Hello Arvind Kaushik!

As you probably know IR and Raman spectroscopy can employed easily for solid samples (powders and such). If your sample is mixed with water, then I would recommend IR spectroscopy, because to obtain an useful spectra with Raman spectroscopy for a sample mixed with water, the samples' concentration should be around 1M (if you have a suspension, then it should be saturated). IR spectroscopy will provide useful spectra even at small concentrations.

The reason Raman is used for biological samples is that visible light is not absorbed by water. Same applies here. IR will not work well for a water based sample. Second, Raman largely responds to symmetric vibrations while IR gives signals for asymmetric vibrations. You would ideally like to see both spectra. Dry powder sample can be done in the IR with a ATR technique. Also if the dry form can be mixed with KBr a pellet sample can be made for transmission in the IR. Raman can be done on a dry sample just by exposing the dry sample to the laser excitation (at a low enough power not to burn it!)

Raman and IR spectroscopy are complimentary techniques and when used together can provide a more detailed and powerful chemical analyses of your sample. That said, Raman is notorious for low resolution when a diluted sample is analyzed - higher concentration and ring structures are favorable. Also the laser employed will affect your acquired signal with regards to resolution and background. For example, use 405 nm laser for biomolecules such as DNA (water peak typically falls in dead zone), 532 nm for inorganic material, 633 nm & 785 nm can be used for both organic and inorganic, 1064 nm useful for dyes, oils or colored polymers.

To your question, I will suggest both techniques if readily available, to get a full picture and cautiously choose the laser depending on the samples being analyzed - that is how concentrated/dilute sample in water is, is the sample organic or inorganic?

Good luck!

Dear Nicholas Edward Schlotter and everyone,

Thank you for your informative comments on the question. Dear Székely István IR spectroscopy should not be preferred for water-based samples as absorption of water lies in the IR region. Hence it will interfere with our sample spectra as pointed out nicely by Nicholas Edward Schlotter .

Dear Theresah Nom Korbieh Zu thank you for providing the important information related to laser wavelengths.

Thanking you all.

Hello Arvind Kaushik

As you put in your last comment, IR spectroscopy is not the best for samples in aqueous solutions. However, if your dry sample is somewhat of a coarse powder you might have problems to acquire good spectra (low signal-to-noise ratio).

You can circumvent this problem by using an ATR (attenuated total reflectance) accessory and drying your sample over the ATR crystal to create an uniform film. Since you can control the sample concentration in the solution and the volume of solution added onto the crystal, both quality, reproducibility, and comparison between samples is significantly improved.

We developed and applied this technique to analyze microorganisms (planktonic microalgae) in the paper linked below. There you can find a description of the method.

Article Fourier Transform Infrared with Attenuated Total Reflectance...

I hope it helps you!

Cheers!

Dear Guilherme Pavan de Moraes ,

Thank you for your important comments on the discussion. The attached paper gives sufficient detail on the FTIR based on the ATR technique.

Thanking you for the refrence.