Simultaneous Determination of Nitrite,Nitrate,Chloride and Phosphate in Milk Powder by Ion Chromatography

The milk powder sample,after dissolution in water was dialyzed dynamically for 1 h.The dialyzate obtained was extracted with ethylether to remove fatty subsances and separated on C18 column to separate partially those non-polar organic compounds.Eluate from C18 column was filtered with filter membrane of 0.45 μm,and the filtrate was ready for IC determination.Ionpac AS9-HC was used as the chromatographic column and a mixed solution of 4.5 mmol·L-1 Na2CO3 and 4.5 mmol·L-1 NaHCO3 was used as the mobile phase.Determination of the 4 anions was carried out by using conductivity detection.Linear relationships between peak area and concentration of the anions were obtained with correlation coefficients of 0.998 2 (for Cl-),0.999 9 (for PO3-4),0.999 7 (for NO-3) and 0.986 7 (for NO-2),and detection limits (S/N=3,mg·kg-1) of 9.36 (Cl-),15.0 (PO3-4),0.1 (NO-3) and 0.02 (NO-2).Values of RSD′s (n=7) found for Cl-,PO3-4,NO-3 and NO-2 were 1.3%,2.6%,2.2% and 7.3% respectively.Tests for recovery were made on 3 kinds of milk powder,giving values over 92%.