Isotope dilution experiments with

Intermediates of Denitrification Isotope Dilution Experiments with... 

We also carried out the isotope dilution experiment with NO on soil (17). Much more NO was added because it was expected that NO would be readily consumed in reactions with soil chemicals. Nonetheless, apparent NO exchange was noted as shown in Table IV. Most of the label was trapped as NO. The sterile controls showed an order of magnitude less gas produced. It was surprising, however, that there was not a trend of more NO with decreasing pH, especially in the sterile control. The data in Table IV seem to support the interpretation of the pure culture data, namely that NO or its enzyme-boimd equilibrium species is an intermediate in denitrification. 

Another approach for determination of the absolute configuration of the monosaccharide components69 involves their interaction with radioactive potassium cyanide and conversion of the products into a mixture of heptonamides. Isotopic-dilution experiments showed the presence of D-gfi/cero-L-manno-heptonamide, thus establishing the D-galacto configuration of the starting hexose. 

The success of the isotope dilution experiment for CO on Pt(lll) was accompanied by a serious difficulty in reconciling the magnitude of the shift, which determines Oy/3Q), with the intensity of the band, which also determines Oy/3Q). When due allowance is made for the resultant surface field and geometric factors (36) in RAIR spectroscopy the intensity is almost consistent with the vibrational polarizability av = 0.057 X3 (39), corresponding to the gas phase intensity, as has been concluded for CO adsorbed on copper films (40) from infrared studies and for CO on Pt(lll) (41) and Cu(100) (42) from high resolution electron energy loss spectroscopy. This value of av is an order of magnitude smaller than that deduced from the frequency shift. 

A suitable calibration strategy has been demonstrated for the determination of selected elements, i.e., Pb, Ag and T1 in a small amount of platinum nanoclusters. In on line isotope dilution analysis with LA-ICP-MS, an isotope emiched tracer solution was nebulized by the Aridus microconcentric nebulizer with desolvator. After about 180 seconds, laser ablation of the investigated platinum nanocluster sample was started and after an additional time of 180 seconds 2 % nitric acid nebulized instead of the tracer solution. The isotope ratios of lead ( Pb/ ° Pb), silver ( ° Ag/ ° Ag) and thallium ( ° T1/ ° T1) - with Cu/ Cu for internal standardization - were monitored during the whole experiment. The measuring time for one experiment was <10 min. 

IR spectroscopy of adsorbed CO/NO combined with isotope dilution experiments in the ad-layer revealed both isolated and clustered coordinatively unsaturated cations on the surface. 

The mechanism of homocysteine biosynthesis by green plants received little attention until the classic isotope dilution experiments of Dougall and Fulton (1967), with cultured cells of Paul s Scarlet Rose, provided evidence... 

FIGURE 2. The pyruvate and the acetate pool are both needed for the formation of the introductory compound of plastidic Mev pathway. Isoprenoid (Car -H PQ) and fatty acid (16 0 -M8 1) formation in developing chloroplasts was stud j.ed in an isotopic dilution experiment. Chloroplasts (31 - 38 pg chi ml vol 1 ml) were supplied with the labeled precursor and increasing cunounts of jnlabeled substrate in the following combinations. Absolute values of C-incorporation j.nt(j> isoprenoids and fatty acids, in nmol C incorporated (mg chi) h -100, are indicated by brackets //. FIG. 2A 5 mM NaH CO (540 KBq)... 

Feeding experiments have also been performed with more advanced putative precursors both for cocaine and tropane biosynthesis. In these experiments, the role of (21) and (22) in cocaine and tropane biosynthesis was examined. No incorporation of the ethyl ester derivative or of the N-acetylcysteamine thioester derivative of (21) into either of the alkaloids, cocaine or scopolamine, was observed [39]. However, isotope dilution experiments did provide evidence for the presence of (21) in Erythroxylon coca [46]. In the case of (22), fed as its ethyl ester, substantial specific incorporations into cocaine [47] and scopolamine [48] were recorded. These results were interpreted as corroborating evidence for the hypothesis that the acetate units in these alkaloids are introduced in a stepwise fashion via Path D2 in Scheme 8 with (22) as an intermediate. 

With the use of IRMs, these false assumptions can be avoided and traceable isotope amount ratios can be generated. At present, however, only for a limited number of elements is an IRM available, as can be seen from Tables 6.1 and 6.2. Natural-like IRMs (Table 6.1) can be used to correct for mass discrimination, whereas enriched IRMs (Table 6.2) are mainly used in tracer studies and isotope dilution experiments. The enriched IRMs offer sufficient quality, but the number of elements covered is too low. This has been discussed at several meetings with the summarized outcome that several enriched isotopes have been exhausted... 

Tir) contributions to Ti have rather complicated and quite different dispersive behavior (see Chap. 6), the separation of two effects from a Ti frequency dispersion curve is a formidable task. It is therefore preferable to identify and separate the intermolecular and intramolecular contributions to relaxation before applying theories. Isotopic dilution experiments [7.73, 7.74] have been done to isolate Tir in liquid crystals. Unfortunately, this technique has not yet been used in conjunction with the field-cycling technique to obtain Tir over a wider frequency range. 

In contrast to the results obtained in animal systems, earher studies suggested that free hydroxyproline may serve as precursor of the hydroxyprohne in actinomycin I (Katz and Goss, 1958, 1959 Katz, Prockop and Udenfriend, 1962). The addition of C-hydroxyproline enhanced formation of actinomycin I suggesting that a direct incorporation of hydroxyprohne could take place (Katz and Goss, 1958, 1959). Isotope dilution experiments have also provided further evidence for this hypothesis (Katz, Prockop and Udenfriend, 1962). In an experiment in which L-proline- C and unlabeled hydroxyproline were supphed to S. antibioticus it was determined that there was a 6-fold dilution of C-label in the hydroxyproline of actinomycin I. Somewhat more direct evidence was obtained with C-hydroxy-L-proline which was incorporated into actinomycin I as peptide-bound hydroxyproline to an appreciable extent (Katz, Prockop and Udenfriend, 1962). The efficiency of incorporation into actinomycin I actually approached that observed in some experiments with L-proline- C. As only a limited amount of C-label was present in the proline (or oxoproline) of actinomycins I, IV and V, it was postulated that free proline is converted to free hydroxyproline which is then incorporated into the antibiotic peptide. If this postulate is true, then the microbial system may differ somewhat from animal... 

In the analysis of seawater, isotope dilution mass spectrometry offers a more accurate and precise determination than is potentially available with other conventional techniques such as flameless AAS or ASV. Instead of using external standards measured in separate experiments, an internal standard, which is an isotopically enriched form of the same element, is added to the sample. Hence, only a ratio of the spike to the common element need be measured. The quantitative recovery necessary for the flameless atomic absorption and ASV techniques is not critical to the isotope dilution approach. This factor can become quite variable in the extraction of trace metals from the salt-laden matrix of seawater. Yield may be isotopically determined by the same experiment or by the addition of a second isotopic spike after the extraction has been completed. 

Van Heuzen, A.A., Hoekstra, T. and van Wingerden, B., Precision and accuracy attainable with isotope dilution analysis applied to ICP-MS theory and experiments, J. Anal. At. Spectrom. 4, 483 (1989). 

In some applications like newborn screening and filter paper blood spots, the internal standard that is labeled cannot be mixed with blood. It can only be present in the extraction solvents. Therefore, only the extracted metabolites can be quantitatively measured. I have denoted a term called pseudo-isotope dilution to account for the differences between traditional isotope dilution and the technique commonly used in newborn screening by MS/MS. A special analysis is capable using this technique, however, in terms of an extraction efficiency experiment. With isotope-labeled standards you can perform an experiment whereby a traditional isotope-dilution technique (internal standard added to liquid blood and spotted) is compared to pseudo-isotope dilution techniques (internal standard is added to the extraction matrix). The ratio of the results of these two analysis (pseudo/traditional) is the extraction efficiency. 

It is evident from Table 1 that certain limiting factors exist. For example, experiments with bromine-82 are limited to a duration of about one week because of the short half-life. At the other end of the scale, experiments with stable carbon-13 are limited to dilutions of less than x 500. Even with radioactive isotopes the maximum specific activity available may limit dilution though not to the same extent. Thus, chlorine-36 can stand dilutions up to x 107 but tritium can improve on this to a factor of x 1012. 

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