Depends on nature of sample and impurity. If the impurity is e.g. volatile, you can observe it quite simply. But some other cases will cause problems (e.g. non-volative impurity in non-volative sample or e.g. some kinds of isomers impurities). In every case is better to compine it with other technique. You can read more in this links or you should specify what kind of sample and what kind of impurities and than maybe some can answer you better.

http://www.sciencedirect.com/science/article/pii/0040603191801437

http://www.perkinelmer.com/CMSResources/Images/44-74556GDE_TGABeginnersGuide.pdf

http://www.drug-dev.com/Main/Back-Issues/IMPURITIES-DETECTION-Impurities-Detection-in-Pharm-549.aspx

Thanks for your response. The material is ZnO nanoparticle, prepared by green synthesis using plant extract.

So inpurities are expected to be organic? If so, I believe it should work. The question is if there is some biosynthetic organic surface modification on these nanoparticles. This can play a key role in this green synthesis and should not be considered as impurity.

Thank you Mr. Lukas, but the thing is preparation method involves combustion wherein the phytochemicals ignite and reduce the precurosr in to NPs. Now my doubt is based on TG curve can I say the np is pure and no organic residue is left from the plant eextract.

You may perform your TG experiment in oxidative atmosphere for comparison. If there will be no mass loss also in this run, your ZnO sample is pure from organic component.

Theoretically yes, you can get the melting point for mostly pure metals, if more than one peak you get, the material is not a pure melt. but the impurity is not so high, TG will give no help.

Thank you very much Sir.

If the impurity is organic the TG will give you weight reduction during the heating up to 800 degreeC. If other metal oxides are present then TG is insufficient. You have to use Raman/ XRD/ Edax.

Dear sir, thanks for your response. If organic impurities are there then the weight will decrease till 800 degree C? But organic compounds melt relatively at low temperature know. Kindly clarify sir.

TGA is not disturbed so far the weight is not changed. If melting occurs it will remain in the boat/sample vial. when it evaporates or degrade you get the weight change. I am clarifying with one example. You take PVC. It will degrade and HCl will be liberated. From wt. of gas released you get the purity of PVC.