Mass Spectrometry & Spectroscopy


Fluorescence-free Raman material identifi cation Dr Melissa J. Gelwicks & Mark Harpster, Metrohm Raman Email: sales.raman@metrohm.com


In Raman spectroscopy, accurate and sensitive identifi cation of chemicals and materials can be compromised by fl uorescence from laser excitation of the target substance itself and/or interferents in the sample matrix. Fluorescence emission in Raman spectra reduces the signal-to-noise ratio and can obscure signature peaks assigned to the unique Raman-active vibrational modes of molecules. This restricts the breadth of illicit and hazardous materials that can be identifi ed for actionable intelligence by fi rst responders, law enforcement agents, as well as military and customs personnel. Recent advances have had some success in mitigating the impact of fl uorescence on data quality; however, compact commercially available Raman devices that provide a universal solution for suppressing fl uorescence are lacking. MIRA XTR DS fi lls this void with a state-of-the-art handheld Raman system that revolutionises material identifi cation in complex environments.


Survey of Fluorescence Suppression Methods


Survey of Fluorescence Suppression Methods


Developers have focused on mechanical and computational solutions for reducing or eliminating spectral interference, resulting in a small handful of fl uorescence-free portable Raman devices that perform with varying levels of success [1-3]. A notable example is the Bruker Bravo™, which uses patented Sequentially Shifted Excitation, or SSE, technology and employs a DBR (distributed Bragg refl ector) diode laser that shifts excitation wavelength as a function of temperature [4,5]. The Raman signal shifts with incident wavelength, but fl uorescence emission does not, and on-board data processing can distinguish the spectrum of elastically scattered light. This system produces high-quality information, but sacrifi ces cost and size in order to accommodate a DBR laser. It also suffers from shortened operational lifetimes due to constant temperature cycling of the laser.


A second example of fluorescence mitigation in handheld Raman devices is the Rigaku Progeny ResQ™. This instrument utilises a 1064 nm laser to excite samples below the UV- visible electromagnetic range in which fluorescence occurs. The disadvantage of long wavelength excitation is that the intensity of the Raman response is inversely proportional to incident wavelength according to λ-4 or 1/λ4 [6] and the resulting signal is 3.4Å~ weaker for 1064 nm than for 785 nm excitation. High-power lasers (420 +/– 30 mW) are employed to compensate for poor signal-to-noise, while lower power lasers (≤ 100 mW) are sufficient for good Raman signal acquisition at 785 nm. In addition, the capture of scattered wavelengths outside of the silicon detection range requires expensive, cooled InGaAs (Indium Gallium Arsenide) detectors. The result is a bulky device that suffers from


reduced operational times in the field due to the increased power requirements. Additionally, test samples (particularly those of dark coloration) are susceptible to damage such as burning when exposed to a high-power laser.


Finally, while 785 nm excitation is less prone to fl uorescence interference than lasers operating in the 400-700 nm range of the spectrum (e.g. green 532 nm lasers), many samples emit levels of background fl uorescence that are suffi ciently strong to conceal Raman signals when excited in the red to near-infrared (IR) region.


Developers have focused on mechanical and compu- tational solutions for reducing or eliminating spectral interference, resulting in a small handful of fluores- cence-free portable Raman devices that perform with varying levels of success [1–3]. A notable example is the Bruker Bravo™, which uses patented Sequentially Shifted Excitation, or SSE, technology and employs a DBR (distributed Bragg reflector) diode laser that shifts excitation wavelength as a function of temperature [4,5]. The Raman signal shifts with incident wavelength, but fluorescence emission does not, and on-board data processing can distinguish the spectrum of elas- tically scattered light. This system produces high-quality information, but sacrifices cost and size in order to accommodate a DBR laser. It also suffers from shortened operational lifetimes due to constant temperature cycling of the laser.


A second example of fluorescence mitigation in hand- held Raman devices is the Rigaku Progeny ResQ™. This instrument utilizes a 1064 nm laser to excite samples


Finally, while 785 nm excitation is less prone to fluores- cence interference than lasers operating in the 400– 700 nm range of the spectrum (e.g. green 532 nm lasers), many samples emit levels of background fluorescence that are sufficiently strong to conceal Raman signals when excited in the red to near-infrared (IR) region.


signal is 3.4× weaker for 1064 nm than for 785 nm excitation. High-power lasers (420 +/– 30 mW) are employed to compensate for poor signal-to-noise, while lower power lasers (≤ 100 mW) are sufficient for good Raman signal acquisition at 785 nm. In addition, the capture of scattered wavelengths outside of the silicon detection range requires expensive, cooled InGaAs (Indium Gallium Arsenide) detectors. The result is a bulky device that suffers from reduced operational times in the field due to the increased power require- ments. Additionally, test samples (particularly those of dark coloration) are susceptible to damage such as burning when exposed to a high-power laser.


below the UV-visible electromagnetic range in which fluorescence occurs. The disadvantage of long wave- length excitation is that the intensity of the Raman response is inversely proportional to incident wave- length according to λ-


4 or 1/λ4 [6] and the resulting


3.462 in


Figure 1. Relative scale of commercially available ‘handheld’ Raman devices.


Figure 1. Relative scale of commercially available “handheld” Raman devices. 2


Raman extraction


Fluorescence in a Raman spectrum contributes undesirable noise that distorts the baseline and obscures Raman peaks. This can be seen in Figure 2, both as a partial effect at 1064 nm and fully at 785 nm. It creates a baseline that cannot be described mathematically, and therefore cannot be subtracted mathematically.


4.978 in


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