Spatial resolution achieved experimentally at the beamline in transmission and “hybrid” macro-ATR modes
As is often the case, what is theoretically possible in “perfect” conditions is often not achieved in the “real world”. Below are experimentally determined spatial resolution results from the beamline in transmission and macro-ATR acquisition modes using a photoresistive polymer USAF target.
Figure: Standard USAF target pattern.
Transmission
Acquisition Parameters:
[3900 - 750 cm-1] wavenumber range
4 cm-1 resolution
64 co-added scans for background (background repeated every 10 sample points)
16 co-added scans for sample measurements
36× objective and condenser (matched; NA = 0.5)
5.6 μm projected aperture (0.2 mm physical size)
1 μm steps in x- and y-direction
Post acquisition (OPUS software):
Spectra were corrected for: CO2 (g), H2O (g) and aqueous solution
Range cut: 3900 - 950 cm-1
Baseline Correction (rubberband, 64 points)
Figure: [Left] Visible image of the USAF target used for data acquisition. Each bar and space was 40 μm. The measurement area is noted by the red rectangle and is a 15×280 grid. [Right] Example spectrum of the photoresistive material of the USAF target after baseline correction (no water vapour correction). Shaded regions are the areas integrated for determining experimental spatial resolution. Exact wavenumbers are listed to the right of the spectrum.
Experimental spatial resolution was determined following a similar procedure to that outlined in Vongsvivut et al. (2019) [Analyst 144, 3226].
Note: Spatial resolution is dependent on wavelength
Figure: [Left] Representative graph to determine spatial resolution using the integrated area between 1280 - 1201 cm-1. The integrated area of the 15 measurement positions of each row was averaged before normalisation. Shown are the lines representing the 5 - 95 % and 10 - 90 % ranges commonly used to determine resolution. Above is the false colour map of the integrated areas of the USAF target. [Right] Experimentally derived spatial resolution for the USAF target as a function of wavelength, with the conversion to wavenumber shown in green. Shown are the experimental averages across the three bars (n = 6) for the 5 - 95 % (diamonds) and 10 - 90 % (circles) ranges for each integrated region (error bars = one standard deviation). For the two lowest wavenumber regions integrated, the plateau for either the maximum or minimum was questionable meaning these values should be regarded with some caution. They are shown on the graph to indicate a trend only. The red lines represent the theoretical resolution achievable, r (solid line) and 2r (dashed line), based on the Rayleigh criterion. 2r is commonly used as a more realistic measure of resolution to account for various optical aberrations.
In transmission, our beamline approximately follows the 2r guideline for spatial resolution for the 5 - 95 % range, and between r and 2r for the 10 - 90 % range
“Hybrid” Macro-ATR
A detailed discussion of the macro-ATR assembly and spatial resolution with the Ge crystal can be found in Vongsvivut et al. (2019) [Analyst 144, 3226]
Below are the experimentally determined spatial resolution results from the same USAF target and projected aperture (2.5 μm) from Vongsvivut et al. (2019)
Note: The theoretical resolution also factors in the refractive index of the Ge hemisphere crystal.