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Combat food fraud: Meet Misa

Combat food fraud: Meet Misa

What’s on your plate?

Food fraud is an ever-present danger around the world. Despite increased regulations, huge scandals still occur regularly, such as deliberately tainted infant formula (2008), or the horse meat affair in the UK due to improper labelling (2013). Other more common examples include the adulteration of highly valued items with lower cost substitutes, or the illegal enhancement of color in foods and beverages with unsafe dyes.

As the population continues to increase, driving the demand for high quality food and beverage choices, so will the amount of food fraud cases. Only a concerted effort to test foodstuffs more frequently in an efficient manner along the supply chain with accurate and precise analytical techniques will bring these cases to light before more people come to harm.

Misa to the rescue

Meet the newest addition to the Metrohm Instant Raman Analyzer family: Misa, the Metrohm Instant SERS Analyzer. Misa is fast, smart, and portable with powerful algorithms that simplify high-tech analyses for non-technicians. Misa is designed with safety in mind, purposefully designed to detect illicit drugs and food additives in complex matrices.

The SERS Principle

Surface Enhanced Raman Scattering (SERS) is an extension of Raman. Perhaps you read in my previous blog post about Raman spectroscopy that «If you can see it, Raman can ID it»… well, SERS amplifies the Raman signal of trace analytes, making it an extremely sensitive method for «ID when you can’t see it.»

When SERS-active analytes adsorb to silver or gold nanoparticles, their Raman signal is enhanced as much as a million-fold, providing incredibly sensitive detection abilities.

SERS is used in biosensor applications, including single-cell sensing, antibody detection, and pathogen monitoring. It can be used to detect chemical warfare agents and illicit drug laboratory residues. Additionally, SERS is a particularly powerful technique for detecting trace contaminants in foodstuffs such as antibiotics fungicides, pesticides, herbicides, illicit dyes, and other additives.

If you know ID Kit for Mira DS, then you already know a little about SERS. SERS is an «enhancement» technique to Raman that enables detection of trace materials. For example, ID Kit was developed as a method for identifying heroin and fentanyl in street drug samples. The cutting agents and added stimulants that constitute the bulk of street heroin fluoresce under investigation with Raman and overwhelm the signal coming from heroin. SERS sees right through the cutting agents and identifies the drug.

Overlaid Raman and SERS spectra demonstrating the ability of SERS to detect the active ingredient in street heroin.

Another example of how Raman and SERS complement each other can be seen with Yaba, a common street drug in southeast Asia. Yaba is a red tablet that contains significant caffeine with a small amount of methamphetamine. When a red Yaba tablet is analyzed with Raman, caffeine and the red dye in the coating are the primary identification targets. This makes sense, because Raman is very good at identifying bulk materials.

However, when a Yaba tablet is subjected to SERS analysis, the story is very different (reminder: these are both also capabilities of Mira DS!) Only SERS can ID the methamphetamine in Yaba and complete the story.

Protecting Consumer Safety with Misa

Consumer safety relies on the ability of food inspectors to detect unwanted additives and assure the quality of the products. Trace detection of food adulterants is traditionally very involved laboratory work, using HPLC, GC/MS, and other techniques requiring extensive sample preparation and scientific training. Misa is designed to simplify food testing, from sample preparation, to sharing results.

The unique capabilities of Misa and SERS analysis in food testing deserve some explanation. Raman is used in food testing in some incredible ways: identifying counterfeit honey, distinguishing scotch from different producers, discriminating between very similar sugars, even making a distinction between grass- and grain-fed beef. However, these are bulk, inherent qualities of a food.

Looking for trace levels of pesticides is a very different science. A successful SERS analyte must interact with nanoparticles—target molecules with amine, carboxyl, and thiol groups often have the required interaction. Fortunately, many food additives fit this definition. Metrohm Raman sponsored a year-long study to identify 82 different food adulterants that can be successfully detected with our SERS substrates. That was just the beginning.

Are you looking for applications suitable for Misa? Check out our free selection of application notes available on the Metrohm website: 

Additionally, reference spectra for several other analytes can be obtained by contacting your local Metrohm  sales organization. 

The next step was to determine the foods which were typically treated with these illicit substances, then how to simplify sample preparation for potentially demanding food matrices. Metrohm Raman is taking two different approaches to this challenge. First, Misa will be released with 17 different «real world» food safety applications (click to download):

Misa is a unique instrument, which is reflected in this broad collection of Application Notes (AN). In addition to standard spectra and experiments, each AN includes a special section titled «Field Test Protocol». Briefly, the Field Test Protocol guides any user through a complete experiment using Misa and the tools in the ID Kits. ID Kits for Misa contain dedicated SERS substrates, in addition to the basic tools required for field testing. These, combined with companion Operating Procedures included on Misa, make food safety testing accessible to anyone, anywhere.

Our second approach to application development for Misa is a very interactive process with our users as we identify the target and food matrix, provide standard spectra for library building, advise sample preparation, and help to optimize results. This approach acknowledges that food is different around the world, adulterants vary, and concerns may be localized. These ANs that accompany Misa at release are intended to give the user an idea of how to use SERS and when it is a useful technique for detection of food contaminants, but custom applications will certainly increase demand for Misa.

Metrohm Raman is excited to introduce you to Misa. Misa has all of the qualities that you appreciate about Mira—intuitive user interface, simple guided workflow, and smart attachments to facilitate onsite testing by non-chemists. Our approach to simplifying food testing includes libraries, dozens of reference spectra, and developed applications targeting food adulterants.

Visit our website

and discover more about how Misa can help the fight against food adulteration scandals.

Post written by Dr. Melissa Gelwicks, Technical Writer at Metrohm Raman, Laramie, Wyoming (USA).

Comprehensive water analysis: combining titration, IC, and direct measurement in one setup

Comprehensive water analysis: combining titration, IC, and direct measurement in one setup

If you perform water analyses on a regular basis, then you know that analyzing different parameters for drinking water can be quite time-consuming, expensive, and it requires significant manual labor. In this article, I’d like to show you an example of wider possibilities in automated sample analysis when it comes to combining different analytical techniques, especially for our drinking water.

Water is the source and basis of all life. It is essential for metabolism and is our most important foodstuff.

As a solvent and transporting agent it carries not only the vital minerals and nutrients, but also, increasingly, harmful pollutants, which accumulate in aquatic or terrestrial organisms.

Within the context of quality control and risk assessment, there is a need in the water laboratory for cost-effective and fast instruments and methods that can deal with the ever more complex spectrum of harmful substances, the increasing throughput of samples, and the decreasing detection limits.

Comprehensive analysis of ionic components in liquid samples such as water involves four analytical techniques:

  • Direct measurement
  • Titration
  • Ion chromatography
  • Voltammetry

Each of these techniques has its own particular strengths. However, applying them one after the other on discrete systems in the laboratory is a rather complex task that takes up significant time.

Back in 1998, Metrohm accepted the challenge of combining different analytical techniques in a single fully automated system, and the first TitrIC system was introduced.

What is TitrIC?

The TitrIC system from Metrohm combines direct measurement, titration, and ion chromatography in a fully automated system.

Direct measurements include temperature, conductivity, and pH. The acid capacity (m and p values) is determined titrimetrically. Major anions and cations are quantified by ion chromatography. Calcium and magnesium, which are used to calculate total hardness, can be determined by titration or ion chromatography.

The results are displayed in a common table, and a shared report is given out at the end of the analysis. All methods in TitrIC utilize the same liquid handling units and a common sample changer.

For more detailed information about the newest TitrIC system, which is available in two predefined packages (TitrIC flex I and TitrIC flex II), take a look at our informative brochure:

Efficient: Titrations and ion chromatography are performed simultaneously with the TitrIC flex system.

Figure 1. Flowchart of TitrIC flex II automated analysis and data acquisition.

How does TitrIC work?

Each water sample analysis is performed fully automated at the push of a button—fill up a sample beaker with the sample, place it on the sample rack, and start the measurement. The liquid handling units transfer the required sample volume (per measurement technique) for reproducible results. TitrIC carries out all the work, and analyzes up to 175 samples in a row without any manual intervention required, no matter what time the measurement series has begun. The high degree of automation reduces costs and increases both productivity and the precision of the analysis.

Figure 2. The Metrohm TitrIC flex II system with OMNIS Sample Robot S and Dis-Cover functionality.

To learn more about how to perform comprehensive water analysis with TitrIC flex II, download our free application note AN-S-387:

Would you like to know more about why automation should be preferred over manual titration? Check out our previous blog post on this topic:

Calculations with TitrIC

With the TitrIC system, not only are sample analyses simplified, but the result calculations are performed automatically. This saves time and most importantly, avoids sources of human error due to erroneously noting the measurement data or performing incorrect calculations.

Selection of calculations which can be automatically performed with TitrIC: 

  • Molar concentrations of all cations
  • Molar concentrations of all anions
  • Ionic balance
  • Total water hardness (Ca & Mg)
  • … and more

Ionic balances provide clarity

The calculation of the ion balance helps to determine the accuracy of your water analysis. The calculations are based on the principle of electro-neutrality, which requires that the sum in eq/L or meq/L of the positive ions (cations) must equal the sum of negative ions (anions) in solution.

TitrIC can deliver all necessary data required to calculate the ion balance out of one sample. Both anions and cations are analyzed by IC, and the carbonate concentration (indicative of the acid capacity of water) is determined by titration.

If the value for the difference in the above equation is almost zero, then this indicates that you have accurately determined the major anions and cations in your sample.

Advantages of a combined system like TitrIC

  • Utmost accuracy: all results come from the same sample beaker

  • Completely automated, leaving analysts more time for other tasks

  • One shared sample changer saves benchtop space and costs

  • Save time with parallel titration and IC analysis

  • Flexibility: use titration, direct measurement, or IC either alone or combined with the other techniques

  • Single database for all results and calculation of the ionic balance, which is only possible with such a combined system, and gives further credibility to the sample results

Even more possibility in sample analysis

TitrIC has been developed especially for automated drinking water analysis but can be adapted to suit any number of analytical requirements in food, electroplating, or pharmaceutical industries. Your application determines the parameters that are of interest.

If the combination of direct measurement, titration, and IC does not suit your needs, perhaps a combination of voltammetry and ion chromatography in a single, fully automatic system might be more fitting. Luckily, there is the VoltIC Professional from Metrohm which fulfills these requirements.

Check out our website to learn more about this system:

As you see, the possibility of combining different analysis techniques is almost endless. Metrohm, as a leading manufacturer of instruments for chemical analysis, is aware of your analytical challenges. For this reason, we offer not only the most advanced instruments, but complete solutions for very specific analytical issues. Get the best out of your daily work in the laboratory!

Discover even more

about combined analytical systems from Metrohm

Post written by Jennifer Lüber, Jr. Product Specialist Titration/TitrIC at Metrohm International Headquarters, Herisau, Switzerland.

Measuring herbicides in drinking water

Measuring herbicides in drinking water

It’s springtime in the northern hemisphere, and with rising temperatures comes increased use of herbicides on agricultural crops and in public spaces. In March 2015, the International Agency for Research on Cancer (IARC) published a report which stated that one such herbicide, glyphosate, was «probably carcinogenic to humans». Ever since, the use of this chemical has been highly controversial. In some countries, including the USA, there are already limit values in effect for the weed killer.

Carcinogenic or not?

Glyphosate is a broad-spectrum herbicide used globally in agriculture. Alongside farming, the chemical is also used to kill weeds in domestic gardens and in public and private spaces kept free from «vegetal invasion», such as railway tracks.

Glyphosate has been used since the 1970s in pesticides and was previously thought to be harmless at typical levels of exposure. However, since the International Agency for Research on Cancer (IARC) – the specialized cancer-research agency of the WHO – found that glyphosate was «probably carcinogenic to humans» (Group 2A) in a report published in March 2015, the chemical repeatedly made headlines [1].

Experts were then divided over whether glyphosate should be reapproved after the expiry of its EU market approval on June 30, 2016. This is because the European Food Safety Authority (EFSA) only recently arrived at the opposed conclusion that it is unlikely that glyphosate is genotoxic or poses a carcinogenic threat [2]. The approval of glyphosate was initially extended by 18 months, but is now allowed to remain in use in the EU until at least the end of 2022 [3].

Determination of glyphosate in drinking water

Because chemicals used in farming can seep through the ground and enter the ground water, limit values are in effect in some countries concerning the concentration of glyphosate in drinking water.

Glyphosate and its metabolite AMPA (aminomethylphosphonic acid) are usually determined by HPLC with post-column derivatization and subsequent fluorescence detection (EPA Method 547), or alternatively by ion chromatography coupled with a mass-selective detector.

Methodology using IC

The following segments explain the initial results of the determination of glyphosate and AMPA in drinking water in the low µg/L range using ion chromatography (IC) with pulsed amperometric detection. The detection limits for glyphosate and AMPA previously attained with pulsed amperometric detection were around ≥ 50 µg/L [4].

Given this improvement in terms of sensitivity, the method outlined here represents a promising approach to the screening of water and food samples for glyphosate and AMPA.

Instrumentation

All determinations were performed with an IC system consisting of a 940 Professional IC Vario ONE with an IC Amperometric Detector and an 858 Professional Sample Processor for automatic sample injection (Figure 1).

Figure 1. Glyphosate and AMPA were determined with the ProfIC IC Vario 1 Amperometry system.

Glyphosate and AMPA were separated on the high-capacity anion separation column Metrosep Carb 2 – 150/4.0, and subsequently detected via flexIPAD (FLEXible Integrated Pulsed Amperometric Detection) using a gold working electrode as a measuring mode in the amperometric detector. The profile of the potential curve produced in one measuring cycle in flexIPAD mode is presented in Figure 2.

Figure 2. Pulse profile of the flexIPAD method: A measuring cycle lasts 0.9 s; measurement of the current is performed during the phase shown in red.

Experiment

The Metrosep Carb 2 column is used mainly for separating and determining carbohydrates, sugar alcohols, alcohols, etc. Its high column capacity, combined with the high pH value of the eluent (approximately pH 10), results in a large difference in retention time for AMPA and glyphosate. This is because, with a pH value of 10, all three acid groups are deprotonated in part of the glyphosate, meaning that it is partially present as a trivalent anion while the metabolite AMPA, which is missing the carboxyl group, is present as a divalent anion.

Results

Figure 3 shows the chromatogram of the determination of AMPA and glyphosate under the conditions used in this application. An aqueous standard solution was injected containing 10 µg/L each of both components.

Figure 3. Separation of AMPA and glyphosate: a standard solution containing 10 µg/L of each component in ultrapure water was analyzed.

The detection limits for both components were determined using the signal/noise (S/N) ratio, i.e., the ratio of the peak height to the baseline noise. At the detection limit, the S/N ratio is 3; with smaller values, secured detection is not possible. The detection limit found for AMPA was considerably lower than 1 µg/L, while the limit for glyphosate was approximately 1 µg/L.

Figure 4 shows a chromatogram of a drinking water sample mixed with 2 µg/L glyphosate and AMPA.

Figure 4. Determination of AMPA and glyphosate in drinking water which was mixed with 2 µg/L of each component.

Summary

For the first time, glyphosate and its primary metabolite AMPA were determined in drinking water in the low µg/L range using ion chromatography with pulsed amperometric detection (flexIPAD). This puts at our disposal a reliable and – compared with HPLC with a mass-selective detector – very inexpensive method for determining the glyphosate and AMPA content in water and foodstuffs. With a detection limit of approximately 1 µg/L, the adherence to limit values for glyphosate can be verified in the USA, Canada, and Australia, among others.

If you want to learn even more about how to measure glyphosate and AMPA via ion chromatography and amperometric detection, download our free white paper «Glyphosate and AMPA in drinking water».

Curious to learn even more?

Check out our webinar:

«Glyphosate and AMPA analysis»

References

[1] IARC Monographs Volume 112 (2015). Retrieved from http://monographs.iarc.fr/ENG/Monographs/vol112/mono112-09.pdf on June 27, 2016.

[2] EFSA press news, 151112 (2015). Retrieved from http://www.efsa.europa.eu/en/topics/factsheets/glyphosate151112 on June 27, 2016.

[3] European Commission: Status of glyphosate in the EU. Retrieved from https://ec.europa.eu/food/plant/pesticides/glyphosate_en on May 25, 2020.

[4] F. Sanchez-Bayo, R. V. Hyne, and K. L. Desseille (2010) Anal. Chim. Acta, 675 125–131.

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

Dissolved oxygen measurement – easier than ever

Dissolved oxygen measurement – easier than ever

Do you know why your drinking water becomes flat after you leave it untouched for a few hours? Or why your orange juice changes its color and darkens a bit when the bottle is left open for a longer time?

One of the key driving factors behind these changes is the amount of oxygen in your beverage.

I would like to share some information with you about the effects (both positive and negative) oxygen has when dissolved in liquids, which parameters affect the dissolved oxygen (DO) content, as well as how to accurately assess the DO concentration.

Why is DO concentration important?

Next to pH and conductivity, dissolved oxygen is one of the most important water quality indicators. Oxygen dissolves in surface water according to its partial pressure (Henry’s law), but also due to aeration processes (e.g., wind, rapids). Additionally, oxygen is introduced into water as byproduct of photosynthesis by plants and phytoplankton. Dissolved oxygen is essential for the survival of fish and any other aquatic organism that breathes oxygen.

The DO content may be reduced when too many bacteria or algae contaminate the water. Bacteria feed on dead algae and other organic material, consuming oxygen and producing carbon dioxide. If all DO is consumed by bacteria, it is called eutrophication. When the DO content in water drops below 5 mg/L, aquatic life is put under stress, and if the concentration is even lower, a large amount of aquatic life can die. Dissolved oxygen can be directly assessed, in-situ in surface water, by the direct measurement technique.

Learn more about dissolved oxygen measurement in surface water by downloading our free application note:

Getting back to the example of your drinking water or orange juice:

Water only tastes good to us when there is a certain amount of oxygen is dissolved into it. When your glass or water bottle is standing around, DO is released as it equilibrates with the atmosphere and additionally it will warm up to the ambient temperature, releasing even more oxygen. This is why the taste of your water turns flat over time.

If you would like an overview of how dissolved oxygen in your water supply can be determined, download our free application note:

Orange juice exhibits the contrary situation. Orange juice (and other fruit and vegetable juices) are kept almost DO free. The reasoning is because oxygen, as an oxidizing agent, has a negative influence on the overall quality, taste, nutritional value, and color of a beverage. The longer you keep your orange juice open to the atmosphere, the more oxygen will dissolve into your juice, until a point. Furthermore, this DO will start to react with other ingredients of your juice. For example, DO will oxidize any present Vitamin C (ascorbic acid, an antioxidant) to dehydroascorbic acid. To prevent quick browning, as well as the flavor and quality of your juice, keep it in a closed bottle.

Do you want to know more about the determination of dissolved oxygen in fruit juices? Download our free application note:

What affects the dissolved oxygen concentration? 

Temperature

The temperature has a large influence on DO concentration. The higher the temperature, the less oxygen is dissolved in the liquid phase. Why? I will explain it to you a bit more visually:

When the temperature of a solution increases, the ions and molecules therein move and vibrate due to the increased energy. This leads to more and more collisions between particles and thus, some of the bonds that hold them together break. As more particles vibrate, more collisions occur, and even more bonds are broken. That also means that the bonds which hold oxygen molecules in the liquid will break, and oxygen will be released from the solution. This results in a decrease in the DO content. The opposite happens if the temperature decreases: particle motion decreases, and therefore the DO concentration increases.

Pressure

For our purpose, here «pressure» refers to the atmospheric pressure. Perhaps you’ve been on the top of a mountain, or inside of an airplane flying at altitude, and had a drink from your water bottle up there. When you were back on the ground, or at the base of your hike, and checked the bottle again, maybe you noticed that it was compressed slightly, or had a suction noise as you opened it again. This is due to the difference in atmospheric pressure, which is inversely proportional to altitude.

As atmospheric pressure decreases, the partial pressure of oxygen also decreases. Therefore at higher altitudes, less oxygen is dissolved in the liquid since the pressure does not hold it there. Oxygen diffuses out of the liquid, the higher we get. When we go to lower altitudes, the DO concentration increases as the atmospheric pressure increases.

Salinity

The salinity also plays a part in the amount of dissolved oxygen which is available in a liquid.

Again consider the ions and molecules present in the solution. When we have a dissolved salt present in the water, these charged ions are very much attracted to the water molecules. Dissolved oxygen has no charge, and is therefore not attracted to anything. The higher the salinity content, the more ions are present. This increased density of particles coerces oxygen to leave the solution as its interaction with water is not so strong.

How can we assess the DO concentration?

There are two possibilities to determine the dissolved oxygen content in liquids, either by direct measurement or by titration. We have summarized the pros and cons for each of the methods in a free white paper which you can download below.

However, I will only cover direct measurement using an optical sensor here. Why? Because you can measure the DO content online or in-situ without tedious sampling and sample preparation and your equipment is almost maintenance-free – you will be surprised how easy it is to use!

The O2-Lumitrode, the optical sensor for DO measurement from Metrohm, is the fastest of its kind on the market. It measures the DO content in liquids in less than 30 seconds! The working principle is based on luminescence quenching.

Let me explain how this works: the sensor cap contains a membrane with an embedded luminophore that is excited by red light. When there is no oxygen present, the luminophore returns to its ground state via emission of luminescence.

If oxygen is present, and these molecules collide with the excited luminophore, the luminophore returns to its ground state emission-free, because the energy is transferred to the oxygen molecule. By evaluating the lifetime of the excited state of the luminophore (by using the phase shift), it is possible to determine the DO content.

The O2-Lumitrode does not need much maintenance—a regular one-point calibration with 100% air saturation is enough. From time to time, we recommend performing a two-point calibration with 100% and 0% air saturation.

Our 913 pH/DO Meter or 914 pH/DO/Conductometer can be equipped with the O2-Lumitrode. Both of these are combined instruments, meaning you can additionally measure pH and/or conductivity alongside dissolved oxygen.

As stated earlier, temperature, pressure, and salinity impact the dissolved oxygen content in liquids. Therefore, the O2-Lumitrode is equipped with a temperature sensor and a pressure sensor so automatic temperature and pressure compensation can be applied for the most reliable results. If you are measuring DO in a saline solution or in seawater, you can measure the conductivity in parallel to DO and switch the automatic salinity compensation on.

The O2-cap must be replaced from time to time, as the luminophore becomes less reactive. This effect is called photo bleaching. However, the sensor will tell you when this is necessary due to its active performance monitoring. Never worry again about inaccurate DO measurements due to poor quality instrumentation.

To summarize, depending on the application and matrix, a wide range of dissolved oxygen can be found. The determination of the DO content fast and accurately is extremely important. Using an optical sensor with a mobile device makes it very easy to assess the DO content in-situ. For the most reliable data, additionally measure the temperature and pressure (and eventually the salinity) in parallel to minimize the effect of these physical parameters on your results.

Want to learn more?

Download our free White Paper:

Determining dissolved oxygen in water: Titration or direct measurement?

Post written by Dr. Sabrina Gschwind, Jr. PM Titration (Sensors) at Metrohm International Headquarters, Herisau, Switzerland.

Determining the total sulfite in food and beverages: faster and easier than ever

Determining the total sulfite in food and beverages: faster and easier than ever

The chances are good that if you’re reading this, you are an analytical chemist or somehow connected to the food science sector. Maybe you have had the lucky experience of measuring sulfite (SO32-) before in the laboratory. I certainly have, and the adventure regarding tedious sample preparation and proper measurement of such a finicky analyte still haunts me today, years later.

Why sulfite?

Sulfite is a preservative added to a vast range of foods and beverages to prevent browning or oxidation. Some individuals are sensitive to sulfite additives and may experience a range of allergic reactions. Therefore, both the U.S. Food and Drug Administration (FDA) and European Union (EU) laws require that the presence of sulfites be declared on food labels when the concentration exceeds 10 mg/L.

To put this into perspective, an Olympic size swimming pool can hold about 2,500,000 liters, meaning anything beyond 25 kilograms (the average mass of one young child!) would need to be reported.

So, which foods contain sulfite?

Many foods and beverages contain sulfite – whether added to prolong the freshness, or occurring naturally as a byproduct from processes like fermentation. Typically, the first things that come to mind are wine, beer, or dried fruit snacks. However, many pickled and otherwise preserved items such as sauerkraut, canned fruits and vegetables, and even frozen foods contain significant levels of sulfites. Processed meats, several condiments, and some prepared doughs are also high on the list of offenders, so beware at your next picnic!

If you think you may be sensitive to sulfites, don’t forget to check the nutrition facts, and try to avoid such foodstuffs.

How is sulfite usually measured?

Several analytical methods exist to measure sulfite in food and beverages, however they suffer from repeatability issues, and can be quite cumbersome to perform.

Traditionally, the optimized Monier-Williams (OMW) AOAC Official Method 990.28 was used for quantification of sulfite in most foodstuffs, but the method detection limit now lies at the regulatory labeling threshold. Automated discrete analysis methods have been reported for sulfite analysis, but they are limited by their strong dependence on sample matrix type. Therefore these methods are less than ideal for laboratories where sulfite analysis is required for a wide variety of food and beverage products.

Methods based on ion chromatography (IC) with conductivity detection exhibit a lack of selectivity combined with an extended analysis time due to separation challenges. A newer method developed by AOAC (Method 990.31) focuses on the use of ion-exclusion chromatography followed by electrochemical (amperometric) detection of samples.

Another issue arises concerning the sensitivity of the detector. After a few injections, fouling from contaminants rapidly decreases the electrode sensitivity. Frequent reconditioning of the working electrode is necessary due to a rising background and baseline noise, and can be accomplished in a couple of ways. Manual polishing and utilizing pulsed amperometric detection (PAD) pulse sequences are the most common choices to recondition the surface of the working electrode, while other methods opt for disposable electrodes to avoid this step altogether.

What has improved?

Metrohm has filed a patent for an innovative, fast, and accurate ion chromatographic (IC) method based on direct current (DC) mode electrochemical detection. It works with the implementation of a unique working electrode conditioning function (patent pending) in the newest version of chromatographic software (MagIC Net 3.3) offered by Metrohm. A great diversity of food and beverage products were analyzed with sulfite recovery values near 100% in all cases. Using a single, robust chromatographic method, any sample can be treated identically, saving time and making laboratory work much easier.

Sample of garlic analyzed for sulfite content (spiked: red, unspiked: black). Recovery was calculated at 100%.
(Click to enlarge)

No matter what type of sample (solid, liquid), the preparation steps are nearly identical, and much simpler to perform than ever before. Additionally, the retention time of sulfite in the method does not shift. This saves even more time for analysts as they do not have to reprocess data. Since the electrode is automatically reconditioned after each analysis, results are both reliable and reproducible. Waste from disposable electrodes is reduced, as well as costs incurred by the materials and excess working hours which would generally be spent performing other manual steps. This is truly a win-win situation for food analysis!

Benefits to QC laboratories and beyond

In real terms, this improved method allows for up to 10x the throughput of samples compared to conventional methods. Previously, the contract laboratories involved in this study could measure 5 samples, with 2 analysts per 8-hour shift (15 samples per 24 hours, if you like). With our patent-pending technique, at 10 minutes per sample, including fully automatic regeneration of the electrode surface, this allows for up to 144 samples to be analyzed every day.

Whether you work in the food and beverage industry, wastewater analysis, or in daily analytical laboratory work, you can appreciate the numerous benefits this method offers. Robustness, reproducibility, time savings, cost savings, and a simpler procedure for sample preparation across the board – are you interested? With our expertise in ion chromatography as well as electrochemistry, among other techniques, Metrohm is able to offer such cutting edge methods for the most challenging applications.

Want to learn more?

Download our free Application Note:

Sulfite determination in food and beverages applying amperometric detection

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

Special thanks are given to Miguel Espinosa, Product Manager Ion Chromatography, at Metrohm Hispania (Madrid, Spain) for his assistance in providing the laboratory data for the study.