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Usually what is the size (in nanometer) of the sample or target substance in IR and Raman Spectroscopy?

Which one has largest size of sample it can scan, how many molecules or atoms are involved? If you can make the size bigger or scan coverage bigger, then the more intense would be the IR or Raman stroke signal? Or no difference in intensity in the case of Raman even if sample size is increased? Please answer separately for IR and Raman since they are different. One uses IR light, the other laser. I guess IR light needs to be focus at a spot just like a laser too.

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  • $\begingroup$ Why do you call it Raman "stroke"? $\endgroup$ – Karl Oct 15 '19 at 6:47
  • $\begingroup$ Because most raman spectrometers only measure the raman stroke.. and not the antistrokes. $\endgroup$ – Jtl Oct 15 '19 at 6:55
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    $\begingroup$ The correct term is Stokes and Anti-Stokes Scattering (named after a person) $\endgroup$ – M. Farooq Oct 15 '19 at 17:33
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This all depends on the exact geometry of your spectrometer light path, if you measure in transmission or reflection, using ATR, etc. AND of course on your sample concentration.

If you want to ask about the minimal size: As small as you can get your focal point or laser spot. A few dozen microns, maybe less. Local sample heating can be a problem. You can buy scanning IR and FT Raman microscopes.

In transmission, you can measure IR on a cm sized sample, given that is is translucent. With Raman, that would be tricky, because you should focus the scattered light again.

Minimum number of molecules for IR is very low, but that also depends on how strong the absorption line in question is. Raman used to be very insensitive, that has become a lot better with FT Raman.

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    $\begingroup$ Raman is a scattering effect. You cannot observe the scattering in the beam direction, because it is completely drowned in your laser light. $\endgroup$ – Karl Oct 15 '19 at 6:01
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    $\begingroup$ That would make no sense, the scattered light goes basically in all directions (there is an angle dependance i remember), and you want to collect as much of it as possible. $\endgroup$ – Karl Oct 15 '19 at 6:44
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    $\begingroup$ I think not. You would have to enlarge every optical part of the instrument. The costs will increase overproportionally, the SNR much less. $\endgroup$ – Karl Oct 15 '19 at 18:08
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    $\begingroup$ Why "nanometers"? We are talking about an optical system here! You should write a new question, in which you explain what this is all about. Mixing substance? Diffusion? This has nothing to do with the original question. $\endgroup$ – Karl Oct 16 '19 at 0:30
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    $\begingroup$ @Jtl Do you know how many orders of magnitude there are between one nanometer and the wavelenght of your Raman laser source, and then how small you can (and should) usually focus a rather powerful coherent light source? $\endgroup$ – Karl Oct 23 '19 at 20:38
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  • IR instruments often work with beam diameter at sample (for macrochamber in transmission) of several mm to a cm or so.

  • The other extreme for "normal" IR instruments is diffraction limited, so in the order of magnitude of 10 μm.

  • There are FTIR instruments with Focal Plane Array (or Line) detectors that simultaneously collect spectra from many such pixels (e.g. 128 x 128), and optics are available to adjust pixel size to your needs (basically again between several mm FOV and diffraction limited pixel size)

  • If you need higher lateral resolution, you can e.g. go for near-field/nano-IR or photothermal spectroscopy.


  • For Raman, the largest spot sizes/measuerment volumes are AFAIK achieved by fiber optic probes. Here, the Kaiser PhAT probe is the one with largest laser diameter I'm aware of (I haven't personally worked with one, though). Other fiber optic probes e.g. this one have lateral diameter of the measurement volume in the range of a few 100 μm. The large spots come at the cost of low NA, and thus low collection efficiency, but this can be partially compensated as the larger measurement volume allows higher total laser power without burning the sample.
  • Microscopic setups have allow high NA and thus also better collection efficiency, but the sampled volume is correspondingly smaller (lateral diameter somewhere between diffraction-limited and a few μm).

  • Thus, for Raman you usually have severe undersampling. Several strategies are used to overcome this, such as continuously moving the sample or doing multiple short acquisitions at different positions. Ideally, you should use knowledge about the heterogeneity of your sample to determine a suitable strategy (how many spots, how far apart, excitation power & time per spot)


For gases, both Raman and IR spectroscopy may be done involving very long interaction path lengths (can be several 100 m).

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