Inductively Coupled Plasma Spectrometry Viktor Kanický Laboratory of Atomic Spectrochemistry Faculty of Science Masaryk University Brno Czech Republic Elemental chemical analysis m Elemental analysis makes it possible ä to verify the presence of an element (qualitative analysis) ä to determine its concentration (quantitative analysis) ä to identify a structure in which it is present (structure anal.) ä to identify a compound in which it is bound (speciation) m WHOWHO analysis ä what (qualitative) ä how much (quantitative) ä where (structure) ä how bound (speciation) m The aim is to relate the composition to the properties Atomic Emission Spectrometry ä AES is one of the oldest analytical methods ä Principles of AES are known since 19^th century ä AES underwent considerable technological development ä Plasmas play a dominant role as radiation sources for AES Definition of a plasma m A plasma is a neutral gas of charged particles which possess collective behaviour. m Practically, any ionized gas can be considered as a plasma. m Presence of free electrons Role of a plasma source in AES m Atomization: from compound to free atoms 3 High kinetic temperature 3 Efficient energy transfer 3 Processes of the order of the ms m Excitation/ionization 3 Energy transfer to higher energy levels 3 Processes of the order of the ns Wavelength and energy Atomic Emission Spectrometry n 1666 - Newton, a sunlight dispersion with a prism, particulate nature of light n 17^thc.-Huygens, wave nature of light n 1678 - Johannes Marcus Marci, a rainbow principle n 1752 - Melvill, a candle flame through a prism n 1802 - Davy, electric arc n 1802 - Wollaston, dark lines in the Sun spectrum n 1817 - Fraunhofer, transmission diffraction grating n 1826 - Talbot, Sr emission in an alcohol flame, recommended for determining of substances Atomic Emission Spectrometry n 1846 - Herschel, Na, K, Ca, Li, Ba, Cu, and Fe could be detected in alcohol flame n 1859 - Bunsen, Kirchhoff, spectral lines emitted by elements, not compounds, emission/absorption n 1860 - Foucault, sodium doublet n 1865 - Balmer, formula for calculating H-wavelenghts n 1869 - Angstrom, reflection diffraction grating n 1869 - Janssen, quantitative spectroscopy n 1877 - Gouy, pneumatic nebulizer to introduce liquids into flames n 1879 - Lockyer, arc and spark spectra Atomic Emission Spectrometry n 1882 - Rowland, concave diffraction grating n 20^thc.- photographic plate for a light detection n 1930 - Gerlach, Schweitzer, internal standard n 1930 - AC current arc, HV spark excitation n 1935 - Thanheiser, Heyes, first photoelectric detection n 1940 - photomultiplier tube, direct-reading analyzers n 1950s- grating spectrometers n 1960 - DC plasma for analysis of liquids n 1965 - Fassel, Greenfield, Inductively Coupled Plasma n 1975 - first commercial ICP-AES with polychromator ICP principle m High-frequency generator 27 - 64 MHz m Discharge initiation by spark - seed electrons accelerated by electromagnetic field m Avalanche ionization Ar + e^- (r) Ar^+ + 2 e^- m Induction coil, 3-5 turns - primary winding m Electrons in plasma - secondary winding m ICP - plasma gas 12 L/min m Centrally introduced carrier argon with aerosol 0.6 - 1 L/min ICP features m Annular (toroidal) plasma m Induction region (10 000 K), skin-effect m Central analytical channel (5000-6000 K) m High temperature and sufficient residence time (3 ms) TH efficient atomization ^m High concentration of Ar^+, Ar^*, Ar^m efficient ionization / excitation (E[ion]= 15.8 eV) m High concentration of electrons 10^20-10^21 m^-3 (0.1% ionization of Ar) >> in flame (10^14-10^17 m^-3) TH low influence of matrix ionization on shift of ionization equilibria TH no typical ionization interferences ICP features 4 Hot annular plasma encloses cooler central channel containing a sample 4 excited analyte atoms in the channel are not surrounded with analyte atoms in lower energy states 4 there is no or only minimum self-absorption in the induction region 4 linearity of calibration extends over 4 to 5 orders of magnitude. ICP excitation * Ar^+ + X (r) Ar + X^+* +/- D E Charge transfer * Ar^m + X (r) Ar + X^+* Penning effects * e^- + X (r) e^- + e^- + X^+ Collisional ionization * e^- + X (r) e^- + X^* Collisional excitation (X - atom of analyte) ICP background and line emission 4Recombination continua 4Molecular bands emission 4Line (I, II) emission 4Bremsstrahlung Ar^+ + e^-OAr* + hn[cont]^ l[max]450 nm Ca^+/Ca*: > 302 nm, 202 nm; Mg^+/Mg*: 257-274 nm, <255nm, <162 nm; Al^+/Al*: 210 nm Spatial distribution of emission in ICP Spatial distribution of emission in ICP Spatial distribution of emission in ICP Spatial distribution of emission in ICP Spatial distribution of emission in ICP l Preheating Zone - PHZ: 4 aerosol desolvation 4 vaporization of solid particles 4 atomization of molecules and radicals l Initial Radiation Zone - IRZ: 4 excitation of atomic lines of low to medium 1^st ionization energies which exhibits here maxima of their axial intensity distributions 4 less intensive ionic (II) emission and low values of their signal-to-background ratios S/B 4 non-spectral (matrix) interferences - enhancement of both atomic and ionic emission in the presence of excess of easily ionisable elements - excitation interferences Spatial distribution of emission in ICP l Normal Analytical Zone - NAZ: 4 higher concentration of electrons and temperature × IRZ 4 excitation of ionic lines exhibiting here maxima of their axial intensity distributions and maximum S/B 4 sufficient intensity of atomic lines with low to medium 1^st ionization energies, higher S/B in comparison to IRZ 4 minimum matrix interferences - combination of effects at nebulization and aerosol transport with interferences in plasma, mostly non-specific depression < 5% under optimum conditions l Tailflame T: 4 lower temperature and electron density than in NAZ 4 recombination reactions, ionization interferences, alkali metals intensive emission Spatial distribution of emission in ICP l Power emitted by a certain surface area of an ICP is measured for a time period (integrated). l Signal intensity is corresponding electrical quantity (photoelectric current, voltage, charge). l Frequency of ICP oscillator influences electron density and excitation temperature. For a certain ICP generator the signal intensity depends on: 4 geometry of plasma torch 4 power input to plasma, P 4 gas flow rates (outer F[p], intermediate F[a], carrier F[c]) 4 observation mode (axial, lateral - observation height) 4 ionization E[i], E[i+1] and excitation energies E[exc] of elements and transitions 4 amount and composition of sample transported into ICP Axial distribution of emission in ICP l Axial intensity distribution exhibits maximum at a certain observation height h depending of electron density and concentration of argon species Ar^+, Ar^* and Ar^m, and E[i] , E[i+1] and E[exc] at which "norm temperature" of the line is achieved. For stable compounds, dissociation energies are also important. l Number density of atoms n[ap] excited on the level p is related to total number density n[a] of atoms by Boltzmann relation (g[p]^a being statistical weight, Z[a] partition function, E[p]^a excitation energy considered form the fundamental state of atom E=0 ). Axial distribution of emission in ICP where partition function (sum over k states) reads Axial distribution of emission in ICP l Ionization equilibrium is described by Saha equation Axial distribution of emission in ICP l Consequently, for atomic line emission intensity yields Axial distribution of emission in ICP l Ionic line emission intensity is described by relation Axial distribution of emission in ICP l As it is approximately Inductively Coupled Plasma Atomic Emission Spectrometry 4 Determination of 73 elements (P, S, Cl, Br, J) 4 Simultaneous and fast sequential measurement 4 High selectivity 4 Low limits of detection (0.1-10 ng/mL) 4 Linear dynamic range (5-7 orders of magnitude) 4 Minimum matrix effects (< +/- 10 % rel.) 4 Introduction of liquid, solid, gaseous samples 4 Conventional flows of liquids (mL/min) or microsamples (mL/min) Inductively Coupled Plasma Atomic Emission Spectrometry 4 Acceptable precision (0.5 - 2 % rel.) 4 Acceptable accuracy (~ 1 % rel.) 4 High sample throughput ~ routinely 10^2 - 10^3 determinations per hour 4 Automation of operation