ISSN:0267-9477    

DOI:10.1039/b207884h

出版商:RSC    

年代:2002

数据来源: RSC

摘要:

1IntroductionIn recent years, we have developed a comprehensive modeling network for analytical glow discharges in dc, rf and pulsed operation mode (e.g.,refs. 1–3). A number of Monte Carlo, fluid and collisional-radiative models were developed for the various plasma species,i.e., electrons, Ar+ions, fast Ar atoms, Ar atoms in various excited levels, sputtered Cu atoms and the corresponding ions, both in the ground state and in various excited levels. The behavior of the Ar gas atoms was generally not calculated; it was simply assumed that there was no major gas flow and that the Ar gas atoms were at thermal velocities uniformly distributed throughout the discharge. It should, however, be mentioned that we have also developed a model to calculate gas heating.4On the other hand, there is in reality always a certain gas flow and, especially in recent developments of glow discharge mass spectrometry (GDMS), a considerable gas flow is produced in the discharge to increase the ion transport towards the mass spectrometer.5Therefore, we wish to extend our modeling network by including the effect of gas flow in the models. The gas flow itself was calculated by a computational fluid dynamics (CFD) program, and this result was used as input into our plasma models in order to calculate the transport of the plasma species, not only by diffusion and migration, but also by convection. In the following, both the CFD program and the incorporation of convection in the plasma models will be briefly outlined, and the results of this coupling will be presented and discussed.

ISSN:0267-9477    

DOI:10.1039/b200746k

出版商:RSC    

年代:2002

数据来源: RSC

摘要:

IntroductionBesides the more traditional thermal ionization mass spectrometry (TIMS), inductively coupled plasma mass spectrometry (ICP-MS) is nowadays increasingly used for isotope ratio measurements. Fields of application include, for instance, the determination of natural isotopic variations in geology, geochronology and archeometry1,2and the measurement of radioactive isotopes.3,4Because of its high sensitivity, ICP-MS in combination with isotope dilution mass spectrometry (IDMS)5–9is also used for reference measurements of elemental amount contents at trace or ultratrace levels.10,11Isotope ratio determinations by ICP-MS have been applied to various sample matrices and elements. A particular advantage of ICP-MS is the ability to ionize most of the elements because of the high plasma temperaturesof up to ∼104K,12making ICP-MS a very versatile method. Another advantage of ICP-MS is the ease of application. Where TIMS usually needs a complex chemical matrix separation, ICP-MS can often be used without extensive sample pre-treatment, which saves time and reduces the risk of contamination.For a long time, the limited precision that can be achieved with quadrupole ICP-MS (0.1–0.3% repeatability) has hampered its use for isotope ratio measurements. However, the introduction of magnetic sector ICP-MS instruments has improved the precision that can be reached with this technique. With single collector magnetic sector instruments, isotope ratios can be measured with a precision of down to 0.05%.3The use of multi-collector ICP-MS instruments (MC-ICP-MS)13allows one to reach a precision of 0.002–0.02%, which is competitive with what TIMS instruments can achieve.14A study was recently carried out comparing the performance of different types of commercially available ICP-MS instrument by means of uranium measurements on identical samples.3The MC-ICP-MS was found to produce uranium isotope ratio measurements with 1–5 times smaller combined uncertainty than a magnetic sector single collector instrument and 10–25 times smaller uncertainty than a quadrupole instrument. However, in this study only a limited number of sources of measurement uncertainty were considered. For accurate isotope ratio measurements by ICP-MS, corrections for detector dead time, instrumental background, mass discrimination and eventually isobaric interferences have to be performed. As these corrections are crucial for the accuracy of the result, precise understanding of them and their accompanying uncertainties is essential. The uncertainties attributed to these corrections, which propagate into the combined uncertainty of the ICP-MSisotope ratio measurement, are dependent on the type of ICP-MS and the particular measurement conditions. Understanding of these sources of uncertainty allows estimating and reporting a realistic uncertainty together with the result, instead ofe.g.only reporting the repeatability. Furthermore, it allows optimization of the measurement conditions to obtain the lowest combined uncertainty of the result.In this study the performance of three different ICP-MS instruments with respect to the combined uncertainty of a copper amount content measurement in a water sample by “direct” IDMS5is compared. For each of the measurement series a comprehensive measurement uncertainty was evaluated and compared for the different instruments and different instrumental settings. The main sources of uncertainty are compared and discussed.

ISSN:0267-9477    

DOI:10.1039/b201443b

出版商:RSC    

年代:2002

数据来源: RSC