Flame Atomic Absorption Dr Lia Asri Reference Skoog, Fundamental of Analytical Chemistry, 9ed, 2013 Atomic Absorption Overview Light Source Hollow-cathode lamp: The cathode contains the element that is analysed. Atomization Desolvation and vaporization of ions or atoms in a sample: high-temperature source such as a flame or graphite furnace ➢ Flame atomic absorption spectroscopy ➢ Graphite furnace atomic absorption spectroscopy 1- Flame atomic absorption spectroscopy: Sample introduction: Flame Atomic Absorption • Flame AA provides a sensitive means for determining some 60 to 70 elements. • The method is well suited for routine measurements by relative inexperienced operators. • Because a different hollow-cathode lamp is required for each element, only a single element can be determined at a time, and this is the major drawback of AA. Process in a Flame AA M* M+ + e_ Ionization Mo M* Excitation MA Mo + A o Atomization Solid Solution Vaporization Region of the Flame for Quantitative Measurements • The optimum region of a flame used in a determination must change from element to element and that the position of the flame with respect to the source must be reproduced closely during calibration and measurement. • Generally, the flame position is adjusted to give a maximum absorbance reading for the element being determined. Region of the Flame for Quantitative Measurements • Figure 28-18 shows the absorbance of three elements as a function of distance above the burner head. • For magnesium and silver, the initial rise in absorbance is a consequence of the longer exposure to the high temperature of the flame, leading to a greater concentration of atoms in the radiation path. • The absorbance for magnesium, however, reaches a maximum near the center of the flame and then falls off as oxidation of the magnesium to magnesium oxide takes place. • Silver does not suffer this effect because it is much more resistant to oxidation. • For chromium, which forms very stable oxides, maximum absorbance is found immediately above the burner. • Chromium oxide formation begins as soon as chromium atoms are formed. Detection Limits and Accuracy The detection limit, DL, is the smallest concentration that can be reported with a certain level of confidence. • Column 2 of Table 28-4 shows detection limits for a number of common elements determined by flame atomic absorption and compares them with results from other atomic spectroscopic methods. • Under usual conditions, the relative error of flame absorption analysis is of the order of 1 to 2%. • With special precautions, this figure can be lowered to a few tenths of one percent. • Note that flame AA detection limits are generally better than flame AE detection limits except for the easily excited alkali metals. Ionisation Interference • M(g) M+(g) + e• A problem in the analysis of alkali metal ions at low flame temperatures and other elements at higher temperatures. • Because alkali metals have the lowest ionisation potentials, they are most extensively ionised in flames. • At 2450 K and a pressure of 0.1 Pa, sodium is 5% ionised. • Potassium is 33% ionised under the same conditions. • Ionised atoms have energy levels which are different to the parent atoms • therefore the analytical signal is reduced. 12 Matrix effects • The amount of sample reaching the flame is dependent on the physical properties of the solution: • viscosity • surface tension • density • solvent vapour pressure. • To avoid differences in the amount of sample and standard reaching the flame, it is necessary that the physical properties of both be matched as closely as possible. 13 2. Graphite furnace atomic absorption spectroscopy Sample holder: graphite tube ➢ Samples are placed directly in the graphite furnace which is then electrically heated. ➢ Beam of light passes through the tube. Basic Graphite Furnace Program THGA transversely heated graphite atomizer Three stages: 1. drying of sample 2. ashing of organic matter 3. vaporization of analyte atoms to burn off organic species that would interfere with the elemental analysis. Basic Graphite Furnace Program THGA Clean Temp Atomization Pyrolysis Dry 1 Dry 2 Time Graphite Flame Advantages Solutions, slurries and solid samples can be analyzed. Much more efficient atomization greater sensitivity Smaller quantities of sample (typically 5 – 50 µL) Provides a reducing environment for easily oxidized elements Inexpensive (equipment, day-to-day running) High sample throughput Easy to use High precision Disadvantages Expensive Low precision Low sample throughput Requires high level of operator skill Only solutions can be analyzed Relatively large sample quantities required (1 – 2 mL) Less sensitivity (compared to graphite furnace) Applications of AAS • Agricultural analysis • soils • plants • Clinical and biochemistry • whole blood, plasma and serum Ca, Mg, Li, Na, K, Cu, Zn, Fe etc. • Metallurgy • ores, metals and alloys 18 Applications of AAS • Lubricating oils • Ba, Ca, Mg and Zn additives • Greases • Li, Na, Ca 19 Applications of AAS • Water and effluents • many elements e.g. Ca, Mg, Fe, Si, Al, Ba • Food • wide range of elements • Animal feedstuffs • Mn, Fe, Co, Cu, Zn, Cr, Se • Medicines • range of elements 20 Problem: Smelter 1. Indikasi polutan: Cr (VI) 2. Apakah memenuhi standar baku mutu limbah untuk dibuang ke lingkungan? 3. Metode? 4. Solusi? 5. Penggunaan adsorbent: efektifitas? 6. Material lain?: efektifitas? 7. Kesimpulan? AAS: Quantitative Analysis Quantitative Analysis • Quantitative analyses are frequently based on external standard calibration. • In atomic absorption, departures from linearity occur more often than in molecular absorption. • analyses should never be based on the measurement of a single standard with the assumption that Beer’s law is being followed • the production of an atomic vapor involves so many uncontrollable variables that the absorbance of at least one standard solution should be measured each time an analysis is performed • Often, two standards are used whose absorbances fall above and below (bracket) the absorbance of the unknown. • Any deviation of the standard from its original calibration value can then be applied as a correction • Standard addition methods, are also used extensively in AAS in an attempt to compensate for differences between the composition of the standards and the unknowns. Quantitative Analysis: Standardization and Calibration • Calibration determines the relationship between the analytical response and the analyte concentration. • This relationship is usually determined by the use of chemical standards. • The standards used can be prepared from purified reagents, if available, or standardized by classical quantitative methods Quantitative Analysis: Standardization and Calibration Standard Calibration ✓ Internal standard method ✓ External standard method ✓ Standard addition method AAS External standard method • A series of standards solutions is prepared separately from the sample • A plot of instrument response versus known analyte concentrations is used to produce a calibration curve Standard Addition Methods • We use the method of standard additions when it is difficult or impossible to duplicate the sample matrix. In general, the sample is “spiked” with a known amount or known amounts of a standard solution of the analyte. • In the single-point standard addition method, two portions of the sample are taken. • One portion is measured as usual, but a known amount of standard analyte solution is added to the second portion. • The responses for the two portions are then used to calculate the unknown concentration assuming a linear relationship between response and analyte concentration • In the multiple additions method, additions of known amounts of standard analyte solution are made to several portions of the sample, and a multiple additions calibration curve is obtained. • The multiple additions method verifies to some extent that the linear relationship between response and analyte concentration holds. • The method of standard additions is a quite powerful method when used properly. • First, there must be a good blank measurement so that extraneous species do not contribute • to the analytical response. Second, the calibration curve for the analyte must be • linear in the sample matrix. The multiple additions method provides a check on this assumption. • A significant disadvantage of the multiple additions method is the extra time • required for making the additions and measurements. The major benefit is the potential • compensation for complex interference effects that may be unknown to the user. Problem: Smelter 1. Indikasi polutan: Cr (VI) 2. Apakah memenuhi standar baku mutu limbah untuk dibuang ke lingkungan? 3. Metode? 4. Solusi? 5. Penggunaan adsorbent: efektifitas? 6. Material lain?: efektifitas? 7. Kesimpulan? Question: AAS The copper in an aqueous sample was determined by atomic absorption flame spectrometry. First, 10.0 mL of the unknown were pipetted into each of five 50.0-mL volumetric flasks. Various volumes of a standard containing 12.2 ppm Cu were added to the flasks, and the solutions were then diluted to volume. a. b. c. d. e. f. Plot absorbance as a function of volume of standard. Derive an expression relating absorbance to the concentrations of standard and unknown (cs and cx) and the volumes of the standards and unknown (Vs and Vx) as well as the volume to which the solutions were diluted (Vt ). Derive expressions for the slope and the intercept of the straight line obtained in (a) in terms of the variables listed in (b). Show that the concentration of the analyte is given by the relationship cx =bcs/mVx, where m and b are the slope and the intercept of the straight line in (a). Determine values for m and b by the method of least squares. Calculate the copper concentration in ppm Cu in the sample using the relationship given in (d).
