Stable-isotope-labeled analogues

S. P. Jindal, T. Lutz, Mass Spectrometric Studies of Cocaine Disposition in Animals and Humans Using Stable Isotope-Labeled Analogues , J. Pharm. Sci. 1989, 78, 1009 -1014. 

Jindal, S. P, Lutz, T. Mass spectrometric studies of cocaine disposition in animals and humans using stable isotope-labeled analogues. J. Pharmacol. Sci. 1989, 78, 1009-1014. 

Alvarez-Sanchez R, Montavon F, Hartung T, Pahler A. Thiazolidinedione bioactivation A comparison of the bioactivation potentials of troglitazone, rosiglitazone, and pioglitazone using stable isotope-labeled analogues and liquid chromatography tandem mass spectrometry. Chem Res Toxicol 2006 19(8) 1106—1116. 

Stable isotope dilution mass spectrometry (IDMS) IDMS is a technique (method) based on the addition to an analytical sample of a known amount of a stable isotopically labelled analogue of a desired analyte as an internal standard. The ratio of the amount of added isotopic analogue and the naturally occurring compound, as measured by mass spectrometry, provides highly accurate values. Definitive accuracy of the IDMS technique requires full equilibration between endogenous natural amounts of the compound... 

When more precise measurements are required, stable-isotope dilution techniques are employed. In this approach to quantitation, a measured amount of a stable-isotope-labelled analogue of the compound of interest (internal standard) is added to the fluid to be analysed. The sample is then prepared for analysis and the mass spectrometer is used to report the ratio of the unlabelled to labelled analogues. With relatively few and minor corrections applied, the concentration of the unlabelled metabolite is easily determined. The technique is insensitive to losses in sample separations and variable derivatization yields as these will affect both analogues in the same proportion. As an example. Figure 2 illustrates the use of this technique in the accvuate determination of the concentration of MMA in human serum. 

For a quantification an addition of stable isotope-labelled analytes, which exhibit the same chromatographic and ionisation behaviour as their unlabelled analogues present in the sample, and which can be... 

A quite reliable method involves the addition of stable isotope-labelled analytes, which exhibit the same chromatographic and ionisation behaviour as unlabelled analogues present in the sample, and which can therefore be distinguished from the latter with the aid of the MS detector separating the different ion masses. However, whereas for many environmental pollutants such as pesticides, pharmaceuticals,... 

Lanckmans, K., Sane, S., Smolders, I., and Michotte, Y. (2007). Use of a structural analogue versus a stable isotope labeled internal standard for the quantification of angiotensin IV in rat brain dialysates using nano-liquid chromatography/tandem mass spectrometry. Rapid Commun. Mass Spectrom. 21 1187-1195. 

Stable Isotope Labeled Internal Standards vs. Structural Analogue Internal Standards... 

Unfortunately, for the majority of small molecule LC-MS/MS analyses, stable isotope labelled internal standards are not available so far. In such cases, compounds with a very similar molecular structure typically serve as internal standard ( homologues or analogues ). Since the ionization properties are substantially determined by functional groups of a molecule, ionization behaviour may differ significantly—even between compounds with very similar over-all molecular structure. Differential clustering, e.g. with sodium, ammonium or formate ions often present in mobile phases may as well impact the parity of ionization yield between analyte and internal standard. Hence the availability of an appropriate homologue is crucial and critical for the development of reliable LC-MS/MS methods in TDM [51]. 

Another requirement for qualitative or quantitative analysis is the use of internal standards (IS) to compensate for sample preparation or chromatographic variability. This is of particular importance in LC-MS analysis, as an adequate IS can also compensate for the negative influence of matrix effects on method precision and accuracy. Stable-isotope-labeled ISs are the most appropriate for this purpose. If a specific deuterated analogue is not commercially available, it could be substituted for deuter-ated substances with similar physicochemical properties to the analyte of interest. However, the use of other marketed pharmaceuticals for this purpose should be avoided, as it cannot be excluded that the patient to be monitored has taken that drug. 

Where a stable isotope-labeled standard is unavailable, the analyst can use either a chemically similar homologue (e.g., incorporating an additional methylene different m/z values to monitor) or a chemically similar analogue (e.g, geometric isomer, same m/z values to monitor) that will need to be chromatographically separated... 

GC-MS is a reliable technique for planar PCBs quantitation because of its improved selectivity, particularly given the availability of C-labeled PCB standards. Using isotope dilution, each individual sample (i.e., unknown samples, calibration standards, quality controls, and blanks) is enriched with stable isotope-labeled analogs of analytes of interest, usually C-labeled for PCBs and pesticides. Chemically, the analytes and labeled analogues behave identically however, they can be distinguished based on their mass differences, thus allowing a complete and automatic recovery correction for each analyte in individual sample. These analyses are typically more accurate, selective, and sensitive than GC-ECD analyses to obtain the sensitivity needed, the mass spectrometers must be operated in the selected ion monitoring (SIM) mode. 

Stable isotope dilution assay. Analytieal method that uses isotope-labelled analogues of the analytes as internal standards to compensate for losses and... 

Remane D, Wissenbach DK, Meyer MR, Maurer HH. Systematic investigation of ion suppression and enhancement effects of fourteen stable-isotope-labeled internal standards by their native analogues using atmospheric-pressure chemical ionization and electrospray ionization and the relevance for multi-analyte liquid chromatographic/mass spectrometric procedures. Rapid Comm Mass Spec 2010 24 859-67. 

It is still debated whether one should make an all-out effort to obtain a stable-isotope labeled version of the compound being investigated or it is adequate to use an analogue of the compound. 

An internal standard (IS) is required to reduce the impact of the system variability on method performance, and its selection is important for quantitative bioanalysis. Structural analogues and stable isotopically labeled (SIL) compounds remain the gold standards of ISs however, structural analogues differ from the compounds of interest and may have different ionization behaviors than the analytes. Even with a closely eluting IS and analyte, they reach the ionization source at different retention times, and short-term variations in the ionization process may be of concern, particularly for ESI. 

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