In applying QASoft, some problems from frequency shift may be encountered. The wavenumber positions of lines in the user's spectra may differ from those in the database spectra. If the integration regions and zeroing regions are wide enough, these differences will probably not seriously affect the quantitative results. However, the frequency differences can cause the subtractions to produce unwelcome "spikes", positive and negative, at the sides of the spectral features. To produce clean difference spectra, the user may wish to perform an X-Shift operation on either the sample spectra or the database spectra. If the spectrometer line widths or line shapes differ from those in the reference spectra, additional undesirable effects will arise in the subtractions. In this case the spectrometer and optical system alignment should be checked to make the lines agree as closely as possible. As in the case of line shifts, however, proper choices of integration regions and zeroing regions can prevent line shape differences from seriously affecting the quantitative result. A VERSATILE X-SHIFT PROGRAM CAN BE FOUND IN THE GRAMS ARITHMETIC FUNCTIONS. Also, see SUBTRACTION ARTIFACTS.