[R] least square fit with non-negativity constraints for absorption spectra fitting

[Xu, Xiuli (NIH/NHLBI) [E]]

I would really appreciate it if someone can give suggestions on how to
do spectra fitting in R using ordinary least square fitting and
non-negativity constraints. The lm() function works well for ordinary
least square fitting, but how to specify non-negativity constraints? It
wouldn't make sense if the fitting coefficients coming out as negative
in absorption spectra deconvolution.

Thanks. 

Xiuli