Noise level in Fast LC

[VEBA]

Dear all,
After switching my LC to more sophisticated (low cell volume,high acquisitin speed etc) I'll get sharp peaks and fast run times.One thing
which makes me little wondering,is noise amplitude .I use PDA-Detector(3µl flow cell).
Detector noise specifations are based on slow response time and aqcuisition.Is it just acceptable to get lower noise values with high speed?
Difference is about 5 to 10 times higher.What kind of experiences you have?Are there much difference between PDA and VWD or between LC brand?
BTW,pulsation from the pump has been tested and is the not the reason for higher noise
Regards,
Veba

[Uwe Neue]

It is normal to get lower noise when you use a slower sampling rate. Effectively, the slow sampling rate is similar to a filter: during the sampling time, the signal accumulates, and the noise averages out. There is no magic in this, and the math works the same independent of the manufacturer of the equipment or whether this is a PDA or a VWD.

To compare noise between detectors from different manufacturers, you need to use the same testing conditions. I believe these conditions are specified in standard spec sheets.

[DR]

Given the recent advent of "PDA wars", you may well have to call some of the vendors to get noise specs at a specific sampling rate as everyone seems to be very interested in using faster rates these days (as well they should be given the moves to higher pressure, lower volume and smaller stationary phase particles). I suspect that once everyone offers a PDA that can sample at ~100Hz, they will resume talking about S/N again.

[reysolutions]

Veba,

I have not seen any high noise values with the Acquity PDA. We used the standard process of setting the sampling rate to maximize sensitivity without impacting peak shape in either the time or wavelength domains. (NLT 20 slices at half-width.)

I have only seen high noise when over-sampling, which also reduces absolute peak sensitivity.

We did see that the Acquity PDA was substantially more sensitive than the Agilent 1100 PDA using the Agilent semi-micro cell. The increase in sensitivity varied with analyte, but was NLT than 4-fold.