An inexpensive modification to a gas chromatography injector liner is reported that facilitates continuous admission of analyte into a gas chromatograph/mass spectrometer (GC/MS) for methods development. The MS methods development liner can be made by making simple modifications to commercially available liners and fits into standard injectors in place of the normal liners without any need to break vacuum in the MS. The injector temperature and gas flow rates are adjusted to provide appropriate analyte levels in the MS, which can be admitted under conditions identical with those of real analyses, including co-admission of column bleed. The device is particularly useful for development of tandem MS methods in GC/MS/MS instruments, which are configured with the GC as the sole sample inlet.