Instrumentation for sample preparation and
sample analysis Sample preparation was performed using the Mars 5 Microwave Digestion System (CEM, Kamp-Lintfort, Germany). Two vessel types (both from CEM) were used: XP1500 vessels (vol. 50 mL, max. 55 bar) and Xpress vessels (vol. 75 mL, self-venting, and no pressure limit). PTFE magnetic stirrers from VWR (Darmstadt, Germany) were used for mixing during the digestion. Sample solution analysis was performed using the ICP-MS 7700x (Agilent Technologies, Waldbronn, Germany) with the following parameters: radio- frequency power—1550 W, sample depth—10 mm, carrier gas—0.65 L/min, nebulizer pump—0.10 rps, spray chamber temperature—13 °C (55.4 °F), and dilution gas—0.40 L/min. Argon was used as plasma, carrier, and dilution gas. In the collision cell, a helium fl ow was used as follows: 0 mL/min in NoGas tuning mode, 4.3 mL/min in He mode, and 10 mL/min in HEHe mode. The measurements were done with three replicates. Table 1 shows the analysis parameters for the elements analyzed. The rinse time was set to 60 sec at 0.3 rps of the nebulizer pump, followed by 30 sec at 0.4 rps. Sample introduction was performed using the ASX-500 autosampler (Cetac, Omaha, NE).
Standard and sample preparation At room temperature, the dried wood material was split apart and then
milled using the MM2000 ball mill (Retsch, Haan, Germany). After adding 250–350 mg of sample and 8 mL nitric acid or aqua regia, the samples were predigested for a period of 20 min. One blank sample was included at every digestion run. The basic microwave digestion procedure was as follows: temperature–time ramp for 20 min with a fi nal temperature of 180 °C (356 °F), then 20–30 min hold time at 1200 W for more than six vessels. Following this, the samples were cooled to room temperature and the vessels were uncapped. The clear sample solutions were transferred to a volumetric fl ask (vol. 100 mL) and fi lled with ultrapure water.
The calibration solutions were prepared in nitric acid or aqua regia (concentration 8%, v/v) with Hg concentrations of 0.05, 0.1, 0.5, and 1.0 μg/L and concentrations of 5, 10, 50, and 100 μg/L for the other ele- ments. The ISTD was prepared at a concentration of 500 μg/L with nitric acid (5%, v/v). All solutions were prepared and stored in vessels made of polyethylene or polypropylene.
Results and discussion Four microwave digestion procedures were tested using two vessel types
and two acid mixtures. The reference material ERM-CD100 was used in all validation experiments to measure the Hg content. The remaining elements analyzed resulted in good calibration for the multielement determination, but their content was not considered further in this study.
First, repeatability testing (intraday precision) was performed for four digestion procedures, and the recovery rate was determined using up to 39 samples. The following experiments were done using microwave digestion with aqua regia and the XP1500 vessels: within-laboratory reproducibility (interday precision), measurement precision, limit of detection (LOD), and limit of quantification (LOQ); method stability and rinsing effects were also determined. The maximum value of the within-laboratory coefficient of variation (CV) should not exceed 11.52%
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AMERICAN LABORATORY • 7 • SEPTEMBER 2013
according to the definition by Horwitz.21 This value was achieved in the majority of the experiments performed.
Repeatability testing, rate of recovery, and within-
laboratory precision Repeatability testing was conducted using more than 30 samples of the reference material and four microwave digestion methods: aqua regia or nitric acid in the XP1500 and Xpress vessels. All measurement results followed a normal distribution, which was verifi ed by the David test; the Horwitz criterion was fulfi lled for all methods, with the exception of the nitric acid digestions in the Xpress vessels. The results of the aqua regia methods showed slightly lower CVs than the nitric acid methods. The high- est standard deviations (SDs) were measured with nitric acid in the Xpress vessels. The average recovery rates were slightly higher in the pressure-tight XP1500 vessels. The recovery rates for all methods were in agreement with the acceptable values for recovery rates (measured value was less from 100 than the fourfold of the CV), according to Kromidas.22
Table 2 shows the measurement results of the four microwave digestion methods; the methods are compared in Figure 1. The within-laboratory precision was determined using microwave digestion with aqua regia in the XP1500 vessels. Therefore, three samples of the reference material and one blank were prepared and measured on fi ve consecutive days. The average
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