Page 59 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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The computer will then multiply the concentration obtained by the relevant dilution factor
automatically, and the concentration in the undiluted sample will be read out.

In many instruments, the computer will control the instrument settings (if a coded hollow-cathode lamp
or a procedure that has been stored in the memory is being used, the computer may set the wavelength
automatically). In addition, the recommended flame composition and the height at which readings will
be taken may also be optimized.

Q. In AAS, where does noise come from?

Q. How can noise problems be minimized?

Q. What is the advantage of logarithmic read-out in AAS?

Q. When is curve correction needed and how can it be performed?

2.3 Sensitivity and Limit of Detection

Limits of detection for FAAS are typically in the range 0.01-0.1 µg ml . A full list is given in Table
-1
2.2. The linear dynamic range (i.e. the maximum range over which the calibration curve is linear) is
limited by the propensity for self-absorption in the flame, and is generally no more than three orders of
magnitude (e.g. from 0.01 to 10 µg ml )
-1

2.4 Interferences and Errors

The use of a line source and the ratio method (i.e. I /I) tend to minimize errors in AAS. Thus, if the
0
wavelength setting is seriously incorrect, it is unlikely that any absorption will be observed. If the
wavelength is incorrectly tuned, the effects on the value of I will roughly equal those on the value of I ,
0
and the error may not be too serious.

Since AAS is a ratio method, many instrumental errors (e.g. long-term source drift, small
monochromator drifts) should cancel out, as I is ratioed to I . However, a stable uptake rate, or
0
aspiration rate, is required. This falls as the viscosity of the solution sprayed is increased. Nebulizer
uptake interferences can be minimized if the dissolved salts content of samples and standards is
-3
approximately matched. For example, when determining µg cm sodium levels in 2 M phosphoric acid,
ensure that the standards are also dissolved in 2 M phosphoric acid, using a blank to check for
contamination.