Abstract A single collector double focusing sector field ICP- MS is evaluated for the determination of isotope ratios. Spec- tral interferences (e.g. 40 Ar 23 Na on 63 Cu) can lead to calculation of inaccurate ratios. The use of high resolution enables such in- terferences to be separated from the isotopes of interest. Exter- nal reproducibilities of < 0.02% are shown for uninterfered iso- topes (measured at low resolution R = 300) and < 0.1% for in- terfered isotopes which required the use of medium (R = 4000) and high resolution (R = 10000). Introduction The analytical strengths of the inductively coupled plasma (ICP) such as the ability to ionize a wide range of elements, rel- atively low inter-element matrix effects, easy sample handling, minimal sample preparation requirements, and fast analysis time, make it an attractive ion source for isotope ratio mass spectrometry [1]. However, the precision obtained with quadru- pole ICP-MS has been limited, primarily due to noise from the ICP. One solution to the problem of ICP noise has been to em- ploy simultaneous measuring of the ion beams of several iso- topes (multiple collector ICP-MS) [2]. However, because plasma noise is not the only weakness of the ICP as an ion source [3], multicollection is not the solution to all of the problems that limit isotope ratio precision. The most significant weakness of the ICP ion source is the occurrence of spectral interferences, especially for low to mid- mass range elements, which become increasingly severe with higher matrix concentrations. When matrix separation or alter- native methods of sample introduction are used to remove these interferences, the principal advantage of MC-ICP-MS over thermal ionization mass spectrometry (TIMS), that of minimal sample preparation, is lost. The general solution to overcome spectral interferences is the use of high resolution [4], which requires that the ICP source be implemented on a double fo- cusing sector-field mass spectrometer. Accuracy of isotope ra- tio determinations is also affected by instrumental mass bias, particularly for low mass elements. Other limitations of ICP- MS instruments for measurements of isotope ratios include in- strumental sensitivity and linear dynamic range (for low con- centration samples and for large isotope ratios). The strengths of sector field ICP-MS for the measurement of isotope ratios (especially for measurements in low-resolu- tion mode) have been shown [5–9]. Recent developments have been made in current sector field ICP-MS further improving the precision and accuracy of isotope ratio measurements [10]. Experimental Instrumentation All isotope ratio determinations were performed with a sector field mass spectrometer with an inductively coupled plasma ion source (ELEMENT2, Finnigan MAT, Bremen, Germany). The ELEMENT2 is the second generation of a single collector dou- ble focusing instrument of reverse Nier-Johnson geometry with three fixed resolution settings [10]. The analyzer design has been optimized for peakshape and transmission leading to in- creased sensitivity in high resolution. With the implementation of a capacitive decoupling (CD) system (a grounded Pt cylinder inserted between torch and load coil) a secondary discharge at the sampling interface (a possible noise source) has been elim- inated. Measurements of the kinetic energy distribution for ions entering the spectrometer have shown a decrease from 20 eV to 2 eV. This reduction of the ion kinetic energy spread increases the transmission through the sector field mass analyzer, result- ing in an improvement in sensitivity by a factor of 10 for me- dium to high-mass elements and by up to a factor of 50 for low- mass elements [11]. The spectrometer housing has been ther- mally stabilized and a new field probe implemented to improve the stability of mass calibration. Sample introduction A specially designed sample introduction system, consisting of a tandem spray chamber arrangement (a ‘cyclone’ coupled to a standard ‘Scott-Double Pass’), was used (Elemental Scientific, Omaha, USA) with a low-flow Micromist microconcentric nebulizer, (Glass Expansion Ltd., Hawthorn, Australia). In this system, the larger droplets produced by the nebulizer are re- moved in the first (‘cyclone’) chamber before entry into the second (‘Scott-Double Pass’) where the droplet size distribu- tion is further reduced before the aerosol is swept into the ICP torch. Results and discussion In the following, the ELEMENT2 sector-field ICP-MS is eval- uated for the determination of isotope ratios, particularly for the difficult cases of isotope ratios of spectrally interfered ele- ments, large isotope ratios and isotope ratios of low mass ele- ments. The optimum instrumental and software parameters are shown in Table 1 for all ratios determined. The ICP plasma pa- rameters and spectrometer lens voltages were optimized before analysis using internationally certified standards of known iso- tope ratios to give minimum mass bias. In order to smooth out signal fluctuations from the plasma, it is necessary with a sin- gle collector instrument to rapidly cycle between all of the iso- topes. This requires the use of short integration times for each peak, which in turn requires high sensitivity to generate accept- able counting statistics. Additionally, in low-resolution mode, the slit design of the ELEMENT2 delivers flat top peaks, an important characteristic for the highest possible isotopic mea- surement precision. The requirement for rapid peak switching rules out magnetic scanning (or jumping) because it is too Fresenius J Anal Chem (1999) 364 : 495–497 – © Springer-Verlag 1999 CONFERENCE CONTRIBUTION M. Hamester () · D. Wiederin · J. Wills · W. Kerl · C. B. Douthitt Finnigan MAT GmbH, Barkhausenstrasse 2, D-28197 Bremen, Germany M. Hamester · D. Wiederin · J. Wills · W. Kerl · C. B. Douthitt Strategies for isotope ratio measurements with a double focusing sector field ICP-MS Received: 11 January 1999 / Revised: 8 April 1999 / Accepted: 11 April 1999