Line Itemwise PO Invoicing using x++

I am using AX 2012.I have below requirement.

For one PO no I have maylines.

PurchTable

#PO12312  Test Purchase Vendorooo1

Purchline

Item1  Testitem1
Item2  TestItem2
Item3  TestItem3
Item4  TestItem4
Item5  TestItem5

PO is in received status.Now I want to invoice only Item3.How we will achieve this using x++.

    //This code will invoice all items for the same PO

    PurchTable purch = PurchTable::find('TestPO');

    poinv = PurchFormLetter::construct(DocumentStatus::Invoice);

    poinv.update(purch,strFmt("Test01%1", purch.PurchId));

    //This code will invoice all items for the same PO