Dear, in FTIR is: when there is no peak in certain area it means there is no exists of this functional group. But if there a peak you can never be certain about the existence of this functional group. That why we repeat the test to get more accurate results.

Regards 

Only vibration that produce a change in the dipole moment are observed as signal on an IR spectrum. 

for example we have  magnitude of  charges which are seperated by distance d, then the dipole moment is M= $ * d ..................... where $ is the magnitude of charge and d is the distance between them so obviously we are going to have a dipole moment. if we are changing the distance then the dipole moment wl be changed and that occured when the bond stretches which change the distance due to which we can observe signal in IR spectrum. 

Now comes to your question you don't see any peak because there is symmertic alkyn which is same on both side which means there is no dipole moment right. the electron donating effect of alkyn group would cancell so there is no dipole moment and so therefore there is no change in dipole moment when carbon-carbon bouble bound stretches and so therefore we don't see a signal hence  this signal is abscent in our IR spectrum. 

Dear Dr.Osama and Dr.Sajjd,

Thanks for your answers. I just want to clarify that the spectra is the mean of a repetition, and the first spectra (produced by BRUKER - alpha) was consistent with the literature as the spectra is of human plasma, which contains multiple components that produces peaks in this area, such as functional groups in carbohydrates, nucleic acids, and amino acids.

Omar,

It appears that there was silicone oil left on the surface of the ZnSe ATR crystal.

Dr. O'Keefe,

I have checked the spectrum of silicone oil after your answer; indeed, it is most probably a remaining silicone oil. Thank you.

Update: I have just noticed that the blood tube used earlier are indeed coated with silicone, unlike those used in the latter samples.