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High Throughput Virtual Slit (HTVS™) Impact on Raman Spectroscopy Limit of Detection (LoD) Performance

Conventional Raman systems have traditionally been limited by a lack of sensitivity due to the design trade-off between resolution and throughput mediated by the input slit of the spectrometer. Tornado’s proprietary high throughput virtual slit (HTVS™) design technique eliminates the spectrometer slit and associated photon losses while maintaining high spectral resolution, resulting in an order of magnitude improvement in sensitivity. This paper demonstrates the detection limits and corresponding minimum required acquisition times of four binary mixtures using Tornado’s HyperFlux™ PRO Plus Raman spectroscopy system.

Instrumentation

A Tornado Spectral Systems HyperFlux PRO Plus 785 Raman spectroscopy system coupled via fiber optics to a MarqMetrix Ball Probe.

Experimental Design

Limit of Detection (LoD) is the lowest quantity of a substance that can be distinguished from the absence of that substance (a blank value) within a stated confidence limit. It varies depending on the combination of the target component and the matrix it is present within. For this purpose, four binary mixtures ranging from 10,000 ppm (1%) to 10 ppm (0.001%) with an average of 15 dilutions including 0 ppm were tested. Laser power at the probe output was tested to be 248 mW. Twenty individual spectra were generated of each mixture based on either a 25 (Benzene & Acetone) or 15 (Propylene Glycol & Sodium Nitrite) averaged exposures. Three PLS calibrations were built over the ranges of 10,000-0, 750-0 and 250-0 ppm. Predictive accuracy was compared via RMSEC (Root mean squared errors of calibration) for the maximum exposure and RMSECV when multiple replicates were available for cross-validation. In each case, the maximum exposure RMSEC was considered the optimum minimized error and then the lowest acquisition time maintaining the same approximate RMSECV and precision was determined.

Results & Discussion

Table 1 below provides an overview of each binary mixture along with spectral range and various averages:

 BENZENE IN ISOPROPYL ALCOHOLACETONE IN ETHANOLPROPYLENE GLYCOL IN WATERSODIUM NITRITE IN WATER
LoD (Limit of Detection)LoD = 104 (10,000-0 ppm)
LoD = 32 (750-0 ppm)
LoD = 24 (250-0 ppm)
LoD = 95 (10,000-0 ppm)
LoD = 49 (750-0 ppm)
LoD = 24 (250-0 ppm)
LoD = 208LoD = 307
AVERAGE TIME (PPM)7.5 second average 10,000-0 ppm
7.5 second average 750-0 ppm
7.5 second average 250-0 ppm
37.5 second average 10,000-0 ppm
187.5 second average 750-0 ppm
750 second average 250-0 ppm
30 second average 10,000-0 ppm30 second average 10,000-0 ppm
SPECTRAL RANGE970–1020 cm-1760–820 cm-1760–1540 cm-11280–1380 cm-1

Table 1: Overview of four binary mixtures

Conclusion

Using a standard 500 mW laser, Tornado’s HyperFlux PRO Plus Raman Spectroscopy system can achieve accurate low level quantitation within acquisition times of 30 seconds or less. The superior performance of the PRO Plus enabled new insights to help overcome these limitations, and allowed for improved quality, better process effi ciency, and a deeper understanding of the incoming materials to enable superior control.