Chemical Interferences in Atomic Absorption Spectrophotometric Measurements

Chem126 Lab – Instrumental Analysis

EXPERIMENT 5Chemical Interferences in Atomic Absorption Spectrophotometric

Measurements

Vanessa Olga J. Dagondon and Ken M. Menez

Department of Chemistry, College of Arts and Sciences,University of the Philippines – Visayas, Miag-ao Iloilo

ABSTRACT

Calcium content in prepared samples containing other species such as K, P, La, EDTA and Al was determined using AAS or Atomic Absorption Spectroscopy. Four schemes were adapted in the experiment: (1) absorbance of Ca standards determined using air/acetylene flame; (2) absorbance of Ca + K standards determined using air/acetylene flame; (3) absorbance of Ca standards determined using N2O/acetylene flame; and (4) absorbance of Ca + K standards determined using N2O/acetylene flame. Addition of K and P contributes to the chemical interference due to the incomplete dissociation of compounds. This can be minimized by using a high temperature flame N2O/acetylene flame instead of air/acetylene and by adding a releasing agent, La. Addition of Al can cause another type of chemical interference using interference due to effects of ionization. This can be minimized by using a low temperature flame, air/ acetylene and by adding a protective agent such as EDTA.

INTRODUCTION

Atomic spectroscopy is a series of different qualitative analyses regarding the concentration of a specific substance in an analyte, with each analysis applied based on the characteristics of the substance.1 As with any atomic spectroscopic method, the identity and concentration of a substance in a sample solution could be determined by exciting the molecules of the solution using a source, such as heat or strong light. This excitation produces neutral atoms in the gas phase, which emit a specific wavelength and intensity of light that gets captured and analyzed by detectors.1 These processes are considered very successful in a wide array of applications in data analysis, with most processes already built into special automated machines.10

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Figure 1. Varian SpectrAA 55B atomic absortion spectrometer. The user interface is located top right, the light source and detector at the lower right, the nebulizer and aspirator (clear tube) at the lower left, and the covered burner at the top left which

includes an exhaust chimney.1

Figure 2. A schematic diagram for the process of atomic absorption spectroscopy.1


This

experiment uses the flame atomic spectroscopy, where an

analyte is taken up by an atomic absorption spectrometer to be excited using different mixtures of acetylene flame. The mixtures depend on the substance being analyzed, as not all substances can be atomized with the same type of flame. Each substance has a certain temperature needed to atomize them in gas phase, as well as a maximum speed for the flame to attain. In the case of this experiment using metallic substances, too high a temperature can ionize the metal while too low will have less metal atoms excited; both decrease sensitivity.9 As only air – acetylene and nitrous oxide – acetylene torches were used in this experiment, the maximum flame speed of air – acetylene is 160 cm s -1 and maximum temperature at 2300 ⁰C while nitrous oxide – acetylene attains a max flame speed of 180 cm s-1 and a max temperature of 2955 ⁰C. 7 An example of an atomic absorption spectrometer using the flame is shown in Figure 1, however a SHIMADZU model was instead used for the experiment. This model had very similar parts as the example, including an easy switch between two cathode lamps.

The process of flame atomic spectroscopy needs the analyte to be dissolved in a solvent; this analyte is then nebulized into the flame. Nebulization is the application of an oxidant gas to force the dissolved substance to spray evenly over the flame.1 The flame will atomize, or turn the sprayed analyte into neutral atoms in gas form by increasing the temperature until the gas forms a plasma; in this plasma the ions stabilize by bonding with free electrons and radicals to form neutral atoms.1 A finer aerosol will easily vaporize the analyte, and a hotter flame will easily vaporize stable compounds; both reduce interferences at this step.9 After excitation, the released energy is converted as light, which travels through a monochromator. The monochromator is a device that helps select one certain wavelength to reach the detector. However, the light coming from the neutral atomic gas is so narrow that it causes gaps in the monochromator slit which greatly deviates the amount of light obtained by the detector; this can be solved by activating a hollow – cathode ray

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tube that delivers radiation patterns similar to the substance being analyzed.1 After the light reaches the detector, it is then analyzed. A simplified process is shown as Figure 2.

Due to different substances having different properties, the metals featured in this experiment have to be dissolved in certain concentrations and have specific flames to atomize.

Aluminum was present in the form of aluminum chloride hexahydrate. Being a more easily dissolvable form of aluminum, it was added directly to the standard solution. The optimal wavelength for aluminum is 396.1 nanometers while using a nitrous oxide – acetylene flame; due to it being partially ionized in the flame, a small amount of potassium chloride was added to the solution as a suppressor.2 Calcium was used as a standard for all the other solutions in this experiment. Used in the form of calcium carbonate, a small amount of nitric acid was added before dilution of water to help it dissolve completely. The optimal wavelength for aluminum is 422.7 nanometers, while using a nitrous oxide – acetylene flame; in air – acetylene flames the interferences of calcium can be reduced by adding lanthanum, while in nitrous – oxide acetylene flames potassium chloride is added to suppress ionization.3 Potassium was present as potassium chloride. The optimal wavelength is at 766.5 nanometers with an air – acetylene flame; this same flame helps eliminate interferences in the sample.4 Lanthanum was present as lanthanum chloride in the experiment. The optimal wavelength is at 441.7 nanometers, with a nitrous oxide – acetylene flame; the interferences caused by partial ionization were suppressed by the potassium present in the solution.5 Phosphorus was present in the experiment as sodium phosphate. The optimal wavelength is at 213.6 nanometers at a nitrous oxide – acetylene flame; due to being an uncommon substance determined in AAS and having a lack of sensitivity in the process only a few studies contain interference information about phosphorus.6

The standard solutions are prepared to obtain a common constant between all other solutions. This standard was based around calcium. If the concentration of the solution is plotted against the absorption of each standard, the slope of the resulting line equals the common constant or molar absorptivity ε. This is used in the Beer – Lambert’s Law, which relates absorbance (A) to the molar absorptivity, path length of light (b), and concentration of the sample solution (c):9

A=εbc (1)The obtained absorbance from the different solutions is then converted to

transmittance:9

T=10−A (2)In vice versa, absorbance could also equal the logarithmic function of transmittance.

To decrease interferences in the samples the same suggestions that were mentioned earlier were used as well as adjusting several aspects of the flame and samples to obtain the maximum sensitivity in high precision.9

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METHODOLOGY

Two series of standard solutions were prepared. The first series contained only calcium carbonate. Five 100 milliliter volumetric flasks were used to contain 0, 1, 2, 3, and 5 ppm of calcium carbonate, respectively. These flasks were labeled from 1A to 5A in the manner of increasing calcium concentration, and then diluted to mark with distilled water. The second series was prepared similar to the first, however an added approximate of 0.0381 grams of potassium chloride was added to each flask. The second series flasks were labeled from 1B to 5B, same in order as the first series.

A set of sample solutions were prepared in 100 mL volumetric flasks, with different mixtures of substances for each. A stock solution of 250 mL calcium was prepared by dissolving 2.4976 grams calcium chloride into 250 mL water in a 250 mL flask. This stock calcium solution was used for each of the following sample solutions:

The first flask, labeled 1S, and all other flasks were each added with 3 mL of stock calcium solution. The second flask, labeled 2S, was added 0.0053 grams of sodium phosphate. The third flask, labeled 3S was added 0.0053 grams of sodium phosphate, and 0.3818 grams of potassium chloride. The fourth flask, labeled 4S, was added with 0.0053 grams of sodium phosphate, 0.3818 grams of potassium chloride, and 0.0017 grams of lanthanum chloride. The fifth flask, labeled 5S, was added with 0.0908 grams of aluminum chloride hexahydrate. The sixth flask, labeled 6S, was added with 0.0908 grams of aluminum chloride hexahydrate and 2.3146 grams of EDTA.

The spectral interference of the each solution was recorded using atomic absorption spectroscopy by air/acetylene and N2O/acetylene flames. Each set of solutions were analyzed by air/acetylene and N20/acetylene flames, with the standards being tested first before the samples.

RESULTS AND DISCUSSION

Two sets of standards were prepared in the experiment: one is composed of calcium in different concentrations and the other one is composed of calcium and potassium in different concentrations. The absorbances of each set of standards were analyzed in the AAS using two flames: air/acetylene and N2O/ Acetylene flame. This resulted to four schemes: Scheme 1 used air/acetylene flame to analyze the calcium standards; scheme 2 used the same flame to analyze the calcium plus potassium standards; scheme 3 used N2O/acetylene flame to analyze the calcium standards; and, scheme 4 used the same flame to analyze the calcium plus potassium standards. These four schemes will result to five calibration curves used to obtain the concentration of calcium in the prepared samples. This variation of conditions is done to identify the effect of the added interference in the prepared samples and also the effect of the type of flame used in the analyses.

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Table 1 summarizes the result of absorbances obtained from each schemes. Notice that in scheme 1 at the concentration of 1 mg/L, there was no recorded absorbance. This is because the absorbance read was negative and therefore would be erroneous. The data was discarded instead. Figure 3 shows the calibration curves obtained from each schemes. Linearity of the calibration curves was not satisfactory. The calibration curve is somehow curved up or has an “upward curvature”. While this may be due to the inaccurate preparation of the standards, it is a fact that it is rare for atomic absorption calibration curves to show ideality (i.e. linear plot). The “upward curvature” in the calibration curves generated is usually observed on the standards of small concentration range.11

Table 2 shows the absorbances recorded in each samples prepared. As shown, there are six samples with the same concentration of calcium, each of which contains different interferences. Table 3 shows the concentration of calcium of the samples obtained from each scheme. It can be observed the fluctuation of the values for the concentration of calcium in each sample despite the fact that they are of the same concentration of calcium when prepared. This shows how much an addition of interference affects the analysis done in AAS.

In the first sample which theoretically contains only 3mg/L calcium, the obtained concentrations from the four calibration curves were much lesser than the theoretical value (Table 3). This indicates that there are indeed errors in the preparation of the calibration curves (i.e. preparation of standards). All the other samples contain the same amount of calcium as that of the first sample. However, the remaining samples contains other components such as P, K, La, Al, and EDTA.

Interferences in atomic absorption fall into six categories: chemical interferences, ionization interferences, matrix interferences, emission interferences, spectral interferences, and background absorption. The most common interferences are chemical interferences. A chemical interference emerges when the sample being analyzed contains a thermally stable compound with the analyte that is not totally decomposed by the energy of the flame and thus, the number of atoms in the flame capable of absorbing light is reduced. 12 There are to general forms of chemical interferences: ionization and incomplete dissociation of compounds.13

The effect of phosphorous and potassium in calcium, as in the second and third sample, is an example of a chemical interference due to incomplete dissociation of compounds. These interferents form compounds which are not completely dissociated at the temperature of the flame and hence prevent the formation of neutral ground state atoms.13To overcome this interference, a higher temperature flame can be used, as in in scheme 2 which uses N2O/Acetylene flame, or La can be added as a releasing agent, as in sample 4. A releasing agent, which can be referred also as a competing cation, reacts with the interferent releasing the analyte.12The presence of Al, as in samples 5, are another

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example of a chemical interference. This time, the interference is due to ionization. To overcome this inference, a lower temperature flame such as air/ acetylene flame must be used, as in in scheme 1, because high temperature flames such as nitrous N2O/acetylene may cause appreciable ionization of the analyte element. The alkali and alkaline-earth metals such as Al are more susceptible to ionization. To control this interference, a suitable cation with an ionization potential lower than that of the analyte is added. A protective agent such as EDTA can also be added to reduce this effect, as in sample 6. A protective agent is a ligand reacts with the analyte forming a relatively volatile complex.

It can be observed that the results of the experiment did not coincide with the theory of the experiment. This can be accounted with the inaccurate preparation of the standards and also of the samples.

Table 1. Absorbance Readings of the StandardsAir/Acetylene Flame N2O/Acetylene Flame

Scheme 1 Scheme 2 Scheme 3 Scheme 4Ca Standard

0 mg/L 0.0005 0.00021 mg/L 0.0020 -2 mg/L 0.0778 0.06133 mg/L 0.0824 0.06615 mg/L 0.1963 0.1780

Ca Standard + K0 mg/L 0.0030 0.00291 mg/L 0.0068 0.00702 mg/L 0.0779 0.08973 mg/L 0.0824 0.09085 mg/L 0.1818 0.1367

Table 2. Absorbance readings of the samplesSample Air/ Acetylene Flame N2O/ Acetylene Flame3 mg/L Ca 0.0824 0.06613 mg/L Ca + P 0.0413 0.01403 mg/L Ca +P + K 0.0519 0.05163 mg/L Ca + P + K + La 0.0508 0.06933 mg/L Ca + Al 0.0292 0.04523 mg/L Ca + Al + EDTA 0.0002 0.0200

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0 1 2 3 4 5 60.0000

0.0500

0.1000

0.1500

0.2000

0.2500

f(x) = 0.0403040540540541 x − 0.0168689189189189R² = 0.93895676448771

Calibration Curve: Scheme 1

Concentration of Ca Standards (mg/L)

Abso

rban

ce

0 1 2 3 4 5 60.0000

0.0500

0.1000

0.1500

0.2000

f(x) = 0.0367986486486486 x − 0.010577027027027R² = 0.945435800471963


Concentration of Ca + K Standards (mg/L)

Abso

rban

ce


(a) (b)

(c) (d)

Figure 3. Calibration curves generated from each scheme: (a) plot of the absorbance obtained using air/acetylene flame against the concentration of Ca standards, scheme 1; (b) plot of the absorbance obtained using air/acetylene flame against the concentration of Ca + K standards, scheme 2; (c) plot of the absorbance obtained using N 2O/acetylene flame against the concentration of Ca standards, scheme 3; (4) plot of the absorbance obtained using N2O/acetylene flame against the concentration of Ca + K standards, scheme 4.

Table 3. Concentration of Ca in the samples in mg/L

Sample Air/ Acetylene Flame N2O/ Acetylene FlameScheme 1 Scheme 2 Scheme 3 Scheme 4

3 mg/L Ca 2.4630 2.5266 2.2004 2.2448

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0 1 2 3 4 5 60.0000

0.0500

0.1000

0.1500

0.2000

0.2500

f(x) = 0.0403040540540541 x − 0.0168689189189189R² = 0.93895676448771



Abso

rban

ce

0 1 2 3 4 5 60.0000

0.0500

0.1000

0.1500

0.2000

f(x) = 0.0343769230769231 x − 0.0095423076923077R² = 0.933180684262194



Abso

rban

ce

0 1 2 3 4 5 60.00000.02000.04000.06000.08000.10000.12000.14000.1600

f(x) = 0.0286 x + 0.0019R² = 0.896881446483941



Abso

rban

ce

0 1 2 3 4 5 60.0000

0.0500

0.1000

0.1500

0.2000

f(x) = 0.0367986486486486 x − 0.010577027027027R² = 0.945435800471963



Abso

rban

ce


3 mg/L Ca + P 1.4433 1.4098 0.6848 0.42313 mg/L Ca +P + K 1.7063 1.6978 1.7786 1.73783 mg/L Ca + P + K + La 1.6790 1.6679 2.2935 2.35663 mg/L Ca + Al 1.1430 1.0809 1.5924 1.51403 mg/L Ca + Al + EDTA 0.4235 0.2929 0.8594 0.6329

CONCLUSION

Atomic Absorption Spectroscopy (AAS) is a technique for measuring quantities of chemical elements present in a sample by measuring the absorbed radiation by the chemical element of interest. The sample is excited by radiation making its atoms absorb ultraviolet or visible light and make transitions to higher energy levels. In this experiment, flame atomic spectroscopy is used to excite the analyte using different mixtures of acetylene flame. The analyte in this experiment is calcium. Calcium content of 6 samples containing interferences and other components were determined. To determine the calcium content in the samples, two sets of calcium standards (Ca standards and Ca+K standards) were prepared to create calibration curves. The absorbances of the standards were determined using air/acetylene and N2O/acetylene flames as indicated in the four schemes followed in the experiment. Four calibration curves were obtained in the experiment. Effects of interference in the determination of calcium in the sample were examined. Potassium and phosphorus caused a chemical interference due to incomplete dissociation of compounds. This interference can be aided by using a higher temperature flame (N2O/acetylene flame instead of air/acetylene) and also by adding Lanthanum which is a releasing agent. Aluminum can cause another type of chemical interference due to effects of ionization. To aid this, a lower temperature flame such as air/ acetylene flame must be used because high temperature flames such as nitrous N2O/acetylene may cause appreciable ionization of the analyte element and also by adding a protective agent such as EDTA. These theories were not reflected in the results of the analysis because of the inaccurate preparation of standards and samples.

LITERATURE CITED

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1. “Introduction”. Determination of Calcium by Atomic Spectroscopy. Chem 334: Quantitative Analysis Laboratory, Colorado State University. March 24, 2016. p. 1 – 2.

2. “Standard Conditions: Al (Aluminum)”. Flame Atomic Absorption Spectrometry: Analytical Methods. Agilent Technologies, Australia. 13th ed. November 2015. p. 16.

3. “Standard Conditions: Ca (Calcium)”. Flame Atomic Absorption Spectrometry: Analytical Methods. Agilent Technologies, Australia. 13th ed. November 2015. p. 24.

4. “Standard Conditions: K (Potassium)”. Flame Atomic Absorption Spectrometry: Analytical Methods. Agilent Technologies, Australia. 13th ed. November 2015. p. 42.

5. “Standard Conditions: La (Lanthanum)”. Flame Atomic Absorption Spectrometry: Analytical Methods. Agilent Technologies, Australia. 13th ed. November 2015. p. 43.

6. “Standard Conditions: P (Phosphorus)”. Flame Atomic Absorption Spectrometry: Analytical Methods. Agilent Technologies, Australia. 13th ed. November 2015. p. 54.

7. Amos, M. D. & Willis, J. B. “Choice of Flame”. Spectrochimica Acta: Use of high – temperature pre – mixed flames in atomic absorption spectroscopy. Pergamon Press Ltd., Northern Ireland. vol. 22. 1966. p. 1327.

8. Martizano, J. “Materials & Procedures”. Chemical Interferences in Atomic Absorption Spectrophotometric Measurements. University of the Philippines Visayas – Miagao Campus, Philippines. March 2016. p. 1.

9. Melville, J. “Theory”. Atomic Absorption Spectroscopy of Metal Alloys. Chemistry 105: Instrumental Methods in Analytical Chemistry, Berkeley College of Chemistry, University of California, California. March 3, 2014. p. 2.

10. Walsh, A. “Introduction”. Spectrochimica Acta: The application of atomic absorption spectra to chemical analysis. Chemical Physics Section, Division of Industrial Chemistry, Commonwealth Scientific and Industrial Research Organization, Melbourne, Australia. Pergamon Press Ltd., London. vol. 7. 1955. pp. 108 – 177.

11. Harvey, D. Modern Analytical Chemistry. United State of America: The McGraw-Hill Companies, Inc.; 2000 [cited 2016 February]. Available from: http://elibrary.bsu.az/

12. Skoog D. A., West D. M., Holler F. J., Crouch S. R. 2014. Fundamentals of Analytical Chemistry Ninth Edition. Canada: Nelson Education, Ltd. 1026p.

APPENDICES

I. Tables

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http://elibrary.bsu.az/


Table 4. Statistical Data for the Schemes 1, 2,3 and 4

Statistical Parameter Scheme 1 Scheme 2 Scheme 3 Scheme 4

M 0.040304054 0.036798649 0.036439189 0.0286B -0.016868919 -0.01057703 -0.01916622 0.0019sr 2.28x10-2 1.96 x10-02 2.28 x10-02 2.15 x10-02

sm 5.93x10-3 5.10 x10-03 5.92 x10-03 5.60 x10-03

sb 1.66x10-2 1.43 x10-02 1.65 x10-02 1.56 x10-02

sc 6.22x10-1 6.42 x10-01 6.91 x10-01 8.34 x10-01

r 0.939 0.9454 0.9267 0.8969

II. Sample Calculations

Least Square Method

*For Scheme 1

Let x be the concentration of the Ca standards and y be the absorbance

x y x2 y2 xy (y1-mx1+b)2

0 0.0005 0 0.00000025 0 0.0003016791 0.0020 1 0.000004 0.002 0.0004594652 0.0778 4 0.00605284 0.1556 0.0001977063 0.0824 9 0.00678976 0.2472 0.000468435 0.1963 25 0.03853369 0.9815 0.000135691

sum 11 0.3590 39 0.05138054 1.3863 0.00156297ave 2.2 0.0718 7.8 0.01027610 0.27726 0.00031259

Sxx=Σ x2−

(Σx )2

n=39− (11 )2

5=14.8

Syy=Σ y2−

(Σy )2

n=0.05138054− (0.3590 )2

5=0.02560434

Sxy=Σxy−(ΣxΣy )2

n=1.3863− [ (11) (0.3590 ) ]2

5=0.5965

m=S xySxx

=0.596514.8

=0.040304054

b=Σyn

−m(Σxn )=0.35905 −(0.02560434 )(115 )=−0.01686892

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Determination of calcium in sample where y = 0.0824

equationof line : y=0.040304054 x−0.01686892

x=0.0824+0.016868920.040304054

=2.4630mg /L

Uncertainty of Measurement

sr=√ S yy−m2SxxN−2=¿√0.02560434−(0.040304054¿¿2)(14.8)

5−2=2.28×10−2 ¿¿

sm=√ sr2Sxx=¿√(2.28×10−2)2

14.8 =5.93×10−3¿

sb=sr√ 1N−¿¿¿¿

¿

sc=¿

uncertainty=√(2.28×10−2)2+(5.93×10−3)2+(1.66×10−2)2+(6.22×10−1)2=6.22×10−1

*All calculations for the remaining schemes are done is the same way shown.

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Chemical Interferences in Atomic Absorption Spectrophotometric Measurements

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