Atomic spectroscopy. G. Galbács. Atomic spectroscopy Principle of operation - PDF

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Atomic spectroscopy G. Galbács Atomic spectroscopy Principle of operation In atomic spectroscopy, generally we study the electronic transitions in atoms, therefore these spectroscopic methods provide analytical

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Atomic spectroscopy G. Galbács Atomic spectroscopy Principle of operation In atomic spectroscopy, generally we study the electronic transitions in atoms, therefore these spectroscopic methods provide analytical information about the elemental composition of a sample. With the exception of a few special methods (see later), the electronic transitions of valence shell electrons in free atoms are studied. Emission, absorption and fluorescence spectra of free atoms are line spectra, which means they consist of very narrow peaks (FWHM is on the order of 0.01 nm or less). A line spectrum is produced, because electronic energy levels in atoms are well defined, quantized. Atomic spectroscopy Principle of operation By the analogy of similar molecular spectroscopies, the transitions involved in an atomic emission, atomic absorption (and atomic fluorescence, not seen in the figure) spectrum can be easily imagined. Selection rules apply. Atomic spectroscopy Atomization No matter what measurement mode we use, the first step in atomic spectroscopy is to break down the sampletoproducefreeatoms.thisis done in the atomizer of an atomic spectrometer, which is a source of high temperature (several thousand of Kelvins, locally). This can be, for example, realized in the form of: flames furnaces electric arcs or sparks plasmas Atomic spectroscopy Atomization In atomic absorption spectroscopy (AAS), the atomizer is only needed to atomize the sample, but electronic excitation is done by an external line light source (hollow cathode lamp, laser, etc.). Consequently, a too high temperature of the atomizer (above ca K) is inadvantageous, because ionization of many of the sample atoms (for example alkalis) would also occur. Remember, that AAS typically based on line absorption of ground state atoms, therefore the AAS signal is proportional to the population of the ground level. In atomic emission spectroscopy (AES), the high temperature source is also responsible for the thermal (collisional) excitation of atoms. The efficiency of collisional excitation increases with the temperature. Also, the emission signal is proportional to the population of the excited levels, so in AES, an as high as possible temperature of the atomizer is required (min. ca. 5000K). Ionization therefore is common, and AES often measures emission from elemental ions too. Atomic spectroscopy Flame atomizers Various combinations of fuel and oxidant gases can be used to produce a flame atomizer. The most populars are acetylene/air, acetylene/n 2 O and propanebutane/air. The sample is introduced into the flame in the form of an aerosol, mixed into the oxidant gas flow. Atomic spectroscopy Graphite furnace atomizers Graphite tube furnaces are heated by electric current (up to a couple of thousands A) in a controlled way, up to about 3000 K. The graphite tube is surrounded by an inert gas to prevent oxidation/burn of the graphite. The sample introduction is done usually by micropipette; a droplet of liquid is placed onto a graphite platform in the tube. Atomic spectroscopy Plasma atomizer (ICP) Inductively coupled plasma (ICP) atomizers are popular, very high temperature atom sources that operate on K temperature work, employing an inert gas environment (typically Ar). Sample introduction is done in the form of an aerosol, mixed with the argon gas. Atomic spectroscopy Liquid sample introduction by nebulizers As was alluded to before, many atomizers require the sample to be introduced in the form of an aerosol (usually wet aerosol, or mist). The most popular devices that produce aerosols from liquids by the action of a pressurized gas are called pneumatic nebulizers. The picture below show a common concentric type pneumatic nebulizer. Atomic spectroscopy Solid sample introduction by laser ablation Laser ablation is a modern way of solid sample introduction into atomic spectrometers. An intense, pulsed laser light is focused onto the surface of the sample, which causes the sample to ablate (break down, evaporate, fragment) in the focal spot. The resulting fine, dry aerosol is the n swept into the spectrometer with the aid of an inert gas flow (e.g. Ar) Flame atomic absorption spectrometry (FAAS) Instrument schematic Flame atomizer (in a slotted burner for best sensitivity) Specific of the analyte Nebulizer Flame atomic absorption spectrometry (FAAS) Instrument schematic optical system AAS instruments incorporate a complex optical system. Thisoptical system helps to free the transmitted intensity of many background radiation (e.g. thermal atomic and flame emission, fluorescence, non-specific absorption, etc.). Several types of such systems are Several types of such systems are in use (Deuterium lamp, Zeeman, Smith-Hieftje); below the operation of the simplest, essential system (rotating chopper) is shown. Flame atomic absorption spectrometer (FAAS) Analytical performance Pros Low efficiency of sample introduction (low signal) Short residence time in the light pathway (low signal) Reasonably low detection limits (ppm-ppb range) Relative ease of use Medium range costs of operation Cons Narrow linear dynamic range (ca. 2 orders of magnitude) Monoelemental method (small sample throughput) Reasonably high sample volume requirement (2-5 ml) Inability to measure non-metals For each analyte we need a different hollow cathode lamp Chemical intereference effects Graphite furnace AAS (GFAAS) The instrument In GFAAS, the graphite furnace replaces the flame atomizer, and there is no need for a nebulizer to introduce the sample. Graphite furnace AAS (GFAAS) Analytical performance Pros High efficiency of sample introduction (high signal) Long residence time in the light pathway (high signal) Possibility for thermal pretreatment of the sample Principal ability to handle liquid and solid samples as well Small sample volume requirement (10-20 µl) Low detection limits (ppt-ppb range) Cons Narrow linear dynamic range (2-3 orders of magnitude) Monoelemental method (small sample throughput) Poor repeatability (5-10%) Increased memory effects Inability to measure non-metals For each analyte we need a different hollow cathode lamp High operating and maintenance costs Flame atomic emission spectroscopy (FAES) The instrument The flame photometer, or flame atomic emission spectrometer (FAES) is the simplest atomic emission spectrometer. The atomizer is a small, circular propane-butane/air flame, and instead of a monochromator, it uses color (interference) filters for wavelength selection. The sample is introduced by a nebulizer. This construction is optimized for cost and the measurement of alkalis. lens filter detector amplifier and display propane-butane gas air waste Flame atomic emission spectrometer (FAES) Analytical performance Pros Low efficiency of sample introduction (low signal) Short residence time in the light pathway (low signal) Reasonably low detection limits (ppm-ppb range) Relative ease of use In principle, it can be run in a simultaneous mode Low costs of operation Cons Narrow linear dynamic range (ca. 2-3 orders of magnitude) Reasonably high sample volume requirement (2-5 ml) Strong ionization intereference effects Only a small number of analytes can be measured ICP atomic emission spectrometer (ICP-AES) Instrument schematic ICP atomic emission spectrometer (ICP-AES) Analytical performance Pros High efficiency atomization/excitation Robust and reliable Principal ability to handle liquid and solid samples as well Low detection limits (ppb range) Very wide linear dynamic range (5-6 orders of magnitude) No or very limited chemical intereferences Multielemental, simultaneous method (sample throughput is high) Ability to measure 80+ elements of the periodic table Cons Moderately high sample volume requirement (2-5 ml) Moderately high purchase and maintenance costs ICP mass spectrometry (ICP-MS) Principle of operation ICP mass spectrometry (ICP-MS) Schematic of the instrument ICP mass spectrometry (ICP-MS) Quadrupole mass analyser and detector ICP mass spectrometry (ICP-MS) Example spectra ICP mass spectrometry (ICP-MS) Analytical properties Pros High efficiency atomization and ionization Robust and reliable Handling ability of liquids and solids Very low detection limits (parts per trillion, ppt) Very wide linear dynamic range (8-9 orders of magnitude) Only a few interference effects Multielemental method (high sample througput) Most elements in the periodic table can be measured (80+) Isotopic information Cons Relatively high sample volume (2-5 ml) High investment and maintenance costs Some isobaric intereference ICP mass spectrometry (ICP-MS) Interferences space charge effect Ions with high inertia (high mass) will be slightly over-represented, because these will repel lighter ions thus the focusing of the latter will be poorer. This effect can be largely eliminated by using an internal standard. ICP mass spectrometry (ICP-MS) Interferences isobar effect Polyatomic ion (interferent) Analyte Analyte Polyatomic ion (interferent) X-ray fluorescence spectroscopy (XRF) Principle of operation In this method, the sample is subjected to continuum X-ray radiation (Bremsstrahlung from an X-ray tube or synchrotron). This radiation, if energetic enough, will eject an electron from a closed electronic shell this vacancy will be filled in by one of the electrons in the atom with a lower bond energy (outer orbital). Energy difference between the two levels will then be emitted as X-ray radiation. This process, of course, takes place in the sample for all atoms and in a cascade-style manner. Emitted radiation is characteristic of the elemental composition. X-ray fluorescence spectroscopy (XRF) Energy and wavelength dispersive systems XRF spectrometers come in two flavours: energy dispersive (ED- XRF) and wavelength dispersive (WD-XRF). The performance of these two types of instruments is different, as can be seen in the next slide. Common (major) properties of all XRF instruments include: solid samples can be best measured samples have to be flat-like chemical information is only obtained from the surface layer (top ca. 1-2 µm) analysis is practically non-destructive light elements (below Na) can be poorly detected ED-XRF instruments are more compact, they can be made fully portable. X-ray fluorescence spectroscopy (XRF) ED-XRF vs. WD-XRF, a comparison ED-XRF WD-XRF energy-based detection wavelength-based detection (X-ray more sensitive (LODs: 1-10 ppm), as monochromator is applied) the detector collects radiation from a less sensitive (LODs: ppm) wider solid angle better spectral resolution fast recording of the whole spectrum slower measurement poorer spectral resolution operation is more complicated operation is easy more costly less costly Automatic analyzers Automatic analyzers Introduction Today, when a large number of samples have to analyzed day-by-day, it common that automatic sample changers are used with practically any instrument. These devices are practically robotic devices, which are capable of a programmed dosibng (injection) of liquid samples, mixing, reagent addition, etc. an x-y positionable, table-based autosampler rotary autosampler Automatic analyzers Introduction Usually however, it takes more to fully automate the analytical process. Sample preparation p also has to be (fully) automated, as this is the step in the analytical process which takes the most time and chemicals. At the same time, effort is made to make the sample preparation (and detection) to work with as small samples as possible, because it conserves chemicals and increases the sample throughput. There are two distinct concepts, along which automatic analyzers are constructed. Discrete analyzers handle samples in parallel; all samples have their assigned analytical channel (cartridge/flow channel, etc.) Flow analyzers work more in a serial fashion; samples are sequentially injected in a carrier flow (together with reagents), and then this will flow through devices (coils, reactors, separators, etc.) which help the mixing, reaction, separation, etc. of components. At the end of the tube there is a detector, which analyzes each sample zone one-by-one. As this concept is based on the operation of pumps and valves, it is sometimes also called Lab-on-a-valve (LOV). Discrete automatic analyzers Example: centrifugal (rotary) analyzers In centrifugal analyzers, each liquid sample has its own radial channel in a disk for sample preparation and detection. Driving of the liquid flow is achieved by the centrifugal force induced when the disk is spinned. Detection of the prepared samples is performed in the outer section of the disk (channels), in a similar manner as CDs/DVDs are read. Flow analyzers Example: flow injection analyzers (FIA) The FIA concept was introduced in the 1970 s by Jaromir Ruzička and Elo Hansen. In this concept, the samples are injected in the carrier flow and pumped forward with ihamulti-channel li l peristaltic i pump.using Ui T or Y-pieces, reagent portions are added to this flow and then typically a reaction coil helps the reaction to complete. During propagation, sample zones will suffer dispersion, hence detector signals (measured by the detector placed at the end of the tube) will be peaks, which decrease in height and widen with the time/length spent in the tube. Detection of the sample zones will have to be therefore done with a tight control of time. Advantage of this system is that the sample throughput is high and it has a great flexibility. Flow injection analyzers The look of it Flow injection analyzers The reaction coil Flow injection analyzers The use of membranes Membranes can be used for: gas/liquid separation dilution liquid/liquid extraction filtering, etc. Flow injection analyzers Dilution and calibration Dilution/calibration can be done in several ways: 1.) electronically (by changing the detection time or the length of tubing) 2.) zone sampling 3.) membrane transport Flow injection analyzers The use of kinetic discrimination Kinetics is an important part of the operation of electrochemical detectors. This is strongly related to the flow rate in FIA. Thus, by changing/optimizing the flow rate, one can e.g.: maximize the net analytical signal (when interferents are present) make the calibration curves to be more linear Flow injection analyzers Other operations Calibration by change of tubing dimensions Precipitation forming Liquid/liquid extraction Flow injection analyzers Applications: determination of rodanide ions Flow injection analyzers Applications: discrete sample introduction in FAAS Advantage: lower sample consumption, higher tolerance towards viscous/concentrated samples.
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