Three elements in one sweep
Sediment samples from the North Sea. Photo: HZG / Daniel Pröfrock
Where does the sediment in the Wadden Sea and the silt in the harbour of Hamburg come from? How are pollutants in the environment distributed? Questions like these can, for example, be answered today though chemically analysing trace elements in sediment or water samples. These methods, however, are still often very time-consuming because the analysis samples must be processed with a great deal of effort. A HZG doctoral candidate, Tristan Zimmermann, has therefore developed a method that vastly speeds up sample preparation. His method will help ease everyday laboratory work for researchers all over the world. His endeavours have now been awarded a prize at a scientific conference.
The origin of the sediment
The Wadden Sea is very unique in that it is constantly in motion. With the alternating ebb and flood, masses of water are virtually pumped back and forth. The mud is rinsed away and deposited at another location. When breaking seas approach the coasts during a hurricane, tons of sediment are whirled up and transported. Scientists at the Institute of Coastal Research of the Helmholtz-Zentrum Geesthacht want to understand these processes. They wish to ascertain where the material comes from, what path the material takes, where it is deposited and how these processes contribute to pollutant distribution.
Taking samples in the mudflat. Photo: HZG / Ina Frings
The answers are supplied by modern chemical analysis methods with which the scientists can determine how much of a certain element, for example, is contained in a sediment sample from the Wadden Sea or in the Elbe silt. Daniel Pröfrock, head of the Department of Marine Bioanalytical Chemistry at the Institute of Coastal Research says, “In essence, we use these methods to determine the elemental fingerprint of a sediment sample from which we obtain the first information on its possible origin as well as the sediment's properties."
Isotopes make the difference
Ultimately, each sediment consists of minute plant remains and small rocks, which have been washed from the stones by weathering, rain or by rivers. The rock, on the other hand, is composed of certain chemical elements according to its age and developmental history and therefore also has it's own type of chemical fingerprint. Elements like lead, neodymium and strontium are particularly interesting components for the Geesthacht scientists.
All three are characterised by the fact that in nature they consist of several “stable isotopes”. Isotopes are those atoms of a chemical element that differ in the number of neutrons and therefore exhibit a different atomic mass. The element strontium, for example, exists in nature with four stable isotopes with a mass of 84, 86, 87 and 88. The relative abundance of these four isotopes differs in nature slightly and depends on the geological and radioactive processes that have occurred according to location over the course of millennia.
he individual isotope content will vary depending on the age and composition of a rock. A rock thus carries a very characteristic isotopic fingerprint based on its place of origin on Earth. Examining the sediment composition in the Wadden Sea using this method, scientists can determine whether the sediment originates from the Elbe’s catchment area or was transported through other river systems.
Department head, Dr Daniel Pröfrock, preparing samples in the laboratory. Photo: HZG / Ulrike Kleeberg
The problem is that because the lead, neodymium and strontium isotopes are only found in tiny quantities in the sediment samples, analysis is thus extremely laborious—mainly due to preparing the samples. The scientists must work with immense precision and separate different chemical elements extremely carefully in order to correctly determine the extraordinarily fine differences in the isotope composition. If they only dissolve a portion of the elements sought from the sample, the isotope ratios are not accurately determined.
The established method is to separate the isotopes of different chemical elements step by step from the remaining interfering elements in painstaking work within the laboratory. In order to do so, the sediment sample is first liquefied using acids. Subsequently, the liquid is transferred to small test tubes using pipettes. The liquid contains what are known as ion-exchange resins to which the different elements from the test solution bind. The interfering elements are gradually separated by adding different reagents. Until recently, this separation had to be carried out separately for lead, neodymium and strontium because all three couldn’t be separated in one go from the remaining elements. “This sample preparation is so laborious," says Tristan Zimmerman, doctoral candidate in Daniel Pröfrock’s department, “that a person can only process about twenty samples in one day."
Extracting three elements simultaneously
In order to ease this laborious preparation, Zimmerman developed a new method with which the samples can be processed much more quickly and the isotopes from the desired elements can be separated from each other automatically. The automated equipment required has only recently been made available on the commercial market. Zimmerman, however, initially needed to modify the apparatus to serve his purpose.
He currently works in the Marine Bioanalytical Chemistry Department, focussing mainly on the elements of lead, neodymium and strontium. As mentioned, the sample preparation and separation of the isotopes for each of these three elements must be carried out manually on an individual basis. Zimmerman, however, now succeeded in carrying out the separation in one step. He utilises a special ion exchange resin that, to date, has been provided by the manufacturer only for separating strontium isotopes.
As Zimmermann determined, the chemical properties of the ion-exchange resin enable all three desired elements to be extracted simultaneously from the sediment sample. “By adding different acids with different acidic strengths, we can dissolve the element isotopes of lead, neodymium and strontium successively from the ion exchanger, thus separating them,” says Zimmerman.
Prize-worthy method saves time
The elements separated from the sample are subsequently examined for their various isotope content independently in another device—the element mass spectrometer. It is the sample preparation in particular that is the limiting factor in environmental studies. Tristan Zimmerman's work helps shorten time-consuming sample preparation enormously. On the one hand, his method filters out the isotopes for the elements lead, neodymium and strontium from a sample in a single step. On the other hand, he has succeeded in automating this process.
HZG doctoral candidate Tristan Zimmermann, developer of the new procedure, taking samples from the Elbe. Photo: HZG / Daniel Pröfrock
“The new method is expected to simplify work in many laboratories worldwide because it saves innumerable hours of sample preparation,” says Pröfrock. It is thus unsurprising that Zimmermann’s work has recently been honoured at a meeting of analytical experts and mass spectrometer users with a prize for best poster. This meeting is organised by the Deutschen Gesellschaft für Massenspektrometrie (DGMS) every two years and is attended by a good 150 experts from industry and research. The poster, which was created in cooperation with colleagues from the University of Natural Resources and Life Sciences in Vienna, was awarded a prize by the Tofwerk firm. The title of the poster is: “Optimization of a new fully-automated sample preparation system for isotopic analysis of sediment digests via MC ICP-MS”.
The history of environmental pollution
Zimmermann’s new method will also substantially ease the work in the HZG laboratories. Neodymium and strontium are elements where the isotopic composition shows the origin of sediments particularly well. Lead isotopes on the other hand, are especially good for researching anthropogenic causes of environmental pollution. Here the researchers take advantage of the fact that lead exhibits a characteristic isotope composition depending upon origin. Researchers can make use of the fact that lead isotopes in the sediment can provide information on whether the measured values are part of the natural soil or have entered the environment as pollutants through human activities.
Researchers can thus detect, for example, lead from automotive exhaust that was still added to gasoline a few decades ago. In quiet cut-off river meanders, which are hardly exposed to currents, we often find neatly layered sediments, which are quite old, and where we can then reconstruct corresponding developments,” says Pröfrock. “If, at the same time, we determine the age of the strata, we can precisely determine the time at which the lead entered the environment—for example, at the beginning of the Industrial Revolution.” Thanks to the methods developed by Tristan Zimmermann, the HZG scientists can obtain vital information from their sediments quite a bit faster in the future.
Text: Tim Schröder, science journalist