Abstract: Methods and apparatuses for rapid detection and quantification of analytes are disclosed. The analytes may be opioids such as fentanyl. The method includes the use of an internal standard. The internal standard is introduced into the unknown sample. The method may involve a separation step prior to an electrochemical detection step. In some embodiments, the separation step is omitted. The analyte may be selectively adsorbed onto a surface of an electrode by maintaining a potential on the electrode during which a solution containing the dissolved analyte flows through the electrode. Performing electroanalysis on the analyte-adsorbed electrode detects and quantifies only the adsorbed analyte. In some embodiments, the internal standard is methyl vanillate.