Select Page
Chemistry of Chocolate

Chemistry of Chocolate

Swiss… Belgian… Pure… Milk…

Here we are in mid-February again, bombarded by chocolate from all sides in preparation for Valentine’s Day on the 14th. Whether in a solid bar, as a chewy truffle, or as a luxurious drink, chocolate has completely infiltrated our lives. Most people can agree that this confectionary treat is fantastic for any occasion – to be given as a gift, to recover after having a bad day, as well as to celebrate a good one – chocolate is certainly meant to be enjoyed.

Even if you don’t like the taste, the chances are high that someone close to you does. So how can you be certain of its quality?

Components of a chocolate bar

For the sake of this article, let us consider the humble chocolate bar, without any extra additions (not to mention any Golden Tickets). This form can be found worldwide in nearly any grocery store or candy shop, generally designated as white, milk, or dark.

All of this variability comes from the edible seeds in the fruit of the cacao tree, which grows in hot, tropical regions around the equator. They must be fermented and then roasted after cleaning. From this, cocoa mass is produced, which is a starting base for several uses. Cocoa butter and cocoa solids are prepared from the cocoa mass and are utilized in products ranging from foods and beverages to personal care items.

As for chocolate bars, these are generally sweetened and modified from the pure form, which is very bitter. Milk (liquid, condensed, or powdered) is added to many types, but does not necessarily have to be present. Varying the content of the cocoa solids and cocoa butter in chocolate to different degrees results in the classifications of dark to white. While some dark chocolates do not contain any milk, white chocolates do to add to the significant amounts of cocoa fat used to produce them.

In general, dark chocolate contains a high ratio of cocoa solids to cocoa butter and may or may not contain any milk. It may be sweetened or unsweetened. Milk chocolate is a much broader category, containing less cocoa solids but not necessarily a different cocoa butter content compared to dark chocolate, as milk fats are also introduced. Milk chocolate is also sweetened, either with sugar or other substitutes. White chocolate contains no cocoa solids at all, but a blend of cocoa butter and milk, along with sweeteners.

Depending on the country, there are different regulations in place regarding the classification of the type of chocolate. If you are interested, you can find a selection of them here.

What makes your favorite chocolate unique?

Of course, more ingredients are added to chocolate bars to affect a number of things like the aroma, texture/mouthfeel, and certainly to enhance the flavor. The origin of the cacao beans, much like coffee, can impart certain characteristics to the resulting chocolate. The manufacturing process also plays a major part in determining e.g., whether the chocolate has a characteristic snap or has a distinct scent, setting it apart from other brands.

In some cases, vegetable fats are used to replace a portion of the cocoa fats, although this may not legally be considered «chocolate» in some countries. The adjustment of long-standing recipes for certain chocolate brands has sometimes led to customer backlash, as quality is perceived to have changed. Truly, chocolate is inextricably tied to our hearts.

Applications for chocolate quality analysis

Nobody wants to give their Valentine a bad gift, especially out-of-date chocolate from a dubious source. Here, we have prepared some interesting analyses for different chocolate quality parameters in the laboratory.

Sugar analysis via Ion Chromatography (IC)

Most types of chocolate contain sugars or sugar substitutes to sweeten the underlying bitterness. Considering different regulations regarding food labeling and also nutritional content, the accurate reporting of sugars is important for manufacturers and consumers alike.

Sugar analysis in chocolate can be performed with Metrohm IC and Pulsed Amperometric Detection (PAD). An example chromatogram of this analysis is given below.

A small amount of commercially produced sweetened milk chocolate was weighed and dissolved into ultrapure water. After further sample preparation using Metrohm Inline Ultrafiltration, the sample (20 µL) was injected on to the Metrosep Carb 2 – 150/4.0 separation column and separated using alkaline eluent. As shown, both lactose and sucrose elute without overlap in less than 20 minutes.

Learn more about Metrohm Inline Ultrafiltration for difficult sample matrices and safeguard your IC system!

In this example, the sugar content was listed on the label as 47 g per 100 g portion (470 g/kg). Lactose was determined to be 94.6 g/kg, and sucrose was measured at 385.6 g/kg. To learn about what other carbohydrates, sweeteners, and more can be determined in chocolate and other foods with Metrohm IC, download our free brochure about Food Analysis and check out the table on page 25!

Lactose content in lactose-free chocolate

The accurate measurement of lactose in lactose-free products, including chocolate, is of special importance to consumers who are lactose-intolerant and suffer from digestive issues after eating it. Foods which are labelled as lactose-free must adhere to guidelines concerning the actual non-zero lactose content. Foodstuff containing less than 0.1 g lactose per100 g (or 100 mL) is most frequently declared as lactose-free.

Determination of lactose in chocolate is possible with IC. Here is an chromatographic overlay of a dissolved chocolate sample with lactose spikes which was analyzed via Metrohm IC using the flexiPAD detection mode.

Milk chocolate, labelled lactose-free measured via Metrohm IC (0.57 ± 0.06 mg/100 g lactose, n = 6).

The sample contained 0.6 mg lactose per 100 g, with measurement of the lactose peak occurring at 13.2 minutes. The black line is the unspiked lactose-free chocolate sample, red and blue are spiked samples of increasing concentration. To prepare the samples, approximately 2.5–5 g chocolate was dissolved in heated ultrapure water, using Carrez reagents to remove excess proteins and fats from the sample matrix. Afterward, centrifugation of the samples was performed, followed by the direct injection of the supernatant (10 µL) into the IC system. Measurement was performed with the Metrosep Carb 2 – 250/4.0 separation column and an alkaline eluent.

Interested in lactose determinations with ion chromatography? Download our free Application Notes on the Metrohm website!

Water determination with Karl Fischer Titration

The amount of water in foods, including chocolate, can affect their shelf life and stability, as well as contribute to other physical and chemical factors. Aside from this, during the processing stage, the amount of water present affects the flow characteristics of the chocolate mass.

AOAC Official Method 977.10 lists Karl Fischer titration as the accepted analysis method for moisture in cacao products.

The determination of moisture in different chocolate products is exhibited in the following downloadable poster. As an example, several samples (n = 10) of dark chocolate (45% cocoa content) were analyzed for their moisture content with Metrohm Karl Fischer titration.  Results were found to be 0.96% water with a relative standard deviation (RSD) of 2.73%. More information about this analysis can be found in our poster about automated water determination in chocolate, or in chapter 11.6 of our comprehensive Monograph about Karl Fischer titration.

Oxidation stability with the Rancimat test

Oxidation stability is an important quality criterion of chocolate as it provides information about the long-term stability of the product. Cocoa contains various flavonoids that act as antioxidants. Although the flavonoid content may vary amongst chocolate type, in general, the greater the content of cocoa solids in the chocolate, the greater its antioxidant effect.

The 892 Professional Rancimat from Metrohm determines the oxidation stability of fat-containing foods and cosmetics. The Rancimat method accelerates the aging process of the sample and measures the induction time or oxidation stability index (OSI).

Chocolate cannot be measured directly with the classical Rancimat method, as no evaluable induction time is obtained. There are many reasons for this: e.g., the fat content is too low. Traditionally, extraction of the fat from the chocolate is necessary, but not always.

Learn more about the Rancimat method on our website, and download our free Application Note about the oxidation stability of chocolate. In this Application Note, the oxidation stability of white, milk and dark chocolate is determined without extraction.

Cadmium in chocolate by Voltammetric analysis

The toxic element cadmium (Cd) can be found in elevated concentrations with high bioavailability in some soils. Under such conditions, cacao trees can accumulate cadmium in the beans. Chocolate produced from the affected beans will contain elevated cadmium levels.

Typical limit values for Cd in chocolate in the European Union are between 100 µg/kg and 800 µg/kg (EU Commission Regulation 1881/2006) depending on the cocoa content of the chocolate. Anodic stripping voltammetry (ASV) can be used to accurately determine trace quantities of cadmium in chocolate down to approximately 10 µg/kg. The method is simple to perform, specific, and free of interferences.

Chocolate samples are first mineralized by dry ashing in a furnace at 450 °C for several hours. The remaining ash is then dissolved in an acidified matrix. The cadmium determination is then carried out on the 884 Professional VA instrument from Metrohm. To learn more about how to perform the analysis, download our free Application Note.

Happy Valentine’s Day from us all at Metrohm!

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

«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.

Trace metal analysis with solid-state electrodes – Part 5

Trace metal analysis with solid-state electrodes – Part 5

In the last part of our series of articles about trace metal analysis using solid-state electrodes, we will have a look at the glassy carbon rotating disc electrode (GC RDE) and its application possibilities.

Did you read the other parts in this series? Find them here!

The Glassy Carbon Rotating Disc Electrode

A rotating disc electrode (RDE) consists of two parts: the electrode tip which is made available in different materials, and a driving axle. The electrode tip is simply screwed onto the axle (Figure 1) to assemble the complete working electrode.

Figure 1. The two parts which make up the RDE. Left: driving axle for RDE. Right: glassy carbon electrode tip, with shaft made of glass.

Glassy carbon (GC) has a long history as solid electrode material for trace metal analysis. In general, GC is carbon with an amorphous structure which is similar to glass or ceramics, but different from graphite or diamond which both have a crystalline structure.

Aside from properties including a high temperature stability and a hardness similar to quartz, glassy carbon is very chemically inert and has a low electrical resistance, making it a versatile electrode material.

In the Metrohm GC electrode tip (Figure 1), the glassy carbon rod is fused within a glass shaft—another inert material. This design creates an electrode tip that is inert against most chemicals and solvents and guarantees measurements with excellent reproducibility due to the seamless intersection between the electrode material and glass shaft.

Modification with a metal film

For trace metal applications, the GC electrode is modified with a metal film, usually mercury or bismuth. The film is plated ex-situ from an acid plating solution which contains about 20 mg/L Hg2+ or Bi3+. Such a solution can easily be prepared from commercially available metal standard solutions and can be used for the plating of several films.

Once the film is deposited on the glassy carbon electrode, multiple determinations can be carried out with the same film. When the performance deteriorates, the exhausted film is simply wiped off and a fresh film is plated. Since only the renewable film is affected by aging processes, the GC electrode itself can be used for a very long time.

Applications using glassy carbon electrodes exhibit excellent reproducibility and stability in combination with very low detection limits.

Figure 2. Glassy carbon rotating disc electrode in a 884 Professional VA instrument from Metrohm.

Applications

Cadmium and lead determinations

The risk of cadmium and lead poisoning from drinking water and the significance of the determination of these two elements has already been discussed in previous posts in this series. To monitor the guideline values of 3 µg/L for cadmium and 10 µg/L for lead, recommended by the WHO (World Health Organization), a detection limit of β(Cd) = 0.3 µg/L and β(Pb) = 1 µg/L would be sufficient.

With the glassy carbon electrode the determination is far more sensitive, featuring a ten-fold improvement on the limit of detection of β(Cd) = 0.02 µg/L and β(Pb) = 0.05 µg/L with a deposition time of 30 s. This limit can be lowered even more with an increased deposition time.

For this extremely sensitive determination, a mercury film is plated on the glassy carbon electrode. The determination of cadmium and lead is carried out by anodic stripping voltammetry (ASV).

To learn more about this application, please check our website.

Free Application Note download: AN-V-225 Cadmium and lead in drinking water – Simultaneous determination on a mercury film modified glassy carbon electrode.

The very low detection limit makes this application especially interesting when it is not only required to monitor limit values but to actually detect concentrations in the ppt (parts per trillion, ng/L) range, e.g. in environmental analysis such as for seawater research.

Nickel and cobalt measurements

Another application with very low detection limits using the GC electrode is the determination of nickel and cobalt. This electrode allows the detection of concentrations down to β(Ni) = 0.05 µg/L and β(Co) = 0.03 µg/L. For this application, the electrode is modified with a bismuth film. The determination of nickel and cobalt is carried out by adsorptive stripping voltammetry (AdSV) using the complexing agent DMG (dimethylglyoxime).

Figure 3. Determination of β(Ni) = 0.34 µg/L and β(Co) < LOD in tap water (30 s deposition time) using the GC RDE.

For decades, this method was successfully executed with the mercury drop electrode. The use of a bismuth film on a glassy carbon electrode offers a non-toxic alternative with a similar sensitivity as the established method. Besides the high sensitivity, this application also shows excellent repeatability.

20 consecutive determinations of β(Ni) = 0.5 µg/L and β(Co) = 0.5 µg/L, carried out on the same bismuth film, showed an average recovery of 105% for nickel, with a relative standard deviation (RSD) of 2.0%. The recovery for cobalt was 112% with a RSD of 3.3%. This makes this method a viable tool in environmental analysis when natural background concentrations, which are often in the ppt (ng/L) range, should be investigated.

For further details about this application, please refer to Application Note AN-V-224: Nickel and cobalt in drinking water – Simultaneous determination in low ng/L range on the GC RDE modified with a bismuth film.

Chromium(VI) monitoring

Legal limits for chromium are relatively high. For example, the guideline value of the World Health Organization (WHO) is 50 µg/L for drinking water. These values usually refer to the total chromium concentration, but there are significant differences in toxicity between Cr(III) and Cr(VI). Even miniscule doses of Cr(VI) are toxic as well as carcinogenic.

Since the beginning of this century, there have been ongoing discussions in the scientific community about whether an additional limit value only for Cr(VI) is required, and what this value should be.

Measuring techniques are needed which allow the determination of Cr(VI) in the ng/L range. Using the glassy carbon electrode modified with a mercury film it is possible to detect Cr(VI) concentrations down to 0.05 µg/L. Cr(VI) is determined by adsorptive stripping voltammetry (AdSV) with DTPA (diethylenetriaminepentaacetic acid) as complexing agent. The recovery of a concentration of β(Cr(VI)) = 0.1 µg/L is 111% with a relative standard deviation of 4.4% (triplicate determination).

If you are interested to learn more, download our free Application Note V-277: Chromium(VI) in drinking water – Ultra-sensitive determination on the mercury film modified glassy carbon electrode (DTPA method).

All the above-mentioned applications can be carried out manually with a 884 Professional VA system (Figure 4), but it is also possible to run small sample series with an automated setup.

Figure 4. 884 Professional VA with two 800 Dosinos for automatic addition of electrolyte and standard solution.

Summary

This was the last post in our five-part series on heavy metal analysis with solid state electrodes. If this or one of the previous posts sparked your interest in one of the applications, do not hesitate to contact your local Metrohm representative.

For a complete overview of the different applications that can be performed with the SSEs exhibited in this series, check out the table below. Click on each application note or bulletin for a free download! 

Overview: Applications with Metrohm SSEs
Element Electrode Application Document Lab Portable
Ag GC RDE Application Bulletin 207

As scTRACE Gold Application Note V-210
Application Note V-211

Bi scTRACE Gold Application Note V-218

Cd, Pb GC RDE (Hg film) Application Note V-225

Cd, Pb SPE (Hg film) Application Note V-231

Cd, Pb Bi drop Application Note V-221

Cr(VI) GC RDE (Hg film) Application Note V-227

Cr(VI) scTRACE Gold (Hg film) Application Note V-230

Cu scTRACE Gold Application Note V-213

Fe scTRACE Gold Application Note V-216

Fe Bi drop Application Note V-222

Hg scTRACE Gold Application Note V-212

Ni, Co scTRACE Gold (Bi film) Application Note V-217

Ni, Co GC RDE (Bi film) Application Note V-224

Ni, Co SPE (Bi film) Application Note V-232

Ni, Co Bi drop Application Note V-223

Pb scTRACE Gold (Ag film) Application Note V-214

Sb(III) scTRACE Gold Application Note V-229

Se(IV) scTRACE Gold Application Note V-233

Te(IV) scTRACE Gold Application Note V-234

Tl scTRACE Gold (Ag film) Application Note V-228

Zn scTRACE Gold Application Note V-215

Post written by Barbara ZumbrägelProduct Manager VA/CVS at Metrohm International Headquarters, Herisau, Switzerland.

History of Metrohm IC – Part 4

History of Metrohm IC – Part 4

In the fourth installment in our series about the history of ion chromatography development at Metrohm over the years, we now focus on some of our combined analysis techniquesThese are unique combinations of Metrohm instruments from different product groups which broaden application possibilities beyond what IC could accomplish alone. Here we will discuss the TitrIC and VoltIC systems.

Did you miss the other parts in this series? Find them here!

TitrIC: combining titration, direct measurement, and IC in one

The combination of titration, pH, and conductivity measurement, as well as ion chromatography in a single analytical system was introduced in the late 1990s. This TitrIC system consisted of a Titrino for the determination of the p- and m-value (alkalinity), pH and conductivity meters, as well as a modular IC from the 732 IC Detector generation.

Figure 1. The first-generation TitrIC system from the late 1990s, including an autosampler for higher sample throughput.

Depending on the analysis requirements, either a second Titrino (for the Ca/Mg water hardness determination) or a second IC channel (for alkali and alkaline earth metal cations as well as ammonium) was selected. The latter enabled an automatic determination of the ion balance for the first time.

The central unit is the autosampler, which directs the water sample to the titration/metering section and to the ion chromatographs. The respective titration software controls the titrators and pH/temperature meters, calculates the results, and transfers them to the IC software (at the time, IC Net). The IC software controls the ion chromatograph and creates the final report. The user only communicates with the master software, making comprehensive analysis a simple task. This combination was available in all Metrohm IC generations (Figures 1 and 2). The most current setup (TitrIC Flex I and II) combines the 930 Compact IC Flex instruments with OMNIS titration.

Figure 2. TitrIC has evolved: TitrIC 7 system with 850 Professional IC series and Robotic Titrosampler has now become the TitrIC Flex (I and II), based on OMNIS titration and the 930 Compact IC Flex series.

Do you want to learn more about the combination of titration, direct measurement, and IC analysis and what it can do for your laboratory efficiency? Take a look at our previous blog post, download a free application note, or check out our brochure full of important information.

VoltIC – Voltammetry and IC

Just as with titration, it also is possible to combine IC and voltammetry in order to further expand application capabilities for enhanced trace measurement of analytes in water. VoltIC Professional integrates these techniques in a single, fully automatic system that is controlled via a common software, providing users with the strengths of both IC and voltammetry. Anions, major cations as well as trace cations may be detected in one determination. 

Back in 2012, the original VoltIC pro system was launched to bridge the gap between these two techniques and to offer a unique solution to the market. In 2013, the new line of 940 Professional IC Vario instruments came along. A few years later in 2016, the 884 Professional VA analyzer was brought to the market, and thus VoltIC on the next level: the current VoltIC Professional system (Figure 3).

Figure 3. VoltIC Professional system developed in 2016 for the enhanced analysis of trace impurities in water samples.

Is VoltIC the right choice for your laboratory? We offer free application notes for download, or check out the Metrohm website and download our brochure!

Next topic:

In Part 5 of our series, we describe the use of ion chromatography for continuous air monitoring with the MARGA from Metrohm Process Analytics.

Download our free monograph:

Practical Ion Chromatography – An Introduction

Post written by Dr. Markus Läubli, Manager Marketing Support IC at Metrohm International Headquarters, Herisau, Switzerland.

Trace metal analysis with solid-state electrodes – Part 4

Trace metal analysis with solid-state electrodes – Part 4

In this series of articles featuring various solid state electrodes, we have introduced the new Bi drop electrode and the scTRACE Gold electrode and their potential in the determination of heavy metals in drinking water. In Part 4, we introduce the next type of sensors available for heavy metal analysis: the screen-printed electrode (SPE) together with two applications for the simultaneous determination of cadmium and lead as well as nickel and cobalt.

Catch up on the series «Trace metal analysis with solid-state electrodes» here:

Screen-printed electrodes (SPEs)

Screen printing microfabrication technology is well established for the manufacture of thick film electrochemical sensors. This technology enables the mass production of reproducible and mechanically robust solid electrodes. The possibility of mass production has great impact on the price and makes the SPEs inexpensive and convenient for the determination of heavy metals.

Printing technology allows production of a maintenance-free reference electrode, making the preparation of the analytical system for the analysis of heavy metals faster and more straightforward. Here, you do not need to refill the reference electrode or clean the sensor after a finished determination. The integration of three electrodes (working, reference, and auxiliary – see Figure 1) on a single platform, with the simultaneous miniaturization of their size and the corresponding device, both supports and facilitates transportation of the equipment to the sampling point and the determination of heavy metals on-site.

Figure 1. Screen-printed electrode (SPE) from Metrohm DropSens.

The simplest and the fastest way to modify the properties of these screen-printed electrodes for heavy metal detection is to deposit (in situ or ex situ) a metal film (either bismuth or mercury) electrochemically on the working electrode. This approach allows flexible use of single sensor type (e.g. carbon) SPEs for a wide range of applications.

Mercury film modified SPEs

The mercury film on the carbon working electrode consists of a very thin layer of mercury adsorbed onto the electrode surface. The mechanisms of accumulation and the stripping are the same as those which occur at a conventional mercury drop electrode.

Bismuth film modified SPEs

Because bismuth is not toxic, there are two environmental friendly methods for the preparation of the bismuth film: in situ plating and ex situ plating. With ex situ plating, a bismuth film is prepared in a separate solution before the first determination. Then the modified electrode is rinsed with ultrapure water and can be further used for the analysis.

Screen-printed electrodes can only be used for a limited number of measurements and have to be replaced at regular intervals. The lifetime and the frequency of replacement depend on the type of electrode and the application.

Figure 2. 946 Portable VA Analyzer (SPE version).

For further information about the 946 Portable VA Analyzer, visit the Metrohm website!

Currently, screen printed electrodes allow the on-site, simultaneous determination of cadmium and lead. Therefore, below I will present a method for the cadmium and lead determination using the ex situ modification of the Metrohm DropSens 11L carbon screen printed electrodes with a mercury film.

In addition to the cadmium and lead determination, a method using an ex situ bismuth film for nickel and cobalt will be introduced. Both measurements (cadmium and lead, as well as nickel and cobalt) can be carried out with the 946 Portable VA Analyzer (version for screen printed electrodes (SPE), see Figure 2) or with any Metrohm VA Stand using the electrode shaft (see Figure 3).

Figure 3. Electrode shaft for screen-printed electrodes (SPE).

With the electrode shaft, you can now use screen-printed electrodes in any Metrohm VA Stand. Whether you are performing voltammetric trace analysis or you want to explore new application fields with your own modified sensor, the electrode shaft allows simple use of SPEs.

To download our free informative flyer, click the button below!

This leaflet explains even more about the handling and troubleshooting of the electrode shaft for screen-printed electrodes.

Applications

Anodic stripping voltammetric determination of cadmium and lead

Cadmium and lead are toxic elements, and their concentration in drinking water has to be monitored. The provisional guideline values in the World Health Organization’s «Guidelines for Drinking-water Quality» are set to a maximum concentration of 3 µg/L for cadmium and 10 µg/L for lead. The Metrohm DropSens 11L carbon screen printed electrode modified with an ex situ mercury film allows the simultaneous determination of cadmium and lead in drinking water samples (Figure 4).

Figure 4. Example for determination of cadmium and lead in tap water spiked with β(Cd) = 2 µg/L and β(Pb) = 2 µg/L.

Using the 946 Portable VA Analyzer with a 90 s deposition time, a limit of detection (LOD) of 0.3 µg/L for both elements can be achieved. This is more than sufficient to monitor the provisional WHO guideline values.

For more information about this application, download our free application note:

The relative standard deviation for 5 measurements in a check standard solution with β(Cd) = 2 µg/L and β(Pb) = 2 µg/L is 14% and 12%, and the recovery rate is 88% and 82% for cadmium and lead, respectively.

Adsorptive stripping voltammetric determination of nickel and cobalt

Nickel and cobalt can be released either directly, or via the wastewater–river pathway, into drinking water systems. In the EU, the legislation specifies 20 µg/L as the limit value for the nickel concentration in drinking water.

The simultaneous and straightforward determination of nickel and cobalt is based on adsorptive stripping voltammetry (AdSV) using dimethylglyoxime (DMG) as a complexing agent. Prior to the first determination, the Metrohm DropSens 11L carbon SPE has to be modified with an ex situ bismuth film. The unique properties of the non-toxic Bi film combined with AdSV results in an excellent performance in terms of sensitivity. When using the 946 Portable VA Analyzer the limit of detection for 30 s deposition time is approximately 0.4 µg/L for nickel and 0.2 µg/L for cobalt, and can be lowered further by increasing the deposition time.

The disposable sensor does not need maintenance such as mechanical polishing or mechanical cleaning. This method is best suited for manual systems.

Figure 5. Example determination of nickel and cobalt in tap water with the modified Metrohm DropSens 11L carbon SPE.

The relative standard deviation for 3 subsequent measurements in a check standard solution (β(Ni) = 2 µg/L β(Co) = 2 µg/L) is 7% and 8% respectively, and the recovery rate is 100% for nickel and 94% for cobalt.

For more information about this application, download our free application note:

Key features of the screen-printed electrodes

  • Mechanically robust, inexpensive, maintenance-free, miniaturized solid electrodes
  • Electrode holder fits all Metrohm VA Stands
    • 884 Professional VA, 797 VA Computrace, 663 VA Stand
  • Solution-proof electrode holder
  • Fast and easy exchange of SPEs
  • Simultaneous determination of Ni and Co, as well as Cd and Pb
  • Limit of detection in low μg/L and even high ng/L range
  • Suitable for on-site diagnostics

What’s next?

In the next part of this series on solid state electrodes, we will have a look at an ultra-sensitive and robust sensor for heavy metal detection: the glassy carbon electrode with some of its associated applications.

Post written by Dr. Jakub TymoczkoApplication Specialist VA/CVS at Metrohm International Headquarters, Herisau, Switzerland.