Question

8- trace elements in the teeth of archeological speciment provides anthropologists with clus about the dite and diseases of ancient people . Porter et al (20020 measured the trace elements strontium from extracted teeth by atomic absorption spectroscopy. A series of solutions were preaperd, each containing 0.750 mg of dissolved tooth enamel. Varying volumes of 100.0 ng/ml Sr stock solution were added to each flask ,and each solution was diluted to a total constant volume of 10.0 ml The final result was a series of flasks containing the following concentration of added Sr.

flask Volume of Sr stock solut. added ,ml concent. of Sr after diluttion ng/ml Blank-concent signal

1 ? 0.00 28.0

2 ? 2.50 34.3

3 ? 5.00 42.8

4 ? 7.50 51.5

5 ? 10.0 58.6

a. suppose 5.00 mg of powdered tooth enamel is dissolved in ultra- pure HNO3 and diluted to 10.0 ml in volumetric glasswear. What volume of this solution must be delivered to each flask , in order to deliver 0.750 mg dissolved enamel to the flask? (show calculations)

b. The Sr standards in the table above were made by delivering volume aliquote of a 100.0 ng/ml Sr stock solution to each 10.0 ml flask . complete the table above to show what volume of Sr stock were added to each flask ,in order to produce the specific concentrations of added Sr?(please show calculations)

c. What type of standardization is being performed here? why is this type of standardization being used , rather that some alternative standardization?

d. Least -squares analysis of the data resulted in the plot shown , where y=3.136x+27.36 and R^2 =0.9973

calculate the concentration of Sr in flask.( show the plot from -10 -5 0 5 10)

e. calculate the Sr content in the original tooth enamel , in units of ppm=micro gram Sr/g enamel. ( please show complete calculation)

  

Answer 1

a) 5.00 mg in 10 ml implies 0.5 mg /ml of the solution so 1.5 ml will have 0.750 mg

b)stock solution has 100ng/ml of Sr. It is being diluted to 10 ml so if you take 1 ml of the stock solution and dilute to 10 ml you will get 10 ng/ml as the 10 ml will contain a total of 100 ng Sr. here you can us the formula

n1v1 = n2 v2   where n1 is the concentration desired from the table and volume is 10 and n2 is 100ng/ml so for example 2.5 x 10 = 100 x v2 or v2 = 25/100 = 0.25 ml

So in the table

entry2       0.25    2.5

entry3       0.5      5.0

entry4       0.75    7.5

entry5       1         10.0

c) This type of calculation is called internal standard calculation of spiking standard experiment. In this the actual concentration is calculated as a differnce from the standard added. It is a better way of calculating trace metals in samples as it helps to identify the peak in the in the absorption experiments and thus background effects are nullified better. Alsothis helps to carry out least squares analysis of the data and so the accuracy of the final data is higher.

d) is not clear as the plot discussed is not shown

e) The table shows the signal obtained from sample containing 0.750 mg enamel and the concentration of Sr added so for example signal where no Sr has been added is 28 and with 2.5 ng/ml of Sr signal is 34.3 so the signal from 2.5ng/ml Sr is 6.3 and doing this calculation for all the other entries in the table and taking an average of the value obtained is 9.7 so 0.750 mg of the tooth contributes to 9.7ng/ml to the 10 ml flask meaning 0.750 mg of the tooth enamel had 97 ng of Sr. So if 0.75 mg has 97 ng of Sr 1g of the tooth enamel will have 129 micro gram of Sr so in ppm it is 129ppm or 129 micro gram /g.