ATOMIC ABSORPTION AND ATOMIC FLUORESCENCE SPECTROMETRY Chap 9 Source Modulation Interferences in Atomic Absorption Interferences in Atomic Absorption Spectral Spectral Chemical Chemical Atomic Fluorescence Spectroscopy Atomic Fluorescence Spectroscopy

AA Source Modulation Need to eliminate emission from analyte atoms Need to eliminate emission from analyte atoms Source beam is chopped Source beam is chopped Chopper placed here

Beam chopper for subtracting flame subtracting flame background emission (a) Lamp and flame emission reach detector (b) Only flame emission reaches detector (c) Resulting signal

ATOMIC ABSORPTION AND ATOMIC FLUORESCENCE SPECTROMETRY Chap 9 Source Modulation Source Modulation Interferences in Atomic Absorption Spectral Chemical Atomic Fluorescence Spectroscopy Atomic Fluorescence Spectroscopy

Spectral Interferences 1) Undesired signals overlapping analyte signal e.g., V at nm near Al at nm ∴ use Al at nm 2) combustion products  broadband absorption subtract signal from a blank subtract signal from a blank 3) matrix interferences (most serious) use higher T use higher T use radiation buffer use radiation buffer add excess of known interferent add excess of known interferent

Other methods of correcting for matrix effects Continuum Source (D 2 ) Correction Method Continuum Source (D 2 ) Correction Method D 2 lamp provides continuum D 2 lamp provides continuum UV abs. is subtracted from analyte signal UV abs. is subtracted from analyte signal Fig Fig. 9-14

Fig 9-14Continuum-source Fig 9-14 Continuum-source background correction

Other methods of correcting for matrix effects Continuum Source (D 2 ) Correction Method Continuum Source (D 2 ) Correction Method D 2 lamp provides continuum D 2 lamp provides continuum UV abs. is subtracted from analyte signal UV abs. is subtracted from analyte signal Fig Fig Zeeman Background Correction Zeeman Background Correction based on splitting of absorption lines based on splitting of absorption lines by a magnetic field by a magnetic field lines absorb different polarizations lines absorb different polarizations

Fig 9-14 Background correction by Zeeman effect B field splits atomic absorption lines (Zeeman effect) Line absorption differs with polarization of source

Chemical interferences (very common) 1) Certain components of the sample decrease the extent of atomization the extent of atomization e.g., SO 4 2- and PO 4 3- hinder atomization of Ca 2+ Add releasing agent: Sr +, La 3+, etc.Add releasing agent: Sr +, La 3+, etc. Add protecting agent: EDTA, hydroxyquinolineAdd protecting agent: EDTA, hydroxyquinoline 2)Ionization interference Occurs when O 2 or N 2 O is oxidantOccurs when O 2 or N 2 O is oxidant Analyte ionizes and releases electrons at high TAnalyte ionizes and releases electrons at high T M ⇌ M + + e -

Table 9-2 Degree of Ionization with temperature Add ionization supressor: K, Rb, Cs saltsAdd ionization supressor: K, Rb, Cs salts Provides high concentration of electrons to flameProvides high concentration of electrons to flame M ⇌ M + + e - shifted to the leftM ⇌ M + + e - shifted to the left

Fig 9-17 Effect of K as ionization suppressor for Ca

Table 9-3 Detection limits (ppb) for selected elements

ATOMIC ABSORPTION AND ATOMIC FLUORESCENCE SPECTROMETRY Chap 9 Source Modulation Source Modulation Interferences in Atomic Absorption Interferences in Atomic Absorption Spectral Spectral Chemical Chemical Atomic Fluorescence Spectroscopy

Absorption and fluorescence by atoms in a flame

Set-up for a Luminescence Experiment If source is at fixed λ and monochromator is scanned an emission spectrum results. If source is scanned, and monochromator is at a fixed λ, an excitation spectrum results. Fig. 7-1 (b) 90°