Principles of instrumentationPrinciples of instrumentation

Prepared by:Prepared by:Ibtisam H. AlAswadIbtisam H. AlAswadReham S. HammadReham S. Hammad

PhotometryPhotometry

--Photometry means Photometry means “the measurement of light“the measurement of light””

If a substance can be converted to a soluble, colored If a substance can be converted to a soluble, colored material, its concentration may be determined by the material, its concentration may be determined by the

amount of color present in the solutionamount of color present in the solution . . - -Photometer & Spectrophotometer are instruments used for Photometer & Spectrophotometer are instruments used for

this type of measutment, in which a photocell or this type of measutment, in which a photocell or photomultiplier tube is used to detect the amount of light photomultiplier tube is used to detect the amount of light that passes through a colored solution from a light that passes through a colored solution from a light sourcesource..

- -The greatest sensitivity is obtained when the light permitted to The greatest sensitivity is obtained when the light permitted to pass through the solution is of a particular wavelengthpass through the solution is of a particular wavelength..

((The wavelength shows the maximum absorbance for the The wavelength shows the maximum absorbance for the solution colorsolution color.).)

Characteristics of LightCharacteristics of Light

11 - -Light is a form of electromagnetic energy that travels in Light is a form of electromagnetic energy that travels in waveswaves . .

22--The wavelength of light is the distance between two beaks The wavelength of light is the distance between two beaks of the light wave, is inversely proportional with its energyof the light wave, is inversely proportional with its energy..

33 - -In the visible region the color of light is a function of its wave In the visible region the color of light is a function of its wave length increasing from violet towards the red colorlength increasing from violet towards the red color

44 - -Objects that appear colored absorb light at particular Objects that appear colored absorb light at particular wavelength and reflect the other parts of the visible wavelength and reflect the other parts of the visible spectrum resulting in many shades of colorspectrum resulting in many shades of color..

exampleexample:: -a substance that absorbs violet light at 400 nm a substance that absorbs violet light at 400 nm

reflects all other light and appears as yellow greenreflects all other light and appears as yellow green..

- -Spectrophotometer takes advantage of the property Spectrophotometer takes advantage of the property of colored solutions to absorb light of specific of colored solutions to absorb light of specific wavelengthwavelength..

- -To measure the concentration of a blue solution, To measure the concentration of a blue solution, light is passed through it at about 590 nm. The light is passed through it at about 590 nm. The amount of yellow light absorbed varies directly in amount of yellow light absorbed varies directly in proportion to the concentration of the blue proportion to the concentration of the blue substances in the solutionsubstances in the solution..

Table-1(wavelengths of various types of Table-1(wavelengths of various types of Radiation)Radiation)

Types of Types of radiationradiation

Approximately Approximately wavelengthwavelength

GammaGamma < <0.10.1

X-raysX-rays 0.1-100.1-10

UltravioletUltraviolet <<380380

VisibleVisible 380-750380-750

InfraredInfrared>>750750

radiowavesradiowaves>>2525 x 107x 107

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Table –2 (the visible Spectrum)Table –2 (the visible Spectrum)

Approximately Approximately wavelengthwavelength

Color of absorbed Color of absorbed lightlight

Color of reflected Color of reflected lightlight

400-435400-435 VioletViolet Green–YellowGreen–Yellow

435-500435-500BlueBlue YellowYellow

500-570500-570GreenGreen RedRed

570-600570-600YellowYellow BlueBlue

600-630600-630OrangeOrange Green blueGreen blue

630-700630-700RedRed GreenGreen

Beer’s lawBeer’s law

When the light of an appropriate wavelength When the light of an appropriate wavelength strikes a cuvet that contains a colored strikes a cuvet that contains a colored sample, some of the light is absorbed and sample, some of the light is absorbed and the rest is transmitted through the sample to the rest is transmitted through the sample to the detector. % percent transmittance which the detector. % percent transmittance which represents the proportion of light reaches represents the proportion of light reaches the detectorthe detector..

% %T = It \ Io x 100T = It \ Io x 100% %

WhereWhere::

IoIo: : is the intensity of light striking is the intensity of light striking the sample the sample..

ItIt:: is the intensity of transmitted is the intensity of transmitted light light..

It Io

Beer’s lawBeer’s law

- -If the concentration of a solution is increased, the It will If the concentration of a solution is increased, the It will decrease and then % T is decreaseddecrease and then % T is decreased..

- -The relationship between the concentration and %T is not The relationship between the concentration and %T is not linear, but if the logarithm of the %T is plotted against the linear, but if the logarithm of the %T is plotted against the concentration, a straight line is obtained )fig1(concentration, a straight line is obtained )fig1(

--The term The term absorbanceabsorbance is used to represent – log % T is used to represent – log % T

A = - log % T = 1/ log % TA = - log % T = 1/ log % T

Beer-Lambert LawBeer-Lambert Law A = a b cA = a b c

Which states that “the absorbance of a solution is directly Which states that “the absorbance of a solution is directly proportional with the concentration of the dissolved proportional with the concentration of the dissolved substancesubstance””

WhereWhere:: - - A is the absorbanceA is the absorbance

- - a is the molar absorbtivity coefficienta is the molar absorbtivity coefficient.. - - b is the light bath through a solutionb is the light bath through a solution..

For x substanceFor x substance : : Ax = a b cxAx = a b cx (1) (1)

For standard substanceFor standard substance:: Ast = a b cstAst = a b cst (2) (2)

Then we can determine the concentration of x substance by measuring both sample and standard absorbance, which can be made by spectrophotometers.

UV – Visible photometryUV – Visible photometry

- -Typical coloremetric instruments contain five Typical coloremetric instruments contain five components components::

11 - -Stable source of radiation energyStable source of radiation energy..

22 - -A transparent container for holding the sampleA transparent container for holding the sample..

33 - -A device that isolates a restricted region of the A device that isolates a restricted region of the spectrum for measurement spectrum for measurement..

44 - -A radiation detector which converts radiant A radiation detector which converts radiant energy to electrical signals energy to electrical signals..

55 - -A signal processor and read out which displays A signal processor and read out which displays the transudated signals, a meter scale, a the transudated signals, a meter scale, a digital meter or a recorder chart. Fig (2) digital meter or a recorder chart. Fig (2)

UV – Visible photometryUV – Visible photometry

11 - -Radiation sourcesRadiation sources

- -In UV regionIn UV region::The most commonly used is deuterium lamp or The most commonly used is deuterium lamp or hydrogen lamp. In which a continues spectrum is hydrogen lamp. In which a continues spectrum is produced by the excitation of deuterium )D2( or produced by the excitation of deuterium )D2( or hydrogen at law pressure, and then produced light hydrogen at law pressure, and then produced light with )160-375( nmwith )160-375( nm..

- -In visible regionIn visible region::

Tungeston filament lamp is the most commonly used Tungeston filament lamp is the most commonly used and produces light at )350-2500( nmand produces light at )350-2500( nm..

22 - -Wavelength selectorsWavelength selectors A- A- FiltersFilters::

may be formed of a transparent dielectric layer such as calcium may be formed of a transparent dielectric layer such as calcium fluoride, the thickness of this layer is controlled carefully and fluoride, the thickness of this layer is controlled carefully and determines the wave length of transmitted light. Or formed of determines the wave length of transmitted light. Or formed of colored glass that absorbs certain portions of spectrum and colored glass that absorbs certain portions of spectrum and transmits others, according to its colortransmits others, according to its color..

B- B- MonochromatorsMonochromators :: Which may beWhich may be::

11 - -Grating monochromatorGrating monochromator:: Grooves formed of relatively broad faces and narrow Grooves formed of relatively broad faces and narrow

unused face )fig 3-A ( unused face )fig 3-A (

22 - -PrismPrism:: Principle:Principle: (fig 3-B) (fig 3-B)::

- - Suppose a radiation of 2 wave length enter from the Suppose a radiation of 2 wave length enter from the entrance slit, they strike the mirror to be reflected on entrance slit, they strike the mirror to be reflected on the dispersing element to produce angular dispersion the dispersing element to produce angular dispersion of light which face a black surface and come out from of light which face a black surface and come out from the exit slit only, by moving monochromator, a specific the exit slit only, by moving monochromator, a specific wave length will pass from the exit slitwave length will pass from the exit slit..

- - For prism by moving it only one For prism by moving it only one will exit will exit

Note:Colorimeters

Photometers

-Used filters as wavelength selector

Spectrophotometer

-Used monochromators as Wavelength selector

3 -Sample containers: - cuvetes that hold the samples must be made of material that

passes radiation in the spectral region of intrest . - Quartz or fused silica may be used in the spectral region

)350-3000 nm(, mean it may be used in the UV, visible and a part of infrared .

- Silicated glass used in )350- 2000 nm( region. - Plastic is used in the visible region

4 -Radiation detectors and read out.

PhototubesPhototubes Photomultiplier tubePhotomultiplier tube

Photoconductivity detectorPhotoconductivity detector silicon diode electrodesilicon diode electrode

- -One of them may be used to transform radian energy into One of them may be used to transform radian energy into electrical energy. Which may be measured by electrical energy. Which may be measured by galvanometer or any or any read out devicegalvanometer or any or any read out device..

Principle of phototubePrinciple of phototube::Photoemissive cathode that tends to emit electrons Photoemissive cathode that tends to emit electrons when irradiated. These electrons flow to the anode when irradiated. These electrons flow to the anode generating current. (Fig 4)generating current. (Fig 4)

General Types of UV-Visible colorimetersGeneral Types of UV-Visible colorimeters

A- A- Photometers (Nondispersive)Photometers (Nondispersive)::

Which use filters as wavelength selector, Which use filters as wavelength selector, which may bewhich may be::

General Types of UV-Visible colorimetersGeneral Types of UV-Visible colorimeters

B- B- Spectrophotometers (dispersive)Spectrophotometers (dispersive)::

Used Monochromator as wavelength selector Used Monochromator as wavelength selector and may be and may be::