Total Hydrocarbons


Summary

"Total Hydrocarbons" is a requirement for most specifications. This term is defined by the analytical method which can only detect volatiles and some semi-volatiles. It is therefore, a test of Total Volatile Hydrocarbons (TVHC), and does not determine hydrocarbons that are condensable (Oil Mist). Misunderstanding and misuse of the terms is a frequent problem in interpreting reports to clients. 

 Hydrocarbons: What are they?

Hydrocarbons are organic compounds made up primarily of carbon and hydrogen, but they can also contain oxygen, nitrogen and halogens. Their origin is generally petroleum but oil can be made from vegetation as well as synthetic polymers. The term of hydrocarbons is technically defined, but in common terms, hydrocarbons can be gases (such as butane), volatiles (liquids that evaporate such as Freon), Oil Vapor (the part of oil that you can smell) and Oil Mist (small aerosols of oil that cling to surfaces).  

In the breathing air industry, hydrocarbons are usually categorized as volatiles (TVHC) which are any organic compounds that can be trapped in and remain suspended in an empty container for subsequent analysis in a laboratory. Oil Mist is the name for oils that can form an aerosol that will remain suspended in the air until it impinges against a surface.   

Sampling and Analysis of Hydrocarbons

Total Volatile Hydrocarbons (TVHC)
Volatile hydrocarbons are compounds that are either gases (such as butane) or as liquids that can evaporate and act as a gas. The usual bulk sample ("bomb sample") is used to capture a sample. The TVHC test is a non-specific indicator rather than a test for a specific compound. Most specifications allow up to 25 ppmv (calculated as methane). This test provides a clue that the air is contaminated with a solvent. Typical TVHC values range from < 0.3 - 12 ppmv. The test we use is more rigorous than most labs, and we often see values of about 7 ppmv. This is because we can also detect fragments from oil breakdown.   

This is significant because when a diver says he smelled something odd in the air (and maybe became nauseas from it) but the lab reported everything normal, the store usually rejects the objection and ends up losing that diver's business. 

When a sample has more than 15 ppmv, we run GC and GCMS to find out what the hydrocarbon is. Examples of compounds that we have found are part of a large list which includes methane, ethane, cleaning agents, fuels, Freon’s, paint and alcohols. Most of these can make one sick or give headaches when breathed under pressure. The extra test is done at no charge because if we don't know for sure what the contaminant is we can't help the client solve the problem. Most labs cannot do that because of insufficient sample size or lack of instrumentation.

(CASE HISTORY)  This dive shop was new, and experienced sporadic high levels of hydrocarbons. GC/MS analysis revealed a group of hydrocarbons that were identified as gasoline. Inspection by the client revealed a water ski company nearby that refueled jet skis. Now the client only makes air when that operation is inactive, and the SCUBA air is clean.

Non-Volatiles - (Oil Mist + Particulate)
Scientists generally agree that volatile hydrocarbons are those that are up to C-14 in length. In reality, larger molecules can also evaporate if given enough time.
The phrase Total Hydrocarbons causes one think that it is a measure of all possible hydrocarbons in the sample including oils. However, the laboratory methods used can only determine the compounds that reach the detector. Since different labs have different techniques, some labs can only detect only the small volatile molecules, while other labs can detect larger molecules. 

It is not uncommon for us to receive a request for Total Hydrocarbons as methane in a gas system where the client is actually trying to validate that the system has no oily residues. This nearly always results in an in-depth discussion as to how the data will be used. If the client is not aware of the distinction, the ramifications could be very serious. An example would be in oxygen system cleaning.

Semi-volatile and Non-Volatile Hydrocarbons
This category is referred to in specifications as: Condensable Hydrocarbons, Oil Mist, Oil Vapor  or Oil Mist + Particulate.   

There is never any confusion as to what that means and how it should be checked.  

Comments on Sampling for Hydrocarbons

1.  If you are attempting to discover if any volatile organic solvents remain in a gas system after you cleaned it, you will need to capture some of the bulk gas from that system and have the laboratory analyze for volatile organics.

2. If you are attempting to discover oily residue remaining in the gas system after it was cleaned, the most foolproof way is to examine the last rinse solution for non-volatile residue (NVR [ Mil-Std-1330-D discusses appropriate procedures and should be consulted]). If you used a water based cleaner (such as citric acid, tri-sodium phosphate or Navy Oxygen Cleaner) you should obtain a quantity of the final rinse solution and send it to a laboratory for analysis. The laboratory will extract that sample and analyze it for non-volatile hydrocarbons. If the cleaner is a volatile solvent (such as isopropanol or Freon TF), send a sample of the final rinse solution to the laboratory where it will be evaporated to dryness and the residue weighed. That NVR is a bulk test, and - correctly done- does not discriminate between condensable hydrocarbons (i.e., oil and flux) and particulate. Particulate is debris that you can see such as rust, charcoal, desiccant and Teflon tape.    

3. If you are attempting to discover oily residue by pushing a test gas (nitrogen, helium or air) through the piping that was just cleaned, be aware that it may reveal particulate residue, but not necessarily any existing oil. There are some situations (retail dive shops or ships) where the only option is to allow gas to travel through the piping and onto a special membrane filter. If this is done, the test gas must flow at as high a velocity as possible, and onto an analytical depth filter membrane. (Thin small pore filters do not allow high flow rates, and do not capture oil very well; they should not be used.)