Gregory C. Turk

Absolute silicon molar mass measurements, the Avogadro constant and the redefinition of the kilogram

The results of an absolute silicon molar mass determination of two independent sets of samples from the highly 28Si-enriched crystal (AVO28) produced by the International Avogadro Coordination are presented and compared with results published by the Physikalisch-Technische Bundesanstalt (PTB, Germany), the National Research Council (NRC, Canada) and the National Metrology Institute of Japan (NMIJ, Japan). This study developed and describes significant changes to the published protocols for producing absolute silicon isotope ratios. The measurements were made at very high resolution on a multi-collector inductively coupled plasma mass spectrometer using tetramethylammonium hydroxide (TMAH) to dissolve and dilute all samples. The various changes in the measurement protocol and the use of TMAH resulted in significant improvements to the silicon isotope ratio precision over previously reported measurements and in particular, the robustness of the 29Si/30Si ratio of the AVO28 material. These new results suggest that a limited isotopic variability is present in the AVO28 material. The presence of this variability is at present singular and therefore its significance is not well understood. Fortunately, its magnitude is small enough so as to have an insignificant effect on the overall uncertainty of an Avogadro constant derived from the average molar mass of all four AVO28 silicon samples measured in this study. The NIST results confirm the AVO28 molar mass values reported by PTB and NMIJ and confirm that the virtual element–isotope dilution mass spectrometry approach to calibrated absolute isotope ratio measurements developed by PTB is capable of very high precision as well as accuracy. The Avogadro constant NA and derived Planck constant h based on these measurements, together with their associated standard uncertainties, are 6.02214076(19) × 1023 mol−1 and 6.62607017(21) × 10−34 Js, respectively.

Consideration and influence of complexed forms of mercury species on the reactivity patterns determined by speciated isotope dilution model approaches: A case for natural biological reference materials

The processes driving the inadvertent transformations of inorganic mercury (iHg) and methylmercury (MeHg) in cryogenically homogenized fresh-frozen (FF) and freeze-dried (FD) biological standard reference materials (SRM) were investigated using alkaline digestion, derivatization and gas chromatography inductively coupled plasma mass spectrometry (GC-ICP-MS). Labile inorganic mercury (201iHg) and methylmercury (Me202Hg) isotopic standards and their cysteine-complexed analogs (201Hg(Cys)2 and Me202HgCys) were used in a double-spike speciated isotope dilution (SID) model to document the influence of complexing ligands on the equilibration, the reactivity and the transformation processes between isotopic mercury species and their endogenous analogs. Cysteine-complexed and labile Me202Hg spiking standards displayed similar equilibration processes in both classes of materials, leading to accurate MeHg determinations with negligible methylation transformations. Labile 201iHg standards provided accurate iHg concentration results in both material series, although an apparent demethylation reaction specific of FF materials was detected, with a negligible effect in FD materials. Cysteine-complexed 201iHg standards led to higher but inaccurate iHg concentrations and higher demethylation yields in both materials. This comparison illustrated a significant influence of complexing ligands on the equilibration processes between labile and/or complexed 201iHg species and their endogenous analogs. The derivatization step was found to catalyze these non-equilibrium conditions with different derivatization yields between labile and complexed iHg species, whereas no differences were observed for MeHg species. When complexation disparities existed in solution between 201iHg species and their endogenous analogs, this process specifically affected the determination of 200/201iHg ratios relative to 200/202iHg, 200/201MeHg and 200/202iHg ratios, which are all involved in the double-spike SID model to establish transformation yields and mercury species concentrations. This process and the influence of demethylating agents were presumably responsible for the apparent demethylation reaction only observed in FF materials. This result gave rise to several questions on the influence of freeze-drying procedures on the lability/complexation patterns of mercury species and/or on the activity of demethylating agents. A complementary analytical step consisting of maximizing the equilibrium conditions between isotopic and endogenous mercury species together with inhibiting the activity of demethylating agent is proposed. When applying these conditions, the two classes of materials displayed negligible transformation reactions, improving their commutability for the simultaneous determination of iHg and MeHg. The approach proposed in this work allowed for the first time documentation of the potential and limitations of speciated isotope dilution procedures to handle the problem of lability/complexation and ligand interactions in natural biological matrices when considering multiple-spike SID approaches.

Improved Calibration Strategy for Measurement of Trace Elements in Biological and Clinical Whole Blood Reference Materials via Collision-Cell Inductively Coupled Plasma Mass Spectrometry

A multi-element quantification strategy based on the method of standard additions incorporating internal standards and collision cell inductively coupled plasma mass spectrometry (ICP-MS) is presented. Approaches to experimental design are discussed in the context of streamlining analytical measurement protocols employing ratio-based standard additions quantification schemes for the certification of multiple elements in Certified Reference Materials, including reduction of the number of required analytical samples and measurement of analytes and internal standards at alternate quadrupole mass resolution settings. This strategy was implemented for the measurement of As, Se, Fe, Mn, Rb, Cu, and Zn levels in a candidate fish tissue NIST Standard Reference Material and measurement of Cd and Pb in two clinical, whole blood Certified Reference Materials. A simple approach to calculating analytical uncertainties for concentration data, as determined using standard additions calibrations, is presented which utilizes regression and prediction uncertainties and quotient propagation of error formulae.

Comparison of clinical methods with isotope dilution inductively coupled plasma mass spectrometry for the new standard reference material 955c lead in caprine blood

The National Institute of Standards and Technology (NIST) has developed Standard Reference Material (SRM) 955c, a new caprine-based, four-level blood standard with certified blood lead levels (BLLs) ranging from 0.4 µg/dL (0.02 µmol/L) Pb to 45 µg/dL (2.2 µmol/L) Pb. Certified values are based on ID-ICP-MS. Strict control and accurate measurement of the procedure blank were necessary to minimize uncertainty for the lowest level (Level 1) and obtain a relative expanded uncertainty (k = 2) of 2.6%. Level 1 is intended to represent a baseline BLL and provides a means to define detection levels and validate methods developed to measure lead at background environmental levels in blood. Level 2 is near 10 µg/dL (0.48 µmol/L), the current threshold defined by the U.S. Centers for Disease Control and Prevention (CDC) for public health action and clinical follow up. The standard has been developed in collaboration with the Wadsworth Center, New York State Department of Health, which provided measurements based on GFAAS and ICP-MS. Results from these clinical methods are statistically compared to the Isotope Dilution (ID) values. The Level 1 SRM 955c standard is below the detection limit of the GFAAS method, but comparison of Level 1 results for the ICP-MS method with the ID-ICP-MS values shows no evidence of statistically significant disagreement, suggesting that the ICP-MS method appears capable of measuring BLLs at background concentrations. Comparison of both GFAAS and ICP-MS results with the ID ICP-MS values for the Level 2 through Level 4 SRM 955c standards similarly shows no statistically significant disagreement between methods at these elevated blood Pb levels. In addition to the SRM 955c data, long-term method performance data are presented for SRM 955b Lead in Bovine Blood and for SRM 966 Toxic Elements in Bovine Blood. The clinical methods performed well within CLIA guidelines for these materials; however, a small negative bias for the ICP-MS value relative to the certified value for SRM 966 requires further investigation.

Using instrumental techniques to increase the accuracy of the gravimetric determination of sulfate

A gravimetric method for the determination of sulfate in a sulfate solution standard by the precipitation of barium sulfate is coupled with the instrumental determination of trace sulfate and precipitate contaminants to improve the accuracy and precision of the analysis. Sulfate in a solution of potassium sulfate is separated by a reverse precipitation with barium chloride in very dilute hydrochloric acid. Coulometry, ICP-MS, and flame atomic emission spectrometry (FAES) are used to quantify the level of the contamination in the barium sulfate precipitate and the solubility loss of sulfate in the filtrate, from which correction factors are calculated. Coprecipitating contaminants contribute about 0.3% to the total precipitate mass, while the analyte lost to the filtrate contributes 0.4%. Despite the poorer precision and accuracy of instrumental methods, the over-all precision and accuracy of the sulfate determination is actually improved, since the instrumental methods are used to determine only a very small part of the analyte. The expanded uncertainty (k= 2) of the method is below 0.2% relative to the precipitate mass.

Thermal lensing spectrophotometry of uranium (VI) with pulsed laser excitation

A double-beam thermal lensing experiment based upon a pulsed dye laser as the excitation beam and a He-Ne laser as a probe beam is described. The technique has been used to detect the small absorptions given by uranyl ions in aqueous solution. The system is capable of detecting absorption coefficients as low as 5 × 10–7 cm–1. Moreover, by changing several dyes, an absorption spectrum can be obtained at concentration levels (ca. 4 × 10–6M) far below (ca. 103×) those obtained by conventional absorption spectrophotometry, thereby allowing the direct study of the chemical equilibria involved. The sensitivity of the present apparatus is limited by short-term fluctuations of the probe laser.

Three-Dimensional Atomic Spectra in Flames Using Stepwise Excitation Laser-Enhanced Ionization Spectroscopy

Stepwise excitation laser-enhanced ionization spectroscopy utilizes two independently tunable dye lasers to populate high-lying excited states of atoms in flames. Two atomic resonances are required, with the upper level of the first-step transition coinciding with the lower level of the second-step transition. Efficient population of a high-lying atomic level is achieved, from which a high rate of collisional ionization can take place. The double-resonance aspect of such excitation adds an extra dimension of spectroscopic selectivity to the measurement. A computer-controlled dual-wavelength LEI spectrometer, including a Fizeau wave-meter for wavelength verification, is used to record three-dimensional spectra–ionization signal as a function of both first- and second-step wavelengths. Examples illustrate the accuracy advantage accorded by the three-dimensional survey.

Laser-Induced Ionization of Atoms in a Power-Modulated Inductively Coupled Plasma

Laser-induced ionization of atoms has been detected in a power-modulated inductively coupled plasma. The measurement is made 1.4 ms after complete interruption of the 40-MHz power to a 400-W plasma. Electrical conductivity measurements between probe electrodes in the plasma during the power-off cycle have been made, demonstrating the decay in plasma background ion/electron concentrations which makes detection of laser-induced ionization possible. Radio-frequency interference from the ICP on the ionization detection electronics is also avoided by this approach. The primary mode of laser-induced ionization was photoionization of the laser-excited atoms, i.e., resonance ionization spectroscopy (RIS). Detection limits of 80 µg Fe/L and 20 µg Ga/L were achieved.

Simultaneous detection of laser-enhanced ionization and laser-induced fluorescence in flames: noise correlation studies

Abstract Random signal fluctuations of simultaneously detected laser-enhanced ionization (LEI) and laser-induced fluorescence (LIF) were investigated and analyzed for correlation with each other and with laser power fluctuations. A greater degree of noise correlation was found between LEI and LIF than between either technique and laser power fluctuations. A background correction method is proposed which subtracts the LIF signal of the background-inducing interfering element from the combined LEI signals from analyte and interferant. Noise correlation between LEI and LIF can be utilized to cancel the noise in the interfering background signal. Broadband spectral interference due to wing excitation of sodium LEI was used as an example to demonstrate the method. Excitation spectra and laser power dependencies of sodium LEI and LIF were also studied, and found to be identical.

Laser-enhanced ionisation spectroscopy in flames and plasmas. Plenary lecture

Recent developments of laser-enhanced ionisation (LEI) spectroscopy are reviewed. This method utilises tunable dye lasers to enhance the rate of collisional ionisation of specific metal atoms in a flame or other atom reservoirs. It can be utilised for trace metal analysis, or for fundamental studies of flames and plasmas. Among the topics highlighted here is the use of double-resonance laser excitation methods to generate three-dimensional atomic LEI spectra. Also discussed are recent results on the use of laser-induced ionic fluorescence as an optical detection method for LEI. An updated list of LEI limits of detection compiled from the literature is included.