Subject: how do you work out the mass of star by its spectrum?

I have some questions on black hole detection (via working out mass of other 
star in binary ‘star’ pair)…So far I have been found out that a star's mass can 
be figured out by looking at it's spectrum. I have found out that each element 
leaves a unique color on spectrum, and so by looking at the colors on a 
spectrum you can work out the elements in a star and (if you knew how much of 
it's element the star was somposed off) you could therefore work out the mass 
of the star. But I don't know how to work out the persentage of each element in 
the star and therefore can't work out it's weight. Here are my questions:

When you try to work out the mass of a star from it’s spectrum (by finding the 
stars element composition (the different elements and their percentage in the 
star) and by finding the weight of these elements), how do you work out what 
percentage each element is in the star? Do you just base your results (on the 
percentage of each element) by how ‘thick’ each line appears on the spectrum 
compared to the others, by doing an overall scale of the spectrum and 
determining it’s ratio compared to other elements? Can you please tell me more 
information about this process?

Re: how do you work out the mass of star by its spectrum?