A comparison of calculation procedures for isotope dilution determinations using gas chromatography mass spectrometry

Abstract

Several data reduction procedures have been used for converting isotope ratios, determined using gas chromatography mass spectrometry, into mole ratios for the purpose of quantitating organic compounds. In this report, these procedures are shown to be related under certain limiting approximations of original sample and internal standard isotope ratios. With the understanding that some of the procedures involve approximations, the incorporation of related systematic errors into calculated mole ratio values was investigated. The results indicate that certain data reduction procedures allow quantitation over wider mole ratio ranges and with internal standards which contain a significant quantity of unlabeled impurity.

Measurement of p-nitrophenol in the urine of residents whose homes were contaminated with methyl parathion.

“…Addition of an isotopically labeled standard ( 13 C 6 -PNP) prior to sample manipulation, a technique known as isotope dilution (29), afforded us many advantages. Chemically, 13 C 6 -PNP behaves almost identically to PNP, but they are distinguishable based upon the 6-atomic mass unit (amu) difference in their masses and respective fragment ions (Figure 2).…”

Paradigms in isotope dilution mass spectrometry for elemental speciation analysis

“…It takes a general hyperbolic form which can be approximated to linear relationship only in the absence of isobaric overlaps [19]. Some 30 years ago isotope dilution mass spectrometry was haunted by the significant amount of improper mathematical approximations that were introduced by users to alleviate problems associated with the calibration graph non-linearity [19][20][21]. Not only the use of unweighted regressions were shown to be highly inaccurate at the origin of the IDMS calibration graphs [21], many approximations to the isotope dilution equations have also lead to inaccurate results [20,22].…”

“…Some 30 years ago isotope dilution mass spectrometry was haunted by the significant amount of improper mathematical approximations that were introduced by users to alleviate problems associated with the calibration graph non-linearity [19][20][21]. Not only the use of unweighted regressions were shown to be highly inaccurate at the origin of the IDMS calibration graphs [21], many approximations to the isotope dilution equations have also lead to inaccurate results [20,22]. With the improvement of personal computers the use of calibration graphs has become nearly extinct and many of the above issues are now largely irrelevant since the IDMS results are now calculated directly from the complex equations using various dedicated computer programs.…”

Quantification of 31 volatile organic compounds in whole blood using solid-phase microextraction and gas chromatography–mass spectrometry☆

“…These linear calibration curves (r 2 typically ≥0.99) spanned three orders of magnitude. Calibration curves were adjusted for ion cross-contamination between native analyte and isotopic analog using a feature similar to the correction described by Colby and McCaman [19]. The lowest calibrators ranged from 5 to 120 pg/mL.…”