Chemical Analysis by Optical Emission Spectrometry

Chemical analysis by Optical Emission Spectrometry (OES) determines with high accuracy the elemental composition of metallic materials and the percentage by weight of those elements. The physical basis of the method lies in the excitation and emission behaviour of atoms: a high-energy electrical discharge (spark or arc) created in the gap between the specimen surface and a tungsten electrode vaporises material from the surface to a depth of the order of microns and excites the atoms in this vapour to higher energy levels. Because the excited atoms are unstable, they emit light (photons) at wavelengths specific to each element as they return to their ground state. This light is the "optical emission" phenomenon that gives the method its name, and it contains the characteristic spectral lines that act, in effect, as a fingerprint of each element.

In practice the specimen is brought into contact with the spark stand (excitation stand) of the spectrometer under an argon atmosphere. The argon shielding gas prevents oxygen and nitrogen in the air from disturbing the analysis, thereby allowing the lines at low wavelengths (particularly carbon, sulphur and phosphorus) to be read. The light emitted during the discharge is separated into its component wavelengths in the optical system by means of a diffraction grating. The intensity of the spectral line belonging to each element is proportional to the concentration of that element in the specimen. Detectors (photomultiplier tubes or CCD arrays) convert these intensities into electrical signals and translate them into concentration values via calibration curves established beforehand with certified reference materials.

In terms of equipment and technique, OES is a multi-channel system capable of measuring a large number of elements simultaneously (such as C, Si, Mn, P, S, Cr, Ni, Mo, V, Cu, Al and Ti) within seconds. The greatest strength of the method is that the carbon content can be determined accurately; this is what distinguishes OES from portable XRF (X-ray fluorescence) instruments, which cannot measure carbon. Carbon directly governs the strength, hardness and weldability behaviour of steel and cast iron, and is therefore of critical importance in material classification. On the other hand, because the OES method leaves a small burn mark on the specimen surface and requires a conductive, flat and clean surface, the test is classified within destructive testing (DT).

At application-specific depth, the quality of the analysis depends largely on specimen preparation. The surface is freed of the outer layer containing oxide scale, paint, coating and segregation by grinding or turning, and is brought to a smooth, metallic bright condition. Separate calibration programmes and matrix matching are used for different material groups (low-alloy steel, stainless steel, cast iron, aluminium, copper, nickel-based alloys); an incorrect matrix selection leads to systematic error. On heat-treated or cast parts, failure to remove the decarburised (carbon-depleted) surface layer sufficiently can make the measured carbon value appear lower than it really is. For this reason each measurement is generally repeated at different points on the specimen and averaged.

At the reporting and acceptance stage, the element percentages obtained are compared with the composition ranges prescribed by the relevant material standard (for example EN 10025 structural steels, EN 10088 stainless steels, or ASTM/AISI grade tables). The analysis result is used for material verification (PMI – Positive Material Identification), for separating mixed materials, for incoming goods inspection, for failure analysis and for confirming the compatibility of weld filler metal. The report contains the value for each element measured, the standard applied, the calibration status and, where applicable, the measurement uncertainty. These data constitute objective and traceable evidence that the material conforms to the declared grade. TÜV AUSTRIA SILA KALİTE performs chemical analysis by optical emission spectrometry within the scope of its TS EN ISO/IEC 17025 accreditation, so that results are reported traceably and reliably within an internationally recognised competence framework.