We detect acetylene and water-vapor lines by using a difference-frequency generator in the spectral region around 3 mm. Both C2H2 and H2O lines belong to fundamental vibrational bands and exhibit a line strength of the order of 10-20 cm/mol. Acetylene molecules were detected either by pure absorption or by first-derivative wavelength-modulation spectroscopy. The minimum detection sensitivity achieved for C2H2 in nitrogen was 4 ppb (parts in 109). Moreover, we discuss the effects of C2H2 pressure reduction in the presence of nitrogen in order to estimate systematic errors in the concentration measurements. Finally, we tested the accuracy of our spectrometer by detecting water vapor present as an impurity in a nitrogen cylinder at a nominal concentration of approximately 5 ppm.