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ATR Attenuated total reflectance From Wikipedia, the free encyclopedia Attenuated total reflectance (ATR) is a sampling technique used in conjunction with HUinfrared spectroscopy UH which enables samples to be examined directly in the HUsolidUH or HUliquidUH state without further preparation. HU[1]U ATR uses a property of HUtotal internal reflectionUH called the HUevanescent waveUH. A beam of HUinfrared UH HUlightUH is passed through the ATR crystal in such a way that it reflects at least once off the internal surface in contact with the sample. This reflection forms the evanescent wave which extends into the sample, typically by a few HUmicrometres UH. The beam is then collected by a detector as it exits the crystal. This evanescent effect works best if the crystal is made of an optical material with a higher HUrefractive indexUH than the sample being studied. In the case of a liquid sample, pouring a shallow amount over the surface of the crystal is sufficient. In the case of a solid sample, it is pressed into direct contact with the crystal. Because the evanescent wave into the solid sample is improved with a more intimate contact, solid samples are usually firmly clamped against the ATR crystal, so that trapped HUairUH is not the medium through which the evanescent wave travels, as that would distort the results. Typical materials for ATR crystals include HUgermanium UH, HUKRS‐5 UH and HUzinc selenideUH, while HUsiliconUH is ideal for use in the Far‐IR region of the HUelectromagnetic spectrum UH. The excellent mechanical properties of HUdiamondUH make it an ideal material for ATR, particularly when studying very hard solids, but its much higher cost means it is less widely used. Sources
1. HU^UH HUFT‐IR Spectroscopy—Attenuated Total Reflectance (ATR)UH. Perkin Elmer Life and Analytical Sciences (2005). Retrieved on HU2007 UH‐ HU01‐26 UH. HUCategory UH: HUSpectroscopy U
• This page was last modified 14:08, 12 July 2007. • All text is available under the terms of the HUGNU Free Documentation License UH. (See HUCopyrights UH for
details.) Wikipedia® is a registered trademark of the HUWikimedia Foundation, Inc UH., a US‐registered HU501(c)(3)UH HUtax‐deductibleUH HUnonprofitUH HUcharity UH.
• HUPrivacy policy UH • HUAbout WikipediaUH • HUDisclaimers UH
You can HUsupport Wikipedia UH by making a HUtax‐deductible UH donation. Inserted from <HUhttp://en.wikipedia.org/wiki/Attenuated_total_reflectanceUH>
Evanescent wave From Wikipedia, the free encyclopedia An evanescent wave is a HUnearfieldUH standing HUwaveUH exhibiting HUexponential decay UH with distance. Evanescent waves are always associated with matter, and are most intense within one‐third wavelength from any acoustic, optical, or electromagnetic transducer. Optical evanescent waves are commonly found during HUtotal internal reflectionUH. The effect has been used to exert optical HUradiation pressureUH on small particles in order to trap them for experimentation, or to HUcoolUH them to very low temperatures, and to illuminate very small objects such as HUbiological cells UH for HUmicroscopy UH (as in the HUtotal internal reflection fluorescence microscope UH). The evanescent wave from an optical fiber can be used in a gas sensor. In HUoptics UH, evanescent waves are formed when HUsinusoidal waves UH are (internally) reflected off an interface at an angle greater than the critical angle so that total internal reflection occurs. The
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Figure 1. Flat crystal holder for Solid samples ; thick depressed holder for liquid samples
Figure 2 Underside of ATR Crystal holder. The two large indentations fit over the mirrors. The small circular holes fit over pins to hold the crystal in place. The pins holes must be aligned with the pins in solid ATR. Failure to align the pins results in a broken crystal and or damage to the ATR apparatus.
Figure 3 ATR apparatus out of instrument. This apparatus shows the micrometer used to press powders and solids against the zinc selenide crystal not shown.
Figure 4 Magnification of the ATR accessory.
Figure 5. ATR mirror assembly removed from apparatus. Do not spill liquids or solids on these mirrors. This ruins the apparatus.
Figure 6. Apparatus with solid ATR crystal properly mounted. To measure a solid place a small amount of sample on to the Zinc Selenide crystal. Press sample against crystal with micrometer. Do not exceed 12 on the micrometer dial
Figure 7 Magnified view of properly mounted ATR flat crystal for solids.
To obtain a spectrum click open the Easy Ominc shortcut on the desktop. Click on the Col Smp icon. Place the ATR crystal on the apparatus. Make sure the crystal is flat and the pins properly aligned Do not put your sample on the crystal yet. You will be asked to collect a background spectrum. Click ok. The instrument will start autogaining. Click on the start collection window.
After collecting the background of the ATR crystal, Place your sample on the crystal ( you only have to cover the 2mm spot on the ATR plate that is amber in color), thenclick ok. The spectrometer will perform an autogain for the signal and you will then have to click on the start collection button. After eight scans the spectrum will appear on the screen. You can then print it. You may want to e-mail the spectrum to yourself as a tiff file or store the spectrum to a flash drive.
Figure 8 Easy Omnic dialogue box for background collection
Figure 9 The autogain screen. To collect the background click on the start collection. Note the single beam spectrum displays water band and carbon dioxide peaks.
Figure 10. Do not click on the OK in the dialogue box until you have placed your sample on the ATR crystal.
Figure 11. Apparatus configured to measure plastic film. Below are spectra showing the effect of pressure on the IR spectrum
Figure 12 No pressure on film micrometer 0
Figure 13 pressure of 2 on micrometer scale. Note improved signal to noise.
Figure 14 Pressure of 6 on micrometer scale. Note much improved signal to noise. A transmittance of 50% vs 89% for a micrometer reading of 2. Never exceed 12 on the micrometer dial. These crystals cost as much as an I phone ~$700.
Zinc Selenide (Raytran, Irtran‐4) Optical Material Information*
Chemical Formula ZnSe
Refractive Index (at 1.00 microns) 2.49
Knoop Hardness (psi) 150
Modulus of Rupture (psi) 8000
Useful Wavelength Range, Transmission (microns)
0.5-20
Useful Wavelength Range, ATR (microns)
0.5-14.3
Maximum Useful Temperature (ºC)
300
Melting Point (ºC) 1525
General properties Withstands limited mechanical and thermal shock; can be cracked by uneven pressure; toxic; yellow.
Chemical Properties Incompatible with acids and strong alkalis; insoluble in water and organic solvents; non-hygroscopic.
Clean with... Alcohol, acetone, water. *This was compiled from various sources. For more details, consult the Chemical Abstracts Service at HUwww.cas.orgUH.
Transmission Spectrum
Harrick Scientific Products, Inc. 141 Tompkins Ave, 2nd floor PO Box 277 Pleasantville, NY 10570
Phone: 914‐747‐7202 Fax: 914‐747‐7209
E‐mail: [email protected]
Inserted from <HUhttp://www.harricksci.com/infoserver/Optical%20Materials/Zinc%20Selenide.cfm UH>
To clean the liquid ATR crystal remove the crystal to the hood ;rinse your sample off the crystal using acetone, or alcohol. Dry the crystal with Nitrogen gas. Do not wipe the ATR crystal with a KIMWIPE. To clean the solid ATR remove the ATR crystal from the apparatus and take to the hood. Brush off the solid into a waste container located in the hood. Rinse the solid ATR crystal with acetone or alcohol. Remember to clean the press. The tip of the press will contain a small amount of your sample. Do not drop the solid on the press onto the mirrors below or you will ruin the ATR accessory.