Replacing an RID with ELSD or DAD/ELSD?

Discussions about HPLC, CE, TLC, SFC, and other "liquid phase" separation techniques.

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We're currently using an RID for analyzing diesel-range petroleum samples to quantify aromatics by ring number (mostly based on ASTM D6591). Normally, I'd strictly adhere to ASTM methodology if not for four things:
  1. Our facilities/maintenance department has no plans to properly regulate the temperature in our lab.
  2. There's no regulatory or company policy force behind strict adherence to the ASTM method if I can show that whatever I come up with is equivalent.
  3. We have another HPLC system using a DAD and ELSD for the same analysis on VGO samples.
  4. Lab real estate is at an incredible premium and I'd love to be able to reclaim bench space by combining instrument functions.
I know ELSD needs a difference in volatility between mobile phase and analyte but I have no idea how much difference or even how that would be quantified (BP? Vapor pressure? Something else?). Is there even a rule of thumb value to start with?
I would assume that I can use ELSD for the diesels since we're already using it for the VGO samples for the same results (mass% 1-ring/2-ring/3-ring/4-ring+ aromatics).
I'd appreciate any input before I dive down the rabbit hole of trying it and seeing what happens.
Please help me understand why UV or DAD detector is not used for detecting aromatics.
Consumer Products Guy wrote:
Please help me understand why UV or DAD detector is not used for detecting aromatics.

I wish I knew. The configuration with the RID was adopted by ASTM for method D6591 and, like all the other lemmings in the petroleum industry, we just blindly followed.

But now that you mention it... Yah, I can't figure out why we wouldn't just use a DAD for both.

Additional information I discovered just before leaving work today:
Nowhere in the Chemstation macro (that was created for us by an outside lab 11 years ago) is the ELSD data used. It's using only DAD data.

We've had the ELSD hooked up and Chemstation has been collecting data from it for 11 years. Heck, it's even in the journal article we based the setup on.


I'm going to remove the ELSD from the configuration tomorrow and see if anything breaks.
We used HPLC with UV detector to monitor aromatics in mineral spirits for a product.

Sometimes we used a special packed GC column with FID.
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