Part 2

Relationship Between Atomic Absorption and Flame Emission Flame Emission: it measures the radiation emitted by the excited atoms that is related to concentration.  Atomic Absorption: it measures the radiation absorbed by the unexcited atoms that are determined. Atomic absorption depends only upon the number of unexcited atoms, the absorption intensity is not directly affected by the temperature of the flame. The flame emission intensity in contrast, being dependent upon the number of excited atoms, is greatly influenced by temperature variations. Prof. Dr. Hisham Ezzat Abdellatef

Atomizers in emission techniques Type Method of Atomization Radiation Source Arc sample heated in an sample electric arc (4000-5000oC) Spark sample excited in a sample high voltage spark Flame sample solution sample aspirated into a flame (1700 – 3200 oC) Argon sample heated in an sample plasma argon plasma (4000-6000oC) Prof. Dr. Hisham Ezzat Abdellatef

Atomizers in absorption techniques Type Method of Atomization Radiation Source Atomic sample solution aspirated HCL (flame) into a flame atomic sample solution evaporated HCL (nonflame) & ignited (2000 -3000 oC) (Electrothermal) Hydride Vapor hydride generated HCL generation Cold vapor Cold vapor generated (Hg) HCL Prof. Dr. Hisham Ezzat Abdellatef

Atomizers in fluorescence techniques Type Method of Atomization Radiation Source atomic sample aspirated sample (flame) into a flame atomic sample evaporated sample (nonflame) & ignited x-ray not required sample fluorescence Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Flame Atomization: In a flame atomizer, a solution of the sample is nebulized by a flow of gaseous oxidant, mixed with a gaseous fuel, and carried into a flame where atomization occurs. The following processes then occur in the flame. Desolvation (produce a solid molecular aerosol) Dissociation (leads to an atomic gas) Ionization (to give cations and electrons) Excitation (giving atomic, ionic, and molecular emission) Prof. Dr. Hisham Ezzat Abdellatef

Processes that take place in flame or plasma Sample Atomization For techniques samples need to be atomized Techniques are useful for element identification Molecular information destroyed by atomization Flame Atomization Sample nebulized Mixed with fuel Carried to flame for atomization T 1

The Atomization Process nebulization desolvation vaporization [M+,X-]aq [M+,X-]aq [MX]solid [MX]gas solution mist atomization atomization excitation or absorption emission [M0]gas [M*]gas [M0]gas [X0]gas (via heat or light) ground state excited state [M+]gas [X+]gas Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Types of Flames: Several common fuels and oxidants can be employed in flame spectroscopy depending on temperature needed. Temperatures of 1700oC to 2400oC are obtained with the various fuels when air serves as the oxidant. At these temperature, only easily decomposed samples are atomized. For more refractory samples, oxygen or nitrous oxide must be employed as the oxidant. With the common fuels these oxidants produce temperatures of 2500oC to 3100oC. Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Burning Velocity: The burning velocities are of considerable importance because flames are stable in certain ranges of gas flow rates only. If the gas flow rate does not exceed the burning velocity, the flame propagates itself back in to the burner, giving flashback. As the flow rate increases, the flame rises until it reaches a point above the burner where the flow velocity and the burning velocity are equal. This region is where the flame is stable. At higher flow rates, the flame rises and eventually reaches a point where it blows off of the burner. Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Flame Structure: Important regions of a flame include: primary combustion zone interzonal region secondary combustion zone Primary combustion zone: Thermal equilibrium is ordinarily not reached in this region, and it is, therefore, seldom used for flame spectroscopy. Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef 2. Interzonal region: This area is relatively narrow in stoichiometric hydrocarbon flames, is often rich in free atoms and is the most widely used part of the flame for spectroscopy. 3. Secondary combustion zone: In the secondary reaction zone, the products of the inner core are converted to stable molecular oxides that are then dispersed into the surroundings. Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Temperature Profiles: A temperature profile of a typical flame for atomic spectroscopy is shown in Fig. 9-3. The maximum temperature is located in the flame about 1 cm above the primary combustion zone. It is important– particularly for emission methods – to focus the same part of the flame on the entrance slit for all calibrations and analytical measurements. Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef secondary combustion zone interzonal region primary combustion (reaction) zone pre heating zone appearance depends on fuel/oxidant length of zones depends on gas flow rate DANGER  gas flow too high- lifts flame off burner gas flow too low- flash back pre heating zone gases heated rapidly primary combustion region contains free radicals but not in thermodynamic equilibrium interzonal region used for spectrometry has free radicals secondary combustion zone combustion products formed i.e. CO and H2O Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Types of fuel/oxidant air/acetylene 2300oC most widely used. nitrous oxide/acetylene 2750oC hot and reducing red feather zone - due to CN very reactive free radical scavenger for 02 → lowers partial pressure of 02 in zone reducing atmosphere C2H2 + 2.502 + 10N2 → 2CO2 + H2O + 10N2 stoichiometric reaction C2H2 + 5N2O → 2 CO2 + H2O + 5N2 Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Why do you need a different burner for different oxidants? because to prevent flash back linear gas flow rate needs to 3 x speed of which flame can travel, burning velocity). Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Role of Chemistry in the Flame  sample atomised by thermal and chemical dissociation H2 + Q → H + H O2 + Q → O + O H + O2 → OH + O O+ H2 → OH + H   equilibrium achieved by 3rd body collision (B)   i.e. N2, O2 H + H + B → H2 + B + Q H + OH + B → H2O + B + Q Free reductions may react with sample to produce atoms i.e. H + HO + NaCl → H2O + Na + Cl Na + Q → Na* 10 Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Flame Atomisation Process Sample must be in the form of a fine mist so as not to put out flame. Breaks down sample into very fine drops to form liquid aerosol or mist. This assist atomisation as sample only in flame ≈ 0.025s Sample drawn up capillary tube at high velocity Sample oxidant Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Suction caused by high flows of oxidant gas and Venturi effect. The high gas flow rate at the end of the capillary creates a pressure drop in the capillary – the pressure in capillary is below atmospheric pressure and sample solution is pulled up. The high speed gas breaks the solution into a fine mist by turbulence as it emerges from capillary. How do we get a better aerosol? use impact bead (glass or alloy) to encourage aerosol formation and remove large droplets. Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Flame absorbance Profiles: Fig. 9-4 shows typical absorption profiles for three elements. Magnesium exhibits a maximum in absorbance at the middle of the flame. The behavior of silver, which is not readily oxidized, is quite different, a continuous increase in the number of atoms, and thus the absorbance, is observed from the base to the periphery of the flame. Chromium, which forms very stable oxides, shows a continuous decrease in absorbance beginning close to the burner tip. Prof. Dr. Hisham Ezzat Abdellatef

Flame absorbance profile for three elements Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Flame Atomizers: Figure 9-5 is a diagram of a typical commercial laminar flow burner that employs a concentric tube nebulizer. The aerosol is mixed with fuel. The aerosol, oxidant, and fuel are then burned in a slotted burner that provides a flame that is usually 5 or 10 cm in length. Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Laminar-Flow Burner Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Advantages: 1. Uniform dropsize 2. Homogeneous flame 3. Quiet flame and a long path length Disadvantages: 1. Flash back if Vburning > Vflow 2. ~90% of sample is lost 3. Large mixing volume Prof. Dr. Hisham Ezzat Abdellatef

Sample introduction techniques Prof. Dr. Hisham Ezzat Abdellatef

Methods of Sample Introduction in Atomic Spectroscopy Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Nebulization Nebulization is conversion of a sample to a fine mist of finely divided droplets using a jet of compressed gas. The flow carries the sample into the atomization region. Pneumatic Nebulizers (most common) Four types of pneumatic nebulizers: Concentric tube - the liquid sample is sucked through a capillary tube by a high pressure jet of gas flowing around the tip of the capillary (Bennoulli effect). This is also referred to aspiration. The high velocity  breaks the sample into a mist and carries it to the atomization region. Prof. Dr. Hisham Ezzat Abdellatef

Types of pneumatic nebulizers Concentric tube; Cross flow Concentric tube; Cross flow Fritted disk Babington Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Cross-flow The jet stream flows at right angles to the capillary tip. The sample is sometimes pumped through the capillary. Fritted disk The sample is pumped onto a fritted disk through which the gas jet is flowing. Gives a finer aerosol than the others. Babington Jet is pumped through a small orifice in a sphere on which a thin film of sample flows. This type is less prone to clogging and used for high salt content samples. Ultrasonic Nebulizer The sample is pumped onto the surface of a vibrating piezoelectric crystal. The resulting mist is denser and more homogeneous than pneumatic nebulizers. Electro-thermal Vaporizers (Etv) An electro thermal vaporizer contains an evaporator in a closed chamber through which an inert gas carries the vaporized sample into the atomizer. Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Liquid samples introduced to atomizer through a nebulizer Pneumatic nebulizer Ultrasonic-Shear Nebulizer Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Atomization Atomizers Flame Electrothermal Special Glow Discharge Hydride Generation Cold-Vapor Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Flame Chemistry Flames are used in atomic emission spectrometry for excitation (emission spectrometry) but in atomic absorption flames are used as Atom Cells to produce gaseous atoms. Why must the atoms not be excited for atomic absorption spectrometry? If the atom is already in the excited state it cannot absorb the light. Prof. Dr. Hisham Ezzat Abdellatef

Different Atomization Sources for Atomic Spectroscopy Typical Source Temperature Source Type 1700 - 3150° C. Combustion Flame 1200 - 3000° C Electrothermal Vaporization (ETV) on graphite platform 5000 -8000° C Inductively coupled plasma (ICP) 6000-1000° C Direct-current plasma (DCP) 2000-3000° C Microwave induced plasma (MI)) non-thermal Glow Discharge plasma (GDP) ~ 40,000° C(?) Spark Sources (dc or ac Arc)   Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Flame Atomizers Superior method for reproducible liquid sample introduction for atomic absorption and fluorescence spectroscopy. Other methods better in terms of sampling efficiency and sensitivity. Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Laminar-Flow Burner Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Atomizers Flame Atomic Absorption Spectrometry Atomization occurs in a flame created by mixing a fuel with an oxidant Analyte and background ions are atomized simultaneously Only a small percentage of the aqueous sample is atomized – much of the sample goes to waste Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Laminar flame atomizer Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef

Performance Characteristics Of Flame Atomizers In terms of reproducible behavior, flame atomization appears to be superior to all other methods for liquid sample introduction. In terms of sampling efficiency and thus sensitivity, however, other atomization methods are markedly better. A large portion of the sample flows down the drain and the residence time of individual atoms in the optical path in the flame is brief (~10-4s). Prof. Dr. Hisham Ezzat Abdellatef

Electrothermal Atomization Atomization of entire sample in short period Average sample time in optical path is seconds Evaporation of sample Microliter volume Low temperature Sample ashed at higher temperature Increase current Sample temperature goes to 2000-3000 °C Sample measured above heated surface High sensitivity for small samples Prof. Dr. Hisham Ezzat Abdellatef

Electrothermal Atomization It provides enhanced sensitivity because the entire sample is atomized in a short period, and the average residence time of the atoms in the optical path is a second or more. A few microliters of sample are first evaporated at a low temperature and then ashed at a somewhat higher temperature in an electrically heated graphite tube or in a graphite cup. Then the current is rapidly increased to several hundred amperes, which caused the temperature to soar to perhaps 2000oC to 3000oC; atomization of the sample occurs in a period of a few milliseconds to seconds. Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Atomizers Electrothermal or Graphite Furnace Atomizer Atomization occurs in an electrically heated graphite tube The graphite tube is flushed with an inert gas (Ar) to prevent the formation of (non-absorbing) metal oxides graphite tube Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Performance Characteristics: Electrothermal atomizers offer the advantage of unusually high sensitivity for small volumes of sample. Typically, sample volumes between 0.5 and 10 L are used; absolute detection limits lie in the range of 10-10 to 10-13 g of analyte. Furnace methods are slow-typically requiring several minutes per element. A final disadvantage is that the analytical range is low, being usually less than two orders of magnitude. Prof. Dr. Hisham Ezzat Abdellatef

Electrothermal atomizer Sample concentration Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Atomization Prof. Dr. Hisham Ezzat Abdellatef From Skoog et al. (2004); Table 28-1, p.840

Atomization and Excitation Atomic Emission Spectroscopy The heat from a flame or an electrical discharge promotes an electron to a higher energy level As the electron falls back to ground state, it emits a wavelength characteristic of the excited atom or ion Prof. Dr. Hisham Ezzat Abdellatef From Skoog et al. (2004); Figure 28-1, p.840

Prof. Dr. Hisham Ezzat Abdellatef Atomic Line Spectra Each spectral line is characteristic of an individual energy transition E = hn = Prof. Dr. Hisham Ezzat Abdellatef

Prof. Dr. Hisham Ezzat Abdellatef Flame atomic absorption spectrometry Beer Lamberts Law   A = log (Po/P) A =  b c where  is the molar absorptivity coefficient in units of mol-1 dm3 cm-1 b is the pathlength in cm and c is the concentration in mol dm-3 In limits (below 0.8 Absorbance) A vs. concentration Po P sample b Prof. Dr. Hisham Ezzat Abdellatef