Ammonia Measurement Using UV Absorption

8/10/2019 Ammonia Measurement Using UV Absorption

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Presented By Amal Agharkar

Ammonia Measurement using UVabsorption


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Differential Optical AbsorptionSpectroscopy

Ammonia measurement is challenging due tointerference of the low concentration stray gasesMeasurement of ammonia with sampling instrumentsis not reliable as ammonia adheres to the surface of

the inlet depending upon the concentration in the inletstream and on the inlet wall.Open path spectroscopy gives better alternative tomany sampling instruments as it eliminates the

necessity of inlets. Absorption of electromagnetic waves causeexcitation in the gas molecules.Differential absorption can be measured over an open

path to calculate the gas concentration using Beer-Lamberts law.


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Modified Beer- Lamberts law (cont.) In addition to the other atmospheric species thereare other atmospheric phenomenon.We need to take into account Rayleighsscattering and Mie scattering.Scattering acts as a continuum absorption fromthe perspective of the instrument and henceextinction coefficients are to be applied to theequation.


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Instrumentation of DOAS systemThis method and instrument was designed in WashingtonState University hence the method is also known as WSUmethod.They designed a state of the art measurement system for

open path spectroscopy.150 W Xe high pressure arc lamp was used owing to itsadvantages over deuterium lamp in terms of stable, lowmodulation UV output.The spectrograph was a homebuilt double crossedCzerny-Turner with a focal length of 37.5 cm on each side.It was designed to minimize scattered light and aberrationsin the focal planes of the parabolic mirror.


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The DOAS Instrument


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Working of the instrumentThe lamp is positioned at the side of the optical path of theprimary mirror and a folding mirror is used to direct thelight beam onto the primary mirror optical axis.The lamp only illuminates an annulus of about 6 cm on the

outer edge of the mirror due to another folding mirror usedto image the light onto the spectrograph entrance slitblocking most of the light from the lamp.The outer annulus of light is directed out over themeasurement path to four 12.7 cm retroreflectors.

The light from the retroreflectors illuminates the innerannulus of the primary mirror by translating the light fromthe outer annulus.The light is then intercepted by the folding flat and directed

into the spectrograph.


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Schematic


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Instrument SpecificationsThe spectrograph has a spectral coverage of about 42 nmThe spectral resolution is 1 nmThe detector is a homebuilt Reticon silicon photodiodearray with 1024 pixels with each pixel 25 m wide and 2.5

mm high.The detector has an analog to digital convertor whichproduces a data number (DN) for 1000 detected electronsThe leakage noise of the detector is 0.004 DN s1 pixel1

The largest variation in instrument precision is caused byillumination changes on the optics caused by thermals inthe atmosphere


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DOAS method

AdvantagesIn-situ measurementWavelength selectivitySimultaneous measurement of variable species

Self-calibration, reading depends only on the absolute crosssection of the gas

DisadvantagesSpatially averaged measurement. Measurement cant betaken at a pointNot all species can be measuredNot as sensitive as other methods.


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OMA 300

Works on the Beer- Lamberts principle It is an integrated monitor which is able tomeasure about 100 compounds.It measures intensity of the UV radiation enteringthe sample and exiting the sample.


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OMA 300 Schematic


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Components of OMA 300

Human Machine Interface: The HMI controlling the

spectrophotometer andcommunication provides a

simple, touch-screen visualinterface. HMI offers the user several

display choices (e.g. standardnumeric display, trendgraph, bar

graph). From this interface, the user can

quickly adjust settings like howfrequently the Auto Zero isperformed, the unit ofconcentration for each


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Flow Cell: The sample (gas or liquid) from

the process stream continuouslycycles through the flow cell via

1/4 Swagelok tube fittings. The standard flow cell is ratedup to 3,000 psi / 150 C andmade from stainless steel 316Lfor corrosion-proof durability.

The path length of the flow cellis specified by companyengineers to optimize themeasurement for the expectedconcentration ranges of theanalytes .


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Fiber Optic Cables: Fiber optic cables are used for

transmission of UV light throughthe flow cell.


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OMA 300 Advantages

Continuously measures ammonia concentrationwith wide dynamic range Totally solid state build with no moving parts modern design for low maintenance Additional software benches available for moreanalytes

DisadvantagesChances of settling of ammonia on the inlet walls ofthe instrument affecting the readings.

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Ammonia Measurement Using UV Absorption