Does anyone have any idea what is the difference between UV-vis absorbance detector and photodiode array detector from user point of view? Advantages and disadvantages? Thanks.

I'd like also to know the difference in detector sensitivity between DAD and UV-VIS detectors. Assuming that they have the same flow cell volume and are operated at the same wavelength and same bandwidth.

As I recall, a PMT (photomultiplier tube; that's generally what's used in UV/Vis detectors) can be about 2- to 10-fold better in sensitivity than a DAD. Photon spectroscopists and engineers...I'm sure you'll speak up if this isn't the case! Admittedly, there are a lot of things about the specific detector design (slit width, electronics, etc. etc.) which will influence your actual sensitivity. Nothing beats a field test on the compounds that you'll actually be analyzing.

Of course, as you probably know, with a DAD you can look at a wide range of wavelengths, essentially simultaneously. Those UV/Vis spectra provide qualitative information--helpful for some applications and nearly worthless for others! But for many folks, that benefit alone makes the $$ outlay and sensitivity loss (if applicable) worth it.

Similar questions arose a few years ago when it was time to acquire a new UV/VIS detector. We opted for a fast scanning detector (Linear, selling under various names). Reason: The sensitivity was at least 2x that of the best DAD; one can get a spectrum point for every nm unit not every 2nd as with DAD; the longest time used to scan a spectrum has been 1/2 sec in our case (is shorter if the scan is over a shorter wave length range and if fewer nm points are scanned), usually enough for on-flow scanning (a DAD can be faster); one can run a chromatogram at several wavelength (DAD does this more elegantly); the price was ~1/2 of the cheapest DAD; Lamps and cells are compatible with a simpler UV/VIs detector in the lab. If this does not hold anymore it would we nice to be informed.

Surely we are talking about signal to noise ratio, not sensitivity, as a reading of 0.1AU on one detector should be roughly the same as any other, everything being equal? Why does sampling frequency change sensitivity? surely sampling less often just means the detector is less able to reproduce the waveform (chromatogram)? Otherwise I agree, its swings and roundabouts, ignoring the fact that more data does not mean more information!
Cheers
Bryan Wallwork

Sorry to be possibly confusing, should have labelled my points 1), 2), 3).... Also, of course "sensitivity" as used above does not refer to a setting on the apparatus. For my purposes I usually use a S/N of 4 as the lower limit so if the DAD would have a signal with S/N = 4 the other detector should at least give S/N = 8. Now, point 1) was about a better S/N, point 2) about higher wavelength frequency, hence, higher wave length resolution.. etc.. Point 1) has nothing to do with point 2), above, that is, I am not relating one to the other.
To eliminate another possible source of misunderstanding: Bryan, you must mean "data does not NECESSARILY . . . " ?

Hans

Dear Anonymous I. (posting the original message),
As you see there are no big differences between DAD and "classical" UV detectors anymore. The newest DAD detectors are very close in sensitivity (S/N ratio) to good UV-VIS detectors, especially those using "single diode technology" (Dionex 340 etc.). The UV/Vis spectrum provided by the DAD detector is very helpful (e.g. for chemical identification of a compound, peak purity test etc.), even if it is not necessary for every application. The stability and sensitivity of DAD detectors enable also the use of micro detector cells (<1 microliter). In my opinion state-of-the-art DAD is THE general detector in HPLC today. I would give preference to UV-Vis detector only if highest detector sensitivities and/or high-resolution spectra are needed.

Hans,
yes I do..thanks
Bryan