Question

2. An analyst is measuring cadmium in drinking water using Atomic Absorption Spectroscopy. The analyst prepares a set of calibration standards between 1 ppb and 50 ppb. The calibration curve is linear between these concentrations and the linear least squares equation is y=0.0127x + 0.0710. The analyst then runs a set of 4 drinking water samples. a) [What kind of standardization was used for this calibration curve: standard addition, internal standard, or external standard?] b) For sample 1, the analyst observes an absorbance of 0.2365. How much cadmium, in ppb, is in this sample? Is this reading valid given the calibration standards that were run? c) For sample 2, the analyst observes an absorbance of 0.0811. How much cadmium, in ppb, is in this sample? Is this reading valid given the calibration standards that were run? d) For sample 3, the analyst observes an absorbance of 0.0922. How much cadmium, in ppb, is in this sample? Is this reading valid given the calibration standards that were run? e) For sample 4, the analyst observes an absorbance of 0.7163. How much cadmium, in ppb, is in this sample? Is this reading valid given the calibration standards that were run? f) [For the samples that were not valid, why not? What could be done to get valid answers from these samples?]

Answer 1

Given Cd concentration were varied from 1 to 50 ppb

Equation for absorbace in terms of cadmium is given by

Absorbance = 0.0127 * Cd + 0.0710

a) Standard addition was the method used for the calibration curve

b)

A	[Cd], ppb
0.2365	13.0315
0.0811	0.795276
0.0922	1.669291
0.7163	50.81102

Given A = 0.2365,

based on the equation above, we finbd the concentration of Cd = (0.2365-0.0710)/0.0127 = 13.0315 ppb

This value of cadmium is well with the limit 1 to 50 ppb of cadmium, so the reading is valid

c) Given A = 0.0811

For this value of absorbance, the concentration of cadmium = 0.795276 ppb.

Since this value of cadmium is outside the range of calibration, this reading is not valid.

d) Given A = 0.0922.

Cd level for this absorbance = 1.6692 ppb

Since the value of Cd is within the calibration range this is valid

e) Given A = 0.7163

Cd level for this absorbance = 50.811

Since this value of Cd is outside the calibration range this is not valid.

f) Doing additional standard samples, say 0.5 ppb and 51 ppb and then using appropriate equation, we can determine the concentration of Cd in these "invalid" samples.