Multiple point internal standard calibration curves between the concentration ratios of fatty acids ( i.e. , C16 :1 , C18 :3 , C18 :2 , C18 :1 , C20 :4 , and C22 :6 ) to internal standards ( C17 :1 for C16 :1 , C18 :3 , C18 :2 , and C18 :1 , C21 :0 for C20 :4 and C22 :6 ) and their corresponding intensity ratios were established with a correlation coefficient of greater than 0.986 .