Introduction
Water plays an essential role for the human body to function. On average, humans are recommended to consume 8 cups of water daily. But are we drinking water that is safe for consumption?
In less developed countries without proper water treatment, the water that people are drinking might be unsafe for consumption. One of the most common effect of drinking polluted water will be arsenic poisoning due the high level of arsenic found.
Therefore, it is important to ensure that the water that we are drinking is safe, thus, we have decided to look into methods to be able to detect contamination of arsenic in water.
What exactly is arsenic & arsenic poisoning?
Arsenic (As):
Chemical element with 33 atomic number
Natural element of earth crust.
Found in air, land & water
Arsenic poisoning:
Due to elevated levels of arsenic in body
Causes several effect such as confusion, leukonychia striata, convulsion, coma and even death!
Chemical element with 33 atomic number
Natural element of earth crust.
Found in air, land & water
Arsenic poisoning:
Due to elevated levels of arsenic in body
Causes several effect such as confusion, leukonychia striata, convulsion, coma and even death!
How did arsenic get into our water supply?
There are a lot of ways arsenic can enter the water supplies. The most common source would be due to industrial pollution or natural deposits in earth. Large amount of arsenic would be released during volcanic activity and arsenic in soil would be dissolved into ground water.
Methods available
1. Graphite furnace atomic absorption spectrometry (GFAAS)
Atoms of interest will absorb the light from the lamp as it passes through the cuvette, which will then be measured by the detector.
Advantages: Very good detection limits, small sample size required, moderate price, very compact instrument and few spectral interference.
Disadvantages: Slower analysis time, no screening ability, 1-6 elements per determination and limited dynamic range.
2. Inductively coupled plasma atomic emission spectrometry (ICP-AES)
Uses inductively coupled plasma source to dissociate the sample into it's constituent atoms or ions, exciting them until they emit light of characteristic wavelength.
Then, the detector will measure the intensity of the emitted light.
Advantages: Easy to use, high productivity, excellent screening abilities and few chemical interference.
Disadvantages: Moderate to low detection limits and spectral interference possible.
3. Inductively coupled plasma mass spectrometry (ICP-MS)
Similar to ICP-AES as it uses plasma source to dissociate the sample into it's constituent atoms or ions. However, this method detect the ions/atoms itself. Ions are extracted from the plasma and passed into the mass spectrophotometer.
The quadrupole analyzer separate the ions based on their atomic mass-to-charge ratio.
Advantages: Excellent detection limits, high productivity, wide dynamic range and easily interpreted spectra.
Disadvantages: Method developmental skill required, high initial capital cost and possible spectral interference.
Summary of methods available
Selected method
Graphite furnace atomic absorption spectrometry
After taking into consideration of each method, we have decided to use GFAAS to determine the arsenic level in tap water. GFAAS is relatively simple, precise and cheap method.
As compared to ICP-AES, it has relatively higher detection limit and compared to ICP-MS, it is relatively cheaper although both have high detection limit.
Materials
2. Nitric acid
3. Water sample
4. Test tube
5. Polypropylene centrifuge tube
6. GFAAS
Instrument Parameters
Procedure
Standard solution
Prepare standard solution with concentration of 1,5,10 & 50 ug/L
(Dilution)
Sample preservation
1 . Filter water samples with 0.45 µm pore diameter membrane filter immediately after collecting the samples.
(Use a portion of the filtered sample to rinse the filter flask, discard this portion and collect the required volume of filtrate.)
2. Acidify the filtrate with (1+1) nitric acid immediately following filtration to pH <2.
Aqueous sample preparation
1. Pipette an aliquot of 20ml of the filtered, acid
preserved sample into a 50 mL polypropylene centrifuge tube.
preserved sample into a 50 mL polypropylene centrifuge tube.
2. Add 0.4 mL (1+1) nitric acid to 20 mL aliquot of sample.
3. Cap the tube and mix.
4. The sample is now ready for analysis. Allowance for sample dilution should be made in the calculations.Risk Assessment
Results & Discussion
1. Results from standard solution will be used to plot calibration graph.
Standard solution of concentration 1,5,10 & 50 ug/L
The higher the concentration of solution, the higher the absorbance.
2. Using the calibration graph from the standard solution, determination of the sample solution concentration is possible.
Nine drinking-water samples from different areas were acquired to analyzed
Safety level: 0.01mg/L or 10ug/L
Tap water 5: 0.0234mg/L
8/9 is safe for consumption
Among the 9 tap water sample used, only tap water 5 contain arsenic concentrations of more than 10ug/ml.
Hence indicating that the water have exceed beyond the safe concentration of arsenic
Figure 1. show the calibration curve of known concentration in ppb. technically, any peaks that exceed the 10ppb curve is considered unsafe.Hence indicating that the water have exceed beyond the safe concentration of arsenic
References
J, C., T, M. and J, O. Determination of trace elements by stabilized temperature graphite furnace atomic absorption. Available at: http://water.epa.gov/scitech/methods/cwa/bioindicators/upload/2007_07_10_methods_method_200_9.pdf
