Jamaican soils are known to be enriched in many elements including arsenic, cadmium and uranium. Suitable reference materials such as NIST 698 Bauxite (Jamaican) do not include reference or information values for measurands of interest such as As, Cd or U. Elevated CRMs such as NIST 2710 introduced effects specific to Instrumental Neutron Activation Analysis such as self-shielding. ICENS therefore decided to develop a natural matrix in-house reference material. The parish of Manchester in Jamaica is known for bauxite mining as well as its naturally elevated levels of several trace elements. It was thought that this would be a suitable area to source the material for the “Soil of Jamaica” quality control material.
Table 1: Comparison of ranges and means for Jamaican and world soils for selected elements
Natural Matrix Source Area & Control Material Preparation
- Sample preparation procedure the same as test soil samples.
- Storage in similar containers housed in reference material cabinet
Instrumental Neutron Activation Analysis (INAA) performed using the SLOWPOKE-2 research reactor at ICENS to irradiate sub-samples.
Figure 3: SLOWPOKE-2 Research Reactor Figure 4: Irradiation Regime Flowchart for “Soil of Jamaica
Quality control material was also irradiated with a suitable certified reference material.
“Soil of Jamaica” was analysed for twenty-four elements of interest and Shewhart charts generated for all the measurands of interest to determine whether the system was under statistical control.
Figure 5: Control Charts for Br, Eu, La and U in in-house reference material “Soil of Jamaica”
Westgard rules were applied to make the charts more sensitive to smaller shifts and to reduce false alarms.
Figure 6: Cs Control Chart for the in-house Reference Material “Soil of Jamaica”
With two observations that surpass the upper warning limit in four runs there was clearly an effect that warranted the attention of the analyst and quality assurance manager for the Cs control chart.
The QCM “Soil of Jamaica” was analyzed under routine analytical conditions for geological samples to characterize the expected measurement variation and range of values. The mean and standard deviation were calculated from these initial analyses. Descriptive statistics were performed on the elemental results, including skewness, kurtosis and normalcy plots to determine whether the data was normally distributed. It was important in the analysis of the control charts to determine whether observations that violated control criteria were the results of analyst performance, performance of instrumentation, improper standardization or some other deviation from standard operating procedure. Figure 7 suggests a systematic error with two instances of rule 9X violations for Zn (illustrated by the red lines and points). Figure 7: Zn Control Chart for the in-house Reference Material “Soil of Jamaica”
The in-house reference material “Soil of Jamaica” appears to be a stable, natural matrix. Of the 24 elements analysed 14 were accepted as recommended values with the remainder regarded as information values. It has proven useful to analysts at ICENS for assessing various analytical methods, assigning values to elements in Jamaican soils and in general supplementing the quality assurance and control mechanisms in place at the Centre. Bode, P., van Dijk, C.P.,(1997). Operational management of results
The authors would like to thank Professor Gerald Lalor for his instruction in this work and the numerous analysts, past and present, who through their efforts at ICENS contributed to this work.
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