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Page 1
Atomic Absorption
Greg Zhang
April 3, 2015
Using Atomic Absorption Spectroscopy to Determine the Amount of Manganese in a Common
Vitamin Tablet
Abstract:
The objective of this experiment is to conduct Atomic Absorption spectroscopy using a line
source spectrometer to analyze the mass of Manganese in a vitamin tablet. We indirectly
determine the amount of Manganese in the tablet by interpolating the concentration of
Manganese in a solution in which the tablet was dissolved in from a linear regression curve of 50
mL dilutions of 0, 1, 5, 10, 20 mL Manganese standard solution (100 ug/mL), for which the
concentration of Manganese is 0, 2, 10, 20, and 40 ppm respectively. The average absorbance of
the dissolved tablet solution was determined to be 0.154, and based on calculations from the
linear regression standard curve the vitamin tablet contains 2.34 milligrams of Manganese.
Introduction:
This particular experiment is important because Atomic Absorption spectroscopy is an
important technique in analytical chemistry that allows for the identification and quantification
of inorganic metal ion analytes dissolved in liquid solutions as opposed to quantifying organic
compounds done in UV-Vis absorbance spectrophotometry. AA spectroscopy also uses a liquid
sample but instead vaporizes the sample to the gas state, because AA is based upon the
absorption of light by free atoms in the gas state, temporarily elevating the atoms to a higher
energy level. It is highly selective because the AA absorption lines are very narrow (on the scale
of a few picometers).
The light source of AA spectroscopy consists of a hollow cathode lamp. The hollow
cathode lamp is composed of a glass tube containing an anode, a hollow cathode, and a filler gas,
usually Neon or Argon, at a pressure of 1-5 torr. The cathode should contain the same kind of
metallic atom as the atom of interest in the sample (AA can only detect one atom at a time),
which in this experiment is Manganese, as light emitted by the particular atoms in the lamp will

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Atomic Absorption
Greg Zhang
April 3, 2015
be then absorbed by the same type of atoms from the sample solution aspirated into the
instrument. The hollow cathode lamp illuminates free atoms in the gas state originally from the
sample and elevates them temporarily to an excited state. The sample solution is vaporized into
free atoms with an air-acetylene flame atomizer at a temperature of about 2300 degrees Celsius.
The sample solution first undergoes nebulization into a spray mist, undergoes desolvation to
become a dry aerosol, and finally volatization into free atoms in the gaseous state. Afterwards,
the light passes through a monochromator (typically Czerny-Turner) which serves the purpose of
selectively transmitting light of a certain wavelength. It is then transmitted to a photomultiplier
tube, a detector is used to measure the exact amount of reduction in light intensity. The
photomultiplier tube is connected to an amplifier and readout that displays the absorbance of the
sample solution.
Using the Beer-Lambert law, we can then determine the concentration of a certain analyte
dissolved in the sample solution using the Beer-Lambert law:

A=lc= log (I / I0) = log (T)
10

10

Atomic absorption ideally relies on the Beer-Lambert law; however, realistically applying
Beer-Lambert’s law in atomic absorption spectroscopy is problematic due to variations in the
atomization from the sample matrix and the non-uniformity of concentration as well as path
length of analyte atoms. The Beer Lambert law is valid only at low concentrations (C<0.05 M);
higher concentrations result in a nonlinear graph of absorbance vs concentration. In the equation,
A represents the measured absorbance and is unitless,  represents the molar absorptivity (a
constant) with units of L mol-1 cm-1, l represents the path length of the sample and is usually 1
centimeter, and finally, c represents the concentration of the compound in solution with units M
or (moles/Liters solution). A variation of the Beer-Lambert’s law, which applies better for AA,

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Atomic Absorption
Greg Zhang
April 3, 2015
allows absorbance to be computed by taking the common logarithm base 10 of the transmittance
T which can further be represented by dividing the intensity of light passing through a sample
with the intensity of before passing through the sample. Beer's law also states (applies for AA)
that for monochromatic radiation, the linewidth of radiation being measured is less than the
bandwidth of the absorbing species.
We indirectly determine the amount of Manganese in the tablet by interpolating the
concentration of Manganese in a solution in which the tablet was dissolved in from a linear
regression curve of 50 mL dilutions of 0, 1, 5, 10, 20 mL Manganese standard solution (100
ug/mL) in which absorbance is plotted vs. Manganese concentration. This is done by measuring
the absorbance of the unknown tablet solution and interpolating it from the linear regression
trendline equation.
Methods:
Materials: Varian SpectrAA-5 Flame Atomic Absorption Spectrophotometer, 12M concentrated
HCl, Manganese standard solution (1000 ug/mL), mortar and pestle, 15 mL glass scintillation
vials, 250mL graduated cylinder, Whatman No. 1 filter paper, 50 mL volumetric flask, 100 mL
volumetric flask, 250 mL Erlenmeyer flask, 10 mL pipette, deionized water and spray bottle,
vitamin tablet, NaOH pellets, 0.2 um syringe filter
The procedure was mostly copied from the lab module posted on eLearning by Dr.
Yanping Qin. All the exact weights of the solutions and the calculations for the prepared
manganese standard solutions and the unknown are documented in the lab notebook for this
experiment as well as the raw data obtained from the readout software.
We begin by preparing 250 mL of 8M HCl solution used for dissolving the tablet. To
make this solution, we dilute concentrated 12 M HCl by adding it to deionized water in a

graduated cylinder. The

M 1 V 1=M 2 V 2

equation shows that we add 167mL of 12M of

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Atomic Absorption
Greg Zhang
April 3, 2015
concentrated HCl to 83 mL of water in order to obtain 250 mL of 8M HCl. In order to make the
Manganese stock solution, we pipet 5 mL of the manganese standard (1000ug/mL or 1mg/mL)

into a 50 mL flask and dilute to the mark (1:10 dilution). The

M 1 V 1=M 2 V 2

equation shows

that the concentration of this solution is 100ug/mL or 0.1 mg/mL. All these calculations are
shown on the separate calculations page
Pipet 0, 1, 5, 10, and 20 mL of this Mn stock (100 ug/mL) into 50 mL volumetric flasks.
Add 25 mL 8 M HCl to each and dilute with deionized water to the mark. Five 50 mL standard
solutions were made: 0 mL Mn standard solution+25 mL 8M HCl+25 mL deionized water; 1 mL
Mn standard solution+25 mL 8M HCl+24 mL deionized water; 5 mL Mn standard solution+25
mL 8M HCl+20 mL deionized water; 10 mL Mn standard solution+25 mL 8M HCl+15 mL
deionized water; and 20 mL Mn standard solution+25 mL 8M HCl+5 mL deionized water.

Again, by using the

M 1 V 1=M 2 V 2

equation, we can figure out the concentration of

manganese in these standard solutions. These standard solutions will be used to generate a
standard calibration curve using the linear regression function in Microsoft Excel. These
equations will be shown on the separate calculations page.
To prepare the unknown vitamin tablet solution, we weigh a vitamin tablet (1.603g) and
place it in a 250 mL Erlenmeyer flask, dissolving it in 25 mL of 8 M HCl (measured with a
graduated cylinder) prepared earlier in the procedure. We bring this solution to a boil on a hot
plate for about 5 min, an afterwards let it cool to room temperature and add 10 mL of deionized
water. We then set up the filter apparatus folding a sheet of Whatman No. 1 filter paper into a
cone, placing it in a glass funnel, and conducting vacuum filtration by pouring the vitamin tablet
solution through the filter apparatus. To increase the quality of filtration, we poured another 20

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Atomic Absorption
Greg Zhang
April 3, 2015
mL of water through the filter apparatus. Finally, we transfer the filtrate to a 100 mL volumetric
flask and dilute to the mark with deionized water. This is a dilute unknown vitamin tablet
solution. All the calculations in making this solution are also shown on the separate calculations
page.
Before aspiration of the samples, filter through a 0.2 um syringe filter if the color of the solution
is not clear (this was not done in our procedure). We transferred the solutions to 15 mL glass
scintillation vials to conduct AA spectroscopy. When testing absorbance using AA, we aspirate
each of the calibration standards and unknown sample with the blank solution (no manganese)
before recording the absorbance for each trial (5 individual trials for each standard and unknown
tablet solution). Deionized water is used to clean the capillary tube.
Results:
Refer to the data tables, graph, and relevant notebook pages. The standard curve generated in the
experiment plotted the absorbance on the y axis vs. the concentration (ppm) of Manganese on the
x axis. The linear regression equation for this standard curve is: y = 0.0067x - 0.0022. Based on
the linear regression equation, we show that the dilute vitamin tablet solution has a concentration
of 23.37 ppm Manganese by plugging in the unknown tablet solution’s averaged absorbance
reading (0.1544). In addition, the calculations showed that there were 2.34 milligrams of
Manganese in the vitamin tablet. You should always work in the linear area of the calibration
curve as nonlinear areas such as testing the absorbance of high concentrations of analyte will
result in inaccurate data based on the linear regression line. The 95% confidence level for
standard error is 2.26 to 2.41 milligrams of Manganese in the vitamin tablet.
Conclusion:
Based on the calculation from the standard curve (on a separate calculations page), we conclude
that the dissolved tablet solution had an average absorbance reading of 0.1544 and based on this,
the vitamin tablet contains 2.337 milligrams of Manganese. This value was noticeably greater

Page 6
Atomic Absorption
Greg Zhang
April 3, 2015
than the “true value” of 2 milligrams of Manganese stated on the experimental module. As a
result, we calculate that the percent error in this experiment is 16.85%. The 95% confidence level
for standard error is 2.26 to 2.41 milligrams of Manganese in the vitamin tablet.
Discussion:
AA spectroscopy is supposed to convey a high amount of accuracy in quantifying concentrations
of inorganic metal ion analytes, but the experimental values for the vitamin tablet showed that
Atomic Absorption Spectroscopy had a rather high error margin calculating the amount of
Manganese in the vitamin tablet. The experimental data showed that the vitamin tablet contained
2.337 mg Manganese (or 1.458 mg Manganese/1 gram tablet), but the module stated that the
tablet contained 2 mg Manganese. Once again, the percent error obtained in this experiment is
16.85%. Also, the 95% confidence level for standard error is 2.26 to 2.41 milligrams of
Manganese in the vitamin tablet. The R2 value/coefficient of determination of the standard curve
was 0.9977, a statistical measure of how well the regression line approximates the real data, for
which the standard curve was very close to 1 (an R2 value of 1 represents that the linear
regression line perfectly fits the experimental data values. This shows that the stock solution
dilutions were done to somewhat good accuracy. There are multiple possible sources of error
that may account for the percent error in this experiment. The instrument readout fluctuated quite
rapidly and never actually stabilized and we resorted to writing down the absorbance shown on
the readout about 10 seconds after pressing “READ” . Doing more trials may serve the purpose
of reducing the standard deviation, as the relative standard deviation was indeed quite high for
the standard solutions consisting of lower amount of Manganese stock.
The percent error may also be due to imperfect experimental technique. In addition, the
experiment did not include a specific section that asked to verify whether the concentration of
the Manganese stock solution is truly 1000ug/mL as the label suggests. Any deviation from this

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Atomic Absorption
Greg Zhang
April 3, 2015
value, even inaccuracy from non-homogeneity of the solution, will result in inaccurate
concentrations of Manganese in the various standard solutions. It is also unknown whether the
vitamin tablet truly contains 2 mg of Manganese as the label suggests. Creating a homogeneous
mixture of solids is a very difficult task, and deviation from the given amount of Manganese is
highly possible unless each tablet was made individually by combining exact amounts of each
ingredient. In addition, we aspirated the standard and tablet solutions by placing the plastic
tubing at the bottom of each solution. The solutions may not have been thoroughly mixed (only
shaken briefly by hand) and the concentration of Manganese ions near the bottom of the
container may be higher than near the top due to the effect of gravity.

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