Reverse isotope dilution

Bechtold WE, Sabourin PJ, Henderson RF. 1988. Reverse isotope-dilution method for determining benzene and metabolites in tissues. J Anal Toxicol 12(4) 176-179. 

The reverse isotope dilution technique can be applied for accurate determination of the Mg contents in a sample, sample, on applying equation 7, by measuring the isotope ratio of a selected pair of stable isotopes, in a weighed mixmre of the sample with an isotopically enriched CRM. The average atomic masses m and the isotopic ratios R of Mg in the enriched CRM and in nature are known. The method was applied for determination of Mg in plant material using a CRM isotopically enriched with Mg, measuring with an ICP/MS instruments. ... 

Values in parentheses obtained by reverse isotope dilution Values in parentheses obtained by gas chromatography " 0-72-h urine 0 8-h urine... 

When the compound to be measured is already radioactive, the preceding method is called reverse isotopic dilution. In this case, the procedure is the same however, the radioisotope is diluted with a stable isotope. 

Carrier Addition or Reverse Isotope Dilution Technique Statement of Problem ... 

Use of internal standards. Mann and Jaworski (31) reported that when the recovery of 1-TI C IAA is monitored during a sample purification procedure, considerable loss of IAA can be detected. Bandurski and Schulze (32) suggested the use of reverse isotope dilution to help quantify the actual loss of IAA during sample analysis. In this procedure, one adds a trace amount of radio-labeled compound which ideally is identical to the compound being monitored. High specific activity is required so that statistically significant amounts of isotope can be detected without having to add an excessive quantity (mass) of internal standard. The amount of internal standard must be less than the amount of PGS. One may then accurately determine the recovery efficiency of the internal standard and thus of the PGS (32). 

Figure 4. Proposed pathways of the degradation of diflubenzuron in agricultural soils and hydrosoils. Abbreviations rid = reversed isotope-dilution ms = mass spectrometry tic = thin layer chromatography.

An important application of isotope dilution in radiochemistry is the determination of a radionuclide by dilution with an inactive nuclide (inactive compound), also called reverse isotope dilution. This apphcation is very valuable if the radionuclide is present in carrier-free form. Again, quantitative separation is avoided a measured amount mi of an inactive isotope of the element to be determined is added and after a non-quantitative separation the amount m2 is measured. The ratio wa/wi is the yield of the separation procedure and the activity of the carrier-free radionuclide (Ax = 0) is obtained from the measured activity A2 ... 

DETERMINATION OF UNKNOWN AMOUNTS OF RADIOACTIVE COMPOUNDS—REVERSE ISOTOPE DILUTION... 

A plant was grown in an atmosphere containing C Os (3 x 10 CPM//i,mole). After several weeks a leaf extract was prepared for glucose-l-phosphate determination via reverse isotope dilution analysis. To 20 ml of the extract, 1.5 mmoles of unlabeled dipotassium glucose-1-phosphate were added. A small amount of dipotassium glucose-1-phosphate was reisolated from the extract and recrystallized to constant specific activity from aqueous ethanol. The recrystallized salt had a specific activity of 2.6 X 10 CPM//jLmole. Calculate the concentration of labeled glucose-1-phosphate in the extract. 

The preceding method is known as reversed isotope dilution analysis when the compound to be measured is already radioactive. The principle remains the same the activity of the subject compound (measured from a fraction), is carried out before and after dilution with the same compound, non-labelled. The calculations are identical. This analysis is used for the determination of the isotopic carrier in a solution of a radionuclide using one of its stable isotopes. 

As defined by reverse isotope dilution analysis and metabolic profiling experiments, unchanged semduramicin sodium was the major component of total residues in liver at the 6 hour withdrawal period. At later withdrawal times, unchanged semduramicin sodium levels declined by a biphasic pattern in liver similar to that of total residues. 

Equations for reverse isotope dilution are very simply derived from the "one-way" isotope dilution case described in Eq. (1) ... 

Spike material has to be an isotopically enriched material, is hence available to limited amounts only and/or can be expensive. Reverse isotope dilution can be used to perform its assay because it can be performed on minute amounts or very small concentrations (ug.mr ). 

Analysis of tissues for lAA as determined by reverse isotope dilution assays reveals however, that brassinolide does not affect auxin uptake or auxin movement within the tissues (41). In this same study we also observed that bean internode sections treated with brassinolide contained significantly less lAA after 2 hours than tissues treated with lAA alone, even though BR potentiated lAA-induced growth by more than 400%. 

---