Atomic Absorption Spectroscopy Prof Dr Hisham E Abdellatef

Atomic Absorption Spectroscopy Prof Dr Hisham E Abdellatef 2011

Light source used for AA • What light source do we use with AA? • Would it be a continuous light source or a line light source? • A line light source is used for AA Prof. Dr. Hisham Ezzat Abdellatef 2

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Absorption of resonance line from vapor lamp Prof. Dr. Hisham Ezzat Abdellatef 5

Hollow-Cathode Lamp, HCL • The hollow cathode lamp (HCL) uses a cathode made of the element of interest with a low internal pressure of an inert gas. • A low electrical current (~ 10 m. A) is imposed in such a way that the metal is excited and emits a few spectral lines characteristic of that element (for instance, Cu 324. 7 nm and a couple of other lines; Se 196 nm and other lines, etc. ). • The light is emitted directionally through the lamp's window, a window made of a glass transparent in the UV and visible wavelengths. Prof. Dr. Hisham Ezzat Abdellatef 6

Hollow Cathode Lamp • Most common source for AA. • Ionization of inert gas at high potential. • Gaseous cations cause metal atoms at cathode to sputter, emitting characteristic radiation. Prof. Dr. Hisham Ezzat Abdellatef 7

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Electrodeless discharge lamps • • A salt containing the metal of interest is sealed in a quartz tube along with an inert gas Discharge lamps, such as neon signs, pass an electric current through the inert gas. The electrons collide with inert gas atoms, ionizing them and accelerating their cations to collide with the metal atoms which then will be excited and decay to lower levels by emitting electromagnetic radiation with wavelengths characteristic to the element. Low-pressure lamps have sharp line emission characteristic of the atoms in the lamp, and high-pressure lamps have broadened lines superimposed on a continuum. Common discharge lamps and their wavelength ranges are: hydrogen or deuterium : 160 - 360 nm mercury : 253. 7 nm, and weaker lines in the near-uv and visible Ne, Ar, Kr, Xe discharge lamps : many sharp lines throughout the near-uv to near. IR xenon arc : 300 - 1300 nm Deuterium lamps are the Uv source in Uv-Vis absorption spectrophotometers. The sharp lines of the mercury and inert gas discharge lamps are useful for wavelength calibration of optical instrumentation. Mercury and xenon arc lamps are used to excite fluorescence. Prof. Dr. Hisham Ezzat Abdellatef 12

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Electrodeless Discharge Lamp • They sometimes provide superior performance to HCL. • Their useful lifetime is longer than HCL • They provide light intensity 10 -100 times more than that of HCL. • They are less stable than HCL Prof. Dr. Hisham Ezzat Abdellatef 14

ATOMIC ABSORPTION INSTRUMENTATION Instruments for atomic absorption spectrometry (AAS) consist of a radiation source, a sample holder, a wavelength selector, a detector, and a signal processor and readout. The sample holder in atomic absorption instruments is the atomizer cell that contains the gaseous atomized sample. Prof. Dr. Hisham Ezzat Abdellatef 15

1. Radiation Sources: It is necessary that band width of the radiation source must be narrow relative to the width of an absorption peak. The problem created by limited width of atomic absorption peaks has been solved by the use of line sources with bandwidths even narrower than absorption peaks. Prof. Dr. Hisham Ezzat Abdellatef 16

Fig. 9 -10 a shows the emission spectrum of a typical atomic lamp source. With a suitable filter or monochromator, all but one of these lines are removed. Fig. 9 -10 b shows the absorption spectrum for the analyte between wavelengths 1 and 2. Passage of the line from the source through the flame reduces its intensity from P 0 to P; the absorbance is then given by log(Po/P), which is linearly related to the concentration of the analyte in the sample. A disadvantage of the procedure is that separate lamp source is needed for each element. Prof. Dr. Hisham Ezzat Abdellatef 17

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Hollow Cathode Lamps: It is the most common source for atomic absorption measurements. This lamp consists of a tungsten anode and a cylindrical cathode sealed in a glass tube that is filled with neon or argon at a pressure of 1 to 5 torr. The cathode is constructed of the metal whose spectrum is desired. Ionization of the inert gas occurs when a potential on the order of 300 V is applied across the electrodes, which generates a current of about 5 to 15 m. A. Prof. Dr. Hisham Ezzat Abdellatef 19

… Hollow Cathode Lamps continued… If the potential is sufficiently large, the gaseous cation acquire enough kinetic energy to dislodge some of the metal atoms from the cathode surface and produce an atomic cloud in a process called sputtering. A portion of the sputtered metal atom are in excited states and thus emit their characteristic radiation as they return to the ground state. Eventually, the metal atoms diffuse back to the cathode surface or to the glass walls of the tube and are redeposited. Prof. Dr. Hisham Ezzat Abdellatef 20

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Electrodeless Discharge Lamps (EDLs): These provide radiant intensities that are usually one to two orders of magnitude greater than hollow cathode lamps. A typical lamp is constructed from a sealed quartz tube containing a few torr of an inert gas such as argon and a small quantity of the metal (or its salt) whose spectrum is of interest. Prof. Dr. Hisham Ezzat Abdellatef 22

…Electrodeless Discharge Lamps (EDLs) continued… The lamp is energized by an intense field of radio-frequency or microwave radiation. Ionization of the argon occurs to give ions that are accelerated by the high-frequency component of the field until they gain sufficient energy to excite the atoms of the metal whose spectrum is sought. Electrodeless discharge lamps are available commercially for 15 or more elements. Prof. Dr. Hisham Ezzat Abdellatef 23

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Atomic Absorption Instrumentation • Hollow Cathode Lamp – Ionization of inert gas by potential – Gas acceleration to cathode – Atoms on cathode into gas state • Some excited • Deexcite with photon emission – Need to excite specific elements for measurement

Atomic Absorption Instrumentation • Electrodeless Discharge Lamps – Inert gas in quartz tube – Excite gas with RF – Similar to cathode expect excitation

How a HCL Works • An applied potential of ~ 300 V DC ionizes the inert gas Ne (g) -----> Ne+* (g) + e • The ionized gas generates a current flow in the lamp • Metal cations (e. g. Fe, Mn, Ca) on the cathode acquire (kinetic) energy from the ionized gas and dislodge into the vacuum • A “cation cloud” forms around the cathode (a process called sputtering) Prof. Dr. Hisham Ezzat Abdellatef 27

How a HCL Works (cont. ) • Some of the sputtered cations are in excited states (M*) and emit light (hn) as they return to ground state (M 0) M* -----> M 0 + hn • sputtered cations redeposit; this occurs mostly on the cathode, but some also deposit on the inner glass surface • Light intensity limitation –> self absorption – As the current increases, M sputtered increases, but the % M* decreases. Unexcited gaseous atoms (M 0) absorb light produced within the lamp, preventing it from exiting the lamp Prof. Dr. Hisham Ezzat Abdellatef 28

Radiation Sources for AAS Electrodeless Discharge Lamps Characteristics • The EDL houses a sealed quartz tube (lamp) containing argon gas and a metal (or metal salt) of interest • The quartz tube is under high vacuum • A radio frequency (or microwave) coil surrounds the lamp • ~ 10 times more intense than a hollow cathode lamp • Unstable output • Only available for about 15 elements Prof. Dr. Hisham Ezzat Abdellatef 29

How an EDL Works • An intense RF (or microwave) field is applied to the sealed quartz tube within the lamp • Ar gas within the tube ionizes and gains kinetic energy from the RF field • Energy is transferred to the metal upon collision • Excited metal returns to ground state, emitting light (hn) Ar (g) -----> Ar* (g) + M (s) -------> M* (g) + Ar (g) hn M (s) Prof. Dr. Hisham Ezzat Abdellatef 30

Spectral Line Sources • Light sources emit spectral lines – Lines in the line spectrum of the analyte being measured • Preferred b/c they represent the precise wavelengths needed for the absorption in the flame – Flame contains this particular analyte • Emitted b/c they contain the analyte to be measured – When lamp is on » Internal atoms are raised to the excited state » Emit their line spectrum when they return to the ground state » This is the light directed through the flame Prof. Dr. Hisham Ezzat Abdellatef 31

Hollow Cathode Lamp • Hollow Cathode Lamp – Most widely used spectral line source – Cathode • Negative electrode • Contains the internal atoms – Hollowed cup – Internal excitation and emission process occurs inside this cup when lamp is on – Anode • Positive electrode – Connected with cathode to a high voltage – Light emitted Prof. Dr. Hisham Ezzat Abdellatef 32

Hollow Cathode Lamp • Hollow Cathode Lamp – Sealed glass tube – Filled with inert gas at low pressure • Neon or argon – How it works (fig. 9. 6, pg 251) • Lamp turned on and argon atoms ionize – Positively charged argon ions then crash into the negatively charged cathode – Causes sputtering » Transfer of surface atoms in the solid phase to the gas phase due to the collisions • More collisions of argon ions with metal atoms cause metal atoms to be raised to the excited state – Light emitted with they drop back to the ground state Prof. Dr. Hisham Ezzat Abdellatef 33

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Hollow Cathode Lamp • Hollow Cathode Lamp – Must contain the element being measured • Usually have number of different lamps in stock – Interchanged in the instrument – Some are multi-elemental • Several different specific atoms present in the lamp – Separated by a monochromator after the flame to isolate the specific spectral line of the analyte Prof. Dr. Hisham Ezzat Abdellatef 35