Geological samples comprising of various rocks, ores and

minerals need to be studied for their elemental chemical data. X-ray  fluorescence  spectrometry is a precise and accurate analytical  technique  for determining elements from atomic No.4 (boron) to 92 (uranium) at concentration levels of 100% to a few parts per million (ppm) in solids, liquids and powders. The non-destructive nature of  anal- ysis, speed, versatility, automation, sensitivity,  sele-ctivity, large sample throughput have made XRF technique

a widely practised and well established technique for the determination of major, minor and trace elements in rock samples

Govil, P K

English

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X-RAY FLUORESCENCE SPECTROMETRYIN GEOLOGICAL SAMPLESP. K. GOVIL'ABSTRACTGeological samples comprising of various rocks, ores andminerals need to be studied for their elemental chemical data. X-rayfluorescence spectrometry is a precise and accurate analyticaltechnique for determining elements from atomic No.4 (boron) to 92(uranium) at concentration levels of 100% to a few parts per million(ppm) in solids, liquids and powders. The non-destructive nature ofanalysis, speed, versatility, automation, sensitivity, selectivity, largesample throughput have made XRF technique a widely practised andwell established technique for the determination of major, minor andtrace elements in rock samples.INTRODUCTIONX-rays are electromagnetic radiations having a wavelength rangeof 10-5 A° to 100 A°. Spectral region of interest in practical XRFspectrometry is 0.1 A° to 20A°.Origin of spectra :(a) Orbital electrons are expelled from inner (K, L, M .....) shellsof the atom.(b) Electrons from outer shells fall into the vacancies.(c) Difference in energy is emitted as an X-ray photon.Scientist, National Geophysical Research Institute. Hyderabad-500007XII-1XRF SPECTROMETRY IS IN GENERAL OF TWO TYPES :Wavelength dispersive XRF spectrometry (WDXRF) : X-ray spectrallines of all elements in the sample are excited. Simultaneously, thenseparated on the basis of their wavelengths prior to detection.Energy dispersive XRF spectrometry (EDXRF) : X-ray spectral linesof all elements in the sample are either excited separately or areexcited simultaneously, then separated by means other than walve-length or energy dispersion.WDXRF SPECTROMETRYPrinciple : XRF spectrometry is a group of non-destructive instru-mental method of qualitative and quantitative analysis of chemical ele-ments based on measurement of the wavelengths and intensities oftheir X-ray spectral lines emitted by secondary excitation.i) The X-ray tube produces the primary X-ray beamii) The primary X-ray beam irradiates the rock sample and exciteseach chemical element to emit its characteristic X-ray spectrum.a) The wavelength of each spectral line is characteristic of theelement present.b) The intensity of each line is directly related to theconcentration of the element present in the sample.iii) The collimator makes the secondary X-rays parallel.The crystal acts as a monochromator.iv) The diffracting crystal specially disperses the secondary X-rayscausing each analyte line to go in a different direction.v) The detector receives each wavelength diffracted by the crystaland for each X -ray photon produces an output pulse of electriccurrent having amplitude proportional to the incident X-rayphoton energy.X11-2vi) The preamplifier and amplifier amplify these electric pulseslinearly.Qualitative Analysisa) The goniometer (collimator-crystal-detector system) is scanned overits full practical range (20-140(l 20).b) The recorder chart records peaks on a grid of intensity vs 20 (X ).c) The elements present are detected from the peak positions (20, X).d) From the peak heights (intensity), the elemental concentration (semiquantitative) can be estimated.Quantitative Analysisa) The goniometer is set at fixed 2 0 R) from a specified element oranalyte line e.g. Fe Ka.b) Fe Ka intensity is measured from each sample and from a seriesof standards in which Fe concentration is known.c) A calibration curve, Fe Ka vs Fe concentration is prepared fromthe standard data.d) Fe Ka intensity from each unknown sample is applied to the curveto calculate Fe concentration.e) This sequence is followed for all the chemical elements to bedetermined.Specimen preparationAny treatment other than simply cutting to appropriate size, towhich the sample is subjected prior to its presentation to the spec-trometer :a) Chemicalb) Physicalc) BothXII-3Specimen forms :1) Solids2) Powders3) Briquettes4) Fusion Products5) Liquids, solution6) Supported specimen(a) Filter paper(b) Mylar filmNeed for preparation1) Not required for qualitative analysis.2) X-ray spectrometer can accommodate the sample inmay forms.3) XRF can analyse the samples in their as - received fromfaster than any other method available.WHY TO PREPARE OR CHANGE THE FORMI a) Rock sample Is a heterogenL. i is mixture of differentminerals present and it is necessary to make thesample homogeneous.b) Surface layer may not be representative of the bulkspecimen.c) Particle size distribution may vary from sample to sample.d) Porosity varies from specimen to specimen.II Additionsa) Internal standardb) Dilution methodc) Standard additionXII-4III. Chemical separation may be required for determining some ofthe trace elements or rare earth elements.IV Available standards must be matched. Standards are notavailable in the as-received form of the sample.ADVANTAGES OF XRF SPECTROMETRYSimplicity of x-ray spectra(a) Relatively a few lines(b) Each element has its own line having relationshipbetween X and Z.c) Independent of chemical state as dealing with innermost electrons of the atomic orbit.2. Method applicable to a very wide range of specimen form.3. Non-destructive method of chemical analysis.4. Applicable to a very wide range of chemical concentrationi.e. ppiii to 100%.5. High precision and accuracy.6. Selectivity and sensitivity.7. Rapid and convenient.8. Cost affective.9. Very little sample preparation is required.LIMITATIONS1. Only a thin surface layer contributes the measured analyteline intensity2. Subject to particle size, surface texture and matrix effect(absorption-enhancement effects)3. Standards must be similar to the samples in physical form,matrix composition, concentration range and particle size.XII-5SOURCES OF ERROR IN XRF ANALYSIS1. Statistical counting error: Arise due to randon timedistribution of x-ray photons.2. Instrumental error : Arises due to short term and long terminstrumental drift, fluctuation and instability in instrumentalcomponents, conditions and parameters(a) x-ray tube kV and mA(b) Intensity of the primary x-ray beam(c) Due to change in temperature, d spacing of the crystalmay change.(d) Gas amplification of gas flow counter due to changesin pressure, temperature and detector potential.(e) Pulse height distribution shift and distortion.3. Operational Error : Caused by the slight non-reproductibilityin instrumental settings like(a) x-ray tube kV mA(b) 28(c) Amplifier gain(d) Detector potential(e) Pulse height selection window4. Specimen error(a) Absorption-enhancement effect(b) Position effect including insertion error(1) Specimen plane(2) Takeoff angle(3) Position(4) Orientation(5) FlatnessXII-6(c) Physical form(1) Thickness (if less than infinitely thick)(2) Heterogeneity of composition (very severe ingeologicalsamples)(3) Hetrogeneity of density(4) Surface texture(5) Particle size5. Spectral line interference.XRF SPECTROMETRY - COMPONENTSA. X-ray tube1. Function : To generate the primary x-ray beam that excitesthe analytical x-ray spectrum.2. Anode targets availableCr, Sc ----------- Light elementsMo. Ag ----------- Medium elements - Rb, SrW ----------- Fe, NiPt, Au ----------- Heavy elementsRh ----------- MultipurposeDUAL ANODE (sc-Mo) ---- Low kV gives Sc spectra whilehigh kV gives Mo spectraB. X-ray tube power supply : Intensity is directly related topower supply.1. Function : To supply to the cathode filament and powerto the x-ray tube.2. Maximum kV out put : Varies with different XRF from60, 75 to a maximum 100 W.XII-7IC. Specimen chamberD. Spectro goniometer1) Collimatora) Function : Blocks divergent x-rays and provides parallelsecondary x-ray beamb) Structure - Spaced parallel metal foils1) Length (2.5 - 10 cm)2) Spacing (0.1 - 1.0 mm)c) Position1) Source collimator (primary) between specimen andcrystal.2) Detector collimator (secondary) between crystal anddetectorCoarse (550 µm) B to CaFine (150 µm) Ti to UA fine collimator gives better resolution i.e. separation of closelines but results in lower intensity.2) Crystala) Function - specially disperses the polychromatic x-ray beamfrom the specimen, causing each spectral line to go in adifferent direction - acts as a monochromatorb) Many crystals are required for different wavelength regions.The longest wavelength a crystal can diffract Is 2 d A° whichis twice its own interplannar spacing.The different crystals commercially available are (2d, increase)LiF 420 2.0 A°LiF 220 2.85 A°LiF 200 4.0 A°Ge 6.53 A°PET 8.73A°T1AP 25.75A°PX-1 to PX-4 (Smaller 2d value crystal results betterresolution)XII-83. Radiation path :a) Air<2.5A° (Ti Ka ) z=9to21b) Vacuum > 6 A° (PK(x ) z = 21 to 92c) Helium - liquid samples - light elementsE. DETECTOR1. Function : Coverts each incident absorbed x-ray photon intoa pulse of electric current having amplitude proportional tothe incident photon energy.2. Typea) Proportional (gas filled-sealed or flow) for determining BtoSe (z = 34).b) Scintillation Z = 50 to 90Both (F&S) are used to determine z=34 to 50.c) Solid state semi conductor, lithium drifted Si.F. READOUT AND DISPLAY1. Preamplifier - amplifies detector output pulses linearly2. Amplifier - further amplifies pulses linearly3. Pulse height selector - Passes all pulses having amplitude greater than the baseline potential through theset 'window'4. Ratemeter5. Recorder6. Scaler - Counts the pulses7. Timer - times the duration8. Printer - print out the accumulated counts from the scaler.Fully automatic XRF spectrometers are of two kinds :1. Sequential : A single collimator - crystal - detector system isprogrammed to index automatically in sequence to the setting for eachXII-9analyte line and its background and accumulate counts for each. Itis very useful in R&D laboratories where one needs to determine avery wide range of elements.2. Simultaneous : upto 22 channels (i.e. collimator - crystal -detector ) are arranged around the specimen each with its own readoutsystem and each channel set for a different analyte line. This kindof system is good where only some limited fixed number of elementsare to be analyzed e.g. cement and steel industries.APPLICATION IN GEOLOGYX-ray fluorescence spectrometry is one of the most powerfulanalytical tool in the study of geological samples. No modern geo-logical institute can function without the presence of XRF spectrome-ter. It is the fastest and non-destructive analytical tool to study thedifferent types of geological samples having different concentrationrange of various elements present. The different rock types can bedivided in four types on the basis of silica content.1. Acidic - SiO 2 66% to 90%2. Intermediate - SiO 2 55% to 66%3. Basic'- SiO 2 45% to 55%4. Ultrabasic - SiO 2 less than 45%All other elements present in the rock samples vary dependingon the variation of silica concentration. XRF technique has beenapplied successfully to a number of geological problems requiring rapidanswers such as the identification of unknown rock types, minerals,ores, geochemical prospecting etc.Sample preparation : All the rocks are heterogeneous mixture ofdifferent minerals present and therefore, it is necessary to have somesample preparation technique to remove the effect of heterogeneity.XII-1tlLkFirst of all the rock sample is finely powdered to -300 mesh size andthen either the fused glass beads (for determining major oxides) orpressed pellets (for determining (.race elements) are prepared.Line Overlapping : The major problem in performing XRF analysisis that of resolving overlapping lines. These interfering lines may beof the elements which are very close in wavelength and cannot beresolved because of the instrumental limitations or the lines diffractedIn higher orders. If the interference is due to the first order lines, itcan be removed by separating the lines by changing the collimatoror crystal used in the analysis. First order lines can easily be resolvedby selecting another crystal of smaller 2 d spacings. Other methodsof resolving lines involve pulse-height discrimination, crystals whichdo not diffract in certain higher orders etc.MATRIX EFFECTThe problem of matrix effect (enhancement or absorption) is verysevere in XRF analysis of rock samples. The intensity of the analytemay be increased or decreased due to the presence of different ele-ments present in the sample and will lead to the wrong results. Thereare different methods for dealing with matrix correction e.g.1. Reference Rock Standards2. Addition Method3. Heavy absorber4. Dilution5. Internal Standard1. Reference Stock Standard : The standards used in making cali-bration curves of intensities vs concentration of the analyte mustbe of similar matrix with reference to the elements present and theirchemical composition. It is not possible to use a curve prepared foracidic rock sample to calculate the elements in basic or ultrabasicconcentration range . Such a procedure gives rise to a family of ana-lytical curves required e.g. basalt standards for basalt samples andgranite standards for granite samples. It is however, often difficult tofind such similar standards . If the materials are too variable in com-position , they can no longer be safely considered as of the same type.Other possibilities to remove or to reduce matrix effects are to pre-pare natural , hemi and synthetic standards of the similar composi-tion or to use preanalysed samples of the same rock type by usingsome classical method of analysis as standard or reference samplesto prepare calibration curves.2. Addition Method : In the absence of suitable similar standards,the method of addition is useful for performing an occasional analy-sis. One has to make a rough estimate of the concentration of theelement sought. Then a small but known amount of the element isadded to an aliquot of the unknown. By measuring the intensitiesof unknown and then the unknown plus the addition, it is possibleto solve for the original composition. This method assumes a linearrelationship between composition and intensity. It is extremely im-portant to make homogeneous dispersion of the element added andone should take great care to grind and mix the powder thoroughly.J+_3. Heavy absorber : Different types of heavy absorbers are usedin making fused glass beads using lithium tetra/meta borate and rocksamples. A heavy element such as lanthanum oxide is added in thefused bead before melting to serve as an absorption buffer. In prin-ciple, a high atomic number element is added in an amount suchthat the variation in concentration of absorbing elements in the samplewill not materially effect the overall absorption of the prepared fusedbead for the element to be determined. The most commonly usedXII-12absorber is lanthanum due to its high absorption coefficient and itsavailability as the oxide at a reasonable cost. Recently it was foundthat lanthanum lines interfere with the sodium lines and it may notbe possible to determine the sodium concentration in rock samplesby using lanthanum absorber. The author has made some studies andit was observed that it is better to use samarium in place of lantha-num if one wishes to determine sodium also in the rock samples.4. Dilution : The adverse effect of different elements in a matrix oneach other can be minimized upto some extent by large dilution. Adilution of 100:1 has shown accuracies comparable to the internalstandard method but in a much simpler way. The major disavantageof dilution technique is the loss of sensitivity and detection limitsspecially for the elements present in lower concentration range.5. Internal Standard : One of the simplest and most effective meansof dealing with matrix effect problem in rock analysis is the internalstandard method. One needs an element either present in the rocksample or added to the various samples and standards in constantamount , which will be enhanced or absorbed to the same extent asthe element being determined. This can easily be done if the sampleis finely powdered or if fusion technique is used. The internal stan-dard element should be as close as possible in atomic number. If theatomic number of the element is Z then the internal standard ele-ment can be Z + 1. For the light elements (Ti and lighter) Z-1 ele-ments will strongly absorb Z elements and therefore, Z + 1 elementsare a better choice. It is extremely important to distribute and mixthe internal standard element homogeneously through the samplewhich requires very small particle size and extensive grinding.DATA COMPUTATIONThe spectrometer measures the X-ray photons and displays asintensity data. This needs to be converted into elemental concentra-tion by using different mathematical models.XI1-1:3The concentration C of an element can be calculated from C =D + E R where R is the net intensity after subtracting background,D is the intercept and E is the slope of the calibration curve afterdoing the regression analysis. This linear equation assumes that thereare no or negligible matrix effects which is possible only in caseswhere elemental concentrations are reasonably equal between samplesand standards. Different calculation models are used in computing theconcentration data. Some of them are :1. Lucas-Tooth Pyne Model : This model is suitable for the analy-sis of samples with small elemental concentration ranges. Thecorrection is based on the intensity of the interfering element and theadvantage is that the correction factors can also be derived for thoseelements for which the concentration is not known.The correction formula is :C1 = (D1 + E1R1) (1 + Y.ia1j R1/ 100)2. De Jongh Model : This model is based on the concentrations ofthe different elements present and uses a set of simulataneous equa-tions. In this model fundamental influence coefficient (alphas) can beapplied. The correction formula is :C, = (D1 + EARA) (1 + Eja1j CJ/100)This method is most extensively used and has proved successfulfor geological samples.The alphas are constant for a given type of rock samples, X-raytube and spectrometer settings. The application of theoretical alphasis extremely useful if only a small number of standards are availableand can not be calculated from regression curves.XII-143. Rasberry - Heinrich Model : This model covers an extrawide range of concentration of different elements. The formula isCi = (D, + E,R,) [ 1 + EJaIj CJ/ 100 + Ekt3Ik Ck/100 (1 + CJ/100)-']The second correction term provides a better fit. This takes help ofcalculation of beta correction factors alongwith alphas but need to becalculated on line.Recently some new softwares have come in the market which canconvert the intensity data into elemental concentration with the helpof pure elemental standards or by using only one standard. The mainproblem faced in the analysis of geological samples is non-availabilityof suitable rock standards for doing regression analysis with fullconfidence. Now this problem is not so accute with the availabilityof "standard less software" though one has to use with caution asthey are not yet proved as quantitative.

X-Ray Fluoresence Spectrometry in Geological Samples

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X-Ray Fluoresence Spectrometry in Geological Samples

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