Assuming the sample is homogeneous, you dont need to analyse the whole sample. Just taking a few small samples from the batch will give you confirmation it is what it is. Run all the usual analyses on 6 smaller samples taken after mixing the large sample to ensure precision. I dont know of anyone who would run 400g sample in a chromatography method. If you have to use it all, dissolving it in a large flask would give you true values, but again you would only use around 1ml in a HPLC vial.