Isotope internal correction, calibration with

Quantification is usually achieved by a standard addition method, use of labeled internal standards, and/or external calibration curves. In order to allow for matrix interferences the most reliable method for a correct quantitation of the analytes is the isotope dilution method, which takes into account intrinsic matrix responses, using a deuterated internal standard or carbon-13-labeled internal standard with the same chemistry as the pesticide being analyzed (i.e., d-5 atrazine for atrazine analysis). Quality analytical parameters are usually achieved by participation in interlaboratory exercises and/or the analysis of certified reference materials [21]. 

Ion fractionation can be very effectively corrected by an internal calibration after spiking a uranium sample with a mixture of isotopes of certified ratio, like IRMM-3636 (Richter et al. 2008) or CETAMA-MIRF-02, or spiking a plutonium sample with a certified mixture of Pu and Pu isotopes. Several batches of mixtures of Pu, and Pu isotopes... 

Elan software appears to be designed with the assumption that if one is analyzing isotope ratios, the element concentration is plentiful, and the isotopic abundances are not very different. Thus, no option is included for an internal standard, nor for blank subtraction. These would be negligible if the above assumptions were true. For uranium, and especially uranium at urine concentrations, these assumptions are not valid. Therefore, no form of quantitative calibration is selected. Each isotope and element will be acquired as Counting (CPS) Only, so that the internal standard corrected blank counts per second may be calculated and subtracted offline with a program such as Microsoft Excel. 

Due to the complexity of the samples (e.g., layered composition or inclusions), it is not straightforward to prepare these almost perfectly matrix-matched standards in many situations and, as discussed before, suitable reference materials are often not available. K that is the case, as commonly occurs in archaeometric research, then it is still possible to obtain reliable quantitative information with non-matrix-matched solid standards, under certain conditions. First, the use of an internal reference element, which is recommended to compensate for any variation in the measuring conditions even if matrix-matched standards are used for calibration, becomes absolutely imperative in this case. The signal intensity for one of the isotopes of a selected element should correct for any differences in ablation yield or transport efficiency, for example between the samples and the standard(s). 

Quantification methods can be based on standard additions or external calibration. Internal standards may be used to correct for some losses of material but for many analyses suitable compounds are unavailable. In LC-MS and LC-MS/MS the ideal internal standards are stable isotope analogs of the target compounds as these behave exactly as the native compound used but can be distinguished by their different masses. For LSD-related compounds with slightly different structures such as methysergide have been used, but trideuterated LSD is available commercially. 

Isotope dilution This relatively new calibration method is occasionally used in specialized forensic applications. The method affords exceptional accuracy and precision, but is limited by the availability of suitable standards. Although the use of deuterated internal standards is sometimes equated with isotope dilution, the techniques are not the same. Isotope-dilution calibration is based on the addition of standards that contained enriched levels of stable isotopes of common elements such as C. The enriched standard, referred to as the spike, is added to the sample and allowed to reach equilibrium. The sample is then analyzed via mass spectrometry. Because the spike is chemically identical to the target analyte, the matrix correction is as good as that achieved with standard-addition calibration. However, until enriched stable isotope analogs of forensic analytes are available and affordable, isotope dilution will not be widely applied in forensic chemistry. 

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