Information Retrieval for Chemists: 84:260 

AND

Cation and Chromatography

retreives only articles which mention both words

Therefore NARROWS your topic.

OR

Cation or Anion

retrieves all articles which mention either word

Therefore broadens your topic.

Always use OR for synonyms.

NOT

Cation not Calcium

retrieves only articles which mention the word Cation but do NOT mention Calcium

This narrows your topic but may eliminate items needed.

Many databases, including SciFinder Scholar no longer require you to use this terminology, rather they are programmed to use it "behind the scenes".  An understanding of what goes on makes it easier to understand the results of your searches. 

Parentheses are very helpful for complex searches and you will find that their use improves your retrieval.  Many databases offer Boolean searching as an option, usually under "Expert" search or "Boolean searching".

They are especially useful if you wish to request all articles on a particular concept when the concept may have several well known synonyms which describe it.

For example: educate/instruct/teach are three common synonyms.

To retrieve all articles which discuss the teaching of algebra in high school, group all your synonyms together (with OR between each one) and surround the statement with parentheses.

"high school" AND algebra AND (teach* or instruct* or educate*)

The reason for using parentheses is that all databases search from left to right and analyze each operator (AND OR NOT) as they reach it. 

ANDs are processed first, then ORs but if you have ( ), the items inside the parentheses are processed first.

"Atomic absorption"  AND  "high performance liquid chromatography" OR hplc

"Atomic absorption"  AND  ("high performance liquid chromatography" OR hplc)