Alan already gave you the right answer - use the sAMAccountName to filter your user.
I would add a recommendation on your use of DirectorySearcher - if you only want one or two pieces of information, add them into the "PropertiesToLoad" collection of the DirectorySearcher.
Instead of retrieving the whole big user object and then picking out one or two items, this will just return exactly those bits you need.
Sample:
adSearch.PropertiesToLoad.Add("sn"); // surname = last name
adSearch.PropertiesToLoad.Add("givenName"); // given (or first) name
adSearch.PropertiesToLoad.Add("mail"); // e-mail addresse
adSearch.PropertiesToLoad.Add("telephoneNumber"); // phone number
Those are just the usual AD/LDAP property names you need to specify.