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Contamination older than sample

Figure 7. Effect (in years along ordinate) of the introduction of 1-30% contamination older (lower in activity) than sample to be dated. Values along the abscissa indicate difference between the true age of sample to be dated and the older contaminant (SO). See discussion in Foiach (75). Figure 7. Effect (in years along ordinate) of the introduction of 1-30% contamination older (lower in activity) than sample to be dated. Values along the abscissa indicate difference between the true age of sample to be dated and the older contaminant (SO). See discussion in Foiach (75).
This technique is useful only when dating objects that are less than 50,000 years old (roughly 10 times the half-life of carbon-14). Older objects have too little activity to be accurately dated. This technique depends on cosmic-ray intensity being constant or at least predictable in order to keep the 14C/12C known throughout the time interval. Also, the sample must not be contaminated with organic matter having a different 14C/12C ratio. [Pg.381]

The impurities of potassium bromide.—Judging from the analyses of potassium bromide by H. Adrian,4 the older commercial varieties were very much more contaminated with impurities than modern samples. For example, H. Adrian found 10 to 15 per cent, of impurities in 10 commercial samples to-day, the German Pharmacopoeia permits 2 per cent, of potassium chloride, and about 0T per cent, of the carbonate. There has been reported 0T3 per cent, of KC1 in English bromide, 4 52 to 5 92 per cent, in American and in both, 0 35 to 1 29 per cent, of moisture. The bromide from Stassfurt is free from iodides. To detect iodides, the soln. is first treated with a little fuming nitric acid, and then shaken with chloroform, if but little iodine is present, the chloroform will not be coloured, but if the bromide be treated with an excess of ferric chloride the iodine will colour the chloroform... [Pg.578]

Deviating results obtained in older studies are, after all, most likely a consequence of the very limited number of usable reflections in diffraction. Usually this number has been lower than 10, and only in the most recent studies on oriented material has the first layer line been exploited. Discrepancies have usually been ascribed to the presence of undoped polyacetylene, either cis or trans, to general inhomogeneity, a small amount of an unidentified phase, or impurities from handling. Especially in isotropic samples, spurious reflections from such contaminants cause positional shifts and altered intensity ratios that caimot be clarified. [Pg.18]

Under normal circumstances, there is no need for the operator to be concerned about routine maintenance of the mass analyzer. With modem turbomolecular pumping systems, it is highly unlikely there will be any pump- or sample-related contamination problems associated with the quadrupole, magnetic sector, or TOP mass analyzer. This certainly was not the case with some of the early instruments that used oil-based diffusion pumps, because many researchers found that the quadrupole and prefllters were contaminated by oil vapors from the pumps. Today, it is fairly common for turbomolecular-based mass analyzers to require no maintenance of the quadrupole rods over the lifetime of the instrument, other than an inspection carried out by a service engineer on an annual basis. However, in extreme cases, particularly with older instruments, removal and cleaning of the quadrupole assembly might be required to get acceptable peak resolution and abundance sensitivity performance. [Pg.160]


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