Atomic Absorption And Emission Spectroscopy. Chapter Four

Chapter Four Atomic Absorption and Emission spectroscopy Atomic ™ spectroscopy is based upon the absorption and emission of electromagnetic radiation by atomic particles. This methods are used for the qualitative and quantitative determination (smaller concentration/ppm to ppb) of more than 70 elements through rapid, suitable, and usually of high selectivity). In atomic emission: atoms decay back to lower levels by emitting light without the presence of a radiation source. atomic absorption: atoms are promoted to a higher energy level, provided by the high temperature of the atomization sources . Fig: Absorption and Emission Eo: ground level E1,2 Exited state 1 Due to this atomization sources atomic spectroscopic methods can be classification: 2 4.1. Atomic Absorption Spectroscopy Atomic absorption spectrometry involves the absorption of radiant energy by neutral atoms in the gaseous state. If the external source radiation is of the appropriate frequency, it can be absorbed by the analyte atoms in the gaseous state and promote them to excited states. All atomic spectroscopic procedures is atomization, a process in which a sample is volatilized and decomposed in such a way as to produce gas-phase atoms and ions; accomplished by using flames and electrothermal atomizers. ™ Thus, atomic absorption are potentially highly specific because atomic absorption lines are remarkably narrow and because electronic transition energies are unique for each element. 3 For Example: absorption of radiation of 285, 330, and 590 nm excites the single outer electron of sodium from its ground-state 3s energy level to the excited 3p, 4p, and 5p orbital's, respectively. After a few nanoseconds, the excited atoms relax to their ground state by transferring their excess energy to other atoms or molecules in the medium. 4 4.1.1 Instrumentation A single beam atomic absorption spectrometer consists of a sharp-line radiation source characteristic of the element of interest, usually a hollow-cathode lamp (HCL), a solution nebulizer and burner, or an electrically heated furnace, and a monochromator, photomultiplier and recording system. Double Beam AAS Instruments account for instability in the source and have a reference beam from HCL to detector. 5 In flame atomization: the sample is first converted into a fine mist consisting of small droplets of solution in nebulizer assembly. The aerosol mist mixes with the combustion gases in the spray chamber where large droplets condense and run to waste. The resulting homogeneous mixture of sample droplets and gases passes to the burner where the flame’s thermal energy desolvates the aerosol mist to a dry aerosol of small solid particles aerosol mist to a dry aerosol of small, solid particles. Subsequently, thermal energy volatilizes the particles, producing a vapor consisting of molecular species, ionic species, and free atoms. 6 Temperature value (adjusted using different fuel/oxidant ratios) and The flame consists of two principal consistency are important zones or cones. The inner cone or primary reaction zone is confined to the region just above the burner orifice; it is here that combustion, atomization and that combustion, atomization and thermal excitation occur. Above the primary zone is the outer cone or secondary reaction zone. Thermal energy in flame atomization is provided by the combustion of a fuel–oxidant mixture. The acetylene – air and acetylene – nitrous oxide flames are used most frequently. 7 Electrothermal Atomizers (ETAAS, GFAAS): The sample is contained in a graphite furnace heated by passing an ™ electrical current through it. To prevent oxidation of the furnace, it is sheathed in gas (usually Ar). The sample is introduced as a drop (usually 10-50 uL) through a small-diameter hole located at the top of the tube. The furnace goes through several steps… Drying: using a current that raises the temperature of the graphite tube to about 110 °C. Drying leaves the sample as a solid residue. Ashing: using a current that raises the temperature to 350–1200 °C, and at these temperatures, any organic material in the sample is converted to CO2 and H2O, and volatile inorganic materials are vaporized. These gases are removed by the inert gas flow. 8 Atomization: the sample is atomized by rapidly increasing the temperature to 2000–3000 °C. Cleanout: waste is blown out with a blast of Ar. (quick ramp up to 3500ºC ) to 3500°C). ™ The analyte’s concentration in the resulting vapor phase may be as much as 1000 times greater than that produced by flame atomization. ™ The improvement in sensitivity, and the resulting improvement in detection limits, is offset by a significant decrease in precision. Radiation Sources for FAAS and ETAAS Hollow Cathode Lamps (HCL): element specific Hollow Cathode Lamps (HCL) ™ are the main source element specific constructed of the same element analyzing. HLC consists of a cathode and anode enclosed within a glass tube filled with a ™ low pressure of Ne or Ar. Since emission or absorption is quantized, this is the same wavelength of light that the analytes will absorb! ™ When a potential is applied across the electrodes the filler gas is ionized (Multielement lamps are available). 9 The positively charged ions collide with the negatively charged cathode, dislodging, or “sputtering,” atoms from the cathode’s surface. Some of the sputtered atoms are in the excited state and emit radiation characteristic of the metal from which the cathode was manufactured. Applications of Atomic Absorption Spectrometry ™ widely used techniques for the determination of metals at trace levels in solution. ™ Only for the routine determination of alkali and alkaline earth metals, is flame photometry usually preferred. Over sixty elements can be determined in almost any matrix by atomic absorption. heavy metals in body fluids, polluted waters, foodstuffs, soft drinks and beer drinks, analysis of metallurgical and geochemical samples, many metals in soils, petroleum products and plastics. Detection limits generally lie in the range 100– 0.1 ppb but these can be improved by chemical pre-concentration procedures involving solvent extraction or ion exchange. 10 4.2 Atomic Emission Spectroscopy ™ Electromagnetic radiation is often emitted when excited particles ™ Electromagnetic radiation is often emitted when excited particles (ions, atoms or molecules) return to lower-energy levels or to their ground states. ™ Atomic emission occurs when a valence electron in a higher-energy atomic orbital returns to a lower-energy atomic orbital. In atomic emission spectroscopy, excitation of analyte atoms is brought about by a flame, an electrical arc or spark, a plasma, and a high-powered laser. ™ During the last decade, however, argon plasma sources have been developed that combine many of the best features of flame sources with the attributes of the classical arc and spark. I™ nductively coupled plasmas are at least 2X as hot as flames or furnaces. Hot enough to excite most elements so they emit light. 6000- 10000 K are common plasma temperatures. Hot enough to prevent the formation of most interferences, break down oxides and eliminate most molecular spectral interferences 11 4.2.1 Instrumentation E ™mission spectrometer consists a sample nebulizer, grating monochromator, photomultiplier detection system and microprocessor controller. Source of energy for atomic emission spectroscopy Stimulus Source for Atomic Emission is plasma. ™ ™ Stimulus Source for Atomic Emission is plasma. ™ A plasma is an electrical conducting gaseous mixture containing a significant concentration of cations and electrons. T ™he concentrations of the two are such that the net charge approaches zero. A ™rgon gas, one of the most widely used plasma species, provides temperature as high as 10,000 K. 12 Three types of plasma considered for atomic emission spectroscopy: ™ Inductively coupled plasma (ICP): the most popular  Direct current plasma (DCP) Direct current plasma (DCP) Microwave induced plasma (MIP). Sample Introduction for plasma sources For the majority of For the majority of applications, the sample is taken into solution and introduced into the plasma as an aerosol in the argon stream. The sample solution is pumped by a peristaltic pump at a fixed rate and converted into an aerosol by a nebulizer. Most (>99%) are collected as Most (>99%) are collected as waste (small fraction that reach the plasma have been) de-solvated to a great extent). 13 Wavelength Selectors for ICP-AES Must have greater resolving power than those for AAS methods (plasmas are ™ hotter, therefore the spectra are more complex). Different detectors for different wavelengths (PMTs still #1 choice, but CCD ™ arrays also common) but CCD arrays also common). The light emitted by an ICP is also more intense! Slits must have greater ™ adjustability. g y Desirable Properties of an emission Spectrometer High resolution (0 01 nm or λ/Δλ > 100 000) Rapid signal acquisition and recovery Low stray light Wide dynamic range (>10 6 ) Accurate and precise wavelength identification and selection Precise intensity readings (<1% relative standard deviation at 500 x the detection limit) 500 x the detection limit) High stability with respect to environmental changes Easy background corrections Computerized operation: readout, storage data manipulation, 14 Applications of Plasma Emission Spectrometry T ™he potential for the employment of plasma emission spectrometry is enormous and it is finding use in almost every field where trace element analysis is carried out. ™ Some seventy elements, including most metals and some nonmetals, such as phosphorus and carbon, may be determined individually or in parallel. ™ The multi-element capability and wide dynamic range of the method are of great value. ™ Plasma emission spectrometry is well established as a routine method of analysis in areas of: Analytical chemistry (Rocks, soils, waters and biological tissue) Geochemistry Quality control of potable waters and pollution studies. 15 CHAPTER FIVE Ultraviolet-visible Spectroscopy 5.1 Introduction Ultraviolet-visible Spectroscopy is based upon absorption of electromagnetic radiation in the visible and ultraviolet regions of the spectrum resulting in changes in the electronic structure of ions and molecules. When ions or molecules are exposed to light having an energy that matches a possible electronic transition or charge transfer energy, some of the light energy will be absorbed as the electron is promoted to a higher energy orbital or as a charge is transferred from donor atom to accepter atom within the ion or molecule. The light having an energy that matches a possible electronic transition or charge transfer energy falls in Ultraviolet-visible region of electromagnetic radiation 16 Electron Transition Absorption Organic molecules and inorganic anions (eg. CO32–) have occupied molecular orbitals - sigma bonding (σ), pi bonding (𝜋) and nonbonding (n), and unoccupied molecular orbitals - sigma antibonding (σ*) and pi antibonding (𝜋*). The unoccupied molecular orbitals often lie close enough in energy to occupied molecular orbitals that the transition of valence electron from an occupied to an unoccupied orbital is possible. 17 Many transition metal ions, such as Cu2+ and Co2+, form solutions that are colored because the metal ion absorbs visible light. The transitions giving rise to this absorption are due to valence electrons in the metal ion’s d-orbitals. For a free metal ion, the five d-orbitals are of equal energy. In the presence of a complexing ligand or solvent molecule, however, the dorbitals split into two or more groups that differ in energy. Example: In the octahedral complex Cu(H2O)62+, the six water molecules perturb the d-orbitals into two groups. The resulting d–d transitions for transition metal ions are relatively weak. 18 Charge- Transfer Absorption: Many inorganic and organic complexes exhibit this type of absorption and are therefore called charge-transfer complexes. A charge-transfer complex consists of an electron-donor group bonded to an electron acceptor. When this product absorbs radiation, an electron from the donor is transferred to an orbital that is largely associated with the acceptor. Familiar examples of charge-transfer complexes include  the iron(llI)/thiocyanate complex responsible for the color of red.  the ferro/ferricyanide complex responsible for the color of Prussian blue. Note. The excited state is thus the product of a kind of internal redox process. This behavior differs from that of an organic chromophore, in which the excited electron is in a molecular orbital that is shared by two or more atoms. 19 5.2 Types of Electron Transitions and their Absorption Characteristics   * and   * transitions: These transitions are of such a high energy that they absorb ultraviolet energy at very short wavelengths (in vacuum UV (max <150 nm)shorter than the wavelengths that are experimentally accessible using typical spectrophotometers. Alkanes - electronic transitions possible are of the σ→σ* type. n  * and   * transitions: They are also rather high-energy transitions, but they do absorb radiation that lies within an experimentally accessible rang (max in the 150-250 nm region)below the cutoff points for the common solvents, so they are not observed in solution spectra. Alcohols and amines absorb in the range from 175 to 200 nm, while organic thiols and sulfides absorb between 200 and 220 nm transitions of the n→σ * type become important. 20 n  * and   * transitions: most common transitions observed in organic molecular UV-Vis, observed in compounds with lone pairs and multiple bonds with max = 200-600 nm. Alkenes and Alkynes. With unsaturated molecules, →* transitions become possible. Alkenes absorb around 175 nm, and alkynes absorb around 170 nm. Carbonyl Compounds. Unsaturated molecules that contain atoms such as oxygen or nitrogen may also undergo n→ * transitions. The typical carbonyl compound undergoes an n → * transition around 280 to 290 nm (ε = 15). Most n → * transitions are forbidden and hence are of low intensity. Carbonyl compounds also have a → * transition at about 188 nm (ε = 900). 21 Electronic Transitions Involving σ, n, and 𝜋 Molecular Orbitals Transition σ → σ* and σ → 𝜋* n → σ* and 𝜋 → σ* 𝜋 → 𝜋* n → 𝜋* Wavelength Range (nm) < 150 150–250 200–500 250–600 Examples C ––C, C––H H2O, CH3OH, CH3Cl C=C, C=O, C=N, C≡C C=O, C=N, N=N, N=O Of these transitions, the most important are the n → 𝜋* and 𝜋 → 𝜋*, because they involve functional groups, or double/triple bonds that are characteristic of the analyte and absorb in the ultraviolet or visible regions that are easily accessible. Why? electrons involved in double and triple bonds of organic molecules are not as strongly held and are therefore more easily excited by radiation with wavelengths in the ultraviolet or visible regions. exhibit useful absorption peaks. Note. Unsaturated organic functional groups that absorb in the ultraviolet or visible regions are known as chromophores. 22 5.3 Absorption characteristics of some simple isolated chromopores Class Trasitio max Log  Class n (nm) Trasitio n max( Log nm)  R‒OH n→σ* 180 2.3 R‒NO2 n→* 271 <1.0 R‒O‒R n→σ* 180 3.5 R‒CHO →* 190 2.0 R‒NH2 n→σ* 190 3.5 n→* 290 1.0 R‒SH n→σ* 210 3.0 →* 180 3.0 R2C=CR2 →* 175 3.0 n→* 280 1.5 R‒C≡C‒R →* 170 3.0 RCOOH n→* 205 1.5 R‒C≡N n→* 160 <1.0 RCOOR’ n→* 205 1.5 RN=NR n→* 340 <1.0 RCONH2 n→* 210 1.5 R2CO 23 You may notice that these simple chromophores nearly all absorb at approximately the same wavelength (160 to 210 nm). The attachment of substituent groups in place of hydrogen on a basic chromophore structure changes the position and intensity of an absorption band of the chromophore. Substituents that increase the intensity of the absorption, and possibly the wavelength, are called auxochromes. Typical auxochromes include methyl, hydroxyl, alkoxy, halogen, and amino groups. Other substituents may have any of four kinds of effects on the absorption: 1. Bathochromic shift (red shift)—a shift to lower energy or longer wavelength. 2. Hypsochromic shift (blue shift)—a shift to higher energy or shorter wavelength. 3. Hyperchromic effect—an increase in intensity. 4. Hypochromic effect—a decrease in intensity. 24 The effect of conjugation on alkenes The bathochromic shift that results from an increase in the length of a conjugated system implies that an increase in conjugation decreases the energy required for electronic excitation. In general, conjugated dienes exhibit an intense band (ε = 20,000 to 26,000) in the region from 217 to 245 nm, owing to a →* transition. 175 217 258 The position of this band appears to be quite insensitive to the nature of the solvent. 25 By studying a vast number of dienes of each type, Woodward and Fieser devised an empirical correlation of structural variations that enables us to predict the wavelength (max for π→* ) at which a conjugated diene will absorb. Homoannular, base 253 nm Acyclic base 217 nm Attached group Homoannul ar (cisoid) Hetroannula r (transoid) Parent  = 253 nm  = 214 nm Double bond extending conjugation +30 +30 Alkyl substituent or ring residue +5 +5 Exocyclic double bond +5 +5 O-Acyl (such as –OCOCH3) 0 0 S-alkyl +30 +30 O-alkyl +6 +6 NR2 +60 +60 Cl, Br +5 +5 heteroannular, base 214 nm 26 27 Solvents Solvent must be UV grade (great sensitivity to impurities with double bonds). Solvents for UV must show high energy cutoffs. Choose solvents that are UV transparent even though a dual beam goes through a solvent blank. Dioxane 220 THF 220 210 CH2Cl2 235 Ether 210 CHCl3 245 EtOH 210 CCl4 265 benzene 280 Acetone 300 Water 205 CH3CN 210 C6H12 Hexane MeOH 210 210 28 5.4 Instrumentation The simplest spectrophotometer is a single-beam filter-photometer or spectrophotometer incorporating prism or grating monochromator, phototube photomultiplier or diode array, glass, quartz or plastic cells. Modern Uv-vis spectrometers are double beam spectrophotometers Matched quartz cuvettes Sample in solution at ca. 10-5 M. System protects PM tube from stray light D2 lamp-UV Tungsten lamp-Vis Double Beam makes it a difference technique 29 5.5 Quantitative application P0(power in) P (power out) B(path through sample) • Transmittance: T = P/P0 • Absorbance: A = -log10 T = log10 P0/P 30 Beer-Lambert Law The Beer-Lambert Law (a.k.a. Beer’s Law): For a single species A = ebc The total absorbance of a solution at any given wavelength is equal to the sum of the absorbance's of the individual components in the solution. A = A1 + A2 + --- +An = ε1bC1 + ε2bC2 + --- + εnbCn This relationship makes it possible in principle to determine the concentrations of the individual components of a mixture even if their spectra overlap completely. A1 = εM1bCM1 + εN1bCN1 A2 = εM2bCM2 + εN2bCN2 31 6.2 Instrumentation Two types of instruments for IR are available in modern laboratories: (1) Dispersive Infrared Spectrometers contain dispersive element (grating or prism) or monochromator to select a small bands of wavelengths. dispersive grating spectrophotometers that are used primarily for qualitative work Dispersive instruments are said to record a spectrum in the frequency domain. (2) Fourier-Transform Infrared Spectrometers contain no dispersing element and all wave- lengths are detected and measured simultaneously using a interferometer usually Michelson interferometer are suited to both qualitative and quantitative infrared measurements. record spactrum in the interferogram that is a time domain spectrum. A mathematical operation known as a Fourier transform (FT) can separate the individual absorption frequencies from the interferogram, producing a spectrum virtually identical to that obtained with a dispersive spectrometer. 32 33