Raman vs FTIR
Comparing these methods of vibrational spectroscopy through the lens of drug checking.​
Vibrational spectroscopic techniques, such as Raman Spectroscopy (Raman) and Fourier Transform Infrared Spectroscopy (FTIR), have been widely used in various industries to answer the question "what's in this substance?" . This includes forensic analytical chemistry, pharmaceuticals, biotechnology, and materials science to name a few. In pharmaceuticals, they are used to characterize the active ingredients of prescription medication products or detect counterfeit drugs. In forensics, Raman and FTIR spectroscopy are used to analyze evidence and identify unknown organic substances in various forms including powders, liquids and fibers. These techniques have become essential tools in these fields due to their ability to provide detailed information about molecular structures and chemical compounds.
Additionally, both techniques are increasingly being used in drug checking for public health, allowing for the identification of substances within the unregulated drug supply in near real-time, helping to prevent overdoses and other harms in communities. Here we will dive into the principles and differences between Raman and FTIR spectroscopy, discussing their strengths and limitations, and exploring Raman's unique advantage with Surface-enhanced Raman spectroscopy (SERS).
​Similarities of Raman and FTIR
Raman and FTIR spectroscopy share commonalities in the way their signal information is generated and interpreted as they are both vibrational spectroscopic techniques. The term "vibrational" originates from the fact that these tools measure the vibrational frequencies of molecules in the form of a spectrum, which are unique to each type of chemical bond present in those molecules. In simple terms, a Raman or FTIR spectrum is like a molecular fingerprint - it's a unique pattern of peaks and troughs that correspond to specific chemical bonds, allowing scientists to analyze and identify the molecules present in a sample. In this way, the techniques can be thought of as “siblings”, or two sides of the same coin.
An FTIR spectrometer with sample analysis (left) and Amplifi ID Raman spectrometer plus accessories (right).
How Raman and FTIR Differ in Analysis
Both techniques are powerful tools for identifying chemical compounds and are typically considered non-destructive with minimal sample preparation. However, they differ in their operational principles and selection rules. FTIR measures the absorption of infrared light from a sample, detecting changes in the dipole moment of molecules, making it sensitive to polar bonds like C=O, O-H, and N-H. In contrast, Raman spectroscopy is based on the inelastic scattering of monochromatic laser light, detecting changes in the polarizability of molecules, making it sensitive to non-polar bonds like C=C, C-S, and aromatic rings.
Put in a more simple way, FTIR is looking for changes in the way the molecules are charged, while Raman is looking for changes in the shape of the molecules. From the perspective of the technician, a spectrum of a sample may show more intense peaks and valleys if analyzed with one technique or the other, depending on the nature of the molecules being tested.
Strengths and Limitiations of Raman and FTIR
The relative advantages of Raman and FTIR spectroscopy stem from the way they analyze the sample and what they are sensitive to. As noted above, FTIR spectroscopy excels in identifying polar bonds due to its sensitivity to changes in dipole moments which can make it ideal for differentiating some families of molecules, but also means it can be hindered by other common polar molecules like water (more below). Raman spectroscopy excels in identifying non-polar bonds but can create interfering light from its own laser with fluorescent molecules.
Water & Fluorescence
FTIR can be limited by solvents like water as this molecule strongly absorb IR light and therefore “block” the signal of other molecules present. Raman spectroscopy does not experience interference from water or “damp” samples but can be limited by fluorescence interference. This is due to the fact that it adds energy to the sample by using a high powered laser, thereby creating potential to emit interfering light from fluorescent molecule in the detection signal.
Limit of Detection
Without enhancements, both techniques face limitations in detecting small concentrations of chemicals due to inherent cut-offs in their analytical sensitivity. Generally, the “limit of detection” or lowest quantity of substance both Raman or FTIR can detect within a sample is 5% wt/wt, depending on the target molecule and mixture complexity.
Sample Preparation
Another consideration is sample preparation. While both techniques are considered non-destructive (i.e. the substance that gets analyzed is not consumed or destroyed) FTIR requires samples to be pressed between its anvil and ATR crystal, which needs direct contact made with those surfaces. While usually not cumbersome, this requires consistent wipes and cleaning between samples and can lead to the possibility of "sample carryover" or cross contamination if not cleaned properly.
Testing surface of an FTIR spectrometer, with the anvil above the ATR crystal. The sample gets compressed between these surfaces for analysis.
With conventional Raman spectroscopy, testing does not require direct contact with the sample. Samples in clear containers and bags can be analyzed directly through their physical barriers which minimizes sample handling and challenges with cross contamination.
Test suspected drugs and unknown substances through clear baggies with Raman spectroscopy by using Amplifi ID's Bulk Scan mode.
Raman's Unique Advantage with SERS
Raman spectroscopy can create a significant advantage for drug sample analysis when adding Surface-enhanced Raman spectroscopy (SERS). SERS dramatically increases the Raman scattering efficiency by utilizing metal nanoparticles, typically made of gold or silver, which enhance the local electromagnetic field when excited by laser light. This enhancement increases the Raman signal by many orders of magnitude, allowing for the detection of very low concentration analytes. From the perspective of the technician, this allows for chemicals to be detected below 1% wt/wt as the signal strength of many active drug molecules becomes much higher. SERS also mitigates the issues experienced with fluorescence as it enhances the Raman scattered signals without enhancing fluorescence ones. In these ways, SERS expands the analytical utility of Raman spectroscopy, enabling the detection of low concentration samples with high specificity, and providing a level of sensitivity that IR spectroscopy cannot achieve due to the lack of comparably developed enhancement techniques.​​
Potent drugs like fentanyl and benzodiazepines are usually present below 5% weight value which is often missed by conventional Raman and FTIR. Conducting a SERS analysis is semi-destructive, meaning that 5mg of a substances needs to be dissolved in a solution during the sample preparation protocol. This amount of substance cannot be returned to the client after the test, although this is a small amount (for context, a "point" of fentanyl, a common dosage, is 100mg).
With Amplifi ID, this protocol is fairly straight forward as it is as simple as preparing a fentanyl test strip or benzodiazepine test strip. After preparing your Test Kit cartridge, insert it into the Trace Scan attachment and conduct a measurement with the Raman.
A prepared Test Kit cartridge being inserted into the Trace Scan attachment, to conduct a SERS/Trace Scan analysis.
The Value of Raman + SERS for Drug Checking
Amplifi ID combines standard Raman (Bulk Scan) and SERS (Trace Scan) functionality.
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The results from both scans can provide semi-quantitative information. If fentanyl is detected in both Trace and Bulk Scan settings, this means that fentanyl is present in large quantities (greater than 5% weight value). If it is only present in the Trace Scan results, it can be assumed the fentanyl makes up less than 5% weight value of the substance.
By using a combination of Bulk and Trace Scan, or conducting both a conventional Raman and a SERS analysis, it is possible to take full advantage of Amplifi ID to generate detailed information on a sample. Bulk Scan can provide insights into the cutting agents and high concentration drugs present, while Trace Scan catches noteworthy substances such as fentanyl, benzodiazepines and xylazine typically found at lower concentrations. The combined results of both can also provide some semi-quantitative insights leading to a robust understanding of a drug's makeup.​