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Chapter: Modern Analytical Chemistry: Spectroscopic Methods of Analysis

Evaluation - Atomic Absorption Spectroscopy

Scale of Operation: Atomic absorption spectroscopy is ideally suited for the analy- sis of trace and ultratrace analytes, particularly when using electrothermal atomization.

Evaluation

Scale of Operation 

Atomic absorption spectroscopy is ideally suited for the analy- sis of trace and ultratrace analytes, particularly when using electrothermal atomiza- tion. By diluting samples, atomic absorption also can be applied to minor and major analytes. Most analyses use macro or meso samples. The small volume re- quirement for electrothermal atomization or flame microsampling, however, allows the use of micro, or even ultramicro samples.

Accuracy 

When spectral and chemical interferences are minimized, accuracies of 0.5–5% are routinely possible. With nonlinear calibration curves, higher accuracy is obtained by using a pair of standards whose absorbances closely bracket the sam- ple’s absorbance and assuming that the change in absorbance is linear over the lim- ited concentration range. Determinate errors for electrothermal atomization are frequently greater than that obtained with flame atomization due to more serious matrix interferences.

Precision 

For absorbances greater than 0.1–0.2, the relative standard deviation for atomic absorption is 0.3–1% for flame atomization, and 1–5% for electrothermal atomization. The principal limitation is the variation in the concentration of free- analyte atoms resulting from a nonuniform rate of aspiration, nebulization, and at- omization in flame atomizers, and the consistency with which the sample is heated during electrothermal atomization.

Sensitivity 

The sensitivity of an atomic absorption analysis with flame atom- ization is influenced strongly by the flame’s composition and the position in the flame from which absorption is monitored. Normally the sensitivity for an analysis is optimized by aspirating a standard and adjusting operating condi- tions, such as the fuel-to-oxidant ratio, the nebulizer flow rate, and the height of the burner, to give the greatest absorbance. With electrothermal atomization, sensitivity is influenced by the drying and ashing stages that precede atomiza- tion. The temperature and time used for each stage must be worked out for each type of sample.

Sensitivity is also influenced by the sample’s matrix. We have already noted, for example, that sensitivity can be decreased by chemical interferences. An increase in sensitivity can often be realized by adding a low-molecular-weight alcohol, ester, or ketone to the solution or by using an organic solvent.

Selectivity 

Due to the narrow width of absorption lines, atomic absorption pro- vides excellent selectivity. Atomic absorption can be used for the analysis of over 60 elements at concentrations at or below the level of parts per million.

Time, Cost, and Equipment 

The analysis time when using flame atomization is rapid, with sample throughputs of 250–350 determinations per hour when using a fully automated system. Electrothermal atomization requires substantially more time per analysis, with maximum sample throughputs of 20–30 determinations per hour. The cost of a new instrument ranges from $10,000 to $50,000 for flame atomization and $18,000 to $70,000 for electrothermal atomization. The more ex- pensive instruments in each price range include double-beam optics and auto- matic samplers, are computer controlled, and can be programmed for multiele- mental analysis by allowing the wavelength and hollow cathode lamp to be changed automatically.

 

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