Research Notes from 5.9.12 conversation with Mathew Gonzales Graduate student Pennsylvania State University Geosciences department
On 5/9/12 I chatted with Mathew Gonzales a graduate student in Pennsylvania State’s Geoscience department about his use of the photographic paper screen for hydrogen sulfide described in Horwell 2004. I was interested to find out, why he used the tool, his protocol for making the photographic strips, his experimental design for using the testing strips, how he is analyzing the results and his overall impressions of the photographic paper assay in his experience.
Why he used the photographic paper strips: As a geoscience research project, Gonzales is investigating the whether the morphology (shape) and growth rate of gypsum--a soft sulfate mineral--is influenced by the ambient concentrations of hydrogen sulfide. He used the photographic paper assay to map the different concentrations of Hydrogen Sulfide found in a cave and intends to correlate them to the morphology gypsum in those sites.
He used the strips specifically because he wanted to be able to map the relative concentrations of H2S affordably. He estimates the costs of each strips was less than a dollar. To make the photographic paper strips semi-quantifiable he used Draeger Tubes along with testing strips. He used diffusion Draeger tubes. Draeger tubes contain materials that react to change color depending on their exposure to particular gases. The rate and extent of color change measures the quantity of the gas. The tubes are activated by breaking the end of the glass tube and allowing gas to enter the tube for a set period of time. Dreager tubes can also come with pumps that draw gas into the tube rather than enabling it to diffuse.
The Draeger tubes are more expensive than the testing strips: $109 for 10 tubes. They are also less sensitive than the photographic paper strips to H2S. There reaction range is 10-300 ppm x h.
Gonzales Protocol for making the H2S strips:
Gonzales did not have access to a dark room, so he made the test strips in a darkened room.
He approximated that he made about 80 strips by cutting up 10 sheets of photographic paper.
He cut the strips before following the preparation procedure described in Horwell.
He like us also found that the test strips don’t completely dry but remain “tacky” after treatment.
Once the strips were “dry” he placed the strips in a U shape inside black film canisters of the kind used for rolls of photofilm and put the caps on.
He marked each canister with a number.
During drying the strips we mildly light exposed but they were all exposed to the same degree so he went ahead with using the strips.
Before trying the test in the field Gonzales place strips around his home to document responsiveness of the test strips to different conditions. He noticed when strips are light exposed as Horwell reported they turn a distinctive grey color that is different from the shades of brown produced by H2S exposure.
Gonzales Experimental Design:
Experimental conditions: Gonzales could not access the cave reliably everyday. The cave is dark so did not present complication of light exposure so he could just uncap the canisters. Gonzales did not actively record humidity and temperature during the experiment as the cave conditions are relatively constant: Gonzales estimates there was nearly 100% humidity in the cave as it regularly drips water inside; the temperature in the cave is relatively stable between 13-15 C. Gonzales note that the ambient H2S concentrations are easily recognizable because of a constant smell. He and his team adapted to smell as they worked so it became less noticeable.
Pre-testing: Before embarking on a full scale experiment he tested a strip to check it worked in these condition. He placed a canister in the most concentrated area of Hydrogen Sulfide in the cave--a gas vent. He removed the canister’s top to open it to the air. He left it there overnight. In the morning he found the test strip had darkened significantly.
Following the pre test Gonzales embarked on a full scale mapping of H2S over 24 hours. He used approximately 40 canisters located in different parts of the cave. He noted locations with the number of canister. Along with the canisters he placed 15-20 draeger tubes around the cave and down into the gas vent in order to look at concentrations of the gas as it rose from the vent. The majority of the draeger tubes where placed vertically down the vent so they could determine the concentrations of H2S rising up the vent. He also placed the draeger tubes around the cave so they were co-located them with canisters. For co-located canisters and draeger tubes he chose a range of predicted exposure areas from areas of high concentration--by the vent--to areas of low concentration.
The goal of co-locating draeger tubes and photostrip canisters was to create a calibration curve by which darkening of the test strips can be correlated to the draeger tube reading and form a distribution curve along which the other photostrips can be placed.
After 24 hours, he reports that he could retrieve useable data from only 30 of canisters as some had fallen from their positions over night. He capped the useable canisters. He could not fix them immediately and so fixed them all 2 weeks later.
Gonzales fixed the strips following Horwell’s protocol. The U shape of the paper presented some problems and the papers need to be weighed to flatten them. He then hung them on a line to dry.
Once dried, he took each strip and glued them to cardboard on which the data, time and location of the strip was recorded.
Gonzales noted differences between the sides of the strips (darker) and the center (lighter), due he feels to the U shape of the strips in the canisters.
Gonzales noted white spots on the strips which he speculates are due to water droplets forming on the strips and preventing H2S from diffusing into the photographic paper.
Following preparing the strips Gonzales scanned in the strips in color at a high resolution-to record “master” before further data analysis. He made sure no auto color corrections occurred on scanned images.
He plans to: Convert the scans to greyscale and in photoshop, assign values between 1-100 on color scale to the various shades of grey to a consistent point across the strips. He intends use the eyedropper tool in area average mode to determine color from the center of the strips.
He will use the draeger tubes paired with photopaper strips to make a distribution curve of H2S concentrations as they correlate to grey scale values between 1 and 250. His guess right now for concentration ranges are: High end 8 ppm low end 1 ppm middle 2-3 ppm
Gonzales believes that values below 1 ppm are hard to resolve with color change in the strips. The draeger tube readings record a maximum concentration of around 8 ppm. These concentrations of H2S are higher than the ppb studied by Horwell (173 ppb to approximately 3000 ppb), to account for that he left the strips out for far shorter periods of time (24 hours versus 7 days or a month). This highlights that the length of time for exposure test should be inversely correlated to expected concentration, i.e. higher expected concentration, shorter testing time.
He believe the test strips to be accurate within the range of +/- 0.5ppm for the strips that are closely correlated to draeger tubes. The tubes themselves have an error rate of 10%. In general, error rates would probably be somewhere near 30 – 40% if trying to correlate color changes to absolute concentrations.
Concerns about False Positives: Gonzales didn’t investigate false positives too much as Hydrogen Sulfide was likely to be the only reactive gas in the cave.
Suggestions for Public Lab research:
For work outside Gonzales suggests placing canisters upside down and to compensate for the limits on gas diffusion by leaving them out longer. It might also be useful to tape something like craft sticks to the sides of the canisters so they can be inserted into soft ground while leaving some space for gas to enter.
He suggests doing a range of experiments around sun exposure with various canister designs.
He thought the idea of an internal control on the strip was neat and could be achieved by taping the strip. Given the gelatinous test strips gas, he thinks the gas would likely diffuse through the strip itself into the controlled area, leaving a blurred edge.