While scanning soil and plant samples using the FTIR spectrometer (Vertex 70, Bruker, Germany) equipped with a microplate reader extension for high throughput screening infrared spectroscopy equipment (HTS-XT, Tensor II, Bruker, Germany) instrument, in some samples reflectance values, reach above 1.0. What are the reasons that cause such differences? How to interpret such values, if it is normal? How to avoid such errors, if it is not correct?
As Hugh says it means that you are achieving higher reflectance from the sample than from the background. All double beam IR spectra are a comparison of sample and background single beam spectra whether in transmission or reflection. When IR microscopes first came into common use people trying to obtain reflection spectra used a gold mirror as a background. This gave very low sample spectra from powders etc. Instead spectroscopic grade KBr was used as a background having more similar reflection properties. Roughly matching particle size also helped.
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