Using external standards

Since a standard additions calibration curve is constructed in the sample, it cannot be extended to the analysis of another sample. Each sample, therefore, requires its own standard additions calibration curve. This is a serious drawback to the routine application of the method of standard additions, particularly in laboratories that must handle many samples or that require a quick turnaround time. For example, suppose you need to analyze ten samples using a three-point calibration curve. For a normal calibration curve using external standards, only 13 solutions need to be analyzed (3 standards and 10 samples). Using the method of standard additions, however, requires the analysis of 30 solutions, since each of the 10 samples must be analyzed three times (once before spiking and two times after adding successive spikes). 

The equation for a normal calibration curve using external standards is... 

The generalized standard addition method (GSAM) extends the analysis of mixtures to situations in which matrix effects prevent the determination of 8x and 8y using external standards.When adding a known concentration of analyte to a solution containing an unknown concentration of analyte, the concentrations usually are not additive (see question 9 in Chapter 5). Conservation of mass, however, is always obeyed. Equation 10.11 can be written in terms of moles, n, by using the relationship... 

When possible, a quantitative analysis is best conducted using external standards. Unfortunately, matrix interferences are a frequent problem, particularly when using electrothermal atomization. Eor this reason the method of standard additions is often used. One limitation to this method of standardization, however, is the requirement that there be a linear relationship between absorbance and concentration. 

When possible, quantitative analyses are best conducted using external standards. Emission intensity, however, is affected significantly by many parameters, including the temperature of the excitation source and the efficiency of atomization. An increase in temperature of 10 K, for example, results in a 4% change in the fraction of Na atoms present in the 3p excited state. The method of internal standards can be used when variations in source parameters are difficult to control. In this case an internal standard is selected that has an emission line close to that of the analyte to compensate for changes in the temperature of the excitation source. In addition, the internal standard should be subject to the same chemical interferences to compensate for changes in atomization efficiency. To accurately compensate for these errors, the analyte and internal standard emission lines must be monitored simultaneously. The method of standard additions also can be used. 

Samples of car exhaust are collected using a 4-L glass bottle evacuated to a level of less than 2 torr. A normal calibration curve using external standards of known Pco is used to determine the Pco in the exhaust samples. 

The %w/w lead in a lead-based paint Standard Reference Material and in unknown paint chips is determined by atomic absorption using external standards. 

Samples of urine are analyzed for riboflavin before and after taking a vitamin tablet containing riboflavin. Concentrations are determined using external standards or by the method of standard additions, fluorescence is monitored at 525 nm using an excitation wavelength of 280 nm. 

Gluodenis describes the use of ICP to analyze samples containing Pb and Ni in brass. The analysis for Pb uses external standards prepared from brass samples containing known amounts of lead. Results are shown in the following table. ... 

Quantitative Analysis Using External Standards To determine the concentration of analyte in a sample, it is necessary to standardize the electrode. If the electrode s response obeys the Nernst equation. 

Quantitative Calculations Quantitative analyses are often easier to conduct with HPLC than GC because injections are made with a fixed-volume injection loop instead of a syringe. As a result, variations in the amount of injected sample are minimized, and quantitative measurements can be made using external standards and a normal calibration curve. 

In order for a solution for the systems of equations expressed in equation 11 to exist, the number of sensors must be at least equal to the number of analytes. To proceed, the analyst must first determine the sensitivity factors using external standards, ie, solve equation 11 for Kusing known C and R. Because concentration C is generally not a square data matrix, equation 11 is solved by the generalized inverse method. K is given by... 

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. 

Table 5.17 Quantitative results obtained for the determination of four diarrhetic shellfish poisons (DSPs) using external standards and the method of standard additions. Reprinted from J. Chromatogr., A, 943, Matrix effect and correction by standard addition in quantitative liquid chromatographic-mass spectrometric analysis of diarrhetic shellfish poisoning toxins , Ito, S. and Tsukada, K., 39-46, Copyright (2002), with permission from Elsevier Science...

Quantitation of anthocyanins has become simple and fast since many anthocy-anin standards became commercially available as external standards in the past decade. When the standards are not available, individual anthocyanins or total monomeric anthocyanins can be determined by the use of a generic external standard such as commercial cyanidin-3-glucoside or other compound structurally similar to the analytes of interest. Individual and total peak areas are measured at 520 nm or their and quantified using external standards by which values are typically slightly different from those via the pH differential method. ... 

In the analysis of seawater, isotope dilution mass spectrometry offers a more accurate and precise determination than is potentially available with other conventional techniques such as flameless AAS or ASV. Instead of using external standards measured in separate experiments, an internal standard, which is an isotopically enriched form of the same element, is added to the sample. Hence, only a ratio of the spike to the common element need be measured. The quantitative recovery necessary for the flameless atomic absorption and ASV techniques is not critical to the isotope dilution approach. This factor can become quite variable in the extraction of trace metals from the salt-laden matrix of seawater. Yield may be isotopically determined by the same experiment or by the addition of a second isotopic spike after the extraction has been completed. 

Use External Standardization for assays and dissolution. Bracketed standards are preferred for improved accuracy. 

Steryl conjugates in liquid chromatographic analyses are most often quantified using external standard methods, which, depending on the... 

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). 

Prior to each series of measurements, it is necessary to ascertain the absence of any driO in the equipment using external standards called monitors and to check the settings of the spectrometer (angular positioning, energy discrimination, etc.). 

The external standard method is the most general method for determination of the concentration of an analyte in an unknown sample. It involves the construction of a calibration plot using external standards of the analyte, as shown in Fig. 1. A fixed volume of each standard solution of known concentration is injected into the HPLC and the peak response of each injection is plotted versus the concentration of the standard solution. The standards used are called external standards because they are prepared and analyzed separately from the unknown sample(s). After constructing the calibration plot, the unknown sample is prepared, injected, and analyzed in exactly the same manner. The concentration of the analyte in the unknown sample is then determined from the calibration plot or from the response factor of the unknown sample versus that of the standard. 

Quantitative analysis is normally performed by preparing calibration curves using external standards. To compensate for instrument drifts, instabilities, and matrix effects, an internal standard is usually added to the standards and to the sample. Multiple internal standards are sometimes used to optimize matching of the characteristics of the standard to those of various analytes. 

Quantitative analyses were performed using external standards n-dodecylbenzene was used for LABs n-C21 fatty acid methyl ester for LASs n-TBP for APs phenanthrene and chrysene for PAHs coprostanol and PCBs congeners lUPAC Nos 28, 52, 101, 118, 138, 153 and 180. NPEOs were quantified, using the NPE30, from the FID profile in coastal waters and from the HRGC-PICESIM trace for wastewaters . In both cases the same response factor was assumed for all the oligomers (1-7). 

Figure 5.10. Typical calibration curve using external standardization (linear calibration through origin). Note that the slope of the curve is equal to the response factor of the analyte.

A quantitative method generates information on the concentration or amount of the analyte (s) in the sample. System calibration (standardization) typically using external standard(s) is required. A quantitative method can also be adopted as a qualitative method. For instance, an assay method can often also serve as an identification method. A quantitative method is more difficult to develop and requires extensive effort for validation. This method type is the focus of this chapter. 

When using external standardization, samples and standards should be analysed more than once, to confirm the reproducibility of the technique. 

An international inter-laboratory study highlighted the importance of using external standards whose formula group profile closely resembles that of the... 

In quantitative NMR spectroscopy the concentration of the analyte is typically determined by comparing the analyte resonance integral to an inert, internal reference with known concentration. In quantitative 2D NMR the relation between the cross peak integrals of the reference and the analyte can be established ether by calibration curve and/or correction coefficients derived from theoretical calculations. It is then quite straightforward to calculate the analyte concentration in the sample. In some cases there are no inert references available that could be applied internally to the sample. In these cases a reference chemical in a coaxial glass insert within the NMR sample tube can be used, so that no contact will take place between the sample and the reference chemical. Other possibility is to use external standard in a separate NMR tube. ... 

Summari2dng the results of FDMS using external standards, electrical or photographic recording, it becomes clear that a shorter analysis time, an easier handling and a higher quality in the trace analysis of metals would be desirable. Therefore, two essential improvements have been introduced ... 

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