Atomic Spectroscopy Outline PDF

Sodium Energy Level Diagram Atomic Emission Spectra  Atomic Absorption Spectra Sodium atoms are capable of absorbing radiation of wavelengths characteristics of electronic transitions from 3S state to higher excited states. Sharp absorption peaks at 5890, 5896, 3302 and 3303 A are observed experi...

Sodium Energy Level Diagram Atomic Emission Spectra  Atomic Absorption Spectra Sodium atoms are capable of absorbing radiation of wavelengths characteristics of electronic transitions from 3S state to higher excited states. Sharp absorption peaks at 5890, 5896, 3302 and 3303 A are observed experimentally. They are corresponding to 3S to 3P and 3S to 4P. It is seen that nonresonance absorption due to the 3P to 5S transitions so weak as to go undetected because the number of sodium atoms in the 3P state is generally small at the temperature of flame. Atomic Fluorescence Spectra Atomic Lines Width  Reasons for Line Spectra Broadening 1. Natural Broadening (Lorentzian Broadening): - Heisenberg Uncertainty Principle: 2. Collisional Broadening: - Pressure Broadening: In dense gases or plasmas, collisions between atoms or molecules can perturb the energy levels of the atoms, leading to broadening of spectral lines. This effect is more pronounced at higher pressures. - Impact Broadening: Collisions between atoms or particles can result in the transfer of energy between them, causing a broadening of spectral lines. This is particularly significant in environments with high collision rates, such as in flames or plasmas. -Quenching: Certain collisions, known as quenching collisions, can transfer energy between atoms, altering their energy levels and broadening spectral lines. 3. Doppler Broadening: 4. Stark Broadening: In the presence of an external electric field, such as in plasmas, the energy levels of atoms can be perturbed, leading to broadening of spectral lines. 5. Van der Waals Broadening: Weak interactions between atoms or molecules due to van der Waals forces can lead to broadening of spectral lines. 6. Instrumental Broadening: In spectroscopic instruments, there are limitations on the resolution, which can lead to apparent broadening of spectral lines.  Major Classes of Spectrochemical Methods Class Radiant Power Concentration Type of Methods Measured Relationship Emission Emitted, Pe or Ie Pe = kc Atomic emission Luminescence Luminescent, Pl or Il Pl = kc Atomic and molecular fluorescence, phosphorescence, and chemiluminescence Scattering Scattered, Psc or Isc Psc = kc Raman scattering, turbidimetry, and nephelometry Absorption Incident, Po or Io -log(P/Po) = kc Atomic and molecular transmitted, Po or Io absorption S=kP Signal (S), P (Radiant Power) k is a constant. Many detectors exhibit a small, constant response in the absence of radiation, which we referred to as dark current (kd). S = kP + kd Spectrochemical instruments are equipped with a compensating circuit that reduces dark current to zero. For such instruments equation is S=kP Power of the radiation emitted by an analyte, after excitation is ordinarily directly proportional to the analyte concentration c. (Pe = kc) So by combination of above two equations S=k'c k' is a constant that can be evaluated by exciting standard of analyte and by measuring S (signal). In case of Absorption methods, it requires two power measurements, beam before analyte and beam after analyte. The two terms are absorbance and transmittance. Transmittance Transmittance is a measure of the fraction of incident light that passes through a sample without being absorbed. Transmittance values range from 0 to 1, where T=0 indicates complete absorption (no light transmitted) and T=1 indicates complete transmission (all incident light transmitted through the sample without absorption). Absorbance Absorbance (A), often denoted as optical density, is a logarithmic measure of the ratio of the radiant power of the incident light (Po​) to the radiant power of the light transmitted through the sample (P): Beer’s Law Beer's Law, also known as the Beer-Lambert Law, is a fundamental principle in spectroscopy that describes the relationship between the concentration of a solution and the amount of light it absorbs. For monochromatic radiation, absorption is directly proportional to the part length (b) through the medium and the concentration (c) of the absorbing species. A=abc a= proportionality constant, the magnitude of “a” will clearly depend upon the units used for “b” and “c”. “b” is often given in terms of centimeters and c in grams per liter. Absorptivity then has units of L g-1 cm-1. When the concentration is expressed in moles per liter and the cell length is in cm, absorptivity is called the molar absorptivity, and given symbol is ε (epsilon) A= εbc ε units L mol-1 cm-1 Beer’s law also applies to a medium containing more than one kind of absorbing substance. Limitations to Beer’s Law 1. Real limitations to Beer’s Law: Beer's Law assumes a linear relationship between absorbance and concentration over a limited range of concentrations. 2. Chemical deviations: Analyte dissociates, associates, or reacts with a solvent to produce a product having a different absorption spectrum from the analyte. 3. Instrumental deviations: Beer’s law observe only with truly monochromatic radiation. 4. Path Length: Beer's Law assumes a constant path length for the light passing through the sample. 5. Temperature and Pressure Effects: 6. Non-Homogeneous Samples: Main Components of Optical Instruments 1. Sources of Radiation 2. Wavelength Selectors 3. Sample Containers 4. Radiation Transducers 5. Signal Processors and Readouts  Atomic Emission Spectroscopy Atomic emission spectroscopy (AES) is a technique used to analyze the elemental composition of a sample by measuring the characteristic wavelengths of light emitted by atoms when they are excited. The main types: 1. Flame Emission Spectroscopy (FES): 2. Inductively Coupled Plasma Emission Spectroscopy (ICP-OES): 3. Arc Spark Emission Spectroscopy: 4. Glow Discharge Emission Spectroscopy (GD-ES): 1. Flame Emission Spectroscopy (FES): - In FES, the sample is introduced into a flame where it is vaporized and atomized. - The atoms in the vaporized sample are excited by the flame's heat. - As the excited atoms return to their ground state, they emit light at characteristic wavelengths. - This emitted light is then analyzed to determine the elemental composition of the sample. 2. Inductively Coupled Plasma Emission Spectroscopy (ICP-OES): - ICP-OES utilizes an inductively coupled plasma (ICP) as the excitation source. - The sample is introduced into the ICP, where it is vaporized and atomized at very high temperatures. - The high temperature of the plasma excites the atoms, causing them to emit light at characteristic wavelengths. - The emitted light is then dispersed by a spectrometer and detected to identify the elemental composition of the sample. (a) Czerney-Turner Grating monochromator (b) Bunsen prism monochromator 3. Arc Spark Emission Spectroscopy: - This technique involves generating a high-voltage electrical discharge (arc) between two electrodes, one of which is made of the sample material. Sample is kept at anode. - The intense heat of the arc vaporizes and excites the atoms in the sample. - As the excited atoms return to their ground state, they emit light at characteristic wavelengths. - The emitted light is then analyzed to identify the elements present in the sample. The terms "arc" and "spark" are often used interchangeably, but they refer to different phenomena. 1. Arc: An arc is a sustained electrical discharge that occurs between two electrodes. It is characterized by a relatively high current and a stable, continuous discharge. The arc is typically formed when a sufficient potential difference (voltage) is applied between the electrodes, causing ionization of the surrounding medium (usually air). Arcs can generate extremely high temperatures, often in the range of thousands of degrees Celsius, leading to the vaporization and excitation of the sample material. 2. Spark: A spark is a brief, high-energy electrical discharge that occurs between two electrodes. It is characterized by a rapid increase in voltage followed by a sudden release of energy. Sparks typically have a shorter duration compared to arcs and are often associated with higher peak currents. 4. Glow Discharge Emission Spectroscopy (GD-ES): - GD-ES involves applying a high voltage to a sample in a low-pressure gas atmosphere. - This creates a glow discharge, which excites the atoms in the sample. - The excited atoms emit light at characteristic wavelengths, which is then analyzed to determine the elemental composition of the sample. - GD-ES is often used for depth profiling of solid samples. The Glow Discharge Technique A glow discharge device is a versatile source that performs both sample introduction sample atomization simultaneously. Briefly, a glow discharge takes place in a low-pressure atmosphere (1 to 10 torr) of argon gas between a pair of electrodes maintained at a dc potential of 250 to 1000 V. The applied potential causes the argon gas to break down into positively charged argon ions and electrons. The electric field accelerates the argon ions to the cathode surface that contains the sample. Ejection of the neutral sample atoms occurs by a process called sputtering. The atomic vapor produced in a glow discharge is made up of a mixture of atoms and ions that can be determined by atomic absorption of fluorescence or by mass spectrometry. In addition, a fraction of the atomized species present in the vapor is in an excited state. Relaxation of the excited species produces a low-intensity glow that can be used for optical emission measurements.  Atomic Absorption Spectrophotometry Schematic diagrams of Atomic Absorption Spectrophotometer Modulated Power Source In atomic absorption instruments, it is necessary to eliminate interferences caused by emission of radiation by the flame. Much of the emitted radiation is removed by the monochromator. Nevertheless, emitted radiation corresponding in wavelength to the monochromator setting is inevitably present in the flame due to excitation and emission by analyte atoms. In order to eliminate the effect of flame emission, it is necessary to modulate the output of the source so that its intensity fluctuates at a constant frequency. The detector then receives two types of signal, an alternating one from the source and a continuous one from the flame. These signals are converted to the corresponding types of electrical response. Hollow Cathode Lamp A hollow cathode lamp (HCL) is a type of light source commonly used in atomic absorption spectrophotometry (AAS). A hollow cathode lamp operates based on the principle of electrical discharge through a gas. It consists of a cylindrical metal tube (the cathode) with a hollow center, filled with an inert gas such as argon or neon, and cathode is made out of metal element to be analyzed. When a high voltage is applied across the electrodes (cathode and anode) in the lamp, inert gas is ionized and moves to cathode and a discharge occurs through the inert gas. This discharge causes the atoms of the analyte element within the lamp to become excited and subsequently emit characteristic light. The light emitted by the excited atoms corresponds to specific wavelengths characteristic of the element present in the lamp. These wavelengths are unique to each element and can be used for excitation of analyte element in AAS. In an atomic absorption spectrophotometer (AAS), the burner plays a crucial role in atomizing the sample and generating a vapor phase of the analyte element for analysis. The primary function of the burner in AAS is to convert the sample solution into a fine aerosol of vaporized atoms. This atomization process is essential for the absorption of light by the analyte atoms in the gas phase. The burner consists of several components: Nebulizer: This component is responsible for generating a fine spray or aerosol of the sample solution. It typically utilizes pneumatic or ultrasonic mechanisms to break the liquid into small droplets. Mixing Chamber: The nebulized sample is mixed with a fuel gas (usually acetylene or natural gas) and an oxidant gas (usually air or oxygen) in this chamber. The mixing of these gases ensures proper combustion and atomization of the sample. Depending on the specific requirements of the analysis, different types of flames can be employed in AAS. The operation of the burner, including the flow rates of the sample solution, fuel gas, and oxidant gas, as well as the positioning of the burner tip, is critical for achieving optimal atomization and signal intensity. Proper optimization ensures efficient atomization of the sample and enhances the sensitivity and accuracy of the AAS analysis. Graphite Furnace