I have some questions about an experiment I will perform, where I will be determining only the Fe content
found in biscuits using FAAS. The paragraph below explains how to prepare a stock solution of 1000 mg/L
#1 Is there a specific way on how to prepare the stock solution? I know I could weigh about 1 gram of Fe
and then dilute it to 1 L using distilled water, but I’m not exactly sure. I might have to use an acid like
HNO3 first but not sure how much and then dilute it to 1 L. They also explain about using aqua regia in the
microwave digestion procedure below with the solid sample, I’m assuming I follow similar steps to prepare
the 1000 mg/L stock solution?
#2 In the table at the bottom, they used various acids to digest the sample at room temperature, where the
aqua regia was the best method to obtain the most Fe content. I think they used those acids to get Fe into its
ionized form but how do I ensure I get it to Fe(III) as described in the article because Fe oxidation states
range from -2 to +6?
#3 My understanding of standard addition is having maybe 3 volumetric flasks, each with the same added
unknown (sample prepared from digestion?), adding in varying amounts of Fe(III) solution, and then
diluting all flaks to the mark using deionized water? Then I can run the three samples using FAAS. I think
that’s how the standard addition curve was prepared but not sure. Also, how did they prepare the 0.5 and 1
microgram/mL Fe(III)? And how much unknown should I add to each 25 mL volumetric flask or does it not
#4 In the table below they also had percent recovery listed. Should I also determine the percent recovery in
my experiment? If so, how is that usually completed?
#5 What is the purpose of blanks and how would I make one for this experiment? They mentioned below
that blanks were used but I don’t know what it would consist of. Would I need to subtract the absorbance
readings of the standard addition curve from the blank curve to get an accurate reading?
#6 I think my hypothesis for this experiment may be different flavors of biscuits will have different metal
content or frozen biscuits will have lower metal content than those refrigerated. I’m leaning towards the
frozen and refrigerated one but wanted your input on this and if I do the frozen one, is there certain proper
collection techniques I need to follow to ensure I don’t contaminate my sample? I plan on buying a certain
brand of biscuit dough in the store that has Fe content, cooking the biscuit in the oven, then placing some in
the freezer and the other in the refrigerator? I’m not sure for how long to leave them in there and just
wanted your input.
I think those are all the questions I have. Let me know if you would like the journal article attached. Thank
Apparatus and reagents
An Analytic Jena Vario 6 atomic absorption spectrometer
equipped with an air–acetylene burner was used for the
determination of iron and zinc. Iron and zinc hollow cathode
lamps were used as the spectral radiation sources. The
wavelengths were 248.3 nm and 213.9 nm for Fe and Zn, respectively.
Instrumental parameters were adjusted according
to the manufacturer’s recommendations.
A CEM MARS 5 (CEM, Matthews, NC, USA) microwave
apparatus equipped with PTFE vessels was used for microwave
The spectrophotometric AOAC official method was carried
out with a Jenway 6105 UV–visible spectrophotometer
at 510 nm for iron determination.
All chemicals were of analytical-reagent grade (Merck).
Distilled-deionized water was used throughout. Stock solutions
(1000 mg/l) of Fe and Zn were prepared from titrisol
standards (Merck) and working standard solutions were prepared
fresh daily by appropriately diluting the stock solutions.
Calibration and calculation
Standard addition was performed after digestion of samples
and blanks. A series of solutions for standard addition
measurements were made to contain 0, 0.5 and 1 microgram/ml
Fe(III) and Zn(II) standards added. After adding an appropriate
amount of analyte, the samples were diluted with deionized
distilled water to 25 ml and iron and zinc were determined
by FAAS. Graphs of the standard addition method
were plotted in terms of absorbances versus iron and zinc
added (microgramg/ml). Iron and zinc content in biscuit samples was
calculated by extrapolating. The results in the paper were
given as the average of at least three independent digestion
procedure. The AOAC spectrophotometric method  was
also performed and the results were compared
Microwave-oven digestion procedure
Accurately weighed 0.5 g samples were placed into vessels
and 5ml of aqua-regia (HCl–HNO3, 3:1, v/v) was added.
The vessels were sealed, placed into the microwave oven
and digestion was performed under an optimized microwave
digestion program (Table 1). After being spiked with standard
iron and zinc solutions, digested samples were diluted
to 25 ml and metal ion concentrations were determined by
FAAS. At least one method blank was carried out using
same microwave-oven digestion procedure. After every digestion,
vessels were cleaned with 10 ml aqua-regia, then
rinsed with distilled-deionized water many times until get
chloride free-rinsed solution.