Techniques to determine elements on dust

Techniques to determine elements on dust

There are a variety of techniques that are available to determine individual elements on dustfall out rate but for the purpose of this report only XRF (X – ray fluorescence) and ICP (Inductively coupled plasma) will be discussed and compared.

Definition of XRF Technology and ICP –MS

XRF Technology – XRF (X-ray fluorescence) is a non-destructive analytical technique used to determine the elemental composition of materials. XRF analysers determine the chemistry of a sample by measuring the fluorescent (or secondary) X-ray emitted from a sample when it is excited by a primary X-ray source. Each of the elements present in a sample produces a set of characteristic fluorescent X-rays (“a fingerprint”) that is unique for that specific element, which is why XRF spectroscopy is an excellent technology for qualitative and quantitative analysis of material composition.

ICP-MS – Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry which is capable of detecting metals and several non-metals at concentrations as low as one part in 1015 (part per quadrillion, ppq) on non-interfered low-background isotopes. This is achieved by ionizing the sample with inductively coupled plasma and then using a mass spectrometer to separate and quantify those ions.

Comparison between XRF and ICP

One of the important distinctions between XRF and ICP is the fact that ICP only reveals individual elements where as XRF determines elements and their oxides, this makes it easy to extrapolate, analyse and compare with relevant standards. More explained below:

1.XRF Advantage: Simple, fast and safe sample preparation without chemical waste.

Measurements by XRF are carried out directly on the solid material with little sample preparations. XRF analyses any type of sample without the need for dilution or digestion and therefore no disposal of chemical waste is necessary. Transferring liquids in wet-chemical methods from one vessel to another can introduce contamination and/or loss of material.

Measuring relatively large sample volumes (100 mg up to 10 grams) results in a more representative characterization of the sample. Also, errors due to sample inhomogeneity are easily minimized by using larger sample volumes. XRF can measure gram quantities without any risk of cross-contamination and therefore the error in an XRF result is much smaller.

2.XRF Advantage: Non-destructive analytical technique

In benchtop XRF spectrometers the sample is excited using an X-ray tube and the characteristic X-rays from the sample are detected and automatically processed by the software. These low-power X-ray tubes don’t produce an extensive amount of X-ray photons or heat and therefore don’t damage the sample or alter its crystal structure. Irregularly shaped samples that fit into the spectrometer can be analysed without the need for destructive sample preparation, like cutting, crushing and grinding. The same sample that was analysed by XRF can later be analysed using other techniques for further investigation, if necessary.

3.XRF Advantage: Low cost of ownership

Taking into account the initial costs of instrument and infrastructure, and running costs of gasses, acids, electricity and waste disposal, XRF benchtop spectrometers are far more cost-effective than ICP. XRF does not require the use of expensive acids, gasses and fume hoods. The only requirement is electricity and in some cases the use of helium to boost the sensitivity for light elements in the sample. Also, the individual components in XRF spectrometers are not exposed to friction or heat and therefore last for many years.

For example, the analysis of oils only requires the use of inexpensive disposable liquid cups. Solid samples, like metals, can be measured ‘as is’ with no sample preparation.

4.XRF Advantage: Analysis at the production site (at-line)

Since no gasses, liquids, acids or fume hoods are necessary to operate the XRF benchtop spectrometer, the instrument can be placed in the production facility, right next to the production line for at-line process control. The instrument is easy to install and with a short and basic training on the software, the user can operate the instrument with confidence.

5.XRF Advantage: No need for daily re-calibration

The latest advances in excitation and detection technology make the current generation of XRF benchtop spectrometers very stable. In comparison with ICP, XRF doesn’t need gasses or liquids to operate. Therefore, changes in the calibrations due to the purity and stability of gasses are not an issue for XRF, making daily re-calibration of the XRF instrument unnecessary.

For XRF spectrometers, gradual instrumental drift over years is easily corrected in the software and does not need a full re-calibration each time the spectrometer is in operation.

  • Speed: Portable XRF analyzers enable quick decisions, including whether to drill or not to drill, equipment relocation considerations, where to focus on the grid, and when to take a proper sample for laboratory analysis.
  • Real-Time Reporting: Field analyses provide for faster delineation of drill targets for timely, defensible data for operational decisions and financial reporting back to management and/or investors.
  • Increased Sample Density: Running more assays in the field allows for finer grid resolution and the ability to send prequalified samples to an off-site laboratory. With improved statistics, this high-density analysis produces a more comprehensive picture of the target than the exclusive use of the traditional bag and lab method.
  •  XRF has a multitude of applications in mining. Exploration: Portable XRF analyzers provide fast acquisition of geochemical data for rapid delineation of anomalous zones and the in-depth, quantitative analysis of metal concentrations for geochemical mapping. Lead times are reduced, which can be critical if the exploration season is short. Quarry operations: Quickly obtaining accurate exploration assay data to guide mining operations is one of the biggest obstacles to optimal productivity. Portable XRF analyzers allow geologists to rapidly acquire and send XRF data to quarry laboratory and operations management personnel allows for easy collaboration and informed decisions. Oil and gas exploration: Portable XRF analyzers are valuable for upstream exploration and production, offering rapid, on-site chemical analysis of rocks, cuttings, and cores that can be used for identifying formations and determining mineral composition of the rock. Industrial minerals: Portable XRF analyzers are an emerging instrument of choice for in-quarry exploration and evaluating the composition of raw materials such as phosphate, potash, gypsum, and limestone for industrial use.
  • XRF is accurate with or without sample preparation. Handheld XRF analyzers are designed for simple point-and-shoot operation. Sample preparation may or may not be necessary depending on your application and acceptable accuracy. XRF without sample preparation can generate reliable data about many elements, particularly base metals, when decisions must be made on the spot. When higher accuracy is required, sample preparation can be accomplished in the field in minutes and then analyzed with portable instruments for lab-comparable results.
  • It keeps getting better! Advances are being made in XRF technology all the time, from improvements in light element analysis to even faster, more accurate results. Review these XRF products to find one that can improve your business. Are you using an XRF analyzer now? Let us know about your experience.

Since XRF measurements rely on quantity, there are limits on the measurements. The normal quantitative limit is 10 to 20 ppm (parts per million), usually the minimum particles required for an accurate reading.

XRF also can’t be used to determine Beryllium content, which is a distinct disadvantage when measuring alloys or other materials that might contain Beryllium.

ICP -MS

Advantages

  • Ability to provide elemental isotopic ratio information.
  • Roughly 25 elements can be analyzed in duplicate and with good precision in 1-2 minutes.
  • Large linear dynamic working range.
  • The effective combination of differing types of ICP instruments coupled with the many varied types of sample introduction allow for customization of techniques for a specific sample type or form of analytes.
  • The lower-cost ICP systems utilize single-quadrupole mass analyzer systems, which are inherently sequential systems, and have relatively low mass resolution.
  • Elements such as Ca and Fe are difficult to determine by conventional ICP.
  • Generally requires a clean room environment for ultra-low detection limits.

 

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