High Resolution Scanning Ion Microprobe Research Articles

Divalent cation distribution in dinoflagellate chromosomes imaged by high-resolution ion probe mass spectrometry

From a variety of analytical electron microscopy experiments, the chromosomes of dinoflagellates are known to contain sizeable amounts of cations, the latter thought to contribute to the neutralization of the negative charge carried by the phosphate groups in the DNA backbone. From previous Ca and Mg chelation experiments, it is also known that these cations are necessary for the compaction and preservation of the chromosome architecture. Similar conclusions have been recently presented by our group concerning mammalian mitotic chromosomes, in studies based on secondary ion mass spectrometry (SIMS) carried out with the University of Chicago high-resolution scanning ion microprobe (UC-SIM). We have now applied this instrument to image the distribution of DNA-bound Ca 2+ and Mg 2+ in dinoflagellate chromosomes, a goal that could not be attained earlier by analytical electron microscopy. Analyzed quantitatively and imaged here by SIMS for the first time, through their cation content, are the chromosomes of the dinoflagellates Prorocentrum micans, Gymnodinium mikimotoi and Gymnodinium dorsum. The cell nuclei were isolated and prepared for SIMS analysis with a minimal protocol (mechanical fractionation in culture medium followed by ethanol drying), which did not expose the samples to artifact-creating, alien chemical agents. By this approach, we have confirmed the earlier findings by several authors, and contributed new structural information provided by our ion probe capability to erode the sample surface layer by layer (SIMS tomography). Dinoflagellates, due to the absence of histones, represent an ideal model system where cations may bind directly with DNA, allowing comparisons to be made with recently reported X-ray crystallography results at atomic resolution. Such comparisons yielded quantitative confirmation that the Ca 2++Mg 2+ concentrations found for e.g. P. micans are consistent with those anticipated to provide complete charge neutralization of naked DNA by cations, also resulting in maximal DNA compaction.

High resolution SIMS imaging of cations in mammalian cell mitosis, and in Drosophila polytene chromosomes

The University of Chicago high resolution scanning ion microprobe (UC-SIM) was used to image, by Secondary Ion Mass Spectrometry (SIMS), the distribution of Ca 2+ and Mg 2+ in the chromosomes of Indian muntjac (IM) deer mitotic fibroblasts. This is part of a systematic study of the cation composition of mammalian cells and chromosomes throughout the cell cycle, after having shown that Ca 2+ and Mg 2+ appear to be important for chromosome condensation and structure at metaphase. We focus here on a detailed description of the metaphase–anaphase transition at narrow time intervals beyond the G2/M border, made possible by controlled cell synchronization procedures. High-density distributions of chromosome spreads showed progressive stages of mitosis, identified by their morphology, within the same UC-SIM field of view. Subtle differences in cation contents between successive mitotic stages could thus be quantified in identical experimental conditions. Preliminary results indicate maximal chromosomal concentrations of Ca 2+ and Mg 2+ at metaphase, and a progressive decrease of the same with advancing stages of anaphase. Ca 2+ and Mg 2+ distributions were also imaged in the polytene chromosomes of Drosophila melanogaster, whose DNA distribution had been previously studied by BrdU labeling. These cations may play a common role in mitosis from lower eukaryotes to mammals.

Cations in mammalian cells and chromosomes: Sample preparation protocols affect elemental abundances by SIMS

The focus of our current research aims at detailing and quantifying the presence of cations, primarily Ca and Mg, in mammalian cells and chromosomes throughout the different stages of the cell cycle, using our high resolution scanning ion microprobe, the UC-SIM. The 45 keV Ga + probe of this instrument, typically ∼40 nm in diameter, carries a current of 30–40 pA, appropriate for surface SIMS studies, but limited in sample erosion rate for dynamic SIMS mapping over cell-size areas, of order 100 μm × 100 μm. Practical and reliable use of this probe toward the above SIMS goals requires a careful matching of the latter factors with the physical and chemical consequences of sample preparation protocols. We examine here how the preferred sample cryo-preservation methodologies such as freeze-fracture and lyophilization affect high resolution SIMS analysis, and, from this standpoint, develop and evaluate the advantages and disadvantages of fast alternate approaches to drying frozen samples. The latter include the use of methanol, ethanol, and methanol/acetic acid fixative. Methanol-dried freeze-fractured samples preserve histological morphology and yield Ca and Mg distributions containing reliable differential dynamical information, when compared with those following lyophilization.

Specific Mg 2+ binding at human and Indian muntjac chromosomal Giemsa bands

Our main research interests have focused on cation–DNA and cation–protein interactions in terms of their roles in higher order chromosomal structure. Our previous study directly analyzed the cation composition of cryo-preserved mammalian interphase and mitotic cells, as well as fractionated untreated and cation-depleted chromosomes at a resolution of 50 nm using the University of Chicago high resolution scanning ion microprobe (UC-SIM). Quantitative direct UC-SIMS signals and subsequent imaging of cryo-preserved cells demonstrated that Na + and K + were associated with chromatin throughout the cell cycle, whereas in contrast Ca 2+ and Mg 2+ exhibited a localization change during interphase and mitosis. Interphase Ca 2+ and Mg 2+ were found mainly throughout the cytoplasm, but at mitosis associated with chromatin. Our findings of chromatin association of Na + and K + support a role of these cations in both interphase and mitotic chromatin compaction, whereas Ca 2+ and Mg 2+ binding points to an essential function in mitotic chromatin compaction. In our present investigation using SIMS images obtained from both human and Indian muntjac metaphase chromosomes, we reveal specific binding of Mg 2+ to chromosomal “p” and “q” arm heterochromatic regions correlating with conventional Giemsa (G-) bands. In addition, detailed Mg 2+ image density peaks along the chromosomes indicate that Mg 2+ peaks correspond to the location of G-bands. Our results support a direct role for Mg 2+ in promoting and maintaining the higher order chromatin structure of heterochromatic regions on chromosome arms represented by G-bands, possibly due to both Mg 2+–DNA and Mg 2+–protein interactions.

Chronic acidosis-induced alteration in bone bicarbonate and phosphate.

Chronic metabolic acidosis increases urinary calcium excretion without altering intestinal calcium absorption, suggesting that bone mineral is the source of the additional urinary calcium. In vivo and in vitro studies have shown that metabolic acidosis causes a loss of mineral calcium while buffering the additional hydrogen ions. Previously, we studied changes in femoral, midcortical ion concentrations after 7 days of in vivo metabolic acidosis induced by oral ammonium chloride. We found that, compared with mice drinking only distilled water, ammonium chloride induced a loss of bone sodium and potassium and a depletion of mineral HCO3(-) and phosphate. There is more phosphate than carbonate in neonatal mouse bone. In the present in vitro study, we utilized a high-resolution scanning ion microprobe with secondary ion mass spectroscopy to test the hypothesis that chronic acidosis would decrease bulk (cross-sectional) bone phosphate to a greater extent than HCO3(-) by localizing and comparing changes in bone HCO3(-) and phosphate after chronic incubation of neonatal mouse calvariae in acidic medium. Calvariae were cultured for a total of 51 h in medium acidified by a reduction in HCO3(-) concentration ([HCO(-)]; pH approximately 7.14, [HCO3(-)] approximately 13) or in control medium (pH approximately 7.45, HCO3(-) approximately 26). Compared with incubation in control medium, incubation in acidic medium caused no change in surface total phosphate but a significant fall in cross-sectional phosphate, with respect to the carbon-carbon bond (C2) and the carbon-nitrogen bond (CN). Compared with incubation in control medium, incubation in acidic medium caused no change in surface HCO3(-) but a significant fall in cross-sectional HCO3(-) with respect to C2 and CN. The fall in cross-sectional phosphate was significantly greater than the fall in cross-sectional HCO3(-). The fall in phosphate indicates release of mineral phosphates, and the fall in HCO3(-) indicates release of mineral HCO3(-), both of which would be expected to buffer the additional protons and help restore the pH toward normal. Thus a model of chronic acidosis depletes bulk bone proton buffers, with phosphate depletion exceeding that of HCO3(-).

Acute acidosis-induced alteration in bone bicarbonate and phosphate.

During an acute fall in systemic pH due to a decrease in the concentration of serum bicarbonate ([HCO(3)(-)]), metabolic acidosis, there is an influx of hydrogen ions into the mineral phase of bone, buffering the decrement in pH. When bone is cultured in medium modeling acute metabolic acidosis, the influx of hydrogen ions is coupled to an efflux of sodium and potassium and a depletion of mineral carbonate. These ionic fluxes would be expected to neutralize some of the excess hydrogen ions and restore the pH toward normal. Approximately one-third of bone carbonate is located on the hydration shell of apatite, where it is readily accessible to the systemic circulation, whereas the remainder is located in less accessible areas. We hypothesize that the surface of bone would respond to acidosis in a different manner than the interior of bone, with depletion of carbonate preferentially occurring on the bone surface. We utilized a high-resolution scanning ion microprobe with secondary ion mass spectroscopy to localize the changes in bone carbonate, as measured by HCO(3)(-), and phosphate and determine their relative contribution to the buffering of hydrogen ions during acute metabolic acidosis. Neonatal mouse calvariae were incubated in control medium (pH approximately 7.44, [HCO(3)(-)] approximately 27 mM) or in medium acidified by a reduction in [HCO(3)(-)] (pH approximately 7.14, [HCO(3)(-)] approximately 13). Compared with control, after a 3-h incubation in acidic medium there is a fivefold decrease in surface HCO(3)(-) with respect to the carbon-carbon bond (C(2)) and a threefold decrease in surface HCO(3)(-) with respect to the carbon-nitrogen bond (CN) with no change in cross-sectional HCO(3)(-). Compared with control, after a 3-h incubation in acidic medium there is a 10-fold decrease in cross-sectional phosphate with respect to C(2) and a 10-fold decrease in cross-sectional phosphate with respect to CN, with no change in surface phosphate. On the bone surface, there is a fourfold depletion of HCO(3)(-) in relation to phosphate, and, in cross section, a sevenfold depletion of phosphate in relation to HCO(3)(-). Thus acute hydrogen ion buffering by bone involves preferential dissolution of surface HCO(3)(-) and of cross-sectional phosphate.

Cation-chromatin binding as shown by ion microscopy is essential for the structural integrity of chromosomes.

Mammalian interphase and mitotic cells were analyzed for their cation composition using a three-dimensional high resolution scanning ion microprobe. This instrument maps the distribution of bound and unbound cations by secondary ion mass spectrometry (SIMS). SIMS analysis of cryofractured interphase and mitotic cells revealed a cell cycle dynamics of Ca2+, Mg2+, Na+, and K+. Direct analytical images showed that all four, but no other cations, were detected on mitotic chromosomes. SIMS measurements of the total cation content for diploid chromosomes imply that one Ca2+ binds to every 12.5–20 nucleotides and one Mg2+ to every 20–30 nucleotides. Only Ca2+ was enriched at the chromosomal DNA axis and colocalized with topoisomerase IIα (Topo II) and scaffold protein II (ScII). Cells depleted of Ca2+ and Mg2+ showed partially decondensed chromosomes and a loss of Topo II and ScII, but not hCAP-C and histones. The Ca2+-induced inhibition of Topo II catalytic activity and direct binding of Ca2+ to Topo II by a fluorescent filter-binding assay supports a regulatory role of Ca2+ during mitosis in promoting solely the structural function of Topo II. Our study directly implicates Ca2+, Mg2+, Na+, and K+ in higher order chromosome structure through electrostatic neutralization and a functional interaction with nonhistone proteins.

Contribution of organic material to the ion composition of bone.

Studies of bone mineral ranging from cadaveric analysis to the use of high-resolution ion microprobe with secondary ion mass spectroscopy (SIMS) have concluded that bone is rich in sodium and potassium relative to calcium. Exposure of bone to acid conditions either in vitro or in vivo leads to an exchange of hydrogen ions for sodium and potassium buffering the acidity of the medium or blood, respectively. Whether these monovalent ions reside within the mineral or organic phases of bone has never been determined. To determine the contribution of organic material to bone ion composition, we dissected calvariae from 4- to 6-day-old mice, removed organic material of some with hydrazine (Hydr), and prepared all bones for analysis using a high-resolution scanning ion microprobe coupled to a secondary ion mass spectrometer. We found that in non-Hydr-treated calvariae (Ctl) there was far more surface sodium and potassium than calcium (23Na/ 40Ca = 15.7 + 1.9, ratio of counts of detected secondary ions, mean + 95% CI, 39K/40Ca = 44.0 + 1.5). Removal of organic material with hydrazine (Hydr) led to a marked fall in the ratio of sodium to calcium and potassium to calcium (23Na/40Ca = 5.9 + 1.4, p < 0.025 vs. respective Ctl and 39K/40Ca = 1.1 + 1.5, p < 0.001 vs. respective Ctl). Similarly, when examining the cross-section of the calvariae there was more sodium and potassium than calcium (23Na/40Ca = 8.6 + 1.6, 39K/40Ca = 26.7 + 1.8). Treatment with Hydr again caused a marked fall in both ratios (23Na/40Ca = 0.3 + 1.6, p < 0.001 vs. respective Ctl and 39K/40Ca = 0.02 + 1.9, p < 0.001 vs. respective Ctl). Thus, within bone the organic material contains the majority of the sodium and potassium. This suggests that the organic material in bone and not the mineral itself is responsible for the acute buffering of the additional hydrogen ions during metabolic acidosis.

Effects of in vivo metabolic acidosis on midcortical bone ion composition.

Chronic metabolic acidosis increases urine calcium excretion without altering intestinal calcium absorption, suggesting that bone mineral is the source of the additional urinary calcium. During metabolic acidosis there appears to be an influx of protons into bone mineral, lessening the magnitude of the decrement in pH. Although in vitro studies strongly support a marked effect of metabolic acidosis on the ion composition of bone, there are few in vivo observations. We utilized a high-resolution scanning ion microprobe with secondary ion mass spectroscopy to determine whether in vivo metabolic acidosis would alter bone mineral in a manner consistent with its purported role in buffering the increased proton concentration. Postweanling mice were provided distilled drinking water with or without 1.5% NH(4)Cl for 7 days; arterial blood gas was then determined. The addition of NH(4)Cl led to a fall in blood pH and HCO(-)(3) concentration. The animals were killed on day 7, and the femurs were dissected and split longitudinally. The bulk cortical ratios Na/Ca, K/Ca, total phosphate/carbon-nitrogen bonds [(PO(2) + PO(3))/CN], and HCO(-)(3)/CN each fell after 1 wk of metabolic acidosis. Because metabolic acidosis induces bone Ca loss, the fall in Na/Ca and K/Ca indicates a greater efflux of bone Na and K than Ca, suggesting H substitution for Na and K on the mineral. The fall in (PO(2) + PO(3))/CN indicates release of mineral phosphates, and the fall in HCO(-)(3)/CN indicates release of mineral HCO(-)(3). Each of these mechanisms would result in buffering of the excess protons and returning the systemic pH toward normal.

Grain boundary chemistry of alumina by high-resolution imaging SIMS

The unique capabilities of the high-resolution scanning ion microprobe developed at the University of Chicago (UC-SIM) are described and its utility is demonstrated in a study of grain boundary chemistry of alumina ceramics. When polycrystalline alumina is doped singly with either MgO or SiO 2, strong segregation of the individual ions to grain boundaries is observed: (1) for Mg segregation C gb/ C grain∼400; (2) for Si segregation C gb/ C grain∼300. However, on codoping with both MgO and SiO 2, grain boundary segregation is significantly diminished by a factor of five or more over single doping as both cations are redistributed into the bulk alumina lattice. A defect compensation mechanism is proposed to explain this mutual solid solubility of Mg and Si in alumina. One important consequence of this chemical redistribution is a change in abnormal grain growth morphology from facetted grains in SiO 2 singly doped alumina, to non-facetted grains with curved boundaries in MgO and SiO 2 codoped alumina. As the Mg/Si dopant ratio exceeds the equimolar concentration, abnormal grain growth development ceases. These findings provide a physical mechanism to explain the role of MgO as a sintering aid to control microstructure evolution in alumina. Another significant consequence of SiO 2 redistribution on MgO doping is an observed improvement in the corrosion resistance of alumina to aqueous HF. Siliceous grain boundary films readily corrode and compromise the intrinsically good corrosion resistance of bulk alumina. MgO doping in amounts greater than the SiO 2 concentration prevents the formation of these corrodable silica-based phases leading to the development of aluminas for use in aqueous HF-containing ambients.

Imaging of BrdU-labeled human metaphase chromosomes with a high resolution scanning ion microprobe.

Detailed maps of the A-T distribution within human mitotic chromosomes labeled with BrdU are obtained with a high resolution scanning ion microprobe through the detection of bromine by imaging secondary ion mass spectrometry (SIMS). Corresponding maps of the emission loci of the molecular ion CN- describe the overall DNA, RNA and protein distribution in the chromosomes. Several chromosome preparations exhibit base-specific banding patterns (SIMS-bands) which mimic the well known G- or Q-bands resulting from conventional staining methods for optical microscopy. SIMS-bands are more noticeable in mitotic cells at the first cell cycle and after in situ denaturation or Giemsa staining. Sister chromatid exchanges (SCE) at the second cell cycle and beyond, occurring both spontaneously and promoted following cell culture exposure to the chemical aphidicolin (an inhibitor of DNA replication), can be visualized readily from the relative label signal intensities between sister chromatids. The comparison of base-specific label maps with CN- maps, in conjunction with the appearance of base-specific banding patterns, is informative about protein survival and/or removal following different chromosome preparation protocols. In addition, the resulting condensation state of the chromosomes can be appraised during SIMS analysis from the sample topography (imaged via the collection of mass-unresolved secondary ions). We demonstrate that imaging SIMS is a powerful complement to existing methods for the study of banding mechanisms and for the elucidation of chromosome structure. The advantages of this novel approach to the systematic and quantitative study of cytogenetic phenomena and methodologies are still largely untapped.

Effects of osteoclastic resorption on bone surface ion composition.

Osteoclasts are responsible for resorption of bone mineral. To determine how osteoclasts alter bone surface ion composition, neonatal mouse bone cells were isolated and cultured in the presence of parathyroid hormone (PTH) on bovine cortical bone. Surface ion composition of the resulting osteoclastic resorption pits was compared with that of unresorbed bone, utilizing a high-resolution scanning ion microprobe. Cortical bone cultured with cells in the presence of PTH had numerous resorption pits. The unresorbed area adjacent to the pits had a ratio of surface 23Na/40Ca of 18.7 + 1.6 (mean counts per second of detected secondary ions +95% confidence interval) and 39K/40Ca of 2.3 + 2.2. At the base of the pits, the ratio of 23Na/40Ca was 1.0 + 2.0 and 39K/40Ca was 0.1 + 1.0 (each different from area adjacent to the pit, P < 0.001). The ratio of 23Na/39K in the unresorbed area was not different from that at the base of the pit. Thus osteoclasts induce a decrease in the ratio of surface ion composition of both 23Na/40Ca and 39K/40Ca but not 23Na/39K in bovine cortical bone. The elevated ratios of 23Na/40Ca and 39K/40Ca on the surface, but not at the base of the pits, indicate adsorption of medium ions onto the mineral. Because osteoclasts foster the release of bone Ca, these results indicate that osteoclastic resorption causes a greater, and approximately equal, release of both 23Na and 39K compared with 40Ca from bone mineral. Osteoclasts appear to remove nonselectively the surface mineral that had been exposed to the medium, uncovering underlying mineral.

Effects of aluminum on bone surface ion composition

Aluminum induces net calcium efflux from cultured bone. To determine whether aluminum alters the bone surface ion composition in a manner consistent with predominantly cell-mediated resorption, a combination of cell-mediated resorption and physicochemical dissolution or physicochemical dissolution alone, we utilized an analytic high-resolution scanning ion microprobe with secondary ion mass spectroscopy to determine the effects of aluminum on bone surface ion composition. We cultured neonatal mouse calvariae with or without aluminum (10(-7) M) for 24 h and determined the relative ion concentrations of 23Na, 27Al, 39K, and 40Ca on the bone surface and eroded subsurface. Control calvariae have a surface (depth approximately 6 nm) that is rich in Na and K compared with Ca(Na/Ca) = 24.4 + 1.4, mean + 95% confidence limit of counts per second of detected secondary ions, K+Ca = 13.2 + 0.9). Aluminum is incorporated into the bone and causes a depletion of surface Na and K relative to Ca (Na/Ca = 9.6 + 0.7, K/Ca = 4.9 + 0.4; each p < 0.001 versus control). After erosion (depth approximately 50 nm), control calvariae have more Na and K than Ca (Na/Ca = 16.0 + 0.1, K/Ca = 7.5 + 0.1); aluminum again depleted Na and K relative to Ca (Na/Ca = 4.1 + 0.1 K/Ca = 1.9 + 0.1; each p < 0.001 versus control). Aluminum produced a greater net efflux of Ca (362 +/- 53, mean +/- SE, nmol/bone/24 h) than control (60 +/- 30, p < 0.001). With aluminum, the fall in the ratios of both Na/Ca and K/Ca coupled with net Ca release from bone indicates that aluminium induces a greater efflux of Na and K than Ca from the bone surface and is consistent with an aluminum-induced removal of the bone surface. This alteration in surface ion concentration and calcium efflux is consistent with that observed when calcium is lost from bone through a combination of cell-mediated resorption and physicochemical dissolution.

Advances in imaging SIMS studies of stained and BrdU-labelled human metaphase chromosomes

We have shown previously that isotope-labelled nucleotides in human metaphase chromosomes can be detected and mapped by imaging secondary ion mass spectrometry (SIMS), using the University of Chicago high resolution scanning ion microprobe (UC SIM). These early studies, conducted with BrdU- and 14C-thymidine-labelled chromosomes via detection of the Br and 28CN- (14C14N-&gt; labelcarrying signals, provided some evidence for the condensation of the label into banding patterns along the chromatids (SIMS bands) reminiscent of the well known Q- and G-bands obtained by conventional staining methods for optical microscopy. The potential of this technique has been greatly enhanced by the recent upgrade of the UC SIM, now coupled to a high performance magnetic sector mass spectrometer in lieu of the previous RF quadrupole mass filter. The high transmission of the new spectrometer improves the SIMS analytical sensitivity of the microprobe better than a hundredfold, overcoming most of the previous imaging limitations resulting from low count statistics.

Physicochemical effects of acidosis on bone calcium flux and surface ion composition.

Net calcium flux (JCa) from bone in vitro is pH dependent. When pH falls below 7.40, through a reduction in [HCO3-], there is both physicochemical and cell-mediated JCa. To characterize the physicochemical effect of acidosis on bone we inhibited the bone-resorbing cells (osteoclasts) with the specific inhibitor calcitonin and studied the effect of acidosis on JCa and bone ion composition using an analytic high-resolution scanning ion microprobe. Neonatal mouse calvariae were cultured for 48 h in physiologically neutral pH medium (Ntl, pH = 7.41, [HCO3-] = 25 nM) or in medium that modeled metabolic acidosis (Met, pH = 7.10, [HCO3-] = 12), each with or without calcitonin (CT, 3 x 10(-9) M). There was net calcium efflux in Ntl (JCa = 631 +/- 36 nmol per bone per 48 h), which increased in Met (1019 +/- 53, p < 0.01); CT inhibited JCa in Ntl (-54 +/- 11, p < 0.01 versus Ntl), which increased in Met (197 +/- 15, p < 0.01 versus Ntl + CT). In the presence of CT the increase in JCa in Met versus Ntl represents physiochemical bone dissolution. The Ntl bone surface (approximately 2 nm in depth) was rich in Na compared to Ca (Na/Ca = 11.9, count/s of detected secondary ions), which fell in Met (Na/Ca = 6.0, p < 0.05); CT caused a further reduction of Na/Ca (3.1, p < 0.01 versus Ntl and versus Met), which was not altered in Met (2.6, p < 0.05 versus Ntl + CT).(ABSTRACT TRUNCATED AT 250 WORDS)

SIMS studies of Ca and Mg distributions in sintered Al2O3

The addition of MgO to α-Al2O3 has been practiced for over 30 years in order to improve greatly the properties of the sintered material. However, a complete understanding of the role of MgO has not been achieved despite significant research efforts by various groups. The most difficult obstacle is microchemical characterization of MgO doped Al2O3. Surface analytical techniques, primarily AES and XPS, have been employed by past researchers in order to analyze the grain boundaries on fractured surfaces of sintered Al2O3. This prior work has met with limited success due to poor sensitivity and spatial resolution. MgO is believed to segregate to the grain boundaries and to retard the grain boundary mobility via a solute drag mechanism; this hypothesis has not been verified experimentally.In this study, the distribution of Ca and Mg was characterized by a high lateral resolution scanning ion microprobe (SIM) developed at The University of Chicago (UC).

Role of second-phase particles in the oxidation of Al-Li alloys

Oxidation behavior of Al-Li alloys, containing a high volume fraction of Ferich second-phase particles, was studied at 530°C and 200°C. Morphological studies showed enhanced growth of Li-rich oxides in the vicinity of the insoluble second-phase particles. Microanalytical depth profiling of the oxide layer with a high resolution scanning ion microprobe indicated rapid diffusion and subsequent oxidation of Li at the free surface. Examination of the alloy surface after removal of the oxide layer suggested that the initial oxide growth occurred in the lateral direction. Secondary ion image depth profiling of the alloy surface after oxide removal revealed Li segregation to the alloy/second-phase interface, supplying Li for accelerated oxidation. A microstructural model of the oxidation process in this case is presented.

Secondary ion imaging of the distribution of δ′ (Al 3Li) in Al-Li alloys

A high resolution scanning ion microprobe has been used, in conjunction with transmission electron microscopy, to study the distribution of the metastable δ' (Al 3Li) phase in Al-Li alloys. Various secondary ions were used for chemical imaging (lateral resolution ∼ 80 nm) and their contrast aspects and formation mechanisms are discussed. δ' particles appeared dark in the 27Al +, 7Li + and 34AlLi + distribution maps despite their higher Li content than the α matrix (aluminum-lithium solid-solution). Maps generated from two molecular ions, 14Li + 2 and 41Li 2Al +, exhibited true chemical contrast in the images: the δ' precipitates appeared bright in accordance with the known Li distribution. Using retrospective image analysis, measurements were made of the secondary ion intensities displayed by the δ' particles and the α matrix. Efforts to calibrate the secondary ion intensities for quantitative analysis are also described. The unusual ion formation mechanism from δ' precipitates illustrates the coupled channel model of secondary particle emission: the observed favored emission of Li 2 causes Li emission to be depressed.

Critical issues in the application of a gallium probe to high resolution secondary ion imaging

The use of liquid metal ion sources to form finely focused probes has enabled analytical imaging by secondary ion mass spectrometry to achieve, in favorable cases, spatial resolution approaching the theoretical localization limits of the method. As a drawback, however, the large intrinsic energy spread of such sources significantly constrains both the minimum size of the probe and the analytical sensitivity-analytical image resolution that are possible. In the context of these limitations, recent applications of a high resolution scanning ion microprobe to studies of Al-Li alloys, high T c superconductors and photoemulsion engineered microcrystals are discussed.

Imaging of Al-Li-Cu alloys with a Scanning Ion Microprobe

In contrast to the inability of x-ray microanalysis to detect Li, secondary ion mass spectrometry (SIMS) generates a very strong Li+ signal. The latter’s potential was recently exploited by Williams et al. in the study of binary Al-Li alloys. The present study of Al-Li-Cu was done using the high resolution scanning ion microprobe (SIM) at the University of Chicago (UC). The UC SIM employs a 40 keV, ∼70 nm diameter Ga+ probe extracted from a liquid Ga source, which is scanned over areas smaller than 160×160 μm2 using a 512×512 raster. During this experiment, the sample was held at 2 × 10-8 torr.In the Al-Li-Cu system, two phases of major importance are T1 and T2, with nominal compositions of Al2LiCu and Al6Li3Cu respectively. In commercial alloys, T1 develops a plate-like structure with a thickness &lt;∼2 nm and is therefore inaccessible to conventional microanalytical techniques. T2 is the equilibrium phase with apparent icosahedral symmetry and its presence is undesirable in industrial alloys.