Server side join issue

I'm trying to add sJoin to my server side script, but if I add any fields with a dot "." in them (eg: 'table1.field') it throws up the error "Undefined index: table1.field"
But the dot is necessary for joining tables!
I don't think it's possible to add array keys with dots in them.
How can I solve this?

Example parts of script: $aColumns = array('table1.field', 'table2.field2');

$sTable = "table1";

$sJoin = " JOIN table2 ON table1.field= table2.field";

$sQuery = "
SELECT SQL_CALC_FOUND_ROWS ".str_replace(" , ", " ", implode(", ", $aColumns))."
FROM $sTable
$sJoin
$sWhere
$sOrder
$sLimit
";