Detector response factors

Much of what is knotm about the structure response of the ECD is based on empirical observations. Clearly, the ability to correlate the response of the detector to fundamental molecular parameters would be useful. Chen and Wentworth have shorn that the information required for this purpose is the electron affinity of the molecule, the rate constant for the electron attachment reaction and its activation energy, and the rate constant for the, ionic recombination reaction [117,141,142]. in general, the direct calculation of detector response factors have rarely Jseen carried j out, since the electron affinities and rate constants for most compounds of interest are unknown. 

A mixture of methyl esters of fatty acids was chromatographed on a Carbowax 20 M column giving the following peak areas and detector response factors ... 

Leggett et al (Refs 22 23) used a similar technique, except that their apparatus was static . TNT samples were placed in a 125ml vial equipped with silicone rubber septum cap. The vial was thermostatted and the sample and its vapor were allowed to equilibrate for 2—4 weeks. Vapor was withdrawn from the head-space with a stainless steel syringe and injected into a gas chromatograph. The concn of TNT in the headspace vapor was determined by manual triangulation of the peak, giving peak area/ volume, and dividing by the detector response factor (peak area/mass), as determined by injection of known quantities of TNT dissolved in benzene... 

Filament temperature Filament resistance Voltage across filament Recorder response Detector response factor... 

If standard mixtures of known weight ratios of components do not give the same ratio of areas, the detector is not responding equivalently to each component. In this case a quantitative analysis of the mixture requires preliminary experimentation. One method is the use of an internal standard (for a more detailed discussion of other procedures, specialist texts should be consulted65,66). Thus for a two-component mixture (A + B), a third component C is selected as the internal standard. Mixtures of A + C and B -I- C are prepared in which the weights of each component in each mixture are known. The relevant chromatogram is recorded and the detector response factors calculated from the following relationships ... 

Ki, the detector response factor, describes the signal generated when particles are present in the eluant as it transits the detector flow-through cell. Detector response arises primarily from scattering of light by the latex particles (15), although a small contribution from light absorption by the sample may occur (16). Polystyrene latex standards of known size and concentration were used to determine Ki factors for conversion of detector response into mass concentration information. 

It is also possible to calculateMi/K a Mp from eqn. 48 and to employ it as an additional correction factor to the peak area, together with the detector response factor (fp) note that the mass of compound i corresponding to the charge of final material introduced into the gas chromatograph is proportional to (Mi/KraMp)Apfp. 

The reaction may be complete in about 2 hr. An aliquot may be removed and analyzed by GC for the intermediate 2-chlorocyclopropanone acetal. In case of a deficiency of sodium amide, a small amount of the chloride may remain but can be removed by distillation. The analysis may be performed on a 0.25 mm i.d. x 25 m capillary GC column (HR-1, Shinwa Chemical Industries, Ltd.) at 80°C. Typical retention times for the chlorocyclopropanone acetal and 3 are 9.9 and 4.6 min, respectively. The amount of the former may be estimated by comparison of its peak area with that of 3 assuming equal detector response factors of the two compounds. 

The application of dual detection [UV and refractive index (RI)] to the SEC analysis of polystyrene-poly(methyl methacrylate) (PS-PMMA) has already been studied in this laboratory (2). Both MWD and CCD were determined using a methodology outlined by Runyon et al. (3). This approach relies on SEC column calibration with narrow polydis-persity standards for each of the homopolymers as well as a measure of the detector response factors for each homopolymer to produce a copolymer MWD. In the case of PS and PMMA this is feasible, but in other block copolymer systems the availability of suitable molecular weight standards may be more limited. In addition, this procedure does rely on true SEC and is not valid for block copolymers for which the universal calibration does not hold true for both blocks in a given solvent system. 

A-5. Calculation of Response Factors. Detector response factors for the 12 compounds studied in this investigation were determined by injecting 23.2 fxL of standard solutions of the compounds into the liquid chromatographic system. These response factors are presented below. 

The area of each peak is obtained from a series of replicate (5+) injections of a mixture containing equal (or known) amounts of all the components. Acceptable precision is essential to obtain satisfactory data. One component is chosen as the reference and the relative responses of the other components are determined by dividing the peak areas by that of the reference component. The detector response factors (Z>rf) may then be used to calculate corrected peak areas (.<4correct) for other analyses involving these components and hence their percentage ratios in the mixture may be... 

The internal standard method is a variation on the above, and is recommended for accurate quantitation. It eliminates the need for accurate injections since a reference standard is included in each sample analysed. An internal standard is selected which has a retention time such that it is eluted in a suitable gap in the chromatogram. The procedure involves analysing a test sample containing known amounts of each component plus a predetermined amount of the internal standard. Since peak area is proportional to the amount of an eluted component and the detector response factor (I>rf)... 

Detector response factors need to be accurately determined for good reproducible quantitative analyses [60]. They may be conveniently obtained by the constant volume method, that is, approximately 10 repeatable volumes of a sample containing an equal amount of the analytes are injected and the mean determined. Drf values are calculated hy normalising each peak area to that of the peak to be used as the reference. 

The detector response will be related to the amount of the analyte in the column effluent though different analytes will respond to differing extents and hence the detector must be calibrated with respect to each of the analytical species of interest. Detector response factors and their determination is covered in detail in Chapters 2 and 10. 

There are two main requirements for processing chromatographic data, accurate digitisation of the analogue detector signal and software to process the data. The software includes algorithms for detections of peaks, correction for base drift, calculation of peak areas and retention time, concentrations of components using stored detector response factors and production of the final analytical report. 

Normalised area% is obtained by first correcting the peak area from the chromatogram, /4chrom. of each component for the relative response of the detector to that component. Detector response factor (Drp) is calculated with reference to a specified reference standard or internal standard see Section 2.7. The corrected area, y4correct of each peak is then used in the equation above to obtain the normalised area%... 

The analysis of alcohol solutions has become very important due to the implications of the Road Traffic Act (1972) which requires the estimation of small amounts of ethanol in blood and/or urine. This experiment demonstrates the most frequently used method—GLC using an internal standard. Before the advent of GC the method most frequently used involved the chromate oxidation of ethanol. The most satisfactory method of quantitative analysis in GC involves the calibration of the detector response for the compound of analytical interest against a reference compound. This internal standardisation technique involves adding a known amount of reference compound to both sample and standard solutions. The detector response factor (I>rf) of an analyte component relative to the internal standard (IS) can be evaluated by running a sample containing the internal standard and each of the sample components, all in accurately known concentration. Thus, the response factor for compound A can be calculated as follows ... 

Prepare a 500 ppm analyte standard solution as follows pipette 1 ml of stock solution and 1 ml of the internal standard solution into a 50 ml volumetric flask then dilute to volume with 40% (v/v) ethanol/water. Prepare sample solutions by diluting 1 ml of internal standard to 50 ml with proprietary whisky. Make duplicate 0.5 pi injections of the standard solution and of all sample solutions. From the peak area data of the standard solutions calculate the detector response factors for each component relative to n-butanol. The parts per million (% v/v) amounts of each component in the sample(s) can be determined. As an alternative to capillary GC the analysis can be carried out on the following packed column—15% PEG400 on supasorb 60-80 (2M). 

A mixture of three volatile compounds (J, B and C) was analysed by GC using a column of 50 cm length. The retention time (/r), the peak width at base line (wj,), the area under each peak and the relative detector response factor, are given in Table 10.17. Under the same conditions a compound with no affinity for the stationary phase took 2.3 min to be eluted. 

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