QUANTITATIVE ANALYSIS OF POWDER MATERIALS USING ENERGY DISPERSIVE X-RAY FLOUROSENCE SPECTROSCOPY

Introduction:

X-ray fluorescence Spectrometry (XRF) is one of the best analytical techniques to perform elemental analysis in all kinds of samples. This technique offers a very simple, fast and relatively inexpensive analysis method in laboratory and production facilities resulting highest accuracy and precision involving fast sample preparation for the analysis of elements from Beryllium (Be) to Uranium (U) in the concentration range from 100% down to the sub-ppm-level. Energy dispersive X-ray fluorescence (EDXRF) is the good choice for dedicated applications in quality and process control with demands for ease of use and compact size.

There are two main sample preparation techniques for measurement of powders with XRF – a) Fused bead and b) pressed pellet. Fused bead techniques incorporate relatively higher cost for sample preparation whereas, pressed pellet is rather cost effective with simplified sample preparation technique. The other option which has recently come out as an inexpensive, simplified and less time consuming sample preparation technique is analysis of loose powders which does not require any chemical processes or involves any physical change in the sample during analysis.

In the present work, an attempt has been made to explore the possibilities of quantitative analysis of minerals samples in their loose powder form using ED-XRF technique. Initially three categories of inorganic materials viz. Limestone, Quartz and Slags have been chosen for this preliminary study.

Experimental:

a) Sample Preparation:

The analysis of loose powdered material usually required that the sample be placed into a plastic sample holder with a plastic support film.  This ensures a flat surface to the X-ray analyzer and the sample to be supported over the X-ray beam.  The more finely ground the sample, the more likely it is to be homogenous and have limited void spaces providing for a better analysis.  Sufficient powder should be used to ensure infinite thickness is obtained for all of the elements of interest.  During the present study each powders (of approximately 2.5 gm) finely ground to 200 mesh particle size were taken into a special cups made for this particular purpose and analyzed directly. Figure 1 shows pictorial representation of all these kind of samples placed within such arrangement during this study.

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b) Instrumental:

During the present study Thermo Scientific ARL QUANT’X (ED-XRF) Spectrometric technique have been used. The basic technical specification of this equipment is as below:

X-Ray Tube Target:   Rhodium (Rh) Standard

X Ray Power         :    50 W

Voltage Range      :    4 – 50 kV in 1 kV steps

Current Range      :    0.02 – 1.98 mA in 0.02 mA steps

Beam Size             :    1 – 8.8 mm adjustable

Detector Type      :   Si (Li) Drifted Detector

However, the actual experimental conditions set for this study is shown in Table 1.

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The calibration curves have been made by selecting at least 10 – 15 samples (previously analyzed by volumetric / ICP-OES)in each case. In most of the cases, the linear co-efficient values found from the graph is more than 0.95-representing a true linear pattern of the curve. Then intensities of the Unknown samples are then measured against that particular calibration curve. Concentration of the unknown samples are  given directly by the Instrument.

Results:

Figure 2 represents the qualitative scan for the limestone, quartz and quartz samples whereas Figure 3-5 shows the calibration curves made for these samples made for different parameters. The individual peaks for the most significant elements present in the sample are clearly reflected and distinguished under the stated condition.

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Table 2 shows the results of certified materials when analyzed by XRF using the above calibration curves respectively. We could observe a very good correlation between the results revealing a very low standard deviation leading to high accuracy and precision.

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Conclusion:

This nondestructive method of analysis is fast and cheaper than wet chemical, ICP-OES as well as XRF with pellets and fusion beads. This study will be continued for other inorganic minerals such as ferrous and non-ferrous ores, refractories etc. Samples such as graphite & mica which are difficult to form pellets even using binders, can be also be analyzed using this technique with a special set of cells and films.

The work has been jointly carried out by Ms. Alakta Saha and Mr. Saikat Mishra, MSK Central Laboratory, Kolkata

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