Inductively Coupled Plasma (ICP) Analysis Techniques

4/28 /2023 1 Inductively coupled plasma (ICP) •The inductively coupled plasma : The higher temperature (6000 –10000 K) and more stable with relatively inert Ar environment than the flame, so less interference •Only for Emsission not absorption •Simultaneous multi -element analysis •...

4/28 /2023 1 Inductively coupled plasma (ICP) •The inductively coupled plasma : The higher temperature (6000 –10000 K) and more stable with relatively inert Ar environment than the flame, so less interference •Only for Emsission not absorption •Simultaneous multi -element analysis •More costive to purchase and to run than AA. •After a spark from a Tesla coil ionizes Ar , free electrons are accelerated by the radio -frequency field. •Electrons collide with atoms and transfer energy to the entire gas, maintaining a temperature of 6 000 to 10 000 K. •The quartz torch is protected from overheating by Ar coolant gas. Inductively coupled plasma cross -sectional view of an ICP burner 4/28 /2023 2 Ultrasonic Nebulizer -Sample solution is directed onto a piezoelectric crystal oscillating at 1 MHz - Vibrating crystal creates a fine aerosol that is carried by Ar - In the next refrigerated zone, solvent condenses and is removed. - Remaining solvent vapor diffuses through the membrane and is swept away by flowing Ar. -Analyte reaches the plasma flame as an aerosol of dry, solid particles. -Plasma energy is not needed to evaporate solvent, so more energy is available for atomization -The concentration of analyte needed for adequate signal is reduced by a factor of 3 to 10. Detection Limit S/N = 2.4 4/28 /2023 3 Temperature is a critical factor in determining -the degree to which a given sample breaks down to atoms (atomization efficiency) -the extent to which a given atom is found in its ground state, excited state or ionized states -describes the relative populations of different states at thermal equilibrium Consider an atom with energy levels E0 and E* separated by DE (Figure 20 -14). An atom (or molecule) may have more than one state at a given energy. The number of states at each energy is called the degeneracy . Here, the degeneracies are g0 and g*. where T is temperature (K) and k is Boltzmann’s constant (1.381 x 10 -23J/K). The Boltzmann distribution applies to a system at thermal equilibrium. -the relative population of any two states is 4/28 /2023 4 Effect of Temperature on Absorption & Emission This is why AA is not very suitable for Na to be determined by the AA. Little absorption is achieved at such high temperature •Absorption arises from ground -state atoms, but emission arises from excited -state atoms. •Emission intensity is proportional to the population of the excited state. •Because the excited state population changes by 4% when the temperature rises 10 K , emission intensity rises by 4% . Effect of Temperature on Absorption & Emission •It is critical in atomic emission spectroscopy that the flame be very stable, or emission intensity will vary significantly. • In atomic absorption spectroscopy, temperature variation is important but not as critical. •Almost all atomic emission is carried out with an inductively coupled plasma, whose temperature is more stable than that of a flame. • Plasma is normally used for emission, not absorption, because it is so hot that there is a substantial population of excited -state atoms and ions. 4/28 /2023 5 •Energy levels of halogen atoms (F, Cl, Br, I) are so high that they emit ultraviolet radiation below 200 nm. •This spectral region is called vacuum ultraviolet because radiation below 200 nm is absorbed by O 2, so spectrometers for the far -ultraviolet were customarily evacuated. •Some plasma emission spectrometers use N 2to exclude air so that the region 130 to 200 nm is accessible and Cl, Br, I, P, and S can be analyzed. Effect of Temperature on Absorption & Emission Atomic linewidth Heisenberg Uncertainty principle : The product of the uncertainties in a pair of complementary variables should be at least h/4 p, So DX. DY where dEis the uncertainty in the energy difference between ground and excited states, dt is the uncertainty in the lifetime of the excited state before it decays to the ground state, and h is Planck’s constant. From this equation: the uncertainty in the energy difference between two states multiplied by the lifetime of the excited state is at least h/4p. If dtdecreases, then dEincreases. Linewidth is limited by the Heisenberg uncertainty principle If energy and time are the two variables, The shorter the lifetime of the excited state, the more uncertain is its energy Werner Heisenberg 4/28/2023 6 The lifetime of an excited state of an isolated gaseous atom is 10 -9s. Therefore, the uncertainty in its energy is Suppose that the energy difference ( D E)between the ground and the excited state of an atom corresponds to visible light with a wavelength of l= 500 nm. D E = hc/l The relative uncertainty in the energy difference is Thus, the relative uncertainty in wavelength dl/Dl is the same as the relative uncertainty in energy: The inherent linewidth of an atomic absorption or emission signal is ~10-4nm because of the short lifetime of the excited state. However in reality, linewidth is larger than 10-4nm dl is the width of an absorption or emission line measured at half the height of the peak. •Two mechanisms broaden the lines to 10 -3to 10 -2nm in atomic spectroscopy: Doppler effect & pressure broadening •Doppler effect An atom moving toward the radiation source experiences more oscillations of the electromagnetic wave in a given time period than one moving away from the source & the atom moving toward the source absorbs high frequency light than that absorbed by the one moving away. The linewidth, dl, due to the Doppler effect, is In a hot flame, atoms move in all directions causing the doppler effect 4/28/2023 7 Linewidth is also affected by pressure broadening from collisions between atoms 14 Hollow Cathode Lamp Monochromators cannot isolate lines narrower than 10 -3–10 -2nm. Monochromator bandwidth (~100x) greater than atomic lines To get narrow lines of the correct frequency, A hollow cathode lamp containing the same element that being analyzed is used. 1 atm = 101325 Pa 4/28/2023 8 How does it work? Hollow Cathode Lamp Apply sufficiently high voltage between the cathode and the anode: (3) Excitation of the cathode element (M) M(g) + Ne += M *(g) + Ne (4) Emission of radiation M*(g) → M(g) + h  (1) Ionization of the filler gas: Ne + e -= Ne ++ 2e - (2) Sputtering of the cathode element (M): M(s) + Ne += M(g) + Ne 16 •The cathode of the hollow cathode lamp (HCL) contains the element being analyzed. •Therefore the atomic radiation emitted by the HCL has the same frequency as that absorbed by the analyte atoms in the flame or furnace. •The linewidth from the HCL is relatively narrow (compared to linewidths of atoms in the flame or furnace) because of low pressure in lamp and lower temperature in lamp (less Doppler broadening). •Thus the linewidth from the HCL is nearly “monochromatic” (compared to sample). •Different lamp required for each element although some are multi -element. Hollow Cathode Lamp 4/28 /2023 9 •An inductively coupled plasma emission spectrometer does not require any lamps and can measure as many as 70 elements simultaneously. •Atomic emission enters the polychromator and is dispersed into its component wavelengths by the grating at the bottom. •One photomultiplier detector is required at the correct position for each element. • Because inductively coupled plasma emission spectrometer does not have hollow cathode lamp, a large number of elements can be analyzed • In one design, atomic emission entering the polychromator is dispersed by the grating. • One photomultiplyer detector is needed at the right position for each element Chapter 20 18 Photomultiplier Tube Electrons emitted from photosensitive surface are accelerated towards more positive dynodes The process is repeated so that more than a million electrons are produced per photon. PMT are very sensitive detectors (usually used for luminescence). 4/28/2023 10 • In another design, only one detector is used. • Atomic emission is reflected from a collimating mirror, making light rays parallel. • A prism disperse the light vertically • A grating then disperses the light horizontally • Now we have 2 -D dispersal of the light • The light then hits the detector, a charge injection device(CID). • Different wavelengths are dispersed across the 262000 pixels of the CID • Each pixel is read any time. Example of spectrum of Fe solution y CID detector All the spectra of Iron are visible 4/28/2023 11