a b s t r a c tAn improved sensitive method was developed and validated for the determination of histamine in food samples by using automated on-line pre-column derivatization coupled with high performance liquid chromatography and fluorescence detection (HPLC-FLD). o-Phthaldialdehyde (OPA) was adopted as derivatization reagent, and a "sandwich" (OPA + histamine + OPA) aspiration mode for the automated online pre-column derivatization was found to efficiently enhance the method sensitivity and precision. Histamine in food samples was efficiently extracted with a methanol-phosphate buffer solution (50:50, v/v) at 60• C for 30 min, and purified with Waters Oasis MCX solid-phase extraction (SPE) cartridge. The limit of quantification for this method is 0.2 mg/kg, which is very sensitive for histamine determination. With the "sandwich" injection program, 3.7% of relative standard deviation (RSD) was achieved by five replicative determinations of a sample blank spiked with 0.25 mg/kg histamine standard. Histamine in food samples such as fumitory skipjack and mackerel was analyzed with relative recoveries over 95% at spiking level of 150 mg/kg, as well as canned tuna fish and cheese with relative recoveries up to 98% at spiking levels of 0.50 and 5.0 mg/kg, respectively. The proposed method was validated with a sample from the Food Analysis Performance Assessment Scheme (FAPAS) as a standard certified material; and the results (140 ± 6 mg/kg) agreed well with the assigned value (139 mg/kg).
Nitrite and nitrate are common inorganic salts in the diet and drinking water. It is generally believed that excessive intake of these substances may result in methemoglobinemia or other diseases. However, the traditional detection methods for nitrite and nitrate in dairy products restrain their applications to routine analysis due to the presence of certain limitations. In order to solve this problem, an improved national food safety standard method for the determination of nitrite and nitrate in dairy products has been studied. After water extraction, protein precipitation and centrifugation, the supernatant was cleaned up by a solid phase extraction (SPE) column. The eluent mainly composed of sodium hydroxide with acetonitrile as organic modifier. External water mode was used for suppressor. An AS 19 column was used as the analytical column, and the oven temperature was 30 degrees C while the cell temperature was 35 degrees C. The detection wavelength was 225 nm and the injection volume was 200 microL. The results showed that good linearity existed when the concentrations of nitrite and nitrate were between 0.005 -0.50 and 0.05 - 1.50 mg/L respectively. The detection limits of nitrite and nitrate were 0.2 and 0.04 mg/kg respectively when using a conductivity detector; while the values were only 0.02 and 0.01 mg/kg using an ultraviolet (UV) detector. The recoveries were between 84.0% and 104.1% when analyzing dairy products. It is a simple, fast and highly sensitive way for nitrite and nitrate detections.
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