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Screensize: 640 x 480 800 x 600 1024 x 768 1280 x 1024 UserName: Password: Anti-Spam Code: Format Mode: Basic  Help  Prompt  Format: Font Andale Mono Arial Arial Black Book Antiqua Century Gothic Comic Sans MS Courier New Georgia Impact Lucida Console Script MT Bold Stencil Tahoma Times New Roman Trebuchet MS Verdana   Size 1 2 3 4 5 6   Color Black Red Yellow Pink Green Orange Purple Blue Beige Brown Teal Navy Maroon LimeGreen Message: * HTML is OFF * Forum Code is ON Smilies [quote][i]Originally posted by sperling[/i] [br]I have a fitting problem that appears to be difficult to implement in Origin. I have a set of absorbance spectra [i]A[size=1]i[/size=1][/i]([i]x[/i]) at various concentrations [i]c[size=1]i[/size=1][/i] and discrete [i]x[size=1]j[/size=1][/i] values. They unfortunately have baseline problems that prevent a straight-forward implementation of Beer's law, i.e. [i]A[/i]([i]x[/i])=[i]a[/i]([i]x[/i])[i]cl[/i], where absorbivity is [i]a[/i]([i]x[/i]) and [i]l[/i] is the path length. (Absorptivity is what I want out of the fit.) While the usual spectroscopic methods for baseline subtraction work reasonably well, they have their own problems. I've found the baselines are well-described by parabolic functions, so each spectrum can be represented as [i]A[size=1]ij[/size=1][/i] = [i]a[size=1]j[/size=1]c[size=1]i[/size=1]l[/i] + [i]f[size=1]i[/size=1][/i] + [i]g[size=1]i[/size=1]x[size=1]j[/size=1][/i] + [i]h[size=1]i[/size=1]x[size=1]j[/size=1][/i]^2 I've generated synthetic spectra that obey this relationship, and Excel solver lands on the correct values. Implementing this in NLFit in Origin 9.1, however, is problematic. I've tried various ways to use global fitting, but I have yet to figure out a way to couple the parameters to get ([i]f[/i],[i]g[/i],[i]h[/i]) for each [i]i[/i] and [i]a[size=1]j[/size=1][/i] for each [i]x[size=1]j[/size=1][/i]. Is this even possible? Am I missing some trick to make this happen? [/quote]   Check here to include your profile signature. Check here to subscribe to this topic.