Phillip Goodall

Slurry nebulization inductively coupled plasma spectrometry-the fundamental parameters discussed

Abstract Slurry nebulization inductively coupled plasma atomic emission spectrometry (ICP-AES) has been investigated to examine how the particle size distribution of the slurry affects analytical accuracy and precision. An empirical upper diameter for the particle size distribution of a slurry of 2–2.5 μm was derived from the analysis of a range of slurries possessing different particle size distributions. A model for slurry particle transport is suggested that gives good agreement with experimental data. The model assumes that for an arbitrarily defined but realistic standard slurry, the maximum allowable particle size is that which allows the occupation of every aerosol droplet by one solid particle. Theoretical considerations and empirical data suggest that for accurate analyses by slurry nebulization, the particle size distribution of the slurry should not exceed a value, determined by density, of 2.9 μm for a material of density 1 g cm−3 falling to 1.5 μm for a material of density 7 g cm−3. Certain carbonaceous materials were shown to be susceptible to micro-flocculation, resulting in 5–10 member assemblies formed from fine primary particles. This led to transport behaviour and analytical recoveries that would normally be associated with slurries of much coarser particle size. With the required conditions of complete dispersion and the correct particle size distribution, slurry nebulization ICP-AES was applied successfully to a range of certified reference materials using simple aqueous standards for calibration.

Ultra-trace determination of cadmium by vapour generation atomic fluorescence spectrometry

The vapour generation of cadmium by reaction in aqueous solution with sodium tetraethylborate was performed using a conventional continuous flow reactor. This vapour generation system was successfully interfaced with atomic absorption and fluorescence instrumentation. A detection limit, with fluorescence detection, of 20 ng dm–3(3σn–1) was obtained after optimization of the chemistry using simplex routines. Interferences from transition metal ions (e.g., NiII, CuII) were observed but were attenuated by the use of citrate as a masking agent. Vapour generation atomic fluorescence spectrometry was successfully applied to the determination of cadmium in potable waters, a National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 1643c Trace Elements in H2O, and Community Bureau of Reference (BCR) Certified Reference Materials (CRMs) 144 Sewage Sludge–domestic and 145 Sewage Sludge–industrial. Full recovery of cadmium spikes was obtained from UK drinking waters. A value of 12.6 ± 0.5 ng cm–3 of cadmium was obtained for NIST SRM 1643c (certificate value = 12.2 ± 0.1 ng cm–3 of cadmium) whilst the cadmium content of BCR 144 and 145 (certificate value 3.41 ± 0.25 and 18.0 ± 1.2 µg g–1, respectively) was estimated at 3.34 ± 0.15 µg g–1(BCR 144) and 18.24 ± 0.7 µg g–1(BCR 145).

Slurry atomization using hydrogen-modified inductively coupled plasmas

The use of molecular gas addition as a volatilization aid in slurry nebulization inductively coupled plasma atomic emission spectrometry (ICP-AES) has been investigated using a combination of rigorous optimization strategies and spectrochemical measurements. The addition of hydrogen resulted in an improvement in accuracy, which corresponded to the elimination of atomization interferences observed during the analysis of a highly refractory material by slurry nebulization ICP-AES. This ability to decompose refractory particles correlated with higher rotational plasma temperatures. This increase in temperature from 2200 to 3900 K was attributed to increased energy transfer from the toroidal to the annular region of the ICP as a consequence of the higher thermal conductivity of hydrogen compared with argon. This process appears to be self-limiting and a mechanism is proposed to account for this behaviour.

Isotopic uranium determination by inductively coupled plasma atomic emission spectrometry using conventional and laser ablation sample introduction

The use of inductively coupled plasma atomic emission spectrometry (ICP-AES) for the determination of 235U: 238U isotope ratios in U–Zr metal alloys (90% m/m U, 10% m/m Zr) is described. Conventional pneumatic nebulization and laser ablation sample introduction techniques were utilized. The results of the determination agreed, within experimental uncertainty, with isotope ratios determined by thermal ionization mass spectrometry (TIMS), e.g., 235U: 238U for conventional nebulization = 2.091 ± 0.006, for laser ablation = 2.092 ± 0.015 and for TIMS = 2.0940 ± 0.0004. The precision, i.e., the relative standard deviation (RSD), of the sequential determination of the intensities of the 235U and 238U components of the U emission line and the resultant isotope ratios was improved significantly by the use of an intrinsic internal standard (Zr), i.e., RSD = 1.6% to RSD = 0.17%.

Improved thallium hydride generation using continuous flow methodologies

The use of continous flow vapour generation to improve the determination of thallium by atomic spectrometry has been investigated. Attempts to generate volatile organothallium compounds were unsucessful but using sodium tetrahydroborate a volatile thallium species was produced. Evidence was obtained to suggest that the vapour phase species was thallium(I) hydride, TlH. At room temperature noisy signals were obtained, but when the reaction manifold was chilled to 0 °C the signals were far more stable. Cooling did not significantly change the sensitivity. A characteristic concentration of 4 ng cm–3 was obtained for continous flow vapour generation determination of thallium by atomic absorption spectrometry (AAS). This is a 1500-fold improvement on the previous sensitivity for batch vapour generation AAS. Continous vapour generation also appears to eliminate any serious interferences from other hydride-forming elements. Flow injection of nitric acid between samples was used to remove memory effects.

Thermochemical effects in hexafluoroethane (Freon 116) modified argon inductively coupled plasmas

Abstract Hexafluoroethane (Freon 116), added to the argon nebulizer gas, has been investigated as a potential volatilization aid for refractory materials in slurry nebulization inductively coupled plasma atomic emission spectrometry (ICP-AES). The introduction of Freon 116 resulted in apparently unusual plasma spectrochemistry, which yielded non-linear calibration curves. These were attributed to classical mass action buffering involving highly stable metal fluoride species. This dictated against the use of Freon 116 as a volatilization aid in slurry nebulization ICP-AES.

Laser ablation–inductively coupled plasma atomic emission spectrometry for the determination of lanthanides and uranium in fuel reconditioning materials: problems, solutions and implications

The successful determination of uranium, lanthanum, cerium, neodymium and yttrium by LA–ICP-AES is reported. LA was performed using a Q-switched Nd:YAG laser operating at 1064, 532 or 355 nm. Ablation at 355 nm was shown to offer optimum performance. The sample matrix consisted of a mixture of lithium and potassium chlorides which, when molten, form the electrolyte for an electro-refining process for the treatment of spent nuclear fuels. Analytical recoveries in the range 96–103% were obtained with a relative standard deviation of 1–3%, i.e., U = 100%, La = 100%, Ce = 96%, Nd = 97% and Y = 103%. The detection limit for uranium was 25 µg g–1 using the optimum LA conditions. The use of solid standards closely matching the sample material was found to be essential if accurate analyses were to be obtained. Calibration using uranium compounds other than the specific compound found in the sample was unsuccessful. For results of the highest quality, single phase materials were produced to allow for accurate internal standardization. Spectral interferences were minimized by the use of high resolution spectrometry. The reliability of the analyses was improved by the introduction of additional aerosol processing in the form of a Scott-type spray chamber between the ablation cell and the ICP torch.

Coincidence laser spectroscopy (CLS) for the detection of ions in ICP-MS (ICP-MS-CLS). A feasibility study

This paper reports a theoretical study of the feasibility of using laser-excited ionic fluorescence in time correlation with ion counting, termed coincidence laser spectroscopy (CLS), for improved specificity in the detection of ions in ICP-MS. The technique is here named ICP-MS-CLS. A number of factors are considered including: the preferred instrumental configuration, simulation of the performance of the optical detector and correlation step in reducing background, the spectroscopy of the selected of isotopes, 10Be+, 55Fe+, 63Ni+, 90Sr+, 99Tc+, 147Pm+, 238U+, 238Pu+ and 241Am+, which might be appropriate candidates for ICP-MS-CLS detection, the laser power required to attain saturation, the effects of ion energy and energy spread on pumping efficiency, the optical abundance sensitivity for adjacent isotopes of the same element, and the detection limits obtainable under a variety of scenarios. The ICP is established as an ideal ion source for elemental mass spectrometry, but as shown here, the ion energy spread produced is too large for optimum optical pumping because the ions are Doppler shifted to an extent that not all of them would be excited efficiently by a narrow-line laser source. This necessitates the inclusion of an ion cooler into the instrumental configuration so that ions maybe brought into resonance with the laser with 100% efficiency. The calculations show that for ions with simple spectra, such as 90Sr+ which can be repetitively pumped by the laser to produce a photon burst, ICP-MS-CLS can reduce the effect of very high backgrounds, 106 cps on mass and 1010 cps at adjacent mass, to low levels and improve detection limits by 2–3 orders of magnitude compared with the normal technique. Optical abundances of 10−5–10−9 are achievable which, combined with the mass abundance sensitivity of 10−5, yields overall abundance sensitivities of 10−10–10−14. This is of the same order as techniques such as accelerator mass spectrometry (AMS) or resonance ionisation mass spectrometry (RIMS). The technique is much less efficient for ions that undergo optical trapping and emit only one photon when pumped and/or exhibit hyperfine structure which distributes the oscillator strength over several hyperfine components. These factors significantly degrade performance and indicate a requirement for further refinement in terms of using two-colour excitation, or quenching of meta-stable levels, to enable the recycling of ions for further pumping.

Approach to the determination of lead by vapour generation atomic absorption spectrometry

A new method for the determination of lead by vapour generation atomic spectrometry is described. This is based upon continuous flow methodology and involves the derivatization of lead, in the presence of an oxidant, with sodium tetraethylborate to yield volatile tetraethyllead. Simplex optimization was applied to this system and to conventional hydride generation. Characteristic concentrations of 0.36 ng cm–3(Pb 283 nm) and 0.145 ng cm–3(Pb 217 nm) for alkyl generation atomic absorption spectrometry were observed. These are approximately 4–5 times better than the equivalent hydride system and superior to all reliable literature values for characteristic concentrations of lead hydride generation atomic absorption spectrometry. Using the 283 nm lead line, limits of detection (3s) obtained for the hydride and alkyl generation systems were 0.62 and 0.07 ng cm–3, respectively. The determination of lead in a standard reference water (NIST SRM 1643c) is reported and good agreement obtained between the certificate value (35.3 ± 0.9 ng cm–3) and the experimentally derived value (34.8 ± 1.4 ng cm–3).

Inductively coupled plasma mass spectrometry, coincidence laser spectroscopy (ICP-MS-CLS), simulation of the transmission efficiency of a 3D quadrupole ion trap for cooling energetic ions from the ICP prior to optical detection

A fundamental requirement of ICP-MS-CLS is a post mass separation ion beam with an energy spread of less than ∼0.2 eV to enable efficient pumping of fluorescent lines with a narrow band (Δν ∼ 100 kHz) laser for the optical detection step. The role of a 3D quadrupole ion trap for ion beam cooling, as an accessory for a new generation of ICP-MS-CLS (inductively coupled plasma mass spectrometry, coincidence laser spectroscopy) (B. L. Sharp, P. S. Goodall, L. M. Ignjatovic and H. Teng, J. Anal. At. Spectrom., 2007, 22, 1447) instrument, was investigated by means of numerical simulation. Whereas in-trap ion cooling is well established, it was found that extraction of cooled ions from the trap introduced additional broadening and a loss of transmission efficiency. Modelling was based on a beam of 20 eV mean energy with a spread of 3 eV which represents a worst case scenario for the ions derived from an ICP source. The efficiency of the cooling process (trapping + extraction), as well as the energy distributions of ions exiting the trap, were calculated and compared for several methods of extraction. Trapping efficiencies of ∼25% were obtained at buffer gas pressures up to 10 mTorr. The addition of virtual reflectrons to the trap resulted in an improvement in the energy distribution of the trapped ions by lowering the wings of the distribution, but did not significantly improve the efficiency. Zero field, rf field only, quadrupolar field and homogeneous field ion extraction were investigated to recover ions from the trap, and of these quadrupolar extraction was best. However, extraction efficiency and ion energy distribution were inversely related so that quadrupolar extraction at Uq = 10 V yielded an exit ion energy distribution of 0.5 eV half width, compared with a trapped width of 0.03 eV, but with only 10% transmission efficiency. Thus, it was demonstrated that a 3D trap can be used to cool energetic ions for post-trap mass or spectroscopic examination, but the low efficiency makes it unsuitable for use with ICP-MS-CLS.