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Ultimate & Proximate

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Coal comes in three main types, or ranks:

  1. lignite or brown coal,
  2. bituminous coal or black coal, and
  3. anthracite or hard coal.

Each type of coal has a certain set of parameters which are mostly controlled by moisture, volatile and carbon content. Proximate analysis determines the fixed carbon, volatile matter, moisture and ash percentages.
The amounts of fixed carbon and volatile combustible matter directly contribute to the heating value of coal. Fixed carbon acts as a main heat generator during burning. High volatile matter content indicates easy ignition of fuel. The ash content is important in the design of the furnace grate, combustion volume, pollution control equipment and ash handling systems of a furnace.
Thus proximate analysis is used to establish the rank/category/type of coals and provide the ratio of combustible to non-combustible constituents.

Certificate of Registration with the NNR

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Radionuclides are present in all naturally occurring minerals and raw materials. Of regulatory importance are the radioactive isotopes in uranium, thorium, radium and potassium. These materials are commonly referred to as Naturally Occurring Radioactive Materials (NORM). In some instances the levels of NORM are quite considerable and thus restrictive procedural controls are required.

Overcoming the “Sell By” Date of an XRF Calibration

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Over short periods of time XRF instrumental drift may be perceived as negligible however long-term drift, although unavoidable, can be sizeable.
Thus an XRF calibration typically has an “Expiry” or “Sell By” date.
It is important that a method of correcting for instrumental drift be associated with every calibration in order to validate the stored calibration data (slopes and intercepts) over a long period of time.
Having to re-calibrate can be very time consuming particularly when, as is the case in a commercial laboratory like UIS Analytical Services, various calibrations, or a wide range of standards within a given calibration, are employed to cover varying material types.
Fortunately there is an alternative to having to re-calibrate called drift correction.
Drift correction is utilised to allow operation over a long period of time by exploiting a previously set-up calibration without re-measuring all the standards.
Drift corrections are based on the measurement of a few reference specimens, known as drift monitors or drift standards, that are run periodically on the XRF.
All drift standards are read at the time the standards for the calibration were initially measured.
It is the relative change in intensity of the monitor(s) that is applied during the analysis of unknown samples that ensures a given sample analysed in January has the same result come Easter holidays.
After a drift correction, the corrected intensity is the same as that used for the calibration.
The drift monitors/standards may consist of one or several specimens (metals, fusion beads etc), each containing one or several analytes of interest, and these monitors need not necessarily be standards actually used in the calibration, as long as they have an intensity (count rate) matching that of the calibration standards.
A drift correction must be applied to the measured intensities of every analyte using one, two or several monitors depending on the spread of the intensity ranges.
It is preferable to incorporate as many analytes as possible in a given monitor and to select monitors that can be used for different calibrations, so as to reduce the instrument time to perform the drift correction, especially in commercial and production laboratories where throughput is of utmost importance to the profits of the business.
Drift correction is strictly speaking not a correct term since the correction of intensities also accommodates for changes such as a different sample diameter (eg changing from 40mm to 32mm), replacement of the flow counter window, new P10 gas cylinder and a new x-ray tube.
It must be remembered that drift correction cannot be used when one type of x-ray tube (eg Rh) is changed for another type (eg Cr) since the matrix corrections will change because of a change in anode. An entire re-calibration will be required.
Examination of the drift intensities over time not only eliminates or reduces long term instrument drift, but also provides the analyst with useful instrument performance data such as a change in instrument instability due to external environmental factors (eg poor laboratory environment), or a change or degradation over time in the response of other XRF components such as the tube, detectors, analyzing crystals or collimators.
As an example, decreasing intensities can be caused by deposition of material on the x-ray tube window (eg loose powder from poorly prepared sample could fall onto and collect on the tube window; or tungsten from the tube filament depositing on the inside of the window); or “dust” accumulating in the collimators.
It is instrumental variation in intensity that is being corrected by performing regular drift corrections / updates, so one must ensure that external factors such as instability of the drift standard, poor reproducibility of its intensity measurements and possible contamination of its analytical surface (eg sodium contamination from touching the surface with fingers after lunch) are avoided.
It is good practice to keep the drift / monitor standards loaded in cassettes or allocated positions on the XRF sample changer to avoid any handling thereof.
Drift correction is a handy tool, but must always be used with care and reason. Simply applying drift correction to a change in count rate without careful monitoring thereof and being aware of the cause of the change in counts could end up being more costly than the time saved drift correcting the problem.
For further information please contact the laboratory operational manager.

Bragg’s Law: X-ray Fluorescence and Nature's Iridescence

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Bragg's Law is the result of experiments into the diffraction of X-rays or neutrons off crystal surfaces at certain angles, derived by physicist Sir William Lawrence Bragg in 1912 and first presented on 1912-11-11 to the Cambridge Philosophical Society. Although simple, Bragg's law has many applications from the study of crystals in the form of X-ray and neutron diffraction, quantitative XRF analysis and the iridescent hues on the tail of a peacock.
In the case of wavelength dispersive spectrometry (WDS) or x-ray fluorescence spectroscopy (XRF), crystals of known d-spacings are used as analyzing crystals in the spectrometer. The crystal separates the various wavelengths of the secondary x-rays emitted by the analysis specimen. When a crystal is bombarded with x-rays of a fixed wavelength (similar to spacing of the atomic-scale crystal lattice planes) and at certain incident angles, intense reflected x-rays are produced when the wavelengths of the scattered x-rays interfere constructively. In order for the waves to interfere constructively, the differences in the travel path must be equal to integer multiples of the wavelength. When this constructive interference occurs, a diffracted beam of x-rays will leave the crystal at an angle equal to that of the incident beam.
The general relationship between the wavelength of the incident x-rays, angle of incidence and spacing between the crystal lattice planes of atoms is known as Bragg's Law , expressed as: n λ = 2d sinΘ where n (an integer) is the "order" of reflection, λ is the wavelength of the incident X-rays, d is the interplanar spacing of the crystal and Θ is the angle of incidence.{mosimage}
Because the position of the sample and the detector is fixed in XRF instruments, the angular position of the reflecting crystal is changed in accordance with Bragg's Law so that a particular wavelength of interest can be directed to a detector for quantitative analysis. Every element in the Periodic Table has a discrete energy difference between the orbital "shells" (e.g. K, L, M), such that every element will produce x-rays of a fixed wavelength. Therefore, by using a spectrometer crystal (with fixed d-spacing of the crystal) and positioning the crystal at a unique and fixed angle (Θ), it is possible to detect and quantify elements of interest based on the characteristic x-ray wavelengths produced by each element.
Iridescence is one of those curious optical games that light loves to play. From pearl earrings to tropical butterflies that appear to be made out of cellophane, iridescence is a
source of luminous beauty across the natural world. The brilliant iridescence (or play of colors) found in nature (buttterfly wings, the pearly glow of a mollusc shell) can be attributed to the diffraction and constructive interference of visible lightwaves which satisfy Bragg’s law, in a matter analogous to the scattering of X-rays in crystalline solids, rather than pure pigment. In animals, iridescence is often termed structural coloration since it derives from the physical structure of the object rather than pigmented chemical compounds. Iridescence happens when light bounces off different reflective surfaces in a semi-transparent substance. As the light rays exit, they interfere with each other, sometimes destructively muting each other and at other times resonating into an impossible burst of color.
Because the final appearance depends on the exact path taken by the light before reaching the eye, iridescent objects change their hue (wavelength) depending on your point of view (or Bragg angle of view) giving them a magical, animated quality. Iridescence is quite common in birds. Here, the surface of the feather - again microscopically - has a scratched or furrowed pattern. This is where the feather does its absorbing and reflecting of light. With a specific pattern of scratches or furrows, the feathers can give off a quite specific prismatic effect. Flowers were previously believed only to use chemical colours, where a pigment absorbs all wavelengths except a few, giving them their apparent colour. Flowers, however, use the same physical structure that makes compact discs iridescent. Unfortunately, because most of the petal iridescence measured is at the ultraviolet end of the spectrum, which insects can see but humans cannot, this raises the intriguing possibility that many flowers are actually iridescent although they do not appear so to the human eye.
So next time you see a bee heading off for an iridescent flower it’s seeing different colours depending on the angle (Bragg's angle) from which the bee is viewing the petals.
For all of it’s magical aesthetic, iridescence is relatively simple to achieve and is found frequently in the inanimate world from the sheen of a puddle contaminated with a thin layer of petroleum and soap bubbles to prized gems and minerals like labradorite and pyrite.

Einstein Explains X-ray Fluorescence and Moondust

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In the early 1960s before Apollo 11, several early Surveyor spacecraft that soft-landed on the Moon returned photographs showing an unmistakable twilight glow low over the lunar horizon persisting after the sun had set. Moreover, the distant horizon between land and sky did not look razor-sharp, as would have been expected in a vacuum where there was no atmospheric haze.
Individual bits of moondust are constantly leaping up from and falling back to the Moon's surface, giving rise to a "dust atmosphere" that looks static but is composed of dust particles in constant motion. The dust is electrostatically charged by the Sun in two different ways: by sunlight itself and by charged particles flowing out from the Sun (the solar wind).
On the daylight side of the Moon, solar ultraviolet and X-ray radiation is so energetic that it knocks electrons out of atoms and molecules in the lunar soil – the photoelectric effect. The positively charged dust (measuring 1 micron and smaller) then repels itself and lifts off the surface of the Moon by electrostatic levitation manifesting itself almost like an "atmosphere of dust", visible as a thin haze and blurring of distant features, and visible as a dim glow after the sun has set (or at moon rise).
Way back in 1905, Albert Einstein (14 March 1879 – 18 April 1955) described light as composed of discrete quanta, called photons. His revolutionary proposal seemed to contradict the universally accepted theory that light consists of smoothly oscillating electromagnetic waves. But Einstein showed that light quanta, as he called the particles of energy, could help to explain phenomena being studied by experimental physicists. Einstein theorized that the energy in each quantum of light was dependent only on the frequency of the incident light and not on its intensity: a low-intensity, high-frequency source could supply a few high energy photons, whereas a high-intensity, low-frequency source would supply no photons of sufficient individual energy to dislodge any electrons. A photon above a threshold frequency has the required energy to eject a single electron, creating the photoelectric effect. This was an enormous theoretical leap but Einstein's theory of light quanta was nearly universally rejected by physicists, and only became accepted in 1919! Einstein’s discovery led to the quantum revolution in physics and contributed towards him earning the Nobel Prize in Physics in 1921.
When a primary x-ray excitation source from an x-ray tube or a radioactive source strikes a sample, the x-ray can either be absorbed by the atom or scattered through the material. The process in which an x-ray is absorbed by the atom by transferring all of its energy to an innermost electron is called the photoelectric effect.
During this process, if the primary x-ray has sufficient energy, electrons are tossed (or ejected) from the inner shells completely out of the atom, creating electron vacancies.  These vacancies present an unstable condition for the atom. As the atom returns to its stable condition, electrons from the outer shells are transferred to the inner shells and in the process give off a characteristic x-ray whose energy is the difference between the two binding energies of the corresponding shells.  
The process of emissions of characteristic “secondary” (or fluorescent) x-rays is called X-ray Fluorescence or XRF.  In most cases the innermost K and L shells are involved in XRF detection resulting in a typical x-ray spectrum from an irradiated sample displaying multiple peaks of different intensities.
The characteristic x-rays are labeled as K, L, M or N to denote the energy shells from which they originated. Another designation alpha (a), beta (b) or gamma (g) is made to identify the x-rays that originated from {mosimage}the transitions of electrons from higher shells. Hence, a Ka x-ray is produced from a transition of an electron from the L to the K shell, and a Kb x-ray is produced from a transition of an electron from the M to a K shell, etc.
Because each element (Fe in hematite, magnetite or ilmenite; Ca in dolomite or gypsum, Si in ferro-silicon or quartz etc) has a unique set of energy levels, each element produces x-rays at a unique set of energies, allowing one to non-destructively measure the elemental composition of a sample.
Since XRF, used routinely at UIS-Analytical Services, is fast and non-destructive to the sample, it is the method of choice for field applications and industrial production for control of materials.
Famous Einstein Quotes:
“Science is a wonderful thing if one does not have to earn one's living at it.”
“No amount of experimentation can ever prove me right; a single experiment can prove me wrong.”

 

XRF Analysis – Choosing a Quantitative or Qualitative Analysis

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Have you ever wondered which analysis to choose as the best means of obtaining XRF results? Understanding the fundamentals is important as accuracy, turnaround time and cost are all influenced by the correct selection of a suitable analysis.

Basic Principles of CHN (Ultimate) Analyses

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The TruSpec CHN is an analytical instrument that determines the C (carbon), H (hydrogen)and N (nitrogen) content of a variety of materials. The sample weight range capability of the Truspec is 50 milligrams to 1.5 grams. The instrument is connected to an external PC and uses a Windows - based software program to control the system operation and data management. There are three phases during an analysis cycle: purge, combustion and analyse.
In the purge phase, the encapsulated sample is placed in the loading head, sealed, and purged of any atmospheric gases that may have entered during sample loading. The ballast volume (zero volume at this point) and gas lines are also purged.
During the combustion phase, oxygen flows into the furnace to aid ignition of the sample. The sample is dropped into a hot furnace (950oC) and flushed with oxygen resulting in a very rapid and complete combustion. The chemistry of the combustion process is as follows:
Coal Sample + O2 + Heat = CO2 + NOX + SO2 + H2O + Ash
The gaseous products of combustion are passed through a secondary furnace called the Afterburner, which is set at 850oC, for further oxidation and particulate removal.The Afterburner contains 3 different reagents that remove potential corrosive compounds and assist in keeping conditions in an oxidising environment.
Alumina pellets present a surface area for combustion gases to stabilise in the dioxide state whilst maintaining an oxidising condition.
The furnace reagent removes sulphur oxide from the flow stream. The life of the furnace reagent depends upon the levels of sulphur contained in the materials analysed.
Magnesium oxide removes halogens from the flow stream. Fluorine and chlorine contained in the sample are removed to prevent corrosion in the system.
Particulate oxides are removed by the quartz wool and honey comb filter. The combustion gases exiting from the Afterburner are then collected in the ballast.
Ultimately the analyses phase; the homogeneous combustion gases in the ballast is now passed via two infrared detectors and a 3cc aliquot loop. Once the gases have equilibrated, carbon is measured as carbon dioxide by the CO2 detector and hydrogen is measured as water vapour in the H2O detector. The gases in the aliquot loop are transferred by a helium carrier flow, swept through copper at 750oC to remove excess oxygen and convert NOx to N2;
O2 + 2Cu = 2CuO
2NOX + 2Cu = 2CuO + N2
The combustion gases now flow through Lecosorb and Anhydrone to remove carbon dioxide and water, respectively. A thermal conductivity detector is used to determine the nitrogen content.
The final results are displayed as weight percentage (%w) or in parts per million (ppm) as selected by the operator.

The Importance of Monitoring XRF Count Rates

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As discussed in a previous article Overcoming the "Sell By" Date of an XRF Calibration (09 July 2009), drift correction is a handy tool in x-ray fluoresence (XRF) analysis, but must always be used with care and reason. Simply applying drift correction to a change in count rate without careful monitoring thereof and being aware of the cause of the change in counts could end up being more costly than the time saved drift correcting the problem.
An important consideration is that analytical error introduced during the drift correction procedure may become unacceptably excessive when increasingly large correction factors are applied to the raw count rates. Such a situation is fortunately recognised when a trained analytical eye is keeping a close watch over the quality standards used to monitor instrumental performance.
A good example of an increasing degradation in x-ray counts over time is the gradual deposition of sample material on the x-ray tube. In wavelength dispersive XRF instrumentation, X-ray tubes are situated either at an angle of 90 degrees above the analytical sample, or at an angle of around 45 degrees below the analytical position, the latter design obviously being more prone to sample deposition occuring on the tube window.
Tube deposition has been found in many-a-lab to occur when analysing loose powders or when introducing the analysis material as a powder briquette. Very interestingly and unexpected, the same is possible when analysing fusion beads (considered to be most suitable sample presentation method in XRF) should the edges of the glass discs "grind" during sample rotation.
The resultant deposition being evident as shown in the adjacent picture of an end-window x-ray tube.
Clearly, an XRF calibration and the count rate drift and correction factors applied thereto requires skillful monitoring to ensure prolonged application of the initial calibration.

On the Trail of Trouble

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Murder. Drama. Intrigue - Sounds a bit like a Sherlock Holmes or Nancy Drew mystery.
Truth be told, by submitting your samples  to UIS Analytical  Services (UIS-AS) you run the risk of exposing them to any or all of the following: from being  savagely beaten (pulverised) to incinerated and melted  down (oxidised and  fused into glass beads), your samples will be subjected to cruel treatment by our highly trained hitmen (laboratory assistants). Why then submit them willingly to such treatments? The answer is simple.
Behind the scenes of the XRF Section of UIS Analytical Services are people who dedicate their working lives to protecting your samples from injustice and seeing to the responsible and trouble-free running of our laboratory.
For most people the mechanics of a lab are beyond their grasp. Sure, there are a lot of people in white lab coats wearing safety protection gear, but what do they do? From arriving at sample reception in a dusty little ziplock bag to being a result in an analytical report, your sample will have been handled by many an employee at UIS-AS.
The sample preparation (crushing, splitting, milling, weighing, roasting, fusing) performed in the XRF section is no mystery to many a client.  The loading of samples on the instrument for analysis and capturing the data is a straightforward task. What follows is far more intriguing. A trained eye needs to evaluate the data for anomalies or possible errors and investigate further.
Errors may, and do, happen. Thinking they don't is like saying the Veyron is just a car or that Hurricane Katrina was just a storm. The importance is to be proactive in this regard. The case of the mismatching QC or the contaminated sample are investigated and the err tracked down and addressed. Once in a presentable format your samples' chemical results are scrutinized for hours on end by more than one trained eye. Any anomalies that come to light are  immediately investigated and the necessary retests requested to determine what might have happened. If a mistake occurs with increased frequency and is thought to be the result of careless handling, it is likely to enjoy much more attention than an isolated error.
The case of contamination can arise from improper cleaning processes or from the equipment itself used during sample preparation. These are easy to identify, address and more importantly, prevent. The case of swapped samples might require more intensive investigation. Were the samples incorrectly labelled during receipt and login? Did the assistant who milled or weighed or fused the samples accidentally swap their labels, or even worse, swap the contents of the sample bags? Could they have arrived at the lab incorrectly identified / labelled by the geologist who took the field sample?
XRF's version of DNA testing to establish parentage is less complicated than that used in forensics. Do not underestimate it, however.
The usual suspects are eliminated first. The sample bags are checked as are the XRF fusion beads. If the bags and beads are not indicative (for example samples  whose colours don't really differ), the investigation heats up. The plot thickens. Re-analysis of the fusion beads will expose an error in loading the samples for analysis on the instrument whereas a complete re-test of the samples will indicate whether the swap occurred during sample preparation and more interestingly at which workstation. At times, after several hours of exhaustive investigation, the anomolous data that was suspected to be due to swapped samples ends up simply being atypical samples, rather than atypical data.
The boundaries of data evaluation are unfortunately not well defined and skill alone does not solve these cases (although it does help). A lot of intuition and a suspicious mind comes in handy. The ability to stick with it under pressure and, of course, a pinch of humour, always helps. Even though data evaluation may become monotonous and routine-bound at times, you can rest assured that the next case is not too far off. And just like in Sherlock Holmes or Nancy Drew, the XRF group never rests until the case is closed.

Human Illnesses & Microbiology Detection Tests

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Many South African citizens, especially those in poverty-stricken areas, still lack access to potable water and rely on untreated or poorly treated water resources e.g. rivers and boreholes. These wells may be/become contaminated thereby posing a significant health risk due to the danger of waterborne diseases and their complications, hence it is vitally important to monitor the microbial quality on a regular basis.

Iron Ore Supremacy

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As a SANAS accredited laboratory, adhering to a quality assurance system is imperative. In addition to stringent internal quality control checks, and the use of certified reference standards, the regular participation in proficiency testing schemes provides verification of the analytical competence of a laboratory.

Analysis Tip

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TIP FOR CLIENTS SUBMITTING SAMPLES TO UIS-AS FOR ANALYSIS
When submitting geological, exploration, beneficiation or any other soil samples for XRF, ICP or Wet Chemical analysis be sure to specify beforehand whether you require the chemical composition based on the sample "as received" by UIS-AS; or whether you require us to pre-dry the sample to eliminate any adsorbed water and carbon dioxide prior to performing the analyses.
In the latter case all results will be reported on the "dry basis" and is probably the better option in situations where a second laboratory is used to perform quality checks of UIS-AS' data and the two laboratories may have different environmental conditions, such as humidity differences; or when samples have been through a beneficiation process and the possibility exists of there being residual surface moisture; or when samples are analysed by more than one laboratory as is the case when shipping raw materials to Clients where both the Supplier and End-user both have to verify the analytical grade to determine the monetary value of the shipment.Atypical data or an unexpected trend in analysis results may occur when samples within an analysis batch contain different surface moisture and are analysed "as received".

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