Determination of N-Acetyl-L-cysteine Ethyl Ester (NACET) by Sequential Injection Analysis

New sequential injection analysis (SIA) methods with optical sensing for the determination of N-acetyl-L-cysteine ethyl ester (NACET) have been developed and optimized. NACET is a potential drug and antioxidant with advantageous pharmacokinetics. The methods involve the reduction of Cu(II) in its complexes with neocuproine (NCN), bicinchoninic acid (BCA), and bathocuproine disulfonic acid (BCS) to the corresponding chromophoric Cu(I) complexes by the analyte. The absorbance of the Cu(I) complexes with NCN, BCA, and BCS was measured at their maximum absorbance wavelengths of 458, 562, and 483 nm, respectively. The sensing manifold parameters and experimental conditions were optimized for each of the Cu(II) complexes used. Under optimal conditions, the corresponding linear calibration ranges, limits of detection, and sampling rates were 8.0 × 10^-6-2.0 × 10^-4 mol L^-1, 5.5 × 10^-6 mol L^-1, and 60 h^-1 for NCN; 6.0 × 10^-6-1.0 × 10^-4 mol L^-1, 5.2 × 10^-6 mol L^-1, and 60 h^-1 for BCA; and 4.0 × 10^-6-1.0 × 10^-4 mol L^-1, 2.6 × 10^-6 mol L^-1, and 78 h^-1 for BCS. The Cu(II)-BCS complex was found to be best performing in terms of sensitivity and sampling rate. Usual excipients in pharmaceutical preparations did not interfere with NACET analysis.