Sputter-depth profiles of model organic thin films on silicon using C 60 primary ions have been employed to measure sputtering yields and depth resolution parameters. We demonstrate that some materials (polylactide, Irganox 1010) have a constant and high sputtering yield, which varies linearly with the primary ion energy, whereas another material (Alq 3 ) has lower, fluence-dependent sputtering yields. Analysis of multi-layered organic thin films reveals that the depth resolution is a function of both primary ion energy and depth, and the sputtering yield depends on the history of sputtering. We also show that ∼30% of repeat units are damaged in the steady-state regime during polylactide sputtering. Crown
A study is presented of the factors affecting the calibration of the mass scale in time-of-flight secondary ion mass spectrometry (TOF-SIMS). At the present time, TOF-SIMS analysts using local calibration procedures achieve a rather poor relative mass accuracy of only 150 ppm for large molecules (647 u) whereas for smaller fragments of Ͻ200 u this figure only improves to 60 ppm. The instrumental stability is 1 ppm and better than 10 ppm is necessary for unique identification of species. The above experimental uncertainty can lead to unnecessary confusion where peaks are wrongly identified or peaks are ambiguously assigned. Here we study, in detail, the instrumental parameters of a popular single stage reflection TOF-SIMS instrument with ion trajectory calculations using SIMION. The effect of the ion kinetic energy, emission angle, and other instrumental operating parameters on the measured peak position are determined. This shows clearly why molecular and atomic ions have different relative peak positions and the need for an aperture to restrict ions at large emission angles. These data provide the basis for a coherent procedure for optimizing the settings for accurate mass calibration and rules by which calibrations for inorganics and organics may be incorporated. This leads to a new generic set of ions for mass calibration that improves the mass accuracy in our interlaboratory study by a factor of 5. A calibration protocol is developed, which gives a relative mass accuracy of better than 10 ppm for masses up to 140 u. The effects of extrapolation beyond the calibration range are discussed and a recommended procedure is given to ensure that accurate mass is achieved within a selectable uncertainty for large molecules. Additionally, we can alternatively operate our instrument in a regime with good energy discrimination (i.e., poor energy compensation) to study the fragmented energies of molecules. This leads to data that support previous concepts developed in G-SIMS. tatic SIMS (SSIMS) is a powerful technique for the analysis of organic and molecular surfaces. Over the last decade, instrumentation has improved significantly so that modern instruments now have very high reliability. Together with reference procedures and methods, the intensity repeatability has improved by over a factor of 10. In a recent VAMAS interlaboratory study [1] conducted by NPL, over 84% of instruments exhibited excellent intensity repeatabilities of better than 1.9%. It was also demonstrated that the comparability of relative ion intensities between different instrument designs can be Ͻ4% by incorporation of the concept of the relative instrument spectral response (RISR) function [1].A significant issue for many analysts is establishing an accurate calibration of the mass scale for time-of-flight (TOF) instruments. In a recent ISO [2] survey of needs for standardization in static SIMS, analysts ranked a procedure for mass calibration as the top priority. Typically, this needs to be conducted for each spectrum since small variations in t...
In desorption electrospray ionisation (DESI) the interaction between the electrospray and the surface is key to two important analytical parameters, the spatial resolution and the sensitivity. We evaluate the effect of the electrospray solvent type, organic solvent fraction with water, analyte solubility and substrate wettability on DESI erosion diameter and material transferral into useful ion signal. To do this five amino acids, glycine, alanine, valine, leucine and phenylalanine are prepared as thin films on three substrates, UV/ozone treated glass, glass and polytetrafluoroethylene (PTFE). Four different solvents, acetonitrile (ACN), methanol (MeOH), ethanol (EtOH) and propan-2-ol (IPA), are used with organic solvent fractions with water varying from 0.1 to 1. These model systems allow the solubility or wettability to be kept constant as other parameters are varied. Additionally, comparison with electrospray ionisation (ESI) allows effects of ionisation efficiency to be determined. It is shown that the DESI efficiency is linearly dependent on the solubility (for these materials at least) and for analytes with solubilities below 1.5 g kg(-1), additional strategies may be required for DESI to be effective. We show that the DESI erosion diameter improves linearly with organic solvent fraction, with an organic solvent fraction of 0.9 instead of 0.5 leading to a 2 fold improvement. Furthermore, this leads to a 35 fold increase in DESI efficiency, defined as the molecular ion yield per unit area. It is shown that these improvements correlate with smaller droplet sizes rather than surface wetting or ionisation.
Ambient mass spectrometry (ambient MS) is a powerful and rapidly growing new field that provides high sensitivity MS directly from surfaces at ambient pressure. There is now a rich evidence base in the published literature of the success of these methods for forensic analysis including: detection of explosives at nanogram levels; chemical composition of counterfeit pharmaceutical tablets, detection of drugs of abuse from biological liquids such as urine and plasma; breath analysis of metabolites; and imaging analysis for document verification and fingerprint identification. Recent developments in miniaturised (shoe-box sized) mass spectrometers have enabled these developments to be translated to portable on-scene detection and first responder usage. In this review, we illustrate and compare the effectiveness of the most popular and promising techniques of desorption electrospray ionisation (DESI), direct analysis in real time (DART), plasma assisted desorption ionisation (PADI) and extractive electrospray ionisation (EESI). Forensic analysis by its very definition must stand up to scrutiny in a court of law. It is therefore essential that measurements are repeatable, valid, traceable and fit for purpose. The establishment of a measurement infrastructure is therefore essential to ensure that the methods used may be accredited and conform to relevant quality systems and procedures.
Desorption electrospray ionization (DESI) is a powerful ambient ionization technique that can provide high-sensitivity mass spectrometry information directly from surfaces at ambient pressure. Although a growing amount of research has been devoted to exploring different applications, there are few studies investigating the basic parameters and underpinning metrology. An understanding of these is crucial to develop DESI as the robust and reliable technique required for significant uptake by industry. In this work, we begin with a systematic study of the parameters affecting the repeatability, sensitivity, and rate of consumption of material with DESI. To do this we have developed a model sample consisting of a thin uniform film of controlled thickness of Rhodamine B on glass. This model sample allowed assessment of optimal sensitivity and spot shape under different conditions. In addition, it allowed us to study the surface in more detail to understand why and how each parameter affects these. Using the model sample to optimize the instrument parameters for DESI led to an absolute intensity repeatability of better than 15%, achieved over a period of 1 day. This model sample provides valuable insight into the electrospray-sample interaction and the desorption mechanism. Confocal microscopy of areas analyzed by DESI allow droplet distribution, material utilization, and spot size to be determined. Studying surface erosion also gives the erosion rate of material, analogous to the sputtering yield in secondary ion mass spectrometry. The results of the study provide a clear description that explains the differences observed with changing electrospray parameters allowing optimization of the technique, for both spatial resolution and sensitivity.
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