  Quantum cascade laser (QCL) frequency combs have revolutionized mid-infrared
(MIR) spectroscopy by their high brightness and fast temporal resolution, and
are a promising technology for fully-integrated and cost-effective sensors. As
for other integrated comb sources such as micro-combs and interband cascade
laser, QCLs have a comb spacing of several GHz, which is adequate for
measurements of wide absorbing structures, typically found in liquid or solid
samples. However, high-resolution gas-phase spectra require spectral
interleaving and frequency calibration. We developed a frequency calibration
scheme for fast interleaved measurements with combs featuring multi-GHz
spacing. We then demonstrate dual-comb spectroscopy with 600 kHz accuracy in
single-shot 54-ms measurements over 40 cm$^{-1}$ using two QCLs at 7.8
$\mathrm{\mu}$m. This work is an important contribution towards fast
fingerprinting of complex molecular mixtures in the MIR. Moreover, the
calibration scheme could be used with micro-combs for spectroscopy and ranging,
both in comb-swept and comb-calibrated setups.
