Ratio Precision

It is also critical to optimize the efficiency cycle of the measurement. With every sequential mass analyzer, there is an overhead time called a settling time to allow the power supply to settle before taking a measurement. This time is often called nonanalytical time, because it does not contribute to the quality of the analytical signal. The only time that contributes to the analytical signal is the dwell time, or the time that is actually spent measuring the peak. The measurement efficiency cycle (MFC) is a ratio of the dwell time to the total analytical time (which includes settling time) and is expressed as follows  

sweepsx dwell timex 100 No. sweepsx (dwell time-I-settling lime)  

Practical Guide to ICP-MS A Tutorial for Beginners, Second Edition 

It is therefore obvious that to get the best precision over a fixed period of time, the settling time must be kept to an absolute minimum. The dwell time and the number of sweeps are operator selectable, but the settling time is usually fixed because it is a function of the quadrupole electronics. For this reason, it is important to know what the settling time of the mass spectrometer is when carrying out peak hopping. Remember, a shorter settling time is more desirable because it will increase the measurement efficiency cycle and improve the quality of the analytical signal.  

FIGURE 21.4 The detector electronics must be able to switch fast enough to detect isotope ratios that require both pulse and analog counting modes (data copyright 2003-2007, all rights reserved, PerkinElmer Inc.). 

Sweeps x Dwell time x 100 No. Sweeps x (Dwell time + Settling time)  

A word of caution should be expressed at this point. Having worked in ICP-MS for almost 30 years, I know that the experience of the person developing the method, running the samples, and performing the demonstration has a direct impact on the quality of the data generated in ICP-MS. There is no question in my mind that the analyst with the most application expertise has a much better chance of getting 

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As discussed before, quadrupole based ICP-MS allows multi-element determination at the trace and ultratrace level and/or isotope ratios in aqueous solutions in a few minutes as a routine method with detection limits of elements in the sub pgml-1 range and a precision for determined trace element concentration in the low % range (RSD - relative standard deviation). The precision for isotope ratio measurements varies between 0.1% and 0.5% RSD. This isotope ratio precision is sufficient for a multitude of applications, e.g., for evidence of contamination of sample with depleted or enriched uranium in urine (this technique is used in the author s laboratory in a routine mode14) or the isotope dilution technique for the quantitative determination of trace element and species concentration after doping the sample with enriched isotope spikes. 

Data acquisition parameters. Precision and accuracy in the measurement of isotope ratios can be improved if the number of measurements is increased (e.g. if the measurement time is increased). Various measurement protocols can be applied and those whereby the time actually spent on measuring the isotope ratios of interest is maximised are preferable. The data acquisition parameters of an ICP-MS device that can be changed to improve the isotope ratio precision... 

Evaluating the Precision Requirements for Isotope Ratio Determination of Archaeological Materials Using Laser Ablation—Time-of-Flight—Inductively Coupled Plasma—Mass Spectrometry Increasing Ratio Precision... 

Detail tests on nuclear models require not only a knowledge of energy, spin and parity of many levels, but also the determination of transition multipolarities and branching ratios. Precise intensities are thus needed. The well shielded anti-Compton spectrometer offers a rather simple solution especially for accurate angular distribution measurements. When the spectra are very complex, like in the case of final doubly odd nuclei, intensities cannot be determined without use of high resolution instruments. The curved crystal spectrometer provides a powerful solution at, unfortunately, non negligible cost. 

The ideal internal standard is the same element as the analyte because it has similar mass, ionization energy, and chemical properties. Therefore, isotope dilution based calibration provides high accuracy as long as isotope equilibration is attained and the measured isotopes are free of spectral overlaps [192,193]. Standards do not need to be matrix-matched. Quadrupole-based ICP-MS instruments can typically provide isotope ratio precision of 0.1% to 0.5%. Much better isotope ratio precision can be obtained by using simultaneous MS detection, such as a multicollector-based instrument or perhaps time-of-flight MS. In comparison to thermal ionization mass spectrometry, ICP-MS provides much higher sample throughput and simpler, faster sample preparation. 

Propagation of errors using isotope dilution ICP-MS has been considered to determine how to optimize the measurements [201]. Comparison of analysis results from external calibration versus isotope dilution can be used to assess the quality of external calibration results and the effectiveness of internal standards with external calibration [202,203]. Because isotope ratio precision depends on the total ion count rate, the use of high-efficiency sample introduction to generate larger signals can improve isotope ratio precision and, therefore, analysis precision [204]. 

Signal-to-Noise Ratio, Precision, and Limit of Detection 99... 

ICP-MS with a hexapole collision cell is advantageous for isotope ratio measurements of difficult to analyze elements." Due to possible interferences with Ar+, ArO+ and dimer argon molecular ions Ar2 " the measurements of the isotope ratios °Ca/ "Ca, Fe/ Fe and Se/ Se by ICP-MS are extremely difficult or completely impossible. Whereas for these isotope ratios precisions between 0.2 and 0.3 % were observed using ICP-MS with hexapole collision cell, a precision of 0.07% was measured for a lOp-gF uranium isotope reference solution 1) in the... 

Collaborative efforts with experts in nutrition and bioavailability have been carried out at NBS and have resulted in the use of TIMS for the measurement of stable Isotope tracers in adults and newborns(, J 0). Typical data for these efforts are summarized in Tables II and III for Ca and Zn isotopes For each of these elements optimum Isotope ratio precisions of about 0 1% (R S D )were achieved which permitted the resolution of extended term kinetic effects that would be impossible to achieve with less accurate and precise mass spectrometry. 

Fig. VI-3 shows a schematie diagram of the electrical conductivity vs. temperature (In tr vs. In T) in the vicinity of the metal-insulator transition. Each of the curves shown in Fig. VI-3 is drawn for a different degree of disorder for a given conducting polymer system, each curve would represent data obtained from a sample with different resistivity ratio, Precisely at the critical point (where the mobility edge is precisely at the Fermi energy), the conductivity follows the power law of Eqn VI-6. On the metallic side, the resistivity remains finite as the temperature approaches zero as indicated in Eqn VI-5. On the insulating side, the resistivity falls below the power law as a result of the exponential dependence that results from variable range hopping see Eqn VI-8 and VI-9.

The sulphides of iron include Fe3S4 (spinel structure), Fe7Sg (pyrrhotite), FeS (troilite), and FeS2 (pyrites and marcasite). Ferrous sulphide rarely has the Fe S ratio precisely equal to unity, though stoichiometric FeS can be prepared. A microcrystalline form prepared by precipitation has been examined by X-ray powder photography and also by electron diffraction. This form (mackinawite) has... 

For the determination of isotope ratios, the precision of TOF-ICP-MS has been studied in a preliminary comparison with other mass spectrometer systems [521]. Typical isotope ratio precisions of 0.05% were obtained, thus overtaking sector field mass spectrometry with sequential detection, for which values of 0.1-0.3% for 63Cu/65Cu in Antarctic snow samples have been reported [522], Similar results were obtained by Becker et al. [523] for Mg and Ca in biological samples (0.4-0.5%). In principle, the features of TOF-ICP-MS may be superior to those of sequential sector field or quadmpole mass spectrometry, however, true parallel detection of the signals as is possible with multicollector systems may be the defini-... 

On the other hand, it was discovered that Pb has an adverse effect that induces production of MF as a by-product, as it speeds up the reaction that produces formaldehyde from methanol. This is the reason that the MMA yield decreases as the ratio of Pb increases (Figure 13.1). Therefore, in order to suppress both decarbonylation and the production of MF as a by-product, it is important to use PdgPbj as the catalytic species, with its atomic ratio precisely controlled at 3 1 Pd Pb, and to ensure that there are no Pb species other than Pd3Pbj present in the catalyst system. To use intermetaUic compounds correctly as catalysts, development of a control technology to maintain the optimum conditions for formation of the active species during catalyst preparation was necessary. 

Finally, the improved isotope ratio precision that can be obtained with MC-ICPMS opens the doors for the isotopic analysis of new elements (e.g., Cu) that might offer further fingerprinting potential [115]. An increase in the number of LA-ICPMS applications that deal with isotopic analysis of archaeological samples might also be predicted. 

It should be taken into account, however, that the isotope ratio precision is typically worse than that with pneumatic nebulization as a means of sample introduction. Recently, Aramendia et al. provided a description of strategies to improve the isotope ratio precision attainable with the combination of LA and single-collector ICP-MS [62]. As the use of LA also affects the degree of mass discrimination, matrix-matched standards are required for adequate mass bias correction [63],... 

This is the ultimate precision attainable and the isotope ratio precision observed in practice should be compared with that predicted by Poisson counting statistics to evaluate whether or not further progress can be made. 

Isotope Ratio Precision with Single-Collector ICP-MS... 

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