Compound Quantitation

Chapter 4.4 addressed various aspects of qualitative identification of pollutants with common analytical techniques, such as chromatography and elemental analysis. Another integral component of environmental analysis is pollutant quantitation. For the data to be valid and usable, the analytes must be not only correctly identified but also properly quantified. 

Two important components of quantitative analysis of environmental samples are the determination of method detection limits and instrument calibration. Understanding how they contribute to data quality will enable us to make decisions related to data validity during the assessment phase of the data collection process. 

Existing definitions of various detection and quantitation limits can be confusing to a non-laboratory person. Despite misleading similarities of these definitions, there is a logic and order to the basic concepts that they express. Various detection limits that we commonly refer to in our daily work (the IDLs, MDLs, and PQLs) are discussed in this chapter in the increasing order of magnitude of their numeric values. Some of these detection limits are determined experimentally and depend on the matrix and the method of preparation and analysis, while others may be arbitrary values selected by the laboratory or the data user. The relationship between these three levels of detection is approximately 1 5 10. 

Environmental laboratories routinely determine IDLs in the course of ICP-AES analysis as a measure of background and interelement interferences at the lowest measurable concentration level above the background noise. The IDL is a trace element analyte concentration that produces a signal greater than three standard deviations of the mean noise level or that can be determined by injecting a standard to produce a signal that is five times the signal to noise ratio (APHA, 1998). 

Consistent with these definitions, there are two methods for IDL determination. The first method consists of multiple analyses of a reagent blank, followed by the determination of the standard deviation of the responses at the wavelength of the target analyte. The standard deviation multiplied by a factor of three is the IDL. This calculation defines the IDL as an analyte signal that is statistically greater than the noise. 

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Applications Off-line SFE-HPLC appears to be applicable and quantitative for a variety of samples in many real -world matrices. The main challenge lies in the use of this technique for the more polar compounds. Quantitative off-line SFE-SFC-UV analysis of HDPE/Ethanox 330 was described after extensive method development (varying modifiers, modifier concentration, temperature) [129]. Soxhlet extraction and SFE-RPLC-UV of PE samples were compared [127]. A sample size (inhomogeneity) problem was pointed out when a SFE reproducibility study was performed on five 3-mg samples of PE. This points to limits... 

Spectroscopy Drug compounds absorb visible, infrared, and UV radiation at frequencies that are characteristic of the compounds. Quantitative measurements can be calculated from the absorbance readings at specific frequencies or wavelengths. 

In HPLC, a sample is separated into its components based on the interaction and partitioning of the different components of the sample between the liquid mobile phase and the stationary phase. In reversed phase HPLC, water is the primary solvent and a variety of organic solvents and modifiers are employed to change the selectivity of the separation. For ionizable components pH can play an important role in the separation. In addition, column temperature can effect the separation of some compounds. Quantitation of the interested components is achieved via comparison with an internal or external reference standard. Other standardization methods (normalization or 100% standardization) are of less importance in pharmaceutical quality control. External standards are analyzed on separate chromatograms from that of the sample while internal standards are added to the sample and thus appear on the same chromatogram. 

On a more fundamental side, much of this chapter has focused on lattice-gas models applied to intercalation systems. The application of such models to metallic intercalation compounds is understood, and indeed the models describe some intercalation compounds quantitatively. But more study is needed of systems where the density of states is low, or where the band picture breaks down. 

Identification of compounds in the river water extracts was based on the coincidence of gas chromatographic retention times and on the equivalence of electron impact and chemical ionization mass spectra with those of authentic compounds. Quantitation was based on standard curves generated for selected compounds. 

These hindered amine bases have pKa 41 -44 and thus are strong enough to deprotonate all carbonyl compounds quantitatively (see following table). 

True profile analysis requires scanning over the whole mass range for the acquisition of all data on excreted compounds. Quantitation has been more challenging on a quadrupole instrument because total ion current peaks are seldom a single component and extracted-ion chromatograms (EICs) when recovered from scanned data are of poor quality due to the lower sensitivity of scanning GC-MS. Thus, we developed profile analysis based on SIM of selected analytes but tried to ensure the components of every steroid class of interest were included. For ion traps the fundamental form of data collection (in non-MS/MS mode must be full -scans). Thus, the quantitative data produced are EICs obtained from scanned data. The EICs are of the same ions used for SIM in quadrupole instruments and the calibration external standards are the same. 

Specificity, in general, is tlie ability of a method to respond only to the substance being measured. This characteristic is often a function of the measuring principle and the function of the analyte under study. A key consideration of specificity is that it must be able to differentiate a compound quantitatively from... 

Quaternization of Heteroaromatic Compounds Quantitative Aspects John A. Zoltewicz and Leslie W. Deady PartI. Discussion... 

Polarimetric analysis is useful for determining the anomeric form of crystalline lactose or related compounds. Quantitation by polarimetric analysis is limited to samples free of other optically active compounds. 

Mason G, Zacharewiski T, Denomme MA, Safe L, Safe S (1987) Polybrominated Dibenzo-p-dioxins and Related Compounds Quantitative In Vivo and In Vitro Structure Activity Relationships. Toxicology 44 245... 

King, R. C., Gundersdorf, R., and Femandez-Metzler, C. L. (2003). Collection of selected reaction monitoring and full scan data on a time scale suitable for target compound quantitative analysis by liquid chromatography/tandem mass spectrometry. Rapid Commun. Mass Spectrom. 17 2413-2422. 

Silicon is a constituent of the main phase as well as of the most common impurities SiC, free Si, Si2N20, Si02, FeSix. Therefore, besides the determination of the total amount of Si, additional methods are needed to determine the amounts of free Si and the other compounds. Quantitative X-ray diffraction (XRD) is often used with modern XRD methods less than 0.1 wt.% free Si [232] and 0.2 wt.% Si2N20 [228] are detectable. The accuracy for SiC is somewhat lower due to the overlapping of the main peak of SiC with / -Si3N4. Free Si can be determined also with volumetric methods [225]. 

Calibration standards can be of two types external standards and internal standards. With external standards, multiple concentrations of the standards are injected, areas are measured, and a calibration curve is platted. Unknown samples are then injected, chromatograms run, and areas are calculated and compared with the calibration curves to determine amounts of each compound present. With internal standards, known amounts of an internal standard are added to each known concentration of standard compound and areas or peak height response factors relative to those of the internal standard are calculated. When unknowns are run, a known amount of internal standard is added to the unknown sample, response factors are calculated relative to the internal standards, and amounts of each unknown present are calculated from the standards calibration factors. Internal standards are usually used to correct for variations in injection size due to different operators and injection techniques. Internal standards can also be used to correct for extraction variation in GC/MS target compound quantitation, this standard is referred to as a surrogate standard. Generally, an internal standard is used for one purpose or the other, not both at the same time. 

After initial calibration has been completed, it must be verified with a second source standard, which has been prepared from material coming from a different source (different manufacturer or lot number). This QC measure, called the second source confirmation, allows detecting errors in the initial calibration standard solution preparation and prevents a systematic error in compound quantitation. Only after this initial calibration verification (ICV) standard has also met its own acceptance criteria may the analyst start analyzing samples. 

The accuracy of compound quantitation depends on the quality of peak resolution. 

The CCV response (calibration) factor cannot be used for compound quantitation, only the average response (calibration) factor of the initial calibration can. The only exceptions to this rule are the CLP SOW methods for organic compound analyses that allow use of CCV for compound quantitation. 

Internal standard calibration is used when the changes in the analytical system are known to be frequent and substantial. To compensate for these changes, internal standards at known concentrations are added to all calibration standards, field samples, and laboratory QC samples prior to analysis. Internal standards are synthetic analogs of specific target compounds or compounds that are similar in nature to the target analytes and that are not found in environmental samples. Internal standard calibration is a requirement of GC/MS methods. Laboratories sometimes use it for GC methods as it significantly improves the accuracy of compound quantitation. 

The equations for compound quantitation are shown in Appendix 22. Most of these equations apply to linear calibration models that rely on average response (calibration) factor for compound quantitation. Calibrations that use linear regression and non-linear polynomial equations read compound concentrations in the analyzed sample aliquot directly from the calibration curve. Once this concentration has been obtained, the final sample concentration can be calculated using the same rationale as for the linear concentration model. 

As individual sample QC checks, internal standards are important in compound quantitation. They should be monitored with the same care as other QC checks. The deterioration of internal standard area counts (the area under a chromatographic peak) reflects the changes in the analytical system that may eventually degrade the quality of analysis to an unacceptable level. EPA Methods 8260 and 8270 require that the area for each internal standard be within the range of —50 to +100 percent of the area of the internal standards in the most recent CCV (EPA, 1996a). This requirement may be used as a general rule for all other methods that use internal standard calibration. 

A rhenium complex, [ReBr(CO)3(thf)]2, has been found able to catalyse the inter-molecular reactions of 1,3-dicarbonyl compounds with terminal alkynes to give the corresponding alkenyl derivatives in excellent yields (Scheme 6).35 These reactions could apply to an intramolecular version and gave the corresponding cyclic compounds quantitatively. Tributylphosphine has been found to be a superior catalyst for the a-C-addition of 1,3-dicarbonyl compounds to electron-deficient alkynes.36... 

Spotting Volume Impact on the Compound Quantitation on Dried Blood Spots on Different Paper/Cards... 

The equilibrium constant Keq of the respective deprotonation equilibrium shows whether a base can deprotonate a C,H-acidic compound quantitatively, in part, or not at all ... 

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