To screen all the chemical compounds in the herbal plant by GCMS is impossible. The reason is too many compounds in the herbal plants. What you can do is to focus on what you interested.

Follow publications from Professor Oliver Fiehn and Professor Lloyd Sumner by Google Scholar to find all the relevant protocols and methods. For sample preparations also refer to JB Harboune's book on Phytochemicals.Too big question for Research Gate !

Please Refer this book

Wagner, H., Sabine, B. & Rickl, V. Plant Drug Analysis:A Thin Layer Chromatography Atlas, Vol. 2. (Springer: Berlin and New York 2009)

Agree w/ all above. What are you interested in? Most extraction processes involve polar and non-polar fractions and then proceed with either/both.

To screen all the chemical compounds found in the herbal plant, there are so methods in the literature. To identify the chemical profiles found in the extracts of the medical plants, I can mention the diverse chromatographic methods. However, as it is customary in the literature, to recognize and characterize the profiles of the essential oils, undoubtedly the GC-MS is the best approach. In short, we can refer to three main criteria in this field involving:

1) Matching their indexes

2) Matching their MS fragmentation patterns

3) The reports of the GC- MS library with a resemblance more than 85%