The sensitivity of Raman at 785 nm also means that lower laser powers can be used. Lower laser powers help to protect sensitive samples from burning or ignition. The silicon detectors used at shorter wavelengths do not need to be cooled, further extending battery lifetimes. The net result is that 785 nm systems can be very small and still provide fast and accurate material identification for long hours in the field.
Previous recommendations to overcome fluorescence
When fluorescence is an issue, 1064 nm laser excitation is often recommended. The tradeoffs include higher laser power, increased sample heating, longer interrogation times, and low Raman scattering efficiency. Often, this means larger instruments with shorter battery lifetimes. Instruments from some manufacturers require longer acquisition times that slow down sampling and can potentially damage the sample.
Is there a better way?
In a word, yes. SSE (Sequentially Shifted Excitation) can be used to remove fluorescent contributions to a Raman spectrum by using a laser that shifts the excitation wavelength as a function of the laser temperature. The result is a very large «handheld» system with a shoulder strap and a high price tag, partly due to the expensive laser used. Aside from the bulk and the cost, another issue with these systems is that the constant temperature cycling of the laser causes the system’s battery to have a short lifetime.
A Metrohm solution
Metrohm Raman has designed a fluorescence rejection system based on its compact MIRA DS package using an IPS single-mode 785 nm laser. The system is capable of producing excellent spectral resolution and flat baseline data with low laser power, short acquisition times, and all of the other excellent functionalities that users have come to expect from MIRA DS.
This fluorescence rejection system is built upon a MIRA DS platform, preserving all of its unique capabilities:
MIRA XTR DS
Figure 1. Comparison of Raman spectra of Gum Arabic powder measured by 1064 nm, 785 nm (MIRA DS), and XTR® (MIRA XTR DS).
Learn more about MIRA XTR DS on our website.
Download our free White Paper below to find out more about the capabilities of MIRA XTR DS.
Classic applications improved with MIRA XTR DS
Traditionally, lidocaine was an issue for 785 nm Raman systems, as its fluorescence prevented both positive identification of lidocaine and detection of cocaine. MIRA XTR DS produces an excellent, fluorescence-free, resolved spectrum of lidocaine (Figure 2).
Figure 2. Comparison of Raman spectra of lidocaine hydrochloride measured by 1064 nm, 785 nm (MIRA DS), and XTR (MIRA XTR DS).
Figure 3. Left: MIRA XTR DS used for no-contact testing. Right: Comparison of Raman spectra of Diphenhydramine measured by 1064 nm SERS, 785 nm SERS (MIRA DS), and XTR SERS (MIRA XTR DS).
But MIRA XTR DS can do more!
With fluorescence mitigation, 785 nm Raman can be used more generally for material identification and chemical analyses.
Microcrystalline cellulose (MCC) is another inert excipient that is commonly used in food production and the pharmaceutical industry. When interrogated with 785 nm Raman, its fluorescence can overwhelm the Raman signal and prevent identification and mixture matching (Figure 4).
Figure 4. Comparison of Raman spectra of MCC measured by 1064 nm, 785 nm (MIRA DS), and XTR (MIRA XTR DS).
Measurement of analytes in ketchup is a particularly interesting application, as it is a highly colored, complex mixture. With 785 nm testing, it shows fluorescence—with 1064 nm testing, it burns. But XTR analysis carries the added benefit of signal enhancement, returning a spectrum that clearly indicates the presence of trace lycopene in ketchup—the chemical that contributes its red color (Figure 5).
Figure 5. Comparison of Raman spectra of ketchup measured by 1064 nm, 785 nm (MIRA DS), and XTR (MIRA XTR DS).
Figure 6. Comparison of Raman spectra of pure honey (left) and imitation honey (center) measured by 1064 nm, 785 nm (MIRA DS), and XTR (MIRA XTR DS). Right: Determination of the ratio of different mixtures of pure honey with adulterants using MIRA XTR DS. (Click image to enlarge.)
A powerful laboratory in the palm of your hand
- Low power 785 nm laser interrogates sensitive samples without risk of ignition or burning.
- Compact, pocket-sized design enables true single-handed operation of the device
- The low power consumption means longer battery life for extended field use
MIRA XTR DS: all the best of handheld Raman with virtually unlimited applications.
Find out more about MIRA XTR DS
Download free white papers and learn more on our website.
 Christesen, S. D.; Guicheteau, J. A.; Curtiss, J. M.; Fountain, A. W. Handheld Dual-Wavelength Raman Instrument for the Detection of Chemical Agents and Explosives. Opt. Eng. 2016, 55 (7), 074103. DOI:10.1117/1.OE.55.7.074103
 Barat, S. A.; Abdel-Rahman, M. S. Cocaine and Lidocaine in Combination Are Synergistic Convulsants. Brain Res. 1996, 742 (1), 157–162. DOI:10.1016/S0006-8993(96)01004-9