Near-Infrared Emission Spectroscopy: Improvements on instrumentation

Near-Infrared Emission Spectroscopy (NIRES) is a technique where radiation emitted by thermally excited samples in the near-infrared spectral (from 1500 to 3000nm) is detected for analytical purpose. Emission methods have some potential advantages over absorption methods, such us, they can be used in probe-free methods since the sample itself is the source of the analytical information, frequently require smaller amounts of samples, allow, if thermal excitation is employed, investigations of the behavior of the sample under heating, and, can be easily applied to opaque solid samples without any prior treatment. The main limitations are the low intensity of the emission signal and the restriction of applications to thermally stable samples.

In 2005 a Near Infrared Emission Spectrophotometer based on AOTF (Acousto-Optic Tunable Filter) was developed at UNICAMP by the Instrumentation and Automation on Analytical Chemistry Group. This spectrophotometer presents the advantage of high energy throughput, a very interesting characteristic considering the low signal intensities obtained in NIRES. A general approach of a NIRES instrument in vertical alignment consist of, from bottom to top, a heater connected to an emission cell where sample is located, CaF2 lens for radiation collimation, AOTF as wavelength selector, CaF2 lens for diffracted beam focusing and PbS detector. The AOTF is driven by a radio frequency (RF) modulated signal in the range of 64-32MHz controlled by a customized Visual Basic software. The NIR emission spectra is obtained by heating a few microliters or miligrams of sample in a constant temperature between 150 and 280°C and scanning the RF signal frequency.

The first NIRES instrument was developed using a homemade radio frequency and the PbS detector was cooled at -10°C. It was applied to material characterization used in chromatography, oxidative stability determination of vegetable oils at frying temperatures, quality parameters determination of diesel oil, distinguish crystalline forms of titanium dioxide and oil lubricant classification. The objective of this work is to continue demonstrate the analytical potential of this technique. For this purpose, the NIRES spectrometer was reassembly using high quality devices: a Brimrose high performance RF was used instead, which allow a lower variation on radio frequency amplitude, and a PbS detector electronic cooled at -20°C was used in order to reduce the noise and increase the signal to noise ratio (SNR). Ten spectra from 2000 to 3000nm, with a nominal resolution of 5 nm, were obtained at a temperature of 200°C for a blackbody with detector position shift of 8° from normal plane in two different modulation (165Hz and 625Hz). The SNR for the raw emission at a maximum (around 2700nm) and lower (around 2285nm) emission wavelengths were estimated by the standard deviation of the intensities. A SNR comparison between the first instrument and the actual assembly showed an improvement of 68% (at 165Hz by increasing signal) and 107% (at 625Hz by reducing noise) for the last at maximum emission wavelengths. Around 2285nm, SNR was kept approximately constant. Preliminary studies with zeolites heated at 200°C demonstrated to be possible perform classification according to their chemical structure.