Burner Design and Premix Burner Systems PDF

Total consumption burner Burner Design: The total-consumption burner mixes the fuel, oxidant, and sample at the flame's base and combusts at the tip of the burner. Fuel: Typically acetylene. Oxidant: Usually air. Process of Nebulization: Fuel, oxidant, and sample all meet at...

Total consumption burner Burner Design: The total-consumption burner mixes the fuel, oxidant, and sample at the flame's base and combusts at the tip of the burner. Fuel: Typically acetylene. Oxidant: Usually air. Process of Nebulization: Fuel, oxidant, and sample all meet at the flame's base. The sample is drawn into the flame using the Venturi Effect: ○ The support gas (oxidant) creates a partial vacuum above the capillary, which pulls the sample up. ○ This results in a fine spray that mixes with the gases and combusts at the tip of the burner. The Venturi Effect: This effect occurs when fluid pressure decreases as it flows through a narrow section of pipe. Combustion in the Flame: The entire sample is aspirated directly into the flame. All steps—desolvation, atomization, and excitation—occur within the flame. Advantages: Effective for aspirating viscous and high-solids samples, such as undiluted serum or urine. Versatile for both low and high-burning velocity flames. Diagram Pre Mix Burner Premix Burner System: ○ The sample solution is aspirated through a nebulizer, forming a fine aerosol. ○ This aerosol enters a mixing chamber where it combines with fuel and oxidant gases. ○ The mixture then flows to the burner head for combustion and atomization. Venturi Effect in Sample Aspiration: ○ The Venturi effect draws the sample through a capillary tube, using the support gas (oxidant) to create the necessary vacuum. Aerosol Handling: ○ Large droplets condense and drain out as waste. ○ Only fine droplets, about 10% of the total aspirated sample, continue to the flame for atomization. ○ Approximately 90% of the sample does not reach the flame and exits through the mixing chamber as waste. Limitations of Premix Burners: ○ Flame Velocity Limitation: Only low-burning velocity flames are suitable. ○ Risk of Flashback: If the burning velocity exceeds the gas flow rate, the flame can travel back into the burner, causing an explosion (flashback). Diagram and disadvantage Parameter Total-Consumption Burner Premix Burner Fuel, oxidant, and sample Fuel and oxidant are pre-mixed meet at the flame base, before reaching the burner. The Nebulization Process where the Venturi Effect Venturi Effect draws in the draws the sample up, sample, with large droplets creating a fine spray that draining out and only fine combusts at the tip. droplets reaching the flame.  The entire sample is Only fine droplets reach the aspirated, but larger flame, enhancing atomization Sample droplet size droplets limit atomization efficiency. The longer path and atomization efficiency. The short path length allows more complete efficiency length results in incomplete atomization and higher vaporization. sensitivity. Larger droplets may not Fine droplets reduce light fully vaporize, causing light scattering and provide stable scattering and atomization, with less Light scattering measurement interference. dependence on flame height Efficiency is sensitive to and flow adjustments. gas flow rate and flame height. It produces a turbulent, Generates a laminar, non-homogeneous flame homogeneous flame, providing that is suitable for flame stable conditions needed for Flame homogeneity photometry but less ideal consistent atomic absorption for atomic absorption due measurements. to instability. Combustion is noisy, often Operates quietly, reducing noise similar to a jet engine, interference and creating a more Noise causing interference for stable environment for analysis. both the detector and the operator. Electrothermal atomization Graphite furnace technique A basic graphite furnace atomizer includes a graphite tube, electrical contacts, a water-cooled housing, and inert gas controls. The graphite tube, positioned in the optical path of the spectrometer, serves as both the heating element and the sample cell. A small sample (5-50 microliters) is placed inside the tube, and heated when an electric current flows through it, provided by contacts at each end of the tube. This setup is enclosed within a water-cooled housing with quartz windows that allow light to pass through. Argon gas flows outside and inside the tube to prevent oxidation, reducing internal flow during atomization for more accurate measurement. One limitation is the temperature gradient: the tube ends are cooler than the center, causing vaporized atoms to condense at the ends. This can result in incomplete removal of sample components, leading to carryover that affects the accuracy of the next sample’s reading. To reduce this effect, a cleanout step with full gas flow and high heat is performed after each sample. However, this can reduce the tube's lifespan due to repeated heating. Process Drying- ashing-atomization Diadavantage and advantage Cold vapour technique Mercury is reduced to its atomic form by reacting with a strong reducing agent, such as stannous chloride or sodium borohydride, in a closed system. Air or argon gas is bubbled through the reaction solution, carrying the volatile mercury atoms out of the reaction flask. Mercury atoms are carried through tubing into an absorption cell in the spectrometer’s light path. As mercury atoms enter the cell, absorbance rises, indicating mercury concentration in the light path. Two Types of Systems Open System: - Mercury vapor flows from the absorption cell to waste. - Absorbance peaks as mercury concentration increases, then decreases as mercury is depleted. - Analytical Signal: Highest absorbance value observed. Closed Loop System: - Mercury vapor is recirculated into the solution and cell. - Absorbance rises until an equilibrium mercury concentration is reached. - Analytical Signal: Equilibrium absorbance. Flow Injection: Automates the cold vapor mercury process Advantage - 100% Sampling Efficiency, ie All mercury is atomized and measured. - Low Detection Limit: Detects mercury at ~0.02 µg/L. - Amalgamation Option Mercury vapor can be trapped on gold gauze, and then heated to release it into the sample cell. This process allows the measurement of even lower concentrations Disadvantage- The concept is limited to mercury. Diagram Hydride generation method Samples are reacted with a reducing agent, typically sodium borohydride, in an external system. The reaction produces volatile hydrides. Unlike mercury, hydrides are not free atoms and cannot directly cause atomic absorption. The sample cell must be heated to break down the hydride gas into free atoms. In some systems, the absorption cell is mounted over the burner head and heated by an air-acetylene flame. Other systems use electric heating. In either case, the hydride gas is dissociated into free atoms, and the atomic absorption rises and falls as the atoms are created and escape from the absorption cell. The highest absorption or integrated peak area is recorded as the analytical signal Advantage Low Detection Limits: Achieves detection limits well below µg/L. High Sampling Efficiency: More efficient, like cold vapor mercury systems. Matrix Separation: Reduces interference by separating the analyte from the sample matrix. Disadvantage Limited Elements: Works only with specific elements (e.g., As, Sb, Se). Parameter Sensitivity: Sensitive to conditions like reaction time, gas pressure, and cell temperature Some common sample components can suppress hydride formation. Interference effects Interferences in atomic absorption can be divided into two general categories, spectral and non-spectral. Non-spectral interferences are those which affect the formation of analyte atoms. Chemical interference Interference Cause: Chemical interference occurs when the sample contains components that form thermally stable compounds with the analyte, preventing complete dissociation into free atoms in the flame. Example: Phosphate forms a stable compound with calcium (calcium phosphate), which doesn't fully dissociate in an air-acetylene flame, reducing calcium absorbance. There are two ways to deal with this problem Solution 1 - Use of a Stabilizing Element: Add a stabilizing element like lanthanum to bind with the interferent (phosphate), allowing the analyte (calcium) to be atomized independently. Solution 2 - Increasing Flame Temperature: Use a hotter flame, like nitrous oxide-acetylene, which can decompose thermally stable compounds, eliminating the need for added stabilizers. Ionization interference In hot flames, extra energy can raise ground-state atoms to an excited state or ionize them, reducing the number of atoms available for absorption and causing ionization interference. Ionization interference is most common in hotter flames like nitrous oxide-acetylene, especially for easily ionized elements (e.g., alkali metals and alkaline earth metals). Solution - Ionization Suppression: Adding an excess of an easily ionized element (like potassium, rubidium, or cesium) creates free electrons that suppress the ionization of the analyte. Example: Potassium added to barium samples in a nitrous oxide-acetylene flame increases the absorption of the ground-state barium atom and reduces ionization. Physical interference Errors are caused by physical factors like sample properties and instrument conditions. 1. Flame: ○ Spray Efficiency: Differences in viscosity and surface tension between sample and standard affect atomization and measurement. 2. Furnace: ○ Sample Dispersion: Uneven sample introduction can lead to inconsistent atomization. ○ Temperature Fluctuations: Uneven heat in the furnace causes measurement variations. ○ Viscosity: Thick samples atomize slowly, affecting readings. 3. Complex Samples: ○ Blood or Juice: Organic components in samples can interfere with atomization and cause errors. spectral interference Spectral interferences are those in which the measured light absorption is erroneously high due to absorption by a species other than the analyte element. The most common type of spectral interference in atomic absorption is "background absorption.” Background absorption occurs when some matrix materials in a sample are not fully atomized. Atoms have very narrow absorption lines, so interference between elements is rare. However, undissociated molecules or solid particles in the sample can cause broadband absorption or light scattering, which overlaps with the atomic absorption of the analyte. This leads to background absorption, which needs to be measured and subtracted from the total absorption to determine the net atomic absorption. An older method called the two-line method was used for background correction. It involved measuring background absorption using a non-absorbing emission line near the analyte’s line but far enough away to avoid atomic absorption. The background absorbance was subtracted from the total absorbance to calculate the analyte signal. However, finding a suitable nearby non-absorbing line is often difficult, and inaccuracies occur if the measurement wavelength is not very close to the analyte’s resonance line Continuum Source Background Correction Continuum Source Background Correction is a technique for Automatically compensating for any background absorption in atomic absorption measurements using a broadband light source. This method uses a continuum light source, which emits light across a broad spectrum, unlike the atomic line source which emits at specific wavelengths. Atomic absorption occurs only at specific wavelengths and doesn’t significantly affect the continuum source's broad emission. However, background absorption, which has broad spectra, affects both the continuum and atomic line sources equally. In this method, light from both sources passes through the sample, monochromator, and detector along the same path. The detector alternates between observing the two sources, and the instrument compares their absorbance. Any absorption affecting both sources equally (background) is ignored, while the true atomic absorption, which affects only the primary source, is measured and displayed Limitation Zeeman Background Correction Zeeman background correction uses the principle that the electronic energy levels of an atom placed in a strong magnetic field are changed, thereby changing the atomic spectra that measure these energy levels. It applies a strong magnetic field to the atomizer, which splits the atomic absorption line into components while leaving background absorption unaffected. Here’s how it works: Magnet Off: The total absorbance (atomic + background) is measured. Magnet On: Only background absorbance is measured since the atomic line shifts due to the magnetic field. The instrument automatically compares these values to calculate the true atomic absorption. Unlike continuum source correction, Zeeman correction uses a single atomic line source, avoiding problems like matching intensities or aligning paths. It corrects background absorption directly at the analyte's wavelength, making it more accurate in complex situations. For example, when measuring arsenic in a graphite furnace with aluminum present, Zeeman correction accurately removes background interference, while continuum correction can give falsely high results. Advantage and disadvantage Method of standard addition Purpose: Allows accurate analyte determination in the presence of matrix interference without eliminating the interference. Procedure: Take aliquots of the sample. Add increasing amounts of a standard to each aliquot. Measure absorbance for each aliquot. Calibration: The absorbance relationship between the sample and standard helps to account for interference effects. Interference Detection: If the spiked sample's calibration line is not parallel to the standard line, interference is present. Limitations: Does not compensate for background, spectral, chemical, or ionization interferences. Raman spectroscopy Raman spectroscopy is a spectroscopic technique based on inelastic scattering of monochromatic light, usually from a laser source. When a photon encounters a molecule, it can be elastically scattered (Rayleigh scattering) or inelastically scattered (Raman scattering). In Raman scattering, the photon transfers energy to or from the molecule, causing it to transition to a different vibrational state. Stokes and Anti-Stokes Scattering: Stokes scattering occurs when the scattered photon has less energy than the incident photon, indicating that the molecule has absorbed some energy and moved to a higher vibrational state. Anti-Stokes scattering occurs when the scattered photon has more energy than the incident photon, indicating that the molecule has released energy and moved to a lower vibrational state. Raman Shift: The difference in energy between the incident and scattered photons is known as the Raman shift. In isotropic media, polarization occurs as the induced dipole moment varies spatially across the molecule. Resonance-enhanced spectroscopy refers to techniques in which the interaction of light with matter is amplified due to resonance. When laser excitation matches an electronic absorption band, intensity increases by 10²–10⁶. RR helps detect concentrations as low as 10⁻⁶–10⁻⁸ M and provides insights into chromophore vibrations, especially those with significant geometry changes between electronic states. Surface-enhanced spectroscopy is a group of techniques that significantly amplify the signal of a molecule adsorbed In the 1970s, it was observed that rough metal surfaces enhanced Raman signals by up to 10⁶. This technique requires highly reflective metals and typically shows a decrease in Raman intensity with higher vibrational frequencies. Instrumentation Excitation Source A laser is used as the excitation source in Raman spectroscopy due to its high intensity and precise wavelength. Common lasers include Argon-ion (488 nm, 514.5 nm), Helium-Neon (632.8 nm) Sample Illumination System and Light Collection Optics Sampling optics, like lenses or microscope objectives, focus the laser on the sample and collect scattered light. Optical fibers are used in portable setups to transmit light between the sample and the instrument. Wavelength Selector Optical filters or monochromators separate Raman-scattered light from the more intense Rayleigh scattering. Edge filters block Rayleigh light, and monochromators disperse the Raman light into its components for analysis. Detector A detector, such as a Charge-Coupled Device (CCD), captures the dispersed Raman light and converts it into an electrical signal for spectral analysis. Application and advantage MASS SPECTROSCOPY Instrumentation Sample Inlet System The sample inlet is responsible for introducing the sample into the mass spectrometer. It often operates under low pressure to ensure the sample can be efficiently ionized. Common methods include molecular leaks or direct injection systems23. Ionization Source This component converts the sample into ions. Various ionization techniques are employed, including: - Electron Ionization (EI): Bombards the sample with electrons to produce positive ions. - Chemical Ionization (CI): Uses ion-molecule reactions to create ions. - Matrix-assisted laser Desorption/Ionization (MALDI) and Electrospray Ionization (ESI) are popular for larger biomolecules12. Mass Analyzer The mass analyzer separates ions based on their mass-to-charge (m/z) ratios. Different types of analyzers include: - Quadrupole Mass Analyzers: Use oscillating electric fields to filter ions based on their m/z ratio. - Time-of-Flight (TOF) Analyzers: Measure the time it takes for ions to travel a fixed distance; lighter ions reach the detector faster. - Magnetic Sector Analyzers: Utilize magnetic fields to bend the path of ions, separating them based on mass13. Detector The detector captures the separated ions and generates a signal proportional to their abundance. Common types include: - Electron Multiplier: Amplifies the signal from incoming ions. - Faraday Cup: Measures current generated by ion impacts. The output is typically displayed as a mass spectrum, plotting relative abundance against m/z ratios Applications of Mass Spectrometry 1. Pharmaceutical Analysis Drug Development: Identifies and characterizes new drugs. Quality Control: Ensures the purity and quality of pharmaceutical products. 2. Clinical Applications Disease Diagnosis: Analyzes biomarkers for diseases like cancer. Therapeutic Drug Monitoring: Measures drug levels in patients to ensure safe dosing. 3. Environmental Science Pollutant Detection: Identifies contaminants in air, water, and soil. Quality Assessment: Monitors soil and water safety. 4. Forensic Science  Crime Scene Analysis: Examines trace evidence like drugs and explosives. Toxicology Testing: Identifies substances in poisoning cases. 5. Food Safety Contamination Testing: Detects pesticides and toxins in food. Nutritional Analysis: Assesses the nutritional content of food products. 6. Research Applications Proteomics: Studies proteins and their functions. Metabolomics: Analyzes metabolic profiles for health research. GIVE SPECIFIC EXAMPLES ATOMIC EMISSION SPECTROSCOPY Atomic Emission Spectroscopy (AES) is an analytical technique used to determine the elemental composition of a sample by analyzing the light emitted when atoms in the sample are excited by a high-energy source. Each element emits light at characteristic wavelengths as its atoms return to the ground state, enabling identification and quantification. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) is based on the principle of light emission from excited atoms and ions. A liquid sample is converted into an aerosol and introduced into an argon plasma generated by radiofrequency (RF) energy. The plasma’s high temperature (6,000–10,000 K) atomizes the sample into free atoms and ions (atomization and ionization) and excites these species. As the excited atoms and ions return to their ground states, they emit light at specific wavelengths. The emitted light is analyzed to determine the type and concentration of elements present. Working and instrumentation An ICP-OES system consists of several key components: ICP Torch: The heart of the ICP-OES, is typically made up of three concentric quartz tubes. Radio Frequency (RF) Generator: Supplies the power needed to sustain the plasma. Nebulizer: Converts liquid samples into an aerosol for introduction into the plasma. - The liquid sample is first introduced into the instrument using a Meinhard nebulizer, a pneumatic device that converts the sample into a fine aerosol by mixing it with high-velocity argon gas. Spectrometer: Analyzes the emitted light to determine the elemental composition. Structure of the ICP Torch: The ICP torch consists of three concentric quartz tubes Outer Tube: This tube provides an outer argon gas flow that cools and stabilizes the plasma. Middle Tube: This tube helps transport the sample into the plasma and is where the primary argon gas flow passes through. Inner Tube: This tube is where the actual plasma is formed and sustains the flow of the sample aerosol. Argon Gas Flow: Argon, an inert gas, is commonly used in ICP systems because it does not interfere with the excitation and emission processes. Argon gas is introduced into the ICP torch through these concentric tubes. Outer Argon Flow: The outer tube helps to control and stabilize the temperature of the plasma by circulating argon around the plasma. Inner Argon Flow: The inner tube transports the sample aerosol into the plasma and helps sustain the high-energy environment. Plasma Formation: Plasma is a hot, ionized gas consisting of free electrons, ions, and atoms. To generate the plasma in the ICP torch, an RF (radio frequency) generator is used. The RF generator provides a high-frequency electric field (typically around 27–40 MHz) that induces a current in the argon gas. The RF coil, which is wrapped around the outer portion of the torch, generates a fluctuating magnetic field. This magnetic field ionizes the argon gas, turning it into a plasma. Ionization of Argon: When the RF energy interacts with the argon gas, it causes the gas atoms to lose electrons, resulting in a mixture of ions and free electrons. This ionized gas forms the plasma, which reaches temperatures of 6,000–10,000 K. Sample Introduction to Plasma: As the sample enters the ICP torch, it is in the form of a fine mist, created by a nebulizer and carried by argon gas into the plasma. The plasma, which is extremely hot (up to 10,000 K), makes the following happen: Atomization: The sample’s molecules break apart into individual atoms because the plasma's heat is strong enough to split chemical bonds. Ionization: Some atoms lose electrons and turn into ions. This is important because ions are easier to excite and emit unique light. Excitation: The high energy in the plasma excites the atoms and ions, causing their electrons to move to higher energy levels. When the electrons drop back to their normal state, they release light specific to each element. The torch design ensures that the plasma remains stable and the heat distribution is uniform, enabling precise and repeatable measurements, and the emitted light from the plasma is analyzed further in the optical system of the ICP-OES to determine the sample's elemental composition. Principle Definition The principle of AAS is based on the absorption of light by free, ground-state atoms in the vapor phase. When a sample is atomized, atoms absorb light of a specific wavelength corresponding to their electronic transitions. The amount of absorbed light is directly proportional to the concentration of the element in the sample, as described by Beer-Lambert's law. Draw diagram Instrumentation Radiation Source: A hollow cathode lamp (HCL) or electrodeless discharge lamp (EDL) specific to the element being analyzed provides light of the required wavelength. Atomizer: Converts the sample into free atoms. Common atomizers include: Flame Atomizer: Uses a fuel-oxidant flame (e.g., acetylene-air). Graphite Furnace: Provides higher sensitivity for trace analysis by using a small, heated graphite tube. Monochromator: Isolates the specific wavelength of light absorbed by the analyte, eliminating interference from other wavelengths. Detector: Measures the intensity of transmitted light (e.g., a photomultiplier tube). The decrease in light intensity corresponds to the amount absorbed by the sample. Signal Processor and Display: Converts the detector's signal into a readable output, typically absorbance or concentration, displayed on a digital screen or computer. Nebulizer (Optional): In flame atomizers, the nebulizer converts liquid samples into a fine mist for better atomization. A hollow cathode lamp is a key light source for atomic absorption spectroscopy. It consists of a hollow cathode made from the metal to be analyzed, sealed in a glass cylinder filled with neon or argon gas at low pressure. When a voltage is applied between the anode and cathode, gas atoms are ionized. The positively charged ions accelerate and collide with the cathode, causing metal atoms to be "sputtered" from the surface. These sputtered metal atoms are then excited by the energy from the collisions, emitting light when they return to their ground state. Over time, the efficiency of hollow cathode lamps decreases as gas atoms are absorbed by the inner surfaces, reducing sputtering and light emission. To extend the lamp's life, some lamps have larger internal volumes or optimized gas pressures. Volatile metals like arsenic or cadmium cause faster wear due to the rapid vaporization of the cathode. When heated, some cathode materials release hydrogen, creating background emission that affects the spectrum. Modern lamps use a tantalum "getter" to absorb hydrogen. Hollow cathode lamps are made from high-purity metals for a pure emission spectrum. Multi-element lamps can be used for several elements but often have lower intensity, which may impact accuracy. Electron discharge lamp While the hollow cathode lamp works well for most elements in atomic absorption, it struggles with volatile elements due to low intensity and short lifespan. For better performance, an "electrodeless discharge lamp" (EDL) can be used. The EDL consists of a quartz bulb containing the element’s metal or salt, placed inside a small RF generator. When power is applied, the RF field vaporizes and excites the atoms, causing them to emit light. EDLs are typically more intense, stable, and long-lasting than hollow cathode lamps, offering better precision and lower detection limits. However, their larger optical image requires instruments with compatible optical systems. EDLs are available for many elements, including arsenic, cadmium, lead, and zinc.