Factors and levels used in the fractional factorial experimental design.

a−1 represents the low level; 0 represents the center point; +1 represents the high level.
bPeak selection mass range represents a continuous mass range. The mid-point of the mass range was 10 kDa. The interval of the mass range was represented by an absolute value. Specifically, the peak selection value for level −1, 0 and +1 were 9 to 11 kDa, 5.5 to 14.5 kDa and 2 to 18 kDa, respectively.