5.19 Detection Limit. Low concentrations of Ni-EDTA near the detection limit gave the following counts in a mass spectral measurement: 175, 104, 164, 193, 131, 189, 155, 133, 151, 176. Ten measurements of a blank had a mean of 45 counts. A sample containing 1.00 uM Ni-EDTA gave 1797 counts. Estimate the detection limit for Ni-EDTA. 5.25. Europium is a lanthanide element found in parts per billion levels in natural waters. It can be measured from the intensity of orange light emitted when a solution is illuminated with ultraviolet radiation. Certain organic compounds that bind Eu(III) are required to enhance the emission. The figure shows standard addition experiments in which 10.0 mL of sample and 20.00 mL containing a large excess of organic additive were placed in 50-mL volumetric flasks. Then Eu(III) standards (0, 5.00, 10.00, or 15.00 mL) were added and the flasks were diluted to 50.0 mL with H2O. Standards to tap water contained 0.152 ng/mL (ppb) of Eu(III), but those added to pond water were 100 times more concentrated (15.2 ng/mL). (a) From the x-intercepts in the graph, calculate the concentration of Eu(III)(ng/mL) In pond water and tap water. (pond water交於-14.6 mL; Tap water 交於-6.0 mL) (b) For tap water, the emission peak area increases by 4.61 units when 10.00 mL of 0.152 ng/mL standard are added. This response is 4.61 units/1.52ng = 3.03 units per ng of Eu(III). For pond water, the response is 12.5 units when 10.00 mL of 15.2 ng/mL standard are added, or 0.0822 units per ng. How would you explain these observations? Why was standard addition necessary for this analysis? 5.31. Verifying constant response for an internal standard. When we develop a method using an internal standard, it is important to verify that the response factor is constant over the calibration range. Data are shown below for a chromatographic analysis of naphthalene (C10H8), using deuterated naphthalene (C10D8 in which D is the isotope 2H) as an internal standard. The two compounds emerge from the column at almost identical times and are measured by a mass spectrometer, which distinguishes them by molecular mass. From the definition of response factor in Equation 5-11, we can write Area of analyte signal/Area of Standard signal = F (concentration of analyte/concentration of standard) Prepare a graph of peak area ratio (C10H8/C10D8) versus concentration ratio [C10H8]/[C10D8] and find the slope, which is the response factor. Evaluate F for each of the three samples and find the standard deviation of F to see how “constant” it is. Sample C10H8(ppm) C10D8 (ppm) C10H8 peak area C10D8 peak area 1 1.0 10.0 303 2.992 2 5.0 10.0 3519 6141 3 10.0 10.0 3023 2819 --

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