Could you post an image of your chromatogram that has the noise?  What concentration of phosphate buffer are you using?

Also, I would be careful rinsing a C18 column with pure water which can cause "phase collapse".

It is important to keep in mind that noise in HPLC surfaces in the detector, so everything witch influences the optical properties of the detector cell may be a source of noise. Very often noise is created by a small air bubble in the detector cell, they can be very hard to eliminate. Some things to check:

Is the lamp energy still high enough? each detector has a way to check this

Is your mobile phase degassed? Sparging with He does wonders, online degassers have a limited lifetime and are not that efficient t( my opinion)

Are the components for the mobile phase suited for the selected wavelength ? if not this has the same effect as a worn off lamp. Changing the mobile phase to a simpler one (no buffer) gives an indication, an other option is to take an UV spectrum of the mobile phase

Do you see pressure variations? A pump problem can give rise to bubble generation or can be cause by a trapped bubble

Do you see cloudiness when you manually make  solution of 90%ACN and 10% Phosphate. One would normally observe precipitation under similar conditions. If your pump is mixing them, this mobile is a poor choice to begin with.

To follow the principle of "Keep it Simple" and- simply try another column under standard conditions. Do you see the same noise, drift etc? If yes, it is an instrumental problem, if not, this is your previous column. If you post the chromatogram, that would help to identify the source of the noise.

To solve the instrumental problem, systematically work your way from the solvent reservoir to to the detector rather than worrying about 50 random variables. There is a tendency among HPLC users to never wash the HPLC bottles. Begin by washing all the bottles. Wash all lines of the degasser, pump injector and the detector with warm water (~ 40 oC) for an hour. Attach a working column and see if this problem has been solved under standard conditions. In the meanwhile wash your dirty C18 column with a water rich mobile phase and re-test after giving an hour of equilibration time with a lower ACN-phosphate content and see if it has restored the column.

Thank you so much forma tour help.

Lukasz, I'm preparing the buffer by myself. And the HPLC equipment has a degasser part. 

Kris: I had never have this problem before :( . when I increased the flowbrate, the pressure alzó increased in a normal way, so, the vacuum bomb is not the probem.

Farooq: 

Tatiana this is not noise it is signal drift.

Does this also appear when You pump a mixture of  70:30 ACN/H2O on one channel (so 100% A containing this mixture, not ACN on A and H2O on B)?

I saw similar baseline caused by thermal problems in the oven, the column was touching the oven wall but the system was working at 60°c. If you have a stand alone digital thermometer with a pt100 sensor or a thermocouple you could verify the regulation of the oven

The most common chemical cause of a wavy baseline is a dirty column. Temperature is another reason. I was having a similar problem with a new column material on the same make of the instrument. After washing the lines (right from the solvent reservoir) to the detector cell with warm deionized water solved most of the problems. The column should be washed separately with a series of solvents while avoiding too much water exposure (ideally on a spare pump). In the meanwhile, you can also arrange to wash the injector section by making large volume injections (~ 80 uL) ten times with water followed by ten injections IPA.

A small remark on the cleaning procedure proposed by Farooq. It is a good idea but be careful with the selection of cleaning solvent. Always consider the  current mobile phase of Your system to avoid precipitation specially in the injection valve and the column. If You don't have a spare pump the column can be washed with the detector disconnected (after cleaning the system first of coarse)

Hello Kriss: thank you for your help, and that line didn't apperar when I used ACN/H2O. But I passed muy samples and they looked good. 

I will tell to the technician to clean the línea.

Thank you so much to all of you ;)

If you run an gradient you should try to use an online cleaning column like the Ghost-Guard-LC. This absorber removes traces of the impurities from the mobile phase and you get an good baselien! We use the Ghost-Guard-LC for all our gradient separations!

I have a problem with the base line noise even after cleaning

Regarding the diagnosis of most baseline instability problems, here are a few free articles which may help others:

• "HPLC Retention Time Drift, Change, Variability or Poor Reproducibility. Common Reasons for it"; https://hplctips.blogspot.com/2015/11/hplc-retention-time-drift-change.html
• "The Three Most Common HPLC Questions and How To Solve Them"; https://hplctips.blogspot.com/2017/03/the-three-most-common-hplc-analysis.html

To provide some constructive comments related to the original question, I would like to add some information to this very old post (4 years) to assist others who may find this. First, very little information was provided by the original poster about the basic HPLC method used (?? column dimensions, exact mobile phase composition and method details??).

• To new users, if you would like to discuss any HPLC or GC method, please include all of the standard basic method conditions in your post. If you do not know what these are, please get some local help before posting. Failure to do so often leads to useless speculation and wasted bandwidth.

General Comments: The problem observed in the supplied screen shot of the chromatogram showed signal instability (a type of variable drift, but I prefer the word instability). It could be caused by a fouled column, temperature instability, poor equilibration, poor method quality, inappropriate instrument settings and so on. We can also see from the screen shot that the Diode array detector's Reference Wavelength software feature was turned ON (this will cause all kinds of problems and invalidate all data collected at 230 and 280nm. Please always turn this 'OFF' and see below article for more info).

"REFERENCE WAVELENGTHS (as used in HPLC UV/VIS)"; https://hplctips.blogspot.com/2011/03/reference-wavelengths-as-used-in-hplc.html

The mobile phase used contained an unknown concentration of phosphate buffer (" phosphate buffer (91℅)"; what molarity is that? What pH? what concentration? We do not express solid concentrations as volume %) and a small amount of ACN. I assume it is isocratic as no other information was presented. As salts can precipitate out when mixed with high concentrations of organic solvents it is useful to know what the concentration is. As the column is run in mostly water, it is likely that the entire HPLC system was contaminated with bacteria and particulate matter over time, causing column fouling. Regular column washing (after each analysis, esp for iso runs) is routine procedure and should also incorporate a solvent which is known to fully dissolve everything in the solution 100% to wash the column clean, before starting any re-equilibration steps. When you use phosphate buffers, I find it useful to include some methanol in your wash steps (in addition to ACN/Water rinses) as methanol does a better job of dissolving most salts and other materials (better than ACN).