TOTAL MERCURY CONTENT continued
Table 2 – Results of repeatability testing and recovery rates for four microwave digestion methods Microwave digestion method
Number of samples
Hg content (average) (mg/kg) SD (mg/kg) CV (%)
Recovery rate (%) Average recovery rate (%)
Aqua regia (XP1500) 33
0.608 0.053 8.69
87-130 102
Aqua regia (Xpress) 35
0.562 0.047 8.35
79-112 94
Nitric acid (XP1500) 33
0.582 0.063 10.88
73-115 97
Nitric acid (Xpress) 39
0.575 0.096 16.73
72-155 96
–25 °C (–13 °F) and measured on the following four consecutive days. The SD values were in the range of 0.01–0.02 mg/kg, the CV was 1.64–3.21 mg/kg, and the recovery rates were 94%–104%.
Spectroscopic analysis of real wood samples:
Figure 1 – Comparison of four microwave digestion methods using Box- Whisker-Plots (the gray shaded area shows the interval of the expected value of the reference material).
values measured were between 0.56 and 0.62 mg/kg, and the recovery rates were between 93% and 102% (average 98%).
Measurement precision, detection limit, and
quantitation limit Ten measurements of the same sample in aqua regia were performed to determine the measurement precision. The CV value was 1.25% and the SD was 0.01 mg/kg. LOD and LOQ were determined with measurements of 10 blank samples. These values were calculated from the sum of the blank measurement value and threefold (LOD) and tenfold (LOQ) of the SD value, respectively. For the measurement solutions, the LOD and LOQ values were 5.74 ng/L and 15.76 ng/L, respectively; in the wood samples (dried matter), the LOD and LOQ values were 2.29 μg/kg and 6.31 μg/kg, respectively.
Method stability To determine method stability, three samples of the reference material
were prepared and the solutions were divided into five parts. One was measured immediately and the remaining four samples were frozen at
Method comparison Using the ICP-MS method based on microwave digestion with aqua regia in the XP1500 vessels, 43 real wood samples of different origin were analyzed. Three roof beams from one building but from different time periods showed very interesting results. The roof beam from a house built in 1910 had an Hg content of 0.377 mg/kg. The Hg content was 0.048 mg/kg in the second roof beam, which was installed on the building in 1985. The third roof beam, from 2010, had an Hg content of 0.004 mg/kg, which is lower than the LOQ (see Figure 2).
Finally, the ICP-MS-based method was compared with two spectroscopic methods using cold vapor ICP-OES and classical ICP-OES, presented in Part 1 of this article (Am. Lab. Aug 2013, 45[7], 6–9).23
The ICP-MS results
for selected wood samples with significant Hg content are shown in Table 3; Figure 3 depicts the comparison between the measurements of all the methods presented in this study. The Hg contents of the samples were lower using cold vapor ICP-OES than with ICP-MS. The SD and CV values were comparable. The results of ICP-OES demonstrated relatively high CV values, because the Hg contents of the majority of the samples were lower than the LOD of the ICP-OES method. Hg analysis using ICP-MS is very sensitive. The LOD and LOQ values were comparable to those of the cold vapor ICP-OES method.
ICP-MS has more of an advantage for multielement analysis. Elements such as As, Pb, Cd, and Cu can be detected in waste wood using ICP-MS. With one measurement, the technique is able to analyze all elements required, which is more time-efficient.
Conclusion A very sensitive and rapid multielement method allows for the determina-
tion of total mercury content and additional elements such as heavy metals AMERICAN LABORATORY • 8 • SEPTEMBER 2013
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