分析化学介绍(analytical chemistry)

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2005/12/25

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专业英语

Analytical chemistry is the analysis of material samples to gain an understanding of their chemical composition and structure.

Types
Analytical chemistry can be split into two main types, qualitative and quantitative:

Qualitative inorganic analysis seeks to establish the presence of a given element or inorganic compound in a sample.
Qualitative organic analysis seeks to establish the presence of a given functional group or organic compound in a sample.
Quantitative analysis seeks to establish the amount of a given element or in a sample.
Most modern analytical chemistry is quantitative. Quantitative analysis can be further split into different areas of study. The material can be analyzed for the amount of an element or for the amount of an element in a specific chemical species. The latter is of particular interest in biological systems; the molecules of life contain carbon, hydrogen, oxygen, nitrogen, and others, in many complex structures.

Techniques
There are a bewildering array of techniques available to separate, detect and measure chemical compounds.

Separation of chemicals in order to measure the weight or volume of a final product. This is an older process and can be quite painstaking.
Analysis of substances with devices using spectroscopy. Measuring the absorption of light by a solution or gas, we can calculate the amounts of several species, often without separation. Newer methods include atomic absorption spectroscopy (AAS), nuclear magnetic resonance (NMR) and neutron activation analysis (NAA).
Many techniques combine two or more analytical methods. Examples of this include ICP-MS(Inductively-Coupled Plasma - Mass Spectrometry), where volatilisation of a sample occurs in the first step, and measuring of the concentration occurs in the second. The first step may also involve a separation technique, such as chromatography, and the second a detection / measuring device.
Techniques that involve volatilisation aim to produce free atoms of the elements making up the sample, which can then be measured in concentration by the degree to which they absorb or emit at a characteristic spectral frequency. These methods have the disadvantage of completely destroying the sample, and any species contained within it. These techniques include atomic absorption spectroscopy and ICP-MS / ICP-AES. These techniques can still be used to study speciation, however by the incorporation of a separation stage before volatilisation.

Methods
Analytical methods rely on scrupulous attention to cleanliness, sample preparation, accuracy and precision.

Many practitioners will keep all their glassware in acid to prevent contamination, samples will be re-run many times over, and equipment will be washed in specially pure solvents.

A standard method for analysis of concentration involves the creation of a calibration curve.

If the concentration of element or compound in a sample is too high for the detection range of the technique, it can simply be diluted in a pure solvent. If the amount in the sample is below an instrument's range of measurement, the method of addition can be used. In this method a known quantity of the element or compound under study is added, and the difference between the concentration added, and the concentration observed is the amount actually in the sample.

Trends
Analytical chemistry research is largely driven by performance (sensitivity, selectivity, robustness, linear range, accuracy, precission, and speed), and cost (purchase, operation, training, time, and space).

A lot of effort is put in shrinking the analysis techniques to chip size. Although there are few examples of such systems competitive with traditional analysis techniques, potential advantages include size/portability, speed, and cost. (Total Analysis System or lab on a chip)

Much effort is also put into analyzing biological systems. Examples of rapidly expanding fields in this area are:

Proteomics - the analysis of protein concentrations and modifications, especially in response to various stresssors, at various developmental stages, or in various parts of the body.
Metabolomics - similar to proteomics, but dealing with metabolites.
Metalomics - similar to proteomics and metabolomics, but dealing with metal concentrations and especially with their binding to proteins and other molecules.

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What is Chemistry ?

Chemistry is frequently defined as the study of matter and the reactions that matter undergoes. Actually, physicists, geologists, and biologists also study matter, but only chemists study the reactions that matter undergoes. For example, only chemists make compounds and try to understand the reactions that produce the compounds. Indeed, a very large segment of chemists are employed by the chemical and pharmaceutical industry for the very purpose of preparing new plastics, coatings, ceramics, drugs, fillers, alloys, and so on. These synthetic chemists must first determine what reaction can be used to synthesize their target compound and then determine what conditions will optimize the yield of the compound in order to make the compound in the most cost-effective way. After the best reaction conditions have been determined, the chemist must determine how to purify the compound, and, finally, the chemist must identify it. This final process of identification usually includes not only being certain that the compound contains the right percentage of the various elements from which it is composed, but also involves the determination of the 3-dimensional structure of the compound.

Structural details are often crucial to the activity of the compound. For example, the compounds dextrophane and levorphane differ in a very subtle way. They are non-superimposable mirror images of one another in the same way that our hands are non-superimposable mirror images of one another. Yet, because of a quirk of the evolutionary process, our bodies are able to recognize this subtle difference and produce a very different response to the two compounds: levorphan is more strongly analgesic and addictive than morphine, whereas dextrophan is neither addictive nor an analgesic. Figure 1 shows the structural formulas of the two compounds (we will discuss the various types of formulas in a later section). In Figure 2, the same molecules are shown as computer generated molecular models. In part (a), ball and stick models are shown, while part (b) shows space-filling models.