|M.Sc Student||Karni Nurit|
|Subject||Matrix Effects in Laser-Induced Breakdown|
Spectroscopy of Calcium in Polyvinyl Chloride
|Department||Department of Chemistry||Supervisor||PROF. Israel Schechter|
|Full Thesis text|
Laser-induced breakdown spectroscopy (LIBS) is a promising analytical method of atomic emission spectroscopy (AES), which allows remote simultaneous multi-elemental analysis, and requires no sample preparation but only optical access to the sample. This method is fast, simple, flexible and allowing case-specific adjustments, for measuring all kinds of samples at all states (gas, liquid, solution, solid) using a single laser pulse. However, this method suffers from matrix effects: The characteristics of the matrix in which the analyte is present affect the analyte signal, which diminish the quantitative capabilities of LIBS. This effect is more pronounced when performing LIBS measurements on solid samples.
The main goal of this research was to achieve a better understanding of the matrix effects in LIBS. Due to the complexity of the system and its numerous relevant parameters, a rather simple case has been selected: This study has been focused on the effect of matrix rigidity on the LIBS signals of the analyte (calcium). The selected matrix was a "neutral" one, namely an organic polymer (PVC). Many samples of different rigidity levels and different calcium concentrations were prepared and tested using LIBS.
The LIBS intensities of the analyte show a dependence on matrix rigidity. Two major different behaviors were observed: one of rigid samples and one of soft. Rigid samples tend to increase the signal with the increasing number of laser shots (each of them on a fresh spot), whereas soft samples show the opposite behavior. Middle-rigidities acted as rigid or soft, depending on the Ca concentration (higher concentration yielded a more soft-like behavior). Interestingly, a higher concentration of Ca further emphasized this matrix effect.
A vast LIBS spectral analysis, as well as crater morphology examination, indicated that the involvement of heat conductivity and thermal losses processes plays an important role in the observed matrix effects. It was suggested that Ca particles in PVC samples act as heat absorbers and heat transmission agents. Absorption measurements in the IR range supported this hypothesis.
We concluded that the difference between rigid and soft matrices in accumulated LIBS measurements is attributed to differences in ablated mass and also to changes in the relative abundance of Ca in the plasma. However, in order to establish and generalize these conclusions, further experiments are necessary.