Page 30 - AAS & AES & FES 01082016_Neat

Qualitative Analysis
The radiation source used in AAS is an HCL or an EDL, and a different lamp is needed

for each element to be determined. Because it is essentially a single-element technique, AAS is
not well suited for qualitative analysis of unknowns. To look for more than one element requires

a significant amount of sample and is a time-consuming process. For a sample of unknown
composition, multielement techniques such as XRF, ICP-MS, inductively coupled plasma-optical

emission spectrometry, and other atomic emission techniques are much more useful and

efficient.
Quantitative Analysis

Quantitative measurement is one of the ultimate objectives of analytical chemistry. AAS
is an excellent quantitative method. It is deceptively easy to use, particularly when flame

atomizers are utilized.
During most measurements of atomic absorption, Beer's law A = kbC is obeyed. The

absorbance is directly proportional to the concentration of atoms in the cell. When the

experimental conditions are fixed, the concentration of atoms in the cell is proportional to the
concentration of the element in the sample solution. Consequently, the measured absorbance is

proportional to the elemental composition of the sample.
Experimental variables such as the aspiration rate into a flame, the region of a flame

through which EMR from the source passes, the flow rates of the gases entering the flame, and

the temperature of the cell during the measurement have an effect upon the proportionality
constant k.

Many spectrophotometers contain dedicated microcomputers. The concentrations of two
or more standards are entered into the computer and the absorbance of the corresponding

solutions is measured. The microcomputer uses the information to calculate the concentration of

a sample solution from its absorbance. The computer uses either the working-curve method or
standard-addition technique. It eliminates the need for the analyst to prepare the graphs. The

elements that can be assayed with AAS and the wavelengths that normally are used for the
assays are listed in Fig.

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