Limited data set

Unlike THg, a limited data set about the occurrence of organomercury compounds in biota of the Ebro River has been noticed. MeHg concentrations were only determined in zebra mussel specimens in the Flix hot spot [32] as well as some fish species living in Flix dam (Diez, unpublished results). 

Based on a very limited data set, it is likely that most of the effects of phenol exposure, including cardiac arrhythmias and central nervous system depression, observed in adults will be observed in children if exposures are comparable. The data are insufficient to determine whether children will be especially sensitive to such effects, however. 

The MEM is a powerful new method which is especially useful in cases with limited data sets (powder diffraction). Monte Carlo simulations have shown that the MEM introduces systematic features into the reconstructed density and caution should be exercised when interpreting fine details of an MEM density. It must be emphasized that because the present MEM algorithms do not contain any models, they cannot filter out inconsistencies in the data stemming from systematic errors. The MEM densities may therefore contain non-physical features not only because of systematic bias in the calculation but also because of systematic errors in the data. 

In the smaller peptide systems, only three reports containing heme peptide reduction potentials have been published however, this limited data set reveals two important aspects of cytochrome design. Figure 12 shows that Huffman et al. (125) have convincingly illustrated a direct 

The integrated H202 concentration was directly related to the all of the radiation measurements, but correlated best with the ultraviolet portion (r2 = 0.985). To use this limited data set, three assumptions need to satisfied  

In an attempt to resolve taxonomic arguments, Craige and co-workers (2Q) reported the presence of DMSP in 12 of 13 unicellular Prasinophyceae they studied, and only 1 of the 10 unicellular Chlorophyceae examined. This limited data set implied a relatively widespread occurrence of DMSP in the marine members of the families. Reed (12) initiated a study to quantitatively determine the concentration of DMSP in marine algae. The data from that study are summarized in Table I. 

The operation of Eq. (3.3) is illustrated by the results given in Table 2 out of 48 molecules of the cc-pVTZ set. They are listed in order of increasing correlation energy. The first column of the table lists the molecule. The next 6 columns show how many orbitals and orbital pairs of the various types are in each molecule, i.e. the numbers Nl, Nb, Nu, Nlb etc. The seventh column lists the CCSD(T)/triple-zeta correlation energy and the eight column lists the difference between the latter and the prediction by Eq. (3.3). The mean absolute deviation over the entire set of cc-pVTZ data set is 3.14 kcal/mol. For the 18 molecules of the CBS-limit data set it is found to be 1.57 kcal/mol. The maximum absolute deviations for the two data sets are 11.29 kcal/mol and 4.64 kcal/mol, respectively. Since the errors do not increase with the size of the molecule, the errors in the estimates of the individual contributions must fluctuate randomly within any one molecule, i. e. there does not seem to exist a systematic error. The relative accuracy of the predictions increases thus with the size of the system. It should be kept in mind that CCSD(T) results can in fact deviate from full Cl results by amounts comparable to the mean absolute deviation associated with Eq. (3.3). 

The X-ray crystal analysis of 5-trimethylsilanyl-4-trimethylsilanylethynyl-l//-pyrazole-3-carboxylic acid ethyl ester was obtained only with / = 0.17 because the crystals of the molecule diffracted extremely weakly and only a very limited data set was available. This means that although the gross stereochemistry of the molecule has been determined, individual bond lengths are not reliable (88JOM247). 

Therefore, there is no evidence to support the contention that the ecological risk assessments must necessarily be based on indigenous species and communities, at least when researchers are interested in threshold concentrations of effects. However, these conclusions are based on very limited data sets, and further research is required to establish the extent to which they can be generalized across geographical locations, taxonomic groups, and compounds. 

Zeolite structures pose unconventional problems for crystal structure refinement. These problems arise from positional disorder pseudo-symmetry, twinning, high mobility of some atoms, and (sometimes) the inaccessibility of single-crystal data. Methods are discussed for investigating split atoms, Si-Al distribution, pseudo-symmetry, and for dealing with parameter correlation and limited data sets. Some additional techniques which have not been applied to zeolite structures are mentioned. 

In DLS computations, constant atomic distances are used and have been very useful. Therefore, it appears worthwhile to constrain the positional parameters of the framework with respect to the known distances for Si-O, Al-O, and 0-0 for normal structure factor least-squares computations. Constrained refinement essentially reduces the number and/or variability of the parameters and can be helpful for work with limited data sets (e.g., for powder diffraction). Constrained refinement has been discussed by Pawley (26, 27). 

---