The ALS Core Environmental Beamlines
Beamline 1.4.3 at the ALS provides state-of-the-art Fourier transform infrared (FTIR) spectromicroscopy in the mid-IR region (0.01 eV to 1 eV). The beam is passed through the moving mirrors of an interferometer and into an IR microscope. The IR light drives the motion of electric charges in the sample or directly excites a suite of vibrations of the molecules within the sample which becomes a “fingerprint” identifying the molecules. As the electromagnetic radiation approaches the resonance frequency of the sample (or portion of the sample), the IR light is efficiently absorbed. A computer-controlled sample stage is used to obtain 2-D spectral 'maps' of these absorptions, with up to a few micron resolution. The penetration depth of the signal depends on the frequency-dependent conductivity of the sample, and can vary from Angstroms (in metals) to micrometers for metallic oxides to tens of kilometers for certain insulators (Hirschmugl, 2002). Analysis of the absorption signatures versus frequency permits identification of the chemical compounds present. Because IR spectroscopy is especially sensitive to vibrations of O-H, C-H, C-O, N-H, and C-N bonds, it has great potential for applications to organic compounds and the chemistry of microbes, as well as for the identification of phases and of adsorbed organic species at specially designed mineral/water interfaces (Brown and Sturchio, 2002).
The IR beamline has been particularly well constructed for such high-resolution imaging of dynamic interactions. In particular, a unique, four axis feedback 'lock-in' beam stabilizer system ensures a well-focused and extremely stable signal, which SR IR beamline over laboratory desktop microIR spectrometers are that the SR source is significantly brighter and much better focused onto a sample, resulting in up to 1000 times better signal to noise than a desktop system. This permits higher spatial resolution investigations of one or a few microbial cells. Additionally, because a specially-designed IR sample mount can also be used at the ALS BL-10.3.2 (where micro X-ray absorption spectroscopy is performed in the 2.5-17 keV range), there is a unique capability at the ALS for combining micro-IR and µXAS/ µEXAFS investigation techniques for environmental studies.
The low infrared photon energy and beam power density do not detrimentally affect living cells (e.g., Holman et al., 2002, 2003). As such, this beamline is extremely useful for investigating live organisms and their interaction contaminants in geologic materials, over time, and with extremely high spectral and spatial resolutions (e.g., Holman, 2000; Geller, 2000). As a good example of the capabilities of the IR beamline to environmental research, Holman et al. (1999) studied the conversion of Cr(VI), which is toxic and mobile, to immobile Cr (III) on a magnetite surface and in the presence of microbes and toluene. The mineral/microbe was exposed to chromate and toluene and then monitored over time with IR spectromicroscopy for changes in Cr(IV), Cr(III), and toluene, as illustrated by the figure above. After five days, the spatial maps showed a significant decrease in the chromate and toluene at the same positions as the living cells, which were identified by amide absorption bands. These results are indicative of a close link between the biodegration of toluene and microbial reduction of Cr(VI). This beamline has also been useful for investigating human cell responses to environmental stresses, including toxins, oxygen, moisture, pH, and temperature (e.g., Holman, 2000).