Determination of Sodium Formaldehyde Sulfoxylate in Food by Capillary Zone Electrophoresis

Sodium formaldehyde sulfoxylate is an illegal food additive and is strictly prohibited in China. However, it is sometimes adulterated in food such as silk noodles, fine dried noodles, crystal sugar and bean curd stick in order to bleach, and improve the texture and antisepsis. To ensure the food safety, a new method for the accurate assay of sodium formaldehyde sulfoxylate in food by capillary zone electrophoresis (CZE) has been developed. An uncoated Φ 75μm fused-silica capillary of 70 cm total length (effective length: 60 cm) under a separation voltage of -12 kV was used. The separation buffer consisted of 50 mmol/L of Na₃PO₄, 10 mmol/L of NaOH, 0.5 mmol/L of cetyltrimethyl ammonium bromide (CTAB) and 1 g/L of polyethylene glycol (PEG) 35 000. The injection pressure and time were 3.45 ×10^-3 MPa and 13 s, respectively. The factors such as the separation buffer concentration and pH, concentration of PEG 35 000 and sample buffer, which affecting the separation and quantitation of sodium formaldehyde sulfoxylate were investigated in detail. The samples were ground into powder, extracted by 5 mmol/L of acetic acid and then centrifuged. The supernatant was directly injected after filtered through 0.45 μm cellulose acetate membrane. The corrected peak areas (Ac) versus concentrations (ρ, mg/L) of sodium formaldehyde sulfoxylate showed a good linear relationship within the range from 15~400 mg/kg with a correlation coefficient (r) of 0.999 1. The limit of detection (LOD) was 4 mg/kg (S/N=3) and the limit of quantitation (LOQ) was 12 mg/kg (S/N=10), respectively. The average recoveries at three spiked levels were in the ranges of 90.0%~95.6% with RSDs all less than 8%. Four food samples (a total of 12) collected from local supermarket and farmer's market were analyzed by the current method. No positive sample was found.