Chromatography Forum

We're running into an issue with the CAD detector autozeroing to be able to detect sample. We have Waters Empower 3 SR1 connected to the CAD through DII v1.6. We have a USB cable from the CAD detector to the computer, and an I/O to serial port cable from the e2695 to computer. We can visualize the output in both chromeleon and empower.

Here is our issue - After allowing the CAD to warmup/stabilize, if our mobile phase is 100% MeCN, we get a readout on the CAD detector hardware of ~30pA (the 'i' value). If we run our actual mobile phase which is a gradient method that starts at 50:50 water:(3:1 MeOH:MeCN), where both water and organic phases also contain 0.1% TFA and 26 mM imidazole, the 'i' value spikes to ~500 pA immediately. When we try and run a blank, the data appears as noise instead of a stable baseline. We've tried to autozero in chromeleon, and it resets the data to 0 pA in the software, but the hardware still reads 500 pA. Is this an autozero issue? Do we need to trigger the autozero on the detector itself? Thanks!

One final note: this method works on a Corona VEO CAD detector, so we were hoping that it would also work on this older model too.

CAD readout should be very low when running 100% ACN, close to 0 when power function is set to 1.8. 30 pA is way too high. You can run 100% ACN without a column to make sure it’s clean. Only volatile additives should be used in mobile phases when using CAD. Bp of imidazole is ~250C. It does not seem suitable for CAD.

I don’t think auto zero works for CAD. A close to zero readout needs to be obtained in order to achieve good sensitivity.

Remove additive and use lcms grade solvents only for cad. Equilibrate for an hour then compare baseline. This will help determine root problem, cad or mobile phase issue
The auto zero doesn’t work like that. The true background will always be displayed. Auto zero button is only triggered in the beginning of each injection

Thanks everyone. We removed the additives and the baseline dropped significantly.