when injection the eluent

[bunnahabhain]

Hello everyone

I use an isocratic method (MeOH/water 65/36, pre-mixed and degassed in ultrasonic bath) over a Phenomenex Luna C18 column. As blank I always inject 20 µl of the eluent. What I did not expect but find reproducibly is a peak at about 2.6 min. First I thout the autosampler was defective because this peak coelutes with air injected from an empty vial (this was a coincidence I found this out).
But this peak occurs on different machines (Agilent 1100, two different Dionex systems), and with different batches of the eluent. Therefore I do not believe in the air hypothesis any more. I cannot believe that three different autosamplers are defective. Also I only see this when running the MeOH/water method.
Any idea what this peak could be?

Thank you,
Jörg

PS.: This peak appears regardless of MeOH/ratio (I tried 45/55 to 75/25).

[HPLCaddict]

Is this peak at t0 aka void? Then it could be due to a small pressure shock that occurs during injection or because of a slight refractive index mismatch between the eluent and the injected solvent (what's the washing liquid for the autosampler btw?).

[bunnahabhain]

No. void is 1.3-1.4 with this column. There is no washing liquid.

Jörg

[tom jupille]

You are probably seeing dissolved air. No suprise; it's quite common. For example:
http://www.shimadzu.com/an/hplc/support ... 37lab.html

[bunnahabhain]

Thank you Tom. This explains everything. I also see the retention time shift using different MeOH7H2O ratios deescribed here.

Jörg