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that can be expected when the method is implemented or transferred [13-15]. For
example, if the method calls for premixing a mobile phase of 60% methanol, then
the high (1) and low (−1) factors might be 58% and 62% methanol, or some simi-
lar range expected to bracket the variability a properly trained analyst using proper
laboratory apparatus can be expected to measure. In the case of instrument settings,
manufacturers' specifications are sometimes used to determine variability. If the
instrument is being used to generate the mobile phase, for example, gradients, or
set the temperature, then the range should bracket those specifications. Ultimately,
the range evaluated should not be selected to be so wide that the robustness test will
purposely fail, but rather to represent the type of variability routinely encountered
in the laboratory.
Table 5.2 lists some factor limits for an isocratic method where the mobile phase
is premixed. Mobile phase composition, flow rate temperature, and wavelength are
all considered. Gradient method factor limits are listed in Table 5.3. Gradient meth-
ods represent a slightly different factor selection challenge; in addition to some of
the factors that must be considered for an isocratic method, gradient timing should
be taken into consideration.
FIgure 5.3a
Example experimental conditions that might be used for a PB design for an
eight-factor HPLC experiment. Figure 5.3a shows the experimental design setup with the fac-
tor names, nominal, upper and lower values, and
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