K. E. Anders Ohlsson

Novel calibration with correction for drift and non-linear response for continuous flow isotope ratio mass spectrometry applied to the determination of δ15N, total nitrogen, δ13C and total carbon in biological material†

With automated analysis of a batch of samples using an elemental analyser isotope ratio mass spectrometer, the instrumental response is often non-linear (e.g., the isotope ratio varies with sample size) and is affected by changes over time (drift). Traditionally, drift and non-linearity effects are compensated for by including several reference samples in the batch with elemental masses, which closely match those of the samples. The novel calibration method presented here corrects for both drift and non-linearity, and thus allows for a wider range of sample masses in the batch, while accuracy and precision is maintained at the level attained by the traditionally accepted calibration procedure without increasing the number of reference samples.

Photographic observation of molecular spectra in inverse Zeeman-effect graphite furnace atomic absorption spectrometry

Time-resolved high-resolution spectrographic measurements were used to record absorption spectra during the vaporisation of a Cu(NO3)2-NH4H2PO4 matrix in a side-heated graphite furnace. Three molecular species, PO, CuH and CN, were identified and the temperature intervals for their existence in the furnace were established. Measurements with and without a magnetic field applied to the furnace showed that only the absorption profiles of certain PO bands were affected. Various analyte emission lines within the spectral regions affected by PO band systems were checked for background-correction errors using a Perkin-Elmer Zeeman-3030 instrument equipped with an HGA-600 furnace. Overcorrection was observed for the 326.1-nm Cd, 253.7-nm Hg, 247.6-nm Pd, 246.3-nm Fe and 328.1-nm Ag lines.

Reduction of bias in static closed chamber measurement of δ13C in soil CO2 efflux

The (13)C/(12)C ratio of soil CO(2) efflux (delta(e)) is an important parameter in studies of ecosystem C dynamics, where the accuracy of estimated C flux rates depends on the measurement uncertainty of delta(e). The static closed chamber method is frequently used in the determination of delta(e), where the soil CO(2) efflux is accumulated in the headspace of a chamber placed on top of the soil surface. However, it has recently been shown that the estimate of delta(e) obtained by using this method could be significantly biased, which potentially diminish the usefulness of delta(e) for field applications. Here, analytical and numerical models were used to express the bias in delta(e) as mathematical functions of three system parameters: chamber height (H), chamber radius (R(c)), and soil air-filled porosity (theta). These expressions allow optimization of chamber size to yield a bias, which is at a level suitable for each particular application of the method. The numerical model was further used to quantify the effects on the delta(e) bias from (i) various designs for sealing of the chamber to ground, and (ii) inclusion of the commonly used purging step for reduction of the initial headspace CO(2) concentration. The present modeling work provided insights into the effects on the delta(e) bias from retardation and partial chamber bypass of the soil CO(2) efflux. The results presented here supported the continued use of the static closed chamber method for the determination of delta(e), with improved control of the bias component of its measurement uncertainty.

Quantitative in situ spectroscopic measurements ana thermodynamic equilibria modelling of CN and C2 in a graphite furnace

Abstract The principal objectives were to measure the in situ absolute steady-state levels of C 2 and CN in a graphite furnace under varying working conditions (temperatures, gas compositions, samples), and to describe the experimental data with a thennodynamic equilibrium model. In an empty atomiser at 3000 K, the partial pressure of C 2 was found to be at heterogeneous equilibrium. For CN, the partial pressure increased with temperature from 5 to 25% of the model value at 2300 and 2600 K, respectively. The CN level was shown, experimentally and by modelling, to be sensitive to the presence of oxygen. The partial pressure of CN exhibited a gradient between the tube wall and the centre of the tube. It is shown that the CN level inside the graphite tube can be used as an indicator for air ingress. The side-heated atomiser was compared in this respect to a Massmann-type furnace. The matrix modifiers Mg (NO 3 ) 2 and (NH 4 ) 2 HPO 4 are shown to reduce the partial pressure of CN under the atomisation step, which may be of importance for atomisation efficiencies of analytes susceptible to cyanide formation.

Diffusion vapour transfer modelling for end-capped atomizers. Part 2. Atomizer with open injection port☆

Abstract For end-cap equipped transverse-heated graphite atomizers (THGA) with integrated contacts used for analytical atomic spectrometry, a model equation describing the diffusional losses of analyte atomic vapour through the tube ends was constructed. The model assumes that the atomic density distribution is stepwise linear along the tube axis and the absence of a sample injection hole. With a quartz tube system, providing controlled experimental conditions at room temperature, the time constant of the diffusion removal function (T R ) of mercury vapour was determined for various open and end-capped tube geometries. These results were also described by an empirical multiple regression equation with a residual standard deviation of 5%. The theoretically predicted T R values, corrected with an empirical factor of 1.33, agreed well (correlation coefficient = 0.996) with the experimentally obtained T R values for the endcapped quartz tubes. For the Perkin-Elmer THGA tubes, the diffusional transfer model was evaluated using the integrated atomic absorbance ratio between various end-capped and open tubes. This is meaningful because the signal ratio for graphite atomizers is closely equal to the corresponding T R ratio. For recommended atomization temperatures the average deviation between these experimental signal ratios and the theoretically predicted ratios for the elements Ag, In, Cd, Co, Hg and Cu was 1–5% for various end-capped tube geometries. The results for the individual elements deviated more from the theoretically predicted ratios mainly because of small differences in the mean gas-phase temperature between open and end-capped tubes. For elements which tend to form molecules in the gas phase at low temperatures and for which the atomization efficiency is increased with the atomization temperature, the experimental ratios tended to be higher than the theoretically predicted values (In, Al, Se, Sn, As), whereas experimental ratios were slightly lower for other elements (Cd, Co, Cu).

Uncertainty of Blank Correction in Isotope Ratio Measurement

Blank correction for isotope ratio measurement on small amounts of substances is often limited by presence of a blank, with an apparent isotopic composition different from that of the sample. For isotope ratios, blank correction is commonly performed either by the regression method, which works without the need for estimation of the blank, or by the subtraction method. With the subtraction method, estimation of the amount and isotope delta of the blank is required, and these estimates could be obtained either by direct, semi-indirect, or indirect measurement. Previously given expressions for the standard uncertainties of indirectly measured blank amounts and blank isotope deltas did not account for covariance between input quantities. In the present work, a previously published data set was re-evaluated, with covariance terms properly included in the calculation of uncertainties. It was shown that covariance effects may yield a significant reduction in uncertainty estimates, both for blank quantities and for blank corrected results. For series measurements on a standard material, it was also shown that the distribution of individual corrected isotope delta values around the average value was approximately normal, with its standard deviation equal to the estimated standard uncertainty of the corrected values. In most cases, it was observed that the regression and subtraction methods yield approximately the same blank corrected average values and uncertainties, regardless of method selected for estimation of blank quantities.

Spatially resolved spectroscopic measurements of CN, C2 and Al in graphite tube electrothermal atomizers under various operating conditions

Direct, spatially resolved, spectroscopic measurements of C2 in side-heated (spatially isothermal) and Massmann-type (spatially non-isothermal) atomizers showed that the partial pressure of gaseous carbon (pC2) is in thermodynamic, heterogeneous equilibrium for temperatures above 2970 K. No evidence for over-equilibrium excesses of C2 was obtained. The pCN exhibits a gradient, decreasing from the bottom to the top of the graphite tube, indicating the importance of air ingress through the injection hole. Further, the pCN is reduced by the presence of aqueous solutions, primarily close to the point of sample deposition, revealing the non-equilibrium distribution of CN in the atomizer, whereas C2 is only marginally affected. Spatially resolved measurements of Al also indicate the importance of O2 ingress and the sheath gas flow rate on the Al atomic absorption signals, indicating that the pO2 largely determines the degree of Al atom formation and not the presence of gaseous, carbon-containing species.

Uncertainty budget for multi-elemental analysis of plant nutrients in conifer foliar material using inductively coupled plasma atomic emission spectrometry (ICP-AES)

Measurement uncertainties evaluated according to GUM were given in an uncertainty budget for the measurement of mass fractions of 12 elements in conifer tree needle materials. The measurement was performed using ICP-AES, with prior microwave digestion of the dried sample material. The uncertainty budget for Ca as an example showed that correction for a systematic error was the main source of measurement uncertainty. The key to reduced measurement uncertainty therefore lies in identifying the main sources of systematic errors, and reducing the uncertainty associated with their correction. The usefulness of the uncertainty budget was demonstrated in its application to method validation, to the design of a quality control program, and finally for guiding method optimization.

Determination of the 15N/14N ratio of ammonium and ammonia in aqueous solutions by equilibrium headspace-gas chromatography-combustion-isotope ratio mass spectrometry

A method for determination of the 15N/14N ratio of total ammoniacal nitrogen (TAN; ammonium and ammonia) in aqueous solutions was developed, primarily intended for use with soil extracts, which have a high TAN level, e.g. from recently fertilised agricultural soils. Ammonium was converted to ammonia by addition of NaOH, followed by nitrogen isotopic analysis of the headspace by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) where complete separation of TAN from the matrix was not necessary. The ammonia concentration in the gas phase was maximised by increasing the temperature and salt concentration and by decreasing the gas liquid ratio in the headspace vials. Isotopic equilibrium was reached after less than 1 h at 80 degrees C. The measured isotopic ratio was constant for solutions containing 30-200 mM NH4-N, corresponding to 950-7000 ng NH3-N detected with the IRMS. The integrated area response at m/z 28 increased linearly with the ammonium ion concentration in the interval 10-200 mM NH4-N. The fractionation factor between the liquid and gas phases was 1.0054 +/- 0.0007 within the linear range, which is in agreement with values reported in the literature, but with a higher precision. Changes in temperature, gas:liquid ratio or salt concentration did not affect the measured ratio, demonstrating the robustness of the developed method.

Transfer of aluminium compounds and spike atomization in a spatially isothermal graphite atomizer

Abstract Aluminium samples were vaporized in a spatially isothermal side-heated atomizer at a slow heating rate ( c . 20 K s −1 ), A single Al absorbance spike was obtained for Al masses ranging from 30 to 1000 ng and multiple spikes for larger masses. Experimental results, using a platform probe technique, showed that Al is redistributed within the atomizer after or at the moment of spike formation but prior to the main Al signal appearance. Experimental results support the hypothesis that the spike is produced by an autocatalytic reaction mechanism. Suspensions of aluminium oxide in water produced an Al spike, being observed only occasionally when cyclohexane slurries were used.