Interferences in AAS

INTERFERENCES IN AAS- SPECTRAL, CHEMICAL AND PHYSISCAL INTERFERENCES

1. INTERFERENCES IN AAS Prepared by- Dr. Mallikarjunaswamy C Assistant Professor Postgraduate Department of Chemistry JSS College of Arts, Commerce and Science Ooty Road, Mysuru-570025

2. 1. Spectral Interferences . Atomic absorption lines are very narrow and absorption lines from different elements almost never overlap ; a much more common problem is molecular absorption . Such absorption can be caused by flame gases/combustion products, undissociated sample-derived molecules and also scattering by particles that may be generated from the sample, especially in electrothermal atomization or in flame AAS when the aspirated sample has much dissolved solids , making it difficult to atomize all of the sample. Unless corrected for, this erroneously measured high absorbance can lead to a gross overestimation of the analyte concentration. INTERFERENCES IN AAS These fall under three classes, S pectral Ch emical, and Physical. We will discuss these briefly and point out their relative effects in emission and absorption measurements. 1. Vapor-phase interferences 2. Condensed-phase interferences 1 Excitation 2 Ionization 3 Dissociation

3. The effects of such background absorption can be corrected for by making sequential measurements of the absorbance due to the background and that due to the analyte + background and determining that due to the analyte by difference . The most common way of doing this is to use an additional light source, a continuum source such as a D 2 lamp , as shown Figure. Such a continuum source background correction method takes advantage of the fact that the background absorption is broad across the entire wavelength span that passes through the slit- monochromator combination (the passband ). When a mirror switches the light source from the HCL to the D2 lamp, the average absorbance is measured across the wavelength region. The atomic absorption line is so narrow compared to this passband that the absorbance due to the analyte atoms is considered essentially negligible. On the other hand, when the absorbance is measured with the HCL, it measures the absorbance due to the analyte atoms as well as the background absorbance at that wavelength .

4. Typically a rotating mirror automatically switches between the two light sources and the difference signal is electronically generated; separate manual measurement or correction is not necessary. A D2 lamp does not provide enough energy above 330 nm, and a quartz halogen lamp needs to be used; low-end instruments do not often provide the ability to use another switchable lamp. The continuum source correction method is far from perfect; mismatches in the correction beam geometry with that of the HCL and lack of uniformity in its spatial and spectral energy distribution and contribution of atomic absorption in the background measurement process, all cause difficulties in high-accuracy measurements. However, it is relatively inexpensive to implement and convenient to use. For example, a vanadium line at 3082.11 Å interferes in the determination of aluminum based on its absorption line at 3082.15 Å. The interference is easily avoided, however, by using the aluminum line at 3092.7 A instead. Spectral interferences also result from the presence of combustion products that exhibit broadband absorption or particulate products that scatter radiation. Both reduce the power of the transmitted beam and lead to positive analytical errors. When the source of these products is the fuel and oxidant mixture alone, the analytical data can be corrected by making absorption measurements while a blank is aspirated into the flame . Note that this correction must be used with both double-beam and single-beam instruments because the reference beam of a double-beam instrument does not pass through the flame

5. Spectral interferences are also observed when components of the matrix other than the analyte react to form molecular species, such as hydroxides and oxides. The resulting absorption and scattering constitutes the sample’s background and may present a significant problem, particularly at wavelengths below 300 nm where the scattering of radiation becomes more important. To help in this problem we must know the composition and nature of the sample’s matrix, the samples then can be prepared using an identical matrix and as a result the background absorption will be same for both the samples under study and standards. Alternatively, the component of the matrix that is responsible for the background can be added in excess to all samples and standards in this way the contribution of the interferent becomes insignificant . Many other types of interferences due to the sample’s matrix can be simply eliminated by raising the atomization temperature . For example, by switching to a higher temperature flame it may be possible to prevent the formation of interfering oxides and hydroxides.

6. Scattering interference Spectral interference because of scattering by products of atomization is most often encountered when concentrated solutions containing elements such as Ti, Zr , and W— which form refractory oxides—are aspirated into the flame. Metal oxide particles with diameters greater than the wavelength of light appear to be formed, and scattering of the incident beam results. Interference caused by scattering may also be a problem when the sample contains organic species or when organic solvents are used to dissolve the sample. Here, incomplete combustion of the organic matrix leaves carbonaceous particles that are capable of scattering light. 2. Physical Interferences. Most parameters that affect the rate of sample uptake in the burner and the atomization efficiency can be considered physical interferences. This includes such things as variations in the gas flow rates, variation in sample viscosity due to temperature or solvent variation , high and variable solids content, Changes in the flame temperature. Sample surface tension changes can affect the size of the nebulized droplets. These can generally be accounted for by frequent calibration, use of internal standards, or standard addition.

7. Chemical Interference - more common than spectral interference 1 ) Formation of Compounds of Low Volatility - Anions + Cations  Salt Ca 2+ +SO 4 2-  CaSO 4 (s) - Decreases the amount of analyte atomized  decreases the absorbance signal - Avoid by : > increase temperature of flame (increase atom production) > add “releasing agents” – other items that bind to interfering ions eg . For Ca 2+ detection add Sr 2+ Sr 2+ + SO 4 2-  SrSO 4 (s) increases Ca atoms and Ca absorbance > add “protecting agents” – bind to analyte but are volatile eg . For Ca 2+ detection add EDTA 4- Ca 2+ + EDTA 4-  CaEDTA 2-  Ca atoms 2 ) Formation of Oxides/Hydroxides M + O  MO M + 2OH  M(OH) 2 - M is analyte - Avoid by : > increase temperature of flame (increase atom production) > use less oxidant A non-volatile & intense molecular absorbance 3) Ionization M  M + + e - - M is analyte - Avoid by: > lower temperature > add ionization suppressor – creates high concentration of e - suppresses M + by shifting equilibrium.

8. Protective agents prevent interference by preferentially forming stable but volatile species with the analyte . Three common reagents for this purpose are EDTA, 8-hydroxyquinoline, and APDC (the ammonium salt of l- pyrrolidinecarbodithioc acid). For example, the presence of EDTA has been shown to minimize or eliminate interferences by silicate, phosphate, and sulfate in the determination of calcium. ionization interferences - Substances that alter the ionization of the analyte also cause ionization interferences. The presence of an easily ionized element, such as K, can alter the extent of ionization of a less easily ionized element, such as Ca. In flames, relatively large effects can occur unless an easily ionized element is purposely added to the sample in relatively large amounts. These ionization suppressants contain elements such as K , Na, Li, Cs, or Rb . When ionized in the flame, these elements produce electrons, which then shift the ionization equilibrium of the analyte to favor neutral atoms.

9. 3. Chemical interferences Chemical interferences are usually specific to particular analytes . They occur in the conversion of the solid or molten particle after desolvation into free atoms or elementary ions. Constituents that influence the volatilization of analyte particles cause this type of interference and are often called solute volatilization interferences. For example, in some flames the presence of phosphate in the sample can alter the atomic concentration of calcium in the flame owing to the formation of relatively nonvolatile complexes. Such effects can sometimes be eliminated or moderated by the use of higher temperatures . Alternatively, releasing agents , which are species (cations) that react preferentially with the interferent and prevent its interaction with the analyte , can be used. For example, the addition of excess Sr or La minimizes the phosphate interference on calcium because these cations form stronger phosphate compounds than Ca and release the analyte .