agilent aas calibration method

A Universal Calibration Method forFlame Atomic Absorption Analysis

Authors

Dr. Joachim Dahmen

Dr. Gerald Fuchs-Pohl

Annelore Bischoff Central Analytical

Laboratory

E.Merck

Darmstadt

Federal Republic of Germany

Dr. Barbara Pohl

Application Note

Atomic Absorption

Introduction

Flame atomic absorption analysis is a relatively simple and quick method for thedetermination of a series of elements in the ng/mL range. G. M. Hieftje, USA, stateswhen he was interviewed by the late J. M. Ottaway, United Kingdom, in 1985 [1].“I would say that the use of flames in atomic absorption will continue to existprominently side by side with furnace techniques. My own feeling is that it is wiseto use flame whenever possible and furnaces should be used only whenever a flamewon’t suffice.”

In flame atomic absorption spectrometry chemical and physical interferences arefewer than in furnace AAS, nevertheless they exist.

Physical interferences are caused by differences in the physical properties (such asdensity, surface tension, viscosity, temperature) of the sample and the calibrationstandard solutions. These parameters control the aspiration rate of the nebulizer,the size distribution of the produced aerosol and thus the volatilization behavior ofthe dissolved sample solution.

Ionization interference, which is also a physical interference, is based on differ-ences in the formation of ions, which are species not detected by normal atomicabsorption. This type of interference occurs especially in the determination of theeasily ionized alkali and alkaline-earth elements.

Chemical interferences are due to the formation of chemical species during nebu-lization and atomization which alter the atomization efficiency compared to aqueousstandard solutions.

Due to the above mentioned interferences, analysis of a complex sample matrixagainst a calibration graph, which is obtained by measurements of aqueousstandard solutions, often leads to systematic errors and incorrect results.

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In this paper both methods are applied to the determination ofvarious elements in diet drinks, diet menus and to the deter-mination of magnesium in human serum. Results from differ-ent calibration procedures are compared to establish theaccuracy of the buffer methods. In the case of the determina-tion of magnesium in human serum an enzymatic method iscompared.

EquipmentAll measurements were carried out with an Agilent SpectrAA-40flame atomic absorption spectrometer and Agilent hollow cath-ode lamps. An air-acetylene flame was used for all determina-tions except for calcium where a nitrous oxide-acetylene flamewas also used. Deuterium background correction was appliedwhenever needed.

ReagentsAll reagents were supplied by E.Merck, Darmstadt, FederalRepublic of Germany.

• Titrisol for the preparation of 1 L sodium, potassium andlithium standard solution after Schuhknecht andSchinkel (Merck-No. 9985), concentrations 0.1 g/Lrespectively, called solution A.

• Cesium chloride/aluminium nitrate buffer solution afterSchuhknecht and Schinkel (Merck-No. 2037), calledsolution B.

• Cesium chloride/lanthanum chloride buffer solution afterSchinkel (Merck-No. 16755).

• Titrisol for the preparation of 1 L magnesium standardsolution (Merck-No. 9949), concentration 1 g/L.

• Titrisol for the preparation of 1 L calcium standard solution(Merck-No. 9943), concentration 1 g/L.

• Titrisol for the preparation of 1 L zinc standard solution(Merck-No. 9953), concentration 1 g/L.

• Titrisol for the preparation of 1 L iron standard solution(Merck-No. 9972), concentration 1 g/L.

• Titrisol for the preparation of 1 L sodium standard solution(Merck-No. 9927), concentration 1 g/L.

• Titrisol for the preparation of 1 L potassium standardsolution (Merck-No. 9924), concentration 1 g/L.

Sample PretreatmentFor the analysis of powdered diet menus and diet drinks asample pretreatment is necessary.

Several methods are available to eliminate these interferencesand to guarantee more accurate results:

1. The use of standard solutions with similar matrix content.

2. The well known standard additions method.

3. Chemical pretreatment such as matrix isolation.

4. The use of a nitrous oxide/acetylene flame instead of anair-acetylene flame.

The performance of most of these methods is very timeconsuming and thus expensive.

A fast universal calibration method was described bySchuhknecht and Schinkel, Analytical Laboratories ofSaarbergwerke, FRG [2]. They recommended, as a “spectro-scopic and physical buffer system”, an addition of cesiumchloride and aluminium nitrate to the samples and standardsolutions in the flame emission spectroscopic determinationof sodium, potassium and lithium.

Cesium was chosen because of its low ionization energy. Itsaddition leads to a large quantity of free electrons in the flame.The ionization of sodium, potassium and lithium is suppressedand their mutual interferences can then be neglected.

The alkaline-earth elements are the main interfering elements inthe determination of sodium, potassium and lithium. This inter-ference is suppressed by anaddition of aluminium nitrate,because compounds with a low volatility are found with alka-line-earth elements. Another effect of the addition of cesiumchloride and aluminium nitrate is the formation of solutions withunique or almost unique physical properties, such as withrespect to density, surface tension and viscosity. Therefore, inthe case of various matrices a one-point calibration is sufficientto obtain accurate results.

A partially-rotated burner with a reduced flame width in thelight path, and therefore reduced sensitivity, is often appliedto analyze higher analyte concentrations.

This emission technique of Schuhknecht and Schinkel [2] isalso applied successfully in flame atomic absorption analy-sis. In 1984 Schinkel [3] extended the method to the flameatomic absorption spectrometric determination of fourteenelements (Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, Sr, Zn)in a variety of sample types by the use of an addition ofcesium chloride and lanthanum chloride.

Both recommended buffers (CsCl/Al(NO3)3 and CsCl/ LaCl3)are commercially available as concentrated solutions whichcan simply be added to the samples and standard solutions.

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Sixty millilitres of a mixture of nitric and hydrochloric acid (90 mLHCl 25% +10 mL HNO3 65%) is added to approximately 10 g ofthe accurately weighed homogenized sample in a 200 mL vol-umetric flask. With continuous shaking the flask is heated ina boiling water bath for 30 minutes. Then approximately 90 mLdeionized water is added, carefully mixed and the digestioncontinued for another 30 minutes. After cooling the solution ismade up to volume with deionized water and centrifuged. Thesupernatant liquids are diluted to a suitable concentrationrange.

Serum samples are analyzed after one dilution step withoutany further pretreatment.

The concentrated buffer solution is added in the last dilutionstep (1:10 of the resulting volume).

Calibration ProceduresAn example for the preparation of five buffered multielementstandard solutions of sodium, potassium and lithium afterthe method of Schuhknecht and Schinkel [2] in the rangefrom 2 to 10 mg/L is given in Table 1. Solution A is themixed standard solution, 0.1 g/L of sodium, potassium andlithium respectively. Solution B is the concentrated buffersolution [2]. Table 2 presents the resulting composition ofthe calibration standard solutions.

Table 1. Preparation of Buffered Calibration Standard Solutions [2] forNa, K and Li, 100 mL Each Made up to Volume with DeionizedWater

Solution A Solution B Deionized water(mL) (mL) (mL)

Blank 0 10 90

Standard 1 2 10 88

Standard 2 4 10 86

Standard 3 6 10 84

Standard 4 8 10 82

Standard 5 10 10 80

Table 2. Compositions of the Buffered Calibration Standard Solutions

Na K Li CsCl Al (NO3)3.9 H2O(mg/L) (mg/L) (mg/L) (g/L) (g/L)

Blank 0 0 0 5 25

Standard1 2 2 2 5 25

Standard 2 4 4 4 5 25

Standard 3 6 6 6 5 25

Standard 4 8 8 8 5 25

Standard 5 10 10 10 5 25

Buffered standard solutions of the other elements areobtained by similar dilution schemes.

A mixture of cesium chloride and lanthanum chloride servesas the buffer system [3].

For one-point calibrations, only one standard with a maximumabsorbance of 0.5 is used. If more than 0.5 absorbance unitsare detected the burner is partially rotated to an angle wherethis maximum absorbance is approximately achieved.

In the standard addition mode two known amounts of the ele-ment to be determined are added to the diluted, unbufferedsamples.

Results and Discussion

1. Diet Menu and Diet Drinks The sample labels of the analysed diet menus and diet drinksare listed in Table 3.

Tables 4–7 present the results of Fe, Zn, Mg and Cameasurements with three different calibration procedures:

A. 1-point calibration [3], concentrations: 1 mg Fe/200 mL,0.2 mg Zn/200 mL, 1 mg Mg/200 mL, 1 mg Ca/200 mL

B. 5-point calibration buffered as per [3]

C. Standard addition method

For the Ca measurements by standard addition a nitrousoxide-acetylene flame and the less sensitive wavelength of239.9 nm were used.

Table 3. Sample Labels

Sample no. Sample label

1 Blackcurrant drink 1

2 Blackcurrant drink 2

3 Beef broth 1

4 Beef broth 2

5 Beef broth 3

6 Pineapple drink 1

7 Pineapple drink 2

8 Orange drink 1

9 Orange drink 2

4

Table 4. Fe Results Using Different Calibration Procedures (mg/200 mL),Wavelength: 248.3 nm, Air-Acetylene Flame

Calibration procedure

A B CSample no. Dilution Result Dilution Result Dilution Result

1 1:2 1.0 1:2 0.95 1:5 0.98

2 1:2 1.3 1:2 1.3 1:5 1.3

6 20:25 0.55 20:25 0.52 1:4 0.59

7 20:25 0.53 20:25 0.51 1:4 0.54

8 20:25 0.53 20:25 0.52 1:4 0.55

9 20:25 0.56 20:25 0.54 1:4 0.52

Table 5. Zn Results Using Different Calibration Procedures (mg/200 mL),Wavelength: 213.9 nm, Air-Acetylene Flame



6 20:25 0.15 20:25 0.14 1:2 0.14

7 20:25 0.16 20:25 0.15 1:2 0.15

8 20:25 0.16 20:25 0.16 1:2 0.16

9 20:25 0.15 20:25 0.15 1:2 0.14

Table 6. Mg Results Using Different Calibration Procedures (mg/200 mL),Wavelength: 285.2 nm, Air-Acetylene Flame, Partial BurnerRotation



1 1:10 8.0 1:10 7.6 1:100 7.3

2 1:10 9.9 1:10 10.2 1:100 9.8

3 1:10 7.9 1:10 7.5 1:100 8.2

4 1:10 6.5 1:10 6.1 1:100 6.7

5 1:10 6.9 1:10 6.5 1:100 7.2

6 1:5 4.6 1:5 4.7 1:25 4.4

7 1:5 4.5 1:5 4.7 1:25 4.4

8 1:5 4.5 1:5 4.6 1:25 4.2

9 1:5 4.5 1:5 4.7 1:25 4.5

Table 7. Ca Results Using Different Calibration Procedures (mg/200 mL),Wavelength: 422.7 nm, Air-Acetylene Flame (method A and B),Wavelength: 239.9 nm, Nitrous Oxide-Acetylene Flame (Method C)



1 1:50 45 1:50 45 1:5 49

2 1:50 55 1:50 55 1:5 61

3 1:50 49 1:50 48 1:5 52

4 1:50 47 1:50 47 1:5 49

5 1:50 51 1:50 51 1:5 57

6 1:50 31 1:50 32 1:4 31

7 1:50 31 1:50 31 1:4 34

8 1:50 30 1:50 30 1:4 33

9 1:50 31 1:50 30 1:5 33

The relative standard deviations of five replicate measurementsare between 0.5 and 2%.

The 1-point calibration procedure [3] is sufficient and its perfor-mance is less time consuming than others. The differencesbetween the results obtained with the 1-point calibrations [3] andthe results obtained by standard additions never exceed 10%.

In the determination of sodium and potassium the followingcalibration procedures were compared:

A. 1-point calibration [3], concentrations: 1 mg Na / 200 mL,1 mg K / 200 mL

B. 5-point calibration, buffered as in [3]

C. 1-point calibration [2]

The results are presented in Tables 8 and 9.

Table 8. Na Results Using Different Calibration Procedures (mg/200 mL),Wavelength: 589 nm, Air-Acetylene Flame, Partial Burner Rotation



1 1:10 8.6 1:10 8.5 1:20 8.8

2 1:20 13.5 1:20 13.4 1:20 10.4

3 1:200 164 1:200 158 1:250 160

4 1:200 135 1:200 132 1:250 125

5 1:200 143 1:200 141 1:250 140

6 1:20 14.8 1:20 15.1 1:20 14.4

7 1:20 14.6 1:20 14.8 1:20 14.8

8 1:20 15.1 1:20 15.6 1:20 14.0

9 1:20 15.7 1:20 16.5 1:20 14.8

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Conclusions

The applications described show only a few examples forthe applicability of the methods of Schuhknecht andSchinkel [2] and of Schinkel [3]. If the analyte is in a mediumconcentration range, it is possible to obtain accurate resultswith acceptably short analysis times by the addition of con-centrated buffer solutions to all the measured solutions.

These universal calibration methods of Schuhknecht andSchinkel [2] and of Schinkel [3] are introduced as additionalworthwhile tools in every flame atomic absorption laboratory.

References

1 G. M. Hieftje, J. Anal. At. Spectrom. 1, 3, (1986).

2 W. Schuhknecht, H. Schinkel, Z. Fresenius. Anal. Chem194, 161, (1963) (in German)

3 H. Schinkel, Z. Fresenius. Anal. Chem. 317, 10, (1984) (inGerman)

Table 9. K Results Using Different Calibration Procedures (mg/200 mL),Wavelength: 766.5 nm, Air-Acetylene Flame, Partial BurnerRotation



1 1:50 44 1:50 43 1:50 41

2 1:100 61 1:100 57 1:100 56

3 1:100 53 1:100 49 1:100 50

4 1:50 40 1:50 38 1:50 38

5 1:50 43 1:50 41 1:50 40

6 1:50 44 1:50 46 1:50 48

7 1:50 43 1:50 45 1:50 47

8 1:50 42 1:50 43 1:50 45

9 1:50 42 1:50 44 1:50 46

In the determination of sodium and potassium no significantdifferences between the calibration procedures wereobserved, so that the 1-point calibrations [3] or [2] werechosen for future determinations.

2. Determination of Magnesium in Serum For the determination of magnesium in serum a 5-point calibra-tion with magnesium concentrations between 1 and 5 mg/Lwas used. The cesium chloride/lanthanum chloride bufferafter Schinkel was added. The burner was rotated to a posi-tion where a maximum absorbance of approximately 0.5was achieved.

Ninety-five samples of human serum and control serumwere analyzed against the buffered calibration graph. Thesame samples were analyzed by an enzymatic method andexcellent agreement was shown with the atomic absorptionresults (Figure 1).

Figure 1. Determination of magnesium in serum, results obtained byatomic absorption versus an enzymatic method [mmol/L].

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© Agilent Technologies, Inc., 1989Printed in the USANovember 1, 2010AA-088

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