Measuring Compositions in Organic Depth Profiling: Results from a VAMAS Interlaboratory Study

Reactive Gas Cluster Ion Beams for Enhanced Drug Analysis by Secondary Ion Mass Spectrometry

In this study, we investigate the formation and composition of Gas Cluster Ion Beams (GCIBs) and their application in Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) analysis. We focus on altering the carrier gas composition, leading to the formation of (Ar/CO2)n or (H2O)n GCIBs. Our results demonstrate that the addition of a reactive species (CO2) to water GCIBs significantly enhances the secondary ion yield of small pharmaceutical compounds in the positive ion mode. In negative ion mode, the addition of CO2 resulted in either a positive enhancement or no effect, depending on the sample. However, an excess of CO2 in the carrier gas leads to the formation of carbon dioxide clusters, resulting in reduced yields compared to that of water cluster beams. Cluster size also plays a crucial role in overall yields. In a simple two-drug system, CO2-doped water clusters prove effective in mitigating matrix effects in positive ion mode compared to pure water cluster, while in negative ion mode, this effect is limited. These clusters are also applied to the analysis of drugs in a biological matrix, leading to more quantitative measurements as shown by a better fitting calibration curve. Overall, the doping of water clusters with small amounts of a reactive gas demonstrates promising benefits for higher sensitivity, higher resolution molecular analysis, and imaging using ToF-SIMS. The effectiveness of these reactive cluster beams varies depending on the experimental parameters and sample type.

Study of the Structure of Hyperbranched Polyglycerol Coatings and Their Antibiofouling and Antithrombotic Applications

While blood-contacting materials are widely deployed in medicine in vascular stents, catheters, and cannulas, devices fail in situ because of thrombosis and restenosis. Furthermore, microbial attachment and biofilm formation is not an uncommon problem for medical devices. Even incremental improvements in hemocompatible materials can provide significant benefits for patients in terms of safety and patency as well as substantial cost savings. Herein, a novel but simple strategy is described for coating a range of medical materials, that can be applied to objects of complex geometry, involving plasma-grafting of an ultrathin hyperbranched polyglycerol coating (HPG). Plasma activation creates highly reactive surface oxygen moieties that readily react with glycidol. Irrespective of the substrate, coatings are uniform and pinhole free, comprising O─C─O repeats, with HPG chains packing in a fashion that holds reversibly binding proteins at the coating surface. In vitro assays with planar test samples show that HPG prevents platelet adhesion and activation, as well as reducing (>3 log) bacterial attachment and preventing biofilm formation. Ex vivo and preclinical studies show that HPG-coated nitinol stents do not elicit thrombosis or restenosis, nor complement or neutrophil activation. Subcutaneous implantation of HPG coated disks under the skin of mice shows no evidence of toxicity nor inflammation.

Quantitative and Qualitative Analyses of Mass Spectra of OEL Materials by Artificial Neural Network and Interface Evaluation: Results from a VAMAS Interlaboratory Study

Quantitative analysis of binary mixtures of tris(2-phenylpyridinato)iridium(III) (Ir(ppy)3) and tris(8-hydroxyquinolinato)aluminum (Alq3) by using an artificial neural network (ANN) system to mass spectra was attempted based on the results of a VAMAS (Versailles Project on Advanced Materials and Standards) interlaboratory study (TW2 A31) to evaluate matrix-effect correction and to investigate interface determination. Monolayers of binary mixtures having different Ir(ppy)3 ratios (0, 0.25, 0.50, 0.75, and 1.00), and the multilayers containing these mixtures and pure samples were measured using time-of-flight secondary ion mass spectrometry (ToF-SIMS) with different primary ion beams, OrbiSIMS (SIMS with both Orbitrap and ToF mass spectrometers), laser desorption ionization (LDI), desorption/ionization induced by neutral clusters (DINeC), and X-ray photoelectron spectroscopy (XPS). The mass spectra were analyzed using a simple ANN with one hidden layer. The Ir(ppy)3 ratios of the unknown samples and the interfaces of the multilayers were predicted using the simple ANN system, even though the mass spectra of binary mixtures exhibited matrix effects. The Ir(ppy)3 ratios at the interfaces indicated by the simple ANN were consistent with the XPS results and the ToF-SIMS depth profiles. The simple ANN system not only provided quantitative information on unknown samples, but also indicated important mass peaks related to each molecule in the samples without a priori information. The important mass peaks indicated by the simple ANN depended on the ionization process. The simple ANN results of the spectra sets obtained by a softer ionization method, such as LDI and DINeC, suggested large ions such as trimers. From the first step of the investigation to build an ANN model for evaluating mixture samples influenced by matrix effects, it was indicated that the simple ANN method is useful for obtaining candidate mass peaks for identification and for assuming mixture conditions that are helpful for further analysis.

Using Focused Ion Beam Time-of-Flight Secondary Ion Mass Spectrometry to Depth Profile Nanoparticles in Polymer Nanocomposites

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a versatile surface-sensitive technique for characterizing both hard and soft matter. Its chemical and molecular specificity, high spatial resolution, and superior sensitivity make it an ideal method for depth profiling polymeric systems, including those comprised of both inorganic and organic constituents (i.e., polymer nanocomposites, PNCs). To best utilize ToF-SIMS for characterizing PNCs, experimental conditions must be optimized to minimize challenges such as the matrix effect and charge accumulation. Toward that end, we have successfully used ToF-SIMS with a Xe+ focused ion beam to depth profile silica nanoparticles grafted with poly(methyl methacrylate) (PMMA-NP) in a poly(styrene-ran-acrylonitrile) matrix film by selecting conditions that address charge compensation and the primary incident beam angles. By tracking the sputtered Si+ species and fitting the resultant concentration profile, the diffusion coefficient of PMMA-NP was determined to be D = 2.4 × 10-14 cm2/s. This value of D lies between that measured using Rutherford backscattering spectrometry (6.4 × 10-14 cm2/s) and the value predicted by the Stokes-Einstein model (2.5 × 10-15 cm2/s). With carefully tuned experimental parameters, ToF-SIMS holds great potential for quantitatively characterizing the nanoparticles at the surfaces and interfaces within PNC materials as well as soft matter in general.

Back to the basics of time-of-flight secondary ion mass spectrometry data analysis of bio-related samples. II. Data processing and display

This is the second half of a two-part Tutorial on the basics of the time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis of bio-related samples. Part I of this Tutorial series covers planning for a ToF-SIMS experiment, preparing and shipping samples, and collecting ToF-SIMS data. This Tutorial aims at helping the ToF-SIMS user to process, display, and interpret ToF-SIMS data. ToF-SIMS provides detailed chemical information about surfaces but comes with a steep learning. The purpose of this Tutorial is to provide the reader with a solid foundation in the ToF-SIMS data analysis.

Back to the basics of time-of-flight secondary ion mass spectrometry of bio-related samples. I. Instrumentation and data collection

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used widely throughout industrial and academic research due to the high information content of the chemically specific data it produces. Modern ToF-SIMS instruments can generate high mass resolution data that can be displayed as spectra and images (2D and 3D). This enables determining the distribution of molecules across and into a surface and provides access to information not obtainable from other methods. With this detailed chemical information comes a steep learning curve in how to properly acquire and interpret the data. This Tutorial is aimed at helping ToF-SIMS users to plan for and collect ToF-SIMS data. The second Tutorial in this series will cover how to process, display, and interpret ToF-SIMS data.

Quantitative analysis of ToF-SIMS data of a two organic compound mixture using an autoencoder and simple artificial neural networks

RATIONALE

Matrix effects cause a nonlinear relationship between ion intensities and concentrations in mass spectrometry, including time-of-flight secondary ion mass spectrometry (ToF-SIMS). Here, two artificial neural network (ANN)-based methods, autoencoder-based and simple ANN methods, were employed for the quantitative and qualitative analyses of a two organic compound mixture via ToF-SIMS.

METHODS

The multilayer model sample contained a mixture of Irganox 1010 and Fmoc-pentafluoro-L-phenylalanine (Fmoc-PFLPA). The sample's positive and negative ion depth profiles were collected through ToF-SIMS. ToF-SIMS-derived cross-sectional image datasets were analyzed using three unsupervised methods, namely principal component analysis (PCA), multivariate curve resolution (MCR), and use of a sparse autoencoder (SAE). The supervised simple ANN method was optimized based on the spectra and validated by predicting the test dataset ratios of Irganox 1010.

RESULTS

The results obtained using the SAE demonstrated linear calibration curves and appropriate material distribution images. The Irganox 1010 and Fmoc-PFLPA positive and negative ion datasets exhibited >0.97 correlation coefficients. The PCA and MCR results demonstrated lower linearity than that of SAE. Moreover, SAE weights indicated the ions important for each organic compound. The simple ANN method accurately predicted the ratios in the test dataset and indicated the important ions.

CONCLUSIONS

Both the supervised and unsupervised methods based on ANN, which were employed in regulating nonlinear relationships, were effective in the quantitative and qualitative analyses of the ToF-SIMS data of the two organic compound mixtures. Regarding qualitative analysis, both ANN-based methods indicated specific ions from the molecules in the sample.

Crystal Structure and Twisted Aggregates of Oxcarbazepine Form III

Polymorphism and crystal habit play vital roles in dictating the properties of crystalline materials. Here, the structure and properties of oxcarbazepine (OXCBZ) form III are reported along with the occurrence of twisted crystalline aggregates of this metastable polymorph. OXCBZ III can be produced by crystallization from the vapor phase and by recrystallization from solution. The crystallization process used to obtain OXCBZ III is found to affect the pitch, with the most prominent effect observed from the sublimation-grown OXCBZ III material where the pitch increases as the length of aggregates increases. Sublimation-grown OXCBZ III follows an unconventional mechanism of formation with condensed droplet formation and coalescence preceding nucleation and growth of aggregates. A crystal structure determination of OXCBZ III from powder X-ray diffraction methods, assisted by crystal structure prediction (CSP), reveals that OXCBZ III, similar to carbamazepine form II, contains void channels in its structure with the channels, aligned along the c crystallographic axis, oriented parallel to the twist axis of the aggregates. The likely role of structural misalignment at the lattice or nanoscale is explored by considering the role of molecular and closely related structural impurities informed by crystal structure prediction.

Three-Dimensional Profiling of OLED by Laser Desorption Ionization-Mass Spectrometry Imaging

Organic light emitting devices (OLEDs), especially in a screen display format, present unique and interesting substrates for laser desorption/ionization-mass spectrometry imaging (LDI-MSI) analysis. These devices contain many compounds that inherently absorb light energy and do not require an additional matrix to induce desorption and ionization. OLED screens have lateral features with dimensions that are tens of microns in magnitude and depth features that are tens to hundreds of nanometers thick. Monitoring the chemical composition of these features is essential, as contamination and degradation can impact device lifetime. This work demonstrates the capability of LDI-MSI to obtain lateral and partial depth resolved information on multicolored OLED displays and suggests the application to other mixed organic electronics with minimal sample preparation. This was realized when analyzing two different manufactured OLEDs, in an active-matrix display format, without the need to remove the cathode. By utilizing low laser energy and high lateral spatial resolution imaging (10 μm), depth profiling can be observed while maintaining laterally resolved information, resulting in a three-dimensional MSI approach that would complement existing OLED characterization methods.

Investigating matrix effects of different combinations of lipids and peptides on TOF-SIMS data

Matrix effects, which cause a change in ion intensity, occur in mass spectrometry methods including time-of-flight secondary ion mass spectrometry (TOF-SIMS). Matrix effects often cause large issues in quantitative analysis because secondary ions related to a particular molecule could be dramatically enhanced or suppressed regardless of the concentration. To investigate matrix effects in biological samples, the authors evaluated mixed lipid {POPC [1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine, molecular weight (MW) 759.6]}, peptide [leu-enkephalin, neo-leu-enkephalin (amino acid sequence: YAGFL, MW 569.3), and neo-angiotensin II (amino acid sequence: DRVYIHAF, MW 1019.5)] samples. Matrix effect features were investigated by analyzing the concentration dependence of secondary ions in lipid-peptide mixed samples to develop a method that enables quantitative analysis using TOF-SIMS. Matrix effects depended on the lipid-peptide combination. Interestingly, some secondary ions possessed an intensity that was highly dependent on concentration.

Quantifying SIMS of Organic Mixtures and Depth Profiles-Characterizing Matrix Effects of Fragment Ions

Sets of matrix factors, Ξ, are reported for the first time for secondary ions in secondary ion mass spectrometry for several binary organic systems. These show the interplay of the effects of ion velocity, fragment chemistry, and the secondary ion point of origin. Matrix factors are reported for negative ions for Irganox 1010 with FMOC or Irganox 1098 and, for both positive and negative ions, with Ir(ppy)2(acac). For Irganox 1010/FMOC, the Ξ values for Irganox 1010 fall with m/z, whereas those for FMOC rise. For m/z < 250, Ξ scales very approximately with (m/z)^0.5, supporting a dependence on the ion velocity at low mass. Low-mass ions generally have low matrix factors but |Ξ| may still exceed 0.5 for m/z < 50. Analysis of ion sequences with addition or loss of a hydrogen atom shows that the Ξ values for Irganox 1010 and FMOC ions change by - 0.026 and 0.24 per hydrogen atom, respectively, arising from the changing charge transfer rate constant. This effect adds to that of velocity and may be associated with the nine times more hydrogen atoms in the Irganox 1010 molecule than in FMOC. For Irganox 1098/Irganox 1010, the molecular similarity leads to small |Ξ|, except for the pseudo molecular ions where the behavior follows Irganox 1010/FMOC. For Ir(ppy)2(acac)/Irganox 1010, the positive secondary ions show twice the matrix effects of negative ions. These data provide the first overall assessment of matrix factors in organic mixtures necessary for improved understanding for quantification and the precise localization of species. Graphical Abstract ᅟ.

Dealing with image shifting in 3D ToF-SIMS depth profiles

The high sputter efficiency and low damage of gas cluster ion beams have enabled depth profiling to greater depths within organic samples using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Due to the typically fixed geometry of the ion sources used in ToF-SIMS, as one digs into a surface, the position sampled by ion beams shifts laterally. This causes a lateral shift in the resulting images that can become quite significant when profiling down more than one micron. Here, three methods to compensate for this image shifting are presented in order to more accurately stack the images to present a 3D representation. These methods include (1) using software to correct the image shifts post-acquisition, (2) correcting the sample height during acquisition, and (3) adjusting the beam position during acquisition. The advantages and disadvantages of these methods are discussed. It was found that all three methods were successful in compensating for image shifting in ToF-SIMS depth profiles resulting in a more accurate display of the 3D data. Features from spherical objects that were ellipsoidal prior to shifting were seen to be spherical after correction. Software shifting is convenient as it can be applied after data acquisition. However, when using software shifting, one must take into account the scan size and the size of the features of interest as image shifts can be significant and can result in cropping of features of interest. For depth profiles deeper than a few microns, hardware methods should be used as they preserve features of interest within the field of view regardless of the profile depth. Software shifting can also be used to correct for small shifts not accounted for by hardware methods. A combination of hardware and software shift correction can enable correction for a wide range of samples and profiling depths. The scripts required for the software shifting demonstrated herein are provided along with tutorials in the supplementary material.

Gas-cluster ion sputtering: Effect on organic layer morphology

Analysis of the surface of thin Irganox 1010 films before and after sputtering with an argon gas-cluster ion beam was performed with AFM and XPS to determine the effect that Zalar rotation has on the chemistry and morphology of the surface. The analysis is based on the change in roughness of the surface by comparing the same location on the surface before and after sputtering. The ion beam used was an Ar n + of size n = 1000 and energy 4 keV. The XPS analysis agreed with previous results in which the ion beam did not cause measurable accumulation of damaged material. Based on the AFM results, the Irganox 1010 surface became rougher as a result of ion sputtering, and the degree of roughening was quantified, as was the sputter rate. Furthermore, Zalar rotation during ion sputtering did not have a significant effect on surface roughening, surprisingly.

Gas Cluster Ion Beams for Secondary Ion Mass Spectrometry

Gas cluster ion beams (GCIBs) provide new opportunities for bioimaging and molecular depth profiling with secondary ion mass spectrometry (SIMS). These beams, consisting of clusters containing thousands of particles, initiate desorption of target molecules with high yield and minimal fragmentation. This review emphasizes the unique opportunities for implementing these sources, especially for bioimaging applications. Theoretical aspects of the cluster ion/solid interaction are developed to maximize conditions for successful mass spectrometry. In addition, the history of how GCIBs have become practical laboratory tools is reviewed. Special emphasis is placed on the versatility of these sources, as size, kinetic energy, and chemical composition can be varied easily to maximize lateral resolution, hopefully to less than 1 micron, and to maximize ionization efficiency. Recent examples of bioimaging applications are also presented.

SIMS of Organic Materials-Interface Location in Argon Gas Cluster Depth Profiles Using Negative Secondary Ions

A procedure has been established to define the interface position in depth profiles accurately when using secondary ion mass spectrometry and the negative secondary ions. The interface position varies strongly with the extent of the matrix effect and so depends on the secondary ion measured. Intensity profiles have been measured at both fluorenylmethyloxycarbonyl-L-pentafluorophenylalanine (FMOC) to Irganox 1010 and Irganox 1010 to FMOC interfaces for many secondary ions. These profiles show separations of the two interfaces that vary over some 10 nm depending on the secondary ion selected. The shapes of these profiles are strongly governed by matrix effects, slightly weakened by a long wavelength roughening. The matrix effects are separately measured using homogeneous, known mixtures of these two materials. Removal of the matrix and roughening effects give consistent compositional profiles for all ions that are described by an integrated exponentially modified Gaussian (EMG) profile. Use of a simple integrated Gaussian may lead to significant errors. The average interface positions in the compositional profiles are determined to standard uncertainties of 0.19 and 0.14 nm, respectively, using the integrated EMG function. Alternatively, and more simply, it is shown that interface positions and profiles may be deduced from data for several secondary ions with measured matrix factors by simply extrapolating the result to Ξ = 0. Care must be taken in quoting interface resolutions since those measured for predominantly Gaussian interfaces with Ξ above or below zero, without correction, appear significantly better than the true resolution. Graphical Abstract ᅟ.

Soft depth-profiling of mixed peptide/lipid samples by means of cluster induced desorption/ionization mass spectrometry-High depth resolution and low matrix effect

Mixed peptide/lipid samples were analyzed with respect to their chemical composition by means of desorption/ionization induced by neutral SO₂ clusters (DINeC) in combination with mass spectrometry (MS). Depth profiles of the mixed films indicated a segregation layer of lipid on top of all samples. The thickness of this layer as obtained by DINeC-MS was in the order of one nanometer what can be seen as an upper limit for the depth resolution of DINeC-MS. The relative amounts of the substance of peptide and lipid derived for the bulk material of mixed samples with different compositions were found to be close to the nominal values indicating a low matrix effect. Throughout the depth profiles, only intact molecular ions [M+H]⁺ as well as dimers of peptides and lipids were detectable, indicating the soft nature of DINeC even when used for depth profiling of biomolecular samples.

Chemical imaging of aggressive basal cell carcinoma using time-of-flight secondary ion mass spectrometry

A set of basal cell carcinoma samples, removed by Mohs micrographic surgery and pathologically identified as having an aggressive subtype, have been analyzed using time-of-flight secondary ion mass spectrometry (SIMS). The SIMS analysis employed a gas cluster ion beam (GCIB) to increase the sensitivity of the technique for the detection of intact lipid species. The GCIB also allowed these intact molecular signals to be maintained while surface contamination and delocalized chemicals were removed from the upper tissue surface. Distinct mass spectral signals were detected from different regions of the tissue (epidermis, dermis, hair follicles, sebaceous glands, scar tissue, and cancerous tissue) allowing mass spectral pathology to be performed. The cancerous regions of the tissue showed a particular increase in sphingomyelin signals that were detected in both positive and negative ion mode along with increased specific phosphatidylserine and phosphatidylinositol signals observed in negative ion mode. Samples containing mixed more and less aggressive tumor regions showed increased phosphatidylcholine lipid content in the less aggressive areas similar to a punch biopsy sample of a nonaggressive nodular lesion.

Reducing the Matrix Effect in Organic Cluster SIMS Using Dynamic Reactive Ionization

Dynamic reactive ionization (DRI) utilizes a reactive molecule, HCl, which is doped into an Ar cluster projectile and activated to produce protons at the bombardment site on the cold sample surface with the presence of water. The methodology has been shown to enhance the ionization of protonated molecular ions and to reduce salt suppression in complex biomatrices. In this study, we further examine the possibility of obtaining improved quantitation with DRI during depth profiling of thin films. Using a trehalose film as a model system, we are able to define optimal DRI conditions for depth profiling. Next, the strategy is applied to a multilayer system consisting of the polymer antioxidants Irganox 1098 and 1010. These binary mixtures have demonstrated large matrix effects, making quantitative SIMS measurement not feasible. Systematic comparisons of depth profiling of this multilayer film between directly using GCIB, and under DRI conditions, show that the latter enhances protonated ions for both components by 4- to ~15-fold, resulting in uniform depth profiling in positive ion mode and almost no matrix effect in negative ion mode. The methodology offers a new strategy to tackle the matrix effect and should lead to improved quantitative measurement using SIMS. Graphical Abstract ᅟ.

Improved mass resolution and mass accuracy in TOF-SIMS spectra and images using argon gas cluster ion beams

The popularity of argon gas cluster ion beams (Ar-GCIB) as primary ion beams in time-of-flight secondary ion mass spectrometry (TOF-SIMS) has increased because the molecular ions of large organic- and biomolecules can be detected with less damage to the sample surfaces. However, Ar-GCIB is limited by poor mass resolution as well as poor mass accuracy. The inferior quality of the mass resolution in a TOF-SIMS spectrum obtained by using Ar-GCIB compared to the one obtained by a bismuth liquid metal cluster ion beam and others makes it difficult to identify unknown peaks because of the mass interference from the neighboring peaks. However, in this study, the authors demonstrate improved mass resolution in TOF-SIMS using Ar-GCIB through the delayed extraction of secondary ions, a method typically used in TOF mass spectrometry to increase mass resolution. As for poor mass accuracy, although mass calibration using internal peaks with low mass such as hydrogen and carbon is a common approach in TOF-SIMS, it is unsuited to the present study because of the disappearance of the low-mass peaks in the delayed extraction mode. To resolve this issue, external mass calibration, another regularly used method in TOF-MS, was adapted to enhance mass accuracy in the spectrum and image generated by TOF-SIMS using Ar-GCIB in the delayed extraction mode. By producing spectra analyses of a peptide mixture and bovine serum albumin protein digested with trypsin, along with image analyses of rat brain samples, the authors demonstrate for the first time the enhancement of mass resolution and mass accuracy for the purpose of analyzing large biomolecules in TOF-SIMS using Ar-GCIB through the use of delayed extraction and external mass calibration.

Peptide Fragmentation and Surface Structural Analysis by Means of ToF-SIMS Using Large Cluster Ion Sources

Peptide or protein structural analysis is crucial for the evaluation of biochips and biodevices, therefore an analytical technique with the ability to detect and identify protein and peptide species directly from surfaces with high lateral resolution is required. In this report, the efficacy of ToF-SIMS to analyze and identify proteins directly from surfaces is evaluated. Although the physics governing the SIMS bombardment process precludes the ability for researchers to detect intact protein or larger peptides of greater than a few thousand mass unit directly, it is possible to obtain information on the partial structures of peptides or proteins using low energy per atom argon cluster ion beams. Large cluster ion beams, such as Ar clusters and C60 ion beams, produce spectra similar to those generated by tandem MS. The SIMS bombardment process also produces peptide fragment ions not detected by conventional MS/MS techniques. In order to clarify appropriate measurement conditions for peptide structural analysis, peptide fragmentation dependency on the energy of a primary ion beam and ToF-SIMS specific fragment ions are evaluated. It was found that the energy range approximately 6 ≤ E/n ≤ 10 eV/atom is most effective for peptide analysis based on peptide fragments and [M + H] ions. We also observed the cleaving of side chain moieties at extremely low-energy E/n ≤ 4 eV/atom.

Channels of oxygen diffusion in single crystal rubrene revealed

Oxygen diffusion channels are imaged in the single crystal organic semiconductor rubrene using Time of Flight Secondary Ion Mass Spectroscopy.