Development of a fully automated slurry sampling introduction system for GF-AAS and its application for the determination of cadmium in different matrices

In the study published in the Analytica Chimica Acta, researchers from the Federal Institute for Materials Research and Testing, Berlin, Germany introduced a novel autosampler extension for slurry sample analysis in Graphite Furnace-Atomic Absorption Spectrometry (GF-AAS), addressing challenges in trace element analysis of solid samples like soils and microplastics.

The system enhances suspension stability using a stirring device, closed vessels to prevent evaporation and contamination, and a cooling unit to minimize solvent and analyte losses. Validation with cadmium analysis in soil and ABS samples demonstrated recovery rates of 94% ± 13% and 104% ± 11%, respectively, highlighting its reliability and accuracy. This innovation streamlines GF-AAS processes, reducing preparation time and error, making it a versatile tool for environmental and polymer sample monitoring.

The original article

Development of a fully automated slurry sampling introduction system for GF-AAS and its application for the determination of cadmium in different matrices

Charlie Tobias, Lennart Gehrenkemper, Thomas Bernstein, Sven Schlau, Fabian Simon, Mathias Röllig, Björn Meermann, Marcus von der Au

Analytica Chimica Acta, Volume 1335, 2025, 343460

https://doi.org/10.1016/j.aca.2024.343460

licensed under CC-BY 4.0

Selected sections from the article follow. Formats and hyperlinks were adapted from the original.

Highlights

• Development of a novel autosampler extension for automated slurry measurements in GF-AAS to increase sample throughput.
• Automated stirring to stabilize suspensions, eliminating the need for surfactants and reducing contamination risk.
• Minimized analytical errors from evaporation using closed, cooled sample vessels.
• Compatibility with organic solvents due to enhanced system design.
• Demonstrated usability by successfully analyzing two contrasting reference materials: a polymer and a soil.

Abstract

Background

Graphite Furnace-Atomic Absorption Spectrometry (GF-AAS) is a powerful technique for trace element analysis, offering high sensitivity and precision. However, its effectiveness is limited by sample preparation challenges for solid samples like soils and microplastics. Traditional methods include sample preparation, such as digestion, which is time-consuming and involves reagents, like acids, contributing to measurement uncertainty and higher carbon footprints. Slurry sampling allows direct analysis of suspensions, offering a more efficient alternative. However, maintaining suspension stability is challenging, requiring robust autosampler systems to streamline the process and enhance analytical performance.

Results

We present a novel autosampler extension for slurry sample introduction into GF-AAS. This system ensures suspension stability with a stirring device and closed vessels to prevent evaporation and contamination, incorporating a cooling unit to reduce solvent and analyte losses. It installs and removes in minutes without additional connections. Validation with cadmium analysis in BAM-U110 (Soil) and BAM-H010 (ABS) showed high reliability. For BAM-U110 (Soil), we achieved recovery rates of 94 % ± 13 % in water suspension. The recovery rate for BAM-H010 (ABS) was 104 % ± 11 % in acetonitrile suspension. These results demonstrate the system's robustness, versatility, and accuracy for different matrices.

Significance

The autosampler extension helps solve key problems in trace element analysis of solid samples, making the process faster and more accurate. It works well with complex materials, making it useful for areas like microplastic or nanoparticle analysis. This improvement also helps meet regulations for monitoring environmental and polymer samples, offering a reliable and flexible tool for high-throughput analysis with fewer errors.

2. Materials and methods

2.1. Materials, reagents and standards

For the calibration, a single element cadmium standard (ρ(Cd) = 1000 mg L⁻¹, Certipur, Merck KGaA, Germany) was used. For the dilution steps, type I reagent grade water obtained from a Milli-Q-System was used (>18.2 MΩ cm; Merck Millipore, Germany). To stabilize the ionic standard, double sub-boiled nitric acid (ω(HNO₃) = 65 %) was used. As modifier for the determination of Cd via high resolution-continuum source-graphite furnace atomic absorption spectrometry (HR–CS–GFAAS), palladium, and magnesium ICP-MS standard solutions were used (ρ(Mg) = 10000 mg L⁻¹; ρ(Pd) = 10000 mg L⁻¹, Certipur, Merck KGaA, Germany). To characterize the newly developed autosampler system, two different reference materials were used: BAM-H010 (acrylonitrile butadiene styrene (ABS), granulate, Federal Institute for Materials Research and Testing (BAM), Germany) and BAM-U110 (contaminated soil, BAM). To obtain small plastic particles for the ABS material, two steps (500 μm and 150 μm sieves) were carried out with a centrifugal mill (ZM 200, Retsch GmbH, Germany) under liquid nitrogen. As sample vials, HPLC vials with PTFE/silicon septa were used. The organic solvents used (methanol (MeOH, LC-MS grade, 99.95 %), ethanol (EtOH, for analysis, 99.9 %), acetonitrile (ACN, LC-MS grade, 99.95 %)) were obtained from Th. Geyer (CHEMSOLUTE, Th. Geyer GmbH & Co. KG., Germany).

2.2. Development and characteristics of the slurry autosampler

The prototype of our newly developed autosampler system, the Automatized Slurry Analyzer (ASA; patent pending: DE10 2024 108 026.4) is presented in Fig. 1.

Analytica Chimica Acta, Volume 1335, 2025, 343460: Fig. 1. Schematic drawing of the newly developed autosampler extension unit for a high throughput analysis of slurry samples. a: PVC sample turning table with silicon inlayer, b: Sample cooling unit: Aluminum plate with water inlet and outlet, c: Light sensor with control unit, d: Motors with power modulator unit, e: Autosampler with trigger. a-e are modified parts while the body of the autosampler (non-marked part) remained unchanged. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

The system is particularly designed for the stabilization of suspensions to allow a reproducible analysis of slurry samples. For this purpose, four brushless motors (Type 0703B 15000 KV, Racerstar Electronic Technology Co., Ltd., China) are utilized as magnetic stirrers by gluing neodymium magnets (diameter 3 mm, height 1 mm) on top of them. The modified stirrer units are placed under the positions where the sample is taken by the injector of the autosampler. To protect stirrer units from possible spills without reducing the magnetic field, the motors are covered by a thin glass-reinforced epoxy laminate layer (FR-4). An ATtiny85-S20U microcontroller (Microchip Technology Corp., USA) is used to define the timings and control the brushless motors. To avoid contamination nickel wire pieces (99.2 %, Evek GmbH, Germany) of different diameters (1 mm and 2 mm) and lengths (4 mm–9 mm) are used as stirring bars. For easy usage, the autosampler is designed to hold standard 1.5 mL HPLC vials. To keep these vials stable and avoid the lifting of the vessels the sample turning table was designed in three layers – PVC/silicon/PVC. The two PVC layers hold the vials in position while the silicon layers prevent them from being lifted after the sample uptake of the needle. As a further modification, a hypodermic needle was installed onto the autosampler to allow the septum of the used HPLC vials to be penetrated and keep the low dead volume within the system. Further, by cooling the aluminum autosampler and using closed HPLC vials the possible loss of solvents is drastically reduced and the time for measurements can be extended without the manual closing of vials with e.g., parafilm as reported by Metzger et al. [13] As a chiller, a Cool-Care 8–16 unit (Van Der Heijden Labortechnik GmbH, Germany, temperature range 5 °C–25 °C) was used. The implementation of a photoelectric sensor enables precise initiation of the motors, thereby minimizing idle rotation time to prolong motor lifespan and prevent overheating. The sensor is connected to an Arduino NANO (ATMEGA328p) as a control unit between the motor controller and the sensor. The system is designed in a way that it can be changed within a few minutes back to the standard AAS autosampler. This allows a fast, easy-to-use, and reproducible way to analyze slurry samples. The developed autosampler can also be used without the stirring, to avoid the evaporation of solvents and thus, increase the accuracy of the utilized analysis method. In addition, the incorporated fluidic system can be also used for heating, e.g., trigger reaction before the sample is taken. This wide range of possibilities in a highly automated fashion makes this autosampler perfectly suited for routine measurements.

2.3. Instrumental analysis

The determination of cadmium was performed on an Analytik Jena contrAA 800 HR–CS–GFAAS system (Analytik Jena GmbH, Germany). Pyrolytically coated graphite tubes with PIN platform (Analytik Jena) were used. The temperature program applied as well as the injection sequence were adapted from Borges et al. [14] Accordingly, the utilized injection loop consisted of 10 μL of ultra-pure water, 25 μL sample, and 5 μL of the modifier mixture (ρ(Pd) = 1.0 g L⁻¹, ρ(Mg) = 0.6 g L⁻¹). The GFAAS temperature program, which was used for all presented data throughout this study, is summarized in Table 1.

Analytica Chimica Acta, Volume 1335, 2025, 343460: Table 1. Temperature program used for the determination of cadmium in soil as well as ABS slurries via HR–CS–GFAAS.

2.4. Sample cooling unit - temperature profile characterization

For the characterization of the temperature profile of the sample cooling unit (see Fig. 1 b), an infrared camera (VarioCam HR, InfraTec GmbH Infrarotsensorik und Messtechnik, Germany) with a 30 mm lens was used. This has a focal plane array (FPA) with an uncooled microbolometer detector consisting of 640 x 480 pixels. The camera covers a spectral range from 7.5 μm to 14 μm. Measurements were taken in the calibrated temperature range from −40 °C to +120 °C. The noise equivalent temperature difference (NETD) is 50 mK at 30 °C. The measurement uncertainty is ±1.5 K or ±2 % for temperatures above 100 °C. The data was recorded at 2 Hz and analyzed using the IRBIS Professional 3.1.80 thermography software.

The thermograms were recorded with a lateral geometric resolution of 0.5 mm at 65 cm between the IR camera and the measurement object.

For the measurement of the reflective aluminum surface, emissivity stickers from Testo SE & Co. KGaA with a defined emissivity (ɛ) of 0.95 were used to measure the aluminum reflective surface as reference points.

4. Conclusion and outlook

The presented newly developed autosampler extension for GF-AAS is a promising tool for streamlining arduous and error-prone sample preparation, particularly the digestion of solid samples. This extension offers a cost-effective alternative for the precise, accurate, economical, and rapid determination of metal contents in solid samples. The system's flexibility and adaptability are evident within the two presented matrices, as it can be easily tailored to different matrices by simply changing the solvent. Furthermore, the system is designed to facilitate temperature control of samples and employs closed vessels, which are readily available from various vendors, enabling easy use of organic solvents. This adaptation of the AAS enables access to emerging markets, such as the investigation of electronic scrap, which is challenging to address using conventional techniques due to its complex composition of glass, polymers, and various other matrix components that are difficult to digest. Additionally, this setup could enable further extensions in the analysis of organic fluorine in the form of adsorbable organic fluorine (AOF) using activated carbon suspensions in the future This capability enables comparison and complementary investigations in the increasingly concerning field of emerging pollutants, specifically per- and polyfluorinated substances (PFAS).