Internal standardization technique

Rice, G. W. Determination of Impurities in Whiskey Using Internal Standard Techniques, /. Chem. Educ. 1987, 64, 1055-1056. 

Radioactivity Analysis. Samples of urine, feces, and tissues were combusted to COo and analyzed for radioactivity (5). By using this method the recovery of radioactivity from samples spiked with C was 95 dt 5%. To determine the radioactivity expired as CO2, 5-ml aliquots of the solution used to trap the CO2 were added to 15 ml of a scintillation counting solution containing 4 grams 2,5-diphenyloxazole (PPO) and 0.1 grams l,4-bis-2(5-phenyloxazolyl)-benzene (POPOP) per liter of 1 1 toluene 2-methoxyethanol. Samples were counted for radioactivity in a Nuclear Chicago Mark II liquid scintillation counter. Counting eflSciency was corrected by the internal standard technique. 

Neither external nor internal standardization techniques make allowance for the different behaviour of samples and chemical standards, due to the matrix effect in the samples or due to the different state of the analyte in the samples and... 

The accuracy of the injection volume measurement can be very important for quantitation, since the amount of analyte measured by the detector depends on the concentration of the analyte in the sample as well as the amount injected. In Section 12.8, a technique known as the internal standard technique will be discussed. Use of this technique negates the need for superior accuracy with the injection volume, as we will see. However, the internal standard is not always used. Very careful measurement of the volume with the syringe in that case is paramount for accurate quantitation. Of course, if a procedure calls only for identification (Section 12.7), then accuracy of injection volume is less important. See Workplace Scene 12.1 for an example of a purge-and-trap procedure for injecting a GC sample. 

Yields were determined by VPC using internal standard techniques, remaining material was starting halide. 

Dose, E. V, and Guiochon, G. A. (1991). Internal standardization technique for capillary zone... 

With attention to the purity of the standards and to the lack of interference of any solvent impurities, the precision of the internal standard method is controlled by the ability to quantitate peak size. That certainly qualifies this technique as the most precise method of quantitative analysis by GC, and where precision is paramount, the internal standard technique should be applied. Its advantages far outweigh the slight increase in effort required for standard and sample preparation. An excellent, detailed, how-to approach for the internal standardization technique as applied to a practical problem has been detailed by Barbato, Umbreit, and Leibrand (35). 

The preceding discussion of sample storage of external liquid standards certainly apply to the standards prepared for the internal standardization technique. There is one further consideration in this regard and that is in the proper selection of the internal standard for a given analysis. The first step is to chromatograph a typical sample and identify the component or components to be analyzed. The internal standard is then chosen such that it must ... 

All of the above points about liquid injection should be considered even when using a standard technique that does not require an accurate volume to be known. Selective evaporation cannot be tolerated even with the internal standard method. The size measurement errors obvious from the above discussion certainly point to the substantial advantage of the internal standard technique for accurate analysis. 

P. C. Barbato, G. R. Umbreit, and R. J. Leibrand, Internal Standard Technique for Quantitative Gas Chromatographic Analysis, Applications Lab Report 1005, August 1966, Hewlett-Packard, Avondale, PA 19311. 

Since trace analysis also includes air or gas samples, it is appropriate to point out that proper addition of an internal standard to this type of sample is difficult. This difficulty lies, not in the mechanical problem of transfer, but in the difficulty of knowing that the precisely intended volume has properly been transferred. However, the internal standard technique is still not widely used here for the same reason it is not generally used in trace analysis. This reason again is because the analyst normally has no prior knowledge of the variation in composition from sample to sample. The continual risk exists that any given sample in a series will have a component, not present in others, which elutes with the internal standard. This occurrence would introduce significant error into the quantitative calculations which result. 

Dose. E.V. and G.A. Guiochon Internal Standardization Techniques for Capillary Zone Electrophoresis. Anatvtical Chemistry. 1154 (June 1991). 

P Haefelfinger. Limits of the internal standard technique in chromatography. J Chromatogr 218 73-81, 1981. 

Reaction conditions Olefin (1 mmol), catalyst (W2 1 mol.% with respect to olefin), H202 (30% aq. solution, 0.2 mmol), CH3CN (0.5 mL), 60 °C. Yield and selectivities were determined by GC or1H NMR with an internal standard technique. Selectivities to the corresponding epoxides were >95% in all cases. Yield (%) = epoxide (mmol) per initial H202 (mmol) x 100. 

The first two methods have been discussed earlier under Gas Chromatography in Chapter 1.3. Isotope dilution method, which is not so frequently employed for quantitation because of cost, is somewhat similar to internal standard technique and is presented below in brief. 

An alternative to MSA in ICP-MS analysis is the internal standard technique. One or more elements not present in the samples and verified not to cause an interelement spectral interference are added to the digested samples, standards, and blanks. Yttrium, scandium, and other rarely occurring elements or isotopes are used for this purpose. Their response serves as an internal standard for correcting the target analyte response in the calibration standards and for target analyte quantitation in the samples. This technique is very useful in overcoming matrix interferences, especially in high solids matrices. 

For the internal standard method, a substance is added at the earliest possible point in the analytical scheme. This compensates for sample losses during extraction, cleanup, and final chromatographic analysis. There are two variations in the use of the internal standard technique. One involves the determination of response factors which are the ratios of the analyte peak response to the internal standard peak response. The second is referred to as response ratios which are calculated by dividing the weight of the analyte by the corresponding peak response. 

S, amount of sulfur/ketone (mol/mol) N, amount of amine/ketone (mol/mol) T, reaction temperature (°C).b Determined by gas chromatography (internal standard technique). The reaction was carried out under reflux. 

R. J. Herberg, Anal. Chem., 35 785 (1963). Statistical Aspects of Liquid Scintillation Counting by Internal Standard Technique. 

It is recommended that the standards and the blank contain the major elements of the sample matrix, for example, aluminium for analysing alumina based samples. The content level of the matrix elements does not necessarily have to be exactly the same as that of the samples, except when interference correction is required. Where possible, it is advisable to check the acid content of the samples and the standards. Where they differ significantly or cannot be controlled, and this leads to interference, the internal standard technique can be used. 

The internal standard method is based on the comparison of signals of the substance to be determined and another substance with similar spectral properties mixed to the calibration and analysis solution in the same concentration. This procedure was introduced to TLC by Klaus in 1972 82) but was not widely adopted. Internal standard techniques lead to more precise results 87,88) by using electronic integrators83) or automated systems 84,85,86). The calculation is based on Eq. (37) where the internal standard factor is calculated from the results in peakheight or area from the calibration track according to Eq. (36)i... 

The internal standard technique is based on the covariance term in the error propagation of this method 89) and avoids the error caused by applying small volumes to the plate. But there is another important aspect of error propagation in the comparison of internal and external standard methods. If the error in measurement is smaller than the error in spotting, the internal standard method is more precise than the external standard method 90>. However, if the error in measurement becomes dominant, the external standard leads to better results. This is the reason why in early quantitative TLC, where peaks had been triangulated and evaluated by the squared... 

Colinearities among the responses are sometimes introduced unintentionally in the course of raw data treatment. It is common to normalize chromatograms to give a sum of peak areas equal to 100 percent. If such data are used to analyze the product profile in a synthetic experiment there will be a purely artificial correlation between the relative amounts of the products in each experiment, i.e. if the amounts of one product increase, the amounts of the remaining products will decrease. Such data can lead to conclusions which are totally erroneous. To avoid such pitfalls, it is advisable to determine product distribution from chromatograms by using internal standard technique. 

The experimental yield was determined by gas chromatography (internal standard technique) directly on the reaction mixture. 

SP-2401" and 3% SP-2250. ° Detectors used by EPA standards procedures, include photoionization (PID)," electron capture (ECD)," Eourier transform infrared spectrometry (PTIR), " and mass spectrometry detectors (MSD)." ° Method 8061 employs an ECD, so identification of the phthalate esters should be supported by al least one additional qualitative technique. This method also describes the use of an additional column (14% cyanopropyl phenyl polysiloxane) and dual ECD analysis, which fulfills the above mentioned requirement. Among MSDs, most of the procedures employ electron impact (El) ionization, but chemical ionization (CI) ° is also employed. In all MSD methods, except 1625, quantitative analysis is performed using internal standard techniques with a single characteristic m/z- Method 1625 is an isotope dilution procedure. The use of a FTIR detector (method 8410) allows the identification of specific isomers that are not differentiated using GC-MSD. 

There are, as yet, no standard methods for determining oxysterols in food. As a conseqnence, considerable variations in cholesterol oxide levels in similar kinds of foods have been reported by varions laboratories (Dutta et al., 1999). For quantification of oxysterols in samples, the nse of an internal standard technique is recommended. For oxysterol separation, capillary columns with a nonpolar stationary phase are most often nsed. For confirmation of oxysterol structure, the use of gas chromatography/mass spectrometry (GC/MS) is highly recommended. During analytical procednres, serious attention must be paid to the possibility of artifacts of oxysterols from cholesterol. In 1992, Wasilchuk et al. developed a procedure for detecting artifactual generation of oxysterols using [ Hg] cholesterol as an internal standard. 

The requirement that the analytes be quantitated using an internal standard mode of instrument calibration is due to the fact that mass spectrometers are intrinsically unstable that is, their response factor varies with time when compared to other GC detectors such as the flame ionization detector (FID). The internal standard technique of instrument calibration is... 

The data presented in Figure 8-1 illustrate the value of the internal standard technique when applied to the determination of boron in plant tissue. The boron spectral line is at 2497.7 A, and the tin line at 2495.5 A is used as an internal standard. The data are replications of the same sample using dc arc excitation and graphite electrodes. 

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