Unstable baseline and inconsistent peak areas for standards

[Marian Minosa-Yertzell]

Hi, I've been having trouble with my chromquest 3.0 system by Thermo-Finnigan. The baseline drifts very low or starts going up really high even without injection. There is no telling when it will happen. Another problem is inconsistent peak heights causing very poor %RSD. Retention time is ok. I've checked the flow rate is stable. The deterium lamp for the detector is new. The flow cell is also new. The guard column was replaced. The column cannot be the cause because when I exchanged the column with another system, the column was fine. With the second column installed, the problem persists. Do you have any other suggestions?

[Uwe Neue]

What's your wavelength, what's the mobile phase?

[Marian Minosa-Yertzell]

The unpredicted big wave on the baseline is gone. I put mobile phase in a vial and injected it through the autosampler. somehow whatever it was that was causing the big wave was flushed out. However, I am still having problems with my standards. I do bracket calibration. So I inject 2 standards in the beginning, then set of samples, then 2 standards at the end. Eveything looks great except for the last 2 standards. the baseline is always bowing to the negative during 9 minutes until 15 minutes. The standards come out within this time frame. bowing of the baseline affects the correllation for the bracket calibration. I am using biorad-aminex column, at wavelength 210. Mobile phase is .0324 M H2S04 any suggestions?

[Marian Minosa-Yertzell]

Correction: mobile phase is .0324 N H2S04

[syx]

Same problem in the same wavelength but in diff mobile phase. In DAD I found wavy baseline on the wavelengths below 220 nm using ACN - MeOH - water (64:18:18) - I have another post in this forum.
We use this method to determine released active substance in dissolution samples:
Dissolution media: 0.09% SLS/0.1N HCl
Conc of 100% released: 10 ppm
Injection volume: 20 ul (using autosampler, with 100% MeOH as rinsing solution)
SST is passed, but trouble is occured when we run calibration standards and samples. They give bad precission, though we injected replicate injections from the same vial.
I still do not have any idea to solve this problem...

[Mark Tracy]

Marian,
This looks like a classic case of late eluting substances that are in your sample but not in your standard. At 210 nm, most everything absorbs, so it could be something innocent looking. Your Aminex column has a relatively large ion-exchange capacity, and your mobile phase is of moderate strength, so various anions could be retained a long time.

Try the following experiment: Figure out how long it takes to run one of your normal sample sets (call it T). Equilibrate your column for 1.5xT. Write a modified method to collect data for 1.5xT. Inject one standard and one sample using the modified method. I'll bet you see a big wave late in the sample, but not in the standard.

Another possibility is a severe mismatch between the ionic strength or pH of the sample and the mobile phase. The above experiment would show similar results.

[syx]

We have fixed the problem: dirty cam. We make it clean and the problem have gone away.