An empirical formula representing the variation of the . quantity with mole fraction of acetonitrile (/) from the values in Table 4-4 could be determined using equation (4-20). The dependence of . versus the mole fraction of ace- tonitrile is shown in Figure 4-25. | pH EFFECT ON HPLC SEPARATIONS 173 TABLE 4-5. Delta Values for Various Methanol Water Compositions 73 74 Volume fraction MeOH 0 Mole fraction 0 8 0 Figure 4-25. Variation in the S quantity with mole fraction of acetonitrile. Figure 4-26. Variation in the S quantity with mole fraction of methanol. An empirical formula representing the variation of the S quantity with mole fraction of acetonitrile x from the values in Table 4-4 could be determined using equation 4-20 . The dependence of S versus the mole fraction of acetonitrile is shown in Figure 4-25. S c MeCN 4-20 An empirical formula representing the variation of the S quantity with mole fraction of methanol x from the values in Table 4-5 could be determined using equation 4-21 . The dependence of delta versus the mole fraction of methanol is shown in Figure 4-26. S c MeOH 4-21 174 REVERSED-PHASE HPLC Similarly the delta values as a function of any volume composition up to 60 v v acetonitrile . is equivalent to volume fraction and methanol can be determined using equations 4-22a and 4-22b . 74 .446fo MeCN S . 1 - MeCW MeCN . 9fMeOH - .11f MeOH s 1 - MeOH - MeOH 4-22a 4-22b Note however that the difference between wpH and sspH is a constant value for each mobile-phase composition and the difference between WpH and SpH depends not only on the type and concentration of mobile-phase composition but also on the particular solution being measured 74 76 . However these sss s values can serve as estimates for convex ting tium wp to sp or wp v to sp v . The authors claim that the S values could be directly used with other electrode systems or by other laboratories given that the residual liquid junction potential of the respective system is negligible 74-76 . This can be a convenient way to convert from the wpH scale to sspH scale as Espinosa et al. have described 73 . .
