Analytic techniques

Gas product was analyzed online by means of an Agilent 6890 model gas chromatograph fitted with TCD (thermal conductivity detector). 

Specific gravity 60/60°F of crude oils and residua was measured by using a pycnometer according to ASTM D-70 method. Energy-dispersive x-ray fluorescence spectrometry technique was used for sulfur content measurements (ASTM D-4294) with a model SLFA-2100 HORIBA spectrometer. Coke-forming tendency of the feedstocks was measured as Ramsbottom Carbon (ASTM D-524), which determines the amount of carbon residue left after evaporation and pyrolysis of oil. The amount of asphaltenes was measured as insolubles in n-heptane according to ASTM D-3279 method. Atomic absorption was used for determining Ni and V contents with a model AA Series Solar spectrometer. Distillation curve of stabilized feeds (TBP distillation) was obtained by means of ASTM D-2892 method. 

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In this section we will discuss only the analytical techniques that are in very general usage without presenting the older chemical methods. 

Finally it is likely that attention will be focused on emissions of polynuclear aromatics (PNA) in diesel fuels. Currently the analytical techniques for these materials in exhaust systems are not very accurate and will need appreciable improvement. In conventional diesel fuels, emissions of PNA thought to be carcinogenic do not exceed however, a few micrograms per km, that is a car will have to be driven for several years and cover at least 100,000 km to emit one gram of benzopyrene for example These already very low levels can be divided by four if deeply hydrotreated diesel fuels are used. 

Chemists frequently work with measurements that are very large or very small. A mole, for example, contains 602,213,670,000,000,000,000,000 particles, and some analytical techniques can detect as little as 0.000000000000001 g of a compound. For simplicity, we express these measurements using scientific notation thus, a mole contains 6.0221367 X 10 particles, and the stated mass is 1 X 10 g. Sometimes it is preferable to express measurements without the exponential term, replacing it with a prefix. A mass of 1 X 10 g is the same as 1 femtogram. Table 2.3 lists other common prefixes. 

Measurements are made using appropriate equipment or instruments. The array of equipment and instrumentation used in analytical chemistry is impressive, ranging from the simple and inexpensive, to the complex and costly. With two exceptions, we will postpone the discussion of equipment and instrumentation to those chapters where they are used. The instrumentation used to measure mass and much of the equipment used to measure volume are important to all analytical techniques and are therefore discussed in this section. 

There is an obvious order to these four facets of analytical methodology. Ideally, a protocol uses a previously validated procedure. Before developing and validating a procedure, a method of analysis must be selected. This requires, in turn, an initial screening of available techniques to determine those that have the potential for monitoring the analyte. We begin by considering a useful way to classify analytical techniques. 

A second class of analytical techniques are those that respond to the relative amount of analyte thus... 

Second, the majority of analytical techniques, particularly those used for a quantitative analysis, require that the analyte be in solution. Solid samples, or at least the analytes in a solid sample, must be brought into solution. 

Atomic absorption, along with atomic emission, was first used by Guystav Kirch-hoff and Robert Bunsen in 1859 and 1860, as a means for the qualitative identification of atoms. Although atomic emission continued to develop as an analytical technique, progress in atomic absorption languished for almost a century. Modern atomic absorption spectroscopy was introduced in 1955 as a result of the independent work of A. Walsh and C. T. J. Alkemade. Commercial instruments were in place by the early 1960s, and the importance of atomic absorption as an analytical technique was soon evident. 

Slavin, W. A Gomparison of Atomic Spectroscopic Analytical Techniques, Spectroscopy 1991, 6, 16-21. 

The potentiometric determination of an analyte s concentration is one of the most common quantitative analytical techniques. Perhaps the most frequently employed, routine quantitative measurement is the potentiometric determination of a solution s pH, a technique considered in more detail in the following discussion. Other areas in which potentiometric applications are important include clinical chemistry, environmental chemistry, and potentiometric titrations. Before considering these applications, however, we must first examine more closely the relationship between cell potential and the analyte s concentration, as well as methods for standardizing potentiometric measurements. 

In potentiometry, the potential of an electrochemical cell under static conditions is used to determine an analyte s concentration. As seen in the preceding section, potentiometry is an important and frequently used quantitative method of analysis. Dynamic electrochemical methods, such as coulometry, voltammetry, and amper-ometry, in which current passes through the electrochemical cell, also are important analytical techniques. In this section we consider coulometric methods of analysis. Voltammetry and amperometry are covered in Section 1 ID. 

In comparison with most other analytical techniques, radiochemical methods are usually more expensive and require more time to complete an analysis. Radiochemical methods also are subject to significant safety concerns due to the analyst s potential exposure to high-energy radiation and the need to safely dispose of radioactive waste. 

An analytical technique in which samples are injected into a carrier stream of reagents, or in which the sample merges with other streams carrying reagents before passing through a detector. 

Gas chromatography/ma.ss spectrometry (GC/MS) is an analytical technique combining the advantages of a GC instrument with those of a mass spectrometer. 

Because a GC and an MS both operate in the gas phase, it is a simple matter to connect the two so that separated components of a mixture are passed sequentially from the GC into the MS, where their mass spectra are obtained. This combined GC/MS is a very powerful analytical technique, the two instruments complementing each other perfectly. 

LC operates in the liquid phase, while MS is a gas-phase method, so it is not a simple matter to connect the two. An interface is needed to pass separated components of a mixture from the LC to the MS. With an effective interface, LC/MS becomes a very powerful analytical technique. 

Emission spectroscopy is a very useful analytical technique in determining the elemental composition of a sample. The emission may be produced in an electrical arc or spark but, since the mid-1960s, an inductively coupled plasma has increasingly been used. 

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