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ATOMIC SPECTROMETRIC

METHODS

PART 9

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Interferences:

Four classes of interferences:

1-Spectral interferences

2-Chemical interferences

3 -Refractory compound formation

4 -Physical interferences

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INTERFERENCES:

1. Spectral interferences Interference occur when either another emission line or a molecular

emission band is close to the emitted line of the test element and not resolved by monochromator

Most danger is from molecular emission e.g from oxides of other elements in the sample

Positive interference occur due to light scatter or absorption by solid particles, unvaporized solvent droplets or moleclar species in the flame.

Example:

Al 308.215 nm , V 308.211nm,

Al 309.27 nm

To avoid the interference by observing the aluminum line at

309.27 nm

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2- Ionization interference

Fraction of alkali and alkaline earth elements and several other elements in very hot flames may be ionized in the flame.

Ionization can be suppressed adding a solution of more easily ionized element example potassium (K) or cesium (Cs).

Ionization interference can usually be overcome either by adding the same amount of the interfering element to the standard solutions or by adding large amount to both the sample and the standard.

Ionization can be detected by positive deviation in the calibration curve

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3-Refractory compounds formation

problems)Phosphate interference(

Calcium pyrophosate)Heat resistant (

i.e: does not dissociateno Ca atoms will be formed in the

flame

72

24323

4 )(

OCaP

hotflame

saltPOCaCaPO

Solution

1-EDTA form chelate with Ca and stable

dissociate in the flame to formCa atom (Ca vapor)

2 -releasing agent by high concentration LanthanumChloride or Strontium nitrate react

with PO4-3 3-High temperature flame e.g: nitrous oxide-

acetylene flame

1-Determination of Ca

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2-Oxides Some elements form stable oxides and hydroxides with O or OH

species in flame their formation can be eliminated by high temperature flame

e.g: nitrous oxide flame (more useful flame)

3-Refractory compounds

- variation in gas flow rate

variation in viscosity

variation in density

variation in surface tension

Due to

Temperature

Solvent variation

High solid content

4-Physical interferences

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F

Sample preparation with flame methods can be kept to a minimum Chemical and spectral interferences are absent

The sample in the form of a dilute and filtrated solution

No difference what the chemical from the analyte is

Several elements can be determined in blood, urine, cerebral spinal fluid and other biological fluids by direct aspiration of the sample

(dilution with water to prevent clogging of the burner)

Chemical interferences can be overcome by simple addition of a suitable reagent solution (dilution) e.g in determination of Ca, serum is diluted 1:20 containing EDTA to prevent interference from PO4

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Na and K are added to ca standard (concentration equal to in the serum) to prevent ionization interference

Sample preparation:

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Electrothermal atomizers are types of minifurnace which a drop of the

sample is dried and then decomposed at high temperature to produce an

atomic vapor cloud (graphite furnace).

1-The sample dried at low temperature (drying )

2-destroy organic mater to produces smoke (pyrolysis) at

The smoke flushed out by inert gas (Ar) 3-sample is thermally atomized (Atomization of sample)

Electro thermal atomizers

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Heating in inert gas (Ar) to: 1-to flush out the smoke 2- to prevent oxidation of graphite or C 3-prevent formation of refractory metal oxides

A light passes over the atomizer A sharp peak of absorbance is recorded as atomic cloud passes

through the light The height of observed peak or its area is directly related to the

metal vaporized Even with pyrolysis before atomization, background correction is

more critical in electrothermal. Background absorption in electrothermal methods is more prominent than flame method due to resuduial organic material vaporized materrix salt

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Detection limits by manufactures of electrothermal atomizers are 10-10 to 10-12 g or less, the concentration detection limite depend on sample volume.

This equal to 1 ng/ml or part per billion

Flame methods used when the element concentration high and adequate sample size. However, the electrothermal techniques are required when:

- the concentration very small or sample size are limitted

- solid sample can be analyzed directly without preparing

solution.