Boyang,

It will be helpful to know what your sample is. Could humidity (addition of water) of the room at the time of measurement affect your scans? Also did you do the repeat measurements immediately or say a week apart? For your measurements, aside wavelength and accumulation, did you maintain the same objective lens, laser power, # of scans? Finally did you normalize the two data points before comparison?

Let me know and I may be of further assistance.

best, Thess

Hi Theresah.

My sample is Na2SO4. The repeat was measured after 2hr of the first measurement. The factors that you mentioned above like objective lens, laser power and scans were all maintained the same. I normalized both spectra, still the total spectra intensity was lower than the first measurement. I really cannot think of what can be the reason

For the same sample detection, Is there any acceptable range for the intensity variation between each measurement?

Many thanks,

Given that all the excitation and spectrometer parameters are described as being exactly the same, one needs to ask about the sample.  As indicated in Theresah's response, is it possible even within the 2 hours between measurements, that changes occurred to the sample?  How was the sample prepared; powder, pressed pellet, film, etc.? Is it kept air-free in an in situ cell, or is it ex situ?  How is the sample mounted and placed in the beam, does the laser always focus on the same spot?  Did you try to look at several spots?  Are you able to check visually via microscope if beam damage has occurred?  How long was the sample exposed to the beam?

All of the above questions are related to the possibility that the sample changed due to external factors or due to the Raman analysis itself.  If any of the above is possible, you could certainly expect changing intensity.

If you can rule out an alteration of the sample, then you have to ask about the sample geometry.  If the laser was not always on the same spot, the geometry of the sample will also have a big effect on the intensity because the diffuse nature of the Raman scattered photons.  The Raman scattered light needs to get to the detector and the sample roughness or angle will have a significant effect on the intensity.

Hi Boyang,

I agree with Charles that your sample could have undergone some form of change (mind you Na2SO4 is hygroscopic) depending on sample preparation (did you scan the powdered sample or prepare a solution of it?). Also did you scan the same spot? did you take multiple scans each time and average it out? was there any carbonization (beam damage) of your sample?

My suggestion. Rerun your experiments as before. This time take a minimum of 10 individual scans each time at different spots or positions within your sample. Do this for both T1=0 and T2= 2 hr.

Normalize the averaged spectra from both T1  and T2 using the norm (vector normalization). Let me know if you still observe the peak difference.

best,

Thank you so much Charles and Theresah. 

the system is an in-situ set up, with fix probe position same powder size cut and volume of sample. the spectra were using for the comparison was an average out of 20 measurements. After measurement the sample was kept in a sealed bag. The only external factor i would think of would be the angle of the beam. 

Also, I was only considering if Na2SO4 absorb the moisture from the surroundings, there would be hydrates peak appears, but forgot about the initial step: deformation of Na2SO4 crystals, which could cause the intensity change.

I am going to repeat the experiment again. see if still have the same problem.

Many thanks to both of you again. 

I have to admit that I have never myself recorded a Raman spectrum, but I was told that the laser output power can vary from day to day. Given that the spectra don't show peak shifts and the intensity ratios stay the same this might be one possible explanation. Are you using a microscope and is your sample polycrystalline? A varying surface quality of your sample might be another.

 Hi Thomas, 

I am using the Raman spectroscopy. I was just wondering if it was because the variation of laser output. Do you know what causes the variation. Is it still relevant to the lab environment?

Many thanks

As I said, I'm not familiar with the practical aspects of Raman spectroscopy, but from what I was told, this seems to be a problem, also relevant to lab environment.

Thank you.  :)