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1、精选优质文档-倾情为你奉上Ultraviolet and visible molecular spectroscopyProbably the first physical method used in analytical chemistry was based on the quality of the color in colored solution. The first things we observe regarding colored solutions are their hue，or color，and the colors depth，or intensity.These
2、 observations led to the technique historically called colorimetry;the color of the solution could identify species(qualitative analysis) while the intensity of the color could identify the concentration of the species present(quantitative analysis).This technique was the first use of what we now un
3、derstand to be absorption spectroscopy for chemical analysis .When white light passes through a solution and emerges as red light，we say that the solution is red.What has actually happened is that the solution has allowed the red component of white light to pass through，whereas it has absorbed the c
4、omplementary color，yellow and blue. The more concentrated the sample solution，the more yellow and blue light is absorbed and the more intensely red the solution appears to the eye. For a long time，experimental work made use of the human eye as the detector to measure the hue and intensity of colors
5、in solutions. However，even the best analyst can have difficulty comparing the intensity of two colors with slightly different hues，and there are of course people who are color-blind and cannot see certain colors.Instruments have been developed to perform the measurements more accurately and reliably
6、 than the human eye. While the human eye can only detect visible light，this chapter will focus on both the ultraviolet(UV) and the visible (Vis) portions of the spectrum. The wavelength range of UV radiation starts at the blue end of visible light(about 400nm) and ends at approximately 200nm for spe
7、ctrometers operated in air. The radiation has sufficient energy to excite valence electrons in many atoms and molecules;consequently，UV radiation is involved with electronic excitation. Visible light，considered to be light with wavelengths from 800 to 400nm，acts in the same way as UV light. It is al
8、so considered part of the electronic excitation region. For this reason we find commercial spectroscopic instrumentation often operates with wavelengths between 800 and 200nm.Spectrometers of this type are called UV/Visible(or UV/Vis) spectrometers.The vacuum UV region of the spectrum extends below
9、200 nm to the X-ray region of the spectrum, at100 A. It has called the vacuum UV region because oxygen，water vapor，and other molecules in air absorb UV radiation below 200nm，so the spectrometer light path3 must be free of air to observe wavelengths200nm.The instrument must be evacuated(kept under va
10、cuum) or purged with an appropriate non-UV absorbing gas such as helium for this region to be used. Vacuum UV radiation is also involved in electronic excitation but the spectrometers are specialized and not commonly found in undergraduate or routine analytical laboratories. For our purposes the ter
11、m UV will mean radiation between 200nm and 400nm，unless stated otherwise.The interaction of UV and visible radiation with matter can provide qualitative identification of molecules and polyatomic species，including ions and complexes. Structural information about molecules and polyatomic species，espe
12、cially organic molecules,can be acquired. This qualitative information is usually obtained by observing the UV/Vis spectrum,the absorption of UV and visible radiation as a function of wavelength by molecules.The shape and intensity of UV/Vis absorption bands are related to the electronic structure o
13、f the absorbing species.The molecule is often dissolved in a solvent to acquire the spectrum . We will look at how we can use the absorption maximum of a chromophore and a set of guidelines to predict the position of the absorption maximum in a specific molecule.We will also consider how the solvent
14、 affects the spectrum of some molecules. As a reminder,the transitions that give rise to UV/Vis absorption by organic molecules are the n*,*，and n*，transitions. Some terms need to be defined. A chromophore is a group of atoms(part of a molecule)that gives rise to an electronic absorption. An auxochr
15、ome is a substituent that contains unshared (nonbonding) electron pairs，such as OH，NH，and halogens. An auxochrome attached to a chromophore with electrons shifts the absorption maximum to longer wavelengths. A shift to longer wavelengths is called a bathochromic shift or red shift.A shift to shorter
16、 wavelengths is called a hypsochromic shift or blue shift. An increase in the intensity of absorption band(that is，an increase in max)is called hyperchromism;a decrease in intensity is called hypochromism. These shifts in wavelength and intensity come about as a result of the structure of the entire
17、 molecule or as a result of interaction between the solute molecules and the solvent molecules. As described at the beginning of the chapter，the types of compounds that absorb UV radiation are those with nonbonded electrons(n electrons) and conjugated double bond systems (electrons) such as aromatic
18、 compounds and conjugated olefins. Unfortunately，such compounds absorb over similar wavelength ranges，and the absorption spectra overlap considerably. As a first step in qualitative analysis，it is necessary to purify the sample to eliminate absorption bands due to impurities. Even when pure，however，
19、the spectra are often broad and frequently without fine structure. For these reasons，UV absorption is much less useful for the qualitative identification of functional groups or particular molecules than analytical methods such as MS，IR，and NMR. UV absorption is rarely used for organic structural el
20、ucidation today in modern laboratories because of the ease of use and power of NMR，IR，and MS. UV and visible absorption spectrometry is a powerful tool for quantitative analysis. It is used in chemical research，biochemistry，chemical analysis，and industrial processing.Quantitative analysis is based o
21、n the relationship between the degree of absorption and the concentration of the absorbing material. Mathematically，it is described for many chemical systems by Beers Law A=abc. The term applied to quantitative absorption spectrometry by measuring intensity ratios is spectrophotometry. The use of sp
22、ectrophotometry in the visible region of the spectrum used to be referred to as colorimetry. UV/Vis spectrophotometry is a widely used spectroscopic technique. It has found use everywhere in the world for research，clinical analysis，environmental analysis，and many other applications. Some typical app
23、lications of UV absorption spectroscopy include the determination of(I)the concentrations of phenol，nonionic surfactants，sulfate，sulfide，phosphates，fluoride，nitrate，a variety of metal ions，and other chemicals in drinking water in environmental testing;(II)natural products,such an steroids or chlorop
24、hyll;(III)dyestuff materials;(IV)vitamins，proteins，DNA，and enzymes in biochemisty. Spectrophotometry in the UV region of the spectrum is used for the direct measurement of many organic compounds，especially those with aromatic rings and conjugated multiple bonds. There are also colorless inorganic sp
25、ecies that absorb in the UV. A good example is the nitrate ion，NO3-.A rapid screening method for nitrate in drinking water is performed by measuring the absorbance of the water at 220 nm and at 275 nm. Nitrate ion absorbs at 220 nm but not at 275 nm;the measurement at 275 nm is to check for interfer
26、ing organic compounds that may be present. Spectrophotometic analysis in the visible region can be used whenever the sample is colored. Many materials are inherently colored without chemical reaction(e.g.，inorganic ions such as dichromate，permanganate，cupric ion，and ferricion) and need no further ch
27、emical reaction to form colored compounds. Colored organic compounds，such as dyestuffs，are also naturally colored. Solution of such materials can be analyzed directly.The majority of metal and nonmetal ions，however，are colorless. The presence of these ions in a sample solution can be determined by f
28、irst reacting the ion with an organic reagent to form a strongly absorbing species. If the product of the reaction is colored，absorbance can be measured in the visible region;alternatively，the product formed may be colorless but absorb in the UV. The majority of spectrophotometric determinations res
29、ult in an increase in absorbance (darker color if visible) as the concentration of the analyte increases. Quantitative analysis by absorption spectrophotometry requires that the samples be free from particulates，that is，free from turbidity. The reason for this is that particles can scatter light. If
30、 light is scattered by the sample away from the detector，it is interpreted as an absorbance.The absorbance will be erroneously high if the sample is turbid. We can make use of the scattering of light to characterize samples as discussed in Section 5. 7，but particulates must be avoided for accurate a
31、bsorbance measurements. Quantitative analysis by absorption spectrophotometry generally requires the preparation of a calibration curve，using the same conditions of pH，reagents added，and so on for all of the standards，samples，and blanks. It is critical to have a reagent blank that contains everythin
32、g that has been added to the samples(except the analyte).The absorbance is measured for all blanks，standards，and samples.The absorbance of the blank is subtracted from all other absorbances and a calibration curve is constructed from the standards. The highest accuracy results from working in the li
33、near region of the calibration curve. These quantitative methods can be quite complicated in the chemistry involved，the number of steps required(extraction，back-extraction，pH-adjustment，precipitation，masking，and many other types of operations may be involved in a method)，and the analyst must pay att
34、ention all the details to achieve accurate and precise results. There is both science and art involvedin performing many of these analyses. Many standard or regulatory methods have published precision and accuracies data in the methods. These are the precisions and accuracies that can be achieved by an experienced analyst. From Undergraduate Instrumental Analysis By James W. Robinson 专心-专注-专业