Download Introduction to the principles of Atomic Spectroscopy

Field Methods
XRF
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Atomic Spectroscopy for Metal Analysis
Introduction to the Principles of Atomic Spectroscopy
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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
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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
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In atomic spectroscopy, higher energy radiation is used to transit inner electrons from
low to high energy states
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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:
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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.
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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.
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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
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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
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Atom is stabilized – emits characteristic X-ray fluorescence unique to the element
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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
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Two main types:
– wavelength dispersive
– energy dispersive
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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
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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
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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
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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
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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
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Field Portable X-ray Fluorescence (FPXRF)
Atomic Spectroscopy for Metal Analysis
Selection of the Proper Atomic Spectroscopic Techniques
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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
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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.