Multidimensional wrote:
they can acquire (scan) an entire range of discreet wavelengths (i.e. 210-600nm) for the DAD (PDA) OR store several discreet wavelengths (i.e. 210[10]; 254[8]; 280[10] etc, often 5 to 8 wavelengths. Each discreet signal is stored alone, no spectra is collected (no need to). THE DAD (PDA) is unique in that it can both collect individual signals and optionally scan a range of wavelengths (these are separate data sets). THE MWD can not "scan" so it can only store the multiple discrete wavelengths. This is easy to do using the available array of diodes within the bandwidth of each selected wavelength. Again, no need to"extract".
You assume that collecting a WL in DAD happens separately from acquiring full spectra. May I ask what's the source of this information? I went through Agilent and Waters operators guides on DAD - none of them mention how they do it. So either you have some insider knowledge, or you're just assuming - same as me
It's entirely possible that DAD always gathers full spectra, then
digitally extracts the WL + BW that we were interested in and then removes spectra. And I have several reasons to believe this is actually the way:
1. I have Agilent DAD files, where WL signal is stored separate in files dad1A.ch, dad1B.ch, etc. And none of these files contain Retention Time - just intensities. It means RT is stored somewhere else - and the only such place is the spectra file (dad1.uv). This fact alone means that there was no separate measurement.*
2. I also have Themo MWD files. Multiple "channels" are written with their Retention Times - some of them are
identical. They don't differ even by a microsecond. Again this means there was only 1 measurement for some wavelengths. There are other channels that could differ though, but maybe it's possible to set up that different measurements save different WLs.
3. Here's a quote from
Waters DAD docs:
The data the detector reports to the database (Empower or MassLynx) can be the average of a number of data points. After calculating absorbance, the detector averages absorbance values based on the requested spectral resolution.
This
kinda implies that there's a stage of gathering data from diodes, and then additional post-processing depending on user settings. But this quote is quite weak, you can't deduce much from it.
4. If spectra is gathered anyway, it doesn't make any sense to make yet another measurement. That would simply gather a subset of the same data. So why measure the same thing twice?
5. Another
a priory thought: it'd make the hardware design more complicated. While it's usually easier to make such things on the software side.
vmu wrote:
DAD can physically measure a "single" wavelength". Each "single" WL has its own photodiode for this purpose (or a range of neighboring diodes (averaged absorbance) for a "single" WL with the bandwidth of several nanometers).
Agree.. But still the fact that in principle it's possible - doesn't yet mean that this is what they're actually doing.
*It's also possible that someone extracted these after the fact - ChemStation allows it, but I don't know if the file format is the same. If you give me an example of a file with DAD + channels that you ran, I can check it as well.