XmlDocument doc = new XmlDocument();
doc.Load(xmlFile);

XmlNodeList nodeList = doc.SelectNodes("//Sample/Analysis");

foreach (XmlNode node in nodeList)
{
    string tgcode = node.Attributes.["TestGroupCode"].Value;
    string tmcode = node.Attributes.["TestMethodCode"].Value;
    string ananame = node.Attributes.["AnaName"].Value;
}