Figure 2. Topographical view of subtractive chromatogram displaying alveolar gradients in a typical normal human subject.

Background air VOCs have been subtracted from the breath VOCs. In this view, rotated in comparison to Figure 1, the x-axis (lower right) displays retention time (sec) on the non-polar primary column, and the z-axis (lower left) displays retention time (sec) on the secondary polar column. Generally, positive peaks represent endogenous VOCs synthesized in the body and negative peaks represent ambient room air VOCs that have been cleared from the body by catabolism and/or renal excretion.