FILTERED NOISE FIELD SIGNALS FOR MASS-SELECTIVE ACCUMULATION OF EXTERNALLY FORMED IONS IN A QUADRUPOLE ION-TRAP

A new wideband resonance excitation technique, termed a filtered noise field (FNF), is demonstrated for selective accumulation of externally formed ions in the rf quadrupole ion trap. Data obtained for detection of trinitrotoluene (TNT) and black powder vapors in ambient air, via atmospheric sampling glow discharge ionization (ASGDI), indicate that a notch-filtered FNF signal applied to the endcap electrodes can prevent accumulation of matrix ions without adversely affecting the ion injection efficiency of analyte species. Consequently, analyte signal enhancement can be realized via extended ion accumulation times even though unwanted ions are present in overwhelming abundance. The TNT molecular anion signal obtained following a 1-s injection period is a factor of similar to 8 greater than that seen without the FNF; S-2(-) and S-3(-) cluster anion signals, obscured without the FNF due to matrix-ion-induced space charge effects, also are isolated completely via the FNF. The subsequently of concatenated FNF waveforms to enhance detection specificity via MS/MS is also illustrated. The FNF-ion trap techniques described here should be generally applicable to many other ionization sources, whether of internal or external configuration.