Spectrometer for Maturity Studies

Vitrinite Reflectance, Kerogen Analyses, Maturity Studies of Sediment and Spore Color Measurement

Peltier-cooled CCD arrays

Peltier-cooled CCD arrays measure between 220 nm and 1000 nm and are suitable for dark-field and fluorescence measurements of weak signals
© A.S. & Co. GmbH

For experts on deposit geology smallest differences in color provide information on the maturity and the origin of spores and pollen (palynofacies). They help in the interpretation of sediments and localities. Microscopically, this differentiation is done via the calculation of the red/green quotient or spectral comparison together with the analysis of peak shifts and other significant curve characteristics. The use of sensitive array spectrometers represents the fastest type of spectra recording so far.

Since a total spectrum of course also includes the value of the reflectance measurement at 546 nm, the known working methods of a vitrinite reflectance measurement can also be transferred to the use of array spectrometers. Direct comparisons between a photomultiplier and an array spectrometer also demonstrate that vitrinite reflectance measurements with an array spectrometer correspond to the standard expected as a scale for classification according to the International ASTM / DIN / ISO.

The SpectraVision concept works colorimetrically on the basis of international standards. The advantages of increased sensitivity of A. S. & Co. microscope systems allow in addition to the classic recording of spectral data, also the recording of fluorescence spectra.

Compliance with the Standard Guidelines by special Calibration Routine

Compliance with the standard specifications is achieved by defining a calibration curve based upon measuring of reflectance standards.

For this purpose, a separate calibration routine was created in the module SpectraVision Histo, which makes it possible to determine the curve from up to 15 reflectance standard. The calculation of a reference linear slope is based on the two-point form, while for using several standards an exponential function leads to more accurate results.

Calibration curve

Advantages of a Sensor Peltier Cooling

SpectraVision uses Peltier-cooled CCD arrays with a resolution of 0.8 nm/pixel. In the vertical direction 80 pixel with a size of 64 square micron collect the light and guarantee the highly system sensitivity. This results in advantages in direct comparison with the photomultiplier measurement.

The cooling system ensures a constant signal/noise ratio during data recording, because the sensor is always at a constant temperature. It is therefore independent of the laboratory conditions. This is of particular importance as geological measurements from different climate zones are often directly compared on the basis of applicable worldwide standards.

Cooled systems with optimized noise reduction can generate more interference-free spectra, even with longer integration times. This leads to a higher sensitivity. In the case of anisotropy detection in polarized light, this is just as imperative as in the detection of weak fluorescence. Otherwise, the observed signals are not of the quality required to assign macerals significantly.

Vitrinite Reflectance and Fluorescence Spectroscopy for Maturity Studies

A progression of maturation in macerals is characterized by increase in vitrinite reflectance with a simultaneous decrease in the fluorescence intensity. The simultaneous determination of both parameters can enhance the significance of analysis, and thus is increasingly gaining importance.

Particular attention is directed to the selection of appropriate fluorescence standards for referencing. The standard materials have in common that their properties change with irradiation. Thus, they are of limited suitability as calibration standards.

As an alternative to the use of fluorescence standards Ottenjann et al. developed another method for the quantitative fluorescence calibration. The halogen spectrum can be defined device independently by the Planck constant and thus provides a stable and reliable long-term fluorescence standard. This method is therefore integrated in the workstations of A. S. & Co..(Ottenjann K, Teichmüller M, Wolf M, 1974. Fortschritte in der Geologie von Rheinland u. Westfalen 24: 2-36).

Working according to the CIELab Standard

Array Spectrometer simultaneously allow

  • the detection of fluorescence and transmission spectra, and
  • the determination of the red/green ratio without changing the filter on the microscope

While the analysis of the fluorescence spectra mainly allows conclusions on the kerogen composition, the recording of transmission spectra also enables to evaluate spores with regard to the maturity of the material (Thermal Maturation Studies).

Microflora from sediments

However, the requirement for this application is to work with light sources the color temperature of which corresponds continuously to the specifications of the CieLab standard. For many microscope illuminations this required quality is not ensured. Therefore it is recommended to use an additional external power supply for halogen measurements in the visible spectral range.

System Configuration

SpectraVision uses cooled CCD arrays with a resolution of 0.8 nm/pixel. Microscope spectrometers with spectral ranges from 310nm to 950nm and 310nm to 620nm are available. The arrays will be merged with the grids to a compact maintenance-free unit and delivered together with the power supply in a 9.5" Rack. PCI or USB interfaces provide the connection to computers, so that the current standard self-recognition routines of the operating system are able to assign the devices during the initial installation.

Especial care was taken not to design the spectrometers as microscope modules in order to maintain the option of flexible upgrading older microscopes. These are often equipped with plenty of mechanical and optical components of extremely high quality, the advantages of which are even nowadays an important additional value.

Specials of the Microscope -Spectrometer Coupling

Peltier cooled CCD spectrometers from A. S. & Co. are connected with the microscope by an optical mono fiber. This ensures a much higher signal transmission from the microscope to the sensor compared to the conventional y-fiber. An integrated light source within the collimator allows the direct illumination of the measuring pinhole, thus avoiding light losses caused by unnecessary beam splits.

collimator with integrated pinhole illumination

The measuring apertures may either be interchangeable ones with fixed diameter or a variable iris diaphragm can be mounted. A centerable fiber collimation allows quick and easy positioning of all optical elements, because the measuring aperture is reflected directly into the eyepiece on a reticle. With this, both the alignment and the sample positioning can be performed. After successful system alignment all positions will be fixed, so that the set-up is adjusted in the medium term. A daily realignment of the setting is not necessary, because the quality of positioning is visible at any time in the eyepiece.

Light guide, aperture systems and fiber collimation

Modified spectro phototube: The standard phototube was optimized to a higher light throughput. Since all optimizations potentiate, the improved light throughput at the tube has a positive impact on the overall performance and reproducibility.

Together with the A. S. & Co. product "Live overlay module" the illumination pinholes and measuring pinholes are reflected direct to the eyepiece. The viewer observes the sample image as well as the aperture, its position, size and sharpness simultaneously live. It requires no extra offline camera image on the monitor, which needs to be re-calibrated each time you change the magnification.

Pinhole live overlay module

SpectraVision System Control and Analysis

The measurement process focuses on adjusting the intensity and measuring the standards and samples.

Depending on the type of microscope, a motorized field stop for stray light suppression may be involved in the process.

To integrate purely manually operated microscopes external shutter and filter changer are available.

A highly stabilized power supply controls the halogen lamp to maintain a constant color temperature.

The system records always complete spectra. They can be displayed either as such, or the integral of 546 + / - 6 nm can be extracted. To measure the degree of maturation in transmission spectrum SpectraVision provides a colorimetric module as an option.

The data analysis is based on the requirements of the international standard and is identical to the functions of the PMT module. The relevant details can be found here.