Page 83 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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Figure 3.7
Isothermal operation using a platform.
modern instruments even have a platform as an integral part of the tube. Using such platforms, some
interference effects can be reduced noticeably.

More recently, an alternative technique has been developed. The use of probe atomization became
popular in the mid-1980s and has been shown to offer the same advantages as a platform. The sample is
pipetted on to a graphite probe and the normal drying and ashing cycles are performed. The probe is
then removed from the tube, which is then heated to the atomization temperature. When this has been
done, the probe is re-introduced into the tube and is heated by the hot gas present, allowing the atoms to
form in an isothermal atmosphere.

3.6.4 Matrix Modification

Matrix modification is a term first coined by Ediger in 1975 [At. Absorpt. Newsl. 14, 127 (1975)],
although 'chemical modification' is now often the preferred term. It is a process by which a chemical or
a combination of chemicals is added to the sample so that the analyte may be separated from the matrix
more easily, hence facilitating interference-free determinations. This may be achieved in two ways. The
first is to accelerate the removal of the matrix by adding a chemical that volatilizes the matrix at a lower
temperature. Examples of agents such as these include ammonium nitrate, nitric acid or oxygen (air).
Ammonium nitrate and nitric acid assist in the removal of chloride ions (see Section 3.5.4) whilst
leaving the analyte in the atomizer. Oxygen or air is used frequently if biological samples are to be
analysed. Gases such as these literally combust the organic matrix and leave the analyte in the atomizer
ready to be atomized into a relatively matrix-free (and hence interference-free) atmosphere.

A large number of matrix modifiers have been developed that thermally stabilize the analyte, allowing
higher ash temperatures to be used without analyte loss. In this way, more matrix may be removed
leaving less to interfere with the analyte's determination. Examples of this type of matrix modifier
include some transition metal ions, e.g. Ni and Pd, which form thermally stable intermetallic
compounds with the metalloids, e.g. As-Ni,