Matching isotopic ratios

Thermal ionization has three distinct advantages the ability to produce mass spectra free from background interference, the ability to regulate the flow of ions by altering the filament temperature, and the possibility of changing the filament material to obtain a work function matching ionization energies. This flexibility makes thermal ionization a useful technique for the precise measurement of isotope ratios in a variety of substrates. 

Both absolute quantitation and relative quantitation of species in mixtures is of interest in some circumstances. Quantitation in a 5-minute analysis can be achieved by addition of an internal standard, ideally the target microorganism grown in special media to incorporate heavy isotopes92-95 and determination of the relative peak heights of pairs of proteins from the analyte and the standard. Isotope-labeled proteins or peptides, selected to match proteins or peptides characteristic of target microorganisms, can also serve as internal standards for isotope ratio measurement. The addition of unmatched proteins or peptides is less reliable for either ESI or MALDI measurements because of unpredictable suppression in the variable mixture. 

Strontium isotope signatures in tooth enamel and bone from five adults and one infant buried in two tombs at Conchopata are very similar. This implies that these individuals consumed food from the same geologic zone, or from geologic zones with similar strontium isotope signatures. Since the strontium isotope ratios in these individuals matched the expected Ayacucho Basin strontium isotope signature, it is most likely that these individuals lived in or near Conchopata for the first and last years of life. 

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. 

Figure 15 Isotope-ratio measurement precision as a function of the number of accumulated counts. Each measured value (circles) represents the relative standard deviation of 10 repetitions. The dotted line represents the precision predicted by counting statistics for a given number of accumulated counts. Because the measured values match the predicted values closely, it is confidently expected that longer integration times will yield even better precision (<0.01 %). (From Ref. 47.)...

I). A parallel study has been conducted to determine the isotopic ratios of lead in galenas (PbS) from known ancient mining sites with the hope that, by matching the sets of ratios, the provenance of the objects could be ascertained (2). The work has all been based on the fact that of the four isotopes of lead ( Pb, Pb, Tb, and Pb), the heaviest three are the stable end products resulting from the radioactive decay of 23 U, and 232Th. 

Most relevant to speleothem investigations is the middle to late Pleistocene - the past half-million years. This is the time scale of U/Th dating. Paleoclimatic signals are often matched to variations in oxygen isotope ratios derived from measurements on deep sea cores. This record has been subdivided into isotope stages that in turn correlate with sea level stands and with temperatures. The version of the isotope stage record shown in figure 1 is the SPECMAP compilation here reconstructed from Bradley (1999). 

A We also developed laser-desorption laser-ionization mass spectrometry for the analysis of adsorbates on surfaces, such as interplanetary dust particles and meteoritic samples. We use one laser to rapidly heat the sample and evaporate molecules from the surface. A second laser intercepts the rising plume of molecules and ionizes those that absorb that color of light. We then weigh the ions using a mass spectrometer. We have analyzed graphite particles extracted from meteorites and found polycyclic aromatic molecules (PAHs). The PAHs have to isotope ratios that match closely the graphite grains, which are believed to be the remnants of the star dust from which our solar system condensed some 4.5 billion years ago. These are the first interstellar molecules observed directly in the laboratory. 

Three of the samples with heavy calculated carbon isotope ratios (Table IV) could be matched with the selected hypothetical dilution water. The calculated dilution ranged from 45 to 85%. For a given pH, the range was smaller (Table IV). The dilution was also calculated for siderite precipitation at pH 7, and the dilution values were similar to those calculated for pH 5 without siderite precipitation (Table IV). For three samples whose calculated isotope ratios were not greatly different from background, and one sample whose mixing curves did not intersect, a different background water composition needs to be selected. One sample (S-20) had an observed carbon isotope ratio that was heavier than observed. This sample may have been diluted by a more contaminated solution with heavier carbon. 

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