miércoles, 15 de abril de 2015

Technological advances in the quantification of trace elements in iron based materials

Despite not being defined in the specifications of chemical composition for the different material qualities, the need to verify and to quantify certain trace elements in iron based materials (cast iron, low and medium alloy steels, as well as stainless steels) is due to the influence that these trace elements can have in the own properties of the material, or even in the various interactions that may occur within other elements, that might negatively affect the main characteristics of these type of materials.

Usually, it is a residual presence (low or very low concentrations of certain elements), which can generate an adverse effect and consequently an undesired influence in the characteristics of such materials.

For example, to be able to quantify trace elements levels as Ce, La and Sb allows establishing limit relations for a specific design or thermal module of a casting, which might develop graphite nodularity problems or certain types of graphitic malformations during solidification, such as chunky graphite or spiky graphite. Another clear example is related to the Pb content in stainless steels, that in mg/kg levels allows advancing possible difficulties or problems during the lamination process. 
The analytical technique that enables this quantification is known as Inductively Coupled Plasma Mass Spectrometry (ICP-MS), characterized by achieving an extraordinary high resolution for very low concentrations.

The mass detector coupled with the collision cell that minimizes the interferences from double loads, oxides or polyatomic molecules, achieve quantification limits of few ppb (parts per billion) for all elements between Li (7 amu) and Uranium (250 amu). Its main advantage is therefore the low detection limits obtained for most of the elements.

The complete process of sample nebulization, argon plasma atomization, electromagnetic separation of neutral molecules, interference elimination using the collision cell, separation of ions according to the mass ratio between the charge and subsequent quadrupole mass detection are perfectly described in the following video.

This level of accuracy allows quantifying these trace elements by a prior sample preparation using acid digestion with ultra high purity reactives, to certify afterwards levels of few ppm (mg/kg) from the different elements in the sample.

One of the main problems in confirming these results is the absence of international certified standards contrasting the presence of these elements to such accuracy levels and in such matrices. To overcome this important gap and to have control samples (QC) with certified results, IK4-Azterlan has done an intensive work putting forth specific efforts in the development of internal control standards from certified synthetic patterns, keeping the matrix present in the samples.

Therefore it is possible to analyze the main elements at trace levels in iron based components and even the most critical elements for this type of materials (such as As, Nb, Cd, Sn, Sb, Ba, La, Ce, Pr, Nd, Pb, Bi), that can be certified within the scope from the Standard UNE-EN ISO/IEC 17025 accreditation from 5 down to 200 ppm.

Example of the calibration curve for Sb121


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