William T. Perkins

Laser ablation inductively coupled plasma mass spectrometry: A new technique for the determination of trace and ultra-trace elements in silicates

Abstract This paper describes recent work applying a laser ablation system coupled to an inductively coupled plasma mass spectrometer (LA-ICP-MS) for the direct analysis of solid geological materials. This work demonstrates the potential of LA-ICP-MS for the determination of a wide range of petrogenetically important trace and ultra-trace elements (including for example REE, Hf, Ta, Nb, Th, U) following a routine method of sample preparation. Powdered geological materials have been prepared as both pressed powder disks and fused glasses; both common methods of sample preparation for X-ray fluorescence (XRF) analysis. The solid materials were sampled by ablation using a pulsed Nd:YAG laser operating at 1064 nm. Analyses can be produced at approximately 10 samples per hour. This instrumental method has limits of detection at or close to those in chondritic meteorites and gives linear calibrations over four orders of magnitude. The accuracy of the technique has been evaluated using reference materials to calibrate the instrument and treating Geological Survey of Japan basalts JB-1a, JB-2, and JB-3 as ‘unknowns.’ Detection limits are better than routine XRF analysis and compare favourably with Instrumental Neutron Activation Analysis. Laser ablation overcomes the problems of sample dissolution employed in standard wet chemical techniques, whilst the fused glasses provide homogeneous solid samples. The fused glass technique has been applied to a wide range of reference materials from ultra-basic rocks through basalts and andesites to granites, as well as syenite, mica schist, and black shale. For all of the elements commonly used to generate multi-element discrimination diagrams the data obtained define straight line calibrations. This method is therefore capable of analysing the complete range of silicate compositions normally encountered with a single calibration (i.e., there is no apparent matrix effect).

Possible interactions between bacterial diversity, microbial activity and supraglacial hydrology of cryoconite holes in Svalbard

The diversity of highly active bacterial communities in cryoconite holes on three Arctic glaciers in Svalbard was investigated using terminal restriction fragment length polymorphism (T-RFLP) of the 16S rRNA locus. Construction and sequencing of clone libraries allowed several members of these communities to be identified, with Proteobacteria being the dominant one, followed by Cyanobacteria and Bacteroidetes. T-RFLP data revealed significantly different communities in holes on the (cold) valley glacier Austre Brøggerbreen relative to two adjacent (polythermal) valley glaciers, Midtre Lovénbreen and Vestre Brøggerbreen. These population compositions correlate with differences in organic matter content, temperature and the metabolic activity of microbial communities concerned. No within-glacier spatial patterns were observed in the communities identified over the 2-year period and with the 1 km-spaced sampling. We infer that surface hydrology is an important factor in the development of cryoconite bacterial communities.

Quantitative analysis of trace elements in carbonates using laser ablation inductively coupled plasma mass spectrometry

Laser ablation inductively coupled plasma mass spectrometry has been applied to the analysis of carbonate materials. Multi-element synthetic standards, prepared both as pressed powders and fused glass discs, were used for calibration. The elements Mg, Mn, Sr, Ba and Pb were added to the pressed powder standards and these elements, together with U, were added to the fused glass standards. Calibration graphs, which are linear over at least three orders of magnitude, were produced using both types of standard but the fused glass discs gave better precision. The accuracy of the technique was evaluated using reference materials. Acceptable values were obtained using the pressed powders [e.g. BCS 393 (limestone) certified values of 905 ppm (Mg), 77 ppm (Mn) and 160 ppm (Sr)], but better accuracy was achieved with fused glass discs [e.g., BCS 393; 957 ppm (Mg), 79.6 ppm (Mn) and 167 ppm (Sr)]. The technique is applied to the analysis of carbonate shell material and demonstrates its potential in environmental monitoring.

Geochemistry of Santorini tephra in lake sediments from Southwest Turkey

A layer of volcanic ash (4 cm maximum recorded thickness) is present at ∼250 cm depth in littoral sediments of Golhisar Golu; a small intramontane lake in Southwest Turkey. The major-element glass chemistry, determined by electron microprobe, is characterised by a rhyolitic composition. Trace-element data, determined by solution inductively coupled plasma mass spectrometry (icp-ms) match previously reported results for proximal pumice samples from Santorini and Crete. Peat immediately below the tephra deposit has yielded radiocarbon ages of 3330±70 yr bp and 3225±45 yr bp (calibrated age range 1749–1406 bc).

Chemical fractionation during infrared and ultraviolet laser ablation inductively coupled plasma mass spectrometry—implications for mineral microanalysis

The silicate glass standard reference material NIST SRM 610 has been repeatedly analysed by UV and IR laser ablation ICP-MS in procedures designed to mimic typical analytical procedures adopted during mineral analysis. Trace element fractionation during both IR and UV laser ablation is observed and relationships between fractionation trends and ionic radius, charge and melting temperature of the elements determined are defined and discussed. In particular the high field strength elements (e.g., Nb, Ta, Zr, Hf) all show decreasing fractionation trends with respect to Si in the NIST SRM 610 glass during repeated laser ablation, whilst the low field strength and large ion lithophile elements (e.g., Rb, Sr, Pb, Ba, Ca) all show increasing trends with respect to Si. For IR laser ablation the degree of fractionation observed in the analysis of the REEs is strongly correlated to their ionic radii. In all the analytical procedures studied, fractionation arising from IR laser ablation is considerably greater than UV laser ablation and simple changes to analytical procedure to reduce trace element fractionation are suggested. For UV laser ablation, the effect of laser focus on analytical precision is assessed. Active focussing of the laser during ablation under computer control significantly improves analytical precision.

The application of laser ablation ICP-MS to the analysis of volcanic glass shards from tephra deposits: bulk glass and single shard analysis

Abstract Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been applied to the chemical analysis of fine-grained (125–250 μm) volcanic glass shards separated from tephra deposits. This has been used both for bulk sample analysis and for the analysis of individual shards. Initial work concentrated on the use of an infra-red (IR) laser operating at 1064 nm, which gave craters of the order of 200 μm and was suitable for the analysis of bulk samples. This technique requires of the order of 80 μg of sample to determine a full suite of trace elements. Modification of the laser optics to enable operation in the ultra-violet (UV, at 266 nm) enables craters between 5 and 50 μm diameter to be produced, and the UV laser couples better with glass than the IR laser. We have applied this UV laser system to the analysis of single shards from Miocene tephra deposits from the Ruby Range in south-west Montana. Detection limits are below 1 ppm for a wide range of petrogenetically significant elements, but are critically dependent upon operating conditions. Calibration is achieved using synthetic multi-element glasses, with internal standardisation provided from electron probe analyses. Analysis of single shards provides a wide range of data from a single sample, enabling (i) magmatic evolution to be discerned within one eruption and (ii) the identification of separate populations of shards within one deposit which may not be apparent from the electron probe data. In this paper we will present a summary of the techniques used for both bulk sample and single shard analysis and compare some new bulk analyses with analyses of glass derived from other analytical methods.

Communication. Mineral microanalysis by laser ablation inductively coupled plasma mass spectrometry

The use of a modified Nd:YAG laser coupled to an inductively coupled plasma mass spectrometer is described for the direct analysis of carbonates, zircon, olivine and feldspars to give a variety of major and trace element data. The modification to the laser produces Q-switched ablation craters with a diameter between 20 and 30 µm compared with 150–200 µm in the un-modified version. This is suitable for direct mineral analysis in rocks. Calibration is achieved by ratioing analyte peak intensities to an internal standard and concentrations are produced by comparison with reference materials. The selection of internal standards is discussed for each mineral type and is dependant upon the assumption that the element chosen is present at a fixed molecular proportion in the mineral, despite other compositional changes. Spatial resolution of micro-analysis by laser ablation inductively coupled plasma mass spectrometry (ICP-MS) at (20–30 µm) is approximately ten times that of an electron probe micro-analysis (EPMA, 2–5 µm) but has the advantage of much lower detection limits and wider dynamic range (from ppm to 10–100% m/m) than routine EPMA analysis. Laser ablation ICP-MS can also offer isotopic information.

Migration of heavy metals in soil as influenced by compost amendments

Soils contaminated with heavy metals can pose a major risk to freshwaters and food chains. In this study, the success of organic and inorganic intervention strategies to alleviate toxicity in a highly acidic soil heavily contaminated with As, Cu, Pb, and Zn was evaluated over 112 d in a mesocosm trial. Amelioration of metal toxicity was assessed by measuring changes in soil solution chemistry, metal leaching, plant growth, and foliar metal accumulation. Either green waste- or MSW-derived composts increased plant yield and rooting depth, reduced plant metal uptake, and raised the pH and nutrient status of the soil. We conclude that composts are well suited for promoting the re-vegetation of contaminated sites; however, care must be taken to ensure that very short-term leaching pulses of heavy metals induced by compost amendment are not of sufficient magnitude to cause contamination of the wider environment.

Comparisons of infrared and ultraviolet laser probe microanalysis inductively coupled plasma mass spectrometry in mineral analysis

The application of an infrared and an ultraviolet laser probe microanalysis ablation system, coupled to an inductively coupled plasma mass spectrometer is described for the analysis of olivine, garnet, phlogopite, magnetite, apatite, calcite, quartz and feldspar. These minerals represent a range of composition, absorption characteristics and physical properties (for example cleavage) and were prepared both as cut blocks and thin sections. The analytical performance of the two laser probe systems is compared for each mineral type in terms of lateral and vertical resolution, duration of ablation, and the importance of mineral orientation. Lower limits of detection are presented for the two laser systems. Lateral resolution in the ultraviolet system is a function of laser power and can be varied from less than 5 to >70 µm. For the infrared system, laser power is essentially fixed and lateral resolution is a function of mineral type and sample preparation. Mineral orientation is an important factor in analysis using the infrared system but for the ultraviolet system this is of no significance. For the ultraviolet laser system, lower limits of detection are dependent on the amount of material ablated which is a function of laser power density and range from hundreds to thousands of ppm at low power to sub-ppm levels at high power. The removal of material by the ultraviolet laser probe system is facilitated by the strong absorption in the ultraviolet region of the electromagnetic spectrum that all the minerals in this study possess. The ultraviolet laserprobe system provides a superior analytical performance for mineral microanalysis when compared with the infrared system.

Trace-element analysis of volcanic glass shards by laser ablation inductively coupled plasma mass spectrometry: application to tephrochronological studies

Abstract The trace-element composition of fine-grained (0.25-0.125 mm) volcanic glass shards in distal tephra beds has been determined using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Accuracy is within 15% for most elements and the precision, as defined by the mean relative standard deviation, is better than 20%. This is very acceptable for tephrochronological studies. The very small amount of glass ablated (∼80 μg) and the short time involved in a single analysis (∼2 min) means that contamination can be easily recognized in the presence of replicate analyses and a detailed definition of trace-element compositional variability of the glass can be obtained. The need to use another trace-element analytical method to obtain the internal standard, Ce in this study, is a disadvantage but preliminary work in progress suggests that 57Fe can be used for this purpose. This would obviate the need to use another trace-element method because the major-element composition of glass shards, as determined by electron microprobe analysis, constitutes a routine part of tephra characterization studies.