datagrid stylesheet

In my SQL I get a value from calculating two fields. I want to place this value in my datagrid. I'm having trouble mapping it in my dg-stylesheet.

VB Code:

Dim cmdView As OleDb.OleDbDataAdapter = New OleDb.OleDbDataAdapter("select check_no, purchase_date, vendor_payee, check_amt, " & _
        [B]"(check_amt-returned_amt) as purchase_amt[/B], returned_amt, approver_id, " & _
        "from pccr.pccr_check_header, pccr.pccr", objCon)
 
Dim grdColStyle5 As New DataGridTextBoxColumn()
        With grdColStyle5
            .HeaderText = "Purchase Amt"
            .MappingName = "purchase_amt"
            .Format = "$#.##"
            .Width = 77
            .Alignment = HorizontalAlignment.Center

VB Code:

Dim cmdView As OleDb.OleDbDataAdapter = New OleDb.OleDbDataAdapter("select check_no, purchase_date, vendor_payee, check_amt, " & _
        "(check_amt-returned_amt) as purchase_amt, returned_amt, approver_id, " & _
        "from pccr.pccr_check_header, pccr.pccr", objCon)
 
dim ds as new dataset
cmdView.Fill(ds, "purchase_amt")
 
Dim ts1 As New DataGridTableStyle
 
ts1.MappingName = "purchase_amt"
ts1.AlternatingBackColor = Color.LightBlue
 
 
Dim grdColStyle5 As New DataGridTextBoxColumn()
 
        With grdColStyle5
            .HeaderText = "Purchase Amt"
            .MappingName = "purchase_amt"
            .Format = "$#.##"
            .Width = 77
            .Alignment = HorizontalAlignment.Center
    end with
 
Me.DataGrid1.TableStyles.Clear()
Me.DataGrid1.TableStyles.Add(ts1)

Regards
Jorge