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Next, I tested the capacity of the GFAAS to detect arsenic at low concentration to ensure accuracy. First, I created a calibration curve ranging from 0 to 10 ppb with a standard concentration of 10 ppb at an interval of 2 ppb with automated dilution by the GFAAS using normal sampling procedure. The resulting data had a large standard deviation among the triplicate values, which made this method statistically not viable for sampling at low concentration with a detection limit at only approximately 8.1 ppb. In order to obtain better accuracy, the mass of arsenic in the analysis has increase. This can be done using two methods. The first test is called characteristic mass. This is a sensitivity test that measure the concentration or weight of arsenic that produces a signal of 0.0044A (1%A) for continuous or peak height measurements. In this test, I measured the characteristic mass to be 24.9 pg/0.0044A-s, where is ideal characteristic mass is 15 pg/0.0044A-s, which is still in an acceptable rangemethod is called multiple injection method. The furnace sequence can be programmed to inject the sample into the furnace chamber multiple times and burn off excess liquid in between each injection. I programmed the furnace to inject the sample 5 times into the furnace in order to increase the mass of arsenic by 5 times. The resulting calibration curve had a polynomial trend line with R-square value of 0.85. The second method is called large injection method. As the name implies, the GFAAS was programmed to inject a larger volume of sample into the furnace chamber. Normally, 20 microliters are injected, but in this test, 50 microliters were used instead. The resulting calibration curve had a strong linear trend with R-square value of 0.998, which showed that the trend line matched that data almost perfectly. In conclusion, large injection method should be used in future studies to evaluate arsenic contaminated samples at low concentration.
Fall 2013 Contributions
This semester the Arsenic Team was in its preparation phase. Since the GFAAS (Graphite Furnace Atomic Adsorption Spectrometer) is currently non-functional, the team concentrated the efforts in preparing the lab for upcoming experiments as well as creating standard testing procedures. I have supported the team by location and searching for necessary chemicals for the upcoming experiments. This semester, we require groundwater solution, which will be used as the basis for our sample solution. We also need diluted arsenic solution, nitric acid and permanganate. I also determined the sample tubes as main vessel for our experiment. This test tube has to fit in both the centrifuge and the carousel tray in the GFAAS. Professor Richardson gave us a variety of centrifugal tubes to test. These tubes come in two sizes both with sloped bottoms: 1,000 uL and 500 uL. Unfortunately, both sizes do not find the carousel slot. The large one is simply too wide, and the small one is too tall. The GFAAS also has its own 500 uL tube, which fits the carousel perfectly. However, it does not fit any centrifuge in available. Therefore, I came up with a hybrid solution, which is to place the small centrifugal tube inside the GFAAS tube when placing in the carousel. However, I will have to ensure that this odd configuration of tubes will not impede the machine in any ways. I also acquired a tube with filter basket, which we will use to emulate filtration.
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