oscillation

[rakesh kumar]

dear all
i m using acetornitrile and water (20:80% w/w) as a mobile phase on reverse phase C18 column. i m getting noise type peaks at constant interval (like oscillations). how can i get rid of this.

[ljc]

See my post with a similar problem (binary solvent and RI detector). What detector are you using ?

[Mark Tracy]

Is the period of the oscillation equal to the period of the pump stroke? If so, check your pump pressure and see if it is also oscillating; that means either a bubble or your pump needs service. If the pump is working correctly, it could be a particle or bubble in the detector flowcell.

[rakesh kumar]

thanks for reply
yea i have cheked the pump pressure it fluctuate slightly. but i m getting stable straight base line without injecting sample. after injection of sample i get peak for 5 minute after then i start facing the problem of oscillation. i m using the UV detector for analysis.

[Mark Tracy]

Is the oscillation present if you inject a blank or only with your samples? Again, does the period of the oscillation equal the pump period? Your pump pressure should be stable to <2% under the worst of conditions. Is this method isocratic?

[josebenjamin]

Rakesh,

Your problem could be due to something common in HPLC. Many times samples contain compounds, other than those of interest, that elute late in the chromatograms. Probably after the first few minutes of interest, those compounds start to elute in the form on long, broad signals, that appear as oscillations in the baseline. These problems are agravated when working in the low UV range, this is, below 220nm.

There are many ways to solve the problem if it is caused by these long-retained compounds. The simplest thing to do is to speed up the flowrate after the signals of interests are eluted, or make a step-change in the composition of the mobile phase to accelerate the elution of those compounds. A recommendation could be to increase the % organic in the mobile phase by 20-25% (if possible, and if you are working with reversed-phase systems).

There are many others thing you could try, but it is not possible to say more without additional information.

Good Luck,

josebenjamin