Detecting an analyte in the presence of an interferent

A method for determining a presence of a narcotic in a mixture comprising at least one interferent, the method comprising:

• a. measuring a voltammetric response of the narcotic with an apparatus, conducted at a specific scanning speed, in a solvent with a certain concentration of electrolytes, and at a defined temperature, thereby determining a voltage at which, in absence of the at least one interferent, a voltammetric signal of the narcotic can be detected;
• b. contacting an electrode with the mixture;
• c. decreasing an amount of oxygen-containing functional groups in the mixture contacted by the electrode in step b by applying a pretreatment potential to the electrode for a duration of at least 5 seconds, the pretreatment potential measuring from −0.4 V to −1.2 V versus Ag/AgCl, for improving a resolution of a the voltammetric signal of the narcotic signal of the narcotic, when present, from a voltammetric signal of the at least one interferent;
• d. measuring a voltammetric response of the mixture with the same apparatus, conducted at the same specific scanning speed, in the same solvent with the same certain concentration of electrolytes, and at the same defined temperature as for step a, the measurement comprising at least the determined voltage;
• and
• e. determining whether the narcotic is present in the mixture by employing a computer system, using peak recognition software, to analyze whether the voltammetric signal of the narcotic, resolved from the voltammetric signal of the at least one interferent, can be detected in the measured voltammetric response.