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Study Notes: Analytical Wavelength and Solvents

One factor that leads to deviations from Beer’s Law is the concentration range of the species of interest. At the working range, the range of concentrations that will produce suitable absorbance value for practical analysis, there is little or no deviation.

However, where the concentration range is above the working range, strong deviations from Beer’s Law occur leading to curvature of the standard curve. In such cases the flame AAS analyst has a number of options:

  1. selecting alternative (less sensitive) wavelengths
  2. rotating the burner across the optical path
  3. doing both.

An example of working concentrations of Cobalt (Co) is shown in the following Table.

Burner Position
Wavelength nm
Relative Sensitivity
Working Range mg/µL
0.06 - 15
1.0 - 240
Rotated 90°
1.2 - 300
Rotated 90°
20 - 4800

Reproduced with permission from Introducing Atomic Absorption Analysis – Varian, Table 2, page 76

Notice that changing the wavelength to a less sensitive wavelength allows a higher working range and permits analysis of samples containing higher concentrations of Co (compare examples 1 and 2).

Notice that rotating the burner across the optical path reduces sensitivity and allows a higher working range and analysis of more concentrated solutions of Co (compare examples 1 and 3).

Notice that combining a less sensitive wavelength with a less sensitive burner position reduces the relative sensitivity by a factor of 320 for Co (compare examples 1 and 4) and allows analysis of more highly concentrated solutions than in the previous two examples.

On the other hand, it is sometimes useful to increase sensitivity of detection, especially with samples of low concentration. One way is to concentrate the sample by evaporation of solvent but this may not always be possible or practicable. Another way, if using GFAA, is to make sequential additions to the graphite tube followed by ashing but not atomisation until the required total sample is added to the graphite tube.

For flame AAS, the use of solutions containing low-molecular-weight alcohols, esters and ketones leads to enhanced absorption peaks. This effect is largely attributable to increased nebuliser efficiency due to lower surface tension and finer drop sizes and a consequent increase in the amount of sample that reaches the flame. More rapid solvent evaporation may also add to the enhancing effect. Leaner fuel-oxidant ratios must be used to offset the presence of added organic material but this results in lower flame temperature and an increase in chemical interferences.



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