Chromatography Forum

There are many LC detectors on the market, RI, ELSD, UV, MSD, FLD, etc.
Some are known as concentration dependent detectors while others are known as mass dependent detectors. Can someone clearly define these terms and classify each of the above detectors.

Thanks

MSD, RI, ELSD - all function based on the amount of analyte passing through them so are "mass dependent" while FLD and UV rely on the absorbance of light by the analyte to generate a signal. As a result, these detectors don't work with all analytes. For a UV (or UV/Visible light detector) to work, the analyte must absorb light within a range of wavelengths. For FLD to work, you must be able to attach something fluorescent to your analyte. The trade off is that these detectors generally have a much wider linear response (or dynamic) range than the more universal detectors.

More commonly found in GC applications, there are additional options (Hall, electron capture, photometric).

More pedantically, concentration-sensitive detectors respond to the *concentration* of analyte in the mobile phase at any given time (as the name implies). Thus, they tend to be flow-rate dependent (imagine shutting off the mobile phase flow while a peak is in the flow cell of a UV detector; the analyte is still in the cell, so the signal stays high).

Mass-sensitive detectors respond to the *mass* of analyte that has gone through the detector (again, as the name implies ). They tend to be flow-rate independent (imagine shutting down the mobile phase flow while a peak is eluting into an evaporative detector. Once you have evaporated the droplets already in the detector -- i.e., almost instantly -- the signal will drop back to zero).

Either kind may have response that varies with analyte structure.

To expand on DR's post, concentration-sensitive detectors include:
- UV-VIS
- fluorescence
- RI
- amperometric (pretty much!)
- electrical conductivity

Mass-sensitive detectors include:
- MS
- the various evaporative detectors
- coulometric
- any of the old "transfer" detectors (moving-belt FID, etc.)
- post-column reaction detectors (usually)

More pedantically, concentration-sensitive detectors respond to the *concentration* of analyte in the mobile phase at any given time (as the name implies). Thus, they tend to be flow-rate dependent (imagine shutting off the mobile phase flow while a peak is in the flow cell of a UV detector; the analyte is still in the cell, so the signal stays high).

Mass-sensitive detectors respond to the *mass* of analyte that has gone through the detector (again, as the name implies ). They tend to be flow-rate independent (imagine shutting down the mobile phase flow while a peak is eluting into an evaporative detector. Once you have evaporated the droplets already in the detector -- i.e., almost instantly -- the signal will drop back to zero).

Either kind may have response that varies with analyte structure.

To expand on DR's post, concentration-sensitive detectors include:
- UV-VIS
- fluorescence
- RI
- amperometric (pretty much!)
- electrical conductivity

Mass-sensitive detectors include:
- MS
- the various evaporative detectors
- coulometric
- any of the old "transfer" detectors (moving-belt FID, etc.)
- post-column reaction detectors (usually)