Instrumental chemical analysis

Distinguish between wet chemical analysis, instrumental analysis, and analysis using physical properties. 

The major weakness of classical tga is that it gives information on weight loss but no chemical information. To overcome this problem, some modern tga instruments have been designed to be interfaced to chemical analysis instruments, the most popular being mass spectroscopy and Fourier transform infrared (ftir) spectroscopy. Both can give extremely useful information but care is needed to avoid problems from secondary reactions of volatile products and from loss of volatile products by condensation in transfer lines. The use of these so-called hyphenated techniques has been reviewed (30). 

Chemical Analysis Instrumental Techniques and Future Trends... 

Ewing, G. W., 1985. Instrumental Methods of Chemical Analysis. McGraw-Hill, New York. 

A wet-process plant maldug cement from shale and hmestoue has been described by Bergstrom [Roc/c Prod., 64—71 (June 1967)]. There are separate facilities for grinding each type of stone. The ball mill operates in closed circuit with a battery of Dutch State Mines screens. Material passing the screens is 85 percent minus 200 mesh. The entire process is extensively instrumented and controlled by computer. Automatic devices sample crushed rock, slurries, and finished product for chemical analysis by X-rav fluorescence. Mill circuit feed rates and water additions are governed by conventional controllers. 

Each type of mass spectrometer has its associated advantages and disadvantages. Quadrupole-based systems offer a fairly simple ion optics design that provides a certain degree of flexibility with respect to instrument configuration. For example, quadrupole mass filters are often found in hybrid systems, that is, coupled with another surface analytical method, such as electron spectroscopy for chemical analysis or scanning Auger spectroscopy. 

In electron-optical instruments, e.g. the scanning electron microscope (SEM), the electron-probe microanalyzer (EPMA), and the transmission electron microscope there is always a wealth of signals, caused by the interaction between the primary electrons and the target, which can be used for materials characterization via imaging, diffraction, and chemical analysis. The different interaction processes for an electron-transparent crystalline specimen inside a TEM are sketched in Eig. 2.31. 

Sampling for Gases and Vapors Many gases and vapors can be sampled by deviees whieh indicate the eoncentration of the substance during sampling or shortly thereafter, without the necessity for chemical analysis. These direct reading devices are convenient and useful when properly calibrated. Other substances cannot be sampled by this method, because no appropriate instrument is available, and indirect methods which require laboratory analysis of the sample must therefore be used. Such analyses are often delayed by days or weeks, depending upon laboratory schedules. 

All bought-in items of plant, especially those supplied without adequate materials certification, should be subject to random inspection. Portable instruments are available for many types of non-destructive examination, chemical analysis and mechanical testing of fabricated items of plant. 

Inhibitor control can be effected by conventional methods of chemical analysis, inspection of test specimens or by instrumentation. The application of instrumental methods is becoming of increasing importance particularly for large systems. The techniques are based on the linear (resistance) polarisation method and the use of electrical resistance probes. They have the advantage that readings from widely separated areas of the plant can be brought together at a central control point. (See Section 18.1.)... 

When a quantity is measured with the greatest exactness of which the instrument, method, and observer are capable, it is found that the results of successive determinations differ among themselves to a greater or lesser extent. The average value is accepted as the most probable. This may not always be the true value. In some cases the difference may be small, in others it may be large the reliability of the result depends upon the magnitude of this difference. It is therefore of interest to enquire briefly into the factors which affect and control the trustworthiness of chemical analysis. 

The sensitivity can, however, be improved if the technique of derivative spectrophotometry (Section 17.12) is employed. The development of inexpensive photoelectric colorimeters has placed this branch of instrumental chemical analysis within the means of even the smallest teaching institution. 

The various instruments used in chemical analysis by x-ray absorption or emission do not all have universally accepted names. To forestall confusion and to show how these instruments are related, they will be summarized schematically. 

Meadowcroft, D. B. In Instrumentation Reference Book, (Ed. B. E. Noltingk), Chapter 6. Chemical analysis — moisture measurement (Butterworth, 1988). 

The very first spectroscopic instruments, from Newton s prism and pinhole to Frauenhofer s simple spectroscope, were constructed to observe luminescence. Even though the great sensitivity of luminescence detection seemed to promise that luminescence would become an important tool for chemical analysis, the fact is that absorption spectroscopy was the first spectroscopic technique to be widely used. At first glance, this may seem surprising since absorption spectroscopy is inherently less sensitive and had to await the development of more complex instrumentation, especially, electronically amplified detection. 

A system has been constructed which allows combined studies of reaction kinetics and catalyst surface properties. Key elements of the system are a computer-controlled pilot plant with a plug flow reactor coupled In series to a minireactor which Is connected, via a high vacuum sample transfer system, to a surface analysis Instrument equipped with XFS, AES, SAM, and SIMS. When Interesting kinetic data are observed, the reaction Is stopped and the test sample Is transferred from the mlnlreactor to the surface analysis chamber. Unique features and problem areas of this new approach will be discussed. The power of the system will be Illustrated with a study of surface chemical changes of a Cu0/Zn0/Al203 catalyst during activation and methanol synthesis. Metallic Cu was Identified by XFS as the only Cu surface site during methanol synthesis. 

The development of modern surface characterization techniques has provided means to study the relationship between the chemical activity and the physical or structural properties of a catalyst surface. Experimental work to understand this reactivity/structure relationship has been of two types fundamental studies on model catalyst systems (1,2) and postmortem analyses of catalysts which have been removed from reactors (3,4). Experimental apparatus for these studies have Involved small volume reactors mounted within (1) or appended to (5) vacuum chambers containing analysis Instrumentation. Alternately, catalyst samples have been removed from remote reactors via transferable sample mounts (6) or an Inert gas glove box (3,4). 

Despite continuing improvements in instrumental methods for chemical analysis, the reliable analysis of... 

F. Rouessac and A. Rouessac, Chemical Analysis Modem Instrumentation Methods and Techniques, John Wiley Sons, New York, NY (2000). 

A. Krause, A. Lange and M. Ezrin, Plastics Analysis Guide - Chemical and Instrumental Methods, Hanser Verlag, Munich (1983). 

Sophisticated instrumental techniques are continually being developed and gradually replace the classical wet chemistry analytical methods. Wet chemical analysis is destructive the sample is dissolved or altered. Nowadays the analyst is highly focused on instrumental methods and chemometrics. Yet, chemical work-up methods (e.g. hydrolysis with alcoholic alkali, alkali fusion, aminolysis, and transesterification, etc.) and other wet laboratory skills should not be forgotten. 

After matrix removal, samples can be measured using various techniques, such as AAS, AES, ICP, etc. Traditional chemical analysis methods, involving separation and gravimetric, titrimetric or polarographic determination of the elements, are being replaced by a wide selection of instrumental methods. 

---