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Performance of these estimators was measured by the total mean squared error (TMSE) averaged over the 1000 replications, and R was defined as the ratio of TMSE values calculated for ML and EB estimates.
Nevertheless, on the basis of these NMR and mass spectral data, Byard (13) identified the selenium urinary metabolite as TMSe.
It is surprising that TMSe, a trace constituent of the urine sample, could be obtained pure from such a simple, nonselective procedure, particularly in the light of later work (see below) showing that the reineckate salt of TMSe is appreciably soluble in water and that additional steps must be taken to obtain acceptable yields in the precipitate.
(14) almost immediately followed that of Byard (13), and similarly identified TMSe in rat urine.
These studies showed that TMSe, originally termed U1, was a common urinary metabolite from all tested selenium sources and constituted 20-50% of the urinary selenium.
Similar studies with rats and radiolabeled [sup.75]Se were also carried out by Kiker and Burk (17) in 1974, and they also reported the presence of TMSe and a second major compound thought to be the same as U2 in the study of Palmer et al.
Byard had earlier suggested (13) that TMSe may be a detoxification end product of selenium metabolism, and this was supported by further work by Palmer's group (16).
Following the collective studies of Byard (13), Palmer's group (14-16), and Kiker and Burk (17), it appeared that TMSe was a major selenium metabolite in rat urine and that at least one other major metabolite (U2) was also present.
In summary, in the early work on selenium in rat urine, two separate studies isolated a major metabolite, apparently to purity, and identified it as TMSe by comparison of spectral data (MS, NMR, and infrared spectroscopy) with those of a synthesized specimen.
These problems appear to have been apparent to the group working under Janghorbani, who carried out studies in the 1980s that reexamined much of the previous work after developing an analysis designed to selectively determine TMSe (19, 22, 37).
It is clear that other researchers were also unhappy with the data being published about selenium species in urine, particularly in terms of the quantification of TMSe, and analytical procedures claiming further improvements in selectively measuring TMSe were reported.
(23) reported a refinement of the cation-exchange/ reineckate precipitation method in which they used buffered cation-exchange chromatography with SP-Sephadex medium and performed the precipitation with trimethylsulfonium ion as carrier to improve the yield of TMSe. The authors stated that without this carrier, the yields of TMSe obtained as the reineckate salt were low, an important result considering the early methods for isolating TMSe.
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