Development of a high pressure digestion technique and a data acquisition/reduction procedure and their application to the ICP-MS analysis of urban sediments and soils from Cebu, Philippines

Diegor, Wilfredo G. (1999) Development of a high pressure digestion technique and a data acquisition/reduction procedure and their application to the ICP-MS analysis of urban sediments and soils from Cebu, Philippines. Masters thesis, Memorial University of Newfoundland.

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Abstract

Trace element analysis in the earth and environmental sciences requires accurate and precise determination of element concentration in geological samples. Inductively coupled plasma-mass spectrometry (ICP-MS) continues to provide reliable data in this area. Analytical concerns in solution ICP-MS include completeness of digestion and volatilization loss. The work embodied in this thesis examined the use of high pressure digestion to address incompleteness of digestion in the analysis of geological materials. -- High pressure digestion vessels (HPDV) fabricated in-house were compared with the existing method at Memorial University of Newfoundland which uses screw top Teflon (STT) jars in digesting several geochemical reference materials (GRMs). ICP-MS analysis of the solutions using the MUN ICP-MS trace element package demonstrated more complete digestion of the marine sediment GRMs PACS-1, MESS-2 and BSK-1, indicated by higher Y, Zr, Nb, HREE, Hf and U values when the HPDV was used. Zirconium was from 40% to 150% higher, Hf was 35% to 120% higher, and the HREEs were generally higher by about 20% in the HPDV than in the routine method. The results indicate better decomposition of resistant minerals especially zircon grains that were likely in high abundance in the sediments. No significant difference between STT and HPDV was found for the other GRMs (basalt, andesite, gabbro, dolente and syenite). Lower element values for the granite material, G-2, were obtained in the HPDV, but Zr and Hf values were still significantly higher than in the STT. Rubidium was poorly recovered in the marine sediments, the reason for which has not been determined in the study. -- An ICP-MS data acquisition and reduction procedure (environmental-exploration package) was developed to determine transition metals and other trace elements in geological samples. Twenty-nine masses comprised the element suite: ⁵¹V, ⁵²Cr, ⁵⁵Mn, ⁵⁹Co, ⁶⁰Ni, ⁶⁵Cu, ⁶⁶Zn, ⁷⁵As, ⁷⁷Se, ⁷⁹Br, ⁹⁸Mo, ¹⁰⁷Ag, ¹¹¹Cd, ¹¹⁸Sn, ¹²¹Sb, ¹²⁵Te, ¹²⁷I ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴⁵Nd, ¹⁶⁷Er, ¹⁶⁹Tm, ¹⁷⁵Lu, ¹⁸³W, ²⁰¹Hg, ²⁰⁸Pb, ²⁰⁹Bi and ²³²Th. Most important interferences were from ⁴³Ca¹⁶O on ⁵⁹Co, ⁴⁴Ca¹⁶O on ⁶⁰Ni, ⁴⁹Ti¹⁶O on ⁶⁵Cu and ⁵⁰Ti¹⁶O on ⁶⁶Zn. Internal standards used were ⁴⁵Sc, ¹¹⁵In and ¹⁸⁷Re. ²³⁸U and ²⁵⁴(UO) were measured to monitor polyatomic ion formation. Sensitivity of the elements was calculated from one of 2 external standards, except for 3 elements which were calibrated through surrogate calibration (Sn using Sb, W using Lu, and Hg using Pb). Calibration was demonstrated to be successful for the pair Sn-Sb; though not as good for W-Lu, probably due to laboratory contamination. Surrogate calibration for the pair Hg-Pb proved unsuccessful, most probably because of volatilization loss of Hg during sample digestion. Analysis of reference materials using the new package showed excellent results for V, Co, Cr, Ni, Sn, Sb, Cu, Zn, Mo, Cd and Pb compared with literature values. Arsenic and Ag results were mostly near detection limits. Comparison with XRF results indicated good agreement between the environmental-exploration package and the XRF. Compared with the MUN ICP-MS trace element package, the new package was in good agreement for the elements Mo, La, Ce, Pr, Nd, Er, Tm, Lu, Pb, Bi and Th. -- The high pressure digestion technique and the ICP-MS environmental-exploration package were applied to the analysis of urban stream sediment and soil samples from the Philippines. Good correlation for La, Ce, Pr, Nd, Er, Tm, Lu, Pb, Bi and Th between the package and the MUN ICP-MS trace results was shown by the high correlation coefficients derived. Nickel and As gave poor results because of the high concentrations in the calibration blanks for Ni and possibly because of volatility for As. Particle size may also have influenced the results for these 2 elements because results for the reference materials indicated good agreement between the XRF and the environmental-exploration package. It was found that the GRMs are significantly finer grained than the stream sediment and soil samples (greater <53 μm fraction), suggesting that there may be a need to grind the stream sediment and soil samples to obtain better results. -- Mapping of empirically derived geochemical classes showed that the spatial distribution of Cr, Ni, Cu, Zn, Pb, Sn, Sb and Cd in the stream sediments correspond to the large commercial zone in the SW portion of the study area. Higher values of Cu and Zn point to an industrial zone to the east. The soil element data correspond to an industrial zone where manufacturing and use of metal parts abound. Arsenic was high in the relatively new land development to the SW, which was a she where storm drainage and airline night soil from the nearby air facility previously collected. Except for Sb and Cd, the element concentration in the stream sediments were high for a limestone terrain. The soil samples likewise indicated input from human activity. The derived geochemical landscape coincided with the known land use in the area.

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