Molarer Response

The most widely used method of analysis for methyl chloride is gas chromatography. A capillary column medium that does a very good job in separating most chlorinated hydrocarbons is methyl siUcone or methyl (5% phenyl) siUcone. The detector of choice is a flame ionisation detector. Typical molar response factors for the chlorinated methanes are methyl chloride, 2.05 methylene chloride, 2.2 chloroform, 2.8 carbon tetrachloride, 3.1, where methane is defined as having a molar response factor of 2.00. Most two-carbon chlorinated hydrocarbons have a molar response factor of about 1.0 on the same basis. 

Differences in the relative proportion of f-PSA and PSA-ACT can affect the result obtained for t-PSA because of the differences in the nature of calibration and the molar response, sensitivity, and specificity of antibodies used in various immunoassays. The efficiency of these immunoassays has been evaluated by several investigators. Because the proportion of free and complexed PSA varies in benign and malignant diseases, these immunoassays measure one form or the other, giving rise to different results for different patient groups. It is very important that data from clinical studies support the proposed intended uses of these assays, since as many as 5 percent of men with a negative free PSA test (free PSA values >25 percent) will have cancer and not be recommended for biopsy. Therefore, a goal for standardization is to detect total and free PSA accurately in equimolar fractions. 

The molar ratio of the different methylated derivatives of neutral sugars is generally calculated from the g.l.c. responses (using flame-ionization detection). In this kind of quantitation, the molar response of the different methylated sugars is assumed to be the same,7 or effective carbon-response is used.53... 

The quantification of short-chain metabolites from nonylphenol ethoxy-lates (NPEOs) has been carried out in different ways because of the lack of individual standards. Thus, several authors have employed commercial mixtures with low ethoxylation degree and assuming similar molar response factors, such as Marlophen 83 with an average of 3.5 mol of ethylene oxide (EO) [1-5], Imbentin-N/7A, a mixture of NPEOi and NPEO2 (75 25) [6-11] or a mixture of miscellaneous origin [12,13], to quantify NPEOi and NPEO2. 

Another difficulty in the gas chromatographic separation of amino acids is the choice of detector and it may be necessary to split the gas stream and use two different detectors. The flame ionization detector, which is commonly used, is non-specific and will detect any non-amino acid components of the sample unless purification has been performed prior to derivatization. In addition the relative molar response of the flame ionization detector varies for each amino acid, necessitating the production of separate standard curves. As a consequence, although gas chromatography offers theoretical advantages, its practical application is mainly reserved for special circumstances when a nitrogen detector may be useful to increase the specificity. 

The same chromatographic parameters were used in determining the molar response of the individual PCB isomers except, that the area responses were determined with a flame ionization detector. The flow rates of the hydrogen and air combustion gases were 30 and 300 mL/min, respectively. Response factors needed to... 

Compounds were quantified by comparing the computer calculated area for the brominated compound with the integrated response for a known amount of octachlo-ronaphthalene. Differences in ionization cross-section, which affect the sensitivity of the mass spectrometer to a given compound, were compensated for by determining the relative molar response (RMR) of authentic compounds to octachloronaphthalene. 

Relative molar response relative to the OCN 404 m/e ion. bAverage of 7 determinations. 

An interesting feature of the electrochemical detection is its relatively small variation in sensitivity for various substances for which it responds. This relatively constant molar response is due to the small number of electrons, usually two or three, involved in electrochemical reactions. This feature is very convenient in trace analysis, because the analyst can predict the sample size, dilutions, and other manipulations drat must be used to produce the desired analytical sensitivity. 

An alternative to quantitative analysis by ICP-MS is semiquantitative analysis, which is generally considered as a rapid multielement survey tool with accuracies in the range 30-50%. Semiquantitative analysis is based on the use of a predefined response table for all the elements and a computer program that can interpret the mass spectrum and correct spectral Interferences. This approach has been successfully applied to different types of samples. The software developed to perform semiquantitative analysis has evolved in parallel with the instrumentation and, today, accuracy values better than 10% have been reported by several authors, even competing with typical ones obtained by quantitative analysis. The development of a semiquantitative procedure for multielemental analysis with ICP-MS requires the evaluation of the molar response curve in the ICP-MS system (variation of sensitivity as a function of the mass of the measured isotope) [17]. Additionally, in the development of a reliable semiquantitative method, some mathematical approaches should be employed in order to estimate the ionisation conditions in the plasma, its use to correct for ionisation degrees and the correction of mass-dependent matrix interferences. 

Compound Retention factor (k) Molar absorptivity ( 280)fe Relative molar response Corrected relative mass response ... 

It obviously applies that m /ms =gj/gs = qjqs>g and q denoting mass fractions and mass concentrations (mass/volume). If molar proportions of the components under analysis are to be determined, the peak areas have to be multiplied by the respective molar response factors the following relationships between molar (superscript n) and mass specific response factors hold ... 

Derivative Relative retention time Relative molar response... 

Tab. 7-1 Relativer molarer Response des ECD fiir 2,3,7,8-substituierte Kongenere der PCDD und PCDF, normicrt auf C18DD (D75) [Schimmel et al. 1993]...

Struktur und Bezeichnung Ballschmitcr Nr. Relativer molarer Response des ECD (a)... 

Schimmel H, Schmid B, Bacher R, Ballschmiter K (1993), Anal. Chem. 65 640-644. Molar response of polychlorinated dibenzo-p-dioxins and dibenzofurans by the mass spectrometric detector"... 

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