I have always used quantify, but have been encountering quantitate more and more in scientific literature. Is quantitate a "valid" verb and a synonym for quantify? Otherwise is there a subtle difference in the meaning of these two words?
I forgot to mention that in some dictionaries both words exist while in others, quantitate is not featured, e.g. http://dictionary.cambridge.org/spellcheck/british/?q=quantitate
Valid word is a difficult thing to quantify. As I've said before, a word exists as long as people use it.
You say in your question that you have read it in literature, so therefore we must conclude that it does exist, and, given the context of its use, is a synonym for quantify.
The meaning according to OED1 is a synonym for quantify, chiefly used in biological or medical contexts, so I feel quite sure that its use is typical.
As can be seen in the above ngram, quantify is more popular than quantitate, but quantitate's use is not insignificant in proportion.
The main difference between the two words is that quantitate is (i.e. can be) used to mean "to perform quantitative analysis upon", which is not really synonymous with quantify. Also, I think it's fair to say, outside of scientific literature, you are unlikely to encounter quantitate.
(1): "quantitate, v.". OED Online. March 2013. Oxford University Press. 20 May 2013 http://www.oed.com/view/Entry/155922?redirectedFrom=quantitate.
I believe this is one of the first uses of the noun quantitation by Arthur Ellis Esq in the Journal of the Royal Statistical Society, Vol. 51, No. 3 (Sep., 1888), pp. 567-598:
I VENTURE to use this word " quantitation " as one which will be readily understood by the Statistical Society, if not by the outer world. It is among objects of the Society, while remaining unsatisfied with bald facts or conclusions, or classification is, to find out " how much " and to what extent facts are facts, or, in other words, to test the meaning of facts. "Quantitation," therefore, expresses in one word-which may be an ugly one but is never-the-less a useful word-what we aim at in an investigation before the notice of the Society. (There is the word " quantification," used by writers on logic, including Sir William Hamilton, applied to the predicate. I remember a student of logic calling this word an invention of the enemy for the confusion of the understanding. Precisely because it applies to logic and not to statistical facts, I am disinclined to use it, and it contains one more syllable which, although one more or less among so many may seem to be no matter, is a disadvantage.)
Purnendu commented above that "quantitate" is one to two orders of magnitude more frequent than "quantify", yet no source was cited. A search of articles on the American Chemical Society's website revealed 9129 results for quantitate and 71388 results for quantify. This simple analysis certainly argues in the other direction. Interestingly, the top result in both searches is an article by David Kurtz in Anal. Chem. 1982, 54 (14), 1456A, titled "To Quantify or to Quantitate", where the term "quantify" is encouraged by the author.
In chemistry one differentiates between quantitative (how much?) and qualitative (what?) analysis. These words have been in vogue since the 19th century. There is no such phrase as quantifying or quantificative analysis. In the Analytical chemistry literature the use of "quantitate" is at least an order of magnitude more frequent than "quantify" quite possibly two orders of magnitude.
Enough pontification!
How many people have to tell you that it's a BS made up word for it to be taken out of the dictionary? Word evolution is certainly a thing, but this particular word isn't evolution. It's bullsh*t, made up by people attempting to puff up their grant proposals by truncating and misusing a perfectly good word. QUANTITATIVE data flows from the QUANTIFICATION of results...it's that simple. This is the same as using the word ORIENTATE in lieu of ORIENT. I promise, no one will think you are stupid if you use the proper word instead of its puffed up imaginary cousin.
