Spectrometry quantitative calculations

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

This technique provides quantitative information about tautomeric equilibria in the gas phase. The results are often complementary to those obtained by mass spectrometry (Section VII,E). In principle, gas-phase proton affinities, as determined by ICR, should provide quantitative data on tautomeric equilibria. The problem is the need to correct the measured values for the model compounds, generally methyl derivatives, by the so-called N-, 0-, or S-methylation effect. Since the difference in stability between tautomers is generally not too large (otherwise determination of the most stable tautomer is trivial) and since the methylation effects are difficult to calculate, the result is that proton affinity measurements allow only semi-quantitative estimates of individual tautomer stabilities. This is a problem similar to but more severe than that encountered in the method using solution basicities (76AHCS1, p. 20). 

The isotope dilution method consists of mixing a natural sample with an artificial spike and measuring the isotopic ratios of the mixture using mass spectrometry, providing very precise quantitative determination of the concentrations of elements in trace quantities. A spike is a solution that contains a known concentration of the element, artificially enriched in one of its minor isotopes. For example, natural Rb samples have 27.84% and 72.16% Rb (Fig. 11.lA). Rb spikes are made by artificially enrich the minor isotope Rb. And a solution with 90% Rb and 10% Rb (Fig. 11.IB) is a Rb spike. Of course, a solution with 99.99% Rb and 0.01% Rb is a better Rb spike. When the known quantities (mass) of the sample and spike are mixed, the resulting isotopic compositions can be used to calculate the concentration of the element in the sample. 

X-Ray Photoelectron Spectrometry. X-ray photoelectron spectrometry (XPS) was applied to analyses of the surface composition of polymer-stabilized metal nanoparticles, which was mentioned in the previous section. This is true in the case of bimetallic nanoparticles as well. In addition, the XPS data can support the structural analyses proposed by EXAFS, which often have considerably wide errors. Quantitative XPS data analyses can be carried out by using an intensity factor of each element. Since the photoelectron emitted by x-ray irradiation is measured in XPS, elements located near the surface can preferentially be detected. The quantitative analysis data of PVP-stabilized bimetallic nanoparticles at a 1/1 (mol/mol) ratio are collected in Table 9.1.1. For example, the composition of Pd and Pt near the surface of PVP-stabilized Pd/Pt bimetallic nanoparticles is calculated to be Pd/Pt = 2.06/1 (mol/ mol) by XPS as shown in Table 9.1.1, while the metal composition charged for the preparation is 1/1. Thus, Pd is preferentially detected, suggesting the Pd-shell structure. This result supports the Pt-core/Pd-shell structure. The similar consideration results in the Au-core/Pd-shell and Au-core/Pt-shell structure for PVP-stabilized Au/Pd and Au/Pt bimetallic nanoparticles, respectively (53). 

The QMS platform combines the identification of proteins with their quantitative detection in one procedure. While protein identification can be deduced from peptide mass fingerprinting (PMF) or MS/MS spectra (see section Mass Spectrometry (MS)), protein quantitation is based on analysing either peak areas or signal intensities, or a combination of both. Several computer programs, in most cases reagent specific ones, are available. For each peak, quantitation values are calculated before differentially expressed proteins are identified by the comparison of control and treated samples. 

Gas chromatography and gas chromatography/mass spectrometry were employed to identify and quantitate individual molecular components. Both 25 and 50 meter glass support coated open tubular (SCOT) and fused silica wall coated open tubular (WCOT) capillary columns (SE.30, BP.l and BP.5 phases) were used with H2 as a carrier gas and F.I.D. detection. Acidic components were derivitized (BF3/methanol) to their methyl esters and hydroxyl groups to their silyl ethers (N,0-bis-(trimethylsilyl)trifluoroacetamide) in order to improve chromatographic separation. Carbon Preference Indices (CPI) were calculated using the equation -... 

Mass spectrometry is a powerful qualitative and quantitative analytical tool that is used to assess the molecular mass and primary amino acid sequence of peptides and proteins. Technical advancements in mass spectrometry have resulted in the development of matrix-assisted laser desorption/ion-ization (MALDI) and electrospray ionization techniques that allow sequencing and mass determination of picomole quantities of proteins with masses greater than 100kDa (see Chapter 7). A time-of flight mass spectrometer is used to detect the small quantities of ions that are produced by MALDI. In this type of spectrometer, ions are accelerated in an electrical field and allowed to drift to a detector. The mass of the ion is calculated from the time it takes to reach the detector. To measure the masses of proteins in a mixture or to produce a peptide map of a proteolytic digest, from 0.5 to 2.0 p.L of sample is dried on the tip of tlie sample probe, which is then introduced into tire spectrometer for analysis. With this technique, proteins located on the surfaces of cells are selectively ionized and analyzed. 

Shui-Tse Chow stated that the only physical method offering the necessary identification selectivity with quantitative capability for the gas chromatographic analysis of heroin in illicit samples is selected 1on monitoring (SIM) mass spectrometry. Deuterated heroin and the ions m/e 369 and 327 for heroin and 375 and 331 of deuterated heroin were used as internal standards and the ions m/e 369 and 327 were quantified and calculated as heroin. The calibration curves for both m/e 375/369 and m/e 331/327 were linear within the concentrations studied. The gas chromatographic analysis was qarried out on a 1.8 m by 6.35 imi O.D. glass column packed with 3 % OV-17 on Chromosorb W HP, 100,120 mesh, at a column temperature of 270°C. For GC-MS 1.8 m by 6.35 mm O.D. glass columns were used, packed with 3 OV-1 on Chromosorb W HP, 100-120 mesh, and a column temperature of 250°C. The results obtained are summarized in Table 14.20. 

Pouchou TL, Pichoir F (1984) Un nouveau modele de calcul pour la microanalyse quantitative par spectrometrie de rayons X I. Application a I analyse d echantillons homogenes. La recherche aerospatiale, No. 3 (Mai-Juin), p 167-192... 

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