Inductively Coupled Plasma Spectrometry Viktor Kanický Laboratory of Atomic Spectrochemistry Faculty of Science Masaryk University Brno Czech Republic

Theory Analyst Instrument Chemistry Synthesis explanation characterization Analysis

Methods of chemical analysis m Main fields of analytical chemistry: ä separation methods ä spectroscopic methods ä electrochemistry m Classification of spectroscopic methods according to carriers of analytical signal: ä electromagnetic radiation (photons) ä particles (ions, electrons)

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 äwhat (qualitative) ähow much (quantitative) äwhere (structure) ähow bound (speciation) m The aim is to relate the composition to the properties

Photon-based spectroscopic methods used for elemental analysis m Atomic Emission Spectrometry (AES) m Atomic Absorption Spectrometry (AAS) m Atomic Fluorescence Spectrometry (AFS)

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 X  X + + e -

Role of a plasma source in AES mAtomization: from compound to free atoms 3High kinetic temperature 3Efficient energy transfer 3Processes of the order of the ms mExcitation/ionization 3Energy transfer to higher energy levels 3Processes of the order of the ns

Wavelength and energy : frequency : wavelength c: light velocity h: Planck's constant

nebulization particles molecules atoms + ions photons vaporizationdissociation Liquid solution Solid sample ionization excitation desolvation nebulization Generating of analytical signal in AES - from sample to photon

Atomic Emission Spectrometry n Newton, a sunlight dispersion with a prism, particulate nature of light n 17 th c.-Huygens, wave nature of light n Johannes Marcus Marci, a rainbow principle n Melvill, a candle flame through a prism n Davy, electric arc n Wollaston, dark lines in the Sun spectrum n Fraunhofer, transmission diffraction grating n Talbot, Sr emission in an alcohol flame, recommended for determining of substances

Atomic Emission Spectrometry n Herschel, Na, K, Ca, Li, Ba, Cu, and Fe could be detected in alcohol flame n Bunsen, Kirchhoff, spectral lines emitted by elements, not compounds, emission/absorption n Foucault, sodium doublet n Balmer, formula for calculating H-wavelenghts n Angstrom, reflection diffraction grating n Janssen, quantitative spectroscopy n Gouy, pneumatic nebulizer to introduce liquids into flames n Lockyer, arc and spark spectra

Atomic Emission Spectrometry n Rowland, concave diffraction grating n 20 th c.- photographic plate for a light detection n Gerlach, Schweitzer, internal standard n AC current arc, HV spark excitation n Thanheiser, Heyes, first photoelectric detection n photomultiplier tube, direct-reading analyzers n 1950s- grating spectrometers n DC plasma for analysis of liquids n Fassel, Greenfield, Inductively Coupled Plasma n first commercial ICP-AES with polychromator

ICP-AES Dispersive system (spectra isolation) ICP (signal generation) Sample introduction system (pneumatic nebulization) Detector High-frequency generator (energy) Data acquisition

ICP principle m High-frequency generator MHz m Discharge initiation by spark - seed electrons accelerated by electromagnetic field m Avalanche ionization Ar + e -  Ar 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 L/min

Induction coil Power 1-2 kW Plasma gas 12 L/min Ar Auxiliary gas L/min Ar Carrier gas with sample aerosol L/min Ar Normal Analytical Zone Plasma torch (quartz) 3 concentric tubes Electromagnetic field frequency 27 MHz, 40 MHz Inductively Coupled Plasma Light passing to spectrometer

ionic lines atomic lines Tailflame Initial Radiation Zone Preheating Zone Lateral viewing Normal Analytical Zone Observation height

Axial viewing Light passing to spectrometer Longer optical path 3-10x better limits of detection

ICP features m Annular (toroidal) plasma m Induction region ( K), skin-effect m Central analytical channel ( K) m High temperature and sufficient residence time (3 ms)  efficient atomization m High concentration of Ar +, Ar *, Ar m efficient ionization / excitation (E ion = 15.8 eV) m High concentration of electrons m -3 (0.1% ionization of Ar) >> in flame ( m -3 )  low influence of matrix ionization on shift of ionization equilibria  no typical ionization interferences

ICP features Central channel 4Hot annular plasma encloses cooler central channel containing a sample 4excited analyte atoms in the channel are not surrounded with analyte atoms in lower energy states 4there is no or only minimum self- absorption in the induction region 4linearity of calibration extends over 4 to 5 orders of magnitude. Induction region

ICP excitation Ar + + X  Ar + X +*   E Charge transfer Ar m + X  Ar + X +* Penning effects e - + X  e - + e - + X + Collisional ionization e - + X  e - + X * Collisional excitation (X - atom of analyte) supratermal concentrations of X +* and X + preferential excitation of ionic spectral lines

ICP background and line emission 4Recombination continua 4Molecular bands emission 4Line (I, II) emission 4Bremsstrahlung Ar + + e -  Ar* + hν cont λ max 450 nm Ca + /Ca*: > 302 nm, 202 nm; Mg + /Mg*: nm, <255nm, <162 nm; Al + /Al*: 210 nm Stable oxides above/below NAZ; OH ( nm); NH 336 nm; NO ( nm); C 2, CN, CO, PO, SO 205 Ar lines nm, mostly by 430 nm, no at nm Ar + + e -  Ar + + e - hν brems λ>500 nm

Spatial distribution of emission in ICP Energy (J) emitted by excited atoms or ions at a transition from upper p to lower q energy level per unit of time (s) from unit volume (m 3 ) to unit solid angle (sr) is emissivity (Wsr -1 m -3 ) where ν pq and λ pq are radiation frequency and wavelength, 4π is total solid angle, A pq is probability of spontaneous emission p  q (number of transitions per sec), n p is concentration of atoms or ions on level p (m -3 ), h is Planck’s constant and c is velocity of light.

ICP channel Element J pq r Spatial distribution of emission in ICP Emissivity J pq corresponds to radial intensity distribution Radiant intensity I pq (Wsr -1 m -2 ) 1 d (Wsr -1 m -3 ) d - plasma layer thickness (m -1 ) Radiant intensity I pq is energy emitted per unit time into unit solid angle by unit area of plasma layer of thickness d, i.e. power per unit area per unit solid angle (Wsr -1 m -2 ). This relates to axial and lateral intensity distribution.

Spatial distribution of emission in ICP Direction of observation Line intensity Ar background intensity 4mm Lateral intensity distribution 30 mm coil T NAZ IRZ PHZ 0 mm Axial intensity distribution Direction of observation Line intensity Ar background intensity LATERAL OBSERVATION

Spatial distribution of emission in ICP TNAZ IRZ 12 mm Direction of observation Line intensity Ar background intensity AXIAL OBSERVATION PHZ

Spatial distribution of emission in ICP l Preheating Zone - PHZ: 4aerosol desolvation 4vaporization of solid particles 4atomization of molecules and radicals l Initial Radiation Zone - IRZ: 4excitation of atomic lines of low to medium 1 st ionization energies which exhibits here maxima of their axial intensity distributions 4less intensive ionic (II) emission and low values of their signal-to-background ratios S/B 4non-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: 4higher concentration of electrons and temperature × IRZ 4excitation of ionic lines exhibiting here maxima of their axial intensity distributions and maximum S/B 4sufficient intensity of atomic lines with low to medium 1 st ionization energies, higher S/B in comparison to IRZ 4minimum 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: 4lower temperature and electron density than in NAZ 4recombination 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: 4geometry of plasma torch 4power input to plasma, P 4gas flow rates (outer F p, intermediate F a, carrier F c ) 4observation mode (axial, lateral - observation height) 4ionization E i, E i+1 and excitation energies E exc of elements and transitions 4amount 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 Atomic line emission intensity is then

Axial distribution of emission in ICP l Ionization equilibrium is described by Saha equation where Z a and Z i are partition functions of atomic and ionic states, n i, n e and n a are ion, electron and neutral atom number densities, m e - electron mass, T ion is ionization temperature and E i ionization energy. Saha equation can be taken into account for evaluation of atomic emission by means of the degree of ionization

Axial distribution of emission in ICP l Consequently, for atomic line emission intensity yields where n 0 = n a + n i is total concentration of particles (atoms and ions) of a certain element. As a result, atomic line intensity increases with temperature in Boltzmann exponential term, however, at the same time the concentration of atoms decreases according to Saha equation due to ionization. The Intensity - Temperature dependence reaches maximum at a norm temperature.

Axial distribution of emission in ICP l Ionic line emission intensity is described by relation l The most significant constituent of the background of Ar ICP is Ar recombination continuum

Axial distribution of emission in ICP l As it is approximately it means that the intensity of recombination continuum background rises with the second power of concentration of electrons, which again increases with temperature, i.e. with power input. Usually the Ar-background rises more steeply than line intensity with power. l Different spatial distribution of emission and different behaviour of lines led to the following classification: 4Hard lines – atomic lines of elements with high 1 st ionization energies and most of ionic lines 4Soft lines – at. lines with low and medium 1 st ioniz. energies 4Between these 2 groups are ionic lines with low and medium 2 nd ionization energies (Ba II nm, La II nm).

Inductively Coupled Plasma Atomic Emission Spectrometry 4Determination of 73 elements (P, S, Cl, Br, J) 4Simultaneous and fast sequential measurement 4High selectivity 4Low limits of detection ( ng/mL) 4Linear dynamic range (5-7 orders of magnitude) 4Minimum matrix effects (< ± 10 % rel.) 4Introduction of liquid, solid, gaseous samples 4Conventional flows of liquids (mL/min) or microsamples (  L/min)

Inductively Coupled Plasma Atomic Emission Spectrometry 4Acceptable precision ( % rel.) 4Acceptable accuracy (  1 % rel.) 4High sample throughput  routinely determinations per hour 4Automation of operation