DataSource Controls :: Using Varibale In The Where Clause?

I am having difficulties using a variable in the "where" clause:

declare @shift_started as datetime
set @shift_started = (SELECT *
FROM OPENQUERY(ADS_RGWP_SERVER, 'SELECT MAX(shift_started) as shift
FROM salestkt AS STK
WHERE NOT EXISTS(SELECT *
FROM SHFTDATE AS SD
WHERE SD.shift_started = STK.shift_started)'))
SELECT *
FROM OPENQUERY(ADS_RGWP_SERVER, ' sum(case when customer_type = ''Inter-Company'' then qty_shipped_today else 0 end) as intercompany_qty,
sum(case when customer_type = ''Charge'' then qty_shipped_today else 0 end) as outside_qty
FROM salestkt
WHERE shift_started ='" & @shift_started & "' ''')

Getting error:

Incorrect syntax near ' & @shift_started & '.