Dich bai bao khoa hoc

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    14-Apr-2017

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Ultraviolet - Visible Spectroscopy (UV/Vis):Principle: It involves the absorption of electromagnetic radiation by the substances in the ultraviolet and visible regions of the spectrum. This will result in changes in the electronic structure of ions and molecules through the excitations of bonded and non-bonded electrons.

Instrumentation: It consists of a dual light source viz., tungsten lamp for visible range and deuterium lamp for ultraviolet region, grating monochromator, photo-detector, mirrors and glass or quartz cells.

NOTE: For measurements to be made under visible region both glass and quartz cells can be used. For the measurements under ultraviolet region, only quartz cell should be used, since, glass cells absorb ultraviolet rays.

There are two types of instrumental designs for this technique as single beam and double beam spectrophotometers. However double beam spectrophotometers are widely used and following is the outline of the instrument:

Applications: It is the most widely used technique for quantitative molecular analysis, for this Beer-Lambert law is applied. Sometimes it is used in conjunction with other techniques such as NMR, IR, etc., in the identification and structural analysis of organic compounds. For qualitative analysis it provides valuable information through the absorption spectrum which is unique for a given compound.

Disadvantages: Samples should be in solution. Mixture of substances poses difficult to analyse and requires prior separation. Interference from the samples matrix makes the measurement difficult.

PolarimetryPrinciple: In the preceding section under spectroscopy interaction of light with the substances leading to absorption, emission and fluorescence has been dealt. Here the phenomena concerns with the rotation of the plane of the plane polarised light when it is passed through the samples which lacks symmetry (e.g. sugar), these substances which are asymmetric in nature are said to be optically active substances. A plane polarised light is that which essentially has its vibration in only one direction or one plane as shown below:

ABC

In the above illustration A refers to the light waves propagating in all directions or planes (multidirectional), B refers to the Nicol prism which cuts all the planes of light and allows the light to come out with vibration in only one plane or direction (unidirectional) and C is the resultant light which is said to be a plane polarised light. It is the light, which is utilised for the measurement of the optical activity of a compound. The plane of this light will be rotated to certain extend depending on the nature of the compound under study and forms the basis of this technique.

Instrumentation: It comprises of a sodium lamp, nicol polarizer, sample tube, nicol analyzer and an eyepiece.

If, the rotation of the plane by a compound is in the clockwise then it is said to be dextro,if it is in the anticlockwise then it is said to be laevo. For any compound the rotation depends on the concentration and the length of the sample tube and importantly on the temperature too.

Applications: It is mainly used as a quantitative tool. It finds extensive application in the analysis of sugar. In pharmaceutical industry it is used for the measurement of concentration of optically active drugs.

Disadvantages: It requires the samples only in solution form. The sample tube, after filling with the sample solution, should be free of bubble or any free particle, otherwise the light path will be affected and hence the accuracy. The sample holder should be thermostatted, since the optical activities of the substances vary with temperature.

Tia cc tm Quang ph nhn thy c (UV-VIS)Nguyn tc: N lin quan n s hp th bc x in t do cc cht trong vng cc tm v c th nhn thy ca quang ph. iu ny s dn n nhng thay i trong cu trc in t ca cc ion v cc phn t thng qua vic kch thch cc electron ngoi quan v phi ngoi quan.Thit b o c: Bao gm ngun nh sng kp, l n Vn-fram cho vng nhn thy c v n -t-ri cho vng tia cc tm, my n sc cch t, b tch sng quang, gng v nhng chu thy tinh hoc thch anh.Ch : Thc hin php o trong vng nhn thy cho c hai t bo thy tinh v thch anh. Trong vng t ngoi, ch c t bo thch anh c thc hin, sau , t bo thy tinh hp th tia cc tm.C hai loi dng c thit k cho k thut ny nh chm n v quang ph chm kp. Tuy nhin quang ph chm kp ang c s dng rng ri v sau y l nhng hnh nh c th ca dng c:

Ngoi ra,n cn c s dng rng ri nht l trong k thut phn tch phn t, p dng theo lut Beer-Lambert. i khi n c s dng kt hp vi cc k thut khc nh NMR, Ir, vv, ... trong vic xc nh v phn tch cu trc ca cc hp cht hu c. Vic phn tch nh tnh th cung cp thng tin c gi tr thng qua ph hp th, ph hp th l duy nht cho mi hp cht hu c .

S bt li: Nhng mu nn trng thi ha tan. Hn hp cc cht c t kh c th phn tch v bt buc c u tin phn tch. S giao thoa t mu gc lm cho php o tr nn kh hn

Phn cc kNguyn tc: Trong phn trc di s tng tc quang ph ca nh sng vi cc cht dn n s hp th, kh thi v hunh quang c x l. y nhng mi quan tm hin tng vi vng quay ca mt phng ca mt phng phn cc nh sng khi n c truyn qua cc mu m thiu i xng (v d ng), cc cht l bt i xng trong thin nhin c cho l hot cht quang hc. mt mt phng phn cc nh sng l ci c bn c rung ng ca n ch theo mt hng hoc mt mt phng nh hnh di y:

A B C

hnh minh ha trn A ni n s truyn sng nh sng ti tt c mi hng hoc mt phng (a chiu), B l lng knh Nicol m n ct tt c mt phng nh sng v cho php nh sng i ra vi s dao ng ch trong mt mt phng hoc mt hng (theo mt phng hng duy nht) v C l nh sng tng hp c gi l nh sng mt phng phn cc. l nh sng m c s dng o hot ng quang hc ca mt hp cht. Mt phng ca nh sng ny s c lun chuyn n mc nht nh ty thuc vo tnh cht ca cc hp cht c nghin cu v hnh thnh c s ca k thut ny.

Thit b o c: N bao gm mt n natri, phn cc nicol, ng mu, phn tch nicol v mt th knh

Nu hp cht quay theo chiu kim ng h th c cho l dextro, nu ngc chiu kim ng h c cho l laevo. Hp cht no cng ph thuc vo nng , chiu di ca ng mu v nhit .

ng dng: N c s dng ch yu nh mt cng c phn tch. c ng dng rng ri trong phn tch ng. Trong ngnh cng nghip dc phm n c s dng o nng ca thuc hot ng quang hc.

S bt li: Yu cu mu ch dng ha tan. ng ng mu , sau khi mu ha tan vo, s c ni ln bong bng hoc cc phn t, hoc ng i nh sng s b nh hng v nh hng chnh xc. ng mu nn c iu chnh nhit v cc hot ng quang hc ca cc cht bin i nhit