the base peak in the mass spectrum (Figure 8b). Figure 7: Positive full-scan ESI LC/MS analysis of a nine component test mixture without Ion Sweep gas. The target analyte, paclitaxel [Taxol (m/z 854)] elutes at 30.1 min. The base peak chromatogram (a) shows a high background that increases with increasing organics in the content of the mobile phase. This high background is due to solvent adducts. The base peak in the mass spectrum (b) is the ammonium adduct (m/z 871) instead of the molecular ion (m/z 854).

Figure 8: Positive full-scan ESI LC/MS analysis a nine component test mixture using Ion Sweep gas. Taxol (m/z 854) elutes at 30.1 min. Use of Ion Sweep gas reduces the background level in the base peak chromatogram (a), as seen by comparing Figure 8a with Figure 7a. Also, the gas reduces ammonium adducts resulting in the molecular ion (m/z 854) as the major ion present in the mass spectrum (b). Figure 9 is the ESI negative ion full MS scan base peak chromatogram of imazethapyr ([M-H]-=m/z 288) without use of Ion Sweep gas.

The negatively charged solvent-ammonium adduct at m/z 136 [2(M-H) -17) where M is the ammonium acetate ion] dominates the ion current, elevating the base peak ion trace and completely overwhelming any analyte signal. The high background from the solvent-ammonium adduct analyte would make identification in cases of unknown or Data-Dependent analysis difficult, at best. The mobile phase modifier, ammonium acetate, is always present in solution phase and, therefore, clusters are always a potential problem; Ion Sweep technology can be used to de-clus
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