1 Atomic Absorption Spectroscopy PHAM VAN HUNG, PhD Atomic Absorption Spectroscopy • AAS is commonly used for metal analysis • A solution of a metal compound is sprayed into a flame and vaporises • The metal atoms absorb light of a specific frequency, and the amount of light absorbed is a direct measure of the number of atoms of the metal in the solution Metal Zn Fe Cu Ca Na λ (nm) 214 248 325 423 589 Atomic Absorption Spectroscopy: An Aussie Invention • Developed by Alan Walsh (below) of the CSIRO in early 1950s. Principles of AAS • The metal vapor absorbs energy from an external light source, and electrons jump from the ground to the excited states • The ratio of the transmitted to incident light energy is directly proportional to the concentration of metal atoms present • A calibration curve can thus be constructed [Concentration (ppm) vs. Absorbance] Absorption and Emission Ground State Excited States Absorption Emission Atomic Absorption • When atoms absorb light, the incoming energy excites an electron to a higher energy level. • Electronic transitions are usually observed in the visible or ultraviolet regions of the electromagnetic spectrum. 2 Atomic Absorption Spectrum • An “absorption spectrum” is the absorption of light as a function of wavelength. • The spectrum of an atom depends on its energy level structure. • Absorption spectra are useful for identifying species. Atomic Absorption/Emission/ Fluorescence Spectroscopy Atomic Absorption Spectroscopy • The analyte concentration is determined from the amount of absorption. Overview of AA spectrometer. Light Source Light Source Detector Detector Sample Sample Compartment Compartment • Emission lamp produces light frequencies unique to the element under investigation • When focussed through the flame these frequencies are readily absorbed by the test element • The ‘excited’ atoms are unstable- energy is emitted in all directions – hence the intensity of the focussed beam that hits the detector plate is diminished • The degree of absorbance indicates the amount of element present Atomic Absorption Spectroscopy Atomic Absorption Spectroscopy • It is possible to measure the concentration of an absorbing species in a sample by applying the Beer-Lambert Law: Abs=−log I I o ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ Abs = ε cb ε ε = extinction coefficient 3 Atomic Absorption Spectroscopy • Instrumentation • Light Sources • Atomisation • Detection Methods Light Sources • Hollow-Cathode Lamps (most common). • Lasers (more specialised). • Hollow-cathode lamps can be used to detect one or several atomic species simultaneously. Lasers, while more sensitive, have the disadvantage that they can detect only one element at a time. Hollow-Cathode Lamps • The electric discharge ionises rare gas (Ne or Ar usually) atoms, which in turn, are accelerated into the cathode and sputter metal atoms into the gas phase. Hollow-Cathode Lamps Hollow-Cathode Lamps • The gas-phase metal atoms collide with other atoms (or electrons) and are excited to higher energy levels. The excited atoms decay by emitting light. • The emitted wavelengths are characteristic for each atom. Atomisation • Atomic Absorption Spectroscopy (AAS) requires that the analyte atoms be in the gas phase. • Vapourisation is usually performed by: –Flames – Furnaces –Plasmas 4 Flame Atomisation • Flame AAS can only analyse solutions. • A slot-type burner is used to increase the absorption path length (recall Beer- Lambert Law). • Solutions are aspirated with the gas flow into a nebulising/mixing chamber to form small droplets prior to entering the flame. Flame Atomisation Flame Atomisation • Degree of atomisation is temperature dependent. • Vary flame temperature by fuel/oxidant mixture. Fuel Oxidant Temperature (K) Acetylene Air 2,400 - 2,700 Acetylene Nitrous Oxide 2,900 - 3,100 Acetylene Oxygen 3,300 - 3,400 Hydrogen Air 2,300 - 2,400 Hydrogen Oxygen 2,800 - 3,000 Cyanogen Oxygen 4,800 Furnaces • Improved sensitivity over flame sources. • (Hence) less sample is required. • Generally, the same temp range as flames. • More difficult to use, but with operator skill at the atomisation step, more precise measurements can be obtained. Furnaces Inductively Coupled Plasmas • Enables much higher temperatures to be achieved. Uses Argon gas to generate the plasma. • Temps ~ 6,000-10,000 K. • Used for emission expts rather than absorption expts due to the higher sensitivity and elevated temperatures. • Atoms are generated in excited states and spontaneously emit light. 5 AAS - Calibration Curve • The instrument is calibrated before use by testing the absorbance with solutions of known concentration. • Consider that you wanted to test the sodium content of bottled water (A = 0.650?). • The following data was collected using solutions of sodium chloride of known concentration 0.760.520.380.18 Absorbance 8642 Concentration (ppm) Calibration Curve for Sodium Concentration (ppm) A b s o r b a n c e 2468 0.2 0.4 0.6 0.8 1.0 Use of Calibration curve to determine sodium concentration {sample absorbance = 0.65} Concentration (ppm) A b s o r b a n c e 2468 0.2 0.4 0.6 0.8 1.0 ∴Concentration Na + = 7.3ppm Sample Problem • The nickel content in river water was determined by AA analysis after 5.00 L was trapped by ion exchange. Rinsing the column with 25.0 mL of a salt solution released all of the nickel and the wash volume was adjusted to 75.00 mL; 10.00 mL aliquots of this solution were analyzed by AA after adding a volume of 0.0700 μg Ni/mL to each. A plot of the results are shown below. Determine the concentration of the Ni in the river water. Determination of Nickel Content by AA y = 5.6x + 20 0 40 80 120 0 5 10 15 Volume of Nickel Added(mL) Absorbance Units Answer: 0.375 μg/mL Infrared Spectroscopy What is Infrared? • Infrared radiation lies between the visible and microwave portions of the electromagnetic spectrum. • Infrared waves have wavelengths longer than visible and shorter than microwaves, and have frequencies which are lower than visible and higher than microwaves. • The Infrared region is divided into: near, mid and far-infrared. – Near-infrared refers to the part of the infrared spectrum that is closest to visible light and far-infrared refers to the part that is closer to the microwave region. – Mid-infrared is the region between these two. • The primary source of infrared radiation is thermal radiation (heat). • It is the radiation produced by the motion of atoms and molecules in an object. The higher the temperature, the more the atoms and molecules move and the more infrared radiation they produce. • Any object radiates in the infrared. Even an ice cube, emits infrared. 6 What is Infrared? (Cont.) Humans, at normal body temperature, radiate most strongly in the infrared, at a wavelength of about 10 microns (A micron is the term commonly used in astronomy for a micrometer or one millionth of a meter). In the image to the left, the red areas are the warmest, followed by yellow, green and blue (coolest). The image to the right shows a cat in the infrared. The yellow-white areas are the warmest and the purple areas are the coldest. This image gives us a different view of a familiar animal as well as information that we could not get from a visible light picture. Notice the cold nose and the heat from the cat's eyes, mouth and ears. Infrared Spectroscopy • Infrared spectroscopy is the measurement of the wavelength and intensity of the absorption of mid- infrared light by a sample. Mid-infrared is energetic enough to excite molecular vibrations to higher energy levels. • The wavelength of infrared absorption bands is characteristic of specific types of chemical bonds, and infrared spectroscopy finds its greatest utility for identification of organic and organometallic molecules. The high selectivity of the method makes the estimation of an analyte in a complex matrix possible. Infrared Spectroscopy The bonds between atoms in the molecule stretch and bend, absorbing infrared energy and creating the infrared spectrum. Symmetric Stretch Antisymmetric Stretch Bend A molecule such as H 2 O will absorb infrared light when the vibration (stretch or bend) results in a molecular dipole moment change Infrared Spectroscopy A molecule can be characterized (identified) by its molecular vibrations, based on the absorption and intensity of specific infrared wavelengths. Infrared Spectroscopy For isopropyl alcohol, CH(CH 3 ) 2 OH, the infrared absorption bands identify the various functional groups of the molecule. Capabilities of Infrared Analysis  Identification and quantitation of organic solid, liquid or gas samples.  Analysis of powders, solids, gels, emulsions, pastes, pure liquids and solutions, polymers, pure and mixed gases.  Infrared used for research, methods development, quality control and quality assurance applications.  Samples range in size from single fibers only 20 microns in length to atmospheric pollution studies involving large areas. 7 Applications of Infrared Analysis  Pharmaceutical research  Forensic investigations  Polymer analysis  Lubricant formulation and fuel additives  Foods research  Quality assurance and control  Environmental and water quality analysis methods  Biochemical and biomedical research  Coatings and surfactants  Etc. • Dispersive instruments: with a monochromator to be used in the mid-IR region for spectral scanning and quantitative analysis. • Fourier transform IR (FTIR) systems : widely applied and quite popular in the far-IR and mid-IR spectrometry. • Nondispersive instruments: use filters for wavelength selection or an infrared-absorbing gas in the detection system for the analysis of gas at specific wavelength. Instrumentation BRUKE TENSOR TM Series Perkin Elmer TM Spectrum One Instrumentation Dispersive IR spectrophotometers Simplified diagram of a double beam infrared spectrometer Modern dispersive IR spectrophotometers are invariably double-beam instruments, but many allow single-beam operation via a front-panel switch. Double-beam operation compensates for atmospheric absorption, for the wavelength dependence of the source spectra radiance, the optical efficiency of the mirrors and grating, and the detector instability, which are serious in the IR region.⇒single-beam instruments not practical. Double-beam operation allows a stable 100% T baseline in the spectra. Dispersive spectrophotometers Designs Null type instrument 8 Sample preparation techniques The preparation of samples for infrared spectrometry is often the most challenging task in obtaining an IR spectrum. Since almost all substances absorb IR radiation at some wave length, and solvents must be carefully chosen for the wavelength region and the sample of interest. Infrared spectra may be obtained for gases, liquids or solids (neat or in solution) The end! . 2,700 Acetylene Nitrous Oxide 2,900 - 3, 100 Acetylene Oxygen 3, 300 - 3, 400 Hydrogen Air 2 ,30 0 - 2,400 Hydrogen Oxygen 2,800 - 3, 000 Cyanogen Oxygen 4,800 Furnaces •. atoms of the metal in the solution Metal Zn Fe Cu Ca Na λ (nm) 214 248 32 5 4 23 589 Atomic Absorption Spectroscopy: An Aussie Invention • Developed by