Question

Please show work and explain

Problems 16&17. Mass spectral measurements were made on 10 replicate low concentration Pb-EDTA samples: 172.3, 132.7, 142.7, 157.7, 126.7, 196.1, 147.7, 152.3, 141.7, and 159.3 counts. The mean and standard deviation of these counts from low concentrations is 153 $15. Ten measurements of a blank had a mean measurement of 100. counts. A sample containing 3.00 UM Pb-EDTA gave a measurement of 770 counts. Estimate the concentration detection limits for Pb-EDTA. 16. What is the concentration limit of detection (uM, two significant figures)? A. 0.20 B. 0.17 C 0.15 D. 0.18 E. 0.25

Answer 1

The limit of detection is usually calculated as 3 times the standard deviation of a blank or low-concentration measurement, divided by the slope of the calibration curve (or by the relationship between signal and concentration).

In this case, the standard deviation for low-concentration measurements is given and it has a value of 15 counts. Three times this value yields 45 counts.

The signal/concentration relationship (since there is no calibration curve) is (we have to subtract the value of the blank from the measurement):

(770 - 100) counts 3M = 223 counts UM

The division of these values results in:

45 counts LOD= 299 counts = 0.20 PM 223 counts