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samples and standards, polyethylene bottles are used, since glass can absorb and

release metal ions, and thus impact the accuracy of this sensitive technique.

Cyclic analysis may be performed that involves the estimation of each interfering

substance in a mixture. Subsequently, the standards for each component in the

mixture are doped with each interfering substance. This process is repeated two or

three times with refined estimates of interfering substance, until self-consistent values

are obtained for each component.

Flame instability requires experimental protocols where determination of an unknown

sample is bracketed by measurements of the appropriate standard, in order to achieve the

highest possible accuracy.

Biological samples are usually converted to ash prior to determination of metals.

Wet ashing in solution is often used, employing an oxidative digestion similar to the

Kjeldahl method (see Section 8.3.2).

12.7.3 Applications

Atomic emission and atomic absorption spectrophotometry

Sodium and potassium are assayed at concentrations of a few p.p.m. using simple filter

photometers. The modern emission spectrophotometers allow determination of about

Energy

4 d

4 p

4 s

3 p

3 s

2 p

2 s

D-line transition

1 s

Fig. 12.19Energy levels of atomic orbitals in the sodium atom. Each atomic orbital can be occupied by electrons

following the rules of quantum chemistry until the total number of electrons for that element is reached

(in case of sodium: 11 electrons). The energy gap between the 3sand the 3porbitals in the sodium atom is

such that it can be overcome by absorption of orange light.

518 Spectroscopic techniques: I Photometric techniques