Identification and separation of protein, contaminant and substrate peaks using gentle-secondary ion mass spectrometry and the g-ogram.

RATIONALE: Secondary ion mass spectrometry (SIMS) is an important technique for the characterization of proteins at surfaces. However, interpretation of the mass spectra is complicated owing to confusion with peaks from contaminants and the substrate which is further compounded by complex fragmentation mechanisms. We test a new development of the G-SIMS method called the g-ogram to separate out spectral components without a priori information about which peaks to include in the analysis and which peaks relate to each component. METHODS: The effectiveness of the g-ogram method is investigated using a model system of lysozyme adsorbed onto a silicon wafer and indium tin oxide substrates. In the method, two SIMS spectra are acquired using Bi(+) and Mn(+) primary ions which create lower and higher fragmentation in the spectra, respectively. The g-ogram separates out components using a separation parameter that is related to the fragmentation energy. RESULTS: The g-ogram separates the spectrum of lysozyme adsorbed onto a silicon wafer into three components: (i) the substrate and PDMS contamination; (ii) a second, but unexpected, contaminant; and (iii) peaks from the protein amino acids. Similar results are achieved for the indium tin oxide substrate. In addition, evidence of fragments from plural amino acids with two candidate peaks at 140.12 Da and 185.08 Da is observed. CONCLUSIONS: The g-ogram method effectively separates out mass peaks relating to the substrate, contamination and protein without any a priori information or subjective decisions about which peaks to include in the analysis (so called 'peak picking'). This is a great help to analysts. We find two possible peaks from plural amino acids but no evidence of pluralities is found for peaks above 240 Da that are generated from when using Bi or Mn primary ions.

Exploring the geochemical distribution of organic carbon in early land plants: a novel approach.

Terrestrialization depended on the evolution of biosynthetic pathways for biopolymers including lignin, cutin and suberin, which were concentrated in specific tissues, layers or organs such as the xylem, cuticle and roots on the submillimetre scale. However, it is often difficult, or even impossible especially for individual cells, to resolve the biomolecular composition of the different components of fossil plants on such a scale using the well-established coupled techniques of gas chromatography/mass spectrometry and liquid chromatography/mass spectrometry. Here, we report the application of techniques for surface analysis to investigate the composition of Rhynia gwynne-vaughanii X-ray photoelectron spectroscopy of two different spots (both 300 µm × 600 µm) confirmed the presence of carbon. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) revealed 'chemical maps' (imaging mode with 300 nm resolution) of aliphatic and aromatic carbon in the intact fossil that correlate with the vascular structures observed in high-resolution optical images. This study shows that imaging ToF-SIMS has value for determining the location of the molecular components of fossil embryophytes while retaining structural information that will help elucidate how terrestrialization shaped the early evolution of land plant cell wall biochemistry.This article is part of a discussion meeting issue 'The Rhynie cherts: our earliest terrestrial ecosystem revisited'.

Higher sensitivity secondary ion mass spectrometry of biological molecules for high resolution, chemically specific imaging.

To expand the role of high spatial resolution secondary ion mass spectrometry (SIMS) in biological studies, numerous developments have been reported in recent years for enhancing the molecular ion yield of high mass molecules. These include both surface modification, including matrix-enhanced SIMS and metal-assisted SIMS, and polyatomic primary ions. Using rat brain tissue sections and a bismuth primary ion gun able to produce atomic and polyatomic primary ions, we report here how the sensitivity enhancements provided by these developments are additive. Combined surface modification and polyatomic primary ions provided approximately 15.8 times more signal than using atomic primary ions on the raw sample, whereas surface modification and polyatomic primary ions yield approximately 3.8 and approximately 8.4 times more signal. This higher sensitivity is used to generate chemically specific images of higher mass biomolecules using a single molecular ion peak.

Topography and field effects in secondary ion mass spectrometry--part I: conducting samples.

Quantitative chemical characterization of surfaces with topography by secondary ion mass spectrometry (SIMS) remains a significant challenge due to the lack of systematic and validated measurement methods. In this study, we combine an experimental approach using a simple model system with computer simulation using SIMION, to understand and quantify the key factors that give rise to unwanted topographic artefacts in SIMS images of conducting samples with microscale topography. Experimental data are acquired for gold wires (diameters 33 to 125 µm) mounted onto silicon wafers. Significant loss of ion intensities and shadowing arise from the distortion of the extraction field, and the chemical analysis over the whole of the sample surface is difficult. For large primary ion incidence angles of ≥55° to the surface normal, a fraction of the primary ions are scattered from the target and impact the substrate, emitting secondary ions that may be mistaken as originating from the wire. For conducting samples, topographic field effects may be reduced by the use of a smaller extraction voltage and an extraction delay. The effects of an extraction delay on ion intensities, mass resolution and time-of-flight are studied, and its application is demonstrated on an anisotropically etched silicon sample. The use of a simple sample holder with a V-shaped groove to reduce topographic field effects for wires is also presented. Using these results, we provide clear guidance to analysts for the diagnosis and identification of topography effects in SIMS, and present key recommendations to minimize them in practical analysis.

Nanotribological characterization of human head hair by friction force microscopy in dry atmosphere and aqueous environment.

Friction force microscopy was employed for the tribological investigation of human head hair in two different environments: a dry atmosphere and de-ionized water. The fibers were immobilized by embedding them in indium. The effects of bleaching, conditioning, and immersion in methanolic KOH were quantified in terms of the relative coefficient of friction (µ). The virgin fibers were clearly distinguished in terms of friction coefficient from the chemically damaged ones in both environments, while all categories of hair exhibited higher friction coefficients in the aqueous environment. Secondary ion mass spectroscopy was used as a complementary technique to examine the presence of fatty acids on the cuticular surface of the different categories of hair as well as the conditioner distribution. Neither bleaching nor 30 min treatment in methanolic KOH was found adequate to completely remove the fatty acids from the fibers' surface. Conditioner species were detected along the whole cuticular surface.