Methods and systems for identifying a chemical species within a substance using nuclear quadrupole resonance (NQR) are described herein. One method includes applying a number of NQR perturbation-detection pulse sequences to the substance. Each perturbation-detection pulse sequence includes a perturbation segment applied at a perturbation frequency and a detection segment applied at a second different frequency. As the sequences are applied, the perturbation frequency, the second frequency, or both are varied for each pulse sequence. The method also includes applying a number of NQR reference pulse sequences to the substance at a reference frequency. The reference frequency is varied for each pulse sequence. A chemical species is identified within the substance by comparing (i) a set of NQR signals generated by the perturbation-detection pulse sequences with (ii) a reference set of NQR signals generated by the reference pulse sequences.