Handheld 785 nm Raman is a well-established material identification technique, most notably in the pharmaceutical and defense and security markets. Now, new capabilities developed by Metrohm Raman are expected to increase the capabilities of handheld Raman in diverse industries. This article will first cover the instrumentation and then conclude with several new applications for 785 nm Raman spectroscopy.

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Figure 1. Comparison of high (green) and low (grey) SNR in the Raman spectrum. High SNR results in better library matches.

Flexible sampling optionsshort analysis timessmall form-factor, and superior identification capabilities are the best known benefits of 785 nm handheld Raman systems. Let’s dive a bit deeper and check out how low laser powers and resolution contribute to this growing list.

Short analysis times and low laser powers both preserve battery life for a system—a necessity for handheld Raman in field applications. Low laser powers also pose less risk of sample degradation for safer analysis of unknown materials.

MIRA’s (Metrohm Instant Raman Analyzer) unique spectrometer design collects data in very short analysis times with an excellent signal-to-noise ratio (SNR). A comparison of high (green) and low SNR (grey) in Figure 1 illustrates how noise in a low-resolution spectrum can occlude peak resolution. Ultimately, high SNR means more peak information for optimal library matching.

A demonstration of how wavelength, laser power, acquisition time, and SNR are related can be found in Table 1 and Figure 2. Observe that 1064 nm Raman requires 440 mW (vs. 50 mW) and nearly 10x the sample acquisition time to compare with 785 nm Raman. At the same laser power (50 mW), SNR of 1064 nm Raman is nearly seven times lower than that of 785 nm Raman. It is clear that the high SNR resulting from the combination of lower laser power and shorter sample acquisition time makes 785 nm Raman the ideal choice for analysts.
 

Table 1. Comparison of data resulting from 785 nm and 1064 nm handheld Raman instruments at varying laser powers. Colors in Table 1 correspond to those in Figure 2.
Figure 2. 785 nm Raman (top spectrum) provides the optimal combination of high SNR and low laser powers in a handheld device for field operations.
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Figure 3. MIRA XTR DS handheld analyzer from Metrohm Raman.

XTR®: The novel fluorescence rejection technique for improved 785 nm performance

Wavelength can be very influential when deciding on a Raman system, e.g., 532 nm for strong signal or 1064 nm for reduced fluorescence – but 785 nm Raman with fluorescence rejection gives users the best of both worlds. Approximately 20–30% of materials fluoresce under excitation at this wavelength. However, patented algorithms on MIRA XTR DS (Figure 3) eXTRact fluorescence from the 785 nm Raman spectrum for fluorescence-free material ID.
 

Find out more about MIRA XTR DS in our free White Paper and related blog article.

Fluorescence-free 785 nm material ID with MIRA XTR DS

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


The benefits of Raman eXTRaction are impressive:

  • Non-technical users can quickly and easily collect high quality data anywhere for diverse material identification applications
  • XTR makes possible the analysis of thousands of highly colored, organic, and/or complex samples
  • High resolution spectra enhance library matching capabilities, providing fast and accurate material ID
  • The incredibly high SNR spectra provided by XTR, even when sampling fluorescent materials, enables direct, sensitive analysis of even low concentration components

Novel sampling capabilities for handheld Raman

To us, flexible sampling means successful data collection in every field scenario: from direct immersion to through-barrier sampling.

Determination of Container Contents – Simple, Guided Method for Materials ID with Raman

Handheld Raman for Acid Attack Prevention – Identification of acids through a novel plastic container


Metrohm Raman also offers options for robot-mounted, no-contact, remote, delayed, and standoff data collection (Figure 4). Ultimately, each of these capabilities is designed to reduce contact with potentially hazardous chemicals.

Figure 4. (L) MIRA XTR DS with Autofocus Standoff Attachment (AFSO). (R) Robotic options are also available to use MIRA remotely in dangerous situations.

Imagine a major chemical spill at a large facility: material ID must occur before remediation can begin, ideally without interaction with unknown substances. In such a scenario, a robot carries MIRA XTR DS with the Autofocus Standoff Attachment (AFSO) into the spill area while operators remain outside. The robot, instrument, and attachment can all be operated remotely to gather relevant information about the nature of the spill.
 

Learn more about Metrohm Raman’s Autofocus Standoff Attachment (AFSO) in the following brochure.

Brochure: Autofocus Standoff Attachment (AFSO)

Reaching new markets with the latest applications

High resolution, fluorescence mitigation, and flexible sampling capabilities are great benefits for handheld Raman users. Here we discuss several application examples to illustrate how handheld Raman is being used beyond the pharmaceutical and defense and security markets.

Folic acid as interrogated by 1064 nm Raman and 785 nm Raman (with and without XTR).
Folic acid as interrogated by 1064 nm Raman and 785 nm Raman (with and without XTR).

Chemicals

Raman can be a powerful tool for synthetic development of chemical compounds or in research laboratories because its specificity makes chemical identification, detection, and characterization quick and easy. However, organic substances can also be some of the most problematic materials under 785 nm interrogation. XTR overcomes this issue, as shown in the example here.

Synthetically useful molecules often have multiple bonds, and are also likely to exhibit fluoresce in the Raman spectrum. Consider folic acid, a B vitamin that is also useful as a synthetic material because of its extended saturation and numerous functional groups. Folic acid does fluoresce under Raman interrogation, but nonetheless, XTR produces a very high-resolution spectrum.

Koolaid® drink mix as interrogated by 785 nm Raman (with and without XTR).
Koolaid® drink mix as interrogated by 785 nm Raman (with and without XTR).

Dyes

Dyes and highly colored materials nearly always fluoresce under Raman interrogation, and the broad, featureless baseline of fluorescence obscures signature peaks. This can be compounded in complex materials and mixtures.

Here, a highly colored food product (Koolaid® drink mix) was interrogated by handheld Raman. The top (blue) spectrum is an excellent example of the strong fluorescence from analysis of dyes with 785 nm Raman. XTR routines extract the signal from target material well enough that Allura Red (FD&C Red 40) can be positively identified in MIRA Cal DS.

Sulfanilic acid as interrogated by 785 nm Raman (with and without XTR).
Sulfanilic acid as interrogated by 785 nm Raman (with and without XTR).

Sulfanilic acid is another great example that shows the advantages of using XTR for analysis of materials that are typically difficult for Raman.

This compound easily forms a fairly stable diazonium salt that is used as a precursor to make dyes and sulfa drugs. Sulfanilic acid is a unique reagent – its pure form does not fluoresce, but its high reactivity ensures contamination in most samples that contain trace amounts of dyes, leading them to fluoresce with Raman. A spectrum full of sharp distinguishing peaks is achieved for sulfanilic acid measured with XTR.

Food and beverages

Authentication continues to be a widespread use for handheld Raman in the food and beverage industry. You may be familiar with authentication of oils with MIRA P. Check out our free White Paper here to learn more.

Facile Verification of Edible Oils with Raman Spectroscopy

Sesame oil as interrogated by 785 nm Raman (with and without XTR).
Sesame oil as interrogated by 785 nm Raman (with and without XTR).

MIRA XTR DS shows its strengths here as well. Sesame oil was used as a primary test material in the development of XTR because it is a darkly colored, organic material that is challenging to analyze with 785 nm Raman (in blue). However, XTR can extract the Raman signal with sufficient resolution to reveal the distinct signature peaks of sesame oil (in green). 

Most edible oils share signature peaks, but relative peak intensities will vary with oil type. A recently published study used MIRA DS to compare relative peak ratios for 1658 and 1442 cm−1 for authentication and quantification of oil mixtures [1]. Compared to the reported baseline method, XTR provides higher spectral resolution and is capable of superior authentication of edible oils.

Quercitin as interrogated by 1064 nm Raman and 785 nm Raman (with and without XTR).
Quercitin as interrogated by 1064 nm Raman and 785 nm Raman (with and without XTR).

Nutraceuticals and dietary supplements

Dietary supplements are often composed of large doses of vitamins, minerals, fiber, and antioxidants that are sourced from vibrantly colored fruits and vegetables. Nutraceuticals and dietary supplements are less regulated than pharmaceuticals, but Raman can be used to confirm the identity of a supplement.

Quercetin is a plant pigment with antioxidant and anti-inflammatory effects that is used as an ingredient in dietary supplements, beverages, and foods. MIRA XTR DS can produce a distinct spectrum from quercetin with intense, well-resolved signature peaks, despite fluorescence. 

Research and education

From the distinction between very similar materials to the detection of target compounds and the comparison of relative peak ratios, handheld Raman also holds promise for research and education applications.

A 2020 study compared handheld and benchtop Raman systems for analysis of plant metabolites and diagnosis of plant stresses in agricultural settings [2]. Researchers concluded that handheld systems collect quality data and were superior for early diagnosis and in-situ, real‑time monitoring of plant stresses in field conditions. This caught the eye of two different Metrohm colleagues, who independently tested an office plant to confirm the data. XTR produces a spectrum with very nice similarities to reported findings (left spectrum below in green), and with far better resolution.

(L) Plant leaf interrogated by 785 nm Raman (with and without XTR). (R) Selection of different leaves and chemical compounds measured by handheld and benchtop Raman systems [2].

SERS Development

We have already discussed how SERS (Surface Enhanced Raman Scattering) adds trace detection and fluorescence mediation to Raman’s long list of benefits in a White Paper and in our Application Notes.


Surface Enhanced Raman Scattering (SERS) – Expanding the Limits of Conventional Raman Analysis

SERS Detection of Brilliant Blue – Overcoming fluorescence issues with MISA


Two new SERS applications illustrate these benefits. The first is a good example of SERS and the analysis of dyes. The second describes how a simple sample clean-up procedure improves detection sensitivity.

Pure saffron as interrogated by 785 nm Raman and SERS.
Pure saffron as interrogated by 785 nm Raman and SERS.

Authentication of saffron

The challenge of identifying low-grade or counterfeit saffron lies in the variety of strategies used to give the appearance of a pure mixture—e.g., added dyes and the inclusion of inauthentic flower parts. This is a demonstration of the power of MISA (Metrohm Instant SERS Analyzer) for simple, portable food authentication.
 

Find out more about MISA in our blog article.

Combat food fraud: Meet MISA


SERS contributes its own inherent fluorescence-reduction capabilities here. In this comparison of Raman and SERS analysis of pure saffron, the SERS background (in orange) is much less affected by fluorescence. This supports very sensitive detection of Sudan 1, a toxic dye used in very low concentrations to mimic saffron’s rich color.
 

Learn more about saffron authentication in the following Application Note.

Trace Detection of Toxic Dye in Saffron – Protecting consumer safety with MISA 

Trace detection of acetamiprid with MISA is possible down to 0.5 µg/g.
Trace detection of acetamiprid with MISA is possible down to 0.5 µg/g.

Pesticides on raisins

Raisins are consumed around the world as a healthy snack. However, the heavy use of pesticides in countries with poor regulation turns this snack into a potentially harmful food product. Acetamiprid is a widely used neonicotinoid pesticide that has a role in bee colony collapse and is now regulated at a maximum residue level of 0.5 µg/g (500 ppb) in Europe.

Simple and effective sample extraction techniques support portable, flexible, on-site SERS analysis. Here, a highly volatile solvent was used to extract the target compound. Quick evaporation of a large volume of supernatant (800 µL instead of the 200 µL generally used) improves detection to ppb levels. With this treatment, peaks for acetamiprid are visible down to 0.5 µg/g (in orange).

Conclusion

This is the evolution of handheld 785 nm Raman: huge innovation in a pocket-sized system. A whole world of broader application possibilities opens up as 785 nm Raman devices begin to adopt and utilize XTR. 

References

[1] Pulassery, S.; Abraham, B.; Ajikumar, N.; et al. Rapid Iodine Value Estimation Using a Handheld Raman Spectrometer for On-Site, Reagent-Free Authentication of Edible Oils. ACS Omega 2022, 7 (11), 9164–9171. DOI:10.1021/acsomega.1c05123

[2]  Gupta, S.; Huang, C. H.; Singh, G. P.; et al. Portable Raman Leaf-Clip Sensor for Rapid Detection of Plant Stress. Sci. Rep. 2020, 10 (1), 20206. DOI:10.1038/s41598-020-76485-5

Author
Gelwicks

Dr. Melissa Gelwicks

Technical Writer
Metrohm Raman, Laramie, Wyoming (USA)

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