Stirred, not shaken: Simple and Efficient Water Analysis

100 containments in surface water Stirred, not shaken: Simple and Efficient Water Analysis

14.02.2020Author / Editor: Oliver Lerch* and Jasmin Zboron* / Dipl.-Chem. Marc Platthaus |

A highly sensitive method for quantitative determination of around 100 contaminants in surface waters with and without sediment has been developed. The key element of this GC-MS/MS-based method is Stir Bar Sorptive Extraction (SBSE) using a special stir bar.

Related Vendor

Gerstel GmbH & Co. KG

Fig.1: Gerstel MPS-SBSE-TDU-GC-MS/MS system used for the determination of around 100 contaminants including priority pollutants as specified in the EU Water Framework Directive (EU WFD).
(Source: Gerstel)

In 2000, the EU Parliament and the Council released Directive (2000/60/EC) [1] that focuses on the protection and improvement of the quality of surface waters and ground water. The directive in its own words “lays down a strategy against the pollution of water. That strategy involves the identification of priority substances that pose a significant risk to, or via, the aquatic environment at (European, Ed.) Union level”. The directive was extended and amended in 2008 (2008/105/EC) [2] and in 2013 (2013/39/EU) [3]. In essence, the Water Framework Directive (WFD) of the European Union (EU) makes it mandatory for all member states to take measures to continually monitor and improve the condition and quality of water bodies in their territory: Various parameters must be monitored, including a range of identified key chemical pollutants. In the EU WFD, priority substances are listed.

EU Environmental quality standards (EQS) specify maximum allowable concentration (MAC) levels for pollutants in surface waters that must be adhered to, at least within a foreseeable future. As the directive is worded: “with the aim of achieving good surface water chemical status in relation to those substances by 22 December 2021 by means of programs of measures included in the 2015 river basin management plans…”. In the Environmental quality Standards that apply to surface waters, there is differentiation between annual average concentration (AA-EQS) and maximum allowable concentration (MAC-EQS) levels specified for “inland surface waters”, such as rivers and lakes, and the levels specified for “other surface waters”, listed as transitional, coastal and territorial waters.

A closer look at the requirements

Commission Directive 2009/90/EC provides technical details for the chemical analysis and monitoring specified in the EU WFD. Minimum performance criteria for analytical methods are specified, such as limits of quantification (LOQs). Compound LOQs lower than 30% of the respective Annual Average EQS (AA-EQS) values are required, for some compounds in the low- to sub-nanogram per liter range.. For example, the required LOQ for benzo[a]pyrene is 0.051 ng/L. Reaching such levels requires a highly efficient analyte concentration technique, such as Stir Bar Sorptive Extraction (SBSE), combined with a highly sensitive analysis technique. In addition, the uncertainty of measurement near the respective EQS must be lower than 50% (k=2) of the specified EQS concentration value. If these criteria cannot be met, due to a lack of suitable analysis methods, the EU-WFD offers some freedom: Monitoring should then be performed using the best available method that doesn’t lead to excessive cost. Stir Bar Sorptive Extraction (SBSE) with the patented Gerstel Twister has long been used for environmental water analysis and has performed well in round robin tests. In 2009, the environmental protection agency of the State of Bavaria, Germany demonstrated SBSE performance in full compliance with the requirements of the German Federal drinking water regulation (TrinkwV 2001). Furthermore, SBSE was previously successfully used for the determination of priority pollutants in surface waters following the EU WFD 2000/60/EU. As a result of a significant investment of time and resources, the SBSE method has been optimized for highly sensitive determination of the GC compatible priority pollutants listed in EU Directive 2013/39/EU — including particle adsorbed pollutants — using one comprehensive method.

A detailed look at Stir Bar Sorptive Extraction (SBSE)

The extraction medium used in SBSE is the Twister. It is a glass coated magnetic stir bar mounted with a substantial sorbent layer. The sorbents used are either polydimethylsiloxane (PDMS) or an ethyleneglycol-silicone polymer (EG-Silicone Twister). The extraction principle of SBSE, like that of liquid-liquid extraction (LLE), relies on partitioning, reaching equilibrium between the aqueous sample and the sorbent phase. In the work described here, PDMS Twisters were used. Analyte extraction and concentration takes place while the Twister stirs the sample. Subsequently, the Twister is removed from the sample, dried, placed in a glass liner for thermal desorption, and the liner is placed in a sealed sample tray position on the Multi Purpose Sampler (MPS). The MPS performs fully automated processing of large sample batches in combination with a thermal desorber. Following thermal desorption, analytes are focused in a GC inlet (Gerstel CIS 4) and subsequently transferred to the GC column in split or splitless mode using programmable heating. This approach ensures that extracted analytes can be transferred up to 100% to the column for extreme sensitivity. The PDMS Twister is well suited for non-polar to medium polarity compounds while the EG-Silicone Twister is mainly used for polar compounds that form hydrogen bonds as proton donor, for example phenols, alcohols and organic acids.

Simple and efficient water analysis

In this work, SBSE, using the Twister, was our method of choice for the GC compatible compounds listed in the EU Water Framework Directive. In addition to reaching the prescribed LOQs, we also wanted to perform quantitative determination of compounds adsorbed on particulate matter, as demanded by the EU WFD, including polyaromatic hydrocarbons (PAHs). The overall analysis requires a special approach: At first, we performed classical SBSE on individual 100 mL samples, which is a large enough volume to generate meaningful results. After adding an internal standard, we let a Twister stir the sample for five hours to extract free organic compounds. The Twister was then removed, an organic modifier was added to the sample along with a second Twister, and the sample extracted at elevated temperature overnight. Using this approach, compounds adsorbed on particulate matter were successfully released and extracted. The two Twisters were transferred to a TDU glass liner and simultaneously desorbed in the Thermal Desorption Unit (TDU) using a temperature program (90 to 300 °C). The analytes were focused in the Cooled Injection System (CIS 4) at -40 °C. The CIS is a PTV type Universal GC inlet, which can be heated very rapidly in order to release analytes to the GC column, in our case a HP 5 ms Ultra Inert (30 m x 0.25 mm x 0.25 μm) from Agilent Technologies. Following separation, the analytes were determined using a Triple Quadrupole MS (Agilent Technologies 7010) in Multi-Reaction Monitoring (MRM) Mode.

Successful determination of water contaminants

The newly developed SBSE-GC-MS/MS method presented here is both efficient and highly sensitive. We were able to determine the GC compatible compounds listed in the EU WFD at the levels specified — or even well below — with three exceptions: cypermethrin, heptachlor, and heptachlorepoxide. Those three are also notoriously hard to determine at the required levels down to sub-pg/L using conventional techniques.

In the following, we list a few examples of required limits of quantitation for surface water following the EU guidelines. For dichlorvos: 0.18 ng/L; using SBSE, we reached 0.073 ng/L. For benzo(a)pyrene: 0.051 ng/L; using SBSE, we reached 0.033 ng/L. For pentachlorobenzene: LOQ of 2.1 ng/L; with SBSE, we lowered it to 0.075 ng/L (see table 1, www.lab-worldwide.com). Comparable results were achieved for around 100 other contaminants relevant for surface water analysis. Relative standard deviations (RSDs) near the respective limits of quantitation were between two and ten percent for the vast majority of compounds with a median of 6.9 percent. Trueness was between 90 and 110 percent in most cases. The accurate and precise quantification of particle adsorbed analytes was tested and confirmed using a certified reference sediment (WEPAL SETOC 745). Certified analysis results were available for PAHs and selected chlorinated compounds.

Additional Information

Assessing Stir Bar Sorptive Extraction

Stir Bar Sorptive Extraction (SBSE) using the Gerstel Twister is simple to perform and highly efficient, resulting in high sensitivity, not only in the case described here, in which priority pollutants listed in the EU WFD were successfully determined. The high sensitivity is mainly due to the large volume of Twister sorbent phase, in this particular case 63 µL, which guarantees a high concentration factor when doing trace analysis. When the Twister is thermally desorbed, the analytes extracted from the sample are transferred quantitatively to the GC, whereas when performing liquid extraction and injection, only a small aliquot is injected. SBSE performed on a 100 mL water sample coupled with thermal desorption and a highly sensitive triple quadrupole mass spectrometer provides extremely low limits of detection, reaching the sub-ng/L range. A further bonus when using Twister technology: The entire method requires only a very small amount of solvent. And since many solvents are toxic, hazardous and harmful to the environment, a reduction in the amount used would seem particularly meaningful and fitting in order to reach environmentally sustainable solutions for chemical analysis.