I think you will find you get better results if you use a shorter path-length cuvette to reduce the absorbance due to the buffer. In the past, I used cuvettes with path lengths less than 1 mm that were assembled from 2 parts when making CD measurements in the far UV.

thank you Adam and Vivek..

It can be done from 20uM and above, it will better to use more than 50uM and upto 200uM for remarkable difference in different environment.  

First of all i want to know what is the molecular weight of ur proteins and peptide. If ur protein is bigger molecules and its molecular weight lies in between 100 or 200 KDa so 1 or 2 microM is enough concentration for Far-UV CD. Similarly, if protein is 10 KDa or 15 KDa like lysozyme so its needed little bit high concentration like 5 to 10 microM. In case of peptides, u have to use higher concentration as sarfuddin already mention. I know sharfu, he has a great expertise in peptide work.

Hi Sarfuddinm I am working actually in the range of concentration.

HI Javed,

My peptide is about 3kDa. I tried to increase my concentration up to 80uM, but the range is between 180-200 is noisy. I tried empty cuvette and with water. The region absobs. I guess it was a problem of type of cuvette I am using or sensitivity of the CD machine in 180-200nm

Dear Javed and Sarfuddinm  thank you for your answers..

Hello Vasantha Kumar M. V,

Why do u want to go below 200 nm. Could u pls tell me the name of buffer and its molarity. you can decrease ur peptide concentration. If possible pls send me the raw data on XL file on my gmail id "[email protected]" may be i can give u some idea. I would like to tell u one more thing if possible use 20 mM Potassium Phosphate buffer. 

HI Javed, I am using 10mM Phosphate buffer, I started with 6uM still I have same problem. I guess may be my sample is not good.(its bit old).

Hi Vasantha, based on your question I am assuming you do not have a small pathlength cuvette and could run only with 1cm cuvette.1cm is commonly used for near UV CD measurement to probe aromatic local tertiary structure. For peptides and proteins, the recommended conc for far UV CD measurement using 0.02cm cell is in the range of 0.3-0.5mg/ml. Hence for 1cm cell, follow the Beers Law would require 50x less conc. 6-10microgm/ml. However you will be limited with your low wavelength cut-off due to the water absorption as the others have mentioned. You may improved with the use of D2O instead of H2O for your phosphate 10mM buffer, however D2O may have an effect on the peptide folding which you cannot rule out. You may also need to do multiple scans to reduce the noise but this is limited to instrumentation. If you have a lower pathlength cell such as 0.02(0.3-0.5mg/ml) or 0.05cm (0.1-0.2mg/ml) cell that would be best to use for a chance to get to 180-200nm region. Good luck!