Attaching a sample code used for invoice..similarly you can write for Purchase-
foreach (Line invoiceLine in x.Line)
{
itm = new LineItem();
if (invoiceLine.Amount != 0)
{
itm.Description = invoiceLine.Description;
itm.Description = String.IsNullOrEmpty(itm.Description) ? "n/a" : itm.Description;
if (invoiceLine.DetailType == LineDetailTypeEnum.AccountBasedExpenseLineDetail)
{
AccountBasedExpenseLineDetail itemLineDetail =
invoiceLine.AnyIntuitObject as AccountBasedExpenseLineDetail;
itm.Quantity = 1;
itm.UnitAmount = invoiceLine.Amount;
itm.AccountCode = itemLineDetail.AccountRef.Value;
}