You should better use single quotes around the char/varchar/nvarchar/text type values/literals used in expressions always.
For understanding how Single and Double quotes are interpretted and when they are aptly used, you should go through the Books online under topic quoted identifiers and read about SET QUOTED_IDENTIFIER (T-SQL) too. That should explain you all about ' and ".
Hope that helps.
- Vijay G
Strive for Perfection