Does anybody know the detection limit of Thermo NIcolet's Nexus series FT-IR (FTIR) spectrometer for thin film applications? I can't seem to find much on their manual or on web.
While I've read from the book 'Handbook of Instrumental Techniques for Analytical Chemistry' that the solid should be at least 1-10 µg. I've seen in literature where FTIRs have been implemented for sub-nanometer films. Any suggestions?
It depends on many factors (e.g. intensity of absorption band, absorbance of substrate, troughput of spectrometer, sensitivity of detector, resolution, scan duration, …).
Generally it should works for thin films. And the case of (sub-)nanometer thin films – I suppose it is not general feature, so the spectrometer will be tuned for specific purposes and particular samples. I expect some multi-reflection technique e.g. Attenuated total reflection (ATR).
Dear Jea Cho, irrespective of the instruments (Nicolet, Bruker, PerkinElmer, etc.) there exists a general FTIR reflective technique for measuring infrared spectra of thin films. The technique is named "grazing angle reflection-absorption". In essence, the probing infrared beam is sent to the sample (a thin film on a reflective support) under 88-89 degrees angle (counting from the normal to the sample). In other words, the beam "grazes" along and through your thin film. Otherwise, IR beam (2-20 um in the mid-IR range) simply "doesn't see" the sample having a sub-um thickness. Another requirement is that your probing IR-beam in such optical geometry must be polarized. It means that one should install an IR-polarizer on the IR-beam tract between the IR-source and the sample. Please seek a "grazing angle" attachment and a polarizer around. Many companies make these. The attachment can be both installed on the sample compartment bottom or hanging in the standard slide-holder of the instrument - it doesn't matter. Good Luck!
All respondents are right I think, it depends on your optics and you can improve sensitivity by increasing the effective thickness the beam sees, either by a grazing incidence beam, or by stacking several layers of the film (doable if your substrate is IR-transparent).
I usually estimate the detection limit by estimating the smallest IR peak I would trust (perhaps 3x the noise level), and multiplying it by the calibration factors from the literature to get the smallest amount of IR-active centers I would trust. That can the be converted to a density if you know the thickness, or a thickness if you know the density.
detection limit depends rather on the technique and less on the FTIR manufacturer.
E.g. For ATR-FTIR spectroscopy (silicon, 12 refelections) you can analyze e.g. thin polymer brush films of PNIPAAM down to 5-10 nm.
References
1. E. Bittrich, S. Burkert, M. Müller, K.J. Eichhorn, M. Stamm, P. Uhlmann, Temperature-Sensitive Swelling of Poly(N-isopropylacrylamide) Brushes with Low Molecular Weight and Grafting Density, Langmuir, 2012, 28 (7), 3439–3448.
2. Müller, M.; Torger, B.; Bittrich, E.; Kaul, E.; Ionov, L.; Uhlmann, P.; Stamm, M.
In-situ ATR-FTIR for characterization of thin biorelated polymer films mehr
Thin Solid Films (2014) In Press
or another example for sensitivity / detection limit:
monolayers of proteins give around a surface concentration of 0.2 microgram/cm2.
Using a silicon ATR crystal with 12 reflections gives you protein monolayer signals with sufficient signal/noise ratio.
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