If a search has been done, and nothing has been found, try other related words. Usually the word added in the keyword list was what the authors used, with a few additional words from the editor adding the references to make the searches a little more comprehensive. These appear in for each 'hit' one after the other and can be printed out for you to file, read, and consider at your leisure. If you do not find a 'hit' in one database, even after considering other keywords, try another database, which might have the compound. Even some sample prep work or TLC will be helpful in developing a separation in HPLC, and vice-versa.
Liquid chromatography is all about solubility and selectivity.
You just have to find the correct combination and it could be in any of the databases. Remember, too, that papers that deal with determination of compounds from a biological fluid will have information on the sample preparation, even though this may not be reflected in the title. The term 'SPE' is used in the keyword list if solid phase extraction is used in such papers. If nothing is noted, then probably liquid-liquid extraction has been used. This is to point out that the more you use the databases, the more you will be familiar with the information contained within the fields.