John G. Williams

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS): a rapid technique for the direct, quantitative determination of major, trace and rare-earth elements in geological samples

Abstract A Nd:YAG laser in fixed-Q (free-running) mode coupled to an inductively coupled plasma mass spectrometer was used to directly analyse pressed powder pellets of seven well-characterised silicate rock reference materials (AGV-1, BIR-1, DNC-1, G-2, NIST 2704, SCo-1 and W-2). The multielement limits of detection (LOD) were in the range 0.05–13 μ g−1 but with a majority of values better than 0.1 μ g−1. Using a narrow range scan LOD for the rare-earth elements (REEs) were between 11–84 ng g−1. Relative responses for the major elements indicate that the chemistry and mineralogy of individual rock samples influence the ablation behaviour, and that samples with very similar chemical and mineralogical compositions exhibit similar elemental responses. The accuracy of major-element determinations, for the materials studied, was generally better than ± 5% with a precision of 10% RSD. First-row transition elements, incompatible elements and, in particular, REEs displayed a high degree of accuracy, with precision of generally

Langmuir probe potential measurements in the plasma and their correlation with mass spectral characteristics in inductively coupled plasma mass spectrometry

A floating Langmuir probe is used to measure the apparent d.c. potential in an inductively coupled plasma (ICP) while the latter is used as an ion source for mass spectrometry (MS). The probe is swung through the plasma to provide potential measurements with some spatial resolution and to obviate cooling of the probe. The d.c. potential in the centre of the plasma is influenced by the presence of the metal sampling cone in the plasma and also by the gas flow through the orifice. In general, the potential correlates with the characteristics of the mass spectra; these parameters depend in a sensitive fashion upon the manner by which the load coil is grounded and shielded. For the load coil geometries investigated in this study, the potential and ion energies generally increase with aerosol gas flow-rate and decrease as power increases. As potential increases the abundance of doubly charged ions generally increases, ArO+/Co+ and Ar2+/Co+ decrease and CeO+ decreases slightly. The measured potential in front of the sampling orifice is generally a few volts below the mean ion energy, which indicates that both measurements are reasonable approximations to the actual d.c. plasma potential.

System optimisation and the effect on polyatomic, oxide and doubly charged ion response of a commercial inductively coupled plasma mass spectrometry instrument

The practical problems of setting up and operating a commercial inductively coupled plasma mass spectrometry (ICP-MS) instrument for routine analysis are discussed and the effect of choice of operating parameters on performance described. With the instrument used, a VG PlasmaQuad, the major parameter variables, plasma power and gas flows and the aperture-load coil spacing are normally kept constant for routine use. Under these conditions a sufficiently uniform response to a range of elements of different chemistries and atomic mass is obtained for the operator to use the instrument without repeated optimisation throughout the working day even though a variety of different sample types and matrices may be run. At fixed plasma power and spacing the only other critical parameter is the carrier gas flow-rate which is adequately stabilised in the instrument used by a precision mechanical flow regulator. The effect upon performance of variation of this flow-rate is shown for elemental response, doubly charged ion ratio and polyatomic ions and it is demonstrated that setting up for maximum elemental response minimises the effect of these interfering species. In practice carrier gas flow-rate is also normally fixed and setting up the instrument consists only of minor trimming of the ion-lens potentials for optimum response which is usually carried out when starting up for a days running. Operated in this way the instrument is used for routine analysis by a variety of users on a wide range of matrices. An indication is given of the performance obtained under these conditions.

Inductively coupled plasma mass spectrometric determination of the absorption of iron in normal women

The determination of iron isotope ratios in blood, without prior sample preparation, using inductively coupled plasma mass spectrometry (ICP-MS) with sample introduction by electrothermal vaporisation (ETV) is described. Following oral administration of 5 mg of enriched 54FeSO4 and intravenous administration of 200 micrograms of 57FeSO4 to non-pregnant women, the 54Fe: 56Fe and 57Fe: 56Fe isotope ratios in serum were measured reliably within 20 min per sample in quintuplicate. Changes in the fractional absorption of iron during human pregnancy could therefore be assessed.

Iron absorption during normal human pregnancy : a study using stable isotopes

The absorption of iron has been determined in nine healthy women studied serially during pregnancy and once post delivery. Following the oral administration of 5 mg aqueous 54FeSO4 plus ascorbic acid and the intravenous injection of 200 micrograms 57FeSO4, the isotope ratios of 54Fe: 56Fe and 57Fe: 56Fe in serum were measured by the use of inductively-coupled-plasma mass spectrometry whereby metal ions are vaporized into an argon plasma without previous blood sample preparation. Mean oral Fe absorption was 7.6 (range 1-22)% at 12 weeks gestation, 21.1 (range 9-58)% at 24 weeks, 37.4 (range 18-56)% at 36 weeks and 26.3 (range 8-54)% at 12 weeks post delivery. All the other biochemical and haematological indices were within normal limits for pregnancy. The significant increase (P less than 0.01) in Fe absorption during normal pregnancy suggests that most women would have the potential to meet the Fe demands of pregnancy without the need for supplementation if dietary Fe has similar availability to the aqueous preparation.

Preliminary assessment of laser ablation inductively coupled plasma mass spectrometry for quantitative multi-element determination in silicates

A Nd:YAG (yttrium aluminium garnet) laser was used to ablate pressed powder pellets of seven silicate rock reference materials for sample introduction into an inductively coupled plasma mass spectrometer. Laser operating parameters, such as mode (fixed-Q or Q-switch), energy and number of shots per site were optimized to meet the criterion of maximum analyte signal without excessive loading of the plasma with ablated material. To compensate for differential laser sampling (i.e., variable amounts of material being removed during each analysis)55Mn was used as an internal standard for multi-element determinations and 137Ba for the rare earth elements (REE), Hf, Ta and W. Relative responses for the major elements indicate that the chemistry and mineralogy of individual rock samples influence the ablation behaviour and that samples with very similar chemical and mineralogical compositions exhibit similar elemental sensitivities. Alkali, alkaline earth and hydride-forming elements also show similar behaviour to the major components. Multi-element detection limits were typically less than a few hundred ng g–1. The accuracy of major element determinations, for the materials studied, was generally better than ±5% relative with a precision of 10% relative standard deviation (RSD). Trace elements in Groups I, II, III, REE, volatile elements or those which exhibit refractory characteristic, displayed good accuracy, with precision of generally <10% RSD. The quantitative determination of major and trace elements in silicate rocks is therefore possible, providing that standards and samples are closely matched both in terms of bulk chemistry and physical (mineralogical) composition.

Determination of trace and ultra-trace elements in saline waters by inductively coupled plasma mass spectrometry after off-line chromatographic separation and preconcentration

A method for the determination of trace and ultra-trace elements in sea-water by ICP-MS, after preconcentration and matrix removal using a commercially available ion-chelation system, was developed and optimized. Cellulose-immobilized ethylenediaminetriacetic acid was used as the column material and elution profiles were determined with an on-line configuration. Recovery experiments were carried out, firstly in a simple matrix (spiked de-ionized water) and secondly in a complex matrix (synthetic sea-water) employing a preconcentration factor of either 10 or 100. In both types of matrix, recoveries of between 90 and 110% were obtained for Cd, Co, Cu, Ni, Pb, U, Y and the 14 REEs. Excess recoveries were obtained for Zn and are thought to result from reagent contamination. Reproducibility was in the range 0.20–7.25% coefficient of variation. Optimized conditions were used to analyse National Research Council of Canada CRMs, Estuarine Water SLEW-1, Open Ocean Sea-water NASS-4 and Nearshore Sea-water CASS-2. Accuracy and precision are related to concentration. Cd, Co, Cu and Ni show good accuracy for concentrations greater than 1 ng ml–1. Both short term instrumental precision and between sample reproducibility are typically better than 10% RSD. Although the rare earth elements are not certified in any of the reference materials, a relatively smooth sea-water normalized plot is seen for SLEW-1.

Noise in inductively coupled plasma mass spectrometry: some preliminary measurements

Fundamental limiting noise sources in a non-commercial inductively coupled plasma mass spectrometer were investigated, using colour photographic evidence and signal processing which is manipulated via computer software. Noise sources were found to be 50 Hz from the mains power supply, peristaltic pump rotation, plasma audiofrequency peaks and their harmonics, and 1/f noise (which is mainly caused by the sample introduction system). These noise sources adversely affect the precision of inductively coupled plasma mass spectrometry (ICP-MS) for a given measurement bandwidth. To reduce their contributions in the noise power spectra (NPS) of the signal, the use of a bonnet device was investigated. This reduced the 1/f noise and peristaltic pump rotation frequency and completely removed the plasma audio frequency peak in the NPS. This latter frequency has been observed by a number of workers in ICP emission spectrometry and is associated with instability at the boundary of the plasma, where it enters the surrounding atmosphere. This audiofrequency peak has been removed in emission spectrometry by using an extended torch arrangement. In this present work the position of the plasma audiofrequency peak in the NPS changed with sampling distance, radiofrequency power and coolant gas flow rate and the peak was removed by the use of a torch extension bonnet.

Communication. Noise sources in inductively coupled plasma mass spectrometry: an investigation of their importance to the precision of isotope ratio measurements

It has long been accepted that the precision of isotope ratio measurements by inductively coupled plasma mass spectrometry (ICP-MS) is poorer than consideration of counting statistics would suggest is possible. This has been attributed to the coupling of a scanning mass analyser to a noisy ICP ion source, the sample introduction system and the ion extraction method. Thus, the ion output from the source varies as the analyser scans between successive masses so that their ratios vary. A variety of noise sources have been identified, mainly from noise power frequency spectra, but the sensitivity of these spectra as diagnostic tools is limited by the statistics of the signals fed to the analysing software, and it becomes necessary to extend the investigation by using isotopic ratio precision as the ultimate criterion. Even when the effect of (previously identified) major sources are greatly reduced, poor precision is still obtained. In this study the effect of poor ion distribution in the plasma and ICP-MS interface is identified as the principal cause and by avoiding this, precision values are found which agree with statistically expected values.

Critical assessment of the effects of skimmer cone geometry on spectroscopic and non-spectroscopic interference in inductively coupled plasma mass spectrometry

Three different designs for skimmer cones, mounted in the interface region of an inductively coupled plasma mass spectrometric (ICP-MS) instrument, were evaluated with respect to their effect on analyte sensitivity, the magnitude of polyatomic ions, refractory oxide ions, doubly charged ions and background and non-spectroscopic matrix effects. Initial results for one particular design, termed ‘PolySkim’, conducted on both a standard commercial and the Surrey design ICP-MS instruments, show a significant reduction in the size of the ArO+ and other polyatomic peaks, with no corresponding deterioration of detection limits across the mass range, although a decrease in sensitivity did occur.