National Centre for Earth Science Studies

S4 PIONEER Calibration curves

S8 TIGER Calibration curves

Instrument is loaded with software containing theoretical elemental line library. By this software, qualitative (identification of major and minor elements) and semi-quantitative analysis of elements (>0.5%) by standardless method for unknown sample is possible. Standardless method has elemental lines calibrated and calibration coefficients stored in the line library. However, for precise and accurate quantitative determination, instrument has to be calibrated taking into consideration our specific needs, sample types and instrument configuration.

Calibration involves identification of optimum conditions of several variable factors for each element, like identifying correct elemental peak and back ground(s), power settings (kV and mA) for excitation of the line spectrum, crystal selection, detector type (scintillation or flow proportional), pulse height, collimator mask, counting time, dead time etc., followed by matrix and inter-element interference correction. This is achieved by repeated analysis of standard samples with known certified values and correcting the variables to yield elemental concentration close to the certified reference values for standards. Calibration curves for major elements based on 19 International standards of basic compositions are given here. These figures illustrate concentration vs intensity, which is, recommended versus obtained values. Excellent correlation of Chemical vs XRF concentration with good linear calibration lines and lower standard deviation supports reliability of analysis and method of sample preparations. .

Accuracy and reliability of results on unknown depend on good calibration (measuring arrangement). It is a lot easier than earlier times, but still a lot of hard work goes into achieving good results. We have achieved good agreement with standard reference materials. With each sample batch, a reference standard is included as a check on reproducibility. These data are provided with analytical results to the user. At present we can analyse complete major and trace element analysis of silicates, oxide rocks and sediments. Major elements results on our XRF for International standards are compared with recommendedvaluesinthefollowingtable.

Due to diverse fields of research at NCESS, XRF lab receives requests presenting special problems to analyse widest possible range of geological materials for a very wide range of compositions. We have put in considerable amount of time in successfully analysing unusual sample types and unusual compositions in soils, river and beach sediments, clay, ilmenite, waste composite etc.

In preparing measurement programmes,NCESS XRFs use reliable calibration standards which are representative of the matrix and target element concentration ranges to be analyzed. Measuring arrangement follows empirical method based on the analysis of standards with known elemental compositions. Sample preparation method is same for a given measurement method. When standards are few (for eg. Zircon, Ilmenite, rutile, clays etc.) alternative “standardless” techniques is adopted using typical composition of the sample. The standardless calibration model is verified and optimized against single standard sample. We are using over 65 certified reference standards to obtain good results for trace elements in all types of geological samples.

Following is the list of International reference standards used in our lab.

Basic rock Composition

BCR2*, BIR1*, DNC1*, BE-N, W2a*, AGV2*, BHVO2*, ANG#, BR*, DR-N#, PM-S#, WS-E#, JA-1A+, JA-2+, JA-3+, JB-1B+, JB-2+, JB-3+, JGB-1, JGB-2

Granitic rock composition

G2*, GSP2*, STM1*, SARM1@, SARM2@, SY3@, RGM*, GA#, GH#, GS-N#, AC-E#, MDOG#, ISHG#, VS-N#, JG-1+, JG-2+, JG-3+, JR-3+, JSY-1+

Sediments and Soils

SCO1*, SDC1*, SO1@, MAG1*, SGR1*, BX-N#, MESS3, PACS2, HISS1, JD-1+, JLK-1+, JSD-1+, JSD-2+, JSD3+, JMS-1+, JMS-2+, VL1′, VL2′, SLB’, JSO-1+

Mineral Analysis

Minerals (ilmenite, rutile, zircon etc) are analysed by standardless method, which is verified and tested against certified mineral standards like SARM 59 (Ilmenite), SARM 61(rutile) and SARM 62 (zircon)

Standard samples Source: *USGS, USA; #: CNRS, France; @NRCAN, Canada; +Venezuela

Analysis are performed on pressed pellets for trace elements and fusion glass disks for major elements.

Pressed pellets are prepared using 40 mm aluminium cups filled with Boric acid crystals as binder. Finely powdered sample (-300 mesh) is sprinkled over boric acid and pressed in a 40-ton hydraulic press to produce a circular 40 mm disk. The pressed powder pellets allow trace element determinations, with limits of detection up to 1 ppm for selected elements. The elements determined presently in our lab areK, Ca, Ti, V, Cr, Co,Mn, Fe, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Ba, La, Ce, Sm, Yb, Th. Light elements like Si, Al, Mg, Na and P are less precise by this method. All major and trace elements are determined on sample pellets for which fused glass disk cannot be made (for e.g. river and marine sediments, soils etc.).

Fused glass Disk is prepared on a Katanax X-300 X-Fluxer triple station fusion instrument in the sample preparation lab of NCESS. One gram of finely powdered sample is mixed with 5 gram of flux (LiT/LiM/LiBR 49.75/49.75/0.50, Pure) in a platinum crucible. It is inserted into the crucible holder which enters the furnace automatically. The door closes and the remaining heating steps are automatically started in sequence and are constantly monitored and displayed. Once the flux is molten at 1050 C, a left-to-right rocking of crucible holder continuously mixes the flux with the sample. When all the sample is dissolved, furnace door opens and the crucible is tilted further to empty its contents into the platinum mold. A set of blowers cool the mold and when completely cooled, the process ends. It is entirely automated and comes preloaded with various fusion programs. It is also possible to create need based programmes.

Fused disks are excellent for analysing major elements as they reduce matrix effects, eliminates particle size effects and provides a homogeneous specimen. Samples which contain higher than normal concentrations of elements like lead, tin, arsenic and/or antimony (as they can seriously damage the platinum ware) and samples which contain organic matter are not suitable for this method of sample preparation. Elements determined by this method are Na, Mg, Al, Si, P, K, Ca, Ti, Mn, and Fe.