Measurement of VOCs for Air Quality Using Widely Tunable Mid-Infrared Laser Source

Combined with Cantilever Enhanced Photoacoustic Detection

Jussi Raittila, CTO, Gasera Ltd. Pittcon 2017, 8. March 2017, 10:05 am

Indoor air quality

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• Most people spend approximately 80% to 90% of their time indoors

•  Indoor air quality has a large impact on health, quality of life and work efficiency

• Numerous indoor air impurities are responsible for respiratory diseases , allergies, intoxication and certain types of cancer

•  Contaminants are caused by e.g. moulds, decomposing floor covering, tobacco smoke, outgassing from furniture

Indoor Air Quality

AIR QUALITY POLLUTANTS

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Contaminant Source Carbonmonoxide(CO) Incomplete combus7on infireplaces, ovens andotherhea7ngappliances, and tobacco

smoking Carbondioxide(CO2) Themetabolismofbuildingoccupantsandpets. Nitrogenoxides(NOx) Sideproductof combus7on. Indoor sources: gasfires, cookingandhea7ngappliances,

smoking Indoor-generatedpar7culatemaFeranddust Carpets, tex7les, food, animal and plant proteins in dust, and occupants (especially in

buildingswithahighdensityofoccupants) Vola/leorganiccompounds(VOCs) Allman-madebuildingmaterialsemitVOCs,especiallywhennewordamaged.

Cleaningproducts. Formaldehyde Buildingmaterials,par/cleboards,householdchemicals,ETS,andcarpetsand

otherhouseholdtex/les. Man-mademineralfibres(MMMF) MMMFareusedininsula7onmaterials,andacous7clinings.Fibresareirritants. Mould (fragments, mouldy material, spores, microbialVOCs)

Mouldgrowthdependsonmoisture:wetstructures,waterleakages,condensa7on,highindoorhumidity

Limonene Freshners,Cleaningproducts,Personalcareproducts InorganicIons Cooking,Smoking Metals Cooking,Smoking,Dust Elementalcarbon(EC),OrganicCarbon(OC) Cooking,Smoking,Dust

PAHs(PolycyclicAroma7cHydrocarbons) Buildingmaterials,Fiberboard,Chipboard,Dust,Cooking,Smoking PCBs(PolychlorinatedBiphenyls) Buildingmaterials,Fiberboard,Chipboard PBDEs(PolybrominatedDiphenylEthers) Plas7cizers,flameretardants

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TECHNOLOGY & INVENTIONS

PHOTOACOUSTIC SPECTROSCOPY

•  Photoacoustic effect was discovered in 1880 by Alexander Graham Bell

•  This theoretical potential has not been reached, since conventional microphones have been used for sensing the pressure pulses

•  Gasera’s novel cantilever sensor technology allows the use of the full potential of the photoacoustic phenomena

Photoacoustic spectroscopy is based on the absorption of light leading to the local warming of the absorbing volume element. The subsequent expansion of the volume element generates a pressure wave proportional to the absorbed energy, which can be detected via a pressure detector.

PHOTOACOUSTIC GAS CELL IR SOURCE

MICROPHONE

IR FILTER

CHOPPER

A typical setup of a conventional PAS system

GAS SAMPLE

GASERA’S KEY INVENTIONS •  Cantilever sensor

•  Over 100 times greater physical movement can be achieved compared to conventional microphone membrane

•  Highly linear response

•  Optical readout system •  Contactless optical measurement based on laser

interferometry •  Measures cantilever displacements smaller than picometer

(10-12 m) •  Extremely wide dynamic measurement range

CONCEPT

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Sensi/vity–Patentedcan7leversensorReliability–Photoacous7cprincipleVersa/lity–canbecombinedwithdifferenttypesoflightsources(NIR-TDL,DFB-QCL,EC-QCL,OPO,BroadbandIRandfilters)

POWERFUL LASER SOURCES FOR VOC DETECTION

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•  Two common VOC fingerprint region can be accessed by either an OPO or an EC-QCL

•  Both OPO and EC-QCL have fairly similar optical characteristics, although the operational principle is completely different

•  OPO has slightly better output power whereas EC-QCL has a broader tuning range

•  For a complex VOC matrix, EC-QCL enables more selective detection of multiple gases due to more isolated spectral features

EC-QCL OPO

CASE STUDIES

BTX MEASUREMENT WITH OPO

•  OPO source from Cobolt AB •  Sample concentrations about 10

ppm •  Pulsed OPO (100 mW) + Gasera

PA201 (discrete sampling) •  Detection limits approx. 10 ppb @

1 second for all compounds •  Multivariate DL below 1 ppb

PNNL

Photoacoustic

VOC FROM FLOOR COVERING WITH OPO •  The damage in the floor coverings due to

moisture is a common indoor air problem •  The emissions of the damaged coverings

lead often to several symptoms to the users of the building.

•  2-ethyl-1-hexanol (2-EH) is the marker compound for the damage

•  Present analysis methods are expensive, time-consuming, limited and unreliable

•  Photoacoustic spectrum between 3398-3458 nm was recorded using a pulsed OPO as a source

•  The spectral shape of 2-EH can be clearly identified in the measured floor covering sample

•  Detection limit of the setup for 2-EH is 125 ppt (0.67 µg/m3) for 1 min measurement time

UNKNOWN GAS WITH EC-QCL

•  A case of an impurity in the air of a production plant

•  A clear impurity was recognized in the measured spectrum

•  Impurity was identified as methanol (fingerprint)

•  The methanol concentration was 3 ppm

•  Detection limit was 0.9 ppb (60 s)

ETHANOL WITH EC-QCL

•  Detection of EtOH in the presence of water and two other target gases is both selective and sensitive

•  Detection limit is in the low-ppb level (60 s) for EtOH and two other target gases (VOC and non-VOC)

VOCs WITH EC-QCL

•  Multi-gas analysis for air quality measurements

•  Tuning range: 1000 – 1250 cm-1

•  Resolution: 1 cm-1

•  3 minutes response time •  ppb-level detection limits (1 –

26 ppb with analysis)

FORMALDEHYDE WITH DFB-QCL

Detection limit (1σ) is 3 ppb for 1-minute response time and 1 ppb for 10-minute response time.

CONCLUSIONS

•  Photoacoustic detection combined with widely tunable mid-IR laser sources provides a versatile platform for various air quality applications

•  High-power EC-QCL in the fingerprint regions enables measurement of many VOCs and also other gases that typically are active in the common fingerprint region

•  Easy to operate, miniaturization possibilities and infrequent maintenance requirement provides additional benefit

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