you can do as below
string skuval = htmlDoc.DocumentNode.SelectSingleNode("//span[@itemprop='sku']").InnerText;
string nameval = htmlDoc.DocumentNode.SelectSingleNode("//span[@itemprop='name']").InnerText;
string mpnval = htmlDoc.DocumentNode.SelectSingleNode("//span[@itemprop='mpn']").InnerText;