Enhanced Elemental Mass Spectrometry of Fluorine and Chlorine via Chemical Ionization in the Afterglow of an Inductively Coupled Plasma
Lesniewski, Joseph Edward
There is an increasing need for trace-level analysis of fluorinated and chlorinated compounds in complex samples due to their prevalence among pharmaceuticals and environmental and food contaminants. Importantly, elemental analysis offers major quantitation advantages in this area by alleviating the need for compound-specific standards. However, the current state-of-the-art elemental analysis method, inductively coupled plasma-mass spectrometry (ICP-MS), faces fundamental limitations in sensitivity for F and Cl detection because the high ionization potentials of these elements reduce formation efficiencies of Cl+ and F+. In this dissertation, new ionization chemistries are reported to overcome the fundamental limitations of ICP-MS. In the new methods, solutions of analytes are introduced into an ICP as aerosols, producing common gas-phase Cl and F small molecules from analytes. These species are then ionized via ion-neutral reactions in the atmospheric-pressure plasma afterglow where plasma gasses cool significantly. Detection of chlorine as Cl- is demonstrated in one method with sensitivities twice as large as that of ICP-MS, highlighting the fundamental advantage of this approach. Notably, introduction of sodium and methanol into the plasma along with organochlorines significantly enhances formation of Cl-. Fundamental investigations of these effects reveal an ionization mechanism where sodium introduction into the plasma leads to formation of a gas-phase NaCl intermediate from organochlorine compounds. NaCl then reacts with a reagent ion derived from methanol oxidation and sodium, [Na(HCO2)2]-, to produce [NaClHCO2]- in the ICP afterglow. This complex releases Cl- inside the mass spectrometer upon ion activation. NaF is hypothesized to form from fluorinated compounds in a similar fashion. However, ionization of this intermediate by [Na(HCO2)2]- is not thermodynamically favorable to produce F-. We therefore explore new reaction chemistries for ionization of NaF and demonstrate a mechanism for formation of Na2F+ via highly-favorable Na+ adduction in the plasma afterglow. Notably, this approach offers two orders of magnitude improvement in ion formation and detection efficiency compared to current ICP-MS/MS methods for F measurement, further underscoring the fundamental advantages of chemical ionization in the plasma afterglow for analysis of high-ionization-potential elements. Finally, matrix tolerance for F detection in this approach is demonstrated through quantitation of fluoride in infant formula.
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