Application of GC–MS retention index in forensic routine

Gas chromatography–mass spectrometry (GC–MS) is widely used for the identification of substances of forensic interest, but it faces challenges due to the vast diversity of available drugs. In this context, the use of the linear retention index (LRI) can help overcome difficulties associated with the identification of a large number of substances, as it is less susceptible to variations in experimental conditions compared to retention time (RT). Building an LRI database allows rapid and simultaneous identification of several compounds, including new and structurally similar substances, thus standardizing analytical identification. This study aimed to evaluate the use of the retention index (RI) in both intralaboratory and interlaboratory forensic routines. Over a 50-week period on the same GC–MS system, RI values were effectively adjusted by reinjecting an n-alkane mix after maintenance that required vacuum breaks, updating the retention times of each substance evaluated (benzocaine, caffeine, cocaine and flunitrazepam). Interlaboratory reproducibility was evaluated between two forensic laboratories using up to 104 analytes. Two oven temperature programs, three GC brands and two analytical columns were evaluated. Under the same analytical conditions, the ΔRI was ≤10 index units (i.u.) for 103 substances, with testosterone enanthate showing a ΔRI = 11 i.u.. The proposed method proved to be reproducible and selective, and the use of RI has demonstrated accuracy as a complementary technique for identifying a wide range of analytes. Furthermore, the sharing of an RI database can strengthen the analytical routine, being a way to quickly obtain information on the identification of new drugs.