There are numerous methods that measure trace levels of H2S. Those that are most common include gold film analyzers, SO2 conversion, colorimetric gas detection tubes, electrochemical detectors, and lead acetate cassette tape gas detectors. This article will aim to help air quality and safety and environmental professionals alike make an informed decision in choosing a H2S analyzer, by discussing the pros and cons of each.
There are a number of things to take into account when selecting a H2S gas analyzer include desired detection level, accuracy of measurements, potential interferences, and how much upkeep is necessary to keep the equipment functioning. To aid in your choice there is Hydrogen Sulfide Analyzer Comparison Chart, which is useful for those wanting to make a quick comparison.
Hydrogen Sulfide Analyzer Comparison Chart
Gold Film Sensor Analyzers Offer Portable and Fixed-Point Solutions that are both Accurate and Precise for Low-Level H2S Analysis
The gold film analysis method has been used to detect H2S since the mid-1980s. Arizona Instrument LLC has developed their own Jerome® brand of H2S analyzer, which works by detecting the change in conductivity of the film from golds sensitivity to hydrogen sulfide. The first step is pumping ambient air over the top of the gold film sensor.
Jerome J605 Portable H2S Analyzer
The hydrogen sulfide in the sample is then absorbed by the film, and the resistance in the sensor increases. This increase is proportional to the mass of H2S present in the sample, and from there it is a simple calculation for the equipment to display the measured concentration in parts per million (ppm) or parts per billion (ppb). The newest model from the Jerome range, the J605, can identify a concentration as small as 3 ppb with a resolution of 20 ppt.
All models in the Jerome range are robust, reliable and simple to operate, with fixed-point and portable varieties available. This makes Jerome the perfect choice for a wide variety of testing applications including regulatory compliance and permitting, odor control monitoring at landfill and wastewater treatment facilities and scrubber efficiency testing. Other industries also use them including semiconductor manufacturing, control room corrosion monitoring, agriculture and livestock production, geothermal emissions monitoring and paper production.
Jerome 651 Fixed-Point H2S Monitoring Solution
An advantage that gold films have over other instruments is that they do not react with hydrocarbons, carbon monoxide (CO), carbon dioxide (CO2), sulfur dioxide (SO2) or water vapor. This enables them to produce consistent and reliable measurements no matter where they are, be it the lab or the field.
Large storage options are available, with some models being able to hold up to 50,000 samples that be exported for further analysis. Jerome models are available to rent or purchase, so they are the perfect choice for any H2S monitoring application, whether long or short term.
Gold film hydrogen sulfide analyzers are inherently non-reactive so interference is rare and can usually be solved through routine maintenance. Some elements and compounds may affect results including Chlorine, ammonia, nitrogen dioxide (NO2) and most mercaptans. In the case of chlorine and ammonia, the level of interference can be reduced by equipping specially designed filters that reduce the concentration of those gases before they reach the sensor.
To keep the analyzer functioning correctly it is recommended that it is calibrated on a least a yearly basis, quarterly if is in more frequent or demanding use. If the chlorine or ammonia filter is being used, they should be regularly replaced, again in accordance to the usage environment.
SO2 Converters are Accurate but only Available as Fixed-Point Solutions
The EPA recommends using SO2 conversion for H2S monitoring, and while it is useful for low level detection (around 0.5 ppb to 10 ppm), the measuring technique is indirect. Firstly, a stream of gas that needs identifying is drawn into the convertor and any SO2 is removed. A catalytic reaction then changes H2S into SO2, which then passes to the next chamber where the molecules are excited through fluorescence via UV light. As this fluorescence is directly proportional to the amount of SO2 in the sample, a photomultiplier tube can detect this and convert it to H2S concentration.
Due to the indirect nature and multiple stages of the process, SO2 convertors are only available as fixed-point models. They are primarily used for ambient fence line monitoring at oil refineries, wastewater treatment plants and landfill facilities, where the large amounts of power they need access to is readily available. The main disadvantage to this system is the lack of portability, but they are also susceptible to interference from various hydrocarbons, nitric oxide (NO) and water vapor.
Colorimetric Gas Detection Tubes are Easy to Use but Results are Subjective
The simple method of operation in a caloremic gas detection tube has seen their widespread use across many industries. A specific volume of the gas that needs analyzing is pumped through the glass detection tube. Then the chemicals in tube react to the gas and change color accordingly. The concentration of the target gas is then given by the depth and length of the color change. This process can be performed quickly, the components are relatively inexpensive, and is easy to operate without specialized training.
Colorimetric Gas Detection Tubes
This simplicity is also its main disadvantage however, as the results are entirely based on color change they can considered subjective and semi-quantitative. Another issue is it is only available as a portable test so cannot be set to automatically record samples.
They also have a limited shelf life, are sensitive to temperature and humidity during storage and use, and are susceptible to numerous chemicals that can impact on results including: various mercaptans, hydrogen chloride (HCl) and other acids and bases, isobutylene, hydrogen peroxyl (HO2) and high concentrations of ammonia. The chemicals responsible for color change may also react with methyl mercaptan, SO2 and nitrogen dioxide (NO2), making it harder to determine the true outcome.
Another potential issue is that several gas detection tubes may be require if you aren’t exactly sure of which gases you’re hoping to find in a sample. This is due to each tube being sensitive to s specific level of the target gas and not every tube will have the same interferences, which makes tube selection a challenge. If you are using several tubes this may increase your costs.
Electrochemical Detectors are Inexpensive but Require Frequent Calibration
Often used for H2S monitoring at landfill and wastewater facilities, electrochemical detectors come in both portable and fixed point forms. They are an easy to use and relatively inexpensive solution for detection of H2S and several other gases at ppm level. They operate by allowing the target gas to pass through a permeable membrane that interacts with an electrode.
Portable Electrochemical Cell Multi Gas Detector
If the target compound is present then an oxidation or reduction reaction will take place within the electrochemical cell, and electrical current produced. This change is proportional to the amount of target chemical and is converted to give the concentration present in the initial sample.
There are some disadvantages to this type system however, as they require continual calibration. They are also affected by heat, humidity, and low oxygen environments, which can all cause drift and degradation of the cell. Interferences include NO2, phosphine (PH3), methyl and ethyl mercaptan, SO2 and other light hydrocarbons.
Lead Acetate Tape Gas Detectors have been Around for Decades but are Susceptible to a Number of Interferences
A longstanding technique that is still the most used method across various industries is lead acetate tape detection. In the presence of the target gas the tape changes color, and specially calibrated optics in the instrument can use small changes in the color change to determine the concentration. The most common usage for lead acetate tape is for monitoring scrubber efficiency and fixed-point H2S detection.
Lead Acetate Detection Strips Circa 1914
There are portable models but it is more commonly found as a static instrument. A main issue is the tapes susceptibility to SO2 and extremes of humidity. If the atmosphere around the tape is too dry the instrument can underreport results, but if it is too moist the tape can absorb some of the moisture and the glass components can fog. This fogging can distort the color received by the optics and potentially affect the final result.
Lead acetate cassette tape gas analyzers have to factor in the cost of cassette replacement on top of their already substantial cost. Depending on how large the rolls are, and the frequency and environment of the sampling, a roll last only around 1-4 weeks. The tapes will also degrade after a certain amount of time and must be stored in a controlled environment to prevent further damage from heat and humidity.
Other Methods of H2S Analysis Exist
There are many other ways of detecting hydrogen sulfide including: Metal oxide semiconductors (MOS), field olfactometers, sulfur chemiluminescence, flame photometric detectors (FPDs) and sulfur titrators. This article doesn’t go into detail about these as the results can subjective, the detection response is slow, specialized training is necessary, complicated operation, lack of portability, or their inability to measure separate sulfur compounds.
Choosing the Correct H2S Analysis Technology is Essential
As this article has hopefully made clear, there are many different methods of detecting hydrogen sulfide, although no single technology may be considered the best for every application. Some of those discussed do offer advantages over others though. The EPA recommends SO2 conversion, but it can only be used as a fixed-point unit. Electrochemical cells are inexpensive but need to be constantly calibrated have numerous interferences.
Colorimetric gas detection tubes are cheap and simple to use, but require an operator at all times with no automation option, with the results being subjective. Lead acetate cassette tape based analyzers have fast response to H2S but can be expensive and need replacement tapes, which can become costly if the detection environment is hostile.
Overall, Jerome gold film sensor H2S analyzers are the only technology commercially available that come in portable and stationary models, and can offer both accurate and repeatable results for unknown, low-level concentrations of H2S. They are sensitive enough to ensure compliance with odor ordinances, and robust and convenient enough for frequent use.
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This information has been sourced, reviewed and adapted from materials provided by Arizona Instrument.
For more information on this source, please visit Arizona Instrument.