Note
Reduction of Spectral Interference between X-ray Peaks Originating from an X-ray Tube and X-ray Fluorescence Peaks in Total Reflection X-ray Fluorescence Analysis
2017 年 57 巻 5 号 p. 953-955
詳細
2017 年 57 巻 5 号 p. 953-955
This paper describes a method for improving the detection limit for zinc by a portable total reflection X-ray fluorescence (TXRF) spectrometer with a tungsten target X-ray tube. Measuring a TXRF spectrum of a small amount of sample in a vacuum remarkably reduced the intensity of the W Lα line (8.40 keV) that originated from the X-ray tube and that partially overlapped with the Zn Kα line (8.63 keV), leading to an improvement in the detection limit for zinc. A detection limit of 0.003 mg/L was achieved when a TXRF spectrum of a 0.05 mg/L zinc standard solution was measured in a vacuum. A TXRF spectrum of a river water sample containing 0.05 mg/L of zinc was also measured, and the Zn Kα line measured in a vacuum was more clearly observed than that measured in air.
Total reflection X-ray fluorescence (TXRF) analysis1) enables to determine trace elements in microliters of solution samples. Streli et al.2) reported that a detection limit of several femtograms was obtained when energy-dispersive TXRF analysis was performed with synchrotron radiation. Sakurai et al.3) reported that a detection limit at 10−16 g-level was achieved when wavelength-dispersive TXRF was used at a synchrotron radiation facility. On the other hand, portable TXRF spectrometers4,5,6) using low power X-ray tubes have been developed since 2006, and a detection limit of 8 pg was obtained for chromium by the portable spectrometer6) with a tungsten target X-ray tube operated at 5 W (tube voltage: 25 kV, tube current: 200 μA). These portable spectrometers make it possible to perform on-site trace elemental analysis.
X-ray peaks attributed to characteristic X-rays from an X-ray tube usually appear in TXRF spectra because of the scattering of incident X-rays reaching an X-ray detector, and these X-ray peaks overlap with X-ray fluorescence peaks when TXRF measurements are performed in air. For example, the W Lα line (8.40 keV) originating from a tungsten-target X-ray tube partially overlaps with the Zn Kα line (8.63 keV). It is difficult to detect trace amounts of elements whose X-ray fluorescence lines overlap with X-ray peaks originating from an X-ray tube. The scattering of incident X-rays from air is reduced when a measurement is performed in a vacuum, leading to reduction in the intensities of X-ray peaks originating from an X-ray tube.7) When a TXRF spectrum of a small amount of sample was measured in a vacuum using the portable spectrometer with the tungsten target X-ray tube, the intensities of the W L lines were remarkably reduced.6) This result indicates that the W L lines were mainly due to the scattering of incident X-rays from air and the sample itself. Therefore, measuring a spectrum of a small amount of sample in a vacuum using a TXRF spectrometer with a low power X-ray tube makes it possible to remarkably reduce spectral interference between X-ray peaks originating from the X-ray tube and X-ray fluorescence lines. In the present study, we show that measuring in a vacuum improves the detection limit for zinc obtained by the portable spectrometer with the tungsten-target X-ray tube compared with that in air. We also show measurement results of a river water sample containing 0.05 mg/L of zinc.
The details of the portable TXRF spectrometer used in the present study were described elsewhere,6) and they are summarized here. A 50 kV Magnum X-ray tube (Moxtek Inc., Orem, USA) with a tungsten target was operated at 25 kV and 200 μA. An X-ray waveguide was used to collimate the X-rays from the X-ray tube. A silicon drift detector VITUS-SDD (Ketek GmbH, Munich, Germany) with an active area of 7 mm2 was used, and a collimator was attached to the detector. A diamond-like carbon (DLC) coated quartz glass substrate8) was used as the sample holder. A 1 μm thick DLC film coating was performed on a quartz glass substrate (Sigmakoki Co., Ltd., Hidaka, Japan) with a diameter of 30 mm, a thickness of 5 mm, and a flatness of λ/20 (λ = 632.8 nm) by Nanotec Co. (Kashiwa, Japan). The sample holder was tilted at 0.04° with respect to the horizontal. A vacuum chamber mainly consisted of acrylic resin9) was used, and the sample holder on which the dry residue of a sample solution was present was placed in the vacuum chamber. A diaphragm pump with a base pressure of 2.7 × 102 Pa was used.
A 0.1 mg/L zinc standard solution and a 0.05 mg/L zinc standard solution were prepared by diluting a 100 mg/L zinc standard solution (Wako Pure Chemical Industries, Ltd., Japan) with distilled water. In order to prepare the dry residue of the 0.1 mg/L zinc standard solution, dropping and drying of 10 μL of the sample solution on the sample holder were performed twice. The same procedure was performed for preparation of the dry residue of the 0.05 mg/L zinc standard solution. A river water sample containing 0.05 mg/L of zinc was prepared by mixing 990 μL of a certified reference material of river water (JSAC 0301-3b) with 10 μL of a 5 mg/L zinc standard solution. The certified value for zinc in JSAC 0301-3b is 0.17 μg/L. Because the zinc concentration in the river water sample prepared in the present study was two orders of magnitude higher than the certified value, the influence of zinc originally contained in JSAC 0301-3b was ignored when the river water sample was prepared. Ten microliters of the river water sample containing 0.05 mg/L of zinc was dropped and dried on the sample holder, and the same procedure was performed for preparation of the dry residue of JSAC 0301-3b. The measurements of all analytes were performed for 600 s. The detection limit for zinc was determined from the following equation:
| (1) |
Figure 1 shows TXRF spectra of the DLC sample holder measured in air and in a vacuum. As shown in Fig. 1, the W L lines were remarkably reduced when the measurement was performed in a vacuum. The intensities of the Ar K lines attributed to air containing 0.9 vol% argon in Fig. 1(b) were lower than those in Fig. 1(a). The Si Kα line from the quartz glass substrate was detected. The Ni Kα line would be from a component of the portable spectrometer. The Cl, K, and Ca Kα lines in Fig. 1(b) would originate from contamination during sample preparation. Figure 2 shows a TXRF spectrum of the 0.1 mg/L zinc solution measured in air and that of the 0.05 mg/L zinc solution measured in a vacuum. Measuring in a vacuum reduced the spectral interference between the Zn Kα line and the W Lα line as shown in Fig. 2, leading to an improvement in the detection limit for zinc. Detection limits obtained from Figs. 2(a) and 2(b) were 0.011 mg/L and 0.003 mg/L, respectively. Figure 3 shows TXRF spectra of the river water sample containing 0.05 mg/L of zinc measured in air and in a vacuum and that of JSAC 0301-3b measured in a vacuum. The W Lα line in Fig. 3(b) was lower than that in Fig. 3(a), and hence the Zn Kα line in Fig. 3(b) was more clearly observed than that in Fig. 3(a). Zinc in JSAC 0301-3b was not detected by the portable spectrometer as shown in Fig. 3(c) because of the low concentration of zinc. The intensity of the W Lα line in Fig. 3(b) was higher than those in Figs. 1(b) and 2(b). A detection limit for zinc obtained from Fig. 3(b) was 0.034 mg/L, and it was higher than that obtained from Fig. 2(b).
Total reflection X-ray fluorescence spectra of a DLC coated quartz glass sample holder measured (a) in air and (b) in a vacuum.
(a) Total reflection X-ray fluorescence spectrum of a 0.1 mg/L zinc standard solution measured in air and (b) that of a 0.05 mg/L zinc standard solution measured in a vacuum.
Total reflection X-ray fluorescence spectra of a river water sample containing 0.05 mg/L of zinc measured (a) in air and (b) in a vacuum and (c) that of a certified reference material of river water (JSAC 0301-3b) measured in a vacuum. Enlarged views around the W Lα lines in Figs. 3(a), 3(b), and 3(c) are shown at the upper right in each figure.
The scattering of incident X-rays from the sample itself has little contribution to enhancements in the intensities of the W L lines originating from the X-ray tube when a spectrum of a small amount of sample such as the dry residue of the 0.05 mg/L zinc standard solution is measured by the portable TXRF spectrometer. When a spectrum of such a small amount of sample is measured in air, the W L lines are mainly attributed to the scattering of the incident X-rays from air. Therefore, as shown in Fig. 2, the intensities of the W L lines were remarkably reduced when the measurement was performed in a vacuum, leading to an improvement in the detection limit for zinc. When a spectrum of a large amount of sample such as the dry residue of the river water sample containing 0.05 mg/L of zinc is measured, the scattering of incident X-rays from the sample itself results in enhancements in the intensities of the W L lines. A detection limit for zinc obtained from Fig. 3(b) was worse than that obtained from Fig. 2(b) because of the higher intensity of the W Lα line. However, measuring in a vacuum reduced the intensity of the W Lα line compared with that in air as shown in Fig. 3. The environmental quality standards for total zinc were established to preserve aquatic organisms in 2003 in Japan, and the environmental standard value for river water is 0.03 mg/L or less. Although the detection limit obtained from Fig. 3(b) was slightly higher than 0.03 mg/L, the detection limit can be improved by increasing a counting time. Therefore, the portable spectrometer will make it possible to detect zinc at around 0.03 mg/L in river water when the measurement is performed in a vacuum.
X-ray peaks in TXRF spectra originating from an anode material of an X-ray tube are mainly attributed to the scattering of incident X-rays from air and the sample itself, and the intensities of the X-ray peaks are remarkably reduced when a spectrum of a small amount of sample is measured in a vacuum with a low power X-ray tube. In the present study, a spectrum of an analyte containing a trace amount of zinc was measured in a vacuum using a portable TXRF spectrometer with a low-power tungsten-target X-ray tube. The intensity of the W Lα line that partially overlapped with the Zn Kα line was remarkably reduced when measuring in a vacuum, leading to an improvement in the detection limit for zinc.
This work was partially supported by JSPS KAKENHI Grant Number 25810088.
