Spike multielement

As mentioned, thermal ionization mass spectrometry is the area in which isotope dilution developed and in which it has received the widest range of applications. One of thermal ionization s major limitations is that it is essentially a single-element technique in no way can it be considered multielement in the sense that numerous elements can be assayed in a single analysis. It is thus highly desirable to mate isotope dilution with multielement analysis capability. Spark source mass spectrometry for years dominated elemental analysis, but the nature of the samples (solids) made use of isotope dilution difficult. Use of a multielement spike was reported as long ago as 1970 by Paulsen et al. [17], however, and more recently by Carter et al. [18] and by Jochum et al. [19,20]. 

Because ICP-MS is multielemental and samples are normally in solution form, use of a multielement isotope dilution spike has many attractive features. Spikes for individual elements can be kept separate and mixed in appropriate concentrations for the problem at hand. Many samples are from the environment, so... 

Sample Solutions. The urine sample solutions all originated from one composite urine sample. The latter was made by combining 150-mL aliquots of the entire first-morning voids of 16, presumably healthy, male subjects. Aliquots of the composite were used to prepare three sets of sample solutions as indicated in Table IV. The solutions in each set were "spiked with internal reference elements and with appropriate volumes of multielement stock solution to make a (multiple) standard addition series (12). The final gallium and yttrium internal reference-element concentrations were 1.0 and 0.1 mg/L, respectively, for both the reference and standard addition solutions. The added analyte concentrations for both the reference and standard addition solutions were 0, 2, 4, 10, and 20 times the approximate "normal analyte concentrations for urine, as listed in Table II. 

Although this technique can yield excellent results, as illustrated in Table 7.4, it is a time-consuming process and for this reason is only used for limited analytes in small numbers of samples. It is advisable that a preliminary semiquantitative analysis be performed so that the magnitude of the spike concentration can be chosen to provide optimal results. In general, this approach is not suitable for multielement analysis. 

In an on-line isotope dilution technique used with LA-ICP-MS, an isotope-enriched spike solution, which can be a multielement solution containing all the spikes of interest, was nebulized using a low-flow nebulizer and introduced through a membrane desolvation unit in front of the sample ceU. The isotope ratio of the spike solution was measured (spike ratio), and after a distinct time, the laser was turned on and the isotope ratio of both the sample and the spike solution (blend ratio) was measured. After turning the laser off, the spike solution was replaced by blank solution and the isotope ratio of the blank was usedfor any blank correction. Using this setup, very accurate determination of trace impurities in small sample sizes was successfully performed. ... 

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