sample introduction systems - university of notre dameasimonet/engv60500/lecture_5_09_15_2011.pdfnew...
TRANSCRIPT
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Sample Introduction Systems
SOLID – LASER ABLATION
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Laser ablation
• Benefits:– spatial resolution of < 10 um– little to no sample preparation required
• Drawbacks:– transient signal– reduced precision/accuracy relative to
solution mode
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What is a laser??
• LASER = light amplification by stimulated emission of radiation
• beam of coherent monochromatic light (i.e. characterized by one wavelength)
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Types of lasers
• carbon dioxide• Nitrogen• Ruby• Nd:YAG• excimer = excited dimer (e.g. ArF gas)
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Laser – general principles• a flash tube supplies photons for amplification through
the gain medium
• atom in gain medium absorbs energy to become excited
• the excited atom is hit by another photon to become “doubly excited” and will release 2 photons of the same wavelength
• light produced reflects back and forth between mirrors
• one of the mirrors is typically partially transparent and the beam of light is emitted through this mirror
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Laser ablation system – general configuration
Montaser (1998)
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Laser Ablation – basic system
Günther & Koch (2008)
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Laser Ablation• The most versatile solid sampling technique for
ICP spectrometry
• Sufficient energy in the form of a focused laser beam is directed onto the sample, material from the surface is sputtered and vaporized
• The plume of vapor and particulate matter is transported in a He carrier gas to the plasma for ionization
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Laser Ablation
• The laser beam is steered in the direction of the laser ablation cell (sample) using mirrors and/or prisms and then focused onto the sample using a lens
• Aperatures along with other optics may be inserted into the beam path in order to improve beam homogeneity
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Laser Ablation
• We require an optical system to observe the sample while it is being ablated –however, this typically consists of a video system to protect the eyes of the operator from potentially damaging laser energy.
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Laser Ablation
• Laser– A minimum power output is required – in the
vicinity of 1 mJ per pulse– Most commonly used laser energies are
based upon the Nd:YAG (Neodymium: Yttrium Aluminum Garnet) laser
– Solid state laser in which the garnet rod is excited using a flash lamp (similar to flash lamps used in photography).
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Laser – Nd:YAG
• Fundamental energy output is at a wavelength of 1064 nm – near Infrared spectrum
• Using suitable crystals, the frequency of the laser output can be multiplied– e.g. doubled – 532 nm
• tripled – 355 nm• quadrupled – 266 nm• quintupled – 213 nm
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Laser• f = c/
• f – frequency• c – speed of light (299,792,458 m/s) – wavelength
• f = E/ h ; E = h ·c/
• E – photon energy• h - Planck’s constant (6.62606896(33)×10−34 J·s)
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Laser Ablation Systems
Longerich (2008)
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Laser ablation systems – new developments
• Recently developed femtosecond (fs = 10-15 seconds) laser systems (e.g. Horn, 2008)
• Characterized by a much shorter pulse width (10,000 times) compared to Nd:YAG and ArF excimer systems since these are characterized by pulse widths in the nanoseconds (10-9 seconds)
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Laser ablation systems – new developments
• Why is pulse width important??
– The much shorter pulse width limits the phenomenon of sample melting at the ablation site.
– However, much higher cost compared to other laser ablation systems and ‘no-turn key’femtosecond model is available
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Laser ablation systems
• Variable power output control is an absolute must!– This is required in order to achieve a suitable
sampling rate, which is dependent on the sample matrix and laser beam size.
• E.g. an analysis using a ‘pit’ or laser beam diameter of 60 microns, an ablation lasting 60 seconds results in a pit 60 microns (@1 Hz) deep (sampling rate of 1 micron/second)
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Laser pits
• Optimal geometry consists of a symmetrical pit and flat bottom
• These can only result from a homogenized beam energy profile
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Laser ablation systems
• In order to maintain reproducibility, energy meters are installed
• Energy meters are either placed in the beam path, or laser beam is split so that a known fraction is directed towards the meter while the remainder is focused onto the sample
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Laser ablation systems
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Laser ablation systems –New Wave Research (Division of ESI)UP213 nm
UP193 nm
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Recently developed 193 nm laser systems
Analyte 193H Resonetics – 193 nm
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Laser ablation cells• Sample is contained in an air-tight cell through
which a flow of gas (He) carries the ablated sample aerosol to the ICP (through tygon tubing).
• The ultimate goal is to design a cell that attains 100% transport efficiency since at this level laser-induced elemental fractionation (LIEF) processes taking place in the laser cell and transport system will be reduced- hence, improved ion signals and results shall be attained
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Laser ablation cells – use of He carrier gas
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Laser ablation pits – are the ‘pits’?
Simonetti et al. (2008)
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Laser ablation pits – are the ‘pits’?
Simonetti et al. (2005)
zircon
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Laser ablation cells
• Basic requirements (somewhat conflicting)
– Large enough to house variably sized solid samples
– Small enough volume in order to attain rapid sample ‘washout’ and high ‘sensitivity’
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Laser ablation cell – “Standard”Cell of UP213 system
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Ablate grains in epoxy mounts vs. thin sections?
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SuperCell™ Accessory Option
1
2
3
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Laser ablation cells –‘Supercell ™’
Sample stage with slide insert Slide insert removed
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Laser ablation cells –‘Supercell ™’
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Laser ablation cells –‘Supercell ™’
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Simonetti et al. (2008)