Flame atomic absorption spectrophotometry (AAS) was applied to determine the total calcium content in soymilk using
a Model TAS990 AAS machine (Hitachi, Tokyo, Japan) with a hollow cathode lamp operating at 240.7 nm as the radiation
source. The lamp current was set at 10 mA. The flame composition was acetylene (gas pressure 2.94 104 Pa) and air (gas pressure 1.28 105 Pa). For an improved measurement accuracy, soymilk samples were first digested with mixed acids (perchloric acid:concentrated nitric acid = 1:4) overnight to convert all forms of calcium into ions. The digested samples were then subjected to AAS as described above. A standard curve was prepared using a series of concentrations (0–25 mg/mL) of CaCl2 dissolved in ultrapure deionized water. The measurement yielded a linear regression equation of
y = 0.031x 0.111 with a correlation coefficient (r) of 0.999 where y is the absorbance and x is the calcium concentration.