diff sampler

Download Diff Sampler

Post on 05-Apr-2018




0 download

Embed Size (px)


  • 8/2/2019 Diff Sampler



    Diffusive SamplerDiffusive or passive samplers are known as the cheapest method of monitoring

    air quality and can give a good overall picture of average pollutant levels in an area. The

    low cost per tube permits sampling at a number of points in the area of interest. Although

    results from single point passive samplers are not as precise as those from automatic

    point monitors, the accuracy and reproducibility of the measurements has increased over

    recent years.

    Diffusive samplers are typically clear plastic tubes open, or with a membrane

    screen, at one end and a pollutant-absorbing chemical matrix or gel at the closed end. Thediffusion tube collects the pollutant during the exposure period (one week or more), at the

    end of which the tube is returned to an analytical laboratory. The time resolution of this

    technique is limited, as it can only provide information on integrated average pollutant

    concentration over the exposure period. They have been widely used for many years in

    personal monitoring and occupational health assessments (Dore and McGinlay, 1997).These kind of devices are very appropriate for gas flux emission measurement,

    they do not require another measurement and can be applied over a wide area, because of

    their low cost. One disadvantage of these devices are that they still require furtheranalysis in laboratory.

    Theoretical understanding of gas movement in diffusive samplers have been

    introduced by Palmes, et.al in 1976. For the axial tube type diffusive sampler, the rate of

    mass transfer down the concentration gradient induced in the air gap, can be derived fromthe Ficks first law (Bates, et.al, 1997):


    dm/dt = rate of mass transfer

    d = diffusion coefficient of gas

    dc/dx = concentration gradient along the axisIntegration of the above equation gives:

    where:mt = mass transferred in time t


    dtdm =

    )( 0 at CC





  • 8/2/2019 Diff Sampler



    Besides the axial tube type sampler described above, to increase the uptake rate of

    gases, the radial type sampler has been introduced. Again Ficks first law may be used todescribe the transfer across the still air zone. In cylindrical coordinates the equation


    where r = radiush = length of the cylinder

    dC/dr = concentration gradient along the radius

    The integrated form is follows:

    Several types of diffusive sampler now commercially available. They can be tubeor badge type, pollutant can be chemically bound by appropriate reagent or physically

    adsorbed by inert media. For further analysis the pollutant may be extracted by thermal

    desorption or solvent extraction. Various types of diffusive sampler can be seen in

    appendix 2.

    Adsorbent For Gases

    Silica gelSilica gel is one of the synthetic amorphous silicas, It is rigid and forms a

    continuous network of spherical particle of colloidal silica. It is synthesised by following


    Na2SiO3 + 2HCl + nH2O 2NaCl + SiO2.nH2O + H2O

    Its properties e.g. surface area, pore volume and strength can be varied by

    varying the silica concentration, temperature, pH and activation temperature. Silica gel

    (along with activated alumina) is a desirable sorbent for drying because of its high

    f d i f ti At l f th t l l
















  • 8/2/2019 Diff Sampler



    - aldehyde

    Activated carbonThe surface properties of activated carbon is non-polar or only slightly polar as a

    result of the surface oxide groups and inorganic impurities. This properties give the

    following advantages:

    - can be used to perform separation and purification processes without priorstringent moisture removal.

    - it adsorb more non-polar and weakly polar organic molecules than othersorbents do

    - the heat of adsorption is generally lower than other sorbents so that strippingof the adsorbed molecules is easier.

    It has a large surface area 300-2500 m2/g which is the largest among all sorbents.

    For application in liquid phases, it should have pore size near or larger than 30 A,whereas for gas phases the pore diameter in the range from 10 A to 25 A. According to

    IUPAC, the pores are subdivided by diameter (d)

    - macropores (d>500 A)- mesopores (20 A

  • 8/2/2019 Diff Sampler



    Activated alumina is a dehydrated (or partially) alumina hydrates, both crystalline

    and amorphous with high surface area. It has the greatest affinity for water, and is appliedin drying of gases and liquids because of its hydrophilic property and large surface area.

    It has been applied as a sorbent for drying gases: Ar, He, H2, alkanes (C1-C3),hydrocarbon, Cl2, HCl, SO2, and NH3. It also may be used for the removal of various

    contaminants from gases, such as trace fluorides, cloride, H2S, alcohol and ethers.

    Molecular sieve carbonMolecular sieve carbon (MSC) is less hydrophilic than zeolites, so can be used

    more efficient in separation process involving wet-gas streams. It has been used for the

    production of nitrogen from air.

    In the preparation and sieving properties study, three approaches were taken:a. Carbonization of polymers such as poly vinylidene chloride (PVDC); saran

    (90/10 mixture of vinylidene chloride and vinyl chloride); and cellulose, sugar

    and coconut shell.

    b. Slightly carbonizing coals, especially anthracites

    c. Coating of the pore mouth of the commercial activated carbon with acarbonised or coked thermosetting polymer.

    Direct Reading Colorimetry Detector TubeColorimetry detector tube or direct reading diffusive sampler is a simple device

    that capable to analyse the airborne contaminant quantitatively based on coloration of

    indicating layer. When the molecule of airborne contaminant reach the reagent layer they

    react chemically with the filling material causing a colorimetric stain. The residual stainlength is a measure of the dosage of the contaminant to be measured, i.e. the product of

    concentration and time.

    Molecules of air contaminants migrate into a diffusive sampler according to the

    laws of gaseous diffusion. When the sampler is exposed in air a concentration gradient isdeveloped between the ambient air and the air inside the sampler.This phenomenon can

    be described by Ficks first law of diffusion as described previously.

    Because of a gas molecule react with the impregnated chemicals, there are

    virtually no free gas molecule in the air inside the sampler. The full concentrationdifference is thus available as a driving force for diffusive sampling. However the

    diffusion pathway for the gas molecule becomes longer as the stain gets longer. It ismean, according to Ficks law, that the mass flow decreases with increasing stain length.

    The increase of the stain length (dl) can be formulated by


  • 8/2/2019 Diff Sampler







    The validity of above relationship can be demonstrated by the calibration curve of the

    direct-reading diffusive sampler using scale figures for the dosage of the contaminant to

    be measured (May, 1989).

    Due to the air movement only depending on diffusion of gases, then a passivedetector tube can be imagined as long term sampler. The problem associated with this

    kind of device is the flow rate of gas can be too low for the reagent system used becauseside reactions can occur on long contact of the gas with the indicating layer.

    For satisfactory function, the reagent must fullfil the following requirements

    ((Leichnitz, 1976):1. the indication color must be a measure of the absolute amount (mass) of gas

    (product of concentration and air sample volume must be constant).

    2. the absorption capacity of the reagent must not be affected by humidity changesand must remain constant during the measuring process.

    3. The rate of reaction of the gas with the reagent system must be much higher thanthe rate of any side reaction with may take place on the surface of the reagentcarrier In many detector tubes, the calibration curve can be represented by the

    following equation:


    l = length of the discolorationm = mass of gas which has reacted in the tube,

    A = constant with the dimension of reciprocal time

    C = sorption capacity of the indicating preparation

    v = flow by volume of air sample in the tube.

    Indicating Layer Material for Detector TubeThe supporting material for the indicating layer in the tube may be silica gel,

    alumina, ground glass, pumice, or resin (Jungreis, 1985). The indicating layer or thereagent system comprises of one or more chemical compound which can produce a

    colour change when react with target contaminant.

    )1( /1vA






  • 8/2/2019 Diff Sampler



    Ozone (O3)

    The indicating layer contains indigo compound which changes colour from blue tocolourless due to oxidation by ozone.

    O3 + Indigo (blue) Isatine (colourless)

    Chlorine and nitrogen dioxide also discharge the color of indigo when present in

    concentration higher than 5 ppm.

    Sulphur dioxide (SO2)The indicating reaction for measurement of sulphur dioxide in the various detector tu