Chemistry 311: Instrumentation Analysis · 2009. 10. 16. · X-ray Spectrometry Instrumentation: Topic 2b: XRF Sources: X-Ray Tube: These are the most commonly used sources for analytical

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Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

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Topic 2b: X-ray Fluorescence Spectrometry Text: Chapter 12 Rouessac (1 week)

4.0 X-ray Fluorescence

Topic 2: Atomic Spectroscopy

Download, read and understand EPA method 6010C ICP-OES


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Topic 2b: XRF Atomic X-ray Spectrometry

Fundamental Principles

Emission of X-rays Absorption Spectra

Mass Absorption Coefficient

X-Ray Fluorescence

Instrumentation Sources Monochromators Transducers

Applications X-ray Fluorescence Methods

Qualitative Quantitative

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Fundamental Principles: X-rays are short wavelength (10-5 Å to 100 Å) EM produced by the deceleration of high-energy electrons or by electronic transitions of electrons in the inner orbitals of atoms. In practice the wavelength range most often used for analytical purposes is 0.1 Å to 25 Å (0.01 nm to 2.5 nm).

Emission of X-rays: There are 4 main sources of “analytical” X-rays; A) Bombardment of a metal target with a beam of high-energy electrons B) X-ray Fluorescence by a material irradiated by X-rays C) Use of an radioactive source D) Synchrotron radiation Source (highly specialized facility not discussed)


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Topic 2b: XRF A)  Bombardment of a metal target with a beam of high-energy electrons e- produced at cathode and accelerated toward a high potential anode (100 kV). Collision e- decelerated and X-ray spectrum is produced. This is dependent only on the accelerating voltage and is independent of target material. Energy of photon is equal to difference in KE before and after collision.

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A) Bombardment of a metal target with a beam of high-energy e- Maximum photon energy corresponds to the instantaneous and complete deceleration of the electron. Described mathematically by Duane-Hunt Law;


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A) Bombardment of a metal target with a beam of high-energy e- Emission behavior of Molybdenum is typical of all elements with atomic numbers (A#) greater than 23 X-ray line spectra relatively simple, with shorter wavelength being K series and longer L series. Elements with A#’s < 23 produce only K series. X-Ray line spectra have a specific “appearance energy” for Mo this is 20 KV. For Tungsten these lines appear at >70KV

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4d10 4p6 4s2

3d10 3p6 3s2

2p6 2s2

1s2


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Topic 2b: XRF B) X-ray Fluorescence by a material irradiated by X-rays The absorption of X-rays produces electronically excited ions, when the ion returns to it’s ground electronic state, characteristic λ are produced. Cutoff λ from the primary X-ray source must be less (greater in energy) than the “absorption edge” of the analyte.

C) Use of an radioactive source X-ray radiation can be produced by radioactive species. γ-rays are high energy EM that is indistinguishable from X-rays.

Another radioactive process is electron capture in which the nucleus captures an electron to form a new atomic species (with lower atomic number). K electrons because of their proximity are captured must often, leaving the K-level electron hole needed for characteristic radiation. Common example is

55Fe → 54Mn + h ν Mn Kα line at 2.1 Å results

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Topic 2b: XRF Absorption Spectra X-rays are absorbed by materials through an photoelectron effect process. Typical absorption spectra are presented below


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Topic 2b: XRF Absorption Spectra The absorption spectra of a given element is relatively simple Observed λ is characteristic of the element and is independent of it’s chemical state. Inner e- far removed from valence e-. Sharp discontinuities are called “absorption edges” The absorption edge for a given band ie., K reflects the difficulty in removing an electron from that orbital. It is more difficult to extract a 1s electron (e-) close to the a nucleus with 82 protons (+82 charge) than it is to extract a 1s electron (e-) close to the a nucleus with 47 protons (+47 charge) ∴ 82Pb has a much lower wavelength (higher energy) K band than does 47Ag.

Mass Absorption Coefficient Beer’s law is also applicable to absorption of X-radiation.

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Topic 2b: XRF X-ray Spectrometry Instrumentation:

Sources: X-Ray Tube: These are the most commonly used sources for analytical work. (see previous diagram, (Skoog Figure 12-7))

Radioisotopes: The nature of the radiation used with these sources is completely dependent on the radioactive material used. Many produce line spectra. Since absorption sensitivity is related to the proximity to specific absorption edges, specific sources are more applicable to specific analysis.

Secondary Fluorescence: This can be quite useful, as discrete lines are produced without the underlying continuum of X-ray tube. However, a primary X-ray tube or Radioisotope source is required to stimulate fluorescence.

Source  λ selector Sample Holder  Detector 


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λ Monochromators and Filters:

Filters: Thin strips of metal can provide effective λ filters.

λ Monochromators: Crystals can be used to produce monochromatic radiation via application of Bragg’s law

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Topic 2b: XRF


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X-ray Transducers: Usually the monitored signals in X-ray spectrometry are of low intensity and frequency, as a result transducers are often operated in a photon counting mode. Most of the detectors in X-ray spectrometry rely on the ionizing nature of X-ray radiation to produce measurable electronic signals. Gas-Filled Transducers: Inert gasses such as Argon, Xenon or Krypton are enclosed metal tube equipped with electrodes that have a high potential applied across them. When X-rays ionize the gas, a current is produced, the nature of which is dependent on the magnitude of the applied potential. 3 types of transducers are obtained (see next page).

Geiger Tube Proportional Counters Ionization Chambers

.

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Topic 2b: XRF X-ray Transducers (cont..):

•  Scintillation Counters: Radiation striking a phosphor produces luminescence that can be monitored and amplified with a photomultiplier tube. •  Semi-conductor Transducers: semiconductor based detectors have a roughly analogous mode of operation to gas filled detectors.


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Gas-Filled Transducers:

Geiger Tube: If the potential is > ~1000V significant amplification occurs (~109). Space charge effects cause a dead-time of 50-200 µsec for this device.

Proportional Counters: Signal gains are less (500 - 10,000) and thus require additional amplification. Dead time is approximately 1 µsec. Signal intensity is dependent on the energy (frequency) of the incident radiation, thus if selected ranges of signals are counted in sequence a frequency domain spectra can be obtained.

Ionization Chambers: Currents are small in this range and thus the sensitivity is also low. Not used in X-ray spectrometry.

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Topic 2b: XRF Signal Processors: Pulse-Height Selectors: Only signals with a preset range of intensities are collected. See Figure 12-13. Pulse-Height Analyzers: Signals with specific energy range have distinct energy ∴ scanning energy range is comparable to scanning frequency (or λ).


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Topic 2b: XRF Applications: X-Ray Fluorescence: The non-destructive nature of this technique makes it very popular especially for qualitative purposes. Semi-quantitative and even quantitative analyses are also possible although these are more difficult.

X-Ray Fluorescence Instrumentation: There are 3 basic types. The later two listed below could be equipped with either a X-ray tube or radioactive source.

Wavelength Dispersive: Since only a small fraction of incident radiation can be effectively dispersed into monochromatic radiation, an intense source is required. ∴ this type of instrument requires a X-ray tube (104 more intense than common Radioactive sources). These can be either sequential (~$60,000) or multi-channel (>$150,000).

Energy Dispersive Non-Dispersive

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Topic 2b: XRF Energy Dispersive: A schematic for a typical energy dispersive instrument is adjacent. Since the source, sample and detector can be placed close to each other signal losses are significantly reduced. Much less expensive (~$15,000 – $20,000).

Non-Dispersive: If a filter or series of filters are placed before the detector, only specific frequencies can be passed, producing a very simple low cost instrument.


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Qualitative and Semi-quantitative Analysis: Qualitative information is obtained by the observed frequency of the radiation. The observed relative intensity of the lines is a rough guide for quantitative determination. A better quick estimate is to use the following relationship;

Px = Ps Wx Where; Px is the observed intensity: Ps is the intensity of pure material; and Wx is the weight fraction of x

Quantitative Analysis: Reasonably accurate quantitative results can be obtained if standards with nearly identical matrices can be used for calibration.


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Topic 2b: XRF Matrix effects: Both bulk and surface elements can absorb X-rays and emit characteristic radiation. For those in the bulk material; the intensity of the excitation absorption is attenuated by the material radiation must pass through before reaching analyte. Furthermore, the fluorescence emitted by the analyte must pass back through material and therefore may also be absorbed. Furthermore, matrix material might also emit interfering radiation.

Calibration: External calibration standards can be used in an identical manner. Dissolving or diluting sample (fusing) may also be used to create a constant matrix.

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Topic 2b: XRF

Disadvantages: •  Low sensitivity (0.01 to 100 %) •  Less applicable for lighter elements (elements below Vanadium A# 23)

• Cost of $5000 to $500000

Advantages and Disadvantages of X-Ray Fluorescence:

Advantages: • Simple spectra • Spectral interferences limited • Non-destructive technique (for the most part) • Many sample types and sizes • Very rapid and convenient

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Chemistry 311: Instrumentation Analysis · 2009. 10. 16. · X-ray Spectrometry Instrumentation: Topic 2b: XRF Sources: X-Ray Tube: These are the most commonly used sources for analytical