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Chemistry of Fireworks

Chemistry of Fireworks

Developed nearly two millennia ago in ancient China, fireworks are increasingly used in cultural celebrations around the world and enjoyed by nearly all ages. As one of the most entertaining forms of chemistry, fireworks appeal to our senses of sight and sound, offering a staggering variety of colors, sizes, shapes, sounds, and so on. We love to watch fireworks because they take our breath away with their magnificence and mystery.

However it is not all fun and games. The business of fireworks (and the field of pyrotechnics in general) is very serious since they should be made as safe as possible to use and also environmentally friendly. Beyond fireworks, other pyrotechnics are found in all kinds of entertainment, like in concerts, movies, and more serious applications for defense and security (e.g., safety measures like flares and signal lights).

What are fireworks made of?

Early fireworks were quite dangerous and were used for protection rather than for celebrations, and hardly resemble the ones we are now familiar with.

It all began back in Ancient China with the invention of gunpowder, which was created from a mixture of charcoal, sulfur, and saltpeter (potassium nitrate). Eventually, as new developments were made to increase the safety and predictability of using these early fireworks, experimentation with colors began and people started using them more for nonviolent purposes. Now there is an entire industry devoted to the development of all kinds of fireworks for consumers and professionals alike.

Learn more about the history of fireworks in the links below:

A firework, or aerial shell as it is also known, basically consists of three main parts aside from the housing: gunpowder and an igniter to make the rocket explode, and inside of the transported capsule on the top there are small garniture pods usually called «stars» (despite being shaped like spheres or cylinders) that include various chemicals for the desired effects. Stars consist of a colorant, a fuel, an oxidizer (oxygen providing substance, e.g., chlorates or nitrates), and a binder to hold the ingredient mixture together in a compact briquette.

The industry has spent a significant amount of time in development to make fireworks explode in shapes like stars and stripes, hearts, or even more complex forms like a cartoon figure, or letters and numbers if timed correctly.

Cross-sectional diagram of a firework capsule filled with star garnitures (72) and igniter (70). [1]

Forming a rainbow of colors

The vibrant colors of fireworks come from the combustion of metal ions which make up to 20% of the components. Metals have been used to color flames even before the invention of modern fireworks (e.g. Bengal fire). Chemically speaking, these metal ions change their electronic state by heating (addition of energy) and then going back to a lower energy state before emitting light of a certain color.

Table 1. List of metals used in pyrotechnics and their colors [2].

Color

Metal

Example compounds

Red

Strontium (intense red)

SrCO3 (strontium carbonate)

Lithium (medium red)

Li2CO3 (lithium carbonate)

LiCl (lithium chloride)

Orange

Calcium

CaCl2 (calcium chloride)

Yellow

Sodium

NaNO3 (sodium nitrate)

Green

Barium

BaCl2 (barium chloride)

B3N3 (boron nitride)

Blue

Copper halides

CuCl2 (copper chloride), at low temperature

Indigo

Cesium

CsNO3 (cesium nitrate)

Violet

Potassium

KNO3 (potassium nitrate)

Rubidium (violet-red)

RbNO3 (rubidium nitrate)

Gold

Charcoal, iron, or carbon black

 

White

Titanium, aluminum, beryllium, or magnesium powders

 

Very prominent here is the yellow color from sodium which is also seen in older street lightbulbs in some countries. Unfortunately, the most vibrant colors formed are also the most toxic for the environment, like strontium (red) and barium (green). These contaminants can be measured in the air, water, and even in the soil—but more on that later.

Find out more information about how fireworks get their colors in the links below:

Safety first

Safety is always a critical issue when discussing fireworks, whether concerning their construction, their use, or their storage. Too many serious accidents have happened over the years involving fireworks.

Learn more about how to handle fireworks in a safe manner here:

Among one of the largest fireworks disasters recorded in Europe was in Enschede (The Netherlands) in 2000. This explosion occurred in the warehouse of the S.E. Fireworks factory, which was located in the center of a residential area as the city grew and continued to build homes around it. An entire neighborhood was razed and the largest of the explosions was felt up to 30 kilometers away.

Because of this incident, sales of larger fireworks in most European countries is only allowed outdoors. Accumulating fireworks at home in preparation for celebrations should be avoided at least in confined environments like basements or apartments. It is better to store them in a ventilated shed or car parking to avoid problems in the case of a fire. Also do not store fireworks for long periods, since most of commercial fireworks are meant to be used within 3–6 months after production because the paper contents can get humid, ionic substances can dissolve and recrystallize, and therefore the likelihood of a failure increases.

In the event of a firework failure: Never have a look immediately! Wait at least 15 minutes at a proper distance and then use a tool to confine it afterwards—never touch it with your bare hands, especially when dealing with exploding fireworks or rockets.

Having said this, fireworks have integrated some safety features over the last several years to work more properly and reliably. For instance, the propellants have been modified from containing black powder to using technology from rockets such as plasticizers for better burning performance during launch, also resulting in less smoke and dust on the ground. A dedicated chain of reactions has to be followed, otherwise it will burn in a harmless way.

Knowledge is power: Prevent accidents with proper analytical testing

In order to help prevent fireworks accidents such as the one in Enschede and countless others, it is crucial to closely monitor different quality parameters including the water content of paper-based fireworks, grain size of the metal particles, and the purity and composition of the colorant, just to mention a few. Adequate quality control provides an entertaining, but safe fireworks experience even in the hands of the general public, when proper protocols are followed.

Metrohm offers several analytical technologies and related applications for this area of research. Analyses can be performed for a wide variety of substances and quality parameters as well as trace materials in the laboratory, on the street, and in the air either via wet chemical methods (e.g., Karl Fischer titration, ion chromatography, voltammetry) or spectroscopic techniques (e.g., near-infrared spectroscopy [NIRS] and Raman spectroscopy).

As mentioned earlier, moisture is an important quality parameter when discussing the safety of explosive materials. Metrohm offers two different techniques for accurate analysis of water content in a variety of matrices which are outlined in the following blog posts.

When it comes to determining the individual concentrations of the main constituents, some wet chemical techniques really stand out. Suppressed anion chromatography is ideal for measuring the ionic components of e.g., firecracker powder, other explosive material, and even in explosion residues for forensic purposes. Coupling an ion chromatograph to a mass spectrometer (IC-MS) opens up even more analysis possibilities. Read more about these studies (and more) by downloading our free Application Notes.

The use of several different metal salts to create the vibrant colors of fireworks can be beautiful but also harmful to our health and that of our environment. Voltammetry (VA) is an electrochemical method suitable for the determination of trace and ultratrace concentrations of heavy metals and other electrochemically active substances. Not only is VA excellent at determining these substances in the laboratory, but also in the field such as for measuring the after effects of a fireworks display or an undesired event. Check out our selection of VA instruments and applications on our website.

Spectroscopic techniques like Raman can help to determine the presence of dangerous explosive materials even when keeping a safe distance by using different instrument attachments. Read our free White Paper about how to use MIRA DS from Metrohm Raman for the purpose of identifying explosives safely.

Environmentally friendly fireworks – a contradiction?

Although fireworks are a very spectacular form of entertainment, there is quite an environmental impact after big cultural events or national holidays. The general atmospheric pollution after a fireworks display has been set off can be seen in an increase of dust and smoke, but also heavy metal content in the air as most contemporary fireworks use these for coloring.

The unburnt material still contains a significant amount of heavy metals. After falling to the ground, this material can dissolve and enter the ground water after it rains. Plastics materials that covered the fireworks for safety reasons are found again as broken shell shrapnel or as microplastics. The combustion of the compounds inside the fireworks leads to increased air pollution in form of aerosols that can be measured and evaluated resulting in heavy metals in the air, fine dust, and even nanoparticles which are extremely harmful for our lungs.

Metrohm Process Analytics has developed the 2060 MARGA (Monitor for AeRosols and Gases in ambient Air) which is used by official agencies and research bodies worldwide to monitor the air quality fully autonomously. This instrument is based on the analytical technique of ion chromatography and can be used as a dedicated continuous air monitoring device that can be left unattended for several weeks at a time, or as a research instrument that can be used for other projects when not monitoring the air quality.

Learn more about the 2060 MARGA and its capabilities in our blog post.

To find out more about the use of Metrohm instruments to monitor the air quality, check out this selection of peer-reviewed articles.

A new «green» firework generation is being developed for both professional and indoor use to try to minimize the heavy metal content and also reduce aerosol forming agents. This makes them more suitable for indoor pyrotechnic shows and for movie production. In regular outdoor shows (e.g. at theme parks), the gunpowder for transport of the capsule has mostly been substituted with an air pressure gun mechanism.

A significant amount of research has gone into substituting heavy metal-based colorants with more environmentally benign substances by increasing the luminosity of lithium derivatives by substituting them for strontium, or by using boron instead of barium or chlorinated compounds.

Finally, the plastic parts commonly used to surround fireworks are planned to be substituted by microcrystalline cellulose mixtures with better plasticizing binders. This leads to a similar stability compared to the current plastic materials, but the cellulose-based containers burn up completely and do not leave harmful materials scattered on the ground.

The future of fireworks shows

All safety measures increase the joy of fireworks not only during, but also after the event—being green and being safe. Foretelling the future, some of these celebrations may now use a cadre of lighted drones in a choreographed dance. This has been happening more steadily as drones fall in price and increase in their handling and programming capabilities. However, fireworks have already been with us for a couple of thousand years, and probably will not disappear any time soon.

Download our free Application Notes

and White Papers related to explosives and propellants

Post written by Dr. Norbert Mayr (Ph.D. in the field of HEDM, pyrotechnics, propellants, and oxidizers), Marketing Specialist & Product Training at Metrohm International Headquarters, Herisau, Switzerland.

Real World Raman: Simplifying Incoming Raw Material Inspection

Real World Raman: Simplifying Incoming Raw Material Inspection

Raw material identification and verification (RMID) is a complicated process for a very important reason: it confirms the quality of the raw materials used in the manufacture of products that you put on and in your body. The complexity of RMID spans the spectrum from analytical techniques and instruments to the testing process, then to the governmental norms and standards that regulate all aspects of RMID including system suitability, extent of sampling, method validation, electronic records, and many others.

With Mira P and Mira Cal P, Metrohm Raman simplifies RMID.

Warehouse verification of incoming materials with mobile Raman in regulated industries involves performing RMID directly at the loading dock. Therefore, chemical analyses that historically would be performed in a laboratory by trained chemists can now be performed very quickly and with great success by nontechnical professionals.

Beginning with System Suitability…

Producers of handheld Raman instruments for RMID must provide suitable calibration and validation routines. Calibration of Mira P with the Calibrate/Verify Accessory (CVA) accomplishes instrument calibration as well as system and performance verification, then summarizes these tests in the System Suitability Test (SST) report for Mira P. CVA ensures that Mira P performs as intended and that users can trust in the generated data quality. Upon completion of the SST, users are assured that all measurements are in accordance with agreed standards.

For more information about instrument calibration, system verification, and performance validation for Metrohm Instant Raman Analyzers (Mira), download our free White Paper!

Moving on to Sampling Flexibility…

RMID methods must accommodate a number of factors to create the most accurate and robust solution for the task. Specific consideration must be given to:

  1. The sampling strategy: how to collect the best quality data, given specific conditions
  2. Sample presentation: including morphology, packaging, and chemical nature

Handheld Raman is recognized as a particularly well-suited technique for RMID, as it offers portable, onsite, no-contact analysis of solid and liquid samples.

RMID for Regulated Industries Part I: General Considerations outlines basic applications of handheld Raman, including sampling considerations and types of evaluation. Metrohm Raman simplifies sampling with Smart Tips for every sample type.

Followed by Method Development…

From Training and Validation Set building and specific recommendations for collecting best quality spectra to dedicated software routines that automatically determine optimal model parameters, Mira P streamlines development of methods for RMID with handheld Raman.

Successful development of a method relies on the inclusion of spectra in libraries and training sets used for RMID. Careful planning in the design phase of the model leads to an easy data collection phase. This data can then be used to determine the best model parameters for robust method development. With Mira Cal P and ModelExpert, even non-technical users can implement accurate, effective RMID methods.

Download our free Application Note AN-RS-031 for more information about Simplified RMID Model Building with Mira Cal P and Model Expert!

And Method Implementation…

Implementation of handheld Raman in RMID, where the majority of materials testing is performed in the receiving area, is a logical step for such a powerful technique. It has become widespread due to some very real advantages.

Massive time savings: acquisition times of less than a minute, coupled with instant, obvious results, permits very high-throughput

Faster turnaround: delivers materials to production sooner

Reduced resources: less demand for laboratory and warehouse personnel and lab consumables, costs, and workloads

Guided Workflows: predefined workflows on Mira P make sampling simple and efficient

With Full Compliance and Utter Confidence.

Just as Mira P has built-in routines to ensure instrument integrity, Mira Cal P is designed to protect customer data integrity and simplify compliance with worldwide norms and standards. All RMID customers require data to be complete, consistent, and accurate, and MiraCal P goes beyond this with full transparency and traceability.

MiraCal P analytical software from Metrohm Raman gives you peace of mind, as it ensures all data processing adheres to several standards. More information about data integrity can be found in our free flyer.

RMID is a complex process. Learn more about how handheld Raman can provide the simplest, most efficient and accurate RMID experience possible.

Learn more about

Handheld Raman spectrometers and SERS analyzers for the lab and the process

Post written by Dr. Melissa J. Gelwicks, Applications Chemist, Metrohm Raman, Laramie, Wyoming, USA.

Recipes with Raman

Recipes with Raman

Many of us have spent more time in the kitchen in the past year than usual, (re)discovering our culinary skills with varying degrees of success. Our pantries have been kept full, and our stoves on for a year (and counting) since our normal, social ways of life have been curtailed by home office regulations, online schooling, and the sweeping closures of bars and restaurants.

Cooking at home can mean a number of things. Some people rely on «Chef Mike» (i.e., the microwave) to prepare their meals, while others turn humble ingredients into haute cuisine dishes. However, most people would probably agree that the keys to delicious and nutritious meals are fresh, high quality ingredients.

What is on your menu today? For breakfast, perhaps toast and some fresh pressed orange juice, lunch is maybe a quiche with tomatoes and cheese, and for dinner, stir-fried vegetables accompanied by a glass of good wine. Hungry yet?

With all of this talk about food, how can you be certain that the ingredients you are using in the kitchen are of the highest quality? You may trust in the grocery store, the brand, or the farmer at your local market, but do you know how different food quality parameters are measured?

One technique provides rapid, non-destructive and specific food quality testing: Raman spectroscopy. Whether you are looking to determine the ripeness of fruits or vegetables, the adulteration of spices or dairy products, or contamination of foods with banned pesticides, Raman spectroscopy is at the cutting edge of food quality analysis.

If you want to refresh your knowledge about Raman spectroscopy, have a look at our previous blog post about Mira, which includes some history about the technique.

To learn more about the analysis of trace adulterants in foods and beverages, read our blog post all about measurement with SERS (surface‐enhanced Raman scattering).

Are you confused about the differences between Raman spectroscopy and SERS? You’re not alone! Check out our blog post about these two techniques and learn about their benefits.

Here, we share a selection of peer-reviewed articles from the scientific community using Raman spectroscopy and portable instrumentation from B&W Tek, a Metrohm Group Company and Metrohm Raman to address quality issues of food. Enjoy your meal! Bon appetit!

~~ Starter ~~

To begin, maybe you would be interested in sharing a bottle of red wine with your companion as you snack on some crispy bread sticks. Red wines are made from red varieties of grapes, whose color is imparted through the crushing process as the skins soak in the sugary juices. Phenolic compounds derived from the grape skins can be beneficial to human health, and can be determined with Raman spectroscopy [1].

It’s not only beneficial compounds but also harmful contaminants that can be measured in beverages with Raman spectroscopy. Fungicides can also be detected in wine with SERS. Download our free Application Note if you want to find out more.

Watch our video below to see how methanol in alcoholic drinks is quantified rapidly without sample preparation – right at the bottle!

Snacking on prepackaged foods when you are on the go, or when you don’t feel like cooking at the moment, is something we have all done. The moisture levels in most of these foods is kept to a minimum, especially in those meant to have long shelf lives. Water content above certain levels allows harmful bacteria to grow, which is one of the major reasons to always consult the date of packaged foods before consumption. Eating contaminated foods can cause severe sickness and even death. It is possible to determine whether such low moisture foods (LMFs) contain harmful levels of these bacteria with SERS [2].

What else do both of these applications have in common? Both of them utilize the portable i-Raman Plus instrument from B&W Tek. For more information, download our free application note: Portable Raman for Quantification of Methanol in Contaminated Spirits.

~~Main Course~~

Depending on what you are in the mood for, anything is possible. Some tomatoes, vegetables, spices, perhaps meat (if you eat it) and a starch are on the menu today, ready to be turned into almost any dish.

Determining whether fresh foods are at peak ripeness can be a tricky process, not necessarily just the change of a color. The ripeness of a fruit or vegetable indicates its antioxidant content, as well as nutrients and other beneficial compounds. Monitoring the ripening process is possible with portable Raman spectroscopy [3], such as the B&W Tek i-Raman Pro.

Some of us like a little heat in our meals. Unfortunately, the adulteration of spices like chili powder (sometimes known as cayenne powder) is common, as cheap and harmful coloring agents are added to achieve more profits at the cost of human health. These synthetic dyes are able to be determined easily even at trace levels with SERS [4].

Download our free Application Note to learn more about the detection of trace levels of Rhodamine B in cayenne powder with SERS.

Some types of cheese command a high price for what seems like just a small pinch. One such type is Parmigiano Reggiano, an Italian cheese with a protected denomination of origin (PDO) quality marker, made in compliance with several production rules. These cheeses are subject to counterfeiting, but luckily this is easy to determine on-site without damaging the sample using handheld Raman spectroscopy [5].

The price of meat varies according to several reasons, even for the same animal source, section (cut), and portion size. Among these is the origin of the meat, as well as how it was produced (e.g., organic or a factory farm). Determining the difference between premium meat products and lower quality ones is possible with handheld Raman systems [6] such as Mira from Metrohm Raman. Not only these differences but also the freshness of meat during the production process can be measured with portable Raman devices [7] like the i-Raman Plus from B&W Tek.

Using lower quality cooking oil with a low smoke point at high temperatures can result in consumption of harmful byproducts formed during cooking. Older oils have a lower antioxidant content as a result of the aging process, and can become rancid when the antioxidant properties vanish. For these reasons, high quality edible oils full of antioxidants are worth much more, but are also susceptible to adulteration with cheaper ingredients. It is possible to not only determine the purity of edible oils by Raman spectroscopy [8] but also the heat stability of different types of oils [9].

For more information about the analysis of edible oils by Raman spectroscopy, download our free Application Notes and our White Paper below!

~~ Dessert ~~

After dinner is over, a hot beverage like tea can be nice to cleanse the palate. How can you be sure that the tea is free of banned pesticides, other than buying from a trusted organic label? SERS allows rapid identification of such substances in tea leaves [10].

To learn more about detecting illegal compounds such as herbicides on tea leaves, download our free Application Note.

The honey you put in your tea or drizzle over your dessert can also be subjected to tampering. Depending on the type of flower or the origin of the honey, costs can vary widely for the same volume. Some honeys (e.g., Manuka) claim to impart certain health benefits, and therefore many lower quality products with cheap sweeteners (e.g., high fructose corn syrup) are falsely labeled as such and sold at a higher price point to unsuspecting consumers. It is possible to detect honey adulteration [11] and even its botanical origin [12] with Raman spectroscopy.

Not only tea and honey, but also coffee and the milk added to it can be analyzed with Raman spectroscopy to determine various quality markers and adulterants.

The protein content of milk can be falsely enhanced with the addition of melamine. This compound is now monitored in dairy products due to scandals which led to deaths from kidney damage. Melamine [13] and other substances which can contribute to ill health effects [14] can be easily determined in milk with SERS.

Want to learn more about Melamine and how to measure it with SERS? Check out our free Application Note for further information.

Download our free Application Note to learn about the rapid detection of the alkaloid trigonelline in coffee, which reduces in concentration the darker the beans are roasted.

The ripeness of fruits and vegetables is not just important information when planning meals, but it is also critical for food transport. Perishable fruits and vegetables are often shipped in an unripened state so they arrive at their destination in top condition.

Freshness in citrus fruits can be determined with portable Raman instruments by measuring the carotenoid content [15].

Aside from the freshness, it is also possible to detect if pesticides, fungicides, herbicides or other harmful substances have been sprayed onto fruits using SERS [16].

Check out our selection of free Application Notes below about the determination of these kinds of substances on different fruits with Misa.

Several food quality parameters can be measured quickly and easily with Raman spectroscopy without the need to open bottles or destroy samples. Portable and handheld instruments make measurements simple to perform nearly anywhere. Visit the Metrohm website to learn more about the possibilities with Raman!

Learn more about rapid food analysis with Raman spectroscopy

Download free applications directly from our website.

References

[1] Dranca, F.; Oroian, M. Kinetic Improvement of Bioactive Compounds Extraction from Red Grape (Vitis vinifera Moldova) Pomace by Ultrasonic Treatment. Foods 2019, 8, 353. doi:10.3390/foods8080353

[2] Pan, C.; Zhu, B.; Yu, C. A Dual Immunological Raman-Enabled Crosschecking Test (DIRECT) for Detection of Bacteria in Low Moisture Food. Biosensors 2020, 10, 200. doi:10.3390/bios10120200

[3] Trebolazabala, J.; Maguregui, M.; Morillas, H.; et al. Portable Raman spectroscopy for an in-situ monitoring the ripening of tomato (Solanum lycopersicum) fruits. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2017, 180, 138–143. doi:10.1016/j.saa.2017.03.024

[4] Lin, S.; Hasi, W.-L.-J.; Lin, X.; et al. Rapid and sensitive SERS method for determination of Rhodamine B in chili powder with paper-based substrates. Analytical Methods 2015, 7, 5289–5294. doi:10.1039/c5ay00028a

[5] Li Vigni, M.; Durante, C.; Michelini, S.; et al. Preliminary Assessment of Parmigiano Reggiano Authenticity by Handheld Raman Spectroscopy. Foods 2020, 9(11), 1563. doi:10.3390/foods9111563

[6] Logan, B.; Hopkins, D.; Schmidtke, L.; et al. Authenticating common Australian beef production systems using Raman spectroscopy. Food Control 2021, 121, 107652. doi:10.1016/j.foodcont.2020.107652

[7] Santos, C; Zhao, J.; Dong, X.; et al. Predicting aged pork quality using a portable Raman device. Meat Science 2018, 145, 79–85. doi:10.1016/j.meatsci.2018.05.021

[8] Liu, Z.; Yu, S.; Xu, S.; et al. Ultrasensitive Detection of Capsaicin in Oil for Fast Identification of Illegal Cooking Oil by SERRS. ACS Omega 2017, 2, 8401–8406. doi:10.1021/acsomega.7b01457

[9] Alvarenga, B.; Xavier, F.; Soares, F.; et al. Thermal Stability Assessment of Vegetable Oils by Raman Spectroscopy and Chemometrics. Food Analytical Methods 2018, 11, 1969–1976. doi:10.1007/s12161-018-1160-y

[10] Yao, C.; Cheng, F.; Wang, C.; et al. Separation, identification and fast determination of organophosphate pesticide methidathion in tea leaves by thin layer chromatography–surface-enhanced Raman scattering. Analytical Methods 2013, 5, 5560. doi:10.1039/c3ay41152d

[11] Li, S.; Shan, Y.; Zhu, X.; et al. Detection of honey adulteration by high fructose corn syrup and maltose syrup using Raman spectroscopy. Journal of Food Composition and Analysis 2012, 28, 69–74. doi:10.1016/j.jfca.2012.07.006

[12] Oroian, M.; Ropciuc, S. Botanical authentication of honeys based on Raman spectra. Journal of Food Measurement and Characterization 2017, 12, 545–554. doi:10.1007/s11694-017-9666-3

[13] Nieuwoudt, M.; Holroyd, S.; McGoverin, C.; et al. Rapid, sensitive, and reproducible screening of liquid milk for adulterants using a portable Raman spectrometer and a simple, optimized sample well. Journal of Dairy Science 2016, 99, 7821–7831. doi:10.3168/jds.2016-11100

[14] Lin, X.; Hasi, W.-L.-J.; Lou, X.-T.; et al. Rapid and simple detection of sodium thiocyanate in milk using surface-enhanced Raman spectroscopy based on silver aggregates. Journal of Raman Spectroscopy 2014, 45, 162–167. doi:10.1002/jrs.4436

[15] Nekvapil, F.; Brezestean, I.; Barchewitz, D.; et al. Citrus fruits freshness assessment using Raman spectroscopy. Food Chemistry 2018, 242, 560–567. doi:10.1016/j.foodchem.2017.09.105

[16] Xie, J.; Li, L.; Khan, I.; et al. Flexible paper-based SERS substrate strategy for rapid detection of methyl parathion on the surface of fruit. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2020, 231, 118104. doi:10.1016/j.saa.2020.118104

Post written by Dr. Sara Seiffert (Product Specialist Spectroscopy at Metrohm Deutschland) and Dr. Alyson Lanciki (Scientific Editor at Metrohm International Headquarters).

Raman vs SERS… What’s the Difference?

Raman vs SERS… What’s the Difference?

If you’ve ever had a conversation with a Raman spectroscopist about the feasibility of a low-concentration sensing application, chances are you’ve heard them say “well, Raman may not be sensitive enough…but maybe SERS will work!” But what’s the actual difference between these two techniques, and why is SERS (surface-enhanced Raman scattering, or alternatively surface-enhanced Raman spectroscopy) recommended for low-concentration applications? Let’s explore the technical differences between Raman and SERS spectroscopies, as well as some of the practical considerations for how we regard the data for each.

In normal Raman spectroscopy, a laser source is incident directly on a sample (Fig. 1a). The laser light is scattered by the bonds of the analyte, and the inelastically scattered light is collected and processed into a Raman spectrum. The non-destructive nature of the technique, the selectivity of Raman bands, and the insensitivity to water make Raman a useful analytical tool for both qualitative and quantitative studies of both organic and inorganic systems.

Figure 1. 

However, for decades Raman spectroscopy was an underutilized technique in real-world applications. This can be attributed to its two major limitations: 1) the inherent insensitivity of Raman, as only ~1 in 106 incident photons are Raman scattered; and 2) fluorescence emission interference, which depends on the nature of the analyte molecule and the excitation wavelength used. Fluorescence is a competing phenomenon that is much more efficient than Raman scattering, and can thus completely overwhelm the Raman signal.

Though they depend on the scattering strength of the analyte molecule and the sample matrix in question, typical limits of detection for normal Raman scattering can range from ~1–10% in concentration. For certain applications such as disease detection or narcotics identification, this limit may be several orders of magnitude higher than what is required! In this case, an application scientist might recommend a SERS measurement. The hardware required would be the same as for a normal Raman measurement, but different sampling is required for SERS analysis. To understand the difference, let’s discuss a bit about the SERS effect.

In the 1970s, several research groups observed that the Raman signal from organic molecules like pyridine was greatly enhanced when adsorbed to a roughened metallic substrate (Fig. 1b) [1–3]. While several theories emerged to account for this observation, it is today generally accepted that the mechanism for enhancement is two-fold: the electromagnetic enhancement mechanism accounts for the dominant contribution, while a chemical mechanism accounts for a smaller portion of the enhancement.

Figure 2.

The electromagnetic enhancement mechanism is enabled by the use of a roughened nanometallic substrate made of a noble metal (usually silver or gold), and the presence of localized surface plasmons, which are quantized oscillations of the valence electrons of the chosen metal. When the laser excites the sample/nanosubtrate complex, it drives the localized surface plasmons into resonance, or excites the “LSPR” (Fig. 2). At this condition, both the laser excitation radiation and the scattered radiation from the sample are amplified. The arrows in Fig. 1b are bolded to show this increase in magnitude. This mechanism can theoretically account for signal enhancement by factors as large as 1011 [4]. The chemical mechanism involves charge-transfers in resonance with the laser excitation wavelength, and typically accounts for a theoretical enhancement factor of up to 104 [5]. Interfering fluorescence can also be quenched by these charge transfers. With the combined enhancement mechanisms we are able to overcome both the inherent insensitivity and fluorescence interference that limits normal Raman scattering. In fact, there are studies which have demonstrated that SERS is able to detect single molecules [6,7]!

Fabrication of these nanostructures has been an increasing area of academic research in the last two decades. SERS substrates can include colloidal suspensions, solid nanospheres, and metal coated on silicon chips. The enhancement tends to be at its height when the analyte molecule is placed at a junction of nanostructures (otherwise known as a SERS “hotspot”), so researchers can tailor the shapes and the plasmonic activity of these substrates to reach even greater levels of enhancement for their research purposes.

There are also commercial SERS substrates that are available for purchase to use for real-world applications. These substrates are designed to be easy-to-use, flexible, and low-cost, but may not be as sensitive as highly ordered substrates. We offer both a paper-based SERS substrate and a chip-based SERS substrate mounted to a glass slide.

After discussion with an application scientist, users may determine that a commercially available SERS substrate is suitable for their application. However, in others greater sensitivity may be required to meet the limits of detection for the application. In this case, local university labs who work on nanofabrication may be able to collaborate on measurements.

Figure 3.

We often get questions such as “Can we use our existing Raman reference library to analyze our SERS spectrum?” Figure 3 shows the difference between a normal Raman spectrum of fentanyl HCl (Fig. 3a), and a SERS spectrum of a saturated solution of fentanyl HCl on a commercial SERS substrate (Fig. 3b). The normal Raman spectrum for fentanyl contains significantly more peaks than the corresponding SERS spectrum. The SERS bands are also noticeably broader than the normal Raman bands. In the case of the SERS spectra, it is not solely the vibrational modes of the molecule that are being probed, but the sample as adsorbed to the substrate. Hence, we may also observe some peaks in a SERS spectrum that can be attributed purely to the substrate. Because of the differences between a SERS spectrum and a normal Raman spectrum, it may be difficult in some cases to use commercial Raman libraries for analysis of SERS spectra. We encourage users who require SERS identification to create their own SERS spectral databases using their substrates. We also include SERS-specific narcotics libraries on some of our TacticID handheld Raman products. For more complicated data analysis, there is also an expansive SERS literature base to draw on.

In low-concentration sensing applications, or instances where fluorescence overwhelms your Raman signal, SERS is an invaluable technique for both researchers and real-world problem solvers alike. For more information, visit our website.

Learn more about SERS

Download free applications, white papers, and more from our website.

References

[1] D.L. Jeanmaire and R.P. Van Duyne, J. Electroanal, Chem84, 1–20 (1977).
[2] M.FleischmannP.J.Hendra, and A.J. McQuillanChem. Phys. Lett. 26, 163-166 (1974).
[3] M.G. Albrecht and J.A. Creighton, J. Am. Chem. Soc. 99, 5215-5217 (1977).
[4] J.P  Camden J. A. DieringerY. WangD.J. MasielloL.D. MarksG.C. Schatz, and R.P. Van DuyneJ. Am. Chem. Soc. 130, 12616–12617 (2008).
[5] R. Pilot, R. Signorini, and L Fabris, “Surface-Enhanced Raman spectroscopy: Principles, Substrates, and Applications”. In: Deepak F.L., editor. Metal Nanoparticles and Clusters: Advances in Synthesis, Properties and Applications. Springer; Cham, Switzerland: 2018. pp. 89–164.
[6] J.A. Dieringer, R.B. Lettan, K.A. Scheidt, and R.P Van Duyne, J. Am. Chem. Soc.129, 16249–16256 (2007).
[7] K. Kneipp, Y. Wang, H. Kneipp, L.T. Perelman, I. Itzkan, R.R. Dasari, and M.S. Feld, Phys. Rev. Lett. 78, 1667-1670 (1997).

Post written by Kristen Frano, Applications Manager at B&W Tek, Newark, DE, USA.

«Analyze This»: 2020 in review

«Analyze This»: 2020 in review

I wanted to end 2020 by thanking all of you for making «Analyze This» – the Metrohm blog for chemists such a success! For our 60th blog post, I’d like to look back and focus on the wealth of interesting topics we have published this year. There is truly something for everyone: it doesn’t matter whether your lab focuses on titration or spectroscopic techniques, or analyzes water samples or illicit substances – we’ve got you covered! If you’re looking to answer your most burning chemical analysis questions, we have FAQs and other series full of advice from the experts. Or if you’re just in the mood to learn something new in a few minutes, there are several posts about the chemical world to discover.

We love to hear back from you as well. Leaving comments on your favorite blog posts or contacting us through social media are great ways to voice your opinion—we at Metrohm are here for you!

Finally, I wish you and your families a safe, restful holiday season. «Analyze This» will return on January 11, 2021, so subscribe if you haven’t already done so, and bookmark this page for an overview of all of our articles grouped by topic!

Stay healthy, and stay curious.

Best wishes,

Dr. Alyson Lanciki, Scientific Editor, Metrohm AG

Quickly jump directly to any section by clicking a topic:

Customer Stories

We are curious by nature, and enjoy hearing about the variety of projects where our products are being used! For some examples of interesting situations where Metrohm analytical equipment is utilized, read on.

From underwater archaeological research to orbiting Earth on the International Space Station, Metrohm is there! We assist on all types of projects, like brewing top quality beers and even growing antibiotic-free shrimp – right here in Switzerland.

Interested in being featured? Contact your local Metrohm dealer for details!

Titration

Metrohm is the global market leader in analytical instruments for titration. Who else is better then to advise you in this area? Our experts are eager to share their knowledge with you, and show this with the abundance of topics they have contributed this year to our blog.

For more in-depth information about obtaining the most accurate pH measurements, take a look at our FAQ about pH calibration or read about avoiding the most common mistakes in pH measurement. You may pick up a few tips!

Choose the best electrode for your needs and keep it in top condition with our best practices, and then learn how to standardize titrant properly. Better understand what to consider during back-titration, check out thermometric titration and its advantages and applications, or read about the most common challenges and how to overcome them when carrying out complexometric titrations

If you are interested in improving your conductivity measurements, measuring dissolved oxygen, or the determination of oxidation in edible fats and oils, check out these blog posts and download our free Application Notes and White Papers!

Finally, this article about comprehensive water analysis with a combination of titration and ion chromatography explains the many benefits for laboratories with large sample loads. The history behind the TitrIC analysis system used for these studies can be found in a separate blog post.

Karl Fischer Titration

Metrohm and Karl Fischer titration: a long history of success. Looking back on more than half a century of experience in KFT, Metrohm has shaped what coulometric and volumetric water analysis are today.

Aside from the other titration blog posts, our experts have also written a 2-part series including 20 of the most frequently asked questions for KFT arranged into three categories: instrument preparation and handling, titration troubleshooting, and the oven technique. Our article about how to properly standardize Karl Fischer titrant will take you step by step through the process to obtain correct results.

For more specific questions, read about the oven method for sample preparation, or which is the best technique to choose when measuring moisture in certain situations: Karl Fischer titration, near-infrared spectroscopy, or both?

Ion Chromatography (IC)

Ion chromatography has been a part of the Metrohm portfolio since the late 1980s. From routine IC analysis to research and development, and from stand-alone analyzers to fully automated systems, Metrohm has provided IC solutions for all situations. If you’re curious about the backstory of R&D, check out the ongoing series about the history of IC at Metrohm.

Metrohm IC user sitting at a laboratory bench.

Common questions for users are answered in blog posts about IC column tips and tricks and Metrohm inline ultrafiltration. Clear calculations showing how to increase productivity and profitability in environmental analysis with IC perfectly complement our article about comprehensive water analysis using IC and titration together for faster sample throughput.

On the topic of foods and beverages, you can find out how to determine total sulfite faster and easier than ever, measure herbicides in drinking water, or even learn how Metrohm IC is used in Switzerland to grow shrimp!

Near-Infrared Spectroscopy (NIRS)

Metrohm NIRS analyzers for the lab and for process analysis enable you to perform routine analysis quickly and with confidence – without requiring sample preparation or additional reagents and yielding results in less than a minute. Combining visible (Vis) and near-infrared (NIR) spectroscopy, these analyzers are capable of performing qualitative analysis of various materials and quantitative analysis of a number of physical and chemical parameters in one run.

Our experts have written all about the benefits of NIR spectroscopy in a 4-part series, which includes an explanation of the advantages of NIRS over conventional wet chemical analysis methods, differences between NIR and IR spectroscopy, how to implement NIRS in your laboratory workflow, and examples of how pre-calibrations make implementation even quicker.

A comparison between NIRS and the Karl Fischer titration method for moisture analysis is made in a dedicated article.

A 2-part FAQ about NIRS has also been written in a collaboration between our laboratory and process analysis colleagues, covering all kinds of questions related to both worlds.

Raman Spectroscopy

This latest addition to the Metrohm family expands the Metrohm portfolio to include novel, portable instruments for materials identification and verification. We offer both Metrohm Raman as well as B&W Tek products to cover a variety of needs and requirements.

Here you can find out some of the history of Raman spectroscopy including the origin story behind Mira, the handheld Raman instrument from Metrohm Raman. For a real-world situation involving methamphetamine identification by law enforcement and first responders, read about Mira DS in action – detecting drugs safely in the field.

Mira - handheld Raman keeping you safe in hazardous situations.

Are you looking for an easier way to detect food fraud? Our article about Misa describes its detection capabilities and provides several free Application Notes for download.

Process Analytics

We cater to both: the laboratory and the production floor. The techniques and methods for laboratory analysis are also available for automated in-process analysis with the Metrohm Process Analytics brand of industrial process analyzers.

Learn about how Metrohm became pioneers in the process world—developing the world’s first online wet chemistry process analyzer, and find out how Metrohm’s modular IC expertise has been used to push the limits in the industrial process optimization.

Additionally, a 2-part FAQ has been written about near-infrared spectroscopy by both laboratory and process analysis experts, which is helpful when starting out or even if you’re an advanced user.

Finally, we offer a 3-part series about the advantages of process analytical technology (PAT) covering the topics of process automation advantages, digital networking of production plants, and error and risk minimization in process analysis.

Voltammetry (VA)

Voltammetry is an electrochemical method for the determination of trace and ultratrace concentrations of heavy metals and other electrochemically active substances. Both benchtop and portable options are available with a variety of electrodes to choose from, allowing analysis in any situation.

A 5-part series about solid-state electrodes covers a range of new sensors suitable for the determination of «heavy metals» using voltammetric methods. This series offers information and example applications for the Bi drop electrode, scTrace Gold electrode (as well as a modified version), screen-printed electrodes, and the glassy carbon rotating disc electrode.

Come underwater with Metrohm and Hublot in our blog post as they try to find the missing pieces of the ancient Antikythera Mechanism in Greece with voltammetry.

If you’d like to learn about the combination of voltammetry with ion chromatography and the expanded application capabilities, take a look at our article about combined analysis techniques.

Electrochemistry (EC)

Electrochemistry plays an important role in groundbreaking technologies such as battery research, fuel cells, and photovoltaics. Metrohm’s electrochemistry portfolio covers everything from potentiostats/galvanostats to accessories and software.

Our two subsidiaries specializing in electrochemistry, Metrohm Autolab (Utrecht, Netherlands) and Metrohm DropSens (Asturias, Spain) develop and produce a comprehensive portfolio of electrochemistry equipment.

This year, the COVID-19 pandemic has been at the top of the news, and with it came the discussion of testing – how reliable or accurate was the data? In our blog post about virus detection with screen-printed electrodes, we explain the differences between different testing methods and their drawbacks, the many benefits of electrochemical testing methods, and provide a free informative White Paper for interested laboratories involved in this research.

Our electrochemistry instruments have also gone to the International Space Station as part of a research project to more efficiently recycle water on board spacecraft for long-term missions.

The History of…

Stories inspire people, illuminating the origins of theories, concepts, and technologies that we may have become to take for granted. Metrohm aims to inspire chemists—young and old—to be the best and never stop learning. Here, you can find our blog posts that tell the stories behind the scenes, including the Metrohm founder Bertold Suhner.

Bertold Suhner, founder of Metrohm.

For more history behind the research and development behind Metrohm products, take a look at our series about the history of IC at Metrohm, or read about how Mira became mobile. If you are more interested in process analysis, then check out the story about the world’s first process analyzer, built by Metrohm Process Analytics.

Need something lighter? Then the 4-part history of chemistry series may be just what you’re looking for.

Specialty Topics

Some articles do not fit neatly into the same groups as the rest, but are nonetheless filled with informative content! Here you can find an overview of Metrohm’s free webinars, grouped by measurement technique.

If you work in a regulated industry such as pharmaceutical manufacturing or food and beverage production, don’t miss our introduction to Analytical Instrument Qualification and what it can mean for consumer safety!

Industry-focused

Finally, if you are more interested in reading articles related to the industry you work in, here are some compilations of our blog posts in various areas including pharmaceutical, illicit substances, food and beverages, and of course water analysis. More applications and information can be found on our website.

Food and beverages
All of these products can be measured for total sulfite content.

Oxidation stability is an estimate of how quickly a fat or oil will become rancid. It is a standard parameter of quality control in the production of oils and fats in the food industry or for the incoming goods inspection in processing facilities. To learn more about how to determine if your edible oils are rancid, read our blog post.

Determining total sulfite in foods and beverages has never been faster or easier than with our IC method. Read on about how to perform this notoriously frustrating analysis and get more details in our free LC/GC The Column article available for download within.

Measuring the true sodium content in foodstuff directly and inexpensively is possible using thermometric titration, which is discussed in more detail here. To find out the best way to determine moisture content in foods, our experts have written a blog post about the differences between Karl Fischer titration and near-infrared spectroscopy methods.

To determine if foods, beverages, spices, and more are adulterated, you no longer have to wait for the lab. With Misa, it is possible to measure a variety of illicit substances in complex matrices within minutes, even on the go.

All of these products can be measured for total sulfite content.

Making high quality products is a subject we are passionate about. This article discusses improving beer brewing practices and focuses on the tailor-made system built for Feldschlösschen, Switzerland’s largest brewer.

Pharmaceutical / healthcare

Like the food sector, pharmaceutical manufacturing is a very tightly regulated industry. Consumer health is on the line if quality drops.

Ensuring that the analytical instruments used in the production processes are professionally qualified is a must, especially when auditors come knocking. Find out more about this step in our blog post about Analytical Instrument Qualification (AIQ).

Moisture content in the excipients, active ingredients, and in the final product is imperative to measure. This can be accomplished with different analytical methods, which we compare and contrast for you here.

The topic of virus detection has been on the minds of everyone this year. In this blog post, we discuss virus detection based on screen-printed electrodes, which are a more cost-effective and customizable option compared to other conventional techniques.

Water analysis

Water is our business. From trace analysis up to high concentration determinations, Metrohm has you covered with a variety of analytical measurement techniques and methods developed by the experts.

Learn how to increase productivity and profitability in environmental analysis laboratories with IC with a real life example and cost calculations, or read about how one of our customers in Switzerland uses automated Metrohm IC to monitor the water quality in shrimp breeding pools.

If heavy metal analysis is what you are interested in, then you may find our 5-part series about trace analysis with solid-state electrodes very handy.

Unwanted substances may find their way into our water supply through agricultural practices. Find out an easier way to determine herbicides in drinking water here!

Water is arguably one of the most important ingredients in the brewing process. Determination of major anions and cations along with other parameters such as alkalinity are described in our blog post celebrating International Beer Day.

All of these products can be measured for total sulfite content.
Illicit / harmful substances

When you are unsure if your expensive spices are real or just a colored powder, if your dairy products have been adulterated with melamine, or fruits and vegetables were sprayed with illegal pesticides, it’s time to test for food fraud. Read our blog post about simple, fast determination of illicit substances in foods and beverages for more information.

Detection of drugs, explosives, and other illegal substances can be performed safely by law enforcement officers and first responders without the need for a lab or chemicals with Mira DS. Here you can read about a real life training to identify a methamphetamine laboratory.

Drinking water regulations are put in place by authorities out of concern for our health. Herbicides are important to measure in our drinking water as they have been found to be carcinogenic in many instances.

Post written by Dr. Alyson Lanciki, Scientific Editor at Metrohm International Headquarters, Herisau, Switzerland.