It is found that most investigators use alcoholic extraction as their initial crude extraction method used methanol to obtain their first crude extract and then proceeded with further fractionation. Traditional methods used in the extraction of botanical samples for phytochemistry investigations often use sequential solvent extraction methods starting with less polar solvents, such as hexane, petroleum ether, ethyl acetate, and chloroform, followed by solvents with increasing polarity such as ethanol and water. So, should be alcoholic and subfractions (petrol ether, hexane,CHCI3, ETOAC, ButOH and H2O ) tested. Because the degree of activity depend of chemical composition of the fractions! 

Some researchers test only the first crude (methanolic) extract mainly for time and economic reasons. BUT remember that crude extracts could hide powerful activities that you will be able to find only in more refined extracts. My advise would be YES, if you can, at least, test all crude extracts and then perform a simple fractionation, let's say, get three fractions: non polar (with hexane), little polar (dichloromethane) and more polar (methanol or ethanol) and then test them all. With this tests you will have more information than only with crude extracts because the molecules that could be active will be more purified. Be careful and only use very pure dissolvents. If you have doubts, download my thesis work.

It actually depends on the target of compound that you want to extract. If you have a lot of plants/samples, and you have no idea which plant will have high activity of your targeted bioassay, you may need to do some preliminary work. I would suggest you to start with crude extract (e.g. methanol or ethanol). It gives you the general idea of how good the sample is. But most of the time, synergistic effect will be taken place in crude extract. For more further identification of the positive result obtained from crude extract, you may need to do fractionation work such as liquid-liquid fractionation technique or column chromatography to further fractionate the plant extract based on different polarity solvent. It actually depends on the nature of the plant. If you have direction on what compound/ group/ class of chemical composition, then you certainly need to do fractionation work (say terpene group). If you know you are going to get essential oil from the sample, then straightly you may use hydrodistillation or soxhlet for extraction.

If you are concerned about athnobotany then going to analyse different fraction is not of yours concern.  But if your plain is of phytchemistry then it is better to go for the screening of all fractions as it is believed different fractions may contain different phytochemicals and will help you find the fraction that has higher potentials of activities. 

Thanks Mr. Muhammad Adil. You will be perhaps better known of this that most of scholars dealing with medicinal or toxic plants choose 4/5 plants and prepare fractions for different plant parts. After doing so they select single fraction for phytochemical characterization. But in my personal opinion, detailed analysis for single plant, selected based on methanol crude extract activity is far better than screening all fractions for all parts of all selected plants. Literature exists adopting this methodology but not commonly referred...I don't know why

Listen to Sodik and Lorena. And don't forget about the biosynthesis! One and the same secondary metabolites can be found in differrent amounts in different parts and, besides, these amounts will vary depending on the vegetative stage of the plant. One important remark: polar and non-polar fractions of same part of the plant  may have different efficacy depending on the route of admstration (i.p., oral or s.c). So if you are not going to limit your research with the determination of  antioxidant or antimcrobial activity in in vitro assay (common case for phytochemists -:)) consider this remark. Remember also, that it is much more convenient for the biologist to work with water-soluble compounds both in in vitro and in vivo. Therefore prepare a good  resesarch plan and discuss it with biologist or pharmacologist before you start your experiments. 

Wish you success 

Please note that to select the most active plant for specific activity, firstly crude extract of different plant parts of individual plants should be tested separately. Once specific part exhibit  considerable activity, one should procced for the fractionation of the active  crude extract. Thereafter, all the fractions should be tested for the activity to identify the most active fraction.   

The methanol-dichloromethane(1:1) crude extract of different parts of each plant should be tested separately. Then the crude extract of the part of a specific plant that shows the highest activity should be given priority for fractionation.

Dear Qureshi

If you are a research scholar pursuing your Ph.D. degree, if you have no constraints of facilities and funds; it is most important that you get hands on experience of different techniques which will help you in the long run. Do not go for short cuts unless time, funds and facilities are the constraints. Even in such cases you should seek help elsewhere. Many University students lacking funds and facilities seek our help and we provide wherever it is feasible. Similarly our students go elsewhere for facilities. The more you work the better you learn. Go for all fractions of as many plants as possible. If you are lucky you can publish several good papers that will help you in finding suitable placement. Natural product chemistry is a gamble. There is no guarantee that you will get desired results from a single species unless it is already known in which case your work loses novelty.

Good luck

Your matter is you want quikly find good results

In research is impossible

you must  a time

you have many variables

 plant

solvents

polarity of coumpouds in plant (oil,  PP, other,..)

activity  antimicrobial

...

I think  you must take  a time and :

tak only  one plant for your study

take solvent  (near for study in  vivo  vitro) water;  ethanol or  DMSO

take longer  time for extraction diffusion at ambient  temperature or under 60°C

filtration

 lyophilsation (T/P)

test on crude

if  activity

you must proceding at fractionnement (TLC, ....HPLC prepa ) 

Good Like

mahrouz marrakech

Check this informative link:

http://www.slideshare.net/rahulbs89/extraction-of-plant-contituents

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