An online field-amplification sample stacking method for the determination of β2-agonists in human urine by CE-ESI/MS

Field amplified sample stacking (FASS) was combined with a simple, rapid, sensitive CE-ESI–MS method to achieve the on-line enrichment and simultaneous determination of Clenbuterol (CLE), Salbutamol (SAL), Terbutaline (TER) and Formoterol (FOR). Samples were diluted in deionized water, and electrokinetic injection (10 kV×50 s) was employed to carry out FASS. With FASS, the four β2-agonists had simultaneously baseline enhancement as much as 319, 332, 297 and 115 fold, respectively. Consequently, satisfactory LODs (S/N=3) of 0.08, 0.1, 0.1 and 0.5 ng/mL for CLE, SAL, TER and FOR were obtained. The separation of the four analytes was performed at 22 kV in ammonium acetate/ammonia (20 mmol/L, pH 9.0), using 7.5 mmol/L acetic acid in isopropanol/water 50/50% (v/v) as sheath liquid. In addition, an excellent linear response was obtained with RSD less than 1.3% for migration times and less than 6.7% for peak areas (n=5). The recoveries of spiked urine samples were in the range of 82.7–101% with RSD lower than 9.8%. The proposed method has been applied to analyze human urine samples successfully.