蔬菜制品中纳检测方案

595 North Harrison RoadBellefonte， PA 16823-0048 USATelephone 800-247-6628·814-359-3441S)SUPELCOFax800-447-3044·814-359-3044email： supelco@sial.com sigma-aldrich.com/supelco Application Note 14A Rapidly Analyze Sodium in Foods， Using SolidPhase Extraction and Cation-Exchange HPLC Sodium in canned corn and tomato soup was monitoredusing cation-exchange high performance liquid chro-matography. Samples were cleaned using SupelcleanLC-18 solid phase extraction tubes. The tube effectivelyretained interfering food sample components， whileallowing sodium ions to elute. Sodium was monitored inthe eluate using a SUPELCOSIL LC-SCX column andconductivity detection. Response was linear over a so-dium concentration rangeof0.1-2mg/mL，and there wasgood correlation between the food label claims and themeasured sodium content. Key Words 。 sodium 。 solid phase extraction。 foods Cation-exchange high performance liquid chromatography is aneffective analytical technique for monitoring sodium ions (1). How-ever， complex sample matrices - such as food products - must becleaned before HPLC analysis or they will clog the column. (Simplefiltration removes particulate matter， but not other matrix compo-nents that could interfere with sodium analysis.) SupelcleanTMLC-18reversed phase solid phase extraction tubes retain the bulk of thepotentially interfering food sample components， while allowing so-diumions to elute. The extraction method is simple， rapid， and reliable. Cans of regular and low sodium whole-kernel corn were drained andthe canning liquids saved. Three-milliliter Supelclean LC-18 solidphase extraction tubes were conditioned with 3.0mL of methanol，and dried under nitrogen. (Up to 12 or 24 tubes can be conditionedsimultaneously， using a VisiprepTM Solid Phase Extraction VacuumManifold equipped with a VisidryTM Drying Attachment.) Particulateand dissolved organic matter was removed from the corn liquid bypassing the sample (about 5mL) through the tube. The collectedeluate was analyzed without further treatment. Similarly， condensed regular and low sodium tomato soups werediluted according to the serving recommendations (1：1)， usingmobile phase (0.05M citric acid). One-milliliter Supelclean LC-18 SPEtubes were conditioned with 1.0mL of methanol and dried undernitrogen. Even after dilution， the highly viscous and particulatesoups clogged the SPE tubes. This problem was alleviated byinserting a plug of silanized glass wool into the SPE tube (afterconditioning)， to trap sample particulate matter. Soup samples(about 3mL) were then passed through the tubes successfully， andthe eluate was analyzed. Cation exchange analyses were carried out on a 25cm x 4.6mmSUPELCOSILTM LC-SCX column， under isocratic conditions， using a 0.05M citric acid mobile phase (pH 2.4). A column temperature of35℃ ensured a stable baseline， and triplicate injections of 1pL(regular product) or 10pL (low sodium product) were made. Sodiumwas detected by conductivity. Results for triplicate injections of the SPE purified food samples aresummarized in Table 1. Response was linear for the range of sodiumstandards used (0.1-2mg/mL)， and there was good correlationbetween the food label claims and the measured sodium content.Chromatograms of HPLC/conductivity analyses for sodium in thecorn liquids and tomato soup samples are shown in Figure A. Figure A. Sodium in Corn Canning Liquids Table 1 Sodium Accurately Monitored inCorn Canning Liquids and Tomato Soup Label Injection Total Na Na Measured Food Product Claim(mgNa/mL) X Volume(pL) e ： = Expected(ug) (pg)(mean， N=3) Corn Liquids Regular 3.12 1 3.12 3.09 Low Sodium 0.10 10 1.00 1.00 Tomato Soup Regular 4.58 1 4.58 4.67 Low Sodium 0.13 10 1.30 1.28 A conductivity detector - used in combination with an ion ex-change HPLC system - will be sensitive to column equilibration andto changes in mobile phase makeup and column temperature.About 50-100mL of mobile phase was needed to achieve columnequilibration. We found it almost impossible to obtain a stablebaseline at ambient temperature， but baselines stabilized at tem-peratures above 30°C. The SPE method used to purify these food samples effectively retainsparticulate and organic matter， while passing sodium， unretained， inthe breakthrough volume of the packing bed. Application of themethod may be complicated by sample viscosity. While corn liquidspassed freely through the SPE tubes， tomato soup required dilutionto prevent back pressure from becoming undesirably high. Sampledilution can， in some cases， affect sodium detection. For many foodsamples， however， this method should be very useful for separatingparticulate material and organic components from sodium， whenmonitoring this ion. Reference 1. Smith， R.L. and D.J. Pietrzyk， Anal. Chem.， 56，610 (1984).Reference not available from Supelco. TrademarksSupelclean， SUPELCOSIL， Visidry， Visiprep 一 Sigma-Aldrich Co. 12-Port Visiprep DL Manifold Fitted withVisidry Drying Attachment Ordering Information： Description Cat.No. Supelclean LC-18 Solid Phase Extraction Tubes 1mL， pk. of 100 57002-U 3mL， pk. of 50 57012 12-Port Visiprep Solid Phase Extraction Vacuum Manifold 57030-U Visidry Vacuum ManifoldDrying Attachment 57100-U Silanized Glass Wool 50g 20411250g 20410 SUPELCOSILLC-SCX Column25cm x 4.6mm， 5pm packing 58997 sigma-aldrich.com/supelco Order/Customer Service 800-247-6628， 800-325-3010 (US only)· Fax 800-325-5052 (US only)· E-mail supelco@sial.comS Technical Service 800-359-3041 (US only)， 814-359-3041· Fax 800-359-3044 (US only)， 814-359-5468·E-mail techservice@sial.com SUPELCO· 595 North Harrison Road， Bellefonte，PA 16823-0048·814-359-3441 The SIGMA-ALDRICH Family 2 SIGMA A ALDRICH Fluka Riede-deHaen SSUPELCO°COTEC T394014A AOP sigma-aldrich.com/supelco We are committed to the success of our Customers， Employees and Shareholders through leadership in Life Science， High Technology and Service.OSigma-Aldrich Co. 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