Hellma's product range covers the most important areas of molecular spectroscopy, including ultraviolet (UV), visible (vis), mid-infrared (IR), fluorescence and Raman spectroscopy. Within the single spectral ranges, the products are differentiated according to their measurement principles:
With a transmission probe or measuring cell, the sample is penetrated only once by the measuring signal. The sample-specific absorption changes the transmission spectrum. This measurement principle is comparable with the measurement in a transmission cuvette and is suitable for the measurement of gases and transparent liquids.
Transflection is a hybrid of transmission and reflection spectroscopy. The light passes through the sample material, is reflected by a reflector and then passes through the sample a second time. A transflection spectrum can be easily compared to a transmission spectrum and is suitable for the measurement of gases and liquids. Due to the reflector, the measuring gap can be changed more easily. For many types of probes, the optical path can be adapted to the application using exchangeable tips.
When performing a reflection measurement, the incident light penetrates into the surface of the sample and is diffusely reflected. Part of the reflected light is collected by one or more fibers and directed to the detector. The reflection measurement is suitable for the measurement of powdery, granulated solids, slurries, and pastes.
ATR stands for Attenuated Total Reflection. The medium circulates around the prism of the probe and, depending on the optical density of the medium; the measuring signal is attenuated with each reflection at the boundary layer between the medium and the prism (evanescent field).
ATR probes are used on highly absorbent samples where measurements with transmission probes are not possible because total absorption would occur. (e.g., dyes).
A fluorescence measurement is an emission measurement. The medium is excited by the incident light and emits light of lower energy in all spatial directions (fluorescence radiation). In order to achieve the best possible separation of excitation and fluorescence radiation, the fluorescence radiation is detected at right angles to the incident radiation. The cylindrical reflector of the probe reflects the excitation light emerging from the sample space back into the center. A plane mirror in the sample space serves to increase the yield of fluorescence light.
Raman spectroscopy is a scattering measurement. The excitation source is monochromatic light, usually laser light in the range of approx. 240 - 1064 nm. The resulting inelastic scatter is detected and analyzed after contact with the medium at an angle of 0°. The elastic scattered light (Rayleigh scattering) is eliminated by an optical filter matched to the wavelength of the laser. The inelastic scatter, the Raman scatter, is wavelength-shifted sent to the detector.
Raman has two major advantages over mid-IR spectroscopy. The first is that Raman spectroscopy is fairly water insensitive allowing for a background-free detection in aqueous solutions. The second advantages that it can employ the robust fused silica and sapphire optics commonly used in the Near-IR and UV-Visible regions. The optics are less expensive and do not degrade over time from the water in the atmosphere. Also, the possibility of using quartz fiber-optics allows a user to couple a probe at a distance from the detector.
The disadvantages of Raman are its sensitivity to fluorescence interference and the difficulty of removing instrument artifacts from the measurements. The decision to use Raman for a specific application depends on a detailed analysis of all relevant factors.