Selective quantitative analysis

Additional advantages can be obtained from the use of SERRS. It is more sensitive and the resulting spectrum can be related back to the molecular resonance spectrum, making for more confidence in assignments. Eluorescence is quenched, signals from the adsorbate are much more intense than from contaminants and there is less dependence on the exact nature of the surface. This makes for unique applications for SERRS, which include simpler, more sensitive and more selective quantitative analysis and single molecule detection. 

Ibrahim and co-workers developed a new method for the quantitative analysis of hypoxanthine, a natural compound of some nucleic acids. " As part of their study they evaluated the method s selectivity for hypoxanthine in the presence of several possible interferents, including ascorbic acid. 

A sample contains a weak acid analyte, HA, and a weak acid interferent, HB. The acid dissociation constants and partition coefficients for the weak acids are as follows Ra.HA = 1.0 X 10 Ra HB = 1.0 X f0 , RpjHA D,HB 500. (a) Calculate the extraction efficiency for HA and HB when 50.0 mF of sampk buffered to a pH of 7.0, is extracted with 50.0 mF of the organic solvent, (b) Which phase is enriched in the analyte (c) What are the recoveries for the analyte and interferent in this phase (d) What is the separation factor (e) A quantitative analysis is conducted on the contents of the phase enriched in analyte. What is the expected relative erroi if the selectivity coefficient, Rha.hb> is 0.500 and the initial ratio ofHB/HA was lO.O ... 

Several standard methods for the quantitative analysis of food samples are based on measuring the sample s mass following a selective solvent extraction. For example, the crude fat content in chocolate can be determined by extracting with ether for 16 h in a Soxhlet extractor. After the extraction is complete, the ether is allowed to evaporate, and the residue is weighed after drying at 100 °C. This analysis has also been accomplished indirectly by weighing a sample before and after extracting with supercritical GO2. 

Selecting and Standardizing a Titrant Most common acid-base titrants are not readily available as primary standards and must be standardized before they can be used in a quantitative analysis. Standardization is accomplished by titrating a known amount of an appropriate acidic or basic primary standard. 

Free Ions Versus Complexed Ions In discussing the ion-selective electrode, we noted that the membrane potential is influenced by the concentration of F , but not the concentration of HF. An analysis for fluoride, therefore, is pH-dependent. Below a pH of approximately 4, fluoride is present predominantly as HF, and a quantitative analysis for total fluoride is impossible. If the pH is increased to greater than 4, however, the equilibrium... 

Quantitative Analysis of Selectivity. One of the principal synthetic values of enzymes stems from their unique enantioselectivity, ie, abihty to discriminate between enantiomers of a racemic pair. Detailed quantitative analysis of kinetic resolutions of enantiomers relating the extent of conversion of racemic substrate (c), enantiomeric excess (ee), and the enantiomeric ratio (E) has been described in an excellent series of articles (7,15,16). 

Introduction An accurate quantitative analysis of the discharge of pollutants from a process must be determined prior to the design and/or selection of control equipment. If the unit is properly engineered by utilizing the emission data as input to the control device and the code requirements as maximum-effluent limitations, most pollutants can be successfully controlled. 

Because X-ray counting rates are relatively low, it typically requires 100 seconds or more to accumulate adequate counting statistics for a quantitative analysis. As a result, the usual strategy in applying electron probe microanalysis is to make quantitative measurements at a limited collection of points. Specific analysis locations are selected with the aid of a rapid imaging technique, such as an SEM image prepared with backscattered electrons, which are sensitive to compositional variations, or with the associated optical microscope. 

Of course, the most reliable and accurate method of quantitative analysis is to calibrate each element with standards prepared in matrices similar to the unknown being analyzed. For a survey technique that is used to examine such a wide variety of materials, however, standards are not available in many cases. When the technique is used mainly in one application (typing steels, specifying the purity of alloys for a selected group of elements, or identifying impurities in silicon boules and... 

The first part of the book eonsists of a detailed treatment of the fundamentals of thin-layer ehromatography, and of measurement techniques and apparatus for the qualitative and quantitative evaluation of thin-layer ehromatograms. In situ preehromatographie derivatization teehniques used to improve the selectivity of the separation, to inerease the sensitivity of deteetion, and to enhanee the precision of the subsequent quantitative analysis are summarized in numerous tables. 

Solvent extraction is generally employed in analysis to separate a solute (or solutes) of interest from substances which interfere in the ultimate quantitative analysis of the material sometimes the interfering solutes are extracted selectively. Solvent extraction is also used to concentrate a species which in aqueous solution is too dilute to be analysed. 

It is crucial in quantitative GC to obtain a good separation of the components of interest. Although this is not critical when a mass spectrometer is used as the detector (because ions for identification can be mass selected), it is nevertheless good practice. If the GC effluent is split between the mass spectrometer and FID detector, either detector can be used for quantitation. Because the response for any individual compound will differ, it is necessary to obtain relative response factors for those compounds for which quantitation is needed. Care should be taken to prevent contamination of the sample with the reference standards. This is a major source of error in trace quantitative analysis. To prevent such contamination, a method blank should be run, following all steps in the method of preparation of a sample except the addition of the sample. To ensure that there is no contamination or carryover in the GC column or the ion source, the method blank should be run prior to each sample. 

Watson, J. T., Hubbard, W. C., Sweetman, B. J., and Pelster, D. R. Quantitative analysis of prostaglandins by selected ion monitoring GC/MS. Advan. in Mass Spectr. in Biochern. Med II. New York Spectrum Publications, 1976. 

Although cSFC shows relatively poor figures of merit (speed, sensitivity, detection dynamic range and sample capacity) as well as a limited application area, its applications tend to be unique. These include solutes that can be solvated with pure SCCO2 and quantified with FID. Linear density programs typical in cSFC are ideal for homologous series found in surfactants, many prepolymers, etc. Selectivity in cSFC, which can be achieved by mobile phase density and temperature programming, relies on selective interactions with the stationary phase. Quantitative analysis in cSFC may be rendered difficult by small injected volumes the use of internal standards is recommended. 

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