This is the example of Qubit™ Protein Assay curve. It relates the concentration (x-axis) with fluorescence (y-axis). What is the curve-fitting method it use for this? Clearly this is not simple linear fitting model.
This is Fig 3 of the PDF that you linked to us, as pointed out in the comment by Anyon. The captio to Fig 3 is:
"Figure 3. The curve-fitting algorithm used to determine concentration in the Qubit™ protein assay. The Qubit® 2.0 Fluorometer generates concentration data based on the relationship between the two standards used in calibration. This plot shows the line corresponding to the curve-fitting algorithm (a modified Hill plot) used in the calculation of concentration data for the Qubit™ protein assay. For reference, the positions of the standards and a set of data points from an actual experiment are shown superimposed onto the line, demonstrating that the curve-fitting algorithm gives accurate values for quantitation."
The curve in the figure also does look like one that you would get from a Hill equation (nevermind the log-log "Hill plot" which is different). The word Hill only appears in that 9-page PDF document once (in precisely the quote figure caption above), so it doesn't look like they wanted to tell you any more.
At the top of the PDF we see two commercial company's names, Invitrogen and Molecular Probes and the PDF is about the trademarked "Qubit™ Protein Assay Kits For use with the Qubit® 2.0 Fluorometer", which means that we're probably not dealing with open accecss science, but with commercial products, which rarely disclose details about calculations to the extent that (some!) scientists more often do when publishing academic papers.
You can try to contact them (all this information was fortunately in the PDF to which you provided a link), but I'm not sure if the model matters to you enough to bother with that:
"Molecular Probes, Inc.
29851 Willow Creek Road
Eugene, OR 97402
Phone: (541) 465-8300
Fax: (541) 335-0504
Customer Service: 6:00 am to 4:30 pm (Pacific Time)
Phone: (541) 335-0338
Fax: (541) 335-0305
Toll-Free Ordering for USA:
Order Phone: (800) 438-2209
Order Fax: (800) 438-0228
Technical Service:8:00 am to 4:00 pm (Pacific Time)
Phone: (541) 335-0353
Toll-Free (800) 438-2209
Fax: (541) 335-0238
Invitrogen European Headquarters Invitrogen, Ltd.
3 Fountain Drive
Inchinnan Business Park Paisley PA4 9RF, UK
Phone: +44 (0) 141 814 6100
Fax: +44 (0) 141 814 6260
Email: [email protected]
Technical Services: [email protected]"
The link that you gave us happened to be hosted on a Smith College server for the "Center for Molecular Biology (CMB)" but there's not "About us" or "People" page so it's hard to see who the contact person would be. Likely the PDF was not on the Smith College website due to them having anything to do with product, other than the fact that it's equipment that's available to their students. The only email address I found on the CMB website after a few clicks and a couple minutes was given here:
"Users will need to be granted access to use the equipment by the CMB’s ITI, Riccardo Racicot, at [email protected]"