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NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 3

NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 3

What is Pyrolysis gasoline or «Pygas»?

Pyrolysis gasoline, also known as Pygas, is a byproduct of naphtha cracking during the production of ethylene and propylene. Pyrolysis gasoline is an easily flammable, colorless liquid with high aromatic contents and represents a mixture of light hydrocarbons (Figure 1). It is a high octane number mixture which contains aromatics, olefins, and paraffins ranging from C5 to C12.

Figure 1. Pyrolysis gasoline (or Pygas) shown here is nearly colorless, but extremely flammable.
Because of its high octane number, Pygas has a high potential for blending in various end-user products.  In addition, pyrolysis gasoline can be used as a component separator for benzene, toluene, and xylene. For this purpose, it is used as a component separation additive.

Pygas contains some undesired conjugated diolefins that when present in high quantities makes them unsuitable as a motor fuel. These conjugated diolefins are highly reactive to polymerization and can plug the downstream refining processes causing unwanted shutdowns and high costs for remediation. These compounds also affect the stability of commercial gasoline. Therefore, the conjugated diolefins content must be controlled.

The content of conjugated diolefins is indirectly measured as the «maleic anhydride value» (MAV), or as the «diene value» (DV). This parameter is usually determined by the Diels-Alder wet chemical method (UOP 326). Furthermore, the determination of Bromine Number (ASTM D1159) in pygas is useful as it indicates the degree of aliphatic unsaturation. These determination methods require several hours and must be analyzed by highly trained analysts. In contrast to using primary methods, near-infrared spectroscopy (NIRS) is a cost-efficient and fast alternative solution for the determination of MAV or DV and Bromine Number in pyrolysis gasoline.

Near-infrared spectroscopy—an ASTM compliant tool to assess the quality of pygas

Near-infrared spectroscopy (NIRS) has been an established method for both fast and reliable quality control within the petrochemical industry for more than 30 years. However, many companies still do not consistently consider the implementation of NIRS in their QA/QC labs. The reasons could be either limited experience regarding application possibilities or a general hesitation about implementing new methods.

There are several advantages of using NIRS over other conventional analytical technologies. For one, NIRS is able to measure multiple parameters in just 30 seconds without any sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes it an excellent method for the determination of both property types.

In the remainder of this post, an available solution for the determination of maleic anhydride value (MAV) or diene value (DV) and Bromine number are outlined which have been developed according to the NIRS implementation guidelines of ASTM E1655 (method development), ASTM D6122 (method validation), and ASTM D8340 (results validation).

Did you miss the first parts in this series about NIRS as a QC tool for the petrochemical industry? Find them all below!

Read our previous blog posts to learn more about NIRS as a secondary technique.

Analysis of Diene Value (DV) and Bromine Number (BN) in pygas with the DS2500 Liquid Analyzer

Historically, NIRS analysis of the diene value and Bromine Number in pygas has been considered to be complicated due to the presence of other non-conjugated dienes as well as alkenes that have similar molecular functional groups. In addition, the majority of the samples are a complex mixture of aromatics and alkanes that varies with process conditions in the ethylene production process, as well with as the different feedstocks used to produce ethylene (e.g., alkanes, naphtha, or gas oil). Also, DV is not reliant on one specific conjugated diolefin, but over a dozen different compounds including cyclopentadiene (a ring structure) and straight-chain diolefins with different chain lengths and side chains. As explained earlier, the diene value is usually determined by the Diels-Alder wet chemical method (UOP 326). Bromine Number (BN) is determined by electrochemical titration at 5 °C (ASTM D1159).

Now, spectroscopic analysis of these parameters in such a complicated system is made successful through a combination of stable NIRS measurements with the DS2500 Liquid Analyzer, and the Partial Least-Squares (PLS) modelling capabilities in the Vision Air complete software package.

Learn more about the Metrohm NIRS DS2500 Liquid Analyzer and Vision Air software here!
Results from NIRS analysis are obtained very rapidly, with no sample preparation required aside from the temperature equilibration of the sample prior to scanning. This makes it possible to monitor and control the process, which is simply not possible using other methods. NIRS measurements do not require highly trained analysts—disposable glass vials are the only things needed for the analysis!

Metrohm offers a related application note for the proper use of NIRS for pyrolysis gas analysis (Table 1).

Table 1. Metrohm’s NIRS solutions for pygas including application details and benefits.
Parameter Reference method   Norm NIRS Application Notes  NIRS benefits
Maleic Anhydride Value (MAV) or Diene Value (DV) Reflux / Hydrolysis / Titration UOP 326 AN-NIR-024 MAV or DV measured within one minute, without requiring sample preparation or use of any chemicals.  A major difference compared to 6–7 hours when using the primary reference methods.
Bromine Number (BN) Cooling / Titration ASTM D1159 AN-NIR-094 Bromine Number measured within one minute without needing chemical reagents or sample preparation.
Learn more about the possibilities of petrochemical analysis with Metrohm NIRS DS2500 Analyzers in our free brochure.

Application example: determination of DV and BN in pygas using the NIRS DS2500 Liquid Analyzer

The diene value and Bromine Number are key parameters for the quality control of pygas.  According to UOP 326, the maleic anhydride is refluxed with the sample in boiling toluene for three hours. Any unreacted maleic anhydride is hydrolyzed to maleic acid, extracted from the reaction mixture, and then titrated with sodium hydroxide. This wet chemical method requires several hours to perform by highly trained analysts. 

For the determination of the Bromine Number according to ASTM D1159, the sample must be cooled down below 5 °C to minimize side reactions like oxidation or substitution.

In contrast to primary methods, near-infrared spectroscopy (NIRS) is a cost-efficient and fast analytical solution for the determination of DV and BN in pyrolysis gasoline.

Figure 2. Quality control of pygas as performed by the Metrohm NIRS DS2500 Liquid Analyzer.
The obtained Vis-NIR spectra (Figure 2) were used to create a prediction model for the determination of DV and BN. The quality of the prediction model was evaluated using a correlation diagram, which displays the correlation between the Vis-NIR prediction and primary method value. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis (Figure 3).
Figure 3. Correlation plots and figures of merit (FOM) for DV and Bromine Number in pygas.
This solution demonstrates that NIR spectroscopy is excellently suited for the analysis of diene value or maleic anhydride value in pygas in less than one minute without sample preparation or using any chemical reagents. In comparison to the wet chemical method in UOP 326, the time to result is a major advantage of using NIRS since a single measurement is performed within one minute instead of taking 6–7 hours with the primary method. Additionally, Bromine Number is easy to measure with NIRS without requiring any chemicals or sample preparation (such as cooling) as mentioned in ASTM D1159.

Want to learn more? Download our free Application Notes.

Summary

Near-infrared spectroscopy is an excellent choice for measuring MV / DV and BN in pygas. A simple feasibility study using your own pyrolysis gasoline samples will quickly indicate if the NIR spectra will be able to be correlated to the primary method values.  Positive aspects of using NIRS as an alternative technology are the short time to result (less than one minute), no chemicals or other expensive equipment needed, and ease of handling so that even shift workers and non-chemists can perform these analyses in a safe manner.

Future installments in this series

This blog article was dedicated to the topic of pyrolysis gasoline (pygas) and how NIR spectroscopy can be used as the ideal QC tool for the petrochemical / refinery industry. Future installments will be dedicated to other important applications in this industry. These topics will include:

 

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

We offer NIRS for lab, NIRS for process, as well as Raman solutions

Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.
Guide to online and inline surface finishing analysis

Guide to online and inline surface finishing analysis

What is surface finishing?

Surface finishing is a series of industrial processes with the main goal to alter the surface of a certain workpiece in order to obtain specific properties. This can be performed chemically, mechanically, or even electronically with the aim of removing, altering, adding or reshaping the material that is being treated.

Industries that use surface finishing techniques

Surface finishing techniques are used by most industries that manufacture industrial parts (e.g., metals, wafers, tools, and more). The use of surface finishing processes has been on the rise globally and is expected to grow further. An article published by Grand View Research (2019) predicted that the market size for metal finishing chemicals is expected to grow to $13.52 billion USD by 2025.

People mostly think about polishing and sanding when surface finishing is brought up, but it is much more than that. Several industries use different processes to treat surfaces with the main objective of obtaining the highest product quality. According to Grand View Research, the top three industries with the biggest market share for metal surface chemicals are automotive and aerospace, semiconductors, and the metal industry (e.g., industrial machinery, construction).

Figure 1 shows that surface finishing is mainly used in the automotive industry. Here, electroplating and electroless plating are the main processes used to protect against corrosion. The electroplating process consists of using electricity to coat a material (e.g. copper) with a thin layer of another material (e.g. nickel). Electroless plating is accomplished with chemical processes that reduce metal cations in a bath and deposit them as an even layer, even on non-conductive surfaces.

Next is the semiconductor industry, which includes the manufacturing and cleaning surface process of electrical and electronic parts as well as silicon wafers. This industry involves plating processes (e.g., electroless plating) as well as chemical cleaning baths. Chemical cleaning baths are used here to remove any contaminants from the wafer surfaces.

Figure 1. Diagram with top five industrial applications that incorporate surface finishing techniques (graphic repurposed from Metal Finishing Chemicals Market Global Forecast to 2021). (Click image to enlarge.)
Finally comes the metal industry, responsible for creating the infrastructure that our modern world depends on. Here, the process of galvanization is used to make metal corrosion- and heat-resistant. Galvanization is an anti-corrosive measure taken with iron and steel (as well as other metals) by applying a protective zinc coating which does not allow oxidation to occur. The zinc also acts as a sacrificial anode which still protects the underlying metal in the event of a scratch in the galvanized surface. Pickling baths are another common surface finishing process for this industry. These acidic baths are used to remove the oxide layer which formed on the surface during the hot strip mill. If the base steel is over-pickled, it can result in pitting of the metal surface, leading to an undesirable rough, blistered coating in the subsequent galvanizing steps and also excessively consumes the pickling acid (e.g. HCl).

Much more than just decorative coatings

Do appearances matter? When talking about products, absolutely! One of the reasons product surfaces are treated is so they have a more pleasant appearance for consumers, but also for more technical reasons that go beyond looks. Since surface finishing processes are used in a broad range of industries, they serve different purposes depending on the uses of the final products.

In the semiconductor industry, any defect on the components (e.g., silicon wafers, microelectronics, printed circuit boards (PCB), etc.) can impact the performance of the final product. Therefore, maintaining the proper concentrations of all components in the chemical cleaning bath ensures a repeatable etching process, which for this purpose means the elimination of surface defects.

Another example includes phosphating baths, which are used to improve corrosion resistance of the product parts used in the automotive and aerospace industry. This process is performed prior to any painting to protect the body structure from environmental factors. Phosphating baths also need to be kept consistent to guarantee the correct (and identical) thickness of the protective layer in each of the products subjected to this process.

Check out our free webinar about how Process Analytical Technology (PAT) brings analytical measurements directly to the process for real-time decision-making, ensuring a high level of control for coating and finishing baths and eliminating unnecessary risk to plant personnel. Learn about real-world case studies and field-tested applications that demonstrate the advantages of optimized bath chemistry and PAT in the surface treatment industry. 

Challenges in surface finishing processes: daily bath maintenance

Like any process, surface finishing has day to day challenges which can be improved upon. Improvement can only come from knowing the bath composition and how it affects the final product. Generally, monitoring the concentration of chemical baths is done via manual sampling and titration in a laboratory on site (in some cases, by a contract lab offsite). While this method works, it can lead to long waiting times from the moment the sample is taken until the final result—therefore the results are no longer representative of the current process conditions. Because of this delay,  bath replenishment can be impaired by over- or under- dosing components, leading to suboptimal bath composition and resulting product quality (Figure 2).

Figure 2. A jagged graph such as this denotes bath quality that suffers from suboptimal conditions. A relatively flat line would suggest a stable bath composition over time, resulting in reproducible high quality surface finishing.
Manual bath analysis and chemical dosage based on old data directly influences the company’s bottom line since the manufacturer loses money either by overusing bath chemicals or producing subpar products. The larger the plating bath volume, the greater the cost of chemicals utilized. Surface finishing baths can be as large as 3500 L (1,000 gallons) or more. Thus, it is extremely important to optimize chemical dosing to reduce unnecessary costs and waste while still providing maximum quality.

If the baths are overdosed, more chemicals are used than necessary which increases overall operational costs. However, if the baths are underdosed based on old data, then the final products may be defective, which results in increased operational costs as well.

Additionally, surface finishing processes involve many hazardous substances. When carrying out any risk assessment, the first resort is the use of personal protective equipment (PPE), and any potential exposure risks should ideally be engineered out of any process.

Automated analysis of the bath components with an online or inline process analyzer completely eliminates the risk of exposure by plant personnel to the hazards associated with the chemicals used, as well as taking care of the sample preconditioning and sampling itself. With a closed loop control, quick measurements are obtained which lead to fast results and response times for optimized process adjustments.

The solution: operate more safely and efficiently with automated process analysis

Process analysis by manual titration typically takes several steps: sample collection, sample preconditioning, volumetric manipulations, calculation, logging and checking results, and finally sending feedback to the process. All of these can be totally eliminated by using online and inline analysis.

The benefits of this are very clear. By limiting the manual handling steps, any risk of exposure to hazardous chemicals is removed. Sampling error, volumetric errors, and end point ambiguity from analyst to analyst are no longer an issue. Furthermore, sampling can be carried out on a timed basis and can be programmed to occur more frequently than possible with manual methods, giving much greater process control.

The analyzer can be used to fully control a process with direct feedback of results for the correct dosage of chemicals to aging baths. Data is automatically recorded and calculated. On-screen plots and signals can warn about deviating process conditions along with alarm outputs to notify operators of bath issues. The user interface is programmed by simple intuitive operation, and can be performed even by non-chemists.

Benefits of online and inline analysis in surface finishing processes:
  • Decrease manual labor – save time and money
  • Safer working environment – avoid contact with hazardous chemicals
  • Faster response time to process changes – better product quality
  • Optimized chemical consumption – less waste, reduced costs
Learn about the differences between inline, online, atline, and offline measurements in our previous blog post.
Metrohm Process Analytics has more than 50 years of experience in process analysis and optimization. The following examples show our expertise with configuring inline and online process analyzers for different surface finishing processes.

Automated monitoring of clean and etch baths

Metal surfaces can have scratches, impurities, and other imperfections which may interfere with further manufacturing processes (e.g., plating or painting). Therefore, clean and etch baths are a key step to obtain clean, polished, and undamaged surfaces.
Figure 3. Trend chart of NH3 and H2O2 concentrations in an SC1 bath. Note the spiking of the baths to maintain their concentrations.
Traditionally, these bath chemicals are measured offline in the lab after taking a sample from the process. However, as mentioned earlier, manual laboratory methods result in long response times in case of process changes (e.g., reaction mixture, moisture levels, …), and the sample preparation can also introduce errors, altering the precision of the analysis. Additionally, it can be quite cumbersome since different operating procedures need to be implemented to analyze multiple parameters including alkalinity, ammonium hydroxide, hydrogen peroxide, and more.
Figure 4. The Metrohm Process Analytics NIRS XDS Process Analyzer is shown here with a diagram of the inline near-infrared spectroscopy (NIRS) system configuration for cleaning bath analysis.
Another example of cleaning baths are mixed acid baths, generally comprised of sulfuric acid, hydrofluoric acid, and nitric acid. Titration only provides the acid value of the sample analyzed; therefore, it is not possible to know how much of a specific acid is present in the baths. However, near infrared spectroscopy (NIRS) is the perfect analytical technique to monitor each acid individually.
Reagent-free NIRS XDS Process Analyzers enable comparison of real-time spectral data from the process to a primary method (e.g. titration) to create a simple, yet indispensable model for process optimization. NIRS is economical and fast, enabling qualitative and quantitative analyses that are noninvasive and nondestructive. Integration of inline spectroscopic techniques allows operators to gain more control over the production process and increase overall safety.

In addition to NIRS process analyzers, Metrohm Process Analytics can design and customize flow-through cells (Figure 5). These clamp on to tubing already present onsite for easy installation with no need to modify the existing setup.

Figure 5. PTFE single fiber clamp-on flow cell from Metrohm Process Analytics.

Automated monitoring of phosphatizing baths

The phosphatizing process produces a hard, electrically non-conducting surface coating that adheres tightly to the underlying metal. This layer protects against corrosion and improves the adhesion of paints and organic finishes to be subsequently applied.

Phosphatization consists of two parts: an etching reaction with phosphoric acid which increases the surface roughness, and a second reaction at the surface between the alkali phosphates and the previously generated metal ions. This coating is quite thin and offers only basic corrosion protection. The addition of metal cations (such as zinc, manganese, and calcium) to the phosphatizing bath results in the formation of very resistant zinc phosphates with a coating thickness between 7–15 times thicker, perfectly suited for outdoor use.

Figure 6. Schematic diagram of the various process stages and baths used in the phosphatizing process. (Click image to enlarge.)

In the cleaning, degreasing, and rinsing baths, and also in the phosphatizing bath itself (Figure 6), the various parameters involved in the process must be kept stable. Conductivity, pH value, free alkalinity, and total alkalinity are among the main parameters that must be determined in the degreasing and rinsing baths. Free and total acids, accelerator, zinc, and fluoride are monitored in phosphatizing baths. The 2060 Process Analyzer from Metrohm Process Analytics (Figure 7) monitors, records, and documents all of these critical parameters at the same time. The combination of different analytical methods within one system as well as the intuitive handling via the well-arranged user interface ensure easy and reliable monitoring of the entire process.

Check out our free related Process Application Note to learn more.

Figure 7. The 2060 Process Analyzer from Metrohm Process Analytics is an ideal solution for online phosphating bath applications.
To sum up, online and inline process analyzers from Metrohm Process Analytics are the ideal solution to automate the analysis of surface finishing processes because of the comprehensive benefits they provide:
  • No manual sampling needed, thus less exposure of personnel to dangerous chemicals
  • Extended bath life by tightening process windows (less chemicals required)
  • Minimize risk of downtime with faster and more precise data
  • Easier compliance with final product requirements by process automation

If you want to learn more about all the applications that we have to offer, download our free application e-book based on 45 years of global installations.

Read what our customers have to say!

We have supported customers even in the most unlikely of places⁠—from the production floor to the desert and even on active ships!
Post written by Andrea Ferreira, Technical Writer at Metrohm Applikon, Schiedam, The Netherlands.
The evolution of handheld 785 nm Raman spectroscopy: Raman extraction from fluorescence interference

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.
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:

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. Comparison of Raman spectra of Gum Arabic powder measured by 1064 nm, 785 nm (MIRA DS), and XTR® (MIRA XTR DS).
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 on our website.

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.

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).

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

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

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).
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).

Figure 5. Comparison of Raman spectra of ketchup measured by 1064 nm, 785 nm (MIRA DS), and XTR (MIRA XTR DS).
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. (Click image to enlarge.)

A powerful laboratory in the palm of your hand

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

Download free white papers and learn more on our website.

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

Post written by Dr. Melissa Gelwicks, Technical Writer at Metrohm Raman, Laramie, Wyoming (USA).
Cyclic voltammetry (CV) – the essential analytical technique for catalyst research

Cyclic voltammetry (CV) – the essential analytical technique for catalyst research

Cyclic voltammetry (CV) is the backbone of most electrochemical research and is an essential electrochemical technique that allows researchers to explore candidate catalysts in greater depth. When coupled with modeling, a systematic goal-focused protocol will supply a range of data that will inform the user of more novel techniques and complex setups. This disciplined approach will save time in the long run, and is especially helpful to those who may have limited access to electrochemical instrumentation in a busy laboratory.
This article provides an overview of possible research goals when using CV along with relevant examples from scientific literature with the approach in action.
Electrocatalysis (ECAT) is defined as the catalysis of an electrode reaction. The electrocatalytic effect leads to an increase of the standard rate constant of the electrode reaction—resulting in a higher current density, or to a decrease in overpotential when other rate limiting steps are involved. The study of an electrocatalytic process requires characterization of the mechanism and kinetics of the electrode reaction. Forced convection methods can offer the advantage of reducing the contributions from mass-transport and providing direct access to the kinetic and mechanistic information.

In the last decade, a greater understanding of critical electrochemical transformations has been established, particularly those that involve water, hydrogen, and oxygen [1]. The expansion of our understanding in this realm was only possible because of the use of critical electrochemical techniques. This has allowed researchers to not only explore a wider variety of catalysts, but explore them in greater detail.

To respond to the potential volume of exploration which may discover more cost-effective and renewable materials that are not at the stage of critical depletion, a systematic approach to analytical research is required.

As always, new techniques are constantly being developed, but the gold standard technique of exploration of catalysts with cyclic voltammetry (CV) is still recommended as the starting point for researchers.
Instrumentation for CV analysis of catalysts from Metrohm Autolab.

Experimental Goals and Procedure Selection

To effectively explore a candidate catalyst, it is important to consider what the experimental goal is and then choose the procedure accordingly. Examples of possible goals are listed in the following sections along with suggested procedures and/or techniques.

Exploring a new system

Determine the (E) stability window of the electrolyte [2]

Method: Perform CV measurement in broad voltage (E) window, using an inert electrode (e.g. glassy carbon) and investigate the general redox behavior of the electrocatalyst material.

 

Investigate the general redox behavior of the electrocatalyst material [2]

Method: Perform CV measurement in a broad voltage (E) window, using a well understood electrolyte and new electrocatalyst.

Determine the electrode surface area for quantitative comparisons [3–5]

Method: Various methods that are material dependent: using a well-defined surface reaction (e.g. stripping or oxide formation) or analysis of electrochemical double layer capacitance (Cdl).

 

Investigate the stability of the electrocatalyst [6, 7]

Method: Perform repetitive CV measurements over several hundred cycles or during several days.

Probing a specific electrochemical reaction

Determine if a reaction is reversible (fast electron transfer kinetics), quasi reversible (slow kinetics), or irreversible (governed by other factors) [8, 9

Method: Perform CV measurements at various scan rate values, then examine dependencies for the peak position (Epeak) and peak height (Ipeak) on the scan rate.

Determine the apparent activation energy of the reaction [10]

Method: Perform CV measurements at various temperatures, then analyze electrochemical Arrhenius plots of log j vs. 1/T.

Combining CV with additional techniques to confirm results and deepen understanding

Determine the molecular structure of products or intermediates at a specific instance of the reaction [9–12]

Method: Perform CV measurement with in-situ spectroscopic determination (spectroelectrochemistry via UV/Vis/NIR or Raman spectroscopy).

Investigate material deposited or removed from the electrode surface during the electrochemical measurement [13]

Method: Measure the mass change at the electrode surface during a CV measurement using electrochemical quartz crystal microbalance (EQCM).

Investigate products and short-lived intermediates via their electrochemical response [14, 15]

Method: Perform bipotentiostat (two working electrodes) measurements in a ring/disk configuration (RRDE).

From action to reactions in the literature

This paper from the Nissan Fuel cell research center (NFCRC) summarizes the analytical approach for reduction of Pt loading in fuel cell catalyst layers (CL) [7]. Using a combined experimental and theoretical approach, they clearly outline the important properties required to measure experimentally or model to reach their goal of reducing the amount of Pt used in the CL. 

Focal parameters for exploration:

 

1. Catalyst Microstructure

Research goal: Determine the electrode surface

Using microscope images combined with the Cdl (double layer capacitance) and ionomer coverage, the researchers were able to analyze and quantify their catalyst layer. They used CV to determine ionomer coverage over the carbon by comparing Cdl values (wet versus dry).

 

2. Transport Properties

Research goal: Investigate material deposited or removed from the electrode surface during the electrochemical measurement

Additional research investigating the electrode surface was performed with CV. Using a rotating disk electrode, the researchers were able to determine the gas transport resistance by measuring the ORR (oxygen reduction reaction). CV also allowed the determination of the Pt roughness factor.

 

3. I-V performance

Research goal: Use CV I-V to calculate the fuel cell performance

I-V performance is a typical measurement for the overall performance of the fuel cell. A potentiostat is needed to measure the actual I-V curve in order to determine the Pt loading so that the I-V performance can be interpreted and compared among various samples.

    This paper illustrates the value of systematic exploration of catalysts with CV to give a comprehensive overview of attributes, structure, and reactions before moving on to more complex setups.

    Your initial investigations with CV may not provide all of the answers at first glance, but you can then move on to more complex setups and experiments with complete insight.

    Curious about electrochemistry?

    Metrohm has you covered.
    References

    [1] Seh Z. W.; Kibsgaard J.; Dickens C. F.; et al. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, 6321. doi:10.1126/science.aad4998

    [2] Kübler, P.; Sundermeyer, J. Ferrocenyl-Phosphonium Ionic Liquids – Synthesis, Characterisation and Electrochemistry. Dalton Trans. 2014, 43 (9), 3750–3766. doi:10.1039/C3DT53402B

    [3] Biegler, T.; Rand, D. A. J.; Woods, R. Limiting Oxygen Coverage on Platinized Platinum; Relevance to Determination of Real Platinum Area by Hydrogen Adsorption. J. Electroanal. Chem. Interfacial Electrochem. 1971, 29 (2), 269–277. doi:10.1016/S0022-0728(71)80089-X

    [4] Trasatti, S.; Petrii, O. A. Real Surface Area Measurements. Int. Union Pure Appl. Chem. 1991, 63 (5), 711–734. doi:10.1351/pac199163050711

    [5] Kinkead, B.; van Drunen, J.; Paul, M. T. Y.; et al. Platinum Ordered Porous Electrodes: Developing a Platform for Fundamental Electrochemical Characterization. Electrocatalysis 2013, 4 (3), 179–186. doi:10.1007/s12678-013-0145-2

    [6] Pilapil, B. K.; van Drunen, J.; Makonnen, Y.; et al. Ordered Porous Electrodes by Design: Toward Enhancing the Effective Utilization of Platinum in Electrocatalysis. Adv. Funct. Mater. 2017, 27 (36), 1703171. doi:10.1002/adfm.201703171

    [7] Xing, L.; Hossain, M. A.; Tian, M.; et al. Platinum Electro-Dissolution in Acidic Media upon Potential Cycling. Electrocatalysis 2014, 5 (1), 96–112. doi:10.1007/s12678-013-0167-9

    [8] Rountree, E. S.; McCarthy, B. D.; Eisenhart, T. T.; et al. Evaluation of Homogeneous Electrocatalysts by Cyclic Voltammetry. Inorg. Chem. 2014, 53 (19), 9983–10002. doi:10.1021/ic500658x

    [9] Sokolov, S.; Sepunaru, L.; Compton, R. Taking Cues from Nature: Hemoglobin Catalysed Oxygen Reduction. Appl. Mater. Today 2017, 7, 82–90. doi:10.1016/j.apmt.2017.01.005

    [10] Barbosa, A. F. B.; Oliveira, V. L.; van Drunen, J.; et al. Ethanol Electro-Oxidation Reaction Using a Polycrystalline Nickel Electrode in Alkaline Media: Temperature Influence and Reaction Mechanism. J. Electroanal. Chem. 2015, 746, 31–38. doi:10.1016/j.jelechem.2015.03.024

    [11] Hernández, C. L.; González García M. B.; Santos , D. H.; et al. Aqueous UV-VIS Spectroelectrochemical Study of the Voltammetric Reduction of Graphene Oxide on Screen Printed Carbon Electrodes. doi:10.1016/j.elecom.2016.01.017

    [12] Görlin, M.; de Araújo, J. F.; Schmies, H.; et al. Tracking Catalyst Redox States and Reaction Dynamics in Ni-Fe Oxyhydroxide Oxygen Evolution Reaction Electrocatalysts: The Role of Catalyst Support and Electrolyte PH. J Am Chem Soc 2017, 139 (5), 2070–2082. doi:10.1021/jacs.6b12250

    [13]  Lee, C-L.; Huang, K-L.; Tsai, Y-L.; et al. A Comparison of Alloyed and Dealloyed Silver/Palladium/Platinum Nanoframes as Electrocatalysts in Oxygen Reduction Reaction. Electrochem. Commun. 2013, 280–285. doi:10.1016/j.elecom.2013.07.020

    [14] Vos, J. G.; Koper, M. T. M. Measurement of Competition between Oxygen Evolution and Chlorine Evolution Using Rotating Ring-Disk Electrode Voltammetry. J. Electroanal. Chem. 2018, 819, 260–268. doi:10.1016/j.jelechem.2017.10.058

    [15] Kocha, S. S.; Shinozaki, K.; Zack, J. W.; et al. Best Practices and Testing Protocols for Benchmarking ORR Activities of Fuel Cell Electrocatalysts Using Rotating Disk Electrode. Electrocatalysis 2017, 8 (4), 366–374. doi:10.1007/s12678-017-0378-6

    Post written by members of the Metrohm Autolab group, Utrecht, The Netherlands.
    Best practice for separation columns in ion chromatography (IC) – Part 2

    Best practice for separation columns in ion chromatography (IC) – Part 2

    The second part of this blog series about best practice for IC separation columns focuses on application related topics that have an impact on the column suitability and stability. First, there is the proper choice of the column that best suits the intended application. Then we turn to the operating parameters which can be modified in order to optimize the separation between analytes, and what the respective effects and possibilities are.

    Choice of column length and diameter

    Metrohm offers a broad range of columns that contain different stationary phases, have different lengths and/or inner diameters. The choice of the stationary phase has a great impact on the selectivity between the individual analytes on the one hand, as well as the stability against different sample matrices on the other hand. Instead, the column length has no impact on the selectivity, but rather on the separation efficiency between the individual peaks.

    Find out more about Metrohm’s wide selection of separation columns for ion chromatography in our Column Catalog.
    Effects of column length

    In the following chromatograms (Figure 1), the effect of the column length on the separation efficiency for the Metrosep A Supp 17 column series is shown. Whenever choosing a column length, you should take the complexity of the intended separation and the presence of matrix components that could disturb the ions of interest into account.

    Figure 1. Effect of column length on the retention times of the standard anions on the Metrosep A Supp 17 column (1: fluoride, 2: chloride, 3: nitrite, 4: bromide, 5: nitrate, 6: sulfate, 7: phosphate). Click image to enlarge.
    Effects of column diameter

    In addition to providing different lengths of IC separation columns, Metrohm also offers most columns in both in 4 mm inner diameter and 2 mm inner diameter (known as «microbore») versions. With regard to this, there are several criteria to distinguish:

    • If you use online systems in a continuous mode (i.e. systems which run unattended for several days in a row such as the Metrohm Process Analytics MARGA system – Monitor for AeRosols and Gases in Ambient air), we recommend using 2 mm IC columns. Due to the reduced flowrate for microbore columns (only 25% of the flowrate for 4 mm columns), the eluent and the regenerant solutions last much longer, which increases the time the instrument can be left unattended.
    • There are applications that require hyphenated techniques such as IC-MS for higher analyte selectivity and sensitivity. In this case, the use of 2 mm columns is ideal. The low flowrate is optimal for the electrospray process, and thus no flow splitter is required before entering the mass spectrometer.
    • Sometimes, only a limited amount of sample is available for injection. In these situations, 2 mm columns are preferred. This is because less dilution/diffusion occurs during the separation process and therefore higher signals are obtained.
    • On the other hand, if your sample contains a high load of matrix components, then selecting a suitable 4 mm IC columns will be a better choice because of the higher capacity available to separate the desired analytes from the matrix.
    Find out more about MARGA and its capabilities for continuous air quality monitoring in our blog post.

    Optimizing the analyte separation

    Next to the column itself, several other parameters can be modified to optimize the selectivity of the separation. These parameters include temperature, eluent components and strength, and organic modifiers.

    Effects of modifying the temperature

    One of the simplest ways to fine tune the separation selectivity in IC is by modifying the temperature of the analysis. This is accomplished by using the integrated column oven in the instrument (if available). Multiple effects can be observed, for instance in anion analysis. As an example, the impact of the temperature on the selectivity is shown in the chromatogram overlay (Figure 2) for the Metrosep A Supp 17 column line.

    Figure 2. Effect of temperature variation on the retention times of a suite of standard anions on the Metrosep A Supp 17 column (1: fluoride, 2: chloride, 3: nitrite, 4: bromide, 5: nitrate, 6: sulfate, 7: phosphate). Click image to enlarge.
    • The monovalent ions such as fluoride, chloride, nitrite, bromide, and nitrate are all accelerated with increasing temperature, indicating that fewer interactions with the stationary phase happen.
    • The behavior of multivalent ions such as phosphate or sulfate is more complicated to describe and will vary with each stationary phase. In general, multivalent ions are retarded more at higher temperatures, which causes the retention times to increase, as can be seen for sulfate. Phosphate on the other hand behaves differently, because of the temperature induced change of the eluent pH in a range close to the pKa value of phosphate. Due to this pH change, the effective charge of the phosphate ion changes as well (in this example, the effective charge is reduced with increasing temperature).
    • The peak shape of the polarizable ions such as nitrite, bromide, and in particular nitrate, is significantly improved at higher temperatures. The reason for this behavior is the reduction of secondary interactions with the stationary phase.
    Effects of modifying the eluent composition and strength

    Eluent composition and strength can be used to change the elution order of several analytes while using the same separation column. In cation chromatography, a retention model was developed by P.R. Haddad and P.E. Jackson, which allows researchers to predict retention times when changing the eluent composition [1].

    Considering that the column remains identical in each determination, no change of ion exchange equilibrium and column capacity is to be expected. Therefore, when changing only the eluent concentration, the following correlation can be used:
    Where:

    • k’ is the retention factor of the analyte of interest
    • c is a constant
    • x is the charge of the analyte
    • y is the charge of the eluent
    • Ey+M is the concentration of the eluent in the mobile phase
    If nitric acid is used as the eluent, y = 1, and the model can be simplified to:
    Applying this formula to practical situations in the laboratory means the following: with increasing the eluent strength, alkaline earth metals are accelerated much faster (x = 2) in comparison with alkali metals (x = 1), and thus it is possible to elute magnesium before potassium. This effect is called electroselectivity.

    Multivalent metal ions are capable of forming complexes with dedicated complexing agents. Therefore, selectivities can be modified by adding complexing agents to the eluent. As an example, dipicolinic acid (DPA) is often used to complex calcium, which leads to a reduction of the effective charge of calcium. As a consequence, the retention time of calcium is reduced and calcium elutes before magnesium in the chromatogram (Figure 3).

    The retention of monovalent cations can be influenced by the addition of crown ether to the mobile phase.

    Figure 3. Effect of DPA concentration in the eluent on the retention times of several cations measured using the Metrosep C 6 column.
    Anion systems are more complex regarding the retention time model, although the same electroselectivity effect can be observed to some extent for anions. However, when changing the eluent strength, the eluent pH also frequently changes, leading to different deprotonation equilibria of multivalent anions (e.g. phosphate). This influences the effective charge of the analyte, and by doing so, also influences its retention in a similar way as previously described for the effects of changing temperature.

    In some cases, the use of a small amount of an organic modifier such as methanol, acetonitrile, or acetone in the eluent can make sense:

    • If bacterial contamination has been an issue before, the addition of 5% methanol to the eluent can help prevent future bacterial growth.
    • When samples containing a lot of organic solvent(s) need to be injected and no sample pretreatment such as extraction or matrix elimination (MiPCT-ME) is possible, it is recommended to add a suitable organic modifier to the eluent to ensure that the organic solvent(s) can be properly flushed out of the chromatographic column.
    • When using IC-MS, it is also recommended to add an organic modifier to the eluent to improve the electrospray process.

    Be aware that the addition of organic modifiers will also affect the separation selectivities. For the standard anions, the effect is similar to that observed with increased temperatures: the peak shapes of the polarizable ions such as nitrite, bromide, and nitrate are improved.

    Organic acids on the other hand may react very differently compared to the standard anions, and their reaction also strongly depends on the type of organic modifier used. Sample chromatograms that show the effect of the organic modifier on retention of analytes are shown in the manual for the Metrosep A Supp 10 column.

    Download the Metrosep A Supp 10 column manual here to see example chromatograms showing the effects of organic modifier on analyte retention time.
    For more information about column care, check out our blog post for different tips and tricks.

    The History of Metrohm IC

    Metrohm ion chromatography: bringing top quality and exceptional analytical performance to the lab since 1987. 
    Reference

    [1Haddad, P. R.; Jackson, P. E. Ion Chromatography: Principles and Applications; Journal of chromatography library; Elsevier; Distributors for the U.S. and Canada, Elsevier Science Pub. Co: Amsterdam, Netherlands; New York: New York, NY, USA, 1990.

    Post written by Dr. Vincent Diederich (Jr. Product Manager IC Columns) and Dr. Anne Katharina Riess (Head of Column Division) at Metrohm International Headquarters, Herisau, Switzerland.