Racemization rates, calibrating

Means of Calibrating Racemization Rates. Theoretically, the racemization technique has an eflFective dating range of 500,000 years in cool temperate environments. One problem in working with these older samples is finding an appropriate calibration sample. Radiocarbon-dated bones 17-20,000 years old provide suitable calibrations for the last... 

Another process that can account for the occurrence of D-amino acids in organisms is racemization, which involves the conversion of L-amino acids into their mirror-image D-form. Estuarine invertebrates have been found to have D-amino acids in their tissues as a result of this process (Preston, 1987 Preston et al., 1997). Rates of racemization of amino acids in shell materials, calibrated against radiocarbon measurements, have also been used as an index for historical reconstruction of coastal erosion (Goodfriend and Rollins, 1998). 

The rate of the ring-opening reaction of 5, " and other substrates have been determined using an indirect method for the calibration of fast radical reactions, applicable for radicals with lifetimes as short as 1 ps/ This radical clock method is based on the use of Barton s use of pyridine-2-thione-Al-oxycarbonyl esters as radical precursors and radical trapping by the highly reactive thiophenol and benzeneselenol/ A number of radical clock substrates are known/ Other radical clock processes include racemization of radicals with chiral conformations, one-carbon ring expansion in cyclopentanones, norcarane and sprro[2,5]octane, a-and p-thujone radical rearrangements, and cyclopropylcarbinyl radicals or... 

In the second approach, the P-CD CSP was connected directly to the mass spectrometer using a thermospray interface. A mobile phase of 0,1 M ammonium acetate (pH 5) and flow rate of 1.2 mL/mtn were employed during the analysis. By using deuterium-labeled racemic terbutaline as the internal standard, linear calibration curves were obtained down to 4 pmol. 

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