ATOMIC ABSORPTION SPECTROPHOTOMETRY COOKBOOK
Section 2
Standard Sample Preparation Method
Praparation of Calibration Curve and Determination Method
Interference in Atomic Absorption Spectrophotometry
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Preparation of Standard Solutions for Atomic Absorption Spectrometry, Lecture notes of Chemistry
Instructions on the preparation of standard solutions for atomic absorption spectrometry using various standard materials and acids. It covers the preparation process for silver, aluminum, boron, beryllium, calcium, cadmium, cobalt, chromium, cesium, copper, lithium, magnesium, molybdenum, nickel, lead, antimony, titanium, and zinc.
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ATOMIC ABSORPTION SPECTROPHOTOMETRY COOKBOOK
Section 2
Standard Sample Preparation Method
Praparation of Calibration Curve and Determination Method
Interference in Atomic Absorption Spectrophotometry
Atomic Absorption Spectrophotometry Cookbook
- Section - Standard Sample CONTENTS
- 3.1 Stock standard
- 3.2 Standard solution for calibration curve
- 3.3 Standard solution preparation method
- Preparation of Calibration Curve and Determination Method
- 4.1 Calibration curve method
- 4.2 Standard addition method
- 4.3 Concentration of calibration curve
- Interference in Atomic Absorption Spectrophotometry
- 5.1 Spectrophotometric interference and its correction method
- 5.2 Physical interference
- 5.3 Chemical interference and its correction method
Fig. 3.1 Change on standing of Fe standard sample
3.3 Standard solution preparation method
- Ag (Silver) 1.0mg Ag/m l Standard material : Silver nitrate (AgNO 3 ) : 1.575g of silver nitrate dried at 110oC dissolved with nitric acid (0.1N) and is diluted with nitric acid (0.1N) to 1000m l accurately.
- A l (Aluminum) 1.0mg A l /m l Standard material : Metal aluminum 99.9% up : 1,000g of metal aluminum is heated and dissolved with hydrochloric acid (1+1) 50m l and is diluted with water to 1000m l accurately after it has cooled. (hydrochloric acid concentration is changed to about 1N.)
Preparation method of solution
Preparation method of solution
- As (Arsenic) 1.0mg As/m l Standard material : Arsenic (III) trioxide 99.9% up : Arsenic (III) trioxide is heated at 105oC for about two hours and is cooled with the desiccator. Its 1.320g is dissolved in the smallest possible sodium hydroxide solution (1N) and is diluted with water to 1000m l accurately.
- Au (Gold) 1.0mg Au/m l Standard material : Gold : 0.100g of high purity gold is dissolved in several m l of aqua regia and vaporized to dryness on water bath. Then, lm l of hydrochloric acid is added and vaporized to dryness. It is dissolved in hydrochloric acid and water and diluted with water to 100m l accurately. Hydrochloric acid concentration is set at 1N.
- B (Boron) 1.0mg B/m l Standard material : Boric acid (H 3 BO 3 ) : 5.715g of pure boric acid is dissolved in water and is diluted to 1000m l.
- Be (Beryllium) 1.0mg Be/m l Standard material : Metal beryllium 99.9% up : 0.100g of metal beryllium is heated and dissolved with hydrochloric acid (1+1) 10m l and is diluted with water to 100m l after it has cooled. Hydrochloric acid concentration is set at 1N.
- Bi (Bismuth) 1.0mg Bi/m l Standard material : Metal bismuth 99.9% up : 0.100g of metal bismuth is heated and dissolved with nitric acid (1+1) 20ml and is diluted to 100ml accurately after it has cooled.
Preparation method of solution
Preparation method of solution
Preparation method of solution
Preparation method of solution
Preparation method of solution
- Cu (Copper) 1.0mg Cu/m l Standard material : Metal copper 99.9% up : 1.000g of metal copper is heated and dissolved with nitric acid (1+1) 30m l and is diluted to 1000m l accurately with 50m l if nitric acid (1+1) and water after it has cooled.
- Fe (Iron) 1.0mg Fe/m l Standard material : Pure iron 99.9% up : 1.000g of pure iron is heated and dissolved with 20m l of aqua regia and is diluted to 1000m l accurately after it has cooled.
- Ge (Germanium) 1.0mg Ge/m l Standard material : Germanium oxide (GeO 2 ) : 1g of sodium hydroxide and 20m l of water are added to 1.439g of germanium oxide and heated and dissolved. It is diluted to 1000m l accurately with water after it has cooled.
- Hg (Mercury) 1.0mg Hg/m l Standard material : Mercury chloride (HgCl 2 ) : 1.354g of mercury chloride is dissolved in water and is diluted to 1000m l accurately with water.
- K (Potassium) 1.0mg K/m l Standard material : Potassium chloride (KCl) : Potassium chloride is heated at 600oC for about one hour and is cooled in the desiccator. Its 1.907g is dissolved in water and diluted to 1000m l accurately with water after hydrochloric acid is added. Hydrochloric acid concentration is set at 0.1N.
Preparation method of solution
Preparation method of solution
Preparation method of solution
Preparation method of solution
Preparation method of solution
- Li (Lithium) 1.0mg Li/m l Standard material : Lithium chloride (LiCl) : 0.611g of lithium chloride is dissolved in water and diluted with water to 1000m l accurately after hydrochloric acid is added. Hydrochloric acid concentration is set at 0.1N.
- Mg (Magnesium) 1.0mg Mg/m l Standard material : Metal magnesium 99.9% up : 1.000g of metal magnesium is heated and dissolved with hydrochloric acid (1+5) 60m l and is diluted with water to 1000m l accurately after it has cooled.
- Mn (Manganese) 1.0mg Mn/m l Standard material : Metal manganese 99.9% up : 1.000g of metal manganese is heated and dissolved with 20m l of aqua regia and is diluted to 1000m l accurately after it has cooled.
- Mo (Molybdenum) 1.0mg Mo/m l Standard material : Metal molybdenum 99.9% up : 1.000g of metal molybdenum is heated and dissolved with hydrochloric acid (1+1) 30m l and a small quantity of nitric and is diluted to 1000m l accurately with water after it has cooled.
- Na (Sodium) 1.0mg Na/m l Standard material : Sodium chloride (NaCl) : Sodium chloride is heated at 600oC for about one hour and is cooled in the desiccator. Its 2.542g is dissolved in water and is diluted with water to 1000m l accurately after hydrochloric acid is added. Hydrochloric acid concentration is set at 0.1N.
Preparation method of solution
Preparation method of solution
Preparation method of solution
Preparation method of solution
Preparation method of solution
- Si (Silicon) 1.0mg Si/m l Standard material : Silicon dioxide (SiO 2 ) : Silicon dioxide is heated at 700 to 800oC for one hour and cooled in the desiccator. Its 0.214g is put in a crucible and is dissolved by mixing 2.0h of sodium carbonate anhydrous and is diluted with water to 100m l accurately.
- Sn (Tin) 1.0mg Sn/m l Standard material : Metal tin 99.9% up : 0.500g of metal tin is added to 50m l of hydrochloric acid. Then heated and dissolved at 50 to 80oC. After it has cooled, it is added to 200m l or hydrochloric acid and diluted with water to 500m l accurately.
- Sr (Strontium) 1.0mg Sr/m l Standard material : Strontium carbonate (SrCO 3 ) : 1.685g of strontium carbonate is dissolved with hydrochloric acid. It is heated to remove carbon dioxide and is diluted to 1000m l accurately with water after it has cooled.
- Ti (Titanium) 1.0mg Ti/m l Standard material: Metal titanium 99.9% up : 0.500g of metal titanium is heated and dissolved with hydrochloric acid (1+1) 100m l and is diluted with hydrochloric acid (1+2) to 500m l accurately after it has cooled.
- Tl (Thallium) 1.0mg Tl/m l Standard material : Metal thallium 99.9% up : 1.000g of metal thallium is heated and dissolved with nitric acid (1+1) 20m l and is diluted with water to 1000m l accurately after it has cooled.
Preparation method of solution
Preparation method of solution
Preparation method of solution
Preparation method of solution
Preparation method of solution
- V (Vanadium) 1.0mg V/m l Standard material : Metal vanadium 99.9% up : 1.000g of metal vanadium is heated and dissolved with 30m l of aqua regia and is concentrated to near dryness. It is added to 20m l of hydrochloric acid and is diluted with water to 1000m l accurately after it has cooled.
- Zn (Zinc) 1.0mg Zn/m l Standard material : Metal zinc 99.9% up : 1.000g of metal zinc is heated and dissolved with nitric acid (1+1) 30m l and is diluted with water to 1000m l accurately after it has cooled.
Preparation method of solution
Preparation method of solution
4.2 Standard addition method Several unknown sample solutions (four or more) of a like quantity, and standard sample solutions of known different concentrations are added. Absorbances of these series of samples are measured. The calibration curve of absorbance versus standard sample solution concentration is prepared as shown in Fig. 4.1 (2). It is extrapolated and the length of the axis of the abscissas from the point inter secting with the axis of the abscissas (concentration axis) to the added concentration 0 is considered as concentration of the unknown sample. Fig. 4.2 shows a preparation example of the sample solution in the standard addition method. Four 100m l measuring flasks are prepared and 10m l of the unknown sample of Mg concentration of 100 x ppm is put in each of the above flasks. 0, 10, 20 and 30m l of Mg standard solution of concentration 1.0ppm are put in each of the above flasks. Then, solvent is added so that the total quantity is 100m l. Samples of Mg concentration x, x+0.1, x+0.2, x+0.3 ppm are now available. They are measured and the calibration curve is prepared as shown in Fig. 4.2 (2) to obtain Mg concentration of x ppm. If this value is multiplied ten times, Mg concentration in the unknown sample can be obtained.
Fig. 4.2 Example preparation of standard solution in standard addition method The advantage of this method is that it decrease analysis errors caused by various interferences based on differences in composition. Because the composition of the calibration curve is close to that of the sample, the calibration curve shows good linearity even in the low concentration area and passes the zero point. Otherwise, an error occurs.
4.3 Concentration of calibration curve The range where the calibration curve shows linearity in atomic absorption spectrometry is generally said to be up to absorbance 0.5 and it is desirable to set the calibration curve at absorption 0.3 or less with some margin given. In the meantime, absorbance sensitivity is shown by 1% absorption value (0.0044 Abs.) or detection limit value in the atomic absorption spectrometry. 1% absorption value is the concentration of the sample which gives absorbance 0.0044 and the detection limit value is the concentration of the sample which gives a signal having amplitude twice as much as the noise width. Because 1% absorbance sensitivity corresponds to 0.004 Abs. when the concentration of the calibration curve is set, the sample concentration with its lower limit of the calibration line being ten-fold concentration of 1% absorption value and with its upper limit being 70 to 80-fold concentration and showing 0.004 to 0.3 absorbance is considered as the optimum concentration range of the calibration curve. If Cd is taken as an example, the concentration range of the calibration curve is 0.12 to 0.96 ppm, because 1% absorption value in flame atomic absorption method is 0.012 ppm as shown in Table 4.1. When the concentration range of the calibration curve is determined from the detection limit value, the concentration range of the calibration curve is about 1000-fold the detection limit value, because the detection limit value is 1/10 to 1/20 of 1% absorption value. When the concentration of the unknown sample is below the concentration range of the calibration curve set by this method, the concentration for determination is to 1% absorption value in flame atomic absorption method It is five times that of the 1% absorption value in the electrothermal atomic absorption method, although accuracy becomes slightly deteriorated. When the concentration of the unknown sample is above the set concentration range, the burner angle is adjusted to lower sensitivity in the flame atomic absorption method. Fig. 4.3 shows the relation between the burner angle and sensitivity. If the burner angle is tilted by 90o, the sensitivity drops to 1/20 and determination can be made to 20-fold the concentration of X the standard condition.
Table 4.1 1% absorption value in the flame and electrothermal atomic absorption methods Flame atomic absorption Electro-thermal atomic absorption Ele- ment
Analysis line wavelength (nm) (^) Gas type (^) concentration (ppm)1% absorption
1% absorption concentration (ppb) Low
1% absorption concentration (ppb) High Ag Al As Au B Ba Be Bi Ca(1) Ca(2) Cd Co Cr Cs Cu Dy Er Eu Fe Ga Gd Ge Hf Hg Ho Ir K La Li Lu Mg Mn Mo Na Nb Ni Os Pb(1) Pb(2)
Air-C 2 H 2 N 2 O-C2H 2 Ar-H 2 Air-C 2 H 2 N 2 O-C2H 2 N 2 O-C2H 2 N 2 O-C2H 2 Air-C 2 H 2 Air-C 2 H 2 N 2 O-C2H 2 Air-C 2 H 2 Air-C 2 H 2 Air-C 2 H 2 Air-C 2 H 2 Air-C 2 H 2 N 2 O-C2H 2 N 2 O-C2H 2 N 2 O-C2H 2 Air-C 2 H 2 Air-C 2 H 2 N 2 O-C2H 2 N 2 O-C2H 2 N 2 O-C2H 2 N 2 O-C2H 2 Air-C 2 H 2 Air-C 2 H 2 N 2 O-C2H 2 Air-C 2 H 2 N 2 O-C2H 2 Air-C 2 H 2 Air-C 2 H 2 N 2 O-C2H 2 Air-C 2 H 2 N 2 O-C2H 2 Air-C 2 H 2 N 2 O-C2H 2 Air-C 2 H 2 Air-C 2 H 2
12
30
16
70
12
30
Flame atomic absorption Electro-thermal atomic absorption Ele- ment
Analysis line wavelength (nm) (^) Gas type (^) concentration (ppm)1% absorption
1% absorption concentration (ppb) Low
1% absorption concentration (ppb) High Pd Pr Pt Rb Re Ru Sb Sc Se Si Sm Sn(1) Sn(2) Sn(3) Sn(4) Sr Ta Tb Te Ti Tl V W Y Yb Zn Zr As(H) Bi(H) Sb(H) Se(H) Sn(H) Te(H)
Air-C 2 H 2 N 2 O-C2H 2 Air-C 2 H 2 Air-C 2 H 2 N 2 O-C2H 2 Air-C 2 H 2 Air-C 2 H 2 N 2 O-C2H 2 Ar-H 2 N 2 O-C2H 2 N 2 O-C2H 2 Air-C 2 H 2 Air-C 2 H 2 N 2 O-C2H 2 N 2 O-C2H 2 Air-C 2 H 2 N 2 O-C2H 2 N 2 O-C2H 2 Air-C 2 H 2 N 2 O-C2H 2 Air-C 2 H 2 N 2 O-C2H 2 N 2 O-C2H 2 N 2 O-C2H 2 N 2 O-C2H 2 Air-C 2 H 2 N 2 O-C2H 2 Air-C 2 H 2 Air-C 2 H 2 Air-C 2 H 2 Air-C 2 H 2 Air-C 2 H 2 Air-C 2 H 2
30
12
15
15 12
15
temperature of the target element. Molecular absorption occurs when NaCl or other salts in the sample evaporate in the molecular form. Absorption of salt molecules occur in the wide wavelength range of the ultraviolet region. (Refer to Fig. 5.1)
Fig. 5.1 Molecular absorption by sodium compound
In measurement of the element having the analysis line in the wavelength range shown in Fig. 5.1, the sum of atomic absorption and molecular absorption is measured to give a big plus error. Such molecular absorption becomes an issue in respect to the percent salt concentration in the flame analysis, and becomes an issue in respect to several hundred ppm salt concentrations. The molecular absorbance is called the background absorbance, and the sum of atomic absorbance and background absorbance is measured by light from the hollow cathode lamp source. If background absorption can only be measured by some means, atomic absorption can only be obtained by doing subtraction of both measured values. Background absorption can be corrected by the following methods. Method by using nearby line At the wavelength slightly shifted from the analysis line of the target element, background absorption occurs, and atomic absorption does not. Therefore, if another hollow cathode lamp,
which gives a nearby spectral line within ±5nm from the wavelength of the target element, only background absorption can be measured. This is the method using the nearby line. A hollow cathode lamp which gives strong light is not always obtained within ±5nm. Even if it is obtained, there is the limitation that atomic absorption cannot occur at the wavelength. Such being the case, it cannot be an accurate background correction method. The method using a continuous light source, as described below, is used as the standard background correction method. Because it has no such limitation, a highly accurate correction can be made.
Method using a continuous light source If a light source, such as a deuterium lamp, is continuously giving off light in the wavelength range of 190 to 430 nm, an accurate background correction can be made. When the wavelength of the spectroscope is set at the wavelength of the target element, the wide wavelength band can be observed in the light of the deuterium lamp. As mentioned before, molecular absorption occurs in a wide wavelength range, and absorption occurs within this region. Also an apparent decrease in the light intensity is observed. The target atom absorbs the light in the center of the wavelength only, and no absorption at a distance of 1/100 angstrom or more. Due to the intensity of the deuterium lamp, the greater part of the light observed is not absorbed. The above shows that only molecular absorption (background absorption) can be measured if the deuterium lamp is used. Thus, atomic absorption can only be measured if subtraction is done from the absorption of the hollow cathode lamp (sum of atomic absorption and background absorption).
Method by self reversal Background correction by self reversal method uses a hollow cathode lamp for self reversal (200-38456-XX) and lights the lamp by supplying high current combined with low current. A in Fig. 5.2 shows lamp current waveform and high current IH is set at 300 to 600mA and low current IL at 60mA or less. It lights the lamp at a frequency of 100 Hz. The spectrum emitted by the lamp current IH becomes two peaks (self reverse) with the depressed center as shown in the upper left of B in Fig. 5.2. This is due to internal absorption by a great deal of atomic clouds scattered from the hollow cathode lamp, as the half-width spreads.