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ANALYTICAL INSTRUMENTATION ASSIGNMENT

Deuterium Lamps & Infrared Spectroscopy

A deuterium lamp is a low-pressure gas-discharge light source often used in spectroscopy when a

continuous spectrum in the ultraviolet region is needed. Deuterium and hydrogen arc lamps are used as

a source of ultraviolet light (short and long wave) for UV spectrophotometers.

Principle of operation

A deuterium lamp uses a tungsten filament and anode placed on opposite sides of a nickel box structuredesigned to produce the best output spectrum. Unlike an incandescent bulb, the filament is not the source

of light in deuterium lamps. Instead an arc is created from the filament to the anode. Because the filament

must be very hot before it can operate, it is heated for approximately twenty seconds before use.

Because the discharge process produces its own heat, the heater is turned down after discharge begins.

Although firing voltages are 300 to 500 volts, once the arc is created voltages drop to around 100 to 200

volts. The arc created excites the molecular deuterium contained within the bulb to a higher energy state.

The deuterium then emits light as it transitions back to its initial state. This continuous cycle is the origin of

the continuous ultraviolet radiation. Because the lamp operates at high temperatures, normal glass

housings cannot be used for a casing (which would also block UV radiation). Instead, a fused quartz, UVglass, or magnesium fluoride envelope is used depending on the specific function of the lamp. The typical

lifetime of a deuterium lamp is approximately 2000 hours.

Special feature

Deuterium lamps have advantages in UV spectroscopy, where stray light often is a significant problem.

Sources like halogen or arc lamps have blackbody type spectra: relatively low UV and high VIS and IR

output. Most detectors show higher response in the visible. As a result the signal produced by scatteredvisible light may exceed the real signal in the UV. But with D2 lamps, a high signal to noise ratio is

obtained for many UV measurements due to the intense continuum from the vacuum UV to 400 nm, and

the low VIS and IR output.

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water cooled deuterium lamp

deuterium lamp

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SPECTROSCOPY is the study of the interaction between radiation (electromagnetic radiation, or light, as

well as particle radiation) and matter. Spectrometry is the measurement of these interactions and an

instrument which performs such measurements is a spectrometer or spectrograph. A plot of the

interaction is referred to as a spectrum.

What is the basic principle of IR spectroscopy? 

IR radiation does not have enough energy to induce electronic transitions as seen with UV. Absorption of

IR is restricted to compounds with small energy differences in the possible vibrational and rotational

states.

For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net change in the

dipole moment of the molecule. The alternating electrical field of the interacts with fluctuations in the

dipole moment of the molecule. If the frequency of the radiation matches the vibrational frequency of the

molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration.

Schematics of a two-beam absorption spectrometer.

Infrared light sources

Instruments for measuring infrared absorption all require a source of continuous infrared radiation and asensitive infrared transducer, or detector.

Infrared sources consist of an inert solid that is electrically heated to a temperature between 1,500 and2,200 K. The heated material will then emit infra red radiation.

The Nernst glower The Nernst glower is constructed of rare earth oxides in the form of a hollow cylinder. Platinum leads atthe ends of the cylinder permit the passage of electricity. Nernst glowers are fragile. They have a largenegative temperature coefficient of electrical resistance and must be preheated to be conductive.

The globar source A globar is a rod of silicon carbide (5 mm diameter, 50 mm long) which is electrically heated to about

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1,500 K. Water cooling of the electrical contacts is needed to prevent arcing. The spectral output iscomparable with the Nernst glower, except at short wavelengths (less than 5 mm) where it's outputbecomes larger.

The carbon dioxide laser 

A tunable carbon dioxide laser is used as an infrared source for monitoring certain atmospheric pollutantsand for determining absorbing species in aqueous solutions.

Infrared Spectroscopy - Detectors

The detectors can be classified into three categories, thermal detectors, pyroelectric detectors and photoconducting detectors.

Thermal detectors 

Thermal detectors can be used over a wide range of wavelengths and they operate at room temperature.Their main disadvantages are slow response time and lower sensivity relative to other types of detectors.

Thermocouple A thermocouple consists of a pair of junctions of different metals; for example, two pieces of bismuthfused to either end of a piece of antimony. The potential difference (voltage) between the junctionschanges according to the difference in temperature between the junctions. Several thermocouplesconnected in series are called a thermopile.

Bolometer A bolometer functions by changing resistance when heated. It is constructed of strips of metals such asplatinum or nickel or from a semiconductor.

Pyroelectric detectors Pyroelectric detectors consists of a pyroelectric material which is an insulator with special thermal andelectric properties. Triglycine sulphate is the most common material for pyroelectric infrared detectors.Unlike other thermal detectors the pyroelectric effect depends on the rate of change of the detectortemperature rather than on the temperature itself. This allows the pyroelectric detector to operate with amuch faster response time and makes these detectors the choice for Fourier transform spectrometerswhere rapid response is essential.

Photoconducting detectors Photoconducting detectors are the most sensitive detectors. They rely on interactions between photonsand a semiconductor. The detector consists of a thin film of a semiconductor material such as leadsulphide, mercury cadmium telluride or indium antimonide deposited on a non-conducting glass surfaceand sealed into an evacuated envelope to protect the semiconductor from the atmosphere. The leadsulphide detector is used for the near-infrared region of the spectrum. For mid- and far-infrared radiation

the mercury cadmium telluride detector is used. It must be cooled with liquid nitrogen to minimizedisturbances.

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