This stable-labeled internal standard is suitable for quantification of 4-ANPP levels in urine or other matrices by LC/MS or GC/MS for clinical toxicology, forensic analysis, prescription monitoring, or urine drug testing applications. 4-ANPP, also known as despropionyl fentanyl, is an intermediate in the synthesis of fentanyl and related opioids and is often found as an impurity in fentanyl preparations.