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Trace metal analysis with solid-state electrodes – Part 3

Trace metal analysis with solid-state electrodes – Part 3

In Part 2 of this series on trace metal analysis with solid-state electrodes, we introduced the scTRACE Gold electrode. The third part of this series explains even more applications which can be performed with this electrode, but this time after modifying the gold micro-wire with a thin layer of another metal.

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

Why modify the electrode material?

As explained for the Bi drop electrode in Part 1 of this series, stripping voltammetry is a two-step measurement.

In the first step, the analyte is deposited on the working electrode. In the case of anodic stripping voltammetry (ASV), the analyte is reduced and forms an alloy with the electrode material (Fig. 1). In the case of adsorptive stripping voltammetry (AdSV), the analyte forms a complex which is adsorbed to the working electrode.

In the subsequent stripping step, the deposit is brought back into solution, giving the analytical signal which is proportional to the deposited amount of analyte. In the case of ASV, the electrochemical reaction is the re-oxidation of the analyte during an anodic scan (Fig. 2). In the case of AdSV, the adsorbed metal complex is reduced during a cathodic scan.

Figure 1. Anodic stripping voltammetry (ASV) with Ag film modified scTrace Gold electrode – deposition of lead (solution stirred).

Figure 2. Anodic stripping voltammetry (ASV) with Ag film modified scTrace Gold electrode – stripping of lead (solution not stirred).

Both steps, deposition as well as stripping, are subject to the principles of kinetics and thermodynamics. Without going into detail, the result is simply that some analytes cannot be determined with certain electrode materials. One way to solve this problem is to modify an existing working electrode with a different material that is more suitable.

Applications

Lead in drinking water

Most of the lead which is present in surface and ground water is of anthropogenic origin, resulting from the leaching of contaminated soils. Lead in tap water, however, often originates from the household plumbing system. Pipes from lead metal were popular in some countries until the 1970s. Although Pb is barely soluble in water, it slowly dissolves in the presence of oxygen. As a result, the allowed limit for lead in tap water can be easily exceeded by a significant amount. Now lead pipes for municipal water transport are forbidden, but there are still houses with old installations intact. The WHO (World Health Organization) recommends a limit value for lead in drinking water of 10 µg/L. In the European Union, the upcoming limit is as low as 5 µg/L.

For the determination of lead, the scTRACE Gold electrode is modified with a silver film. The film is plated ex situ from a separate plating solution. Once plated, it can be used for multiple determinations. When the film is depleted, it can be removed, and then a fresh film is plated again. A side effect of the silver film is that the scTRACE Gold electrode lasts longer, since aging processes mainly affect the renewable silver film. In a repeatability study, determining 10 µg/L Pb with 3 different electrodes on 4 different days (total number of determinations = 10) the average recovery of Pb was 96% with a relative standard deviation of 5%.

Using the 884 Professional VA it is possible to measure lead concentrations in water down to 0.4 µg/L, allowing a simple and reliable determination of even the future limit in the European Union.

With the 946 Portable VA Analyzer, the limit of detection is only slightly higher: 0.6 µg/L. However, the mobile use offers the possibility for close monitoring of individual installations without the need to preserve samples and send them in to a central lab. Furthermore, the concerned resident gets an immediate result on the spot.

Free Application Note download: AN-V-214 Lead in drinking water – Straightforward determination by voltammetry using a gold microwire electrode.

If you want to learn more about our voltammetry product lines, lab as well as mobile, check out our website!

Nickel and cobalt in drinking water

Similar to lead, nickel concentrations present in water sources can be increased by human influence as well. Plumbing fixtures and faucets are often plated with a thin layer of nickel for protection against corrosion, even if the finish is made of chromium. Furthermore, nickel is part of many alloys from stainless steel to nickel brasses and bronze. Nickel steel alloy cookware or nickel pigmented dishes can also cause increased nickel levels. The maximum allowed level in drinking water in the European Union is 20 µg/L whereas the WHO recommends a limit of 70 µg/L.

For the voltammetric determination of nickel and cobalt, an ex situ plated bismuth film on the scTRACE Gold electrode is used as working electrode. Nickel as well as cobalt are determined in the form of their DMG (dimethylglyoxime) complex. This method had already proven its reliability with the mercury electrode, and therefore this application can now be transferred to a mercury-free electrode. With a detection limit of 1 µg/L with the 946 Portable Analyzer, and even lower at 0.2 µg/L with the 884 Professional VA lab instrument, the method is surely sufficient to monitor the compliance with legal requirements. The recovery for 1 µg/L Ni in a standard solution is about 99% (mean of 10 determinations) with a relative standard deviation of 5%.

Free Application Note download: AN-V-217 Nickel, cobalt in drinking water – Straightforward determination by voltammetry using a gold microwire electrode.

Chromium(VI) in drinking water

The problem of chromium(VI) in drinking water was brought to the attention of the general public with the movie «Erin Brockovich» in 2000, starring Julia Roberts. The plot is based on a true story, which happened in the small community of Hinkley, California, where the local energy provider contaminated the groundwater with the carcinogenic hexavalent chromium. The company attempted to cover up the incident, but an increased number of tumors and other health problems among the residents could finally be traced back to the contaminated drinking water.

Contamination with Cr(VI) in the environment is usually the result of improper handling of various industrial processes, especially abandoned waste dumped from galvanic chromium plating. The WHO recommends a maximum limit of 50 µg/L total chromium for drinking water

After modifying the scTRACE Gold electrode with an ex situ plated mercury film, Chromium(VI) can be determined as a complex with DTPA (diethylenetriaminepentaacetic acid). The recovery of a standard containing 30 µg/L Cr(VI) is 115% (mean of 3 determinations) with a relative standard deviation of 2%. Using the 946 Portable VA Analyzer it possible to determine concentrations down to 2 µg/L Cr(VI), allowing the on-site determination, providing immediate results without delay.

Free Application Note download: AN-V-230 Chromium(VI) in drinking water – Sensitive determination on the mercury film modified scTRACE Gold (DTPA method).

Summary

Talking about all of the applications that are possible with the scTRACE Gold electrode would go beyond the scope of this blog. The table here gives an overview of several elements for which methods with the scTRACE Gold are currently available from Metrohm. Your local Metrohm representative can assist in case of questions regarding the determination of one of the elements, or analysis in a specific matrix.

Figure 3. The scTRACE Gold electrode from Metrohm is suitable for trace analysis of several elements in water.

Overview: Applications with the scTRACE Gold
Element Application document
As(total) Application Note V-210
As(III) Application Note V-211
Hg Application Note V-212
Cu Application Note V-213
Pb Application Note V-214
Zn Application Note V-215
Tl Application Note V-228
Fe Application Note V-216
Ni, Co Application Note V-217
Bi Application Note V-218
Sb(III) Application Note V-229
Cr(VI) Application Note V-230

What’s next?

In Part 4 of this series, I will discuss the use of screen-printed electrodes (SPEs) which had already been introduced in the blog post «Virus detection using screen-printed electrodes». However, this time the focus will be on the determination of heavy metals using these disposable electrodes.

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

Trace metal analysis with solid-state electrodes – Part 3

Trace metal analysis with solid-state electrodes – Part 2

In the second part of our series on «heavy metal» analysis with solid-state electrodes, the focus lies on the scTRACE Gold electrode. Gold electrodes have been used in electrochemistry for decades. However, the scTRACE Gold has a very special design. Originally developed to improve the voltammetric determination of arsenic, the electrode has also proven to be suitable for the determination of a number of other elements, such as copper, iron, lead, and even the toxic chromium(VI).

How does it work?

The working electrode is a gold micro-wire (Fig. 1), which is thinner than a human hair. This special form of electrode leads to a very short initial preparation time. Different from other gold electrodes, the scTRACE Gold is ready for use within a few minutes.

Another advantage of this electrode is that it comes with the reference and the auxiliary electrode printed on the rear side of the sensor (Fig. 2). That does not only save on costs for the two additional electrodes required in a voltammetric system, it also makes maintenance for the reference electrode obsolete.

Figure 1. Close-up view of the gold micro-wire working electrode on the scTRACE Gold.

Figure 2. Close-up view of the reference and auxiliary electrode on the rear side of the scTRACE Gold.

Applications

The high level of sensitivity and a straightforward setup makes voltammetry a valuable tool in drinking water analysis.

Availability of clean drinking water is one of the major concerns of the 21st century. Besides microbiological contaminations, such as bacteria and viruses, the presence of heavy metals in drinking water can be a health risk. The first step in providing clean water is to identify contaminants, since health-threatening concentrations of heavy metals are not visible. For many heavy metals, limit values in drinking water are specified by authorities like the U.S. Environmental Protection Agency (EPA) or the European Commission. Water quality laboratories often use ICP (inductively coupled plasma) to monitor metal concentrations in drinking water.

Voltammetry is one of the few analysis techniques which offers a comparable sensitivity. Needing only basic infrastructure and low running costs, voltammetry is a viable alternative to monitor some key elements. In the following sections, some selected application examples are shown which demonstrate the capabilities of the scTrace Gold electrode in water analysis.

Arsenic in drinking water

Arsenic gained some global notoriety when water wells were built in Bangladesh to avoid diseases caused by microbiological contaminations in the surface water. Rather than suffering from cholera or hepatitis, people were instead afflicted with chronic arsenic poisoning.

It would be somewhat careless to believe that arsenic is only a problem in less developed countries. Actually, arsenic can be found nearly everywhere in the earth’s crust. Though as Paracelsus already knew, «the concentration makes the poison».

Therefore, the crucial question is how much of this arsenic finds its way into the water table. The WHO (World Health Organization) recommends a concentration of 10 µg/L arsenic as the maximum level in water that is intended for human consumption. This figure is also the legal limit in many countries.

Want to know more about arsenic? Then download our free article: «Arsenic – The multipurpose poison».

The voltammetric determination of the limit value of 10 µg/L shows a recovery of about 92% (n = 10 determinations) with a relative standard deviation of 6.5%. With a detection limit of 1 µg/L, which is one tenth of the legal limit, voltammetry using the scTRACE Gold electrode offers a reliable and cost-effective way of monitoring the arsenic content of drinking water.

For more information, download our free application note AN-V-210: Total arsenic in mineral water – Straightforward determination by voltammetry on a gold microwire electrode.

The scTRACE Gold electrode can be used with the 884 Professional VA as well as with the 946 Portable VA Analyzer. The 884 Professional VA is designed for laboratory use. The system is extremely flexible and can be adapted to user requirements. The modular setup also allows a later extension of the instrument from manual to fully automated.

As the name says, the 946 Portable VA Analyzer is intended for mobile use. It allows for on-site determination, directly at the sample source.

Copper in surface water

Under normal circumstances, copper in drinking water is not a problem. The legal limits are comparably high, WHO recommends a maximum concentration of 2 mg/L. An example from the field illustrates where the determination of copper in water can nevertheless be advisable.

The production of distilled alcoholic beverages (e.g., gin, whiskey, brandy, schnapps) involves single or multiple distillation of the raw material, which is done in copper stills. Cleaning out the copper apparatus and draining the rinsing water to a river can contaminate the environment with copper.

Although regulatory limits in effluents are usually higher than in drinking water, the copper limits can still be exceeded if the water is not treated properly prior to discharge. Since pollution from this cleaning is not a continuous process but only occurs periodically, it is difficult to detect and even harder to confirm, especially in less accessible areas.

Here, mobile voltammetry using the scTRACE Gold with the 946 Portable VA Analyzer can make a valuable contribution to the protection of the environment due to the reliable determination of low concentrations of copper.

For a concentration of 5 µg/L, the mean recovery of 10 determinations is approximately 107%, with a relative standard deviation of 2%. Concentrations down to 0.5 µg/L copper in the water can be determined directly at the point of sampling. This allows immediate re-sampling in case of suspicious results, and can furthermore help to locate the source of the pollution. In this way, chances increase to identify the source and hold the responsible entity accountable.

For more information, download our free application note AN-V-213: Copper in drinking water – Straightforward determination by voltammetry using a gold microwire electrode.

Iron in water

According to WHO, iron does not pose a health concern in levels typically found in drinking water. In contrast, it is an essential element for human nutrition. Nevertheless, many countries specify a maximum contaminant level between 200 µg/L and 300 µg/L.

The reason is simply that higher concentrations have a negative effect on the taste of the water, and will stain laundry and sanitary appliances. 

With a detection limit of 10 µg/L, the voltammetric determination of iron offers a straightforward method for monitoring the iron concentration of the water supply. The recovery of a voltammetric determination of 20 µg/L Fe is in the range of 91% (n = 10 determinations) with a relative standard deviation of 1%.

For more information, download our free application note AN-V-216: Iron in drinking water – Straightforward determination by voltammetry using a gold microwire electrode (DHN method).

What’s next?

In part two of this series, I introduced the scTRACE Gold electrode which I will also continue to discuss in Part 3. In the next installment, I will focus on applications which are carried out after electrochemical modification of the gold micro-wire.

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

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).

Trace metal analysis with solid-state electrodes – Part 1

Trace metal analysis with solid-state electrodes – Part 1

This new series of blog posts covers a range of new sensors suitable for the determination of «heavy metals» using voltammetric methods.

The quantification of heavy metal ions plays an important role in many applications, including environmental monitoring, waste management, research studies, or even in clinical tests. Heavy metals occur naturally, but the rise of industrialization and urbanization in the past two centuries are responsible for increased levels in our environment. These dangerous elements are released and accumulate in the soil, and in ground or surface water. They enter the food chain directly from drinking water or through bioaccumulation in plants and animals. It is for this reason that pregnant women are discouraged from eating seafood, on the basis of mercury (Hg) accumulation through the food chain.

The degree of toxicity depends on the type of metal, its biological role, and most importantly, its concentration. Increased concentrations of lead, iron, cadmium, copper, arsenic, chromium, or nickel in drinking water are most often responsible for human poisoning. To highlight the toxicity of certain heavy metals in drinking water and to protect human health, guideline values or limit values for the heavy metal concentration in drinking water have been set by international organizations as the World Health Organization (WHO) or by such authorities as the U.S. Environmental Protection Agency (EPA) or the European Commission.

Several techniques have been developed for heavy metal ion analysis in the past. Commonly used techniques include atomic absorption spectrometry (AAS), inductively coupled plasma (ICP), or fluorescence spectrometry. However, these techniques require expensive equipment combined with high maintenance costs and trained personnel. Therefore, a price-effective, straightforward and sensitive method that allows detection of metal ions in water samples is highly desired.

Stripping voltammetry is the right solution for these challenges providing a simple, rapid, and cost-effective alternative for the aforementioned techniques that is also suited for untrained personnel. In addition, detection limits in the ng/L range and the possibility to determine the trace levels of heavy metals in the field make it so interesting and valuable.

The principle of stripping voltammetry

Voltammetric determination of heavy metals consists of two steps. In the first step, the analyte is preconcentrated on the surface of the working electrode as shown using the example of anodic stripping voltammetric determination of lead (Pb) in Figure 1.

Figure 1. Anodic stripping voltammetry – deposition of lead (solution stirred).

In the subsequent stripping step (Figure 2), the analyte is released. This can be achieved by oxidation or reduction depending on the method used for the determination. This step generates the analytical signal, which has to be proportional to the deposited amount of analyte.

Figure 2. Anodic stripping voltammetry – stripping of lead (solution not stirred).

Besides anodic stripping voltammetry, cathodic stripping voltammetry or adsorptive stripping voltammetry are also possible to utilize and work in a similar manner. All of these methods have something in common: every voltammetric determination is as good as the sensor used for the measurement. Therefore, in this series of posts we want to introduce our powerful sensors and demonstrate the outstanding performance with a few typical applications.

Need for new sensors

The need for heavy metal ion determinations in the field, sensor costs, and environmental issues are the main triggers for research on new sensors in voltammetry. Non-toxic and inexpensive materials are preferred for new sensors. The properties of these materials, however, can lead to some restrictions. First is the limited number of elements that can be detected on a particular electrode material (e.g., gold, carbon or bismuth). In addition, it is difficult to determine several elements simultaneously at the same mercury-free sensor. The choice of the most suitable electrode material in combination with the optimum sensor design helps to overcome these issues.

Bismuth as an alternative electrode material

In the past, there were many attempts to find less toxic electrode materials than mercury for the determination of heavy metal ions, but none have achieved exceptional electroanalytical performance. Twenty years ago (2000), an American researcher by the name of Joseph Wang reported a bismuth film electrode for the first time (Joseph Wang, 2000).

Figure 3. Bismuth crystal.

After this initial revolutionary report, bismuth-based electrodes prepared as in-situ and ex-situ films on solid-state electrodes such as carbon, have been growing in popularity. The broad electrochemical window and low toxicity of bismuth were key factors. In addition, bismuth is able to form alloys with quite a high number of heavy metals and it exhibits high hydrogen overpotential, similar to mercury. These properties are particularly interesting for stripping voltammetry. The hydrogen evolution is suppressed very efficiently with the consequence that noise-free measurements at negative potentials can be carried out. Bismuth electrodes based on bismuth films are a good option. However, film deposition is an additional step that is time-consuming.

New sensor in VA: the Bi drop electrode

With the Bi drop electrode, a novel solid-state electrode is now available for the determination of heavy metal ions in drinking water. A bismuth drop of approximately 2 mm diameter serves as the working electrode within the voltammetric measurement.

The electrode works without the need for polishing or film deposition—only electrochemical activation is required. This significantly shortens the entire analysis time. Once activated, series of heavy metal determinations with high repeatability in the low μg/L and even ng/L range are possible.

The Bi drop electrode allows for mercury-free monitoring of the limit values of the heavy metals cadmium, lead, nickel, cobalt, and iron in drinking water. Since the electrode does not require mechanical treatment, it is especially suitable for online applications. Another advantage of the Bi drop electrode is fact that cadmium and lead as well as nickel and cobalt can be determined simultaneously.

The sensor is cost-efficient, stable, extremely sensitive, and is able to deliver more reproducible results than other previously examined bismuth-based electrodes. To demonstrate the broad possibilities and flexibility of the Bi drop electrode, examples for anodic stripping voltammetry, adsorptive stripping voltammetry, and direct voltammetric determination will be presented and discussed.

Applications

Anodic stripping voltammetric determination of cadmium and lead

To reduce the toxic effects of cadmium on the kidneys, skeleton, and respiratory system, as well as the neurotoxic effects of lead, 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.

Figure 5. Example for determination of cadmium and lead in tap water spiked with β(Cd) = 2 µg/L and β(Pb) = 2 µg/L.

A completely mercury‑free sensor, the Bi drop electrode allows the simultaneous determination of cadmium and lead in drinking water without any additional film plating step. With a 60 s deposition time, a limit of detection (LOD) of 0.1 µg/L for cadmium and 0.5 µg/L for lead can be achieved. This outstanding sensitivity is more than sufficient to monitor the provisional WHO guideline values.

Not only is the sensitivity impressive, but also the reproducibility and accuracy. The relative standard deviation for 10 measurements in a check standard solution (β(Cd) = 1 µg/L and β(Pb) = 5 µg/L) is 5% and 3%, and the recovery rate is 90% and 100% for cadmium and lead, respectively.

Direct determination of iron

The presence of iron in drinking water can lead to an unpleasant, harsh metallic taste or reddish-brown stains. In addition, «iron bacteria» which can grow in waters containing iron as low as 100 µg/L, create a reddish-brown slime that can clog plumbing and cause an offensive odor. Over a longer period, the formation of insoluble iron deposits is problematic in many industrial and agricultural applications, such as water supply, system cooling, or field irrigation. To avoid these problems, the U.S. Environmental Protection Agency (EPA) defines the Secondary Maximum Contaminant Level (SMCL) for water treatment and processing plants as 300 µg/L iron in drinking water.

Figure 6. Example for determination of iron in tap water spiked with β(Fe) = 20 µg/L.

The voltammetric determination of the iron triethanolamine complex on the non-toxic Bi drop electrode does not require enrichment. The system uses catalytic signal enhancement, allowing both the detection at very low levels with a limit of detection of 5 µg/L and measurements in a wide range of concentrations up to 500 µg/L.

This method is best suited for automated systems or process analyzers, allowing fully automatic determination of iron in a large sample series and providing stable results. The relative standard deviation for 10 measurements in a check standard solution (β(Fe) = 50 µg/L) is 3% and the recovery rate is 111%.

Adsorptive stripping voltammetric determination of nickel and cobalt

The main sources of nickel pollution are from electroplating processes, metallurgical operations, or leaching from pipes and fittings. Catalysts used in the petroleum and chemical industries are major application fields for cobalt. In both cases, the metal is either released directly, or via the wastewater–river pathway into the drinking water system. Therefore 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). The unique properties of the non-toxic Bi drop electrode combined with AdSV results in an excellent performance in terms of sensitivity. The limit of detection for 30 s deposition time is approximately 0.2 µg/L for nickel and 0.1 µg/L for cobalt, and can be lowered further by increasing the deposition time.

Figure 7. Determination of nickel and cobalt in tap water spiked with β(Ni) = 0.5 µg/L and β(Co) = 0.5 µg/L.

This method is best suited for automated systems or process analyzers, allowing fully automatic determination of these metals in large sample series and providing stable and accurate results. The relative standard deviation for 10 subsequent measurements in a check standard solution (β(Ni) = 1 µg/L β(Co) = 1 µg/L) is 4% and 5% and the recovery rate is 106% for nickel and 88% for cobalt.

Key features of the Bi drop electrode

  • Non-toxic, completely mercury-free alternative for trace metal determination
  • Simultaneous determination of Ni and Co, as well as Cd and Pb
  • Limit of detection in low μg/L and even ng/L range
  • Suitable for automated and online systems

What’s next?

In the next installment, we will take a look at a cost-efficient and semi-disposable sensor for heavy metal detection: the scTRACE Gold and its associated applications.

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