Sequential-injection extraction for sample preparation

This report introduces a flow-based extraction method where an aqueous sample and organic solvent are injected sequentially into an extraction coil, then mixed and separated due to the differential flow velocities of the aqueous and organic phases. A 500 μl aqueous sample is propelled through a 50 μl segment of organic solvent whose flow is impeded due to hydrophobic interactions with the walls of a Teflon extraction coil. This wall drag allows the faster moving aqueous sample to penetrate through and ultimately separate from the slower organic solvent. These steps are repeated with a back extraction into a second aqueous segment (100 μl) that is collected and analyzed with high-pressure liquid chromatography (HPLC). The configuration of this novel sequential-injection extraction (SIE) system allows ease of changing reagents and uses minimum organic solvent volumes, as it avoids continuous pumping of reagents. Barbiturates (phenobarbital, amobarbital, pentobarbital and secobarbital) and serotonin re-uptake inhibitors (SRIs) — venlafaxine, paroxetine, sertraline and norsertraline — were extracted as model acidic and basic compounds from urine into a 1 : 4 (v : v) mixture of 1-octanol and butyl chloride and back extracted into 0.45 M NaOH and 0.18 M H3PO4, respectively. Absolute recoveries of the analytes were: venlafaxine 29%; paroxetine 30%; norsertraline 25%; sertraline 20%; phenobarbital 11%; amobarbital 24%; pentobarbital 23%; and secobarbital 27%. The sample throughput, including extraction and back extraction, was 20 samples h−1.