Isotope-dilution mass spectrometry calibration

Conventional calibration MDRD equation [used only with those creatinine methods that have not been recalibrated to be traceable to isotope dilution mass spectrometry (IDMS)]... 

Isotope dilution mass spectrometry (IDMS) can be applied with most of the ionisation methods used in mass spectrometry to determine isotope ratios with greater or lesser accuracy. For calibration by means of isotope dilution, an exactly known amount of a spike solution, enriched in an isotope of the element(s) to be determined, is added to an exactly known amount of sample. After isotopic equilibration, the isotope ratio for the mixture is determined mass spectrometrically. The attraction of IDMS is its potential simplicity it relies only on the measurement of ratios. The... 

A single measurement of a calibration sample can give the concentration of the test solution by a simple ratio. This is often done in techniques where a calibration internal standard can be measured simultaneously (within one spectrum or chromatogram) with the analyte and the system is sufficiently well behaved for the proportionality to be maintained. Examples are in quantitative nuclear magnetic resonance with an internal proton standard added to the test solution, or in isotope dilution mass spectrometry where an isotope standard gives the reference signal. For instrument responses As and /sample for internal standard and sample, respectively, and if the concentration of the internal standard is Cjs, then... 

Calibration for the head-space gas chromatography method is based on calibration curves with individual amines in distilled water. Isopropylamine is the internal standard [28]. For the isotope dilution mass spectrometry method [2H9]-TMA is used as the internal standard [27]. 

Table 2 Comparison of Cholesterol Reference Method Laborato- tute, NIST National Institute of Standards and Technology (US ry Network (CRMLN) traceability model to the International Or- NMI), IDMS isotope dilution mass spectrometry, ACL accredited ganization for Standardization (ISO) model. BIPM International calibration laboratory, AK Abell-Kendall Bureau of Weights and Measures, NMI National Metrology Insti-...

Some abuse drugs have been extracted from urine by SFE [viz. cocaine and its metabolites (134) and amphetamine and methamphetamine (135). In the first instance, the levels measured using SFE showed analyte recovery better than 70% for cocaine, better than 40% for benzoylecgonine, and better than 85% for ecgonine methyl ester from whole blood and urine. The limits of detection and quantitation were 1 and 10 ng, respectively, based on a 200-pL sample. Regarding amphetamine (AP) and methamphetamine (MA), an in situ SFE and chemical derivatization procedure followed by GC-isotope dilution mass spectrometry in urine was described. The mean recoveries achieved were 95% (RSD = 3.8%) for AP and 89% (RSD = 4%) for MA. The calibration graphs were linear within 100-500,000 ng/mL, varying the limits of detection and quantitation from 19 to 50 and from 21 to 100 ng/mL, respectively. 

GC-ICP-MS has the potential to facilitate simultaneous multielemental speciation analysis because species of Se, Pb, Hg, and Sn have volatile forms and can be analyzed in a single run. The use of ICP-MS as a detector enables calibration by isotope dilution mass spectrometry as weU as providing very low limits of detection (pg-ng range). 

Webb, K.S., VAM report on calibration and traceability for primary chemical methods application of isotope dilution mass spectrometry (IDMS) to organic ultra-trace analysis (1994). 

Thienpont, L.M., Van Nieuwenhove, B., Stockl, D. and De Leenheer, A.P., Calibration for isotope dilution mass spectrometry - description of an alternative to the bracketing procedure, J. Mass Spectrom., 31, 1119-1125 (1996). 

For some radionuclides, only the direct measurement is needed, based on calibration with a radionuclide standard and reference to the measured sample mass. For other radionuclides, the isotope ratio to its stable element is needed. An example of a more complex situation is measurement of the 14C/12C isotope ratio of an environmental or archeological sample in comparison to the modern atmospheric CO2 value. This value must be adjusted for anthropogenic 14C produced by atmospheric testing of nuclear weapons and by other nuclear operations, and also for changes in atmospheric CO2 with cosmic-ray flux fluctuations over time. For an element such as Pu, which has no stable isotope, the total quantity is measured by isotope dilution mass spectrometry in which the sample is traced (or spiked) with 242Pu or 244Pu (see Section 17.3.3). 

Chapter 8 provides the reader with the basic principles of isotope dilution mass spectrometry used for elemental analysis and also discusses more advanced features of this calibration approach, such as its use in direct solid sample analysis and in elemental speciation work, wherein not the total amount but that of various chemical species of a target element need to be determined. 

Stable isotope dilution mass-spectrometry (MSID) is the most accurate technique for determining lanthanide abundances in geochemical materials.. The superior quality of the method may be attributed principally to the inherent sensitivity of mass-spectrometers, and to the use of the ideal internal standard, namely, an artificially enriched isotope of each element to be determined. The utilization of isotopic internal standards virtually eliminates such potential analytical problems as quantitative recovery and instrument calibration. The sensitivity of the mass spectrometer is such that the lower limit of measurable abundance is usually controlled by the purity of the reagents used in preparing the sample for analysis. 

The use of a definitive analytical method can also be used to establish standard reference materials. Definitive methods are ones that can produce exacting quantitative data without the need to compare measurements to a calibration standard. The gravimetric analysis method is a definitive technique. Isotope-dilution mass spectrometry, which is extensively used by NIST and other agencies producing certified standard reference materials, is also considered to be a definitive method of analysis. As discussed in Chapter 7, isotope dilution quantitation can be effectively used with ICP-MS. Therefore, a laboratory with ICP-MS instrumentation can produce reference materials in specific sample matrices for selected elements by using the isotope dilution technique.These standard reference materials still must be considered secondary standards, because they are usually not traceable to existing certified standards. 

CRMs issued by NIST (previously NBS) and IRMM (previously CBNM) based on calibrated isotope ratio mass spectrometry. The indicates the sample is a spike (i.e. can be used for isotope dilution). Further information are presented in www.irmm.jrc.be and www.nist.gov. 

Mestek, O., Koplik, R., Fingerova, H., and Suchanek, M. (2000). Determination of thallium in environmental samples by inductively coupled plasma mass spectrometry Comparison and validation of isotope dilution and external calibration methods.J.y4mi/./l(. Spectrom., 403-408. 

Vanhaecke, E, Boonen, S., Moens, L., and Dams, R. (1997). Isotope dilution as a calibration method for solid sampling electrothermal vaporization inductively coupled plasma mass spectrometry./. Ana/. At. Spectrom. 12(2), 125. 

In isotope dilution inductively coupled plasma-mass spectrometry (ID-ICP-MS) the spike, the unspiked and a spiked sample are measured by ICP-MS in order to determine the isotope ratio. Using this technique, more precise and accurate results can be obtained than by using a calibration graph or by standard addition. This is due to elimination of various systematic errors. Isotopes behave identically in most chemical and physical processes. Signal suppression and enhancement due to the matrix in ICP-MS affects both isotopes equally. The same holds for most long-term instrumental fluctuations and drift. Accuracy and precision obtained with ID-ICP-QMS are better than with other ICP-QMS calibration... 

Applications The application of the isotope dilution technique is especially useful in carrying out precise and accurate micro and trace analyses. The most accurate results in mass spectrometry are obtained if the isotope dilution technique is applied (RSDs better than 1 % in trace analysis). Therefore, application of IDMS is especially recommended for calibration of other analytical data, and for certification of standard reference materials. The technique also finds application in the field of isotope geology, and is used in the nuclear industry for quantitative isotope analysis. 

An excellent possibility for quantifying analytical data in inorganic mass spectrometry is isotope dilution analysis (IDA) based on more precise isotope ratio measurements. IDA uses highly enriched isotope spikes of analytes of known concentration for calibration and is the method of choice if a high accuracy for element concentrations is required. The principles and applications of this method will be discussed below. 

Jonckheere, J. A., De Leenheer, A. P., and Steyaert, H. L. (1982). Statistical evaluation of calibration curve non-linearity in isotope dilution gas chromatography/mass spectrometry. Anal. Chem. 55, 153-155. 

The ideal internal standard is the same element as the analyte because it has similar mass, ionization energy, and chemical properties. Therefore, isotope dilution based calibration provides high accuracy as long as isotope equilibration is attained and the measured isotopes are free of spectral overlaps [192,193]. Standards do not need to be matrix-matched. Quadrupole-based ICP-MS instruments can typically provide isotope ratio precision of 0.1% to 0.5%. Much better isotope ratio precision can be obtained by using simultaneous MS detection, such as a multicollector-based instrument or perhaps time-of-flight MS. In comparison to thermal ionization mass spectrometry, ICP-MS provides much higher sample throughput and simpler, faster sample preparation. 

Institute of Science and Technology has been described by Bowers et al [3]. A recent report describes using isotope dilution to calibrate analysis of solid samples using electrothermal vaporization with inductively coupled plasma mass spectrometry (ICP-MS) [4]. 

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