Bands in the Extreme Ultra-Violet Spectrum of a Helium Discharge
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Of late, mass spectrometric interface with materials is increasingly strengthened with advances in atomically precise clusters of noble metals. Advances in instrumentation along with recent developments in synthetic approaches have expanded the chemistry of clusters, and new insights into matter at the nanoscale are emerging.
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High-resolution MS coupled with soft ionization techniques enable efficient characterization of atomically precise clusters. Apart from that, techniques such as ion mobility, tandem MS, etc.
Growth, nucleation, and reactivity of clusters are also probed by MS. Some of the recent advancements in this field include the development of new hyphenated techniques. Finer structural details may be obtained by coupling MS with spectroscopic tools, such as photoelectron spectroscopy, vacuum ultraviolet spectroscopy, etc. With such advancements in instrumentation, MS can evolve into a universal tool for the characterization of materials.
The present review captures highlights of this area. Mass spectrometry MS , a century-old discipline concerning the investigation of matter using ions, is undergoing revolutionary changes. While the systems being examined went through systematic changes from atomic and molecular species to proteins and macromolecules in the course of evolution of MS, the perception of matter itself underwent a drastic transformation in this period. Materials science is becoming increasingly molecular today, and constituents of molecular matter are acquiring new properties leading to novel applications.
Investigations require newer tools, and MS has evidently met the needs in this area. This has happened due to efficient methods of ionization of large molecular systems 1 , 2. The first use of MS dates back to a century ago when Sir J. He also found the first evidence of isotopes of an element 5.
Bands in the Extreme Ultra-Violet Spectrum of a Helium Discharge.
Later in , Aston received a Nobel Prize in Chemistry for his discovery of isotopes in several nonradioactive elements 5. Initially, MS gained importance in the analysis of organic molecules, and ionization techniques like electric discharge and electron impact EI were used in such studies. Slowly, scientists started using MS for the analysis of sugars, alkaloids, and peptides 6. The development of hyphenated techniques like gas chromatography—mass spectrometry GC—MS 7 , 8 further strengthened the use of MS as an analytical tool.
For the study of large molecules like proteins, there was a need for the development of softer ionization techniques. Such needs led to the development of electrospray ionization ESI 1 , and for this breakthrough development, a Nobel Prize was awarded to John Fenn in Koichi Tanaka was also awarded a Nobel Prize in the same year for his development of soft laser desorption SLD and its applications for ionization of macromolecules The use of time-of-flight TOF mass analyzers came into use in the year In , fourier-transform ion cyclotron resonance FTICR analyzers were introduced 13 , which exhibited higher-resolving power compared with the TOF analyzers Though MS is mainly used for proteomics these days, it also became popular for studying materials.
In this review, we have captured how MS has enriched the field of materials science. Extended solids have been the materials of recent past. They had unique tools for structural characterization, principally revolving around diffraction techniques of various forms The molecular systems on the other extreme had MS as their integral or most essential tool for compositional to a lesser degree structural as well analysis.
As extended solids became molecular in their building blocks, as evident from the recent advances in nanoscale matter, analytical requirements to understand composition reached newer scales. Molecular materials of the past were composed of smaller constituents weighing a few hundred mass units. Many of them were stable under the harsh conditions of electron impact MS, and therefore were investigated extensively using sector-based instruments. Coupled with methods of ion activation, fragmentation gave a wealth of information on molecular structure These studies were complemented by nuclear magnetic resonance NMR spectroscopy, and structural details of molecules with NMR active nuclei could be understood with precision 19 , This was expanded to even solution-state structure determination of macromolecules.
The other extreme of structural insight came from electron diffraction, which was instrumental in understanding structures of simple molecules at the early part of the last century Soon, structural details of small molecules in the gas phase were understood mostly by spectroscopy, particularly for those molecules in the atmosphere as well as in interstellar space. Electron diffraction of another kind became a prominent analytical tool with the advent of electron microscopy EM. In addition to single-crystal diffraction, NMR, electron diffraction, and spectroscopy, there are a number of scattering and analytical tools involving X-rays, electrons, positrons, neutrons, photons, and ions to unravel the structure and properties of materials.
While this review does not intend to cover comprehensively any of these techniques, we wish to note that the evolution of materials demands new kinds of techniques for compositional and structural analysis. This has happened because constituent units in advanced materials became more and more discrete while increasing in complexity.
Along with this change, constituents of matter expanded from diatomics to polyatomics to macromolecules to nanoparticles. While the need to study all of them in isolation became intense, experimental methods of MS in ionization and ion analysis also got evolved. The importance of MS as a tool for characterization of materials 22 , 23 , 24 has got strengthened with the advancement in the science of atomically precise nanoclusters NCs 25 , 26 , which are materials with atomic precision. Such clusters exhibit unique electronic and optical properties, and have precision in its compositional structure in the metal core and the protecting ligand layers.
However, due to their extremely small size, size determination by other techniques like transmission electronic microscopy TEM or powder XRD has been less reliable. In contrast, MS can accurately identify precision in their compositions. Moreover, advancement in instrumentation has enabled the determination of inherent properties of clusters like electron affinity EA , ionization energy IE , electronic transitions, etc.
In this review, we will discuss the recent advances in those emerging directions, and elucidate how MS is evolving into a promising tool for materials characterization. This has become possible due to advancements in various areas of ionization, mass analysis, detection, sensitivity, resolution, etc.
The landmark developments in these aspects are captured in Table 1. One of the most popular classes of materials of modern science, fullerenes, was discovered using a mass spectrometer When a pulsed laser evaporated a solid disk of graphite, cooling of the resulting carbon species by a high-density helium flow resulted in the formation of carbon clusters, which were then detected by a TOF MS Fig. The clusters were produced into a flow of He gas. In iii, the He density was less and in ii, it was increased to torr which resulted in an enhancement in the peaks of C 60 and C The spectrum i was obtained by maximizing the cluster—cluster reactions and thermalization Peaks x , y correspond to Mo x S y.
Gas-phase clusters of noble metals like Au n or Ag n were also investigated by MS Furche et al. Lechtken et al. Later, in , Li et al.
agunmaiprep.gq Along with such studies on gas-phase bare metal clusters, MS slowly evolved into a powerful tool for the characterization of ligand-protected noble metal NCs also. After Brust reported a new method of synthesis of thiol-protected gold NPs in 46 , scientists started the synthesis of monodisperse NPs with molecule-like optical absorption features.
In , Whetten et al. The LDI measurements suggested that the gold cores consisted of — atoms, which was also consistent with their TEM measurements.
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Though LDI was used as the primary characterization tool in the s, softer techniques like matrix-assisted laser desorption ionization MALDI and ESI became more popular for the ionization of intact ligand-protected clusters. In , Schaff et al. Following this, several other reports came on the same cluster until in , it was reassigned as Au 25 SG 18 by Negishi et al. With advancement in instrumentation, resolution of the mass spectral measurements improved, which enabled successful characterization of the cluster.
MS has largely been used in the characterization of gold clusters, particularly due to their high stability under ambient conditions. Though ESI efficiently ionized water-soluble gold clusters, organic-soluble clusters often showed poor ionization efficiency in this technique. In order to overcome this, scientists implemented new approaches like ligand exchange with ionizable ligands 50 , using Ce SO 4 2 51 , or CsOAc to enhance the ionization 52 , In most cases, with the choice of appropriate matrices like sinapinic acid, cinnamic acid, etc.
In , Dass et al. However, due to lesser stability, mass spectral characterization of silver clusters has always been challenging.
As Ag has two isotopes and , silver clusters show broader isotope patterns. In , Harkness et al. Recently, in , Bakr et al. High-resolution ESI MS gave the accurate molecular formulae of these clusters, which matched exactly with their composition found in crystal structures. This highlights the importance of using MS as a versatile tool for the characterization of the NCs. The crystal structures of many such clusters are not yet available. Experimental and calculated isotope patterns of Au 25 PET 18 are shown in the inset Apart from research in the field of Au and Ag NCs, the applicability of MS has also been extended to the field of clusters of other noble metals, eg.
MS has also been proved to be useful in case of non-noble metals; eg. Over the years, there has been a tremendous improvement in instrumentation, which has enabled to obtain HR mass spectra of clusters with minimum fragmentation. This also increases the sensitivity of the measurement. Precise composition of the core and the ligands and charge states of the cluster can be determined accurately by using HRMS.
The compositions are further confirmed from the isotope patterns of the metals Au, Ag, Pt, Pd, etc. Comparison of the experimental and calculated isotope patterns is presented in the insets Apart from the conventional ESI MS analysis, HR mass spectrometers of the present day are also equipped with several other advanced features that enable further studies on the gas-phase cluster ions.
Some such recent studies are summarized below. MS coupled with IM has proved to be an important tool for structural characterization, and has enhanced research in many areas of biochemical and biophysical studies In the IM cell, the ions are passed through buffer gases like He, N 2 , etc. As a result, species having the same mass but different size and shape exhibit different collision cross-section CCS , hence show different drift times and get separated. IM—MS is capable of studying the conformational dynamics present in a system, and has largely been used to understand the folding and unfolding mechanism in proteins To understand the mechanism, the extent of curvature in the structure of the products at each step was modeled by comparing the CCS of the computed structures with the experimental CCS observed in IM—MS Fig.
IM—MS has also been largely used for understanding the structures of polyoxometallates. Surman et al. Baksi et al. A schematic of the instrumental setup showing the formation of aggregates of the cluster under increased pressure conditions in the trap, and its subsequent separation in the IM chamber is presented in Fig.
Chakraborty et al. The influence of the ligand shell in isomerism was studied by using [Ag 44 SR 30 ] clusters protected by different ligands, which showed different number of isomers. Also, Au 25 SR 18 and Ag 25 SR 18 clusters showed a single peak in IM, suggesting that isomerism is highly selective to the structure and the symmetry of the cluster. Isotope patterns of the monomer, dimer, and trimer of Au 25 PET 18 cluster are also shown The peaks show similar isotopic distribution, confirming the isomeric nature Kappes et al.
In a recent study, Daly et al.