Portable arsenic test kit for drinking water

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Does anyone have experience with getting false positive results from arsenic test kits?

I tested the water with a portable kit and the results showed 0.025 - 0.05 mg/l of arsenic.
I sent the sample to the lab and their test showed 0 mg/l.

What could be the reason for the difference in the results?  

Which arsenic test kit do you use? 
Can you recommend one which gives the most reliable results?

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7 Answers

  1. Portable test kits do have a limit of accuracy and we are talking about very low results here. what is the range the test kit covers? If the results obtained are less than 5% of the full range of the test kit, you should not take the result as gospel.

  2. I've used colorimetric tests for arsenate (molybdenum-blue method if I remember correctly) and you get the same response from phosphate. So maybe phosphate is causing an interference in your testkit?

    Cheers,

    Jorgen

  3. The most common cause of high results in Arsenic field kits is interference from Hydrogen Sulfide.  Most field kits will have a method for removing this interference.  One method is to pass the gas stream through lead acetate which reacts with the hydrogen sulfide to produce lead sulfide but doesn't react with the arsenic. The lead acetate is commonly used to soak some cotton which is placed in the gas stream.  You need to make sure that all of the gas passes through the cotton ball.   Another common method is to oxidize the hydrogen sulfide to sulfate before starting the test.  Some methods field utilize this approach.  The strong oxidant then needs to be removed usually by the addition of an organic to prevent its interference in the reduction of the arsenic to arsine gas.  It is important to follow instructions carefully.  Many firms make these test kits.  Hach test kit 28228-00 uses the lead acetate method and Hach test kit 28000-00 utilizes the oxidation method

  4. Numbers of commercial kits, are available in markets.

     t he Arsenator  and the Merckoquant,  use a chemical technique: Arsenic in water reacts with a test reagent to produce arsine gas. A color-changing strip reveals the quantity.

     

    https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&cad=rja&uact=8&ved=0ahUKEwiDh7SL5cvRAhUBvY8KHUHhA50QFghHMAU&url=http%3A%2F%2Fcen.acs.org%2Farticles%2F90%2Fweb%2F2012%2F02%2FNew-Portable-Kit-Detects-Arsenic.html&usg=AFQjCNGUan2oFuaZzXOf4BlnBllZr1vTdQ&sig2=l6p-8lGNfSSlONusLSWTMg

  5. There are numerous arsenic testing kits being used in the field , like; 

    a. Merck & Wagtech Field Arsenic Testing Kit

    b. ARSOlux - Arsenic Field Testing Kit

    c. Arsenic Quick™ II, 50 Tests and 481303-5 – Arsenic Quick™ II
    Mini, 5 Tests

    Please also read my paper ( attached )  on " Monitoring and Impact Evaluation System for Arsenic
    Mitigation Interventions - Arsenic Contamination Areas of
    Pakistan" in which I have discussed latest arsenic field testing kits 

    1 Comment

    1. Thank you Islam - i will read through your papaer and if i have more wquestions i will get back to you - i was using your c. option fyi - Cheers Bibi

  6. Hi Bibi, of course insitu measurements are practical for having direct outcomes, but I saw great variations in results from As kits. We normally double check the insitu results in the lab by using stabilization techniques by means of a small resin cylinder and preserve the water sample until analysis in the laboratory. The preservation procedure aims to maintain the distributions of natural species and their concentrations. The As stability problems can be overcome when the separation of As(V) from As(III) is carried out on site using a chloride-form strong base anion exchange resin. The pH of each sample has to be measured prior to As speciation and has to be in the range of < 8. In this pH range, As(III)is completely protonated as H3AsO3, and As(V) is present in ionic forms (i.e., H2AsO4– and HAsO42-). Therefore, the resin retains the As(V) and allows As(III) (i.e., H3AsO3) to pass through the resin column. Best regards, Jaap van Nes

    1 Comment

    1. Hi Jaap - thank you for your useful information - the borehole i tested are in a semi secure location in Northern Nigeria and i am not sur eif my organisation would be able to locate resins you discuss - id there any other alternative. The project site is 2 days flying from Abuja - 1 day to Madiduguri (Capital of Borno State) and due to flight time the next day to Abuja - many thanks for your comments - Best - Bibi

  7. Firstly, I would ask the lab that ran the sample what their LOD and LOQ is for arsenic. They may quote you these values as they apply to the machine used for the measurment, most likely ICP if it is an environmental lab. I would also check, if it is an environmental lab, what the NELAP certification requirements are for the measurement of arsenic in water. These requirements tend to be significantly less precise than the precision of the instrument itself and are more representative of the precision of the result you received from them. 

    Secondly, I wouldn't trust a result from a single run of a field kit for arsenic. Any individual run of a field kit analysis can be influenced by mistakes/interferences and could be an outlier. Repeat the field kit diagnostic multiple times (at least 3) and average the results together. Compare these results with the information you get from the lab to determine if you truly have an arsenic problem.

    1 Comment

    1. Thank you Adam this is very useful  - can you explain briefly the significance of the LoD, Loq and NELP would have on the results- i already repeated the arsenic test on the first sample within 20mins of the first result - it gave the same mg/L as the first one, 2 days later i went to the field and did the test in-situ as well as recorded the temp of the water to make sure it was within the range the manufactures of the portable test kit recommended and it was- and got the same results as the first test, then 10 days later a rd sample was taken and tested the next day, again recording the temp of the water - all 4 test came back with the same results