Distortionless Enhancement by Polarization Transfer (DEPT) is a carbon based NMR experiment that can aid in distinguishing between primary, secondary, tertiary, and quaternary carbons.
The combination of DEPT and C-13 can define how many protons are attached to a particular carbon. Varying the flip angle provides different information about the number of attached protons. DEPT-45 experiment shows -CH, -CH2, and -CH3 groups all as positive signals. DEPT-90 experiment only shows -CH groups as positive signals. DEPT-135 shows -CH and -CH3 as positive signals while -CH2 are shown as negative signals. Quaternary carbons do not show in these experiments since they are not attached hydrogen. Above is an example using the molecule N-(4-ethylphenyl)acetamide and a recreation of how the DEPT spectra should appear for comparison of methods.
Attached Proton Test (APT) is similar but less sensitive method in comparison to DEPT in determining the number of attached protons. In this experiment, positive signal indicate that the carbon is either attached 1 or 3 protons. Negative signal indicates the carbon is attached to either 0 or 2 protons. This test is unique in comparison to the DEPT experiments since it shows the quaternary carbon. Above is the APT spectrum of N-(4-ethylphenyl)acetamide in comparison to the DEPT spectra.