Note
Portable Total Reflection X-ray Fluorescence Spectrometer with a Collodion Film Sample Holder
2015 Volume 55 Issue 8 Pages 1794-1796
Details
2015 Volume 55 Issue 8 Pages 1794-1796
Grazing incidence X-ray fluorescence analysis with a collodion film sample holder was performed by using a portable total reflection X-ray fluorescence spectrometer. The spectral background was low when using this sample holder, and a Cr detection limit obtained with this sample holder was comparable to that obtained with a diamond-like carbon coated quartz glass sample holder which was usually used as sample holder for the portable spectrometer. A river water sample was also measured with the collodion film sample holder, and 10.0 μg/L of Cr and 5.1 μg/L of Mn were detected. Collodion film sample holder can be used as a disposable sample holder for trace elemental analysis using the portable spectrometer.
Total reflection X-ray fluorescence (TXRF) spectrometry1) is useful for simultaneous determination of multiple elements in solution samples. In the case that TXRF spectrometry of metallic materials is performed, these materials are usually needed to be digested by acid prior to measurements. Methods to digest these materials used in atomic absorption spectrometry (AAS), inductively coupled atomic emission spectrometry (ICP-AES), and inductively coupled mass spectrometry (ICP-MS) can be applied to TXRF spectrometry. Total reflection X-ray fluorescence spectrometry makes it possible to reduce the amount of metallic samples and acids needed to prepare sample solutions because the dry residues of microliters of solution samples are measured in solution analysis. A detection limit of 0.31 fg was obtained when using synchrotron radiation.2) Using a TXRF spectrometer with a few kW X-ray tube, a detection limit of 0.7 pg was achieved for Rb.3) Even when a bench top TXRF spectrometer with a several tens of watts X-ray tube was used, a detection limit of about 1 pg was obtained for Ni.4) Portable TXRF spectrometers5,6,7,8) have been developed since 2006, and a portable spectrometer achieved a detection limit of 8 pg for Cr when an X-ray tube was operated at 5 W.8) In TXRF spectrometry, the incident X-rays illuminate a sample at a glancing angle which is smaller than the critical angle for total reflection of the incident X-rays on the sample holder. For example, the critical angle of 25 keV X-rays on the surface of quartz glass is 0.07°. A sample holder having a flat and smooth surface is needed to be used to achieve high X-ray reflectivity. Furthermore, a sample holder which contains almost no impurities should be used. Because polished quartz glass substrate has a flat and smooth surface and it contains almost no impurities, it is usually used as sample holder. However, it is expensive and therefore it is needed to be reused after cleaning. If a residue of a prior sample remains after cleaning, a quantification result of a subsequent sample will be influenced by the residue. The possibility that this problem happens can be eliminated by using a disposable sample holder, and cheap materials should be used as a disposable sample holder. Schmitt et al.9) demonstrated TXRF spectrometry with a Perspex sheet sample holder. Hoffmann et al.10) used a Mylar film placed on a quartz glass substrate as sample holder. Yamaguchi et al.11) used a polyester film as a disposable cover for a silicon wafer sample holder. If a thin film which is attached to a substrate having a large hole is used as sample holder, the scattered X-rays and fluorescent X-rays from a substrate which reach a detector will be significantly reduced. Furthermore, the scattered X-rays from sample holder may be reduced using such a thin film sample holder even when a part of the incident X-rays illuminates a sample at glancing angles above the critical angles. Collodion film is usually used as sample support for transmission electron microscope (TEM), and this film is able to be attached to a substrate with a hole. Collodion film can be produced at low cost and therefore it can be used as a disposable sample holder for TXRF spectrometers. In this note, a collodion film was used as sample holder for a portable TXRF spectrometer. A spectrum measured with the collodion film sample holder was compared with that measured with a diamond-like carbon (DLC) coated quartz sample holder12) usually used as sample holder for the portable spectrometers. We show that the collodion film can be used as a disposable sample holder for trace elemental determination.
A portable TXRF spectrometer was used for measurements. The details of the portable spectrometer were previously reported,6) but they are briefly outlined here. An air-cooled X-ray tube 50 kV Magnum (Moxtek Inc., Orem, UT) having a tungsten target was used as excitation source, and it was operated at 5 W (Tube voltage: 25 kV, Tube current: 200 μA). White X-rays (the characteristic X-rays and the continuum X-rays) from the X-ray tube were collimated by an X-ray waveguide before illuminating a sample. Spectra were measured using a Si-PIN detector X-123 (Amptek Inc., Bedford, MA) having an effective detection area of 13 mm2 in this note although an X-123 with an effective detection area of 7 mm2 was used in the previous paper.6) A glancing angle of the incident X-ray beam was adjusted by tilting a sample holder, and it was set to 0.04°. However, the influence of the angular divergence of the incident X-ray beam was not considered when adjusting a glancing angle. The collodion film sample holder was prepared by the following steps:
1. A 2% collodion solution was prepared by diluting a 10% collodion solution (Wako Pure Chemical Industries, Ltd., Osaka, Japan) with 3-methylbutyl acetate.
2. A 5 μL portion of the 2% collodion solution was dropped on distilled water.
3. After a collodion film was formed on distilled water, the collodion film was attached to an acrylic resin plate with a hole. This acrylic resin plate had a length of 30 mm, a width of 30 mm, and a thickness of 5 mm. The size of the hole was about 15 × 15 mm2.
4. After the collodion film was attached to the acrylic resin plate with the hole, the thin film was dried.
A DLC coated quartz glass sample holder (DLC sample holder) was also used for measurements. In the previous paper,12) a TXRF spectrum of a certified reference material of river water (JSAC 0302-3)13) measured with a DLC sample holder was compared with that measured with a quartz glass sample holder, and a detection limit of Cr was improved when using the DLC sample holder. In this note, a 1 μm thick DLC film was coated on a quartz glass substrate with a flatness of λ/20 (λ=632.8 nm) (Sigmakoki Co., Ltd., Hidaka, Japan). This quartz glass substrate had a length of 30 mm, a width of 30 mm, and a thickness of 5 mm. This DLC coating was conducted by Nanotec Co. (Kashiwa, Japan). A sample containing 1 ng of Cr was measured for 600 s in air. To prepare this sample, a 1 μL portion of a standard solution containing 1 mg/L of Cr was pipetted and dried on the collodion film sample holder or the DLC sample holder. A certified reference material of river water (JSAC 0302-3b) was also measured. The certified concentrations of K, Ca, Cr, Mn, and Fe were 0.48 mg/L, 13.0 mg/L, 10.0 μg/L, 5.1 μg/L, and 59.6 μg/L, respectively. A 10 μL portion of the river water sample was pipetted and dried on the collodion film sample holder. This pipetting and drying were repeated three more times, and the dry residue of the total amount of 40 μL of the river water sample was measured for 600 s in air.
Figure 1 shows a representative spectrum of the collodion film sample holder. No sample solution was dropped on the sample holder. The Ar Kα line was due to the air containing 0.9% of Ar. The W L-lines were from the tungsten target of the X-ray tube. The Fe Kα line and the Ni Kα line were also detected. Figure 2 shows representative spectra of the sample containing 1 ng of Cr on the DLC sample holder and that on the collodion film sample holder. The net intensity of the Cr Kα line in Fig. 2(a) was 3294 counts/600 s, and that in Fig. 2(b) was 4086 counts/600 s. The background hump in the X-ray energy range above about 17 keV appeared in Fig. 2(a). Because of the angular divergence of the incident X-rays,8) a part of the incident X-rays in this energy range would illuminate the DLC sample holder at glancing angles above the critical angles. The background hump in Fig. 2(a) would be mainly resulted from the increase in the penetration depth of the incident X-rays in this energy range into the DLC sample holder. The spectral background in the energy range from 10.3 keV to 19.7 keV in Fig. 2(b) was higher than that in Fig. 2(a). On the other hand, the spectral background in the X-ray energy range from 3.5 keV to 9.1 keV in Fig. 2(b) was lower than that in Fig. 2(a). The background intensity of the Cr Kα line in Fig. 2(a) was 6691 counts/600 s, and that in Fig. 2(b) was 5656 counts/600 s. The weak scattered X-rays from the thin collodion film would lead to the low spectral background. The low spectral background obtained with the collodion film sample holder also indicates the possibility that the reflectivity of the incident X-rays on this sample holder was high. A detection limit of Cr was 0.07 ng when the DLC sample holder was used, and that was 0.06 ng when using the collodion film sample holder. This result indicates that the collodion film can be used as sample holder for trace elemental analysis using the portable spectrometer. In a previous paper,7) a detection limit of 12 pg was achieved for Cr when the measurement was performed in air using the portable spectrometer with a quartz glass sample holder. As described in Experimental, the use of the DLC sample holder can lead to improvement of the detection limits of the portable spectrometer. However, the Cr detection limit obtained with the DLC sample holder in this note was higher than that presented in the previous paper. This would be because optical axis adjustment in this note was not appropriate. The higher detection limit was also because the measuring time in this note (600 s) was one third of that in the previous paper. The Fe Kα line and the Ni Kα line were detected when using a quartz glass sample holder and a DLC sample holder in the previous paper,12) and these two fluorescent X-rays seemed to originate from a component of the portable spectrometer. The Fe Kα line was not clearly detected in Fig. 2(a), and the net intensities of the Fe Kα line and the Ni Kα line in Figs. 1 and 2(b) were higher than those in Fig. 2(a). A part of these two fluorescent X-rays in Figs. 1 and 2(b) would be from contamination during preparation of the collodion film sample holder. Figure 3 shows a spectrum of the dry residue of JSAC 0302-3b on the collodion film sample holder. As shown in Fig. 3, the trace amounts of Cr and Mn were detected, and Si, S, K, and Ca were also detected. The net intensity of the Fe Kα line in Fig. 3 was higher than those in Figs. 1 or 2(b). This result indicates that Fe in the river water sample was detected.
Representative spectrum of a collodion film sample holder.
Representative spectra of (a) a sample containing 1 ng of Cr on a diamond-like carbon coated quartz glass sample holder and (b) that on the collodion film sample holder.
Representative spectrum of the dry residue of the total amount of 40 μL of a certified reference material of river water (JSAC 00302-3b) on the collodion film sample holder.
When using a collodion film as sample holder for a portable TXRF spectrometer, a detection limit of several tens of pg was achieved for Cr. This detection limit was comparable to that obtained with a DLC sample holder. Because collodion film is able to be produced at low cost, it can be used as a disposable sample holder for trace elemental analysis using TXRF spectrometers. The use of the collodion film sample holder makes it possible to measure a large number of samples without cleaning a sample holder or providing a large number of sample holders. Therefore, the collodion film sample holder can be useful for performing on-site trace elemental analysis using the portable spectrometer. The portable spectrometer with the collodion film sample holder can be applied to on-site simultaneous analysis of trace elements in metallic materials.
This work was supported by the 22th ISIJ (The Iron and Steel Institute of Japan) Research Promotion Grant. We thank Prof. Jun Kawai (Kyoto University, Kyoto, Japan) for helpful comments about collodion film.
