Improved High-Sensitivity Transflection Fiber Probe Design

This article demonstrates art photonics’ ameliorated transflection fiber probe design and additionally uncovers the probe’s improved functionality in both experimental and industrial applications. Possible applications of the probe range from biopharmaceutical analysis [1], real-time reaction monitoring [2], analytical characterization and the production and development of biofuel.

These applications are possible thanks to the probe’s specialization of transmission spectroscopy in liquids at long distance in the ranges of UV - VIS and VIS - NIR.

Measurements were performed on Isopropanol and Ethanol solutions of various concentrations in order to analyze the sensitivity of the fiber probe. These substances were selected thanks to the large amount of research done on their spectra as well as their universal, industry-wide application. A sensitivity to solute concentrations as low as 5% could be reliably determined using the experimental data.

Description

“Double-pass fiber probes”, is the common name given to transflection fiber probes, which transmit light through a sample, before reflecting this light off a highly polished optical mirror. The mirror transmits the light back through the sample, where it travels into the detection fiber.

This repeated movement therefore gives it the name of a “doublepass probe”. The adaptable design of the probe permits it to measure both transmission and reflection, thus leading to the name transflection.

The probe is an insertion probe, which sees the probe directly submerged in the liquid sample to be studied, due to the source and return fibers being located on the same side of the sample [3]. This process is thus in contrast to methods which use flow cells or cuvettes. Available with a number of removable shaft heads, with slit with of 2, 5, or 10mm (optical pass lengths of 4, 10, and 20 mm, respectively), these duel pass fiber probes have a bifurcated design, connecting the prove to a light source and spectrometer.

Figure 1. Transflection fiber probe (a) and shaft head (b).

Methods

An Ocean Optics NIRQUEST spectrometer and art photonics’ improved transflection fiber probe performed these measurements. The probe was first immersed in solutions of both Ethanol and Isopropanol of descending concentration (100%, 80%, 40%, 20%, 10%, 5%, 2.5% and 1%), at which stage of the process three spectra were noted and averaged for later evaluation. 6 ms was the integration time used for the measurements.

The Ethanol solutions begin at an 80% concentration as an 80% Ethanol stock solution was used and further diluted in order to create the other Ethanol solutions. Three spectra of distilled water were also recorded for reference.

Measurements

These values show the averaged NIR spectra for each measured concentration of Isopropanol (fig. 2) and Ethanol (fig. 4) respectively. In addition, the NIR spectrum of distilled water (used to dilute the solutions) was deducted from each measurement (fig. 3 & 5).

Finally, to calculate the location of peaks based on their relative prominence pp, a peak finding algorithm [4] was applied to the water subtracted spectra. A value of pp = 0.25 for Isopropanol was used and for Ethanol, it was a value of pp = 0.5. The results of the algorithm are displayed through the graph’s color coded points. The data the algorithm obtained was subsequently used to analyze two exemplary peak decays, shown in fig. 6 and 7.

Isopropanol

Averaged spectra of 100% - 1% concentrated Isopropanol solutions.

Figure 2. Averaged spectra of 100% - 1% concentrated Isopropanol solutions.

Water spectra subtracted from averaged spectra of 100% - 1% concentrated Isopropanol solutions with peak calculation.

Figure 3. Water spectra subtracted from averaged spectra of 100% - 1% concentrated Isopropanol solutions with peak calculation.

Ethanol

Averaged spectra of 80% - 1% concentrated Ethanol solutions.

Figure 4. Averaged spectra of 80% - 1% concentrated Ethanol solutions.

Water spectra subtracted from averaged spectra of 80% - 1% concentrated Ethanol solutions with peak calculation

Figure 5. Water spectra subtracted from averaged spectra of 80% - 1% concentrated Ethanol solutions with peak calculation

Peak Decay

Isopropanol peak decay at ~ 1150 nm with linear and second order poly-nomial fit.

Figure 6. Isopropanol peak decay at 1150 nm with linear and second order poly-nomial fit.

The data was fitted with a linear- and second order polynomial function, which offered the following results:

Linear: y = 0.1853 ⋅ x
Polynomial: y = 0.0015 ⋅ x2 + 0.0522 ⋅ x + 0.3841

Ethanol peak decay at ~ 1300 nm with linear and second order polynomial fit.

Figure 7. Ethanol peak decay at 1300 nm with linear and second order polynomial fit.

Once again, the data was fitted with a linear-, and a second order polynomial function, yielding the following results:

Linear: y = 0.3040 ⋅ x
Polynomial: y = 0.0031 ⋅ x2 + 0.0865 ⋅ x + 0.2571

Results

The results of the recorded spectra display a signal with minimal noise and well-defined peaks. A non-linear correlation is suggested by the analysis of the Isopropanol/Ethanol peak decays at the previously depicted wavenumbers. The probe can be used to accurately detect unique peaks up to a solute concentration of around 5% by using the visually represented data in the water subtracted spectra.

Conclusion

To conclude, the fiber probe permitted the accurate and dependable detection of a solute down to a concentration of at least 5%. At this point, results start to diverge. The enhanced design of the transflection fiber probe offers a clear signal with well-defined spectra using the given spectrometer as well as an integration time of 6 ms. The need for curve smoothing algorithms is thus eliminated, which, by extrapolating from the original data, cause a loss of information.

Detection of decreasing concentrations of solute gets markedly more difficult, as evidenced by the non-linear correlation observed during the peak decay analysis. Nonetheless, before drawing a definite conclusion, further peaks should be analyzed. Finally, the design of the fiber probe, in particular the shaft head - although it does not prevent the problem from occurring entirely- renders it significantly easier to see and remove air bubbles from the window/reflector in the optical pass, in comparison to other insertion fiber probe designs. This prevents the recording of incorrect measurements.

References

  1. John M. Chalmers; Peter R. Griffiths (2007) ”Sampling Techniques and FiberOptic Probes”. Handbook of Vibrational Spectroscopy, Online. DOI: 10.1002/9780470027325.s8902
  2. S. Küppers, “Application of Optical Spectroscopy to Process Environments”. Handbook of Spectroscopy (2014)                                                    
  3. Terry R. Todd (2006). ”Fiber-optic Probes for Near-infrared Spectrometry”. Handbook of Vibrational Spectroscopy, Online. DOI: 10.1002/9780470027325.s2705
  4. Anonymous, SciPy Community (2019) https://docs.scipy.org (Last visited: 24.6.2019 13:06)

This information has been sourced, reviewed and adapted from materials provided by art photonics GmbH.

For more information on this source, please visit art photonics GmbH.

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