2. I’ve been using cheap plastic test tubes, bought in bulk from Fisher Scientific, and when I sample I use two swabs, each one going over the entire specimen in all the likely spots. Break off the swab ends, put them both directly into the tube. No intermediate ziplock bag to label and throw away.
3. Take your “known negative” sample right there with the same distilled water so you can have lots of confidence in your negative.
PREPPING THE TEST
1. Cut up Parafilm to make lids for the test tubes. The Parafilm I’ve got has squares printed on it and cutting them in quarters is perfect.
2. Put on gloves. The mercury bromide test papers are not healthy.
3. Cut up test papers in 1/8 pieces…four strips lengthwise and then each in half. Then snip a slit in the end and fold to make a little propeller-like thingy that will prevent the strip from falling into the tube. I call these “helicopters” and I make a bunch ahead of time and keep them in a beaker with a watchglass for a lid.
4. Use one empty test tube as a model to make all the lids you need using the squares of Parafilm you’ve cut, slit with scalpel, insert a “helicopter” into each one.
5. Set your tubes in a rack in the fume hood, set the little lids in front of each
6. Make a reporting sheet for your results
7. Tape down plain paper strips to the rack to keep it clean. Remove the topmost sample swab from each beaker using a straight-beaked tweezers and set it on the paper right in front of the tube it came from. Make sure other swab is fuzz-down in the tube. In this way, you’ve got a backup swab but you’re being super efficient about making not making more mess and labels.
8. Run your known negative and a known positive EVERY time to give confidence in results. I’ve been saving the spare swab from strongly positive specimens to use as my known positive in future tests.
9. Have all reagents ready to go. The reaction makes a gas as soon as all the components come together, and that is what the test paper is sensitive to. You’ve got to have all your ducks in a row to get that lid on quickly. The acid dropper that came with the acid in the Merckoquant test kit is annoying. Remove it with straight beaked tweezers, slowly using one beak to lift gently around all sides. I use plastic pipettes for the acid and base and little glass beakers to hold them when not in use. Acids and bases scare me, so I have a fanatical little routine of diluting and rinsing and throwing away the pipettes after a few rounds of testing.
10. Put on goggles
11. Put 2 drops KOH in each tube Leave tubes in rack while doing this. We got our 1 molar solution made up by the local pharmacist. NOTE: this version of the test DOES NOT WORK if you don’t use the KOH. This is a major difference between the Hawks method and Odegaard et al.
12. Add 7 drops HCl to each tube. Keep careful track when reloading the dropper if you run out partway through counting your drops. Leave tubes in rack while doing this.
13. Hold the rack with one hand and gently rattle tubes in the holes with the other. This just makes me feel secure that all the liquids are getting on the swab OK.
14. Lift out each tube and add scoop (approx 0.5g) of zinc dust. I find the coordination works better if you are trying to bring two hands together than if you’re trying to hit the target with one hand and the tube is in the rack. It also makes the next step easier. Since I had an old Merckoquant test kit, I’ve been using that nice scoop, but if I didn’t have it I would scheme a way to have the right quantity of zinc ready ahead of time. Adding the zinc makes the reaction happen.
12. Quickly but without panic, grab each lid by one corner and poke the paper into the tube, squeezing down the edges of the Parafilm to seal the lid in place, set back into rack.
13. Contents should be bubbling vigorously. If they are not, something is wrong. Bubbling has nothing to do with a positive or negative result, it just means the test is running properly.
14. References suggest results are reliable after 30 minutes. I usually wait overnight, just for convenience.
REPORTING THE RESULTS
15. Dipping the paper in tap water and immediately holding it up to plain white paper helps a lot to see faint yellow positives.
16. The test is not known for false positives. If your negative control did not turn and your positive did, you ought to be OK.
17. The degree of color change is not indicative of the amount of arsenic on the sample. Your sampling technique or the application method of arsenic over various areas are examples of varying factors that make this test qualitative and not quantitative.
18. The “helicopters” of test paper are hazardous waste. The rest of the materials are not, particularly if you only throw them away 10 at a time and you keep your extra positives for future testing. With the Odegaard et al method, you’ve potentially got a bunch of contaminated water to deal with.
TROUBLESHOOTING AND NOTES
I’ve gotten positive results on really small specimens, including a hummingbird that was only about an inch square.
Things that test positive for arsenic can also have insect infestations. A bird mount that tested positive had a previous infestation in the feet. Tons of larval casings and frass. Cathy Hawks tells me that feet were generally not treated on traditional bird mounts and have a bit of meat left intact which can be attractive to insects.
I am still investigating, but it seems that a number of specimens in our collection that were freeze dried are coming up positive for arsenic. Cathy Hawks told me that the craze for freeze drying really got going after arsenic was going out of favor, but I guess that doesn’t mean that the two are mutually exclusive.
Once, I accidentally ran my two swabs at once, using only the normal amount of reagent. The result still came out positive.
UPDATE January 18, 2011. Cathy Hawks just sent me a link to an arsenic test kit on the website for Gallade Chemical: