ALS offers ASTM Method D8230-19 for the characterization of siloxanes in air, vapor and gaseous matrices
FREQUENTLY ASKED QUESTIONS
What are the most common siloxanes found in LFG/biogas?
Trimethylsilanol, octamethylcyclotetrasiloxane (D4), and decamethylcyclopentasiloxane (D5) are commonly reported from landfill gas samples. D4 and D5 are often observed in biogas, while trimethylsilanol is not as common.
Can the samples be collected and submitted in a Tedlar bag instead of sampling with a tube?
Yes, ASTM D8230 permits sample collection with passivated stainless steel canisters, sorbent tubes or gas sampling bags such as Tedlar bags. Note, the standard holding time for a Tedlar bag sample is 72 hours from sample collection to analysis. Reporting limits for samples collected with a Tedlar bag will be higher than those for samples collected on a sorbent tube, due to a smaller sample size.
Will high humidity affect the sample results?
Some of the siloxane compounds are soluble in water (most notably Trimethylsilanol and D3). Therefore, high humidity (>90%) may hinder the overall siloxane recovery. Condensation inside the tubes may also reduce the performance of the sorbent. In instances where condensation is visible, a Tedlar Bag may be a more suitable sampling approach.
What about inclement weather?
In very cold temperatures, recoveries can be lower. To mitigate the impact of the temperature on the samples, the sample train can be shortened, insulated tubing can be used and/or a Tedlar bag can be used.
Are there advantages in one collection method over the other?
ASTM D8230 permits three sampling approaches. Tedlar bags are cost effective, easy to use. However, the holding time is 72 hours, and they often must be shipped as a Dangerous Good due to the flammability of the sample.
Canisters are easy to use, are evacuated so no pump is necessary, yield lower reporting limits than a Tedlar bag typically. Holding time is 30 days. They often must be shipped as a Dangerous Good due to the sample volume and flammability of the sample.
Sorbent tubes are cost effective. Sampling is a little more involved, as you need to know the flow rate and time sampled to calculate the final volume collected. However, the holding time is 14 days, they do not require DG shipping, and they yield often the lowest reporting limits.
What is the sample hold time?
Sorbent tubes are stable for up to 14 days after sampling and may be stored at room temperature prior to analysis. Tedlar bags may be stored for 72 hours prior to analysis.
Which siloxanes will be on my report?
There is no specific target analyte list in ASTM D8230. The list reported varies among laboratories from 5 to 22 siloxane compounds.
ALS reports the following silxoanes:
- Hexamethyldisiloxane (L2)
- Hexamethylcyclotrisiloxane (D3)
- Octamethyltrisiloxane (L3)
- Octamethylcyclotetrasiloxane (D4)
- Decamethyltetrasiloxane (L4)
- Decamethylcyclopentasiloxane (D5)
- Dodecamethylpentasiloxane (L5)
- Dodecamethylcyclohexasiloxane (D6)
What units are used for the results?
Some methods report total siloxanes but do not speciate them, others speciate them in various units: mg/m3, mg Si/m3, ppmV, ppmV as Si or μg Si/m3 CH4.
ALS reports results in:
- as Silicon μg/m3
- Total Silicon