External Standard Quantification

Quantification External standard External and internal standard... 

A calibration curve, also called a standard line, is a plot of a series of numbers that represents the observed signals from a set of samples with known concentrations of the analyte (y-axis) plotted against those concentrations (x-axis). There are three common methods of quantification external standard, internal standard, and standard addition. 

To analyze pesticides from the sample, several GC techniques were used GC with FID and EC detectors and GC/MS with external standards. Pesticides are mostly analyzed using split/splitless technique where higher amount of injected solution exits the gas chromatograph without decomposing therefore by quantification of the several pesticides in the filter, we found out how harmful is exposing analysts to pesticide compounds during the GC analysis. 

A further critical point are the intensities correlated to spectra of the pure elements. Calculated and experimentally determined values can diverge considerably, and the best data sets for 7 measured on pure reference samples still show a scatter of up to 10%. The use of an internal standard or a simultaneously measured external standard seems to be the most successful way to reducing the inaccuracy below 10%. (Eor a more detailed discussion of background subtraction and quantification see, e.g., Seah [2.9].)... 

Quantification is by reference to an external standard solution of the feedstock appropriate to the sample to be analyzed. The method assumes that the free oil has the same refractive index as the appropriate feedstock material. 

For details of the clean-up of the pyrolysate of the DIN oven see reference 12. Identification and quantification of PBDD/F was performed by GC/MS (refs. 8-12). This was done for all brominated PBDD and PBDF from mono- through octabromo compounds using external standards which were either prepared (refs. 11-13) or purchased. There exists a total of 210 brominated compounds of PBDD/F. Since not all isomers are available a complete isomer-specific determination could not be performed. 

HBCD determinations were carried out simultaneously with mono- to hepta-BDEs. Technical HBCD consists of three diastereomers a, (3 and y-HBCD. HBCD can be determined by GC-MS, but until now, the three diastereomers have not been separated by this technique. However, as apparently the response factors of the three diastereomers do not differ very much [25], HBCD can be quantified as total HBCD by GC-MS. Quantification was carried out by external standard. 

Quantitative analysis of AP/APEO by HPLC-FL can be performed with external standard solutions of mixtures of AP or APEO. Initially quantification of oligomeric mixtures was based on the elaborate procedure of normal-phase analysis with subsequent quantification of all oligomeric peaks [27]. Kiewiet et al. [28] have described the general principle of quantification of ethoxymers in reversed-phase LC with spectroscopic detection in detail using the example of derivatised alcohol ethoxylates. Based on this method the quantitative analysis of... 

The quantification of surfactants in environmental samples needs further development, particularly in so far as quality assurance of the analysis is concerned. Since the majority of the individual isomers and oligomers involved are not yet available as standards, quantification has to be based in part on external standards of commercially available mixtures. As this holds for both LC-FL and LC-MS analyses, both suffer from this shortcoming. Yet, samples analysed with both the methods have shown good agreement of resulting data. 

In solution, vitamin D (both D2 and D3) isomerizes to previtamin D and forms a temperature-dependent equilibrium mixture [520], which leads to quantification problems. Previtamin D is difficult to quantify because of interference from co-eluted contaminants. The reversibility of the isomerization is very slow, therefore the percentage of previtamin can be considered constant during the entire analysis. The quantification of the potential vitamin D can be performed using an external standard that has undergone saponification procedure as the sample [521]. Vitamin D2 and D3 can be used as an internal standard to quantify the other one. Indeed, the isomerization rates of vitamins D2 and D3 are virtually the same thereby the previtamin D/vitamin D ratio will be the same for both vitamers at any temperature. The isomerization problem can be resolved by... 

Quantification of all compounds is by relating the intensity of specified ions to that of the internal standard stigmasterol [m/z 394 (M-90)]. Instrument calibration was achieved by running the external standard daily. 

Analytes are identified on the basis of retention time compared to standards and with the addition of the suspected compound to the sample (55). The diode array detector has been used recently as an additional aid in the identification of sweeteners and the determination of peak purity (56). Quantification is performed by the internal or external standard method on the basis of peak height or area. 

One of the important aspects of any analytical method is its calibration and, therefore, much effort has been put into SPME calibration. As it is not always practicable to employ traditional calibration methods (external standards, internal standards, and standard addition) owing to the sometimes significant matrix effects in complex samples, equilibrium calibration has been suggested as an alternative. In SPME, however, it would normally take rather a long time to achieve equilibrium calibration. If sensitivity were not a concern in an analysis, reduction of extraction time would be desirable, that is, the extraction could be stopped before equilibrium but this would thus demand a new approach to calibration. In this regard, as a way of circumventing matrix effects in environmental analysis, several diffusion-based calibration methods have been recently developed for quantification in SPME.30... 

Quantification by external standard is the most straightforward approach because the peak response of the reference standard is compared to the peak response of the sample. The standard solution concentration should be close to that expected in the sample solution. Peak responses are measured as either peak height or area [41 ]. 

True quantification of pyrazines in the powdered sarples presented seme difficulty. Due to sensitivity problems in sampling reconstituted sairples, the decision was made to purge the powdered NFCM. An internal standard could not be easily added to a dry sample, so quantification was accomplished calculating concentrations relative to response factors determined for external standard solutions. Standardized concentration units were determined using the following relationships ... 

Headspace GC-MS is the preferred method for the analysis of very volatile migrants. Practically the same GC conditions can be used as for GC-MS. Due to the coupling to MS, identification is also relatively easy. The heating time and temperature are the main experimental variables. The major drawback of headspace GC-MS is quantification. As a result of the principle of headspace GC-MS, i.e., partitioning of compounds between gas phase and liquid phase, the chemical properties will have a significant influence on the partition of each molecule between gas phase and liquid phase. Therefore, quantification is almost solely possible by using external standards of the same compound (Grob and Barry 2004). 

The oleoresins were analyzed by high performance liquid chromatography following the standard procedure of the Normalization French Association (AFNOR) (7). The quantification of each component was carried out by using the external standard vanillylnonanamide. 

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