High Ion Mass Research Articles

ChemInform Abstract: FTMS of Natural Polymers.

The ability of Fourier-transform mass spectrometry to produce spectra of some natural polymers - peptides, lignin and humic acid - that include stable, high-mass ions formed by laser desorption/ionization (LDI) and matrix assisted laser desorption/ionization (MALDI) is demonstrated.

MALDI linear TOF mass spectrometry of PEGylated (glyco)proteins

PEGylation of proteins is a fast growing field in biotechnology and pharmaceutical sciences owing to its ability to prolong the serum half-life time of recombinant proteins. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS) has been shown to be a powerful tool in the analysis of several PEGylated small proteins. Here we present data obtained with a standard secondary electron multiplier (SEM) and a high mass (HM) detector combined with a MALDI linear TOF MS system for the detection of PEGylated (glyco)proteins in the range of 60-600 kDa. Examples of MALDI TOF MS of small (interferon alpha2a), middle (human serum albumin (HSA)) and high molecular mass proteins (coagulation factor VIII and von Willebrand factor (vWF), both heavily glycosylated proteins) are presented. The particular challenge for the analysis was the heterogeneity of the (glyco)proteins in the high molecular weight range in combination with additional PEGylation, which even introduced more heterogeneity and was more challenging for interpretation. Nevertheless, the performance of MALDI linear TOF MS with both detector systems in terms molecular weight and heterogeneity determination depending on the m/z range was superior to the other methods. Although the SEM was able to obtain information about protein PEGylation in the mass range up to 100 kDa (e.g. PEGylated HSA), the HM system was crucial for detection of HM ions (e.g. PEGylated recombinant vWF), which was impossible with the standard SEM.

Behaviour of tetraalkylammonium ions in high‐field asymmetric waveform ion mobility spectrometry

High-field asymmetric waveform ion mobility spectrometry (FAIMS) is an ion-filtering technique recently adapted for use with liquid chromatography/mass spectrometry (LC/MS) to remove interferences during analysis of complex matrices. This is the first systematic study of a series of singly charged tetraalkylammonium ions by FAIMS-MS. The compensation voltage (CV) is the DC offset of the waveform which permits the ion to emerge from FAIMS and it was determined for each member of the series under various conditions. The electrospray ionization conditions explored included spray voltage, vaporizer temperature, and sheath and auxiliary gas pressure. The FAIMS conditions explored included carrier gas flow rate, electrode temperature and composition of the carrier gas. Optimum desolvation was achieved using sufficient carrier gas (flow rate > or = 2 L/min) to ensure stable response. Low-mass ions (m/z 100-200) are more susceptible to changes in electrode temperature and gas composition than high mass ions (m/z 200-700). As a result of this study, ions are reliably analyzed using standard FAIMS conditions (dispersion voltage -5000 V, carrier gas flow rate 3 L/min, 50% helium/50%nitrogen, inner electrode temperature 70 degrees C and outer electrode temperature 90 degrees C). Variation of FAIMS conditions may be of great use for the separation of very low mass tetraalkylammonium (TAA) ions from other TAA ions. The FAIMS conditions do not appear to have a major effect on higher mass ions.

Irradiation with heavy-ion particles changes the cellular distribution of human histone acetyltransferase HAT1

Hat1 was the first histone acetyltransferase identified; however, its biological function is still unclear. In this report, it is shown for the first time that human Hat1 has two isoforms. Isoform a has 418 amino acids (aa) and is localized exclusively in the nuclear matrix of normal human keratinocytes (NHKs). Isoform b has 334 aa and is located in the cytoplasm, the nucleoplasm, attached to the chromatin and to the nuclear matrix. Immunohistochemical analyses revealed that the bulk of Hat1 is confined to the nucleus, with much lesser amounts in the cytoplasm. Cells undergoing mitotic division have an elevated amount of Hat1 compared to those that are non-mitotic. Senescent cells, however, exhibit a higher concentration of Hat1 in the cytoplasm compare to proliferating cells and the amount of Hat1 in the nucleus decreases with the progression of senescence. NHKs exposed to hydrogen peroxide (H(2)O(2)) or to a beam of high mass and energy ion particles displayed bright nuclear staining for Hat1, a phenotype that was not observed in NHKs exposed to gamma-rays. We established that the enhanced nuclear staining for Hat1 in response to these treatments is regulated by the PI3K and the mitogen-activated protein kinase signaling pathways. Our observations clearly implicate Hat1 in the cellular response assuring the survival of the treated cells.

Protolith age and provenance of metasedimentary rocks in Variscan allochthon units: U–Pb SHRIMP zircon data from the Orlica–Śnieżnik Dome, West Sudetes

AbstractNew U–Pb sensitive high-mass resolution ion microprobe (SHRIMP) data from detrital zircons within the Orlica–Śnieżnik Dome provide new insights into the stratigraphic and palaeogeographic position of assumed relict Precambrian basement preserved in the Variscan collisional orogen of the West Sudetes, SW Poland. Hitherto, the Młynowiec Formation and the Stronie Formation within the Orlica–Śnieżnik Dome were assumed to represent two metavolcano-sedimentary successions of Proterozoic and early Palaeozoic age, respectively. However, when previous U–Pb data and mapping data from the Orlica–Śnieżnik Dome are combined with the new detrital zircon isotopic ages both from paragneisses within the Młynowiec Formation and from light-coloured quartzites and mica schists within the Stronie Formation, the result strongly suggests that the protoliths of these two formations actually form a continuous succession. This continuous succession is herein designated the Młynowiec–Stronie Group. The rocks of this group were deposited during middle Cambrian–early Ordovician times (c.520–470 Ma), presumably at the northern edge of West Gondwana after the 10–20 Ma period of tectonic quiescence that followed the terminal stage of the Cadomian collisions. Monotonous Młynowiec metagreywackes form the lower part of the succession, and the lithologically diverse schistose Stronie Formation forms its upper part. The change from greywacke (Młynowiec) to volcano-sedimentary (Stronie) facies coincided with the onset of rather short-lived volcanic activity which climaxed around 505–495 Ma and which supplied the volcanogenic components to the Stronie Formation. No ‘Cadomian unconformity’ has been observed in the region. Xenocrystic zircons from the Młynowiec–Stronie Group retain records of Archaean (3.0–2.3 Ga), Palaeoproterozoic (2.1–1.8 Ga) and Neoproterozoic to early Cambrian (660–530 Ma) zircon-forming events. These zircon ages, together with the lack of 1.7–1.2 Ga zircon ages, suggest that the source areas for the metasedimentary rocks may have been the West Africa craton, which therefore differs from the Amazonian affinity of the adjacent Brunovistulia Terrane. Nevertheless, two zircons,c.1.0 and 1.1 Ga old, respectively, indicate that the Młynowiec–Stronie Group sedimentary basin must have still been within the delivery reach of detritus ultimately derived from the Grenvillian-age belt(s). The detrital components of the supracrustal formations of the Orlica–Śnieżnik Dome were mainly derived from Neoproterozoic zircon-bearing crystalline rocks that were accreted to, and included in, the Cadomian basement in several intrusive pulses that culminated at 660–640 Ma, 620 Ma and 570–530 Ma.

Structure of Titan's ionosphere: Model comparisons with Cassini data

Solar extreme ultraviolet and X-ray radiation and energetic plasma from Saturn's magnetosphere interact with the upper atmosphere producing an ionosphere at Titan. The highly coupled ionosphere and upper atmosphere system mediates the interaction between Titan and the external environment. New insights into Titan's ionosphere are being facilitated by data from several instruments onboard the Cassini Orbiter, although the Ion and Neutral Mass Spectrometer (INMS) measurements will be emphasized here. We present dayside ionosphere models and compare the results with both Radio and Plasma Wave–Langmuir Probe (RPWS/LP) and INMS data, exploring the sensitivity of models to ionospheric chemistry schemes and solar flux variations. Modeled electron densities for the dayside leg of T18 and all of T17 (dayside) had reasonable agreement with the measured RPWS electron densities and INMS total ion densities. Magnetospheric inputs make at best minor contributions to the ionosphere for these flybys, at least for altitudes above about 1000 km. At lower (<1100 km) altitudes, the total ion densities measured by the INMS are less than the electron densities measured by the RPWS/LP which could be due to heavy (>100 daltons) ions, which the INMS is not able to detect. Qualitatively, INMS spectra exhibit the same ion species and 12 amu family separations for the dayside ionospheres of T17 and T18 as were seen in the mass spectra measured during T5 (nightside). However, the relative abundance of high-mass ( m>50) ion species is about 10 times less for the dayside T17 and T18 passes than it was for the polar nightside T5 flyby, which can perhaps be explained in several ways including differences in neutral composition, less dissociative recombination on the nightside than on the dayside (due to lower electron densities and affecting heavier ion species more than lighter ones), and transport of longer-lived high-mass species from day-to-night.

Degradation Mechanism of Secondary Electron Emission in Plasma-Exposed MgO Films

Degradation mechanism of secondary electron emission (SEE) properties in plasma-exposed MgO films was studied using three types of plasma ion; He+, Ne+, and Ar+. As the mass of impinging ions increased from He+ to Ar+, the SEE coefficient (γ) of MgO decreased from 0.0227 to 0.0175. Synchrotron radiation photoemission spectroscopy revealed that the Mg-to-O ratio significantly decreased from 1.00 to 0.79 in He plasma, to 0.73 in Ne plasma, and to 0.66 in Ar plasma. This was due to the preferential sputtering of Mg atoms by the high-mass ions, leading to the production of Mg vacancies (V centers) near the surface of MgO and the decrease in γ.

Heavy negative ions in Titan's ionosphere: Altitude and latitude dependence

One of the unexpected results of the Cassini mission was the discovery of negative ions at altitudes between 950 and 1400 km in Titan's ionosphere with masses up to 10,000 amu/q [Coates, A.J., Crary, F.J., Lewis, G.R., Young, D.T., Waite Jr., J.H., Sittler Jr., E.C., 2007. Discovery of heavy negative ions in Titan's ionosphere. Geophys. Res. Lett., 34, L22103, doi:10.1029/2007GL030978; Waite Jr., J.H., Young, D. T., Coates, A. J., Crary, F. J., Magee, B. A., Mandt, K. E., Westlake, J. H., 2008. The Source of Heavy Organics and Aerosols in Titan's Atmosphere, submitted to Organic Matter in Space, Proceedings IAU Symposium no. 251]. These ions are detected at low altitudes during Cassini's closest Titan encounters by the Cassini plasma spectrometer (CAPS) electron spectrometer. This result is important as it is indicative of complex hydrocarbon and nitrile chemical processes at work in Titan's high atmosphere. They may play a role in haze formation and ultimately in the formation of heavy particles (tholins), which fall through Titan's atmosphere and build up on the surface. During Cassini's prime mission negative ions were observed on 23 Titan encounters, including 7 in addition to those reported by Coates et al. [Coates, A.J., Crary, F.J., Lewis, G.R., Young, D.T., Waite Jr., J.H., Sittler Jr., E.C., 2007. Discovery of heavy negative ions in Titan's ionosphere. Geophys. Res. Lett., 34, L22103, doi:10.1029/2007GL030978]. Here, we also examine the altitude and latitude dependence of the high-mass negative ions observed in Titan's ionosphere, and we examine the implications of these results. We find that the maximum negative ion mass is higher at low altitude and at high latitudes. We also find a weaker dependence of the maximum mass on solar zenith angle.

Analysis of polychlorinated biphenyls in human serum by gas chromatography–mass selective detection operating at high ion source temperature

Optimization of analytical instrumentation is essential when analyses of persistent organic pollutants in human serum are performed at ultra-trace levels. This research describes the analysis of polychlorinated biphenyls (PCBs) in serum using gas chromatography coupled with a mass selective detector (GC/MSD) in the selected ion monitoring (SIM) mode. We selected PCB-58 and PCB-186 as internal standards (ISs) for the method development, and newborn calf serum (NCS) was chosen as the matrix. The matrix was fortified with PCB congeners and extracted by an automated solid phase extraction system using C18 sorbent. The extracts were analyzed at two ion source temperatures, 230 and 300 °C. The use of high ion source temperature increased the abundances of high-mass ions, and the response factors in SIM mode for PCBs. An excellent linearity from 0.5 to 100 ng/ml at ion source temperature of 300 °C was demonstrated, with a calculated detection limit of 0.1 ng/g serum. Seven replicate fortifications of newborn calf serum, at three spiking levels of 1, 10, and 50 ng/g of serum, gave mean recoveries of 110%, 85%, and 98%, with average relative standard deviation (RSD) values of 5.4%, 7.3%, and 4.7%, respectively.

Low-molecular-weight model study of peroxide cross-linking of ethylene-propylene (-diene) rubber using gas chromatography and mass spectrometry: I. Combination reactions of alkanes

The combination reaction of linear and branched alkanes, initiated by dicumylperoxide, has been studied as a model for the combination cross-linking reaction of peroxide-cured terpolymerised ethylene, propylene and diene monomer. Both gas chromatography–mass spectrometry (GC–MS) and comprehensive two-dimensional GC–MS (GC × GC–MS) analyses have been employed to analyse the isomeric reaction products. The identification of these products based on their MS fragmentation patterns is quite complex, due to the high tendency of random rearrangements. Careful elucidation of the high-mass ions at optimised ionisation energy (55 eV) has resulted in proposed structures for the different isomeric reaction products. The structure assignment by MS is in agreement with the GC × GC elution pattern and with the result of a theoretical model to predict the boiling points and, thus, the GC retention times. In addition, a model that provided a direct correlation between chemical structure and retention times was developed and this was found to provide a useful fit. Quantification of the identified reaction products by GC separation and flame ionization detection allows classification according to the hydrogen abstraction sites for the alkanes by dicumylperoxide. The selectivity for hydrogen abstraction generally follows the expected order, but a higher reactivity was observed for the methylene group next to a primary methyl group, while a reduced reactivity of the methylene group next to ethyl and to methyl groups was observed. The used approach proved to be a very powerful tool to enhance our understanding of the mechanism of peroxide cross-linking of (branched) alkanes.

Time‐of‐flight secondary ion mass spectrometric analysis of the interface between bone and titanium implants

Implant healing into bone tissue is a process where the mature bone grows towards and eventually fuses with the implant. In this study we investigated implant healing during 4 weeks with focus on the implant-tissue interface. Our main interest was to study the mineralization process around the implant. Titanium discs were implanted in rat tibia for 2 and 4 weeks. After implantation cross sections of bone and implant were made using a low-speed saw equipped with a diamond wafering blade. One section from each sample was stained with basic fuchsin and micrographed by light microscopy (LM). The other section was analyzed with imaging time-of-flight secondary ion mass spectrometry (TOF-SIMS) using a Bi(3)(+) cluster ion source. This ion source has recently been shown to enable identification of high-mass hydroxyapatite (HA) fragment ions (m/z 291-653) in bone samples. The LM images were used to identify areas suitable for TOF-SIMS analysis. Three areas were selected for mass spectral analysis, corresponding to interface region, bone and soft tissue, from which positive ion spectra were recorded. In the areas identified as bone, high-mass HA fragments ions were found after both 2 and 4 weeks. In the soft tissue area, no high-mass ions were found after 4 weeks. However, after 2 weeks HA-related ions were identified in mineralized spots in areas defined as soft tissue. After 4 but not after 2 weeks, high-mass HA fragment ions were found in the interface region. In conclusion, differences were observed regarding mineralization between 2 and 4 weeks of implantation and between different regions surrounding the implants. Imaging TOF-SIMS analysis using a Bi(3)(+) cluster as ion source enables identification of high-mass HA fragment ions at implant-tissue interfaces in bone. This technique might therefore be useful for biocompatibility assessment and for studying the mineralization process at implant surfaces.

Formation of High-Mass Cluster Ions from Compound Semiconductors Using Time-of-Flight Secondary Ion Mass Spectrometry with Cluster Primary Ions

The detection of high-mass, nonstoichiometric, GaxAsy and InxPy secondary ion clusters using time-of-flight secondary ion mass spectrometry is reported for the first time. The GaxAsy and InxPy clusters are detected in both positive and negative ion spectra and extend to masses of at least 6000 dalton (Da). Consecutive clusters differ by the addition of one gallium (indium) atom. This leads to nonstoichiometric clusters at high mass (i.e., Ga15As3 at 1270 Da) which are metastable above a critical mass. The relative secondary ion yields of high-mass GaxAsy clusters detected using several primary ion sources (Cs+, Bi+, Bi3+, Bi32+, Bi52+, C60+, and C602+) are compared. The relative secondary ion yield of high-mass GaxAsy clusters is significantly enhanced by the use of cluster primary ions and the best relative secondary ion yield is obtained using Bi3+ primary ions. An application of the high-mass GaxAsy clusters is presented, in which these clusters are utilized to distinguish between contaminant levels of Ga and bulk GaAs structure in a depth profile of a MnAs/GaAs heterojunction. These results illustrate improved analysis of inorganic materials using cluster primary ions and break the paradigm of stoichiometric secondary cluster ion formation for SIMS of inorganic compounds.

Protolith age and timing of Precambrian magmatic and metamorphic events in the Priest River complex, northern Rockies

U–Pb dating of detrital zircons, magmatic zircons, and metamorphic zircon overgrowths and titanites from the Priest River complex, USA, reveal the ages of high-grade metasedimentary rocks, intrusive ages of associated igneous rocks, and the timing of Precambrian metamorphic events. Sensitive high-mass resolution ion microprobe (SHRIMP) U–Pb dates of detrital zircons from the Hauser Lake gneiss and Gold Cup Quartzite are predominately Paleoproterozoic with some Archean grains. The Hauser Lake gneiss contains detrital zircons with nearly concordant ages as young as 1511 Ma. Isotope dilution – thermal ionization mass spectrometry (ID–TIMS) and chemical abrasion – thermal ionization mass spectrometry (CA–TIMS) dating of magmatic zircon from amphibolites in the Hauser Lake Gneiss yield 1470–1430 Ma crystallization ages based on discordant data, with Cretaceous lower intercepts. The U–Pb zircon systematics are very complex and reflect multiple periods of magmatic and metamorphic growth. A combination of mechanical abrasion, chemical abrasion (CA–TIMS), and SHRIMP methods were required to identify the major geochronological components. These data, in addition to one Nd model age, strengthen correlations between the Hauser Lake Gneiss and the lower Belt–Purcell Supergroup and the Gold Cup Quartzite with the Neihart Quartzite, which underlies the Belt–Purcell Supergroup. Four SHRIMP-dated metamorphic zircon overgrowths give concordant Grenville dates with a weighted average of 1127 ± 110 Ma. This Grenville-aged metamorphic event is recorded by many isotopic systems in the Belt–Purcell basin and reflects a static thermal event, possibly driven by magmatism at depth.

Analysis of metastable ions in the ToF-SIMS spectra of polymers

This paper describes a simplified method to determine the parent and daughter ions of metastable peaks observed using a single stage reflectron ToF-SIMS instrument. The method requires two or more spectra, acquired with different reflector potentials and relies on the use of the apparent mass of the metastable, rather than the time-of-flight required by previous methods. We demonstrate the origin of metastable peak shapes, which depend upon the mass ratio of daughter and parent ions, the kinetic energy released during the decay process and the kinetics of the decay process. We highlight the difficulty of obtaining information from these shapes. The metastables of four common polymers, poly(ethyleneoxide), poly(lactide), poly(methylmethacrylate) and poly(tetrafluoroethylene) are used to provide comprehensive maps of the important in-flight fragmentation reactions. These maps demonstrate that the polymer structure is generally retained for high mass ions and the origin of the C 4H 9O 2 + secondary ion in poly(lactide), a prominent but puzzling ion, is explained.

Oxide Materials with Low Thermal Conductivity

A heuristic approach to identifying candidate materials with low, temperature‐independent thermal conductivity above room temperature is described. On the basis of this approach, a number of compounds with thermal conductivities lower than that of 8 mol% yttria‐stabilized zirconia and fused silica have been found. Three compounds, in particular, the Zr3Y4O12delta phase, the tungsten bronzes, and the La2Mo2O9phase, exhibit potential for low thermal conductivity applications. As each can exhibit extensive substitutional solid solution with other, high atomic mass ions, there is the prospect that many more compounds with low thermal conductivity will be discovered.

Surface and Depth Profiling Investigation of a Drug-Loaded Copolymer Utilized To Coat Taxus Express2 Stents

The surface of a styrene-b-isobutylene-b-styrene triblock copolymer, containing a solid-phase drug, was studied by time-of-flight secondary ion mass spectrometry employing 15-keV Ga+ and 20-keV C60+ ion sources. This polymer/drug system has direct application in the cardiac stent arena, where it has been used to treat restenosis or renarrowing of arterial walls after stent or angioplasty procedures. Overall, the results illustrate the successful use of a cluster ion beam for greatly enhancing the high-mass fragment ion and molecular ion intensities from the surface and bulk of the polymer system. The use of C60+ also established the ability to remove common overlayers like poly(dimethylsiloxane), which was not possible using a Ga+ ion source. Furthermore, the use of C60+ allowed depth profiles to be obtained using primary ion dose densities in excess of 6 x 1014 C60+/cm2. Resultant sputter craters reached depths of approximately 2 microm and possessed relatively flat bottoms without the need for sample rotation. AFM and profilometry studies support the relatively gentile removal of surface species via phase contrast and topographic imaging. In addition, the findings suggest that relatively high ion doses do not significantly alter the phase distribution or surface topography of the polymer surface; however, a slight increase in surface roughness was detected.

Negative gas‐phase ion chemistry of silane: a quadrupole ion trap study

Silicon clusters are of considerable interest for their importance in astrophysics and chemical vapour deposition processes, as well as from a fundamental point of view. Here, we present a quadrupole ion trap study of the self-condensation ion/molecule reactions of anions of silane. In the high-pressure regime, several ion clusters are formed with increasing size: the largest ions detected are Si5Hn- (n = 0-3). Selective ion isolation and storage allowed detection of the main reaction sequences occurring in the reacting system. The most frequent condensation step is followed by single or multiple dehydrogenation, this latter being particularly observed for the high-mass reactant ions. As a consequence, the most abundant ions in the mass spectra are those with a low content of hydrogen, namely Si2H-, Si3H-, and Si4H-. These results are discussed with reference to literature data on silicon cluster anions and related systems.

Mass spectrometric identification and characterization of antimony complexes with ribose-containing biomolecules and an RNA oligomer

Mass spectrometric techniques have been used to study the interaction of inorganic Sb(V) with biomolecules containing a ribose or deoxyribose moiety. Electrospray (ES) mass spectra of reaction mixtures containing inorganic Sb(V) and one of several biomolecules (adenosine, cytidine, guanosine, uridine, adenosine-5'-monophosphate, adenosine-3',5'-cyclic monophosphate, ribose, or 2'-deoxyadenosine) afforded high-mass antimony-containing ions corresponding to Sb(V)-biomolecule complexes of stoichiometry 1:1, 1:2, or 1:3. The complexes were characterized by collision-induced dissociation (CID) tandem mass spectrometry (MS) using ion-trap multistage MS. The CID results revealed that Sb(V) binds to the ribose or deoxyribose moiety. Structures are proposed for the Sb-biomolecule complexes. Analysis of the reaction mixtures by reversed-phase chromatography coupled on-line to either inductively coupled plasma (ICP) MS or ES-MS showed that in solution Sb(V) forms complexes with all the analyzed biomolecules with vicinal cis hydroxyl groups. Evidence (from size-exclusion chromatography ICP-MS and direct infusion ES-MS) of complexation of Sb(V) with an RNA oligomer, but not with a DNA oligomer, supports the suggestion that the presence of vicinal cis hydroxyl groups is critical for complexation to occur. This is the first direct evidence of complexation of Sb(V) with RNA. Results obtained by studying the effect of changing reaction conditions, i.e. pH, reaction time, and Sb/biomolecule molar ratio, on the extent of Sb-biomolecule formation suggest the reaction may be of physiological importance. Selected reaction monitoring (SRM) and precursor-ion-scanning tandem MS were investigated to determine their potential to detect trace levels of the Sb-biomolecule complexes in biological samples. Application of SRM MS-MS in combination with high-performance liquid chromatography enabled successful detection of an Sb-adenosine complex that had been spiked into a complex biological matrix (liver homogenate).

Comparison of detection efficiencies of negatively charged gold–alkanethiolate-, gold–sulfur- and gold-clusters in ToF-SIMS

In order to improve quantification of high mass ions, the influence of cluster composition on detection efficiencies has been studied using a TOF-SIMS IV with the extended capability of postaccelerating ions up to 20 keV. In this experimental study, we focus on the comparison of detection efficiencies for three types of negatively charged secondary cluster ions: gold–alkanethiolate-clusters (Au x M y ), gold–sulfur-clusters (Au x S y ) and gold-clusters (Au x ). The clusters were sputtered from self-assembled monolayers of hexadecanethiols on gold substrates using 10 keV Ar + primary ions. The detection efficiencies were derived on the basis of a function for the secondary electron yield and a fourth-order approximated Poisson probability distribution for electron propagation and amplification within the microchannel plate. In addition to the well-known dependence of detection efficiencies on ion mass and energy, which has already been studied for positively charged ions, we were able to show a similar behaviour for the investigated negatively charged secondary ions. We have observed major variations among the three types of clusters at similar mass and energy as predicted in a theoretical approach. The observed differences are due to the different composition of the investigated clusters which has a major influence on the kinetic ion induced electron emission within the microchannel plate. For the first time it was possible to experimentally verify these predictions for detection efficiencies.

Depth profiling of taxol-loaded poly(styrene- b-isobutylene- b-styrene) using Ga + and C 60+ ion beams

The surface of a triblock copolymer, containing a solid-phase drug, was investigated using 15 keV Ga + and 20 keV C 60 + ion beams. Overall, the results illustrate the successful use of a cluster ion beam for greatly enhancing the molecular ion and high-mass fragment ion intensities from the surface and bulk of the polymer system. The use of C 60 + also established the ability to see through common overlayers like poly(dimethyl siloxane) which was not possible using atomic ion sources. Moreover, the use of C 60 + allowed depth profiles to be obtained using primary ion dose densities in excess of 6 × 10 14 C 60 +/cm 2. Resulting sputter craters possess relatively flat bottoms without the need for sample rotation and reached depths of ca. 2 μm. AFM results illustrate the more gentile removal of surface species using cluster ions. Specifically, phase contrast and topographic images suggest the relatively high ion doses do not significantly alter the phase distribution or surface topography of the polymer. However, a slight increase in rms roughness was noticed.