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Sophisticated sample holder

7th June 2019


Table 1
Table 2

Michael Nelson presents a case study on the measurement of boron and fluorine in water solution by micro-droplet method using a smart sample holder

When elements in a liquid are analysed by X-ray fluorescence (XRF) spectrometry, the direct liquid analysis method, where a liquid sample is poured into a liquid cell with sample film and measured under helium, is typically used.

The wavelengths of the element lines of boron (B-Kα) and fluorine (F-Kα) are so long, however, that they are absorbed by the sample film, meaning that boron (B) and fluorine (F) in water cannot be analysed by the direct liquid analysis method with XRF.

The micro-droplet technique using the Rigaku Ultra Carry method enables boron and fluorine analysis in water. The Ultra Carry sample retainer is designed for analysis of trace elements in water solution. Since the paper pad and film are very thin, the observed background during measurement is very low, which significantly improves the peak-to-background ratio. The paper pad captures water, accommodating a drop of 500 μL. After drying, the specimen is measured under vacuum. Since water analysis using the Ultra Carry method can be conducted under vacuum without sample film, boron and fluorine in water can be analysed by XRF.

Boron and fluorine analysis in water with the Ultra Carry method is conducted as described below.

Sample and sample preparation

Standard solutions of boron and fluorine are diluted to make calibration standards. On an Ultra Carry pad, 500 μL of each calibration standard is dropped and dried overnight. The dried Ultra Carry pad is placed on a backscatter elimination cup made of aluminium and then set into the sample holder of the analyser.

Since the Ultra Carry pad is very thin, backscatter from the inside of the sample holder passes through and is observed. The backscatter elimination cup blocks backscatter from the sample holder.

Instrument

The Rigaku ZSX Primus and ZSX Primus IV instruments are floor-standing sequential wavelength dispersive X-ray fluorescence (WDXRF) spectrometers, which have advantages in high spectral resolution and high sensitivity from light to heavy elements. The instruments are designed to offer flexibility and versatility for a wide range of applications.

The spectrometers referenced here are equipped with an Rh target X-ray tube. The maximum tube power is 4kW for the spectrometers. Up to 10 analysing crystals can deliver rapid quantitative determination of major and minor atomic elements, from beryllium (Be) to uranium (U).

Instrument control software provides users with easy operation. The flowbar system fully supports setup of qualitative and quantitative analysis.

The ZSX Primus IV spectrometer was used for measurement in this example. The measurement conditions are shown in Table 1.

Analysis results

Calibration curves of boron and fluorine in water using the Ultra Carry method were generated. The calibration summary is shown below in Table 2

The accuracy of calibration was calculated by the following formula:

Ci:    certified value of standard sample
Cˆi:   calculated value of standard sample
n:     number of standard samples
m:     degree of freedom(2: linear, 3: quadratic).

Conclusions

The results show that the Ultra Carry filter paper is applicable to boron and fluorine analysis in water. The LLD of boron and fluorine in water is 8.4 ppm and 0.4 ppm respectively. Since the water solution is dried on Ultra Carry, samples can be measured under vacuum, enabling analysis of elements from boron to uranium without the need of helium gas.

Ultra Carry also facilitates analysis of trace elements lower than 0.1 ppm, or at ppb level, in water solution. Since sample preparation is very simple, it is possible to reduce analytical error derived from sample handling and also shorten the total analysis time.

Michael Nelson is with Rigaku





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