A novel instrument for electron-gas secondary neutral mass spectrometry (SNMS) is described which is designed to enable in situ x-ray photoelectron spectroscopy measurements as an option. By operating the postionizing SNMS plasma inside an UHV chamber, residual gas signals are reduced to the mass-independent background in the order of a few 10(-1) cps. A comparison between SNMS spectra taken with the existing INA-S and the novel INA-X instrument reveal an increased detection power down to some 100 ppb. C and O as examples of analytically difficult elements become detectable down to the 10 ppm regime.