Fig. 1

Comparison of manual versus automated fractionation of SIP density gradients. Fractionation is the process of dividing a SIP density gradient into multiple fractions. A For “manual” fractionation, 22 independent density gradients were fractionated by visually counting and collecting droplets in microcentrifuge tubes using the method described in Blazewicz et al [17]. Buoyant density (g/ml) for each fraction is measured via refractometry and is represented by a single dot. The number of fractions collected from manual density gradients was variable (range 15–27 fractions per gradient; only fractions 2–19 are displayed). B For automated fractionation, 24 independent density gradients were fractionated robotically using an Agilent Technologies fraction collector, which automatically divides the gradients into fractions of a set volume (~236 μl) and dispenses them into a 96-well plate. Automated density gradients consistently produced 22 fractions per gradient (only fractions 2–19 are displayed). Fractions at the beginning and end of the gradient (fractions 1, 20–22) were excluded as these densities are altered by the water used to displace the gradient and are not typically used for molecular analysis