chapter 5 uv/visible absorption spectroscopy- part 2 540-uv... · photoacoustic spectroscopy •...

Chapter 5

UV/visible Absorption spectroscopy-

Part 2

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Apparatus and Instruments used for

Uv/Vis measurements

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EMR

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Deuterium Lamps

Used to obtain ultraviolet absorption spectra. Above 350 nm its

output is too weak for accurate measurements of absorption. The

irradiance is too low for fluorescence measurements. This is a low

pressure discharge lamp that produces light by the reaction,

D2 + e D2* 2D + hn

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Quartz Halogen Lamps

Used to obtain visible absorption spectra. Its output below 300 nm is too weak for accurate measurements of absorption. The irradiance is too low for most fluorescence measurements. Power supply: 12 V dc at 4 A, and highly regulated.

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Xenon Arc Lamps

it is difficult to obtain high quality spectra in those regions. This is a high Used to obtain fluorescence excitation, emission spectra, and make quantitative measurements. Below 300 nm the output is too weak to make accurate measurements. The lines between 450 and 500 nm and above 650 nm make pressure discharge lamp.

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Mercury Arc Lamps

Used to perform quantitative fluorescence measurements, or obtain

emission spectra. The line output precludes obtaining excitation

spectra. Note how the 2537 line is inverted! The UV, blue and green

lines emit 10-100 times more light than a xenon arc lamp. This is a

high pressure discharge lamp.

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Calibration Pen Lamps

Pen lamps can be purchased with a wide variety of gases that emit

sharp line spectra. The line output is commonly used to calibrate

wavelength separators, such as grating monochromators.

A mercury pen lamp emits intense lines in the ultraviolet through the

blue at 2537, 3130, 3650, 4047, and 4358 . To calibrate in the red use

a neon lamp.

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Absorption Filters

The figure shows the transmission curve for three bandpass

filters. 51715 passes everything except the deep UV. It would

be used to reject the mercury 253.7 nm line. 51670 passes a

band from the near UV to the green, while 51660 passes a band

from ~280 to 400 nm. These would be used with fluorescence

excitation.

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Absorption filters

The figure shows the transmission curve for two high pass filters.

51294 rejects wavelengths below 500 nm and passes those above.

5131 performs the same function with a cutoff wavelength of ~580 nm.

Note that it is nearly impossible to make a low pass colored glass

filter.

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An interference filter is created when light is multiply reflected between two parallel surfaces. The beams exiting the filter must be in phase. Only specific wavelength can pass through

The pass band varies with the order, N. To restrict transmission to only one order, the exit surface is most often a colored glass filter.

Interference Filter

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Interference Filter

Center wavelengths can be from the UV to the infrared. Pass bands

are narrower than absorption filters, but broader than grating

monochromators. The filter above is centered at 415 nm with a 10 nm

FWHM and a transmission ~33%. As the bandwidth narrows the

transmission drops.

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Prisms

Because of a complicated mathematical relationship between wavelength and bend angle, prisms are seldom used in modern instruments. For maximum dispersion (angular resolution), visible light is separated with a glass prism and ultraviolet is separated with a quartz prism. One common design is the constant deviation, or Pellen-Broca, prism shown in the figure. Constructed from one piece of material, it can be viewed as three separate prisms. For a given orientation, only one wavelength exits at a 90 deviation. To select a different wavelength the prism is rotated about the juncture of the dotted lines, P.

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P

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Diffraction Grating

m = 0

+1

+2

-1

-2

+1

a

+1

+2

-1

-2

+1

a i

r

A diffraction grating is created by a spatial modulation in phase or amplitude of an incoming plane wave. When exiting the grating, the input is separated into several plane waves traveling at angle to the original direction of propagation - these are called diffraction orders. The number and intensity of the orders depends upon the functional form of the modulation. The figure at the top is a transmission grating, while that at the bottom is a reflection grating. The diffraction angle depends upon order, m, wavelength, l, and spatial modulation period, a.

sin m m a l

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Blazed Gratings

+1

+2

-1

-2

+1

a

i

r

By forming the bottom of the grooves into a saw tooth shape, the angle of reflection can be made to correspond to one of the diffraction orders. One order then blazes, that is, it has most of the incoming optical power. Show below are three gratings blazed at 300, 400 and 500 nm. Note how the efficiency drops on either side of the blaze wavelength.

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Grating Resolution

7.3 : 9

The angular distribution of a monochromatic beam of light reflecting off a diffraction grating is a sinc function (sinx/x). The node spacing of the sinc function is called the angular resolution, D, where N is the number of grating grooves and a is the groove spacing. The goal is to minimize D, which is accomplished by maximizing the grating width, Na. You can also increase the order, which increases m. Grating spectral resolution is defined as, where Dl is the range of wavelengths occurring over D, and l is the center wavelength. High resolution means small Dl. The angle at which light leaves a grating leads to an ambiguous wavelength, e.g. 600 nm, m = 1; 300 nm, m = 2; 200 nm, m = 3.

2

cos mNa

l

D

R mNl

l D

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Commercially Available Gratings

The vast majority of commercially available gratings are plastic replicas. The master grating is coated with an epoxy resin to form a negative image. A thin layer of plastic coats the negative and cured. When removed from the negative, the replica is mounted on glass for rigidity and coated with aluminum. Ruled gratings are made with a spacing from 25 mm-1 to 1,800 mm-1. The upper size is 1515 cm, with 55 cm being typical. Concave ruled gratings can be manufactured with great difficulty. Ruled gratings have a scattered light figure of 10-3. Holographic gratings are made by coating the surface of glass with a photoresist. Two laser beams irradiate the surface at an angle to each other creating interference fringes. Bright fringes polymerize the photoresist. The surface is washed with an organic solvent to dissolve remaining monomer, then coated with aluminum. Spacings up to 2,400 mm-1 are possible, as well as concave gratings. Holographic gratings have a scattered light figure of 10-4.

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• Reverse bias creates a depletion

layer that reduces the

conductance of the junction

nearly to zero

• When radiation impinges on the

chip holes and electrons are

formed in the depletion layer and

those provide a current that is

proportional to radiant power

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53 IC = Integrated Circuit

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General Instrument Designs

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Spectronic 20 optical diagram

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1. Double- beam-in-space configuration

This requires two detectors that must be

perfectly matched

2. Double- beam-in-time configuration

Sample and reference measurements are

separated in time. Rapidly rotated mirror or

“chopper” is used. Only one detector is used

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General instrument designs double beam-in-space

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General instrument designs duoble beam-in-time

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Multichannel photon detector

It consists of an array of tiny photosensitive

detectors that are arranged in a pattern that

all elements of a beam of radiation that has

been dispersed by a grating can be

measured simultaneously

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Multichannel diode array spectrometer

(Multichannel photon detector)

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Characteristics of UV/Vis Methods:

Wide applicability to organic and inorganic systems

Sensitivities to 10-4 to 10-7 M

Moderate to high selectivity

Good accuracy, about 1-3% relative uncertainty

Ease and convenient data acquisition

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Derivative and Dual wavelength Spectrohotometery

• The use of dual dispersing systems are arranged in

such a way that two beams of slightly different

wavelengths (typically 1 or 2 nm) fall alternatively

onto a sample cell and its detector; no reference

beam is used.

• The ordinate parameter is the difference between the

alternate signals, which provides a good

approximation of the derivative of absorbances as a

function of wavelength (DA/ Dl).

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Dual Wavelength spectrophotometer

•Two beams of differing wavelengths having same

intensity are passed alternatively through a single

sample cell (previous types used single- wavelength)

•Two monochromators are used. One monochromator

may be used but light from the monochromator is

chopped and the monochromator is shifted between

the two wavelengths

• A single covette is used thus, scattering and

instrumental stray light are cancelled

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Dual-Wavelength Spectrophotometer using two monochromators

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Derivative spectroscopy has been mostly

used in the ultraviolet and visible regions for

qualitative identification of species, in which

the enhanced detail of a derivative spectrum

makes it possible to distinguish among

compounds having overlapping spectra.

Dual-wavelength spectrophotometry has

proven particularly useful for extracting

ultraviolet/visible absorption spectra of

analytes present in turbid solutions, where

light scattering obliterates the details of an

absorption spectrum.

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Derivative Spectroscopy

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Photoacoustic Spectroscopy

• Photoacaustic spectroscopy is based on light absorption effect.

• This effect is observed when a gas in a closed cell is irradiated with a chopped beam of radiation of a wavelength that is absorbed by the gas.

• The absorbed radiation causes periodic heating of the gas, which in turn results in regular pressure fluctuations within the chamber.

• If the chopping rate lies in the acoustical frequency range, these pulses of pressure can be detected by a sensitive microphone.

• Photoacoustic or optoacoustic spectroscopy, which was developed in the early 1970s, provides a means for obtaining ultraviolet and visible absorption spectra of solids, semisolids, or turbid liquids.

• Acquisition of spectra for these kinds of samples by ordinary methods is usually difficult at best and often impossible because of light scattering and reflection.

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• In photoacoustic studies of solids, the sample

is placed in a closed cell containing air or

some other nonabsorbing gas and a sensitive

microphone.

• The solid is then irradiated with a chopped

beam from a monochromator.

• The photoacoustic effect is observed provided the radiation is absorbed by the solid; the

power of the resulting sound is directly related

to the extent of absorption.

• Radiation reflected or scattered by the sample

has no effect on the microphone and thus does

not interfere. This latter property is perhaps the

most important characteristic of the method.

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Photoacoustic Spectrometer

• Light source is modulated at an audio frequency

• The sample absorbs the radiation and becomes a

heat source producing alternating regions of

compressions and refractions in the enclosed gas

that is acoustic or sound wave

• The acoustic signal is converted into electrical

signal by microphone

• Solid sample is directly irradiated with the

modulated source resulting in production of

acoustic waves in the surrounding gas

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