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An introduction to Ultraviolet/Visible Absorption Spectroscopy

Lecture 24

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Instrumentation• Light source - selector

• Sample container

• Detector

• Signal processing

• Light Sources (commercial instruments)

– D2 lamp (UV: 160 – 375 nm)

– W lamp (vis: 350 – 2500 nm)

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SourcesDeuterium and hydrogen lamps (160 – 375 nm)

DD22 + E + Eee → D → D22* → D’ + D’’ + * → D’ + D’’ + hh

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(a) A deuterium lamp of the type used in spectrophotometers and (b)

its spectrum. The plot is of irradiance Eλ (proportional to radiant power) versus

wavelength. Note that the maximum intensity occurs at ~225 m.Typically,

instruments switch from deuterium to tungsten at ~350 nm.

Deuterium lampUV region

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(a) A tungsten lamp of the type used in spectroscopy and its spectrum

(b). Intensity of the

tungsten source is usually quite low at wavelengths shorter than about 350 nm. Note that the intensity reaches a maximum in the near-IR region of the spectrum.

Visible and near-IR region

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The tungsten lamp is by far the most common source in the visible and near IR region with a continuum output wavelength in the range from 350-2500 nm. The lamp is formed from a tungsten filament heated to about 3000 oC housed in a glass envelope. The output of the lamp approaches a black body radiation where it is observed that the energy of a tungsten lamp varies as the fourth power of the operating voltage.

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Tungsten halogen lamps are currently more popular than just tungsten lamps since they have longer lifetime. Tungsten halogen lamps contain small quantities of iodine in a quartz envelope. The quartz envelope is necessary due to the higher temperature of the tungsten halogen lamps (3500 oC). The longer lifetime of tungsten halogen lamps stems from the fact that sublimed tungsten forms volatile WI2 which redeposits on the filament thus increasing its lifetime. The output of tungsten halogen lamps are more efficient and extend well into the UV.

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Tungsten lamps (350-2500 nm)

Why add IWhy add I22 in the lamps? in the lamps?

W + IW + I22 → WI → WI22

Low limit: 350 nm

1)Low intensity2)Glass envelope

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3. Xenon Arc Lamps Passage of current through an atmosphere of

high pressured xenon excites xenon and produces a continuum in the range from 200-1000 nm with maximum output at about 500 nm. Although the output of the xenon arc lamp covers the whole UV and visible regions, it is seldom used as a conventional source in the UV-Vis. The radiant power of the lamp is very high as to preclude the use of the lamp in UV-Vis instruments. However, an important application of this source will be discussed in luminescence spectroscopy which will be discussed later.

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Sample Containers

Sample containers are called cells or cuvettes and are made of either glass or quartz depending on the region of the electromagnetic spectrum. The path length of the cell varies between 0.1 and 10 cm but the most common path length is 1.0 cm. Rectangular cells or cylindrical cells are routinely used. In addition, disposable polypropylene cells are used in the visible region. The quality of the absorbance signal is dependent on the quality of the cells used in terms of matching, cleaning as well as freedom from scratches.

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Instrumental Components

• Source

- selector (monochromators)

• Sample holders

• Cuvettes (b = 1 cm typically)

1. Glass (Vis)

2. Fused silica (UV+Vis)

• Detectors

– Photodiodes

– PMTs

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Instrumental designs for UV-visible photometers or spectrophotometers. In (a), a single-beam instrument

is shown. Radiation from the filter or monochromator passes through either the reference cell or the sample cell before striking the photodetector.

Types of Instruments

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1. Single beam

–Place cuvette with blank (i.e., solvent) in instrument and take a reading 100% T

–Replace cuvette with sample and take reading % T for analyte (from which absorbance is calc’d)

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Most common spectrophotometer: Spectronic 20.

1. On/Off switch and zero transmission adjustment knob

2. Wavelength selector/Readout

3. Sample chamber

4. Blank adjustment knob

5. Absorbance/Transmittance scale

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End view of the exit slit of the Spectronic 20 spectrophotometer pictured earlier

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• Single-Beam Instruments for the Ultraviolet/Visible Region

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• Single-Beam Computerized Spectrophotometers

Inside of a single-beam spectrophotometer connected to a computer.

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2. Double beam (most commercial instruments)

– Light is split and directed towards both reference cell (blank) and sample cell

– Two detectors; electronics measure ratio (i.e., measure/calculate absorbance)

– Advantages:

• Compensates for fluctuations in source intensity and drift in detector

• Better design for continuous recording of spectra

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General Instrument DesignsDouble Beam: In - Space

Needs two detectors

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General Instrument DesignsDouble Beam: In - Time

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Merits of Double Beam Instruments

1.Compensate for all but the most short term fluctuation in radiant output of the source

2.Compensate drift in transducer and amplifier

3.Compensate for wide variations in source intensity with wavelength

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Dual Beam Instruments