Paper spray ionization for ambient inorganic analysis

Abstract

The feasibility of using paper spray ionization as an ambient ionization method for inorganic analysis was investigated. Aqueous solutions were spotted onto paper triangles and high voltage was applied to induce electrospray. In-source collision-induced dissociation assisted in breaking up clusters to produce mass spectra dominated by atomic ions and their corresponding oxides and hydroxides and high-resolution exact mass measurements were used to identify atomic ions in the presence of organic interferences. A feasibility test with a standard inductively coupled plasma mass spectrometry (ICP-MS) test mixture demonstrated detection of 21 of 23 elements as M(+•) or [M + OH](+). The two undetected elements (boron and lithium) were below the minimum detectable m/z setting for the time-of-flight mass spectrometer used for these experiments. Untreated filter paper showed residual contamination from group I and group II elements. Excellent linearity was observed over the concentration range 0.1-1000 ppm for nickel(II) nitrate solutions containing 10 ppm cobalt(II) nitrate as internal standard. The method was applied to the analysis of lead-free solder, a bismuth subsalicylate tablet, permanent magnet materials and lead chromate pigment in a three-cent US postage stamp issued in 1948. Paper spray ionization was successfully applied to qualitative inorganic analysis. Quantitative analysis was shown to be possible with the use of internal standards. Initial detection limits were much higher than for comparable inductively coupled plasma and glow discharge methods, ranging from the high parts-per-billion range to the low parts-per-million range. The detection limits for some elements were limited at low levels by the presence of interferences that were not separated by the resolving power of the mass spectrometer. Despite these limitations, the simplicity and potential portability of paper spray may find some applications for elemental analysis.