Presentation on theme: "Dong-Sun Lee / cat-lab / SWU 2010Fall version Chapter 28 Atomic spectroscopy."— Presentation transcript:
Dong-Sun Lee / cat-lab / SWU 2010Fall version Chapter 28 Atomic spectroscopy
Atomic spectroscopy deals with the absorption, emission, or fluorescence by atom or elementary ions. Two regions of the spectrum yield atomic information- the UV-visible and the X-ray. As atoms have no rotational or vibrational energy, transitions occur only between electronic levels and bandwidths in atomic spectra are very narrow. Atomic spectroscopic methods normally are classified according to the type of spectral process involved and the method of atomization used.
Absorption, emission, fluorescence Schematic representation of absorption, emission, and fluorescence.
Origin of three sodium emission lines (resonance line) (a)Partial absorption spectrum for sodium vapor. (b)Electronic transitions responsible for the absorption lines in (a)
Atomic energy level diagram.
Atomic absorption spectrometry Atomic absorption is the process that occurs when a ground state atom absorbs energy in the form of electromagnetic radiation at a specific wavelength and is elevated to an excited state. The atomic absorption spectrum of an element consists of a series of resonance lines, all originating with the ground electronic state and terminating in various excited states. Usually the transition between the ground state and the first excited state is the line with the strongest absorptivity, and it is the line usually used. Transition between the ground state and excited state occur only when the incident radiation from a source is exactly equal to the frequency of a specific transition. Part of the energy of the incident radiation P o is absorbed. The transmitted radiation P is given by P = P o e(k b) where k is the absorption coefficient of the analyte element and b is the horizontal path length of the radiation through the flame. Atomic absorption is determined by the difference in radiant power of the resonance line in the presence and absence of analyte atoms in the flame. The width of the line emitted by the light source must be narrower than the width of the absorption line of the analyte in the flame.
The concentration value in the AAS is based on the Lambert-Beer law: with : E - extinction, T - light transmission expressed as a percentage (transmission), ID - intensity of the transmitted signal, I0 - intensity of the original signal, e - extinction coefficient (proportionality factor), c - concentration l - layer thickness (= distance traversed by the light ray through the flame/atomic cloud).
Atomic emission spectrometry with flame A solution if the sample is sprayed into a flame possessing the thermal energy required to excite the element to a level at which it will radiate its characteristic line emission spectrum. For an atom or molecule in the ground electronic state to be excited to a high electronic energy level, it must absorb energy from the flame via thermal collisions with the constituents of the partially burned flame gases. Upon their return to a lower or ground state, the excited atoms and molecules emit radiation characteristic of the sample components. Band spectra arise from electronic transitions involving molecules. For each electronic transition there will be a whole suite of vibrational levels involved. This causes the emitted radiation to be spread over a portion of the spectrum. Band emissions attributed to triatomic hydroxides(CaOH) at 554 nm and monoxides (AlO, strongest band at 484 nm) are frequently observed and occasionally employed in FES. The boron oxide system gives very sensitive bands at 518 and 546 nm. A photodetector measures the radiant power of the selected radiation that is correlated with the concentration of analyte in the sample and in standards.
Basic components of an atomic spectrophotometer
Thermo Elemental SOLAAR 969 AA spectrometers
Sources Hollow cathode lamp These lamps consist of a cylindrical metallic cathode(the same element as that being analyzed) and tungsten anode sealed in a glass tube containing neon or argon at a pressure of about 1 to 5 torr. When high voltage is applied between the anode and cathode, the filler gas is ionized and positive ions are accelerated toward the cathode. They strike the cathode with enough energy to “sputter” metal atoms from the cathode surface into the gas phase. The free atoms are excited by collisions with high-energy electrons and then emit photons to return to the ground state. This radiation has the same frequency as that absorbed by analyte atoms in the flame or furnace. A hollow cathode lamp.
Atomic absorption bandwidths are so narrow, generally in the range to nm. The narrowest band of wavelengths that can be isolated from a continuum with best monochromator is about 0.5 nm. At a proper conditions, the bandwidth of emitted radiation with hollow cathode lamp is even narrower than the atomic absorption bandwidth. Comparison of atomic absorption and monochromator spectral bandwidths. Relative line widths for copper emission and absorption.
Doppler broadening : The wavelength of radiation emitted or absorbed by a fast moving atom decreases if the motion is toward a detector and increases if the atom is receding from the detector. The linewidth, ,due to the Doppler effect, is given approximately by (7×10 –7 )(T/M) –1/2 where is the frequency(Hz) of the peak, T is temperature(K), M is the mass of the atom. Pressure broadening : Pressure, or collisional, broadening arises from collisions of emitting or absorbing species with other atoms or ions in the heated medium. Line broadening The linewidth of the source must be narrower than the linewidth of the atomic vapor for Beer’s law to be obeyed.
Cause of Doppler broadening. (a) When an atom moves toward a photon detector and emits radiation, the detector sees wave crests more often and detects radiation of higher frequency. (b) When an atom moves away from a photon detector and emits radiation, the detector sees crests less frequently and detects radiation of lower frequency. The result in an energetic medium is a statistical distribution of frequencies and thus a broadening of spectral lines.
Atomic absorption of a narrow emission line from a source. The source lines in (a) are very narrow. One line is isolated by a monochromator. The line is absorbed by the broader absorption line of the analyte in the flame (b) resulting in attenuation ( c) of the source radiation. Since most of the source radiation occurs at the peak of the absorption line, Beer’s law is obeyed.
The mass spectrum of a standard rock sample obtained by laser ablation / ICP-MS.
Processes occurring during atomization. Solution of analyte Nebulization Spray(small droplets) Desolvation Solid/Gas aerosol Volatilization Gaseous Excited h molecules molecules molecular Dissociation(reversible) Atoms Excited atoms h atomic Ionization(reversible) Atomic Excited ions h atomic ions A) Schematic diagram of a premix burner. B) End view of flame.
Processes leading to atoms.
Continuous sample introduction methods. Region of a flame
Properties of flames. Fuel Oxidant Temperature o C Maximum burning velocity(cm s –1 ) Natural gas Air 1700~ ~ 43 Natural gas Oxygen 2700~ ~ 390 Hydrogen Air 2000~ ~ 440 Hydrogen Oxygen 2550~ ~1400 Acetylene Air 2100~ ~266 Acetylene Oxygen 3050~ ~ 2480 Acetylene Nitrous oxide 2600~
Comparison of detection limits for various elements by flame absorption and flame emission methods. Flame emission Sensitivity Flame absorption more sensitive about the same more sensitive Al, Ba, Ca, Eu, Cr, Cu, Dy, Er, Ag, As, Au, B, Ga, Ho, In, K, Gd, Ge, Mn, Mo, Be, Bi, Cd, Co, La, Li, Lu, Na, Nb, Pd, Rh, Sc, Fe, Hg, Ir, Mg, Nd, Pr, Rb, Re, Ta, Ti, V, Y, Ni, Pb, Pt, Sb, Ru, Sm, Sr, Tb Zr Se, Si, Sn, Te, Tl, Tm, W, Yb Zn E. E. Pickett and S. R. Koirtyohann, Anal. Chem., 1969, 41(14), 42A.
Inductively coupled plasmas By definition, a plasma is an electrical conducting gaseous mixture containing a significant concentration of cations and electrons. (The concentrations of the two are such that the net charge approaches zero.) In the argon plasma employed for emission analyses, argon ions and electrons are the principal conducting species, although cations from the sample will also be present in lesser amounts. Argon ions, once formed in a plasma, are capable of absorbing sufficient power from an external source to maintain the temperature at a level at which further ionization sustains the plasma indefinitely; temperatures as great as 10,000 K are encountered. Three power sources have been employed in argon plasma spectroscopy. One is a dc electrical source capable of maintaining a current of several amperes between electrodes immersed in a stream of argon. The second and third utilize powerful radio-frequency and microwave-frequency fields through which the argon flows. Of the three, the radio-frequency, or inductively coupled plasma (ICP), source appears to offer the greatest advantage in terms of sensitivity and freedom from interference. On the other hand, the dc plasma source (DCP) has the virtue of simplicity and lower cost. Both will be described here. The microwave induced plasma source (MIP) is not widely used for analysis (primarily because it is not available from instrument manufacturers).
Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES, or ICP). ICP-AES, often referred to simply as ICP, is a multi-element analysis technique that uses an inductively coupled plasma source to dissociate the sample into its constituent atoms or ions, exciting them to a level where they emit light of a characteristic wavelength. A detector measures the intensity of the emitted light, and calculates the concentration of that particular element in the sample. When undergoing ICP analysis, the sample experiences temperatures as high as 10,000 o K, where even the most refractory elements are atomized with high efficiency. As a result, detection limits for these elements can be orders of magnitude lower with ICP than with FAAS techniques, typically at the 1-10 parts-per-billion level.
Principles of ICP An ICP source consists of a flowing stream of argon gas ionized by an applied radio frequency field typically oscillating at 27.1 MHz. This field is inductively coupled to the ionized gas by water-cooled coil surrounding a quartz torch that supports and confines the plasma. A sample aerosol is generated in an appropriate nebulizer and spray chamber and is carried into the plasma through an injector tube located within the torch. The sample aerosol is injected directly into the ICP, subjecting the constituent atoms to temperature of about 6000 to 8000 o K. Because this results in almost complete dissociation of molecules, significant reduction in chemical interferences is achieved. The high temperature of the plasma excites atomic emission efficiently. Ionization of a high percentage of atoms produces ionic emission spectra. The ICP provides an optically thin source that is not subject to self absorption except at very high concentrations. Thus linear dynamic ranges of four to six orders of magnitude are observed for many elements. The efficient excitation provided by the ICP results in low detection limits for many elements.
Advantages of an ICP source 1. The analytes are confined to a narrow region. 2. The plasma provides simultaneous excitation of many elements. 3. The analyst is not limited to analytical lines involving ground state transitions but can select from first or even second ionization state lines. For the elements Ba, Be, Mg, Sr, Ti, and V, the ion lines provide the best detection limits. 4. The high temperature of the plasma ensures the complete breakdown of chemical compounds (even refractory compounds ) and impedes the formation of other interfering compounds, thus virtually eliminating matrix effects. 5. The ICP torch provides a chemically inert atmosphere and an optically thin emission source. 6. Excitation and emission zones are spatially separated : this results in a low background. The optical window used for analysis lies just above the apex of the primary plasma and just under the base of the flame-like afterglow. 7. Low background, combined with a high S/N ratio of analyte emission, results in low detection limits, typically in the parts-per-billion range.
Inductively coupled plasma optical emission spectrometry (ICP-OES) is a major technique for elemental analysis. The sample to be analysed, if solid, is normally first dissolved and then mixed with water before being fed into the plasma. Atoms in the plasma emit light (photons) with characteristic wavelengths for each element. This light is recorded by one or more optical spectrometers and when calibrated against standards the technique provides a quantitative analysis of the original sample. ICP.
Instrumental Lay-out An ICP-OES instrument consists of a sample delivery system, an IC plasma to generate the signal, one or more optical spectrometers to measure the signal, and a computer for controlling the analysis. The most common sample delivery system consists of a peristaltic pump and capillary tube to deliver a constant flow of analyte liquid into a nebulizer.
The device which produces the IC plasma is commonly referred to as the ICP torch. It consists of two to four Argon flows depending on the manufacturer: Nebuliser gas (inner Argon flow), at about 1 L/min, carries the analyte aerosol Sheath gas (JY patent), for producing a laminar flow to improve low excitation energy elements eg group I & II elements Auxiliary gas (if present), lifts the plasma above the injector tube, used when measuring organics Plasma gas, at about L/min, sets the plasma conditions, eg excitation temperature The argon and analyte flow into a toroidal radio frequency (RF) field, usually at MHz. The plasma is ignited by a Tesla spark.
Schematic diagram showing the different regions in the IC Plasma Inductively coupled plasma source
Temperature profile of an ICP. Schematic diagram of an ICP torch.
ICP torch and components of an ICP torch Author: Geoff Tyler, Jobin-Yvon Horiba, France, First published on the web: 15 May Demountable Torch The ICP torch must be kept clean and open. Periodically, then, the torch is dismounted for cleaning.
Vaporization, Atomization and Excitation Aerosol vapour is transported to the plasma Vapour desolvates Atomization occurs within the plasma Atoms get excited to atomic and ionic states Rich spectra produced because of presence of both atomic and ionic lines Because of their different excitation energies, different emission lines will have maximum intensities at different vertical positions in the plasma
Spray chamber The droplets coming from the nebuliser can vary greatly in size, from less than 1 µm to more than 10 µm. Since droplets going into the IC plasma should be kept below 5 µm in size, it is necessary to remove the large droplets. This is done in a spray chamber. The liquid spray from the nebuliser enters the spray chamber. By sheer size, the larger droplets fall to the bottom of the chamber and exit through the drain. The finer droplets in the vapour are transported to the plasma. Various types of spray chambers commonly used are: Scott Cyclonic Inert Cooled Low Volume
Electrothermal atomization : the graphite furnace The electrically heated furnace offers greater sensitivity than that afforded by flames and requires a smaller volume of sample. The main part of the atomizer is a small graphite tube about 5 cm in length and 1 cm in diameter. The maximum recommended temperature for a graphite furnace is 2550 o C for not more than 7 sec.
A graphite furnace atomizer.
Flame Atomic Absorption Spectrometry (FAAS) In flame atomic absorption spectrometry, either an air/acetylene or a nitrous oxide/acetylene flame is used to evaporate the solution and dissociate the sample into its component atoms. When light from a hollow cathode lamp (selected based on the element to be analyzed) passes through the cloud of atoms, the atoms of interest absorb the light from the lamp. This is measured by a detector, and used to calculate the concentration of that element in the original sample. The use of a flame limits the excitation temperature reached by a sample to a maximum of approximately 2600 o C (with the N 2 O/acetylene flame). For many elements this is not a problem. Compounds of the alkali metals, for example, and many of the heavy metals such as lead or cadmium and transition metals like manganese or nickel are all atomized with good efficiency with either flame type, with typical FAAS detection limits in the sub-ppm range. However, there are a number of refractory elements like V, Zr, Mo and B which do not perform well with a flame source. This is because the maximum temperature reached, even with the N 2 O/acetylene flame, is insufficient to break down compounds of these elements. As a result, flame AAS sensitivity for these elements is not as good as other elemental analysis techniques.
Atomic absorption spectrophotometer Single beam design Double beam design
Background correction Background signal arises from absorption, emission, or scatter by everything in the sample besides analyte ((the matrix), as well as absorption, emission, scatter by the flame, the furnace, or the plasma. Background correction methods : Beam chopping D 2 lamp The difference between absorbance measured with the hollow cathode and absorbance measured with the D 2 lamp is the absorbance due to analyte. Zeeman When an atomic vapor is exposured to a strong magnetic field (0.1 to 1 tesla), a splitting of electronic energy levels of the atoms takes place, which leads to formation of several absorption lines for each electronic transition. Smith-Hieftje --- based on the self-reversal, or self-absorption, behavior of radiation emitted from hollow cathode lamps when they are operated at high currents.
A beam chopper for subtracting the signal due to flame background emission. Resulting square-wave signal.
Interference Types of interference 1) Spectral : unwanted signals overlapping analyte signal 2) Chemical : chemical reactions decreasing the concentration of analyte atoms 3) Ionization : ionization of analyte atoms decreases the concentration of neutral atoms.
Methodology : E stablishing a relationship between absorbance and concentration Standard curve method Standard addition method Internal standard method Standard calibration curve for copper. Internal standard calibration curve.
Graphite Furnace Atomic Absorption Spectrometry (GFAAS) This technique is essentially the same as flame AA, except the flame is replaced by a small, electrically heated graphite tube, or cuvette, which is heated to a temperature up to 3000 o C to generate the cloud of atoms. The higher atom density and longer residence time in the tube improve furnace AAS detection limits by a factor of up to 1000x compared to flame AAS, down to the sub-ppb range. However, because of the temperature limitation and the use of graphite cuvettes, refractory element performance is still somewhat limited.
Features of the electrothermal (nonflame) atomizers 1. Only small amounts (10 –8 to 10 –11 g absolute) of analyet are required. 2. Solid can be analyzed directly, often without any pretreatment. 3. Small amounts of liquid samples, 5 to 100 l, are needed. 4. Background noise is very low. 5. Sensitivity is increased because the production of free analyte is more efficient than with a flame.
Flame, furnace, and ICP detection limits.
Detection limit of ICP-MS
ICP emission calibration curves for several metals.
The toxicological effect of metallic mercury: Tea party from Lewis Carroll’s Alice in Wonderland.
Biological concentration of mercury in the environment.
Applications Trace elements in a wide variety of aqueous matrices: drinking water, river, lake and ground water, waste water and effluent, and seawater. Trace elements in solids after digestion: sediment, soil, sludge, road dust, air particulate matter, plant tissue and grain, rocks and minerals, etc. Trace elements in samples of body fluids, including blood, plasma, and urine. Isotope ratio measurement
Sample Requirements and Preparation Minimum liquid sample volume required varies from a few l to a few mL, depending on the number of elements to be determined and the concentration levels required (i.e. if sample can be diluted). Ideally 10 ml. Solid samples must be dissolved before they can be analyzed. (Note: when the laser ablation accessory will be available, solid samples can be analyzed directly). Digestion facilities for solids are available. Minimum solid sample size for dissolution varies from 0.1 mg to 20 mg, depending on the sample matrix, the homogeneity of the sample, the digestion method and the concentration level of the elements interested. The content of the total dissolved solid (TDS) in the final solution to be analyzed should be less than 1%. However, the high sensitivity of the HR ICP- MS allows high dilutions, and hence the ability to analyze samples with high TDS.