Dynamic Field Name in Datatable

Hi

   In below line i have given field name in Dataset. But i have field name in Variable , I want to give variable name . Can it be done.

Convert.ToDecimal(dt.Rows[i]["FixedPercentage"].ToString())

I am trying like below but it is giving message Input String is not in correct format

M_fieldname = "PurchaseAmount"

m_Amount = Convert.ToDecimal(dt.Rows[i]["FixedPercentage"].ToString()) * Convert.ToInt32(dt0.Rows[i0][m_fieldname].ToString());

Convert.ToDecimal(dt.Rows[i][m_fieldname].ToString())

Thanks