Composition, detecting changes

Narashimhan, S., R.S.H. Mah, A.C. Tamhane, J.W. Woodward, and J.C. Hale, A Composite Statistical Test for Detecting Changes of Steady States, AlChE Journal, 32(9), 1986, 1409-1418. (Fault detection, steady-state change)... 

During the last several years, a number of new instrumental surface techniques have been developed that are quite effective in detecting changes in the surfaces of minerals that have undergone chemically induced or natural geologic alteration. These techniques are quite sensitive (approximately 0.1-0.5% atomic concentration for x-ray photoelectron and Auger spectroscopy, for example), and they make it possible to monitor very small amounts of elements that may be present in the near surface material. Any change in the surface with respect to chemical composition may readily be measured qualitatively... 

Fig. 2.48. Separation of an extract by HPLC under optimized conditions in a 250 mm X 2 mm i.d., 5 pm particle, C18 column. The mobile phase was a gradient prepared from 0.03 per cent TFA in water (a) and acetonitrile (b) mobile phase composition (%) was changed from 90a 10b to 64a 36b in 35 min. The flow rate was 0.2 ml/min, the temperature 25°C, and detection was performed at 210 and 330 nm. Peak assignments 1 = pseudochlorogenic acid 2 = neochlorogenic acid 3 = chlorogenic acid 4 = cryptochlorogenic acid 5 = cynarin 6 = cynaroside 7 = scolymoside 8 = 3,4-di-O-caffe- oylquinic acid 9 = 1,3-di-O-caffeoylquinic acid 10 = 4,5-di-O-caffeoylquinic acid 11 = cynaropikrin. Reprinted with permission from M. Hausler et al. [148].

The discrepancy in numbers between natural and synthetic varieties is an expression of the usefulness of zeolitic materials in industry, a reflection of their unique physicochemical properties. The crystal chemistry of these aluminosilicates provides selective absorbtion and exchange of a remarkably wide range of molecules. Some zeolites have been called molecular sieves. This property is exploited in the purification and separation of various chemicals, such as in obtaining gasoline from crude petroleum, pollution control, or radioactive waste disposal (Mumpton, 1978). The synthesis of zeolites with a particular crystal structure, and thus specific absorbtion characteristics, has become very competitive (Fox, 1985). Small, often barely detectable, changes in composition and structure are now covered by patents. A brief review of the crystal chemistry of this mineral group illustrates their potential and introduces those that occur as fibers. 

When the mobile phase composition is changed in LC, the sensitivity of the detector to different solutes may be altered. For example, the UV spectra of solutes may shift upon a change in the composition of the mobile phase [584]. Especially if the detection wavelength is on the flank of an absorption band, this may easily lead to variations in the peak area that exceed the 20% difference which we used to discriminate between the different solutes in figure 5.35. 

Plastic is usually avoided as it can absorb constituents from the oil, thus altering the composition of the oil. Also, this can cause the container walls to swell and weaken. Chemicals in the plastic may also be absorbed by the oil, thus altering its composition again. When components of the plastic become dissolved in the oil, detectable changes often occur in its odour. 

Beckett described inductively coupled plasma mass spectrometry (ICP-MS) as an off-line detector for FFF which could be applied to collected fractions [ 149]. This detector is so sensitive that even trace elements can be detected making it very useful for the analysis of environmental samples where the particle size distribution can be determined together with the amount of different ele-ments/pollutants, etc. in the various fractions. In case of copolymers, ICP-MS detection coupled to Th-FFF was suggested to yield the ratio of the different monomers as a function of the molar mass. In several works, the ICP-MS detector was coupled on-line to FFF [150,151]. This on-line coupling proved very useful for detecting changes in the chemical composition of mixtures, in the described case of the clay minerals kaolinite and illite as natural suspended colloidal matter. 

Lipoyl-4-p-aminobenzoate and the product, p-aminobenzoate, were separated on a Nucleosil 3Ci8 column (4 mm x 50 mm). Solvent A was 0.1% trifluoroacetic acid and solvent B was methanol. The column was equilibrated with solvent A. In the first minute the composition was changed to 30% B, and then to 100% B by minute 11. This composition was held for 1 minute before returning to the starting conditions. The flow rate was 1 mL/min. Detection was by fluorescence using excitation and emission wavelengths of 276 and 340 nm, respectively. 

Typical molecular weights of all synthesized copolymers were in the range of 22,000-33,000 g/mol with polycarbonate standards used for calibration. These slight differences in molecular weights did not produce detectable changes in solvent-resistance for the same copolymer compositions as determined by traditional solvent-resistance measurements.16... 

The other two principles that detect changes in the solvent properties are conductivity and density measurements. Conductivity as a detection principle can only be used for ionic substances. The detection range is quite high, but the detector is sensitive to changes in solvent composition and shows a baseline shift if gradient elution is applied. 

Although high levels of fluoride (<3,000 ppm) kill most bacteria, there is little evidence that common levels of fluoride (1-10 ppm) alter the types of bacterial species or their relative concentrations in biofilms. Mutans and other streptococci in the biofilm may switch to asaccharolytic fermentation (Sect. 1.3.2). A fluoride-mediated reduction in bacterial acid production, ensues without a detectable change in bacterial biofilm composition. Correspondingly, fluoride has no effect on the development of gingivitis or its progression to periodontitis. 

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