by
lmh » Thu May 31, 2018 10:21 am
The obvious missing manufacturers from your list are Waters and Shimadzu. All manufacturers have several grades of instrument, and if sensitivity is important to you, then you should be considering the better-quality PDAs.
PDAs are naturally measuring the full spectrum continuously, but the software you're using will possibly place limitations on how you handle chromatograms at different wavelengths. For example, Agilent's Chemstation doesn't make it easy if you need to work with more than 5 (I think) different wavelengths, which it expects you to specify in the acquisition method (data are then collected at these wavelengths independently of spectra, which may also be collected; the advantage is that you can collect data far more rapidly to give a better estimate of peak shape. If you're used to VWDs, then Chemstation will feel quite natural). Meanwhile Shimadzu's software doesn't collect chromatographic data at fixed wavelengths separately at all; it simply collects spectra and expects you to specify, during data-processing, what wavelengths you need.
There are all sorts of other bells and whistles on the software side that might matter to you, or might be irrelevant. For example, Shimadzu can only subtract a single wavelength from all wavelengths you need for analysis, if you're in the habit of subtracting a blank, but they have some rather nice built-in stuff that, if you choose to use it (you don't have to) extends the dynamic range by deliberately looking at the "wrong" wavelength in very concentrated samples, and compensating for the reduced extinction coefficient by multiplying by the ratio of extinction coefficients at the "wrong" and optimal wavelengths as determined in a standard.
Software is actually as important as hardware; if you and your coworkers don't like the software, the instrument won't be a success. Another big difference is whether data are stored in individual files (Chemstation, Shimadzu) or a database (Waters, Thermo's Chromeleon, or Agilent's newer software. I still don't know which approach is best, but it has consequences for how you work and how you store your data.
Do check you can get appropriate support and training; Waters, for example, seem to regard PDA as specialist, and run separate (expensive) courses on how to use it.