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Determination of Magnesium by atomic absorption.

Akshay Charegaonkar

Lab Partners:

Kelly Scheuer

Kevin Smith

Group #5

2nd February and 8th February, 2006

Laboratory instructor: Prof. Thomas Allston

Abstract:

A Perkin Elmer AAnalyst 100 atomic absorption spectrometer was used to prepare the calibration curve of Mg in water.(Figure 1). This curve was used to measure the concentration of Mg in urine of students in lab group #5. The concentrations were determined to be Akshay- 188 ± 1 ppm, Kelly 116 ± 1 ppm and Kevin 43 ± 1 ppm

( Table II). The effect of using a volatile organic solvent on the absorbance was studied. Ethanol solvent was found to increase absorbance (Table IX). Using a larger slit width (@ nm) and a smaller slit width (.2 nm) was found to reduce absorbance relative to the 0.7 nm slit width used to prepare the calibration curve (Table X). 1 ppm Mg solutions were analyzed in the presence of 200 ppm of Na and K atoms respectively. These interfering species were seen to increase absorbance by absorbing at the 285.3 nm wavelength used to excite Mg atoms. (Table VII).

Theory:

Atomic absorption spectroscopy is an analytical technique by which the concentration of a particular atom or atomic ion in a solution can be measured. The technique is especially useful for determining the concentration of metal ions in liquids.

The method is based on the principle of absorption of a photon of radiation by an atom or molecule by which it is goes to a thermodynamically unfavorable electronically excited state. The excited species then gives off this energy by several pathways, such as florescence, phosphorescence and vibrational relaxation.

The species to be analyzed is dissolved in a solution which is generally aqueous. Organic solvents also can be used.

The solution is atomized and introduced into a flame. The flame temperature can be changed by changing the fuel flow rate or the fuel itself. The high temperature of the flame atomizes the species being analyzed. A hollow cathode lamp is used to emit specific wavelengths that are characteristic absorption wavelengths for the metal being analyzed. The amount of light given off by this lamp into the sample is carefully measured.

This is achieved by using a hollow cathode lamp that has the cathode made of the same metal as the one being analyzed. These atoms absorb the radiation and are excited into a higher electronic state. The atoms relax into the ground state by several pathways, including fluorescence, phosphorescence and vibrational relaxation. An insignificant number of atoms relax by emitting the same frequency photon, and these can be ignored during analysis.

A photomultiplier tube is used to detect the amount of light passing through the flame. The difference in light is the light absorbed by the atoms being analyzed. Since the amount of light absorbed is directly proportional to the concentration of the metal ions in solution, a calibration curve of the metal can be prepared. This curve can be used to find the unknown concentration.

Magnesium metal is being analyzed in this experiment. Therefore, a hollow cathode lamp containing magnesium will be used. A calibration curve will be prepared by diluting a 1000 ppm stock magnesium solution to produce other solutions of known concentration. The absorbances of these solutions will be measured at wavelength 285.2 nm, emitted by the hollow cathode lamp. A Beer-Lambert’s law plot is constructed as the calibration curve.

Figure 1: Beer’s law [3]

It is seen that absorbance is directly proportional to the concentration of Magnesium in the solutions when all other experimental parameters are held constant. Therefore we expect to see a linear calibration curve. The curve will be made ranging from 0.05 ppm Mg to 1 ppm Mg. The average concentration of urine in a healthy adult is 100 to 300 mg. Therefore the urine samples are diluted with distilled water in a ratio of 1: 100 to ensure that the concentration of the analyzed sample lies within our calibration curve.

It is noted that human urine also contains a relatively high concentration of sodium (Na) and potassium (K) ions.

The energy level diagram of Na shows an absorption line at 285.3 nm while the energy level diagram of potassium could not be found. However, we expect that Na and K will behave alike because they are both alkali earth metals. We expect to see absorption due to these atoms, because they absorb at the wavelength used to study Mg, which is 285.2 nm. Two 1 ppm Mg solutions, containing 200 ppm Na and K respectively will be made and analyzed. We expect a greater absorbance reading than the 1 ppm Mg in water solutions because of the absorbance due to these ions.

The ethanol/ water solvent is expected to increase absorbance because the lower surface tension of ethanol will lead to smaller droplets and higher nebulizer efficiency.

The experiment uses an acetylene-air flame which reaches a temperature range of 2100-2400 Celsius. The flow rate of the fuel gases has to be carefully controlled. If the flame velocity is too low, the flame tends to move back into the burner. A high rate may lead to the flame blowing off the burner itself. A stable flame is essential for accurate spectroscopy. The flame is not homogenous and shows three distinct regions: The inner core, which tends to be the hottest, the interzonal region where metals may be oxidized, and the outer mantle in which the atoms are blown out of the flame. Magnesium forms stable, non-absorbing oxides in the interzonal region. Therefore it is generally studied in the inner core.

Figure 2: General flame zones and the absorbance of various metals as a function of distance from the burner head.[1]

The number of molecules in the atomic stage is directly dependent on the flame temperature. We expect to see an increase in absorbance when the fuel setting is increased. But this effect will only be seen up to a certain point, at which the oxidizer becomes the limiting reagent.

The experiment is performed at a constant slit width of 0.70 nm. The slit serves to limit the bandwidth of the radiation emitted by the source. Increasing the slit width will increase the bandwidth and result in a loss of spectral detail. We also expect to see a lower absorbance value, because more light reaches the sample. Most of this light is not of the wavelength absorbed by the metal, but is still factored into the absorbance calculation. Therefore we also expect to see a decrease in absorbance values for larger slit widths and an increase for narrower slits.

Instrument:

Figure 3: Schematic diagram of the processes involved in the functioning of an atomic absorption spectrometer.[1]

The aspiration process involves an acetylene-air mixture pumped at a high velocity over the capillary tube dipped into the solution being analyzed. The mixture passes to the nebulization chamber where the aqueous solvent forms an aerosol in air. The liquid droplets are then evaporated into vapor which passes to the flame. Atomization of the molecular species takes place in the flame.

Figure 4: The schematic diagram of a hollow cathode lamp showing the various components.

The hollow cathode lamp uses Mg as the cathode material so that we have Mg emission wavelengths passing into the sample. The emitted wavelengths passed through the flame containing the species to be analyzed, in this case Mg. The Mg atoms in the flame absorb part of this radiation and the rest passes into the theromocouple detector. The absorbance value is proportional to the concentration of Mg atoms in the solution.

Preparation of solutions:

A 100 ppm stock solution of Mg was further diluted with distilled water to make the solutions for the calibration curve.

Table III: Method of preparation of standards used to make the calibration curve of Mg Flask Mg concentration 9ppm) Volume of 100 ppm stock Mg solution (mL) Volume of distilled water (ml) 1 0.05 0.025 49.975 2 0.075 0.0375 49.9625 3 0.1 0.05 49.95 4 0.15 0.075 49.925 5 0.2 0.1 49.9 6 0.25 0.125 49.875 7 0.1 0.25 49.75

Solutions 1,2 and 3 were found to be contaminated and were discarded. The prepared 10 ppm Mg solution was diluted further to mare samples 1a, 2a and 3a .

Table IV: Method of preparation of solutions 1a, 2a and 3a. The solutions were prepared again. Flask Mg concentration (ppm) Volume of prepared 10 ppm Mg solution Volume of distilled water (mL) 1a 0.05 0.25 49.75 2a 0.075 0.375 49.625 3a 0.1 0.5 49.5

The urine samples were initially diluted in a ratio of 1: 100. However, the concentration of Mg in the urine samples of Akshay and Kelly were larger than our calibration curve. Therefore the samples were diluted further to achieve a 1:200 dilution factor. The urine sample of Kevin was not diluted further.

Table V: Method of preparation of urine samples to be analyzed for the concentration of Mg Student Volume of urine (mL) Volume of distilled water (mL) Dilution factor Akshay 1 199 1:200 Kelly 1 199 1:200 Kevin 1 99 1:100

Data:

Table VI: The data collected for preparing the calibration curve of Mg for the determination of the concentration of Mg in urine.

Absorbance vs. concentration of Mg Standard Concentration of Mg ( ppm) Absorbance (A) 1 0.05 0.02 2 0.075 0.029 3 0.1 0.037 4 0.15 0.052 5 0.2 0.068 6 0.25 0.089 7 0.5 0.168 8 1 0.308

Figure 5:

Calibration curve of Mg in water made for the determination of concentration of Mg in urine samples plotted using Microsoft Excel.

Table II: The absorbance values for the urine samples for lab group #5 Student Absorbance of urine sample Akshay 0.295 Kelly 0.185 Kevin 0.143

Calculations:

Equation of calibration curve is

y = 0.3045x + 0.0079

I: Akshay

Y = 0.295

Solving calibration equation for x, we obtain

X = 0.9428 ppm

Since the urine sample was diluted by a actor of 1: 200, we multiply the above result by 200 to obtain the value for concentration of magnesium

Akshay = 188.57 ppm Mg in urine

Similarly,

Kelly = 116.32 ppm Mg

Kevin = 44.38 ppm Mg

Table VII: Effect of using 100 ppm Na and K solutions, used to check for interference by those atoms in the analysis of urine. Solution Interfering species Concentration of Mg (ppm) Absorbance (A) 1 None 1 0.308 2 200 ppm Na 1 0.362 3 200 ppm K 1 0.369

Table X: The effect of varying slit widths on the absorbance of a 1 ppm Mg solution Solution Slit width (nm) Concentration of Mg (ppm) Absorbance (A) 1 0.7 1 0.308 2 0.2 1 0.235 3 2 1 0.108

Table IX: Effect of using water as a solvent vs using volatile organic solvent Ethanol Solution Solvent Concentrration of Mg (ppm) Absorbance 1 Distilled water 1 0.308 2 1:1 Ethanol + water 1 0.447

Table XI: Effect of varying the input of fuel (ethylene) Solution Concentration of Mg (ppm) Fuel scale reading Absorbance (A) 1 1 1.5 0.177 2 1 2 0.238 3 1 2.5 0.257

Discussion:

The Perkin Elmer AAnalyst 100 spectrometer used is a double beam instrument. In this instrument the beam is split by a mirror and the reference beam is not sent through the flame. The sample beam passes through the sample and is recombined with the reference beam and the two beams are together sent to a grating monochromator. The hollow cathode lamps tend to drift in intensity over time. Since we have a reference beam to subtract from, the drift problem is resolved.

The calibration curve obtained by making standards of Mg was linear. This shows us that the technique used here follows Beer’s law very closely. The urine samples of the lab group were found to vary widely in the concentration of Mg, which is possibly due to a difference in diet, amount of water consumed and the difference in kidney function.

The presence of Na and K atoms is seen to increase the absorption. This is due to the fact that Na has an emission line at 285.2 nm [5]. The energy level diagram of K could not be found but it is assumed that K behaves similarly to Na because they are both alkali earth metals. The increase in absorbance due to these atoms was seen to be 16 %. This value is obtained for 200 ppm solutions of these metals. The urine samples contain significantly less concentration and were also diluted in ratios of at least 1 : 100. This would lead to a very low concentration of K and Na in the analyzed urine samples. Therefore we do not correct for the absorbance due to these atoms.(Table VII)

The 1 ppm Mg is made using 1:1 Ethanol/water as a solvent. The surface tension of pure ethanol is 22 mN/m while the surface tension of pure water is 72 mN/m [4]. A lower surface tension will lead to a higher nebulizer efficiency, smaller drops of the solvent and consequently more magnesium atoms will reach the flame [2] . Ethanol is also more volatile than water and will evaporate rapidly. This will also lead to a higher absorption peak [2] for the 1 ppm Mg solution in ethanol/water as compared to a 1 ppm Mg in pure water solution. (Table IX)

The slit width changes produced the expected results. As the slit width gets bigger, a larger bandwidth of light passes through the sample, but is not absorbed. Therefore we see a fall in absorbance readings. An extremely small slit width limits the amount of light going through the sample and therefore reduces absorbance that way (Table X).

Varying the amount of fuel going into the flame produces the expected results as well. We see that the absorbance rises linearly as the fuel is increased (Table XI). This is due to the fact that with more fuel the flame burns hotter and more uniformly. [1]. Therefore more and more Mg atoms are atomized and we see an increase in absorbance.

However we note that the oxidant settings are not touched, and therefore at one point increasing fuel flow rate will not increase absorbance any further. We note that changing the fuel settings changes the absorbance significantly, and therefore changes in flame intensity should be reduced as much as possible.

Refrences

1 : http://www.cis.rit.edu/class/scha311/

2:http://www.mgwater.com/convert.sh

3: pediatrics.aappublications.org/cgi/content/full/112/1/e70

4: http://ptcl.chem.ox.ac.uk/~rkt/lectures/liqsolns/liquid_surfaces.html