Characterization of articular calcium-containing crystals by synchrotron FTIR

SummaryObjectiveSixty percent of synovial fluids from patients with severe osteoarthritis (OA) contain calcium pyrophosphate dihydrate (CPPD) or basic calcium phosphate (BCP) crystals. These bioactive crystals can be particularly difficult to accurately identify in complex biologic systems, such as in vitro models of crystal formation. We sought to determine if synchrotron Fourier Transform Infrared spectroscopy (sFTIR) could be used to identify and characterize calcium-containing crystals in mineralization models.MethodsCPPD and BCP crystals from porcine models of crystal formation were examined with an FTIR Microscope attached to a synchrotron light source. As a comparison, crystals from human synovial fluids were also examined. The sFTIR spectra generated were compared with known spectra of multiple forms of BCP and CPPD crystals, as well as spectra generated by synthetic CPPD and BCP crystals and cartilage proteoglycans, alone and in mixtures.ResultssFTIR readily identified CPPD and BCP crystals in porcine models as well as in fresh synovial fluids. Brushite was also present in human and porcine samples, and whitlockite was seen in some porcine samples. Mixtures of minerals were commonly found in a single crystal aggregate in both human and porcine samples. In spectra from many CPPD crystals, the peak at the 1134cm−1 found on the standard spectrum for CPPD was diminished. Addition of spectra from cartilage proteoglycans to those of synthetic CPPD crystals dampened the peak at this frequency region, much as this peak was diminished in biologically derived CPPD crystals.ConclusionsFTIR analysis allows for accurate identification of CPPD and BCP crystals generated in vitro and will be a useful research tool to study articular crystals

A high resolution interferometer for use with synchrotron radiation

We present the first results from a new instrument which is designed to work at very high resolution in the infrared spectral region but whose principles can be used in the vuv/soft-ray region. The instrument is based on an interferometer. For the vuv/soft-xray region the beamsplitting is achieved by wavefront division which takes advantage of the spatial coherence of synchrotron radiation. Normally the highest frequency in the spectrum from an interferometer is determined by the sampling frequency, which has to be at least twice that of the highest frequency in the spectrum. We show that this can be avoided if the spectral range is deliberately restricted by a grating or by the use of an undulator. In this case the interferograms contain a known and restricted range of frequencies which led to an unambiguous assignment during the Fourier processing required to generate the spectrum