Milk is the main basic foods for infants, because they contain essential nutrients (proteins, lipids, carbohydrates) and micronutrients (minerals, vitamins, enzymes) 1. The ratio of calcium to phosphorus (Ca/P ratio) in milk and dairy products is an important factor for the absorption of calcium. The absorption of calcium deteriorate when the Ca/P ratio is too low in the diet 2. According to the official standards (CODEX STAN 72-1981) of FAO/WHO, the ratio of Ca/P in infant formula should be in the range 1-2 3. Inductively coupled plasma mass spectrometry (ICP-MS), one of the most sensitive techniques, has been used extensively for determining trace elements in a wide range of foodstuffs 4-8. Unfortunately, the determination of Ca or P by ICP-MS suffers from high background problems: 40 Ca + (96.97 %) is interfered by 40 Ar + , 44 Ca + (2.06 %) is interfered by 12 C 16 O2 + and 14 N2 16 O + , 42 Ca + (0.64 %) is interfered by 40 ArH2 + and 31 P + (100 %) is interfered by 14 N 16 OH + , 15 N 16 O + and 14 N 17 O + 9,10. Some authors used the cool or cold plasma technique to alleviate the Arbased interferences in ICP-MS analysis 11,12. However, it cannot eliminate the interfering N-oxide or hydroxides ions on P determination 13. Recently, it has been proved that the dynamic reaction cell (DRC) technique coupling with ICP-MS can alleviate effectively mass spectral interferences 14-17. Chen et al.

Signal enhancement of elements with different ionization potential by organic solvent in inductively coupled plasma mass spectrometry was investigated. In the presence of the 3 % methanol or 4 % glycerol, the enhancement factors were in the range of 1.5 to 2.8 compared with the aqueous solution for high ionization potential elements (e.g., As, Se etc.), while the factors were less than 1.5 for the low ionization potential elements (e.g., Ce, In etc.). Signal enhancement caused by sample nebulization-transport efficiency improved was less than 1.5, which was obtained by detecting the sample aerosol collected with cold trap. The degrees of ionization of As and Se in organic solvent were increased to 69-71 and 56-58 %, respectively. However, In and Ce were only slightly increased with organic solvent. Our results demonstrated that signal enhancement of elements with high ionization potential was mainly attributed to the increased degree of ionization in plasma, while the low ionization potential elements were mainly attributed to the improved sample nebulization-transport efficiency.