Chromatography Forum

I've had some problems wrapping my head around the evaporative light scattering (ELS) detectors as a "mass detector" and I hoped someone could provide a new perspective. I can understand how an ELS would give similar responses for two analytes with similar molecular weights, for example compound (A) = 200 g/mol and compound (B) = 195 g/mol. Where it starts to get fuzzy is when you have a compound with twice the molecular weight, such as compound (C) = 400 g/mol. In my mind this would generate half the amount of "particles" that (A) or (B), which would provide a response by the detector of 1/2.

Would an ELS actually provide the same response for a mixture of sucrose and a protein at the same concentration (mg/mL)?

Thank you in advance for any insight you might provide on this topic.

In my mind this would generate half the amount of "particles"

That's the misconception.

In an evaporative detector, the column effluent is sprayed out of the nebulizer to form droplets. As a first approximation, 1 droplet = 1 particle at the end. The number of droplets is governed by the viscosity and the surface tension of the liquid. Unless the analyte concentration gets high enough to affect those, its identity doesn't matter.

You are making an assumption that all compounds form a similar particle. In classical ELS this is not the case. Think of it like this, if you compared an un-popped popcorn kernal and a popped popcorn kernal, both have the same mass, yet the popped kernal will scatter much more light than the un-popped. The response of a given compound in ELSD is dependant on that particular compound, the composition of the mobile phase at the time of nebulization, the detector conditions (temperature, gas flow ect.), and concentration (this is part of the reason why ELSD in a non-liniar technique).

So, the simple answer to this question :Would an ELS actually provide the same response for a mixture of sucrose and a protein at the same concentration (mg/mL)?" is NO

Would an ELS actually provide the same response for a mixture of sucrose and a protein at the same concentration (mg/mL)?" is NO

Ok, you caught me! But the difference would not be proportional to the molecular weight (one to two orders of magnitude) and in practice is much smaller than the kinds of differences you can encounter in UV.

Individual molecules are not scattering light. Optimum nebulization provides particles in the low micron size range
for maximum sensitivity. Clearly these are large aggregations of molecules. And it takes about the same mass of
million Dalton molecules as mass of 400 dalton molecules to make a micron size particle.