The evolution of handheld 785 nm Raman spectroscopy: Raman extraction from fluorescence interference

MIRA DS (Metrohm Instant Raman Analyzer) is a handheld Raman system that identifies materials using 785 nm laser excitation. The advantages of using 785 nm Raman are well understood. Excitation with shorter wavelengths produces strong Raman scattering with short acquisition times. This results in a high signal-to-noise ratio and provides excellent spectral resolution with lower power draw. These are just some of the reasons that handheld Raman has become so popular over the last two decades.

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.

Learn more about how MIRA became mobile in our previous blog post.

How MIRA Became Mobile

However, while this is considered the «sweet spot» for both a strong signal and fluorescence mitigation among possible wavelengths, approximately 10% of Raman active materials fluoresce under interrogation with 785 nm Raman systems [1]. For example, Gum Arabic is a widely used filler and binding agent. When sampled with 785 nm systems, its fluorescence overwhelms the Raman signal (more on this subject later). Similarly, cutting agents (e.g., sucrose found in street drugs) fluoresce and can prevent positive identification of the target substance. Dyes can be problematic in the analysis of tablets, foodstuffs, art, and plastics as well. Often, weak Raman features can still be observed in fluorescent materials with 785 nm interrogation, but fluorescence mitigation is crucial for library matching.

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:

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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).

MIRA XTR DS is the evolution of Raman spectroscopy. It combines the smaller size, higher resolution, and lower power consumption of a 785 nm Raman instrument with patent-pending advanced algorithms to eXTRact Raman data, even from spectra that have strong fluorescence!

Figure 1 contains Raman spectra from a fluorescent material, Gum Arabic powder, with traditional 785 nm and 1064 nm laser excitation, in addition to MIRA XTR DS. The improvement in resolution with XTR is obvious. Notice the very flat (uncorrected) baseline in the XTR spectrum on the bottom. This is crucial for library matching with a Pearson correlation, where the dot product between spectra and non-zero baselines contribute strongly to the correlation.

Learn more about MIRA XTR DS here.

MIRA XTR DS: The evolution of handheld Raman for fluorescence-free material identification

Applications for MIRA XTR DS include Sensitive Site Exploitation / Intelligence Surveillance Reconnaissance (SSE/ISR) of clandestine labs and determination of synthetic routes to illicit products. MIRA XTR DS is designed for real world scenarios like the analysis of methamphetamine lab residues and identification of narcotics in street drug samples. This includes ID of narcotics, despite cutting agents that fluoresce and fail analysis at 785 nm. ORS™ combined with fluorescence rejection means that MIRA XTR DS can also delicately interrogate sensitive materials like colored explosive compounds.

Download our free White Paper below to find out more about the capabilities of MIRA XTR DS.

Want to know more? Download our white paper

This free white paper presents a survey of fluorescence suppression methods, the benefits of MIRA XTR DS, and application examples (e.g., hazardous chemicals, illicit drugs, ingredients and products used in the food and beverage industry, and manufactured materials).

Figure 2. Comparison of Raman spectra of lidocaine hydrochloride measured by 1064 nm, 785 nm (MIRA DS), and XTR (MIRA XTR DS).

Classic applications improved with MIRA XTR DS

Lidocaine [2] is a local anesthetic that can also be used to cut cocaine because it enhances the immediate numbing sensation that many cocaine users associate with a high quality product. Since cocaine is typically present at only ~30% in street samples, its signal can be occluded by other components in the mixture. However, positive identification of common cutting agents like lidocaine can lead to further investigation of a suspect sample.

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).

Diphenhydramine is another example of a common OTC drug that, when detected, may suggest darker dealings. It can be abused on its own, but it is also a potential precursor in the synthesis of methamphetamine. Diphenhydramine exhibits some fluorescence when interrogated with 785 nm Raman (Figure 3), but it is also typically present in mixtures with inert ingredients that fluoresce. For this type of analysis, SERS can be used to detect trace amounts of a substance. This is an excellent showcase for MIRA XTR DS, because it can perform both 785 nm Raman and SERS tests, while most 1064 nm systems currently on the market cannot be used for SERS analysis.

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).

What’s the difference between Raman and SERS? Read our blog article to find out!

Raman vs SERS...What's the Difference?

But MIRA XTR DS can do more!

With fluorescence mitigation, 785 nm Raman can be used more generally for material identification and chemical analyses.

Figure 4. Comparison of Raman spectra of MCC measured by 1064 nm, 785 nm (MIRA DS), and XTR (MIRA XTR DS).

Microcrystalline Cellulose

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 5. Comparison of Raman spectra of ketchup measured by 1064 nm, 785 nm (MIRA DS), and XTR (MIRA XTR DS).

Ketchup

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).

Honey

Another important application demonstrates how MIRA XTR DS can distinguish imitation honey from the pure, unadulterated form in the pursuit of fraudulent food products, and that it shows promise for quantitative analysis. MIRA XTR DS extracts Raman spectra from materials that typically show fluorescence with 785 nm excitation, this time with sufficient resolution to detect different ratios of mixtures (Figure 6).

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.

A powerful laboratory in the palm of your hand

https://metrohm.scene7.com/is/image/metrohm/5312-crop-800px?ts=1675862660293&$sq-500$&dpr=off

Historically, Raman users dealt with fluorescence by using instruments with a 1064 nm laser. MIRA XTR DS combines the smaller size, higher resolution, and lower power consumption of a 785 nm laser with revolutionary machine learning to eXTRact Raman from fluorescent samples. The benefits are considerable!

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

References

[1] 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

[2] 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

Contact

Dr. Melissa Gelwicks

Technical Writer
Metrohm Raman, Laramie, Wyoming (USA)