A computer program for the deconvolution of mass spectral peak abundance data from experiments using stable isotopes (original) (raw)
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Journal of Analytical Atomic Spectrometry, 2008
were characterised on a multi-collector ICP-MS instrument both in terms of isotopic composition and concentration. The isotopic composition of the spikes was determined using either a natural abundance or certified standard for mass bias correction, except for lead where thallium internal correction was used. It was observed that the use of weighted regression lines for mass bias correction provided similar or, in most cases, lower uncertainties in the isotopic composition of the spikes in comparison with unweighted regression lines or single ratio measurements. For the establishment of the spike concentration by reverse isotope dilution analysis, isotope pattern deconvolution was evaluated and mass bias could be corrected internally in each blend by minimising the variance of the multiple linear regression model. It was also observed that isotope pattern deconvolution with internal mass bias correction provided lower uncertainties in the concentration of the spikes in comparison with the usual procedure based on external mass bias correction.
Analytical Methods for Non-Traditional Isotopes
This chapter is devoted to the analytical methods employed for making high precision isotope ratio measurements that preserve naturally occurring mass-dependent isotopic variations. The biggest challenge in making these types of measurements is deconvolving mass-dependent isotopic fractionation produced in the laboratory and mass spectrometer, from naturally occurring mass-dependent isotopic fractionation, because the patterns of isotope variation produced by these processes are identical. Therefore, the main theme of this chapter is the description and mathematical treatment of mass-dependent isotopic variations and the possible pitfalls in deconvolving instrumental mass bias from naturally occurring mass-dependent isotopic variations. This chapter will not attempt to catalog methods for isotopic analysis of different elements. These details are better discussed in later chapters where 'element specifi c' analytical issues are covered. Rather, the effort in the chapter will be to focus on those specifi c items that make mass analysis of non-traditional isotopes challenging and unique, and the methods that can be employed to make precise and accurate isotope ratios.
Isotope abundance ratio measurements by inductively coupled plasma-sector field mass spectrometry
Journal of Analytical Atomic Spectrometry, 2012
This tutorial reviews fundamental aspects of isotope abundance ratio measurement by inductively coupled plasma-sector field mass spectrometry (ICP-SFMS). After a synopsis of the scope of isotope abundance ratio measurement and a summary introduction to the factors affecting precision and accuracy, attention is turned to noise sources. Detailed theory behind Poisson or counting statistics and plasma flicker noise components is given, since much of the observed imprecision can be attributed to these sources. Using single collector instruments, ion beams from different isotopes are sampled in rapid sequence, and so ratioing of the signals will be subject to fluctuations derived from intensity variations, i.e., flicker noise. It is demonstrated that flicker noise can, under specified circumstances, become the limiting factor for the attainable precision. Furthermore, the practice of partitioning dwell times, ostensibly to optimize precision based on isotopic abundances and assumed Poisson statistics, is shown to be flawed and actually requires accounting for flicker noise. In addition to random uncertainty, various offset factors may contribute to systematic error in measured isotope abundance ratios. Two of these, namely mass scale shift and spectral interferences are ameliorated using ICP-SFMS. The former is eliminated when operating under conditions providing flat-topped peaks, such that the minor drift in mass calibration typical of the technique becomes inconsequential and the intensity remains the same. Isotope abundance ratio measurements are subject to three further important offset factors. First is abundance sensitivity, which quantifies the extent of peak tailing to neighboring masses and can present a considerable source of offset. Second is mass bias, resulting from the fact that all sector field devices exhibit increasing sensitivity with ion mass, and various empirical methods used to correct for this effect are compared and contrasted. Third is detector dead time, which affects mass spectrometers equipped with ion counting systems. Although a well-understood phenomenon, all current methods for determining the dead time on the basis of experimentally measured isotope abundance ratios are likely to yield biased estimates. Finally, the capabilities of ICP-SFMS for the determination of isotope abundance ratios are placed in perspective by making a brief comparison with other techniques.
Mass-spectrometer bias in stable isotope ecology
Limnology and Oceanography: Methods, 2008
Stable isotope analysis (SIA) is recognized as a powerful analytical tool with numerous ecological applications. This has been highlighted by the increase in popularity of the isotope ratio mass spectrometry (IRMS) technique and the large number of studies reporting isotopic data. Comparisons of new isotopic data with previously published results and the use of large volumes of isotopic ratios in meta-analyses to explain isotopic variance are commonplace. Such data often originate from different IRMS instruments and are assumed to be readily comparable as all instruments are calibrated to International Atomic Energy Agency (IAEA) standards. To test the validity of this assumption, we analyzed a single ecological sample (homogenized cod muscle, Gadus morhua) on eight anonymous IRMS instruments and found significant variation in both δ 15 N and δ 13 C. We used a one-way analysis of variance (ANOVA) with random effects to estimate the average variability of laboratory results within and among instruments. Overall, 74% of variation in δ 15 N and 35% of variation in δ 13 C of a single ecological sample was explained by differences in the IRMS instrument used. In light of these findings, researchers are encouraged to submit their own sample reference to provide an independent check on variation between runs and between instruments; consistent discrepancies between instruments should be corrected through linear regression. Comparisons of data obtained from multiple instruments should acknowledge inter-instrument variation as a potential source of error.
Correcting mass isotopomer distributions for naturally occurring isotopes
Biotechnology and Bioengineering, 2002
In one method of metabolic flux analysis, simulated mass spectrometry data is fitted to measured mass distributions of metabolites that are isolated from cultures with defined feeding of 13 C-labeled substrates. Doing so, simulated mass distributions must be corrected for the presence of naturally occurring isotopes. A method that was recently introduced for this purpose consists of consecutive correction steps for each isotope of each element in the considered compound. Here we show that all isotopes of each individual element must, however, be corrected in one single step. Furthermore, it is shown that the source of information with respect to isotopic compositions of the elements needs to be chosen with care.
Measurement of light stable isotope ratios by SIMS
International Journal of Mass Spectrometry, 1998
Mass bias occurring during analysis of the light stable isotopes of oxygen, carbon, and sulfur in geological materials by secondary ionization mass spectrometry has been investigated. The effects of instrumental parameters (primary ion beam, secondary ion energy, and polarity) were evaluated by measuring sulfur isotope ratios in conductive sulfide minerals. The role of analyte chemical composition (matrix effect) on mass bias was investigated in sulfides (sulfur), silicates and oxides (oxygen), and carbonates (oxygen and carbon). For oxygen and carbon, various correlations between mass bias and matrix parameters have been identified. The application of several empirical models for prediction of oxygen isotopic mass bias indicates that for silicates, depending on mineral composition, bias can be predicted with an accuracy that is typically within two times that of the precision. However, extension of these models to other matrices has proved problematic, indicating that additional factors are important. (Int J Mass Spectrom 178 (1998) 81-112)
The Purdue Rare Isotope Measurement Laboratory
Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms, 1994
Purdue University has brought into operation a new NSF/NASA facility dedicated to accelerator mass spectrometry. Based on a 7.5 MV FN tandem, toBe, 26Al, and 36Cl are being measured at a rate of 1500 samples per year. Research involves primarily 1) earth science studies using cosmogenic radionuclides produced in the atmosphere and measured in rain, groundwater, and soils, 2) Quaternary geomorphology and climatology studies using in-situ produced radionuclides, 3) planetary science studies using a wide variety of meteorites and radionuclides, and 4) biomedical tracer studies using laAl.