Electron capture atmospheric pressure photoionization mass spectrometry: analysis of fullerenes, perfluorinated compounds, and pentafluorobenzyl derivatives

Abstract

An electron capture (EC) ionization mechanism has been found to be highly efficient in negative-ion atmospheric pressure photoionization (APPI) for the analysis of compounds with positive electron affinity (EA). Using negative-ion APPI, we first report the sensitive detection of natural electrophores with limited polarity, such as fullerenes and perfluorinated compounds, by mass spectrometry (MS). Using direct infusion on a quadrupole time-of-flight (QTOF) mass spectrometer, the limits of detection (LODs) for C(60) and perfluoromethylcyclohexane were determined to be 0.15 pg (0.2 fmol) and 1 femtoliter (fL) ( approximately 1.5 pg or 4.3 fmol), respectively. As the EA of the analyte increases, the detection sensitivity is enhanced. Making use of the accurate mass measurement capability of the QTOF mass spectrometer, we were able to investigate the elemental composition of the ions in each spectrum and attribute the observed high sensitivity to an EC-initiated ionization process. The proposed EC ionization mechanism is further supported by the observation of a dissociative EC reaction of pentafluorobenzyl (PFB)-derivatized phenols. The analysis of phenols by EC-APPI of their PFB derivatives resulted in very high sensitivity, with the lowest reported LOD of approximately 0.17 pg (0.5 fmol) being for 2,4-dinitrophenol. For future LC/EC-APPI-MS applications, the effect of additives and solvents on sensitivity was also tested and reported.

Ionization mechanism of negative ion-direct analysis in real time: A comparative study with negative ion-atmospheric pressure photoionization

“…Figure 1 demonstrates a NI-DART spectrum of ϳ100 pL PFK in toluene. Compared with the previously reported NI-APPI spectrum of ϳ667 fL PFK in toluene [25], NI-DART was ϳ100 times less sensitive than NI-APPI. Furthermore, the NI-DART spectrum of PFK was more like a NICI spectrum of PFK where an even number of fluorines was easily found for the high m/z ions, while the NI-APPI spectrum of PFK was more like an electron ionization (EI) spectrum of PFK where mostly an odd number of fluorines were found for any ions.…”

“…At the same time, they also possess positive EAs. Our previous study [25] demonstrated that NI-APPI was efficient in the ionization of aliphatic PFCs. Dominant [M -F] Ϫ ions were observed, which was attributed to an EC mechanism for the formation of M Ϫ· ions, and thereafter instant in-source decomposition of the M Ϫ· ions for the formation of [M -F] Ϫ ions.…”

“…Our previous study [25] demonstrated that NI-APPI was highly efficient in the ionization of C 60 , C 70 , and C 60 F 48 , and a limit of detection (LOD) of 0.15 pg was achieved for C 60 . Because highly abundant M Ϫ· ions were observed, the ionization was attributed to an EC mechanism.…”

“…So far, LC/PI-APPI-MS has been demonstrated to be effective in the analysis of hydrophobic lipids [17,18] and compounds containing conjugated aromatic rings [8, 19 -24]. On the other hand, our recent studies [25,26] have demonstrated that NI-APPI is highly efficient in the ionization of low-polarity compounds possessing positive electron affinities (EAs), implying that LC/NI-APPI-MS may permit the analysis of nonvolatile, low polarity compounds with the sensitivity and selectivity that gas chromatography negative ion chemical ionization mass spectrometry (GC/NICI-MS) delivers for volatile nonpolar compounds.…”

“…However, unlike NI-DART, these electrons are stabilized through their collision with ultra-high-purity (UHP) nitrogen, which is usually used as nebulizer and/or heater gas in an enclosed APPI source chamber, rather than atmospheric gas in open air. Therefore, it is understandable that electron capture (EC) and dissociative EC ionization mechanisms were found highly efficient when NI-APPI was used to analyze low-polarity compounds possessing positive EAs [25,26]. At the same time, however, experimental evidence also suggested constant existence of O 2 Ϫ· [26 -28], an EC product of oxygen that must come from the atmosphere, in an APPI source.…”