Noisey chromatograms with Atlas and Waters

[Cliff Mitchell]

Hi All

When using our Waters HPLC’s (Alliance 2690 and 996 PDA) we see extremely noisy baselines. We use Thermo's Atlas as a data acquisition system, and record the analog channel out and convert it with a Thermo A/D box. To the best of my troubleshooting, it is not a chemical noise issue, and not related to flow through the detector. The mobile phase is acetonitrile and buffer (ammonium acetate). The detection wavelength is 278nm. I have explored increasing the bandwith of the PDA detector, and it does decrease the noise in the chromatogram, but not to a sufficient level. The Hamming filter does have a dramatic effect on the noise, but the baseline has a lot of square blockish features in it.

In the chromatogram below, the green trace is from an Agilent 1100 VWD, and the blue trace is the Waters 996 PDA, and they are on the same scale. The firmware for the 996 is the most current version, but I suspect that the noise is in the signal leaving the detector. Any comments?

I just wanted to see if anyone else has experienced this before.

Cliff

[Dan]

Cliff,

This may be a question for data systems and not chromatography.

This looks like the same problem that we have encountered. The problem traces back to an incompatability between the data rates/processing for the Waters detectors and the CDS. This only occurs when the detector has both channels enabled; everything is fine when only one channel is enabled. It occurred when we used the PeakPro CDS and still occurs now that we use the Atlas CDS, but only with the Waters detectors (both the UV and the PDA). We don't have the problem with the dual channel Agilent 1100 UV detector.

You probably have both channels enabled on the 996. The problem occurs even if you have the analog output for the second channel off. The problem exists if both channels are "enabled".

We asked Waters for assistance and found that you can improve the signals by varying the acquisition/processing parameters of the detector and the acquisition rate for Atlas. Doesn't always work, so I can't suggest the best parameters. It has been a bit of trial and error. The best option is to just disable the dual channel use. If you need the second channel, the use either a second detector (we have systems with both a PDA and a UV) or use the Agilent 1100 dual channel UV.


Regards,
Dan

[DR]

Assuming that you have a CDS that is capable of collecting PDA data from the 996, I would suggest using that for your single wave runs too as the analog outputs of Waters PDAs aren't the best. Digital signals from Waters PDAs are reputed to be much better than analog (quite possibly because of the potential for mis-matches between DAD & A/D sampling rates - is sort of like filming a TV and seeing those black bars migrating up the screen).

Also - check all wiring - it could be something with a connection between the 996 & the A/D.

[danko]

Hi Cliff

I’m not familiar with “Atlasâ€

[Cliff Mitchell]

I wondered if that would get noticed. The two chromatograms are not of the same sample, the blue is a standard and the green is a sample. The are nearly identical in concentration, but the sample is prepared from an acetone stock and the standards is from acetonitrile. Below is another chromatogram, both sample are standards, blue is Waters with Atlas, and brown is Agilent 1100 with Atlas. Both chromatograms are on the same scale, and the concentration of each sample is (nearly) identical.

I did have both channels active and I am investigating turning off channel 2. I'll let you know what I find.

Cliff

[khw]

Just another idea: What reference wavelength are you using?