A new method for the determination of iron oxidation states by resonant and non‐resonant ionization mass spectrometry

Abstract

Different compositions of micrometric particles of powdered iron oxides (Fe,-,O, Fe,O, and Fe,O,) are examined by time-of-flight laser microprobe mass spectrometry. The positive-ion cluster distributions show a strong correlation with the stoichiometry of iron in the oxide. This correlation is more striking when the resonant process of iron at 293.7 nm is used. The bond dissociation energy of Fe---0 in the bulk Fel-,O also affects the intensity distributions. It is very important to study and to identify the iron oxides because they could play a role in the toxicity of certain molecules which could be removed on the oxides.There is tremendous interest in gas-phase studies of well-characterized transition-metal systems'-4 to enhance our understanding of the chemistry and sometimes of the toxic, carcinogenic effects of transition metal oxide clusters and compounds.s The analysis of inorganic solids by time-of-flight laser microprobe mass spectrometry (TOF-LMMS) shows the formation of clusters in the mass spectra which can be used for the identification of inorganic particles.'-' A systematic study of the types of cluster induced and of their relative abundances is needed to indicate to what extent the mass spectra can be used as 'fingerprints' of the analysed compound. io-is Recently, resonance ionization spectroscopy coupled with mass spectrometry detection (RIMS) has been revealed in several laboratories as an interesting analytical technique.The outstanding features of resonance ionization spectroscopy (RIS) are sensitivity and selectivity with respect to the element or isotope under study, which make it a valuable tool for many applications in trace detection and in analysis of toxic and radioactive materials.''-23 The idea was then developed to use lower wavelengths (UV range) to ionize atoms in the gas phase by a two-photon ionization process (A(oi, ~o ' e -) A + )~~ for which two photons of the same laser pulse are successively absorbed by the atoms in the laser plume (screen effect) exciting them into their own ionization continuum. Increased sensitivity is consequently obtained with this kind of process. More recently, Nogar et al. showed the potential use of RIMS as a tool for the detection of copper in isolated plant cells. 2sIn this paper, we have applied resonant ionization of iron to study the non-stoichiometric Fe,-,O (to 1200 "C : 0.852 < (1 -x) < 0.953), Fe,04 and Fe203 compounds in positive mode by TOF-LMMS. The positiveion mass spectra are characterized by many differences between the clusters detected and their intensities. Critical parameters of the ionization are the laser wavelength, the energy of the pulse and the resonant character of the ionization. The determination of iron oxidation states has been performed by this technique which needs only the use of one UV-tunable laser for both desorption and resonant photoionization. The present investigation has been performed instrumen-* Author for correspondence.CCC 095 1-4 198/95/030225-05

High irradiance laser ionization mass spectrometry for direct speciation of iron oxides

“…The other reason for the increase in the variety of cluster ions could be the effect of intense aggregation reactions in the plasma under high laser irradiation. As illustrated by Maunit et al [24,25], laser irradiation of iron oxides could generate singlet oxygen ( 1 O 2 ) in the plasma. 1 O 2 plays an important role in the aggregation formation of oxide clusters.…”

“…Iron oxide speciation analysis was performed previously using LMMS with a laser desorption ionization mechanism [24,25]. The conclusion was that the neutral species FeO plays an important role in the formation of various cluster ions.…”

“…Allen et al [23] described speciation of arsenic oxides using high-mass cluster ions that are unique to the oxidation state of each oxide sample in the negative ion mass spectrum. Maunit et al distinguished various iron oxides by analyzing the distribution of clusters and explained cluster formation mechanism using LMMS [24,25]. De Ville d'Avray et al investigated ferrite nanoparticles by laser desorption ionization mass spectrometry and found that more high m/z peaks were produced in Fe 3 O 4 particles compared with Fe 2 O 3 [26] However, the above studies were conducted with laser irradiation less than 10 8 W/cm 2 , which indicates that the laser-target interaction mechanism probably involves a desorption-ionization process accompanied by the emission of isolated neutrals, ions, and electrons.…”

Potential of laser mass spectrometry for the analysis of environmental dust particles—A review

“…Generally, the relative distribution of cluster ions produced by UV laser ablation is used. Thus, it was possible to distinguish different titanium [86,87], molybdenum [88], iron [89,90], lead [91,92], arsenic [93], chromium [92,[94][95][96][97][98] and nickel [92,99] compounds. To achieve transition metal oxidation state identification, the main part of the reported methodologies considered the relative distribution of cluster ions with a fixed number of metal atoms but different amounts of oxygen.…”

Laser ablation mass spectrometry of inorganic transition metal compounds. Additional knowledge for the understanding of ion formation

“…Finally, dissociative processes may also take place as a result of excess of internal energy from the desorption event, or collision with neutrals [19]. These processes have been proposed to explain the formation of highly oxygen deficient [20] or highly oxidized [21] species after laser ablation.…”