Hi,
May I have the guidance of concerned seniors about the following commonly used detectors in HPLC? I will sincerely appreciate it if you can shed light on their terminology, shortcoming, and benefits compared to one another. Why there is a need to call them differently if some of them share the same mode of action?
Thanks in advance.
i- UV / UV-Vis Detectors
ii- Fixed Wavelength Detector
iii- Diode Array Detectors
iv- Photodiode Array Detectors
v- Multiple Wavelength Detectors
vi - Variable Welength Detectors
RI, UV, PDA, Fluorescent, Transport, ELSD, Chiral, Electro-Chemical, Mass detector
The other detectors are subsets of UV/UV-vis detectors, number "i" in your post.
A fixed wavelength detector used only a single wavelength, typically 254 nm as this was the brightest emission from the lamps used at that time, and also is a common absorbance wavelength for compounds.
A variable Wavelength detector allowed the user to choose a detection wavelength by changing the angle of a dispersion grating.
Diode array = photodiode array. These detectors essentially use a camera sensor that detects the dispersion from a grating over all wavelengths at once. Some of the detectors are CMOS now, rather than photodiodes. These allow measurement of a spectrum as well as absorbance at a single wavelength. Depending on the software, one can see a chromatogram after the run is finished on other wavelengths.
There are two categories for the UV detector a) a fixed wavelength and b) a variable wavelength detector. Thirty-forty years ago, you will find a fixed one where you can use only one wave length (say 254 nm) the entire run. If you have the second compound having the different wavelength (say 365), you need to change the filter and inject the sample. Sometime that filter may not be the maximum absorption wavelength of your compound, but that is what you have. The give you more flexibility and better sensitivity, the instrument maker came up with the variable one where you can dial the wavelength of your choice for ease of use and maximum sensitivity. This light source from the lamp will fall on a prism to split the white light into the rainbow color (different wavelength). You let the specific light (nm) pass thru the sample cell and collect the data (this call a variable UV detector). The more advance variable UV detector is the diode array detector where you pass the white light to the sample cell and separate the white light into different wavelength and determine what wavelength is absorbed by the sample. The detector will detect the whole "array" of light so you get a better picture. It is easy then to choose what wavelength to use in the method and you may use it to identify the peak as different compound has different absorbance profile. Please check out this youtube video
https://www.youtube.com/@ardentscientific7080
Thanks all. I sincerely appreciate you taking the time and helping me (and maybe someone else following the thread) to understand the different typesof UV/UV-Vis detectors.
My major question, if I may say, was what's the difference between DAD and MWD? I have seen several YouTube videos and articles but failed to understand the difference. The only thing which I was able to understand that location of flow cell is critical and the reason why different detectors behaves differently.
Can someone would be kind enough to put the schematic diagram of the detectors to help in better understanding. Thanks again.
In modern instruments? DAD and MWD are the same name for the same thing.
Jack Silver I know at least one company that markets both DAD and MWD.
looking at the details it is nearly the same box, except that the machine marketed as DAD gives You access to the 3 D field and the spectra and the MWD box only gives you access to four preselected wavelengths. - I bought the MWD when funds were just enough to get that and got it upgraded (for the price difference) a few years later when that difference was available.
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