Principles Chemical imaging

1 principles

1.1 terminology
1.2 types of vibrational chemical imaging instruments

1.2.1 mid-infrared chemical imaging
1.2.2 near-infrared chemical imaging
1.2.3 raman chemical imaging


1.3 fluorescence imaging (ultraviolet, visible , near infrared regions)

principles

chemical imaging shares fundamentals of vibrational spectroscopic techniques, provides additional information way of simultaneous acquisition of spatially resolved spectra. combines advantages of digital imaging attributes of spectroscopic measurements. briefly, vibrational spectroscopy measures interaction of light matter. photons interact sample either absorbed or scattered; photons of specific energy absorbed, , pattern of absorption provides information, or fingerprint, on molecules present in sample.

on other hand, in terms of observation setup, chemical imaging can carried out in 1 of following modes: (optical) absorption, emission (fluorescence), (optical) transmission or scattering (raman). consensus exists fluorescence (emission) , raman scattering modes sensitive , powerful, expensive.

in transmission measurement, radiation goes through sample , measured detector placed on far side of sample. energy transferred incoming radiation molecule(s) can calculated difference between quantity of photons emitted source , quantity measured detector. in diffuse reflectance measurement, same energy difference measurement made, source , detector located on same side of sample, , photons measured have re-emerged illuminated side of sample rather passed through it. energy may measured @ 1 or multiple wavelengths; when series of measurements made, response curve called spectrum.

a key element in acquiring spectra radiation must somehow energy selected – either before or after interacting sample. wavelength selection can accomplished fixed filter, tunable filter, spectrograph, interferometer, or other devices. fixed filter approach, not efficient collect significant number of wavelengths, , multispectral data collected. interferometer-based chemical imaging requires entire spectral ranges collected, , therefore results in hyperspectral data. tunable filters have flexibility provide either multi- or hyperspectral data, depending on analytical requirements.

spectra typically measured imaging spectrometer, based on focal plane array.

terminology

some words common in spectroscopy, optical microscopy , photography have been adapted or scope modified use in chemical imaging. include: resolution, field of view , magnification. there 2 types of resolution in chemical imaging. spectral resolution refers ability resolve small energy differences; applies spectral axis. spatial resolution minimum distance between 2 objects required them detected distinct objects. spatial resolution influenced field of view, physical measure of size of area probed analysis. in imaging, field of view product of magnification , number of pixels in detector array. magnification ratio of physical area of detector array divided area of sample field of view. higher magnifications same detector image smaller area of sample.

types of vibrational chemical imaging instruments

chemical imaging has been implemented mid-infrared, near-infrared spectroscopy , raman spectroscopy. bulk spectroscopy counterparts, each imaging technique has particular strengths , weaknesses, , best suited fulfill different needs.

mid-infrared chemical imaging

a set of stones scanned specim lwir-c hyperspectral imager in thermal infrared range 7.7 μm 12.4 μm. minerals such quartz , feldspar spectra recognizable.

mid-infrared (mir) spectroscopy probes fundamental molecular vibrations, arise in spectral range 2,500-25,000 nm. commercial imaging implementations in mir region employ hyperspectral imagers or fourier transform infrared (ft-ir) interferometers, depending on application. mir absorption bands tend relatively narrow , well-resolved; direct spectral interpretation possible experienced spectroscopist. mir spectroscopy can distinguish subtle changes in chemistry , structure, , used identification of unknown materials. absorptions in spectral range relatively strong; reason, sample presentation important limit amount of material interacting incoming radiation in mir region. data can collected in reflectance, transmission, or emission mode. water strong absorber of mir radiation , wet samples require advanced sampling procedures (such attenuated total reflectance). commercial instruments include point , line mapping, , imaging. mid-infrared chemical imaging can performed nanometer level spatial resolution using atomic force microscope based infrared spectroscopy (afm-ir).

remote chemical imaging of simultaneous release of sf6 , nh3 @ 1.5km using telops hyper-cam imaging spectrometer

for types of mir microscope, see microscopy#infrared microscopy.

atmospheric windows in infrared spectrum employed perform chemical imaging remotely. in these spectral regions atmospheric gases (mainly water , co2) present low absorption , allow infrared viewing on kilometer distances. target molecules can viewed using selective absorption/emission processes described above. example of chemical imaging of simultaneous release of sf6 , nh3 shown in image.

near-infrared chemical imaging

the analytical near infrared (nir) region spans range approximately 700-2,500 nm. absorption bands seen in spectral range arise overtones , combination bands of o-h, n-h, c-h , s-h stretching , bending vibrations. absorption 1 2 orders of magnitude smaller in nir compared mir; phenomenon eliminates need extensive sample preparation. thick , thin samples can analyzed without sample preparation, possible acquire nir chemical images through packaging materials, , technique can used examine hydrated samples, within limits. intact samples can imaged in transmittance or diffuse reflectance.

the lineshapes overtone , combination bands tend broader , more overlapped fundamental bands seen in mir. often, multivariate methods used separate spectral signatures of sample components. nir chemical imaging particularly useful performing rapid, reproducible , non-destructive analyses of known materials. nir imaging instruments typically based on hyperspectral camera, tunable filter or ft-ir interferometer. external light source needed, such sun (outdoor scans, remote sensing) or halogen lamp (laboratory, industrial measurements).

raman chemical imaging

the raman shift chemical imaging spectral range spans approximately 50 4,000 cm; actual spectral range on particular raman measurement made function of laser excitation frequency. basic principle behind raman spectroscopy differs mir , nir in x-axis of raman spectrum measured function of energy shift (in cm) relative frequency of laser used source of radiation. briefly, raman spectrum arises inelastic scattering of incident photons, requires change in polarizability vibration, opposed infrared absorption, requires change in dipole moment vibration. end result spectral information similar , in many cases complementary mir. raman effect weak - 1 in 10 photons incident sample undergoes raman scattering. both organic , inorganic materials possess raman spectrum; produce sharp bands chemically specific. fluorescence competing phenomenon and, depending on sample, can overwhelm raman signal, both bulk spectroscopy , imaging implementations.

raman chemical imaging requires little or no sample preparation. however, physical sample sectioning may used expose surface of interest, care taken obtain surface flat possible. conditions required particular measurement dictate level of invasiveness of technique, , samples sensitive high power laser radiation may damaged during analysis. relatively insensitive presence of water in sample , therefore useful imaging samples contain water such biological material.

fluorescence imaging (ultraviolet, visible , near infrared regions)

emission microspectroscopy sensitive technique excitation , emission ranging ultraviolet, visible , nir regions. such, has numerous biomedical, biotechnological , agricultural applications. there several powerful, highly specific , sensitive fluorescence techniques in use, or still being developed; among former flim, frap, fret , flim-fret; among latter nir fluorescence , probe-sensitivity enhanced nir fluorescence microspectroscopy , nanospectroscopy techniques (see further reading section). fluorescence emission microspectroscopy , imaging commonly used locate protein crystals in solution, characterization of metamaterials , biotechnology devices.