Size of target substance in IR and Raman Spectroscopy

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.




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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* 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](https://en.wikipedia.org/wiki/Nano-FTIR) or [photothermal spectroscopy](https://en.wikipedia.org/wiki/Photothermal_spectroscopy).




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* For Raman, the largest spot sizes/measuerment volumes are AFAIK achieved by fiber optic probes. Here, the [Kaiser PhAT probe](https://www.kosi.com/na_en/products/raman-spectroscopy/raman-probes-sampling/phat-probe-head.php) 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](http://www.inphotonics.com/probeRPB.htm) 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)




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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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