## Question

Recall that it is mandatory that your pre-lab assignment for the specific milestone be approved by your TA before you begin performing any experimental work.

1) The prep-room provides you a 10% HNO3 (by volume) calibrated standard solution with a Hg2+ concentration of 1000.0 ppm (by mass). Construct a detailed plan for how you will produce the series of four 50.0 mL mercuric standards as specified in the experimental section for this milestone. Note: you have access to 50.0 and 100.0 mL volumetric flasks, 0.5, 1.0, 2.0, 5.0, 10.0 mL volumetric transfer pipettes, and concentrated HNO3. Recognize, it is not essential that your standard be exactly equal to, for example, 1.0 ppm; it must only be near the desired value but, more importantly, a value that you know exactly. For ppm calculations for this question you can assume the density of solutions to be exactly 1.0 g/mL. Do not neglect the use of HNO3 in your standards preparation – the matrix must be the same for all of your solutions or your calibration curve would be meaningless due to matrix effects. For this experiment you will have access to both 20 – 100 uL and 100 – 1000 uL micropipettes in addition to the volumetric transfer pipettes. Note: if you use the micropipettes, you should verify their accuracy and precision using the density of water to verify that they are acceptable for the level of accuracy you desire.

2) Assume you have determined the lower concentration limit of detection (LOD) for mercury via your AAS protocol to be 5.0 ppm; i.e. the method cannot statistically distinguish solutions with mercury concentrations of 5.0 ppm or lower from that of a “blank” solution containing 0 ppm of mercury. In your protocol development you also determine that the minimum volume of sample solution that you can analyze via AAS is 20.0 mL. Assuming the dried fish sample contains 1000 ng of mercury per gram of dried tissue, what is the minimum mass of dried fish tissue that you would have to employ in order to prepare a solution with a mercury concentration above the LOD? For this question you can assume the density of all solutions to be 1.0 g/mL and all ppm concentrations are by mass – thus 1 ug / mL = 1 ppm.

3) While in the previous question you assumed a 5.0 ppm LOD, in this question you will utilize sample data provided to calculate the sensitivity, resolution, and LOD for the method; providing an opportunity to mirror the experimental and analysis process that you will employ to complete the first milestone. For this question you will need to download and analyze two .csv data files posted on the D2L site titled “Hg_AAS_Standards.csv” and “Hg_AAS_Blanks.csv”.

The “Hg_AAS_Standards.csv” file contains two columns of data. The first column is the concentration (in parts-per-million by mass) of prepared mercury standards. The second column is the measured absorbance from the AA spectrophotometer for each standard. Construct a calibration curve for this data and find the best-fit linear model for the response.

The “Hg_AAS_Standards.csv” file contains a single column of data; specifically 25 absorbance measurements, obtained from the AA spectrophotometer, for a mercury-free “blank” solution. You should use this set of data to determine the sample-to-sample measurement variation which is necessary for calculating the lower concentration LOD and resolution.

Utilize these two sets of data to calculate and report i) the method sensitivity – i.e. the slope of the calibration curve (include units!), ii) the lower concentration limit of detection (LOD) for this method – see your quantitative chemical analysis textbook for assistance, and iii) the resolution of the method. It is typically important to know the minimum difference in sample concentration that can be resolved with a certain method – i.e. the resolution of the method. For calculating the resolution, utilize:

Resolution ~ (sample standard deviation of signal from “blank”) / (method sensitivity or slope ) (Eq. 1)

4) Based on goals of this first milestone (consider the Overview, Pre-laboratory assignment, and Technical Report sections), construct a detailed list specifying the data that you must acquire in order to complete this first milestone – determine the sensitivity, resolution, and LOD. Make sure to use this list in the laboratory.

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