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Field Methods XRF Copyright © 2013 by DBS Atomic Spectroscopy for Metal Analysis Introduction to the Principles of Atomic Spectroscopy • Atomic spectroscopy: – mainly deals with high energy absorption/emission of individual atoms Atomic Spectroscopy for Metal Analysis Introduction to the Principles of Atomic Spectroscopy Introduction to the principles of Atomic Spectroscopy • In molecular spectroscopy, low-energy radiation (IR, VIS, UV) causes molecules to vibrate/rotate or outer electrons to transit from low to high energy states • In atomic spectroscopy, higher energy radiation is used to transit inner electrons from low to high energy states • High energy radiation is provided by: (a) flame in flame atomic absorption spectroscopy (FAA) (b) electrothermal furnace in flameless graphite furnace atomic absorption spectroscopy (GFAA) (c) plasma in inductively coupled plasma-optical emission spectroscopy (ICP-OES) (d) X-rays in X-ray fluorescence spectroscopy (XRF) Atomic Spectroscopy for Metal Analysis Introduction to the Principles of Atomic Spectroscopy 3 major types of atomic spectroscopy: • Absorption – light of a wavelength characteristic of the element of interest radiates through the atom vapor. The atoms absorb some of the light. The amount absorbed is measured. • Emission – sample is heated to excitation/ionization of the sample atoms. Excited and ionized atoms decay to a lower energy state through emission. Intensity of the light emitted is measured. • Fluorescence – a short wavelength is absorbed by the sample atoms, a longer wavelength (lower energy) radiation characteristic of the element is emitted and measured Atomic Spectroscopy for Metal Analysis Introduction to the Principles of Atomic Spectroscopy Introduction to the principles of Atomic Spectroscopy 3 major types of atomic spectroscopy: • Absorption – light of a wavelength characteristic of the element of interest radiates through the atom vapor. The atoms absorb some of the light. The amount absorbed is measured. • Emission – sample is heated to excitation/ionization of the sample atoms. Excited and ionized atoms decay to a lower energy state through emission. Intensity of the light emitted is measured. • Fluorescence – a short wavelength is absorbed by the sample atoms, a longer wavelength (lower energy) radiation characteristic of the element is emitted and measured Atomic Spectroscopy for Metal Analysis Introduction to the Principles of Atomic Spectroscopy Atomic X-Ray Fluorescence • X-ray fluorescence is a two-step process 1. Excitation of inner electrons via X-rays 2. “jump ins” of the electrons from higher energy levels to fill vacancies • Atom is stabilized – emits characteristic X-ray fluorescence unique to the element • XRF instrument measures the photon energy from the fluorescence to identify the element and the intensity of the photon to measure the amount of element in the sample Atomic Spectroscopy for Metal Analysis Instruments for Atomic Spectroscopy Atomic X-Ray Fluorescence • Two main types: – wavelength dispersive – energy dispersive • Consists of a polychromatic source (X-ray tube or radioactive material), sample holder, photon detector (Si-semiconductor) Atomic Spectroscopy for Metal Analysis Instruments for Atomic Spectroscopy Atomic X-Ray Fluorescence • Wavelength dispersive XRF (WDX) – fluorescence radiation is separated according to wavelength by diffraction on an analyzer crystal before being detected, can detect multiple elements at the same time • Energy dispersive XRF (EDX) – energy of a photon of a specific wavelength is detected Atomic Spectroscopy for Metal Analysis Instruments for Atomic Spectroscopy Atomic X-Ray Fluorescence • Energy dispersive XRF – consists of a polychromatic source (X-ray tube or radioactive material), sample holder, detector – Smaller and cheaper than wavelength dispersive XRF – Ideal for field investigations – No moving parts Atomic Spectroscopy for Metal Analysis Instruments for Atomic Spectroscopy Atomic X-Ray Fluorescence • • • • XRF is unique among all atomic spectroscopic techniques in that it is non-destructive Good for elemental composition analysis For quantitative analysis require reference standard with similar matrix to that of the sample Detection limits are in the ppm (mg/kg) range • Field Portable X-ray Fluorescence (FPXRF) Atomic Spectroscopy for Metal Analysis Selection of the Proper Atomic Spectroscopic Techniques • Important factors: – Detection limit – Working range – Sample throughput – Cost – Interferences – Ease of use – Availability of proven methodology Ions Found in Natural Waters Conc. Range Cations (mg L-1) 0-100 Ca2+, Na+ Anions Cl-, SO42-, HCO3- 0-25 Mg2+, K+ NO3- 0-1 Fe2+, Mn2+, Zn2+ PO43- 0-0.1 Other metal ions NO2- Reeve, 2002 References • • Csuros, M. and Csuros, C. (2002) Environmental Sampling and Analysis for Metals. CRC press, Boca Raton, Fl. Kalnicky, D.J. and Singhvi, R. (2001). Field portable XRF analysis of environmental samples. Journal of Hazardous Materials, Vol. 83, No. 1–2, pp. 93-122.