Air Pollution Monitoring Frequently Asked Questions

How long should I sample the diffusion tubes for?
  • Inorganics – these tubes should be exposed for 2-4 weeks. 
  • VOCs – these tubes can be exposed for 1-4 weeks for passive exposure. Alternatively these tubes can be pumped for periods from a few minutes to a few hours.

Ideally, samplers would be placed at breathing height, but in order to reduce loss of tubes due to theft, tubes may be placed at a height of 2-4m. Concentrations of pollutants may decrease with distance from the source so tubes placed too high may underestimate the actual concentrations to which the public are exposed.

Please ensure that your exposure sheet is correctly filled in. Samples should be returned to Gradko International, St Martins House, 77 Wales Street, Winchester, SO23 0RH. United Kingdom.

TEA is the absorbent used in Nitrogen dioxide diffusion tubes. TEA stands for Triethanolamine and may be combined with water or acetone to form the absorbent.

Yes, all 6 acid gases which can be detected by this tube can be analysed at the same time, or you may choose to select only the gases of interest. Acid gas options are Hydrogen fluoride, Hydrogen chloride, Hydrogen bromide, Phosphoric acid and Sulphuric acid.

Although VOC tubes are designed to measure many compounds simultaneously, sometimes several tubes are required. This is mainly due to two reasons:

  • Compounds with large differences in boiling points usually require different sorbents to ensure that the sample is retained by the tube;
  • The properties of some compounds require different analysis procedures so different tubes are required.

Limit of detection is the lowest concentration of the compound that can be identified during the analysis process. If the compound is present in lower concentrations than the limit of detection, it cannot be analysed. Guideline limits of detection can be found on the relevant technical data sheet or specific values can be obtained by contacting us.

Uncertainty is an unavoidable part of any measurement. Measurement uncertainty is a measure of the quality of a result. It demonstrates how well the result represents the true value of the quantity being measured.

During estimation analysis, the compounds detected are compared to a non-specific standard, usually toluene, to calculate their concentrations in the air. It is assumed that the compounds behave in the same way as the standard during the analysis. This is useful for trend analysis and where unknown compounds need to be identified and measured but is not an exact measurement of the compound.

During quantitative analysis the compounds detected are compared to a standard of the same compound to determine their concentrations in the air. This is only available for the analysis of named compounds, not for the analysis of unknown compounds. This requires additional procedures so a charge applies.

No, not all of our tubes can be shipped outside the UK. The following require export licenses:

  • NO2 Acetone (50% TEA/Acetone)
  • Combined NO2 & SO2
  • Acid Gases

Gradko is in no way affiliated with obtaining these licenses. 

Featured Product

Washers - Pack of 10

These washers fit around the diffusion tubes allowing them to sit comfortable inside our circular clips.

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