A method of and apparatus for analyzing a substance takes a
stream of ions in said substance and supplies the ions to a collision
cell including a
quadrupole rod set for guiding the ions and a
buffer gas. An RF
voltage is applied to the
quadrupole rod set to guide ions. An additional
alternating current signal is applied to the
quadrupole rod set at a frequency selected to cause
resonance excitation of the secular frequency of a desired
ion, whereby said desired ions are excited and undergo collision with the
buffer gas causing fragmentation. The
alternating current signal is then modulated, whereby periods in which said
alternating current signal is applied alternate with periods in which said alternating signal is not applied. The
ion spectrum after fragmentation is collected to generate one set of data for one spectrum, representative of the
ion spectrum when the alternating current signal is applied, and another set of data for another spectrum, representative of the ion spectrum when the alternating current signal is not applied. These two spectra can then be subtracted.