Hi, folks!
I wanna measure the FT-IR spectra of some amino acids. I am going to prepare the plates made from the mixtures of amino acids crystals and KBr under 10 tons pressure. But I don't want to destroy the original crystal structure of amino acid. Does anybody know whether the crystal structure of amino acids can survive under such a large pressure?
Thank you very much in advance!
Yin
Dear Yin,
I don’t know what aminoacid you are going to press, but the possibility that you will receive a mixture of different polymorphs is very high. Look at the citation from our article below, please:
“on increasing pressure up to 2.5 GPa, the structure of gamma-glycine compressed anisotropically, so that a/c ratio decreased. At 2.74 GPa the reflections of a new phase could be observed, although gamma-polymorph was still the major component at this pressure. The new high-pressure phase was present as the main component in the pressure range 4.17–7.85 GPa, but even at 7.85 GPa the weak peaks of the strongest reflections of the low-pressure phase (gamma-glycine) were still present in the diffraction patterns”
E.V. Boldyreva, S.N. Ivashevskaya, H. Sowa, H. Ahsbahs, H.-P. Weber Effect of hydrostatic pressure on the gamma-polymorph of glycine. 1. A polymorphic transition into a new delta-form // Z. Kristallogr. (2005). 220, 50–57.
It's clear that you want to do transmission FT-IR spectroscopy by preparing a KBr pellet using your mixture of amino acid crystals. Does it have to be in transmission mode? Iff you can tolerate ATR-IR, then you might want to try that first. Also, it's unlikely that your crystals will melt if you're doing all this at room temperature, if that's what you were worried about.
Hi, Folks!
I am so sorry for this very late response! And thanks a lot for your very insightful comments on m question!
Unfortunately, our FT-IR doesn't have ATR component. ATR-FTIR is not available for me.
According to the comments suggested by Svetlana, my pressure is two orders of magnitude less than the GPa. Perhaps it doesn't hurt. I will test its Raman before and after the pressure. But, there is one question for Aurelio. Usually, which Raman shift region (cm-1) is the most sensitive to its crystal structure?
Wish a successful new year ahead to everyone!
Yin
After a back-of-the-envelope calculation, I would say that the uniaxial pressure that the hydraulic press imparts to the sample substance matches about 7 MPa. This figure was estimated for the 10 Mkgf of force applied, and considering a typical disk-shaped pellet, with a diameter of 13.0 mm. Such a pressure does not seem high enough to impose permanent effects, susceptible to modify the vibrational spectra of typical samples. Even considering that particles of a soft crystalline substance may be smashed to some extent, that should chiefly affect a microscopic scale orders of magnitude larger than that effectively analysed by FT-IR spectroscopy. Harder particles tend to be protected from shape distortion owing to the plasticity of the embedding matrix. This (coarse) estimate was obtained considering that the sample substance occupies around 1% of the disk surface, a seemingly reasonable assumption compatible with about 1% wt/wt dilution ― an often chosen dilution factor for the sample powder added to KBr. The uniaxial pressure that the press imparts to the pellet would reach about: 10·103 kgf / [π·(1.3 cm / 2)2 ] = 7.5·103 kgf / cm2 = 7.3·103 atm. This uniaxial pressure is shared by the sample substance and embedding matrix. The part effectively supported by the sample substance would reach about 73 atm, or 7.4 MPa. Often, this can be further reduced ― down to 3 MPa or so ― by somewhat decreasing the applied force (say, to 8 or 9 Mkgf) and/or by further diluting the sample (say, to 1:200 wt/wt).
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