Attached proton test (APT)

The attached proton test is a 1D ¹³C NMR experiment that is used as an aid to assignment by separating carbons unattached to protons and CH₂ signals from CH and CH₃ signals. The APT experiment yields methine (CH) and methyl (CH₃) signals positive and quaternary (C) and methylene (CH₂) signals negative. It is slightly less sensitive than DEPT but a single experiment shows all carbon signals at once unlike DEPT that supresses quaternary carbons and requires up to three different acquisitions to yield full results. Fig. 1 shows the APT spectrum of ethylbenzene.

Fig. 1. APT spectrum of ethylbenzene showing CH and CH₃ positive while CH₂ and quaternary C are negative

The APT pulse sequence is shown in fig. 2. The delay between the first two pulses is set to one over the carbon proton coupling constant. For CH₂ (which is the deciding factor for this experiment) this is usually about 125 Hz so the delay is 8 ms. Between the last two pulses and between the last pulse and the acquisition 100μs is usually sufficient.

Fig. 2. Pulse sequence for APT