There are a number of techniques that could be used for the determination of  the chemical constituents of your dried  leaf samples. The excellent text which is useful in this matter is:PHYTOCHEMICAL METHODS,2000, BY J. Harborne, of READING UNIVERSITY, ENGLAND.He was  the Professor of phytochemistry during my graduate study and he was the top expert in this respect.

LC-MS will give you insights, but the phytochemical profile will depend on the extraction (e.g. solvent) you use. Do you have a specific family of compounds (for example flavonoid, terpenoid) that you want to screen in particular?

take care dehydration not more than 40 C, or 55 C under vaccum

Have a look at phenol-explorer.eu You can download an excel sheet containing the complete set of polyphenols contained in the database (http://phenol-explorer.eu/downloads) and also a complete list of polyphenols with: name, molecular weight, chemical formula, and the number of associated foods and composition data.

For beginning, many times, it helps.

I am doing extraction in Methanol. I want to compare coumarins and xanthone in two plant sample.

FFor a quantitative comparison between samples, if you have standards for the compounds of interest, HPLC-UV-DAD probably is the better and less expensive choice. Otherwise you need an MS instrument able of quantitation such as a triple-Q.

at first, which plant and which family, to specify the major phytochemicals. then collect standard or authentic compounds. afterthat select which column and solvent system to fractionate compounds

Phytochemicals have different polarities. LC with gradient elution can assist to get out different phytochemicals of varying polarity. GC-MS will fragment the compounds owing to their specific patters depending on the characteristics of each. You will definitely need to have an insight of which phytochemicals you may have in the plant material as you will need to know the fragmentation patterns for comparison.