Composition, analysis

There are mainly two methods for chemical composition analysis hydrolysis method and MS method. Hydrolysis method is by adding ot-Cal to induce hydrolysis reaction and after the reaction is analyzed by HPLC MS is the direct detection of the product composition mass/charge ratio and can more accurately determine the composition of the product chemical components. 

Take 100 mg of the product powder, dissolve it in 2 mL of 50 mmol/L acetic acid buffers (pH 6), and add sufficient quantities of a-Gal (50 nkat) and stir to dissolve. The reaction temperature is 30°C, reaction time is 36 h and the shaking speed is 100 r/min. After the enzymatic reaction, put the container into boiling water bath for 10 min and the a-Gal can be fiiUy inactivated. Add a certain amount of ultra-pure water to the inactivated mixture, and transfer into the centrifuge tube and centrifuge at 5,000 r/min for 5 min. Take the supernatant for HPLC analysis. The results are shown in Fig. 4.17. 

Comparing with the HPLC peak retention time of standard samples, it can be concluded that the main component is galactose (peak retention time of the standard is 5.068 min and that of the product is 4.975 min) and fi-CD (peak retention time of the standard is 13.537 min and that of the product is 13.551 min). Quantification of the amount of galactose and fi-CD by external standard method obtained that the mole ratio of the two components is 1 1. It can be speculated that the main product is a single-substituted galactose-j8-CD derivative. 

LEIS is characterized by the use of low-energy (0.1-10keV) projectiles, usually inert gases (He and Ne) or sometimes alkali ions. The composition can be obtained from a modified form of Eq. (3.3.6), which is 

This is a very surface-sensitive technique capable of resolving stmcture and composition in the first and sometimes second atomic layer of the surface. This sensitivity arises from a large scattering cross section experienced by low-energy heavy ions and charge exchange. 

The yield equation modified for charge exchange in LEIS is given in Eq. (3.3.18). is the probability that the ions leaving the surface are in a charged state. To overcome this uncertainty in practical analysis, the most reliable approach is to use standards [4, 5]. Thus to measure the composition of the surface layer of an alloy (PtNi) or of Pt deposited on an Rh surface, the yields from dean elemental surfaces can be obtained and related to the number of atoms on the modified surface. Using these standards, it is possible to measure the concentration of two or more elements on a surface. Consider the case of a surface composed of elements a and b. On the pure standard surface in each case 

Analyzing a mixed surface provides the surface composition via 

Similar approaches can be used for most impurity concentration determination and, in general, adsorbate covered surfaces however, if the adsorbate sits above the surface, it is possible that it will shadow some surface layer atoms. In this case, a shadowing factor, S, for the degree to which the adsorbate shadows the first layer of the substrate needs to be incorporated into the yield equation for scattering. 

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To type crude oils (see Figure 2.13). This method uses an extremely accurate compositional analysis of crudes to determine their source and possible migration route. As a result of the accuracy It is possible to distinguish not only the oils of individual accumulations in a region, but even the oils from the different drainage units within a field. If sufficient samples were taken at the exploration phase of a field, geochemistry allows one to verify cross flow and preferential depletion of units during later production. 

Typical analysis in the laboratory consists of sample validation, a compositional analysis of the individual and reoombined samples, measurement of oil and gas density and viscosity over a range of temperatures, and determination of the basic PVT parameters Bo, Roand B. ... 

A container full of hydrocarbons can be described in a number of ways, from a simple measurement of the dimensions of the container to a detailed compositional analysis. The most appropriate method is usually determined by what you want to do with the hydrocarbons. If for example hydrocarbon products are stored in a warehouse prior to sale the dimensions of the container are very important, and the hydrocarbon quality may be completely irrelevant for the store keeper. However, a process engineer calculating yields of oil and gas from a reservoir oil sample will require a detailed breakdown of hydrocarbon composition, i.e. what components are present and in what quantities. 

Both vapor-phase chromatography and high performance Hquid chromatography, along with nuclear magnetic resonance spectroscopy, have been used for isomer and composition analysis. 

However, for the past 30 years fractional separation has been the basis for most asphalt composition analysis (Fig. 10). The separation methods that have been used divide asphalt into operationally defined fractions. Four types of asphalt separation procedures are now in use ( /) chemical precipitation in which / -pentane separation of asphaltenes is foUowed by chemical precipitation of other fractions with sulfuric acid of increasing concentration (ASTM D2006) (2) solvent fractionation separation of an "asphaltene" fraction by the use of 1-butanol foUowed by dissolution of the 1-butanol solubles in... 

Tracer Diffusivity Tracer diffusivity, denoted by D g is related to both mutual and self-diffusivity. It is evaluated in the presence of a second component B, again using a tagged isotope of the first component. In the dilute range, tagging A merely provides a convenient method for indirect composition analysis. As concentration varies, tracer diffusivities approach mutual diffusivities at the dilute limit, and they approach selr-diffusivities at the pure component limit. That is, at the limit of dilute A in B, D g D°g and... 

Chromatographs can perform a total composition analysis for a sample. It is possible but inconvenient to provide an analog input for each component. Furthermore, it is often desirable to capture other information, such as the time that the analysis was made (normally the time the sample was injected). 

Size cut enhances resolution, optically important aerosol analysis, low artifact potential, particle bounce amenable to automated compositional analysis automated versions available large networks under development... 

Another confusing issue is that of depth resolution. It is a measurement of the technique s ability to clearly distinguish a property as a function of depth. For example a depth resolution of 20 A, quoted in an elemental composition analysis, means that the composition at one depth can be distinguished from that at another depth if there is at least 20 A between them. 

Quantitative compositional analysis from EDS or EELS, and crystal structure analysis from CBED... 

The STEM instrument itself can produce highly focused high-intensity beams down to 2 A if a field-emission source is used. Such an instrument provides a higher spatial resolution compositional analysis than any other widely used technique, but to capitalize on this requires very thin samples, as stated above. EELS and EDS are the two composition techniques usually found on a STEM, but CL, and even AES are sometimes incorporated. In addition simultaneous crystallographic information can be provided by diffraction, as in the TEM, but with 100 times better spatial resolution. The combination of diffraction techniques and analysis techniques in a TEM or STEM is termed Analytical Electron Microscopy, AEM. A well-equipped analytical TEM or STEM costs well over 1,000,000. 

Electron Probe Microanalysis, EPMA, as performed in an electron microprobe combines EDS and WDX to give quantitative compositional analysis in the reflection mode from solid surfaces together with the morphological imaging of SEM. The spatial resolution is restricted by the interaction volume below the surface, varying from about 0.2 pm to 5 pm. Flat samples are needed for the best quantitative accuracy. Compositional mapping over a 100 x 100 micron area can be done in 15 minutes for major components Z> 11), several hours for minor components, and about 10 hours for trace elements. 

STEM can provide image resolution of thin specimens rivaling TEM, but in addition can provide simultaneous crystallographic and compositional analysis at a higher spatial resolution than any other widely-used technique. Any solid material may be examined, provided that a specimen can be prepared that is less than about 100 nm in thickness. 

Auger electron spectroscopy is the most frequently used surface, thin-film, or interface compositional analysis technique. This is because of its very versatile combination of attributes. It has surface specificity—a sampling depth that varies... 

In the discussion so far we have considered the typical LEIS experiment only, i.e. large angles of incidence of exit relative to the surface plane. Under these conditions, in general, quantitative composition analysis is possible, because the ion-target interaction can be considered as a binary collision, because of the absence of matrix effects (see below). 

Surface composition analysis by LEIS is based on the use of noble gas ions as projectiles, making use of the superb surface sensitivity of LEIS under these conditions. A consequence of this surface sensitivity is that the LEIS energy spectrum consists of lines, one per element, if the masses differ sufficiently. The lines are narrow, because inelastic energy losses play a minor role here. Thus, the information on the atomic species present is deduced from the energy of the back-scattered ions, which can be converted to the mass of the scattering center. (Eig. 3.55 [3.141]). In Eig. 3.55 it is shown that the mass range, where LEIS is sensitive, depends on the projectile mass. 

A further example where quantitative surface composition analysis is possible for a non-trivial surface is shown in Fig. 3.58, where for the systems Ta -i- O and Nb -i- O adsorption the ion signal from the metal is shown as a function of the ion signal from O. In this binary example Eq. (3.43) are valid for the concentration ... 

The most common ions observed as a result of electron-stimulated desorption are atomic (e. g., H, 0, E ), but molecular ions such as OH", CO", H20, and 02" can also be found in significant quantities after adsorption of H2O, CO, CO2, etc. Substrate metallic ions have never been observed, which means that ESD is not applicable to surface compositional analysis of solid materials. The most important application of ESD in the angularly resolved form ESDIAD is in determining the structure and mode of adsorption of adsorbed species. This is because the ejection of positive ions in ESD is not isotropic. Instead the ions are desorbed along specific directions only, characterized by the orientation of the molecular bonds that are broken by electron excitation. 

The atom probe field-ion microscope (APFIM) and its subsequent developments, the position-sensitive atom probe (POSAP) and the pulsed laser atom probe (PLAP), have the ultimate sensitivity in compositional analysis (i.e. single atoms). FIM is purely an imaging technique in which the specimen in the form of a needle with a very fine point (radius 10-100 nm) is at low temperature (liquid nitrogen or helium) and surrounded by a noble gas (He, Ne, or Ar) at 10 -10 Pa. A fluorescent screen or a... 

Run-of-the-mill instruments can achieve a resolution of 5-10 nm, while the best reach 1 nm. The remarkable depth of focus derives from the fact that a very small numerical aperture is used, and yet this feature does not spoil the resolution, which is not limited by dilfraction as it is in an optical microscope but rather by various forms of aberration. Scanning electron microscopes can undertake compositional analysis (but with much less accuracy than the instruments treated in the next section) and there is also a way of arranging image formation that allows atomic-number contrast, so that elements of different atomic number show up in various degrees of brightness on the image of a polished surface. 

Several techniques which combine imaging with spectrometric (compositional) analysis have now been explained. It is time to move on to straight spectrometry. 

FIGURE 5.5 (a) The hydroxy amino acids serine and threonine are slowly destroyed during the course of protein hydrolysis for amino acid composition analysis. Extrapolation of the data back to time zero allows an accurate estimation of the amonnt of these amino acids originally present in the protein sample, (b) Peptide bonds involving hydrophobic amino acid residues snch as valine and isolencine resist hydrolysis by HCl. With time, these amino acids are released and their free concentrations approach a limiting value that can be approximated with reliability. 

The complexity of oil fractions is not so much the number of different classes of compounds, but the total number of components that can be present. Even more challenging is the fact that, unlike the situation with other complex samples, in which only a few specific compounds have to be separated from the matrix, in oil fractions the components of the matrix itself are the analytes. Figure 14.1 presents an estimation (by extrapolation) of the total number of possible hydrocarbon isomers with up to twenty carbon atoms present in oil fractions. Although probably not all of these isomers are always present, these numbers are nevertheless somewhat overwhelming. This makes a complete compositional analysis using a single column separation of unsaturated fractions with boiling points above 100 °C utterly impossible. 

Pennington, Composition Analysis Procedures For PBXN-1, PBXN-2, PBXN-3, PBXN4 and PBXN-5 , NOTS TP 3337, NAVWEPS 8409 rioA/n jo in a c +f onvM.im a... 

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