General Spectroscopy (AES, ICP-AES, AAS, Jablonski) 2024-2025 PDF

UCB009-CHEMISTRY Introduction to Spectroscopy (AES, ICP-AES, AAS, Jablonski) I. A brief outline of General Spectroscopy: Introduction to spectroscopy Daily life applications Spectroscopy: principles and observables Electromagnetic radiation Types of spectroscopy Principles...

UCB009-CHEMISTRY Introduction to Spectroscopy (AES, ICP-AES, AAS, Jablonski) I. A brief outline of General Spectroscopy: Introduction to spectroscopy Daily life applications Spectroscopy: principles and observables Electromagnetic radiation Types of spectroscopy Principles and observables in spectroscopy Light-matter interactions Absorption and emission spectra Transitions in atoms and molecules upon light irradiation II. A brief outline of AES, ICP-AES, AAS and Jablonski diagram: Introduction to Atomic emission spectroscopy (AES) and its principle Sequence of events in the flame and effect of temperature Fuel-oxidant combinations and limitations of AES Introduction to ICP-AES and its working principle. Advantages of ICP-AES Introduction to Atomic absorption spectroscopy (AAS) and its principle A brief introduction to hollow cathode lamp Differences between AES and AAS, Advantages and disadvantages of AAS Introduction to fluorescence, phosphorescence and other photophysical processes Introduction to Jablonski diagram, processes and timescales 2024-2025 ODD Semester Differences between fluorescence and phosphorescence UCB009 (Chemistry) Applications of fluorescence  What is Spectroscopy? It is the branch of science which deals with the study of interaction of electromagnetic radiation with matter To know the molecular structure (qualitative analysis) To determine the quantity of species present (quantitative analysis) 2024-2025 ODD Semester UCB009 (Chemistry)  What is Spectroscopy? To study changes in the properties of light when it interacts with matter. 2024-2025 ODD Semester UCB009 (Chemistry) Electromagnetic Spectrum: 2024-2025 ODD Semester UCB009 (Chemistry) Electromagnetic Radiation: Revision of fundamental concepts Frequency (ν): It is defined as the number of times electrical field radiation oscillates in one second. The unit for frequency is Hertz (Hz). 1 Hz = 1 cycle per second Wavelength (λ): It is the distance between two nearest parts of the wave in the same phase i.e. distance between two nearest crests or troughs. The relationship between wavelength & frequency can be written as: c = ν λ As photon is subjected to energy, so E = h ν = h c / λ 2024-2025 ODD Semester UCB009 (Chemistry) Types of spectroscopy Atomic Absorption Flame Emission UV-Visible Fluorescence IR Phosphorescence Line spectrum Band spectrum 2024-2025 ODD Semester UCB009 (Chemistry)  Spectroscopy: Principles and Observables The principle is based on the measurement of the spectrum of a sample containing atoms/molecules. Spectrometer is an instrument used to measure/observe the spectrum of a sample. Spectrum is a graph of the intensity of absorbed or emitted radiation by sample versus frequency (ν) or wavelength (λ). 2024-2025 ODD Semester UCB009 (Chemistry) How do we study changes in the properties of light when it interacts with matter? 2024-2025 ODD Semester UCB009 (Chemistry) Atomic absorption and emission spectra Excited State Ground State Electromagnetic Radiation and Matter: Interaction Atomic Transitions Electronic Transitions Line spectrum Intensity (A.U.) Valence Electrons Absorption h Wavelength (nm) Ground state Emission h Excited state Ground state Electromagnetic Radiation and Matter: Interaction Molecular Orbitals Molecular Transitions ❖ Atomic Orbitals ❖ Overlap Band spectrum Intensity (A.U.) Excited state Energy Wavelength (nm) Eelectronic Erotational Ground state Evibrational Note: This will be discussed in detail later during molecular spectroscopy Atomic Emission Spectroscopy (AES) 2024-2025 ODD Semester UCB009 (Chemistry) Atomic emission spectroscopy: Introduction Atomic emission spectroscopy is a special area of emission spectroscopy in which a flame/spark/plasma is used to excite the atoms. (Note: if flame is used as an excitation source, then it is known as flame emission spectroscopy (FES)) For a few elements, such as the alkali metals Na and K, the thermal energy is hot enough to not only produce ground-state atoms but also raise some of the atoms to an excited electronic state. So flame emission spectroscopy is used for the detection of alkali metals and some of the alkaline earth metals. In Flame emission spectroscopy, only one element can be detected at a time in the presence of other elements. 2024-2025 ODD Semester UCB009 (Chemistry)  Flame emission spectroscopy: Principle Absorption of heat energy by ground state atoms present in the flame results in the excitation of valence electrons of atoms. This valence electrons come back to the ground state with the emission of a photons. Wavelength and intensity of emitted photons help in qualitative and quantitative analysis of the sample. E2 E = h Measure the emitted light E1 2024-2025 ODD Semester UCB009 (Chemistry) Sequence of events in FES: Nebulization Desolvation Volatilization Sample solution Spray Heat Dry aerosol Free atoms Heat Nebulization: The solution of the metal salt is sprayed into the flame. Desolvation: The solvent evaporates, leaving the finely powdered salt. Sublimation: Vaporization of the salt. Occurs in flame Atomization: Conversion of ions into free gaseous atoms. Excitation: The valence electron is raised to a higher energy state. Relaxation & Emission: The excited electrons return to the ground state, and light is emitted. Measurement: The wavelength and intensity of the emitted light are measured. 2024-2025 ODD Semester UCB009 (Chemistry) Effect of temperature on FES: Boltzmann Distribution : N*/ N0 = A e - ∆E/k T N* : Number of atoms in an excited state (Intensity) N0 : Number of atoms in the ground state ∆E : E1-E0 = Difference between two energy states k : Boltzmann constant T : Temperature of flame A : Constant for particular atom Thus, TEMPERATURE plays an important role in FES. High temperature = High Excitation = High Intensity (Caution) “TEMPERATURE OF THE FLAME” depends on a) Fuel & Oxidant b) Fuel: Oxidant Ratio 2024-2025 Number of atoms in excited state depends on the flame ODD Semester temperature. UCB009 (Chemistry) Fuel and Oxidant combinations: Fuel Oxidant Flame temperature (°C) Propane Air 1900 Propane Oxygen 2800 Hydrogen Air 2100 Hydrogen Oxygen 2800 Acetylene Air 2200 Acetylene Oxygen 3000 Limitations of FES (i) Does not give information about the molecular form of the sample (ii) FES is mainly applicable to alkali & alkaline earth metals (iii) Multiple elements can not be detected simultaneously Note: The second and third limitations, mentioned above, can be overcome if we change the excitation source from flame to plasma. 2024-2025 ODD Semester UCB009 (Chemistry) Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) Is it safe to drink TAP WATER?? NO !!! Because of the presence of different heavy metals/trace elements Basics of AES: Revision of fundamental concepts Atomic emission spectroscopy (AES) or optical emission spectroscopy uses quantitative measurement of the optical signals when atoms relax from an excited state to the ground state to determine analyte concentration. Different excitation sources in AES Plasma Arc/Spark Flame The energy emitted is directly proportional to the concentration of the analyte present in the solution. Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) ❖ ICP-AES utilizes plasma as an excitation source. A plasma is an electrically neutral, highly ionized gas that consists of ions, electrons, and atoms. ❖ Inductively coupled plasma (ICP) is a type of high-temperature plasma generated by electromagnetic induction, usually coupled with argon gas. ❖ The plasma can reach temperatures up to 10,000 Kelvin. ❖ Hot enough to excite most elements. ❖ Hot enough to prevent the formation of most interferences, break down oxides, and eliminate most molecular spectral interferences. ❖ Sensitive approach and most widely applied to determine trace elements. Advantages of ICP-AES It has a wide elemental coverage It has extremely low detection limits (ppt/ppm) or (ng/L to mg/L) Approximately 10 - 40 elements per sample can be analyzed simultaneously Atomic Absorption Spectroscopy (AAS) 2024-2025 ODD Semester UCB009 (Chemistry)  Atomic Absorption Spectroscopy: Introduction Most powerful technique for the determination of trace metals in solution 70-80 elements can be detected Determination can be made in the presence of many other elements but only one element can be detected at a time Wide applications 2024-2025 ODD Semester UCB009 (Chemistry)  Atomic Absorption Spectroscopy: Principle A sample solution containing known metal ions, when introduced into the flame and irradiated with light of their own specific wavelength, will absorb light proportional to the density (concentration) of ground state atoms in the flame. Flame Specific Wavelength (Hollow cathode lamp) Note: Number of gaseous atoms in the ground state is always greater thanODD 2024-2025 99.9% of Semester the total gaseous atoms at any instant inside flame. UCB009 (Chemistry) Basic difference between AAS and FES: Set-up perspective http://faculty.sdmiramar.edu/fgarces/LabMatters/Instruments/AA/AA.htm Recall the difference between AAS and FES: Difference between AAS and FES FES AAS  Advantages of AAS Atoms of a particular element can absorb radiation of their own wavelength – No spectral interference Much larger No. of atoms contribute in the AAS signal. Variation in flame temperature has less effect on absorption intensity 70-80 elements can detected  Disadvantages of AAS A different Hollow cathode lamp for each element is required Elements that form the stable oxides eg. Al, Ti, W, and Mo, do not give very good results  Jablonski energy diagram Fluorescence Phosphorescence & Other Photophysical Processes 2024-2025 ODD Semester UCB009 (Chemistry) The fate of an excited molecule A-A or /\- New Products hv A [A]* Photo Chemcal Processes Molecule Excited New Products Molecule Rearrangements etc. A Molecule Photo Physical Processes A molecule (A) on absorption of desired wavelength of light gets excited to an intermediate excited state [A]*. This short lived excited intermediate state has two possibilities to undergo to achieve the stability. (a) Photochemical Processes – Organic reaction, Rearrangement etc. (Not Part of Course) (b) Photophysical Processes Explained by Jablonski Energy Diagram Jablonski energy diagram S3 IC T3 S2 IC IC T2 S1 IC Absorption T1 Phosphorescence Fluorescence S0 Term symbols for photophysical processes For any molecule in the ground state, spin quantum number s = ½ - ½ = 0 Substituting this value in ground energy state (E0) of molecule as shown below (E0 = 2s+1 = (2 × 0) + 1 = 1, represented by S) For ground state, this energy state is represented by (S0) On absorption of suitable energy, one of the electrons from molecular orbital gets exited to higher energy level with two possible orientations – Parallel or Antiparallel as shown below. Possibility -1 : s = ½ - ½ = 0 Possibility - 2 : s = ½ + ½ = 1 E1 = 2s+1 = (2 × 0) + 1 = 1, E1 = 2s+1 = (2 × 1) + 1 = 3, (S1) (T1) S1 S1 Spin Allowed Spin Forbidden Transition T1 Transition S0 S0 S0 Singlet (S1) Excited State Triplet (T1)Excited State Ground State Anti parallel Spin of electrons Parallel spin of electrons Photophysical processes in the Jablonski diagram 1. Radiative Processes Fluorescence: A process in which an excited molecule comes to ground state, from the same spin state (S1 to S0), by releasing energy in the form of light is called Fluorescence. As per quantum mechanics, this is an allowed transition having a time range of 10-10-10-8s. Phosphorescence: A process in which an excited molecule comes to ground state, from a different spin state (T1 to S0), by releasing energy in the form of light is called Phosphorescence. As per quantum mechanics, this is a forbidden transition having a time range of 10-6-10-3s. Note: Both “Fluorescence & Phosphorescence” fall under a broad classification of a range of well-known radiative processes known as Luminescence. The scope of the present course covers only above mentioned two processes. Photophysical processes in the Jablonski diagram 2. Non-radiative processes It involves transition from S3 → S2 or S2 → S1 or T3 → T2 or T2 → T1. It does not involve emission of any radiation and hence, is called Non-radiative transition. It only involves emission of heat. Internal Conversion (IC): In this process, energy loss occurs in the form of heat. It involves transition from S3 → S2 or S2 → S1 or T3 → T2 or T2 → T1. It occurs in less than 10-11 seconds. Intersystem Crossing (ISC): It involves transition from S3 → T3, S2 → T2 or S1 → T1. Both of these transitions are forbidden. Summary of all photophysical processes Jablonski energy diagram Photophysical Process Lifetime Scale Types of Transition Absorption or Excitation 10-15 s S0 → Sn state Radiative, Spin allowed Internal Conversion (IC) 10-12-10-10 s Sn → Sn-1 state Non-radiative, Spin allowed Intersystem Crossing (ISC) 10-10-10-9 s Sn → Tn state Non-radiative, Spin forbidden Fluorescence 10-10-10-8 s S1 → S0 state Radiative, Spin allowed Phosphorescence 10-6-10-3 s T1 → S0 state Radiative, Spin forbidden Difference between fluorescence & phosphorescence Fluorescence Phosphorescence Fluorescence is the absorption of energy by Phosphorescence is the absorption of energy atoms or molecules followed by immediate by atoms or molecules followed by delayed emission of light or electromagnetic emission of electromagnetic radiation radiation. Fluorescence is fast process. Lifetime is Phosphorescence is delayed process. short compared to phosphorescence Lifetime is much longer compared to fluorescence Emission wavelength of fluorescence Emission wavelength of phosphorescence observed at shorter wavelength compared to observed at longer wavelength compared to phosphorescence fluorescence S1 → S0 state, Spin Allowed Transition T1 → S0 state, Spin Forbidden Transition Fluorescence is observed in solids, liquids. Phosphorescence is observed only in solids. Applications of fluorescence and phosphorescence ❖Applied in Fluorescent lamps (LED). ❖ Spectroscopy/chemical sensors: To determine the concentration of the analyte to a very low detection limit, up to ppb/ppt level. ❖ Useful for many biological applications such as fluorescent labeling and pharmaceutical applications. ❖ Forensic applications ❖As materials for display on electronic devices