Knowledge


2018.02.13
Improvement in Sensitivity and Quantification by Using a Portable Total Reflection X-Ray Fluorescence Spectrometer Equipped with a Silicon Drift Detector
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Excerpted from Adv. X-Ray. Chem. Anal., Japan: 42, pp. 115-123 (2011)
H. Nagai, Y. Nakajima, S. Kunimura, J. Kawai

We have adapted a silicon drift detector (SDD) to a portable total reflection X-ray fluorescence (TXRF) spectrometer using polychromatic X-rays from an X-ray source of single-digit watt.
Analytical performance was greatly improved in PB ratio and the sensitivity for lighter elements when compared with the former spectrometer equipped with a Si-PIN detector.
In this article improved sensitivity of the new spectrometer is described, and additionally the evaluation of the current wave-guide, a newly developed sample preparation device for precise quantitative evaluation and an application to the analysis of suspended particles are reported.

fig1b

Portable type TXRF analyzer: Ourstex 200TX

This analyzer is a compact type, only 8kg weight.
It is easy to be movable at on-site measurement, requires AC100V power. shows its structural photo. For the X-ray tube, transparent compact X-ray tube is used with 50kV and 200µA of maximum voltage and tube current, respectively. Tungsten is used as the target of the X-ray tube. Electronic cooling type Silicon Drift Detector (SDD, light reception area; 7mm2, Be thickness; 8µm) is used as the detector. The SDD is composed of united structure of Si-PIN with FET, and ring-shaped electrode gradient on the surface. It can be effectively achieved to drift the electron to the centered anode.
By this reason, decrease of thermal noise, higher improvement of electron collecting effect and decrease of leak current etc. are attainable. This improvement contributes to obtain higher energy resolution ability, and PB ratio becomes bigger. Hence, the measurement of high calculating ratio is achievable without decreasing the resolution ability. In general, the wider the light reception area is, the lower the resolution ability is in case of the Si-PIN detector, while in case of the SDD, it has a great capability that it is almost no lowering of energy resolution ability. On the contrary, it has a certain demerit that higher energy X-ray may be easy to transparent because the crystal thickness is thin as in 450µm, so the detecting efficiency is not too high at the high energy area.

fig1b

X-ray wave-guide is used for achieving paralleled of incident X-ray illuminating from the X-ray tube.
X-ray wave-guide is a photo-element that enables to implement paralleled of incident X-ray by using 2 pieces of silicon wafers as shown in (a). As a part of incident X-ray is totally reflected by using the mirror-face silicon wafer, stronger X-ray is obtained than using usual slit. The interval between the silicon wafers is designed and made as 10mm horizontal width and 10µm vertical width 3).
(b) shows the outlook of the X-ray wave-guide.

fig1b

Produced wave-guide has a possibility of variation of the slit width by affecting the manufacturing fabrication accuracy and the edge that was cut the silicon wafer. Therefore stainless steel cutter is set at the outlet of the X-ray wave-guide, and by knife-edge scanning method, the evaluation of the slit width of the X-ray wave-guide has implemented. Evaluation of the slit width was determined by obtained data that were differentiated and the full width half maximum (FWHM) value that was calculated by Gaussian fitting. The result is shown in .
Calculated slit width was estimated as 16.5µm, which was close to 10µm that was assumed at the initial designing time.
Quartz optical flat was used in the sample plate (accuracy of reflection wave surface: λ/20).
Incident angle was adjusted as setting at ca. 0.05°that is most suitable for transition metals, by the Goniometer that is installed at under the sample plate 6).

fig4b

PB (Peak-Background) ratio improvement by adopting SDD

SDD was used in this analyzer. Energy resolution ability of the SDD is 132eV in the FWHM of Mn-Kα ray. (Formerly used Si-PIN detector’s resolution ability is 180eV in the FWHM of the same ray)
Comparison of spectrum between the Si-PIN detector and the SDD is shown in . Spectrum is standardized by the peak intensity of W-Lα ray that is the excitation source.
Measuring sample:
Sc, Cr, Co, As and Sr are in 0.5ppm aqueous solution respectively. The solution is dropped on the optical flat as 2mL and it is dried (1ng).
As a measuring result, energy resolution ability as well as the PB ratio in the SDD was more improved than those in the Si-PIN detector.
illustrates respective PB ratio. In Co, PB ratio in the Si-PIN detector is 0.921, while 1.796 on the SDD is detected, almost 2 times higher improved.
In the Si-PIN detector, it was impossible to separate the peaks between Ar-Kα and Ar-Kβ ray, and no detect for very little peaks; The SDD enabled to detect the separation of the peaks between Ar-Kα and Ar-Kβ ray, and detect very little peaks effectively.
Lower limit of detection (LLD) values estimated from obtained data for Sc, Cr, Co, As and Sr in 1800 seconds measurement are, 29pg, 20pg, 14pg, 51pg and 192pg, respectively.
The following theoretical formula was used for the calculation of the LLD value 7).

where W: content (ng), t: measuring time (sec),
INET: Net intensity of the peak (cps),
IBG: background intensity (cps)

fig5bfig6b

The LLD value of Co obtained this time was the same value as in previous reported one 6) by Kunimura et al.
For this reason, as the reception light surface area is 7mm2 in the SDD, X-ray intensity was lower than the Si-PIN detector, which has the reception light surface area of 13mm2. It depends on the droplet size though, significant detection sensitivity may be obtained by using bigger caliber of the SDD.

Quantitative improvement by using the droplet supporting device

On the analysis of the total reflection X-ray fluorescence spectrometry, sample preparation is the most important factor in evaluating the analysis quantitatively.
In this analyzer, sample solution is dropped on the optical flat and its dries residual is measured. It is quite important that the sample solution should be dropped on the center position of the optical flat precisely, in order not to lower the quantitative accuracy by changing the X-ray intensity significantly by the relationship with the taking-un angle of the detector. We fabricated the droplet supporting device such as in .
The device has a small hole that enables to insert the point of the micro-pipette tip and it can insert always at a constant depth. Hence, it enables to drop the solution always at the center part of the optical flat, and the quantitative accuracy improves much higher.
The material used in the device is acryl resin with consideration of the contamination, and the device is reusable by washing treatment.
To verify the effect of using the droplet supporting device, two conditions were compared:
Measuring the change of the X-ray intensity of Cr-Kα in Cr standard solution prepared as in 10ppm and drops 5mL:
1) with the droplet supporting device
2) without the device
The results are illustrated in . Compared to the case of without the device, the change of the X-ray intensity shows smaller in the case of using the device.

fig7btable1b

In order to evaluate quantitative verification of the analyzer, drawing the calibration curves of Co and Cr was executed.
In , the calibration curves obtained from the experiment are shown.
Sample solution preparation:
Standard solutions for atomic absorption spectrometry with Cr and Co were diluted and adjusted as in 500ppb, 100ppb, 50ppb, 25ppb and 10ppb, respectively.
Also, constant amount of Sc solution as the internal standard is added and 10µL of the solution is dropped on the optical flat. The sample is dried on it, and is measured. As a result, good linearity relation is obtained both for Cr and Co, and it can be confirmed that sufficient quantitativity is approved even in the lower concentration range

fig8b

Sensitivity improvement for light elements

In order to confirm analytical sensitivity of the analyzer in the light elements field, P analysis in the solution was implemented. With the formerly used Si-PIN detector, it has lower energy resolution ability that the peak of P-Kα ray is hidden in the tailing part of the peak of Si-Kα ray that is due to the optical flat. So trace amount of P analysis was not achievable. Now the experiment for the analyzer with the SDD is implemented whether trace amount of P analysis is achievable or not.

Standard P solution is diluted as in 1ppm, 0.5ppm and 0.1ppm, respectively. As the measuring sample, 5mL of it is dropped on the optical flat, and after drying measurement is executed. Absolute amount of sample is 5ng, 2.5ng and 0.5ng, respectively.
Measured P spectra are shown in . It can be confirmed that even in 0.5ng concentration range, sufficient detection is achievable.
The LLD value estimated by 0.5ng peak intensity and the background intensity with the formula (1) becomes 286pg in 1800 second measurement. It can be confirmed that high sensitive measurement is attainable even in the light element field in air atmospheric environment.

This achievement could be concluded as:
1) Energy resolution ability improves much higher by the SDD adoption
2) On the structure of total reflection X-ray fluorescence spectrometer, the distance between the sample and detector is extremely short; the detector is located on upper of the sample; and it is no affection of the absorption by the polymer film, etc. that is used as sample holding in usual fluorescence X-ray spectrometer.

fig9b

Application field

As the examples of application fields measured by a portable total reflection X-ray fluorescence spectrometer, there are bunch of reports as in measured soil eluding water 8), leachate from tea or toys, etc 9).
In this report, the possibility for element analysis in air particulate matter is examined.
Optical flat is on the Petri dish and it is left at the sampling place 1 hour remains. Accumulating particles on the optical flat is directly measured.
Sampling place is along national route in Neyagawa City, Osaka Pref., that is relatively high traffic-laden street.
Measured result spectra are shown in . Compared to the blank, the elements of Al, S, K, Ca, Ti, Cr, Mn, Fe, Zn etc. were detected. Most likely, as the sampling place is high traffic-laden street, elements from automobile exhaust gas and frictional metal by the break pad etc. may be detected presumably.
By improving the sampling method, this analyzer might be utilized at the application of environmental monitoring of airborne particulate matter.

fig10b

Conclusion

By loading the SDD as the detector of a portable total reflection X-ray fluorescence spectrometer utilizing non-color X-ray generating from small X-ray tube in a few watt, energy resolution ability is improved, and compared to the former used Si-PIN detector, PB ratio improves about 2 times higher. Moreover, analysis sensitivity of P in light element field is improved; it is obtained 286ng as the lower limit detection.
Stable sample preparation is achievable by utilizing the droplet sample supporting device; and it enabled to improve quantitative accuracy drastically.
This analyzer is compact and light-weighted, so it may be possible to implement measurements in case of having difficulty of sample storage and/or transferring the sample, such as the air particulate matter at site (ex. at Antarctic), as well as by measuring the hair of the cattle at site, cattle’s health condition may be determined as routinely.

References

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