Quantification stable isotope dilution analysis

Aranda, M., Morlock, G. (2007) New method for caffeine quantification by planar chromatography coupled with electrospray ionization mass spectrometry using stable isotope dilution analysis. Rapid Communications in Mass Spectrometry RCM, 21,1297-1303. 

Guth, H. (1996) Use of the Moving Capillary Switching System (MCSS) in combination with stable Isotope Dilution Analysis (IDA) for the quantification of a trace component in wine. Poster at the 18th International Symposium on Capillary Chromatography, Riva del Garda, Italy, May 20-24, 1996. 

Accurate quantification is not straightforward as the amounts of different aroma compounds extracted are related to their partition ratio between the food and its headspace. Stable isotope dilution analysis using a labeled internal standard can be used [7]. 

The development of new fiber coatings in the near future should further improve the specificity of SPME and overcome some of the observed matrix effects. Quantification by stable isotope dilution gas chromatography/mass spectrometry (GC/MS) may assist in improving analytical performance. Along with the possible application of micro LC and capillary LC columns to in-tube SPME, the development of novel derivatization methods and the potential for the analysis of fumigant pesticides, SPME appears to be a technique with a future in the analysis of pesticide residues in food. 

Since alterations in global DNA methylation are implicated in various pathobio-logical processes, a gradient IPC-ESI-MS/MS method with a volatile IPR was used to determine cytosine and 5-methylcytosine in DNA quantification relied on stable isotope dilution [58], Muscular dystrophies caused by various mutations in the dystrophin gene are amenable to easier prenatal diagnosis via a multiplex polymerase chain reaction (PCR)/IPC assay [59]. Some guidelines for the analysis of genomic DNA by IPC-ESl-MS can be found in Reference 60. 

Mass spectrometric analysis using negative chemical ionization techniques affords greater sensitivity over electron impact methods in that a very intense ion expressing the molecular weight of the compound of interest is produced. Quantification using the stable isotope dilution method is exact and reproducible. The method does not rely on the efficiency of extraction, but rather on the ratio between the compound of interest and the internal standard, which remains constant in an individual sample. 

A stable isotope dilution assay using mass spectrometry to measure insulin, proinsulin, and C-peptide has been developed. The difference in mass among the three analytes allows specific measurement of each protein. Comparison of patient samples revealed that most, but not all, results were higher by immunoassay than mass spectrometry. Thus immunoassays may overestimate insulin, particularly at low concentrations. The high protein concentration in the serum requires extraction of proteins (e.g., by immunoaffinity) and purification by high-performance liquid chromatography (HPLC) before quantification by mass spectrometry. This method is not suitable for routine laboratory analysis. 

Determination of absolute quantities of peptide levels mostly employ the stable isotope dilution (SID) methodology coupled to mass spectrometric analysis. SID involves the use of stable isotope-labeled standards wherein quantification is performed by comparing the mass spectrometric profiles of analytes to their corresponding labeled standards. It is important to choose the internal standard carefully to accurately measure absolute quantities from signal intensities. 

The quantification of ochratoxin A, at levels within the range 0.25-10 ng/ml from wine by HPLC-fluorescence detection, was described [193]. RP-HPLC - fluorescence method for the detection of ochratoxin A in wine with a detection limit of 0.05 ng/ml was also published [194]. A stable isotope dilution assay by LC-MS/MS was developed for quantification of the ochratoxin A by using [D5]-ochratoxin A as internal standard with a low detection and quantification limits of 0.5 and 1.4 pg/kg, respectively [195]. The LC-MS/MS method (ESI and APCI) was also applied to the analysis of contaminated coffee samples by ochratoxins A and B with absolute minimum detection limit around 10-20 pg per injection. Fragment ions from the [M+H]+ and [M+Na]+ ions of... 

Quantitative determinations will be precise only if full corrections for losses occurring during extractions and possible detector instabilities are made. The mass spectrometer measuring the mass of molecules, the use of auxins labeled with stable isotopes provides almost ideal IS for accurate quantifications. The stable isotope dilution method [22] consists in measuring by Multiple Ion Monitoring (MIM) the ratio of normal to heavy isotopes added at the beginning of the analysis. The absolute amount of auxin in plant extract is then obtained [see 19]. 

Isotope dilution mass spectrometry (ID-MS) is widely accepted as a quantification procedure of proven accuracy in elemental analysis and isotope ratio measurements [4]. Several areas of research in nuclear science, geochronology, medicinal chemistry, environmental science, and agricultural science have benefited from this technique. ID-MS is applicable to all elements that have at least two stable isotopes. Monoisotopic elements can be analyzed only if they have a long-lived natural or artificial radioisotope. For example, iodine and thorium have been determined with spikes of the long-lived isotopes 29i and 25 Th, respectively [44]. TI-MS and ICP-MS are the methods of choice for accurate ID-MS analysis. ICP-MS has the advantage that several elements can be analyzed simnltaneously under the same experimental conditions. Other ionization techniqnes discussed in this chapter have also been coupled with ID-MS. 

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