Passive samplers IAQ2 Flashcards
What are the basic principles of passive samplers?
- Pollutant sampled onto medium - - at rate controlled by molecular diffusion
- without pump - After exposure (days to weeks), sampler closed and returned to laboratory
- Absorbed pollutant determined
What are the two methods used to determine the absorbed pollutant?
- Spectrophotometry
- ion chromatography
What is the operating principle of the passive sampler?
- NO2 diffuses up tube and absorbed on TEA (Triethanolamine) coated mesh
- Identified reaction product - triethanolamine N-oxide
What is the reaction scheme of the operating principle of the passive sampler?
2NO2 + N(CH2CH2OH)3 + 2OH- ->2NO2- + -O-N+(CH2CH2OH)3 +H2O
- Reaction will not occur in extremely dry air
- OH- occurs from dissociation of TEA in water
In what formations does the NO2 participate in?
- Troposphere O2
- Peroxyacyl nitrate (PAN)
- Nitrate aerosols
What are the advantages of passive samplers?
- Simple
- Lightweight
- Cheap devices
- Operate with no power source
How is the preparation done for NO2 monitoring?
- 2 stainless steel wire meshes placed into coloured cap
- 50 μl of 20% w/v TEA injected on meshes
- 0.5 ml wetting agent added to 20 ml of TEA (aq) - acts as absorbent of NO2
What is the chemical formula of Triethanolamine (TEA)?
N(CH2CH2OH)3
How are the samplers positioned?
3 samplers positioned at 1.5 - 4m from surface
What is the purpose of the three samplers in NO2 monitoring?
- 2 samplers exposed, without colorless cap
- 1 sampler col.cap not removed, used used as field black
What is the exposure time of the passive samplers?
4 weeks
Using spectrophotometry what are the observations made on NO2 using passive samplers?
- Nitrite in solution, NO2 extracted as nitrite ion from TEA
- Reacts with sulphanilamide, forms diazonium compound
- On coupling with NEDA forms purple azo dye
Reaction done in spectrophotometry for NO2 monitoring?
2NO2 + N(CH2CH2OH)3 + 2OH- -> 2NO2- + -O-N + (CH2CH2OH)3 + H2O
What is NEDA?
N-1-naphthylethyldiamine dihydrochloride
How long does it take for full color development and at what wavelength is it spectrophotometrically measured?
15 minutes required before optical absorbance is measured at 543 nm
How is the Nitrogen Monoxide determined?
Does not react with Saltzman reagent.
- Oxidizes using acidified potassium permanganate
How does the analysis of NO2 occur using spectrophotometry?
- colorless cap removed
- Saltzman reagent added containing:
a) 2% w/v solution sulphanilamide in b) 5% w/v orthophosphoric acid
c) 0.14 % NEDA solution
How is the concentration of ambient NO2 calculated?
By the amount of extracted nitrite for samplers.
What method is used to analyze NO2 in short term exposure?
Using chemiluminescence analyzer
1. Luminol in alkaline solution reacts with gaseous NO2
2. Produces intense chemiluminescence with maximum emission at 425 nm
3. for sensitive NO2 analyzers
4. detection limit: <30ppt
What equipment is used in ion chromatography?
- Dionex chromatograph
- anion separator
- conductivity detector
What is used in ion chromatography analysis?
- anion exchange column
- solution of sodium bicarbonate
- sodium carbonate (Dionex eluent solution)
How to calculate the NO2 concentration?
NO2 = 1.67 x 10^3 W/t
W: nitrite (μg) subtracted from black value
t: collection time (hrs)
How is the NO determined using Analyst samplers?
- NO oxidised to NO2
- Absorbed as nitrite by ion chromatography
- NOx conc. calculated
- NO subtracted from NOx and NO2 determined by passive samplers
NOx = NO + NO2
What is the equation to calculate NOx?
NOx = 2.89 x 10^3 W/t (μg/m3)
What are the two properties of NO?
- Thermodynamically favored species at HIGH temperature
- Reason why NOx in combustion exhausts
What are the two properties of NO2?
- Thermodynamically favored at LOW temperatures
- The dominant species in ambient air
How do you calculate the total uptake of a sampler ΔM (g)?
Depends on conc. of pollutant Co (g/cm3), and exposure time Δt (s).
ΔM = S x Co x Δt
S: sampling rate (cm3/s) - depends on geometry
What is the passive sampling equation?
S = D A / L
S: sampling rate
D: diffusion coefficient
A: area
L: length
What is the equation for the volume of a passive sampler and the concentration?
V = S x t = DA/L x t
C = m/V = m/ (DA/L x t)
