Simultaneous LC-MS/MS quantification of oxycodone, tramadol and fentanyl and their metabolites (noroxycodone, oxymorphone, O- desmethyltramadol, N- desmethyltramadol, and norfentanyl) in human plasma and whole blood collected via venepuncture and volumetric absorptive micro sampling

•An accurate and precise LC–MS/MS method was developed to quantify opioid analgesics.•This method is capable to quantify eight analytes simultaneously.•The method is applicable to plasma and whole blood collected via venepuncture or volumetric absorptive micro sampling (VAMS) devices.•This method could help to set the base practice for opioid Therapeutic Drug Monitoring in older patients.