Cr in Tablets by Electrothermal AAS A2 Last Revised:
August 2014
1. REAGENTS & EQUIPMENT 1.1 1000 mg/L Cr 1.2 Mineral tablets 1.3 5M Aristar nitric acid (for digestion) 1.4 0.1% Aristar nitric acid (for rinsing) 1.5 1% Aristar nitric acid (for solution preparation) 2. PROCEDURE The exercise takes two sessions, the first for planning the method details and the second for the analysis. You are required to develop a method for the analysis of microgram levels of chromium in the supplied mineral tablets, by standard addition. This method must include the following: 
sample preparation 
standard preparation 
validation 
temperature program 
autosampler volumes 
standard addition parameters The digestion process for the tablets is 15 minutes gentle boiling in 20 mL of 5M Aristar nitric acid. You will need to refer to the tablet bottle to determine expected levels of Cr, and the specifications page for chromium from the instrument manual for the characteristic mass and temperature program (download from webpage). The following questions are intended to make the planning process easier. Method development questions 1. What mass of Cr is supposed to be in each tablet? 2. If you dissolved the Cr content of one tablet in a 100 mL volumetric flask, what would the Cr concentration be? 3. From the instrument manual sheet provided, what absorbance is expected from 10 uL of 7.5 ug/L Cr? 4. 5. 6. 7. A2 What absorbance would be expected from: (a) 5 uL of 7.5 ug/L (b) 5 uL of 15 ug/L (c) 10 uL of sample solution (from Q2) How will you prepare the sample to achieve an absorbance 0.2 from the analysed portion of 10 uL? This includes the initial measuring out of the solid tablet, and dilutions. Standard addition will be used in the analysis, where the instrument will be programmed to take 0, 5, 10 and 15 uL of a standard, added to 10 uL of your sample. What concentration of Cr standard is required, if the 5 uL addition is to add an extra 0.2 Absorbance (10 uL will add 0.4 etc)? Given 1000 mg/L Cr standard, how will you prepare the standard in Q6? p2 8. 10. 11. 12. What mass of Cr would be appropriate as a recovery check? How will you add this to the tablet at the start of the analysis? Hint – use 100 uL of one of the standards from Q7. What changes to the supplied temperature program (Cr in dilute acid) should be made for this analysis? What results will you need to record? You are required to prepare a Results Sheet before Week 2. A2 p3 3. REPORT Calculations The software for the EAAS does standard addition on batches in an unusual way. It only does the full set of additions on the first sample, and then simply measures the absorbance of the other samples. Its calculations are then based on using the slope of the line of best fit graph for the full addition. Mass of analyte in aliquot = Sample absorbance ÷ LOBF slope 
identify any of the replicate values that are clearly wrong 
calculate the average absorbance for each solution 
calculate the mass added in each of the standard additions 
plot a standard addition graph for the first sample 
determine the slope of the line of best fit graph 
for each sample & RC, calculate the mass of Cr in the analysed aliquot 
calculate the concentration of Cr in each original tablet solution 
calculate the mass of Cr in an average tablet 
calculate the % recovery Discussion 
account for any differences between expected and found Cr levels in the tablets 
comment on the recovery check value 
suggest improvements to the method 
explain what would have to be done if you had a batch of samples of different matrices that you wanted to analyse for the same analyte by standard addition on this instrument Questions 1.
What is the toxic form of chromium? How does that differ to the chromium in these tablets? 2.
What dietary role is played by chromium? 3.
What is the recommended daily intake of chromium? 4.
Why is standard addition preferred for ultra‐trace analysis? A2 p4