A comparison of double-focusing sector field ICP-MS, ICP-OES and octopole collision cell ICP-MS for the high-accuracy determination of calcium in human serum

Human serum is routinely measured for total calcium content in clinical studies. A definitive high-accuracy and low-uncertainty method is required for reference measurements to underpin medical diagnoses. This study presents a novel octopole collision cell ICP-MS, high-accuracy, methodology and comparison of that technique with double-focusing sector field ICP-MS and an ICP-OES method. Double-matched isotope dilution mass spectrometry (IDMS) was employed for ICP-MS techniques and an exact matching bracketing technique using scandium as an internal standard was used for ICP-OES analysis. Medium resolution mode was utilised for double-focusing sector field ICP-MS analysis to resolve the dominant interferences on the ⁴⁴Ca/⁴²Ca isotope pair. Hydrogen reaction gas was employed to chemically resolve a number of polyatomic interferences predominantly through charge transfer reactions in the octopole collision cell. Comparison data presented for NIST CRM 909b human serum analysis from all three techniques demonstrates highest accuracy (99.6%) and lowest uncertainty (1.1%) for octopole collision cell ICP-MS. Data from ICP-OES using a non-IDMS technique produces comparably accurate data and low-uncertainties. The much higher total expanded uncertainties for double-focusing sector field ICP-MS compared with octopole collision cell data are explained by lower precision on the measurement of the ⁴⁴Ca/⁴²Ca isotope ratio. Data for octopole collision cell ICP-MS submitted for an international blind trial comparison (CCQM K-14) demonstrated excellent agreement with the mean of all participants with a low expanded uncertainty.     

Frequency distribution of radiation emitted from excited atomic systems

General expressions for the time-dependent probability amplitudes of the quantum states of two arbitrary, interacting atoms are calculated when one atom is initially in an excited p state and the other atom is in an s ground state. The lifetimes of the excited states and the line shape of the emitted radiation are obtained as functions of both the atomic separation and the energy difference between the excited states of the two atoms. The emission line shape is shown to be doubly peaked and to agree with the line shape of the radiation scattered by a system of two interacting atoms. The expressions for the lifetimes of the excited states are found to be identical to those obtained for the radiation scattering situation.     

High-performance liquid chromatography of amino acids, peptides and proteins CXXV. Molecular dynamics simulation of n-butyl chains chemically bonded to silica-based reversed-phase high-performance liquid chromatography sorbents

The molecular dynamics method has been applied to investigate the conformations of n-butyl ligands immobilised onto an amorphous silica surface analogous to those utilised with silica-based RP-HPLC sorbents. Three systems were constructed which corresponded to ligand densities of 1.64, 2.67 and 3.69 μmol/m². A number of parameters related to the structure of the sorbent materials were derived in order to characterise the molecular properties of each system. These parameters included the hydrocarbon layer thickness, the frequency of gauche conformations, the distance distribution for carbon atoms and the diffusion coefficients of individual atoms in the n-butyl ligands. From these properties, the positions of chains with respect to the surface as well as their mobility were estimated. It was found that at higher densities, the n-butyl chains are predominantly perpendicular to the surface while at low density they are highly tilted or lying almost parallel to the surface. The degree of ligand flexibility decreased with increasing surface density. Mobility of individual carbon atoms as well as chain disorder increased with distance from the surface for all ligand densities. The simulated properties of n-butyl chains immobilised to a silica surface correlated well with results obtained by Fourier transform IR and ¹³C-cross polarisation magic angle spinning NMR experimental methods and statistical predictions of the behaviour of immobilised chains.     

High-resolution spectroscopy of induced chiral dimers: a study of the dimers of ethanol by Fourier transform microwave spectroscopy

We present the first recording of the high-resolution spectrum of an induced chiral dimer. Three conformers of the induced chiral dimers of ethanol have been observed using a pulsed molecular-beam Fourier transform microwave spectrometer. The rotational constants of the normal isotopomers of the three species have been determined to be (a) A=5113.826(5), B=1329.7214(4), and C=1257.5151(3) MHz, (b) A=5086.459(5), B=1316.6508(4), and C=1243.6329(4) MHz, and (c) A=4851.608(5), B=1369.7558(6), and C=1243.4184(4) MHz. The observed species have been assigned to calculated structures via Kraitchman double substitution analyses and ab initio calculations. The Kraitchman analyses and the fitted centrifugal distortion parameters suggest that the deuterium bond is significantly stronger than the hydrogen bond in the dimers of ethanol.     

The reaction of N-aminophthalimide with isothiocyanates

N-Aminophthalimide ( I ) reacted in refluxing isopropyl alcohol with a number of isothiocyanates to give the related 1:1 addition products, N-(3-substitutedthioureido)phthalimides III. On the other hand, heating I directly with an excess of neat arylisothiocyanates produced the N-arylphthalimides IV. As shown for IIIa, the 1:1 addition products are conveniently deblocked by the Ing-Manske procedure to yield the 4-substituted thiosemicarbazide.     

Current mass spectrometry strategies for selenium speciation in dietary sources of high-selenium

This document reviews the most relevant mass spectrometry approaches to selenium (Se) speciation in high-Se food supplements in terms of qualitative and quantitative Se speciation and Se-containing species identification, with special reference to high-Se yeast, garlic, onions and Brazil nuts. Important topics such as complexity of Se speciation in these materials and the importance of combining Se-specific detection and molecule-specific determination of the particular species of this element in parallel with chromatography, to understand their nutritional role and cancer preventive properties are critically discussed throughout. The versatility and potential of mass spectrometric detection in this field are clearly demonstrated. Although great advances have been achieved, further developments are required, especially if speciatedcertified reference materials (CRMs) are to be produced for validation of measurements of target Se-containing species in Se-food supplements.     

High-performance liquid chromatography of amino acids, peptides and proteins. CXV. Thermodynamic behaviour of peptides in reversed-phase chromatography.

The thermodynamic behaviour of three peptides, bombesin, beta-endorphin and glucagon, was studied under reversed-phase high-performance liquid chromatographic conditions. Experimental data related to the interactive surface contact area (S values) and solute affinity (log k0) were derived over a range of temperatures between 5 and 85 degrees C. These experimental conditions allowed changes in the secondary structure of the solute to be monitored. The influence of the nature of the stationary phase ligand on the relative conformational stability of the three peptides was analysed by acquiring data with n-octadecyl silica (C18) and n-butyl silica (C4) sorbents. Values for the relative changes in entropy and enthalpy associated with the interactive process were also determined. The results provide further insight into the factors involved with the stabilization of secondary structure and the mechanism of the interaction of peptides with hydrophobic surfaces.     

Influence of temperature on the behaviour of small linear peptides in capillary electrochromatography

The influence of temperature, T, on the retention times, peak widths, peak symmetry coefficients and theoretical plate numbers of two small linear peptides, [Met5]enkephalin and [Leu5]enkephalin, has been studied with capillary electrochromatography (CEC) capillary columns of 100 microm I.D. and 250 mm packed length with a total length of 335 mm, containing 3 microm Hypersil n-octadecyl bonded silica. With increasing column temperature from 15 to 60 degrees C, the electroosmotic flow (EOF) and the column efficiencies increased, whereas the retention coefficients (Kcec) of both peptides decreased. A linear relationship was found between the EOF value and the square root of the temperature over this temperature range, with a linear regression correlation of 0.998. Non linear Van 't Hoff plots (In Kcec versus 1/T) were observed for these peptides between 15 and 60 degrees C, suggesting that a phase-transition occurred with the n-octadecyl chains bonded on the silica surface, affecting the CEC retention behaviour of these peptides. In CEC systems, the Kcec values of peptides incorporate contributions from both electrophoretic migration and chromatographic retention. Positive and negative Kcec values can, in principle, thus arise with these charged analytes. However, the Kcec values of the enkephalin peptides under all temperature conditions studied were positive with an eluent composed of water-50 mM NH4OAc/AcOH, pH 5.2-acetonitrile (5:2:3, v/v) and therefore the chromatographic component dominates the retention process with these small peptides under these conditions.