Introduction

The increasing endeavor to reduce fossil raw materials has led to an increasing recognition of recycling and thus circular economy has become a significant factor in plastic material management over the past several decades. Polyethylene (PE), polypropylene (PP), polystyrene (PS) and polyethylene terephthalate (PET) represent the largest percentage of the plastic fraction of municipal solid waste [1]. Composites of such materials allow for specific tuning of parameters such as permeability to oxygen, moisture, and other substances, which may influence the quality of a packaged product [2, 3]. However, composite films pose a challenge in the recycling process, as the individual compounds can only be separated from each other with significant effort if at all. This leads to the presence of plastic impurities in recyclates of a main type. Given that contaminants affect quality factors such as mechanical properties and therefore the reusability of recyclates, it is important to detect and subsequently reduce such contaminants by optimizing the process parameters [4]. Cascales et al. demonstrated that contamination of recycled PE by a thermoplastic starch polymer even at a low level of 2.5 wt.% negatively affects tensile properties and others [5]. Nevertheless, it should be noted that not only contaminants but also the number of mechanical recycling cycles inflicts a detrimental effect on the quality of a recyclate [6].

To determine the grade of contamination in a recyclate, analytical techniques such as Fourier transform infrared with attenuated total reflection mode (FTIR-ATR), differential scanning calorimetry (DSC) or near-infrared (NIR) assisted flake analysis are employed [4, 7,8,9]. However, it is essential to carefully consider the advantages and drawbacks of each technology in relation to the specific composition of the recyclate. For instance, FTIR-ATR does not allow for an accurate quantification of PET concentrations within the relevant sub-percent range [7]. Additionally, more complex analytical laboratory systems involving mass spectrometers (MS) are employed to investigate plastics [9,10,11].

Aiming for highly sensitive and easy-to-operate analytical systems of compact size, ion mobility spectrometers (IMS) operating at atmospheric pressure should be considered. Originating from vapor and gas-phase analysis in the early 1970’s [12], similar to MS, IMS can be equipped with a pyrolyzer (Py) and a gas chromatograph (GC) to analyze solid or liquid samples. This approach has been employed to investigate bacteria and bio aerosols by Py-GC-IMS in the past [13,14,15]. In a pyrolyzer, a sample quantity in the microgram range is rapidly heated to a temperature of several hundred to a thousand degrees Celsius in the absence of oxygen and thus broken down into more volatile constituents [16]. Subsequently, the gaseous pyrolysate enters the GC as a defined sample volume. In the GC, the sample is separated into the individual constituents due to their individual retention when traveling through the GC column. Connected to the GC, MS and IMS can serve as a detector, offering an orthogonal separation technique. The advantage of MS is of course the possible identification of individual components of the pyrolysate. However, the disadvantage is the high instrumental effort, including laboratory space and vacuum pumps. If the source materials of the recyclate are known, contaminations only need to be monitored instead of identified to ensure consistent recyclate quality. Consequently, IMS, functioning as a GC detector with sufficient analytical detection capabilities and the advantage of less instrumental effort compared to MS, represent a suitable choice. Over recent decades, IMS have been established as suitable instruments for quality assurance monitoring purposes [17,18,19,20,21]. Currently, IMS can achieve detection limits in the sub-parts per trillion range (pptv), while new production methods enable further reduction in manufacturing costs [22,23,24,25,26,27,28].

One particular IMS embodiment uses a drift tube for ion separation [29]. In such drift tube IMS, a gaseous sample enters a reaction region where the sample is usually ionized by reactant ions in chemical gas phase reactions [29]. The reactant ions are formed from the neutral gas inside the reaction region using a proper ionization source [29]. Passing an ion gate, a defined ion cloud is transferred into the drift tube, where the different ion species of the ion cloud separate under the influence of a constant electric field based on their ion mobilities before they discharge at a detector. The resulting discharge current plotted over time forms an ion mobility spectrum giving the individual ion mobility of the ion species. As shown in Eq. (1), ion mobility \(K\) is the proportionality factor between the applied electric field \(E\) within the drift region and the mean drift velocity \({v}_{d}\) of the ions traveling through a neutral drift gas. In practice, the electric field \(E\) is determined by the length of the drift region \(L\) and the applied drift voltage \({U}_{d}\). The mean velocity of an ion is equal to the quotient of the drift length \(L\) and the drift time \({t}_{d}\).

$${v}_{d}=K\times E=K\times \frac{{U}_{d}}{L}=\frac{L}{{t}_{d}}$$
(1)

The mean drift velocity \({v}_{d}\), and thus the separation of the individual ion species along the drift tube, is the result of constant collisions of the ions with neutrals in the drift gas and the acceleration by the electric field \(E\). Since neutral particles are an essential part of the separation process in drift tube IMS, the density of the neutrals has a direct effect on the ion mobility. To compensate for the temperature \(T\) and pressure \(p\) influence, a reduced ion mobility \({K}_{0}\) (see Eq. (2)) is frequently calculated to normalize \(K\) by the means of a standard temperature \({T}_{0}= 273.15 {\text{K}}\) and standard pressure \({p}_{0}= 1013.25 {\text{hPa}}\) and to facilitate comparison of ion mobilities.

$${K}_{0}=K\times \frac{{T}_{0}}{T}\times \frac{p}{{p}_{0}}=\frac{{L}^{2}}{{U}_{d}{\times t}_{d}}\times \frac{{T}_{0}}{T}\times \frac{p}{{p}_{0}}$$
(2)

A variety of ionization sources are currently employed in ion mobility spectrometry. For gaseous samples, X-ray, corona discharge, photoionization and plasma sources are described in the literature [29,30,31,32,33,34]. Besides some low energy photoionization types, the ionization source continuously ionizes the gas within the reaction region, thereby forming reactant ions, predominantly water clusters [29]. The reactant ions subsequently ionize the compounds of interest. Due to the complex interaction of factors influencing this ion chemistry, such as the composition of the sample gas, temperature and the humidity, some compounds are more easily ionized than others are. For example, ionizing alkanes and alkenes by atmospheric pressure chemical ionization can be challenging [30,31,32]. In addition to the precise control of the ion chemistry, proper selection of the ionization sources, such as photoionization or corona discharge, can also lead to a higher response to alkanes and alkenes compared to radioactive sources [11, 33]. However, for the purposes of this study, the limited response of some ion sources to certain compounds is not a drawback but an advantage.

In this work, we present a first feasibility study using a Py-GC-IMS operating at atmospheric pressure, with the objective of monitoring the PET contaminations in PE recyclates. The recyclates are composed of virgin PE mixed with different ratios of waste material from shredded dog food bags, which foil is a composite of PE and PET. During pyrolysis, PE normally breaks down into hydrocarbon chains of different lengths [10, 34]. The predominant compounds in the pyrolysis of PE are alkenes, with the precise ratio of alkenes, dienes, and alkanes dependent on the final pyrolysis temperature, typically between 600°C and 1000°C, and the heating rate [35]. This study aims to determine if an ionization source that is less sensitive to alkenes, dienes, and alkanes can be employed to identify contaminations in polyethylene (PE) recyclates, given that the background matrix is expected to produce only minimal signals.

Experimental

The following sections provide detailed information about the used PE recyclates and instrumentation. All self-built components described below were developed at Leibniz University Hannover, Institute of Electrical Engineering and Measurement Technology, Department of Sensors and Measurement Technology, Hannover, Germany.

Material

In this study, different PE recyclates were prepared by mechanical recycling at Leibniz University Hannover, IKK—Institute of Plastics and Circular Economy, Garbsen, Germany. Virgin polyethylene (PE) LLDPE 118NE from SABIC® (Hamburg, Germany) was blended with 25 wt.%, 20 wt.%, 15 wt.%, and 10 wt.% of pre-consumer waste material derived from shredded dog food bag foils. The waste material was provided by the waste management facility and recycler, PreZero Stiftung and Co. KG (Porta Westfalica, Germany). Prior to blending, the dog food bag foils were subjected to a cold washing procedure at Reantec GmbH (Kempen, Germany). Subsequently, they were ground and blended with virgin PE at IKK using a double screw extruder BluePower ZE28 (KraussMaffei Extrusion, Laatzen, Germany). The foils contain polyethylene (PE) and polyethylene terephthalate (PET) as well as inks derived from the former product design and labeling. In addition to the recyclates, virgin PE, virgin PET, and the pure foils are included in the analysis. In order to perform pyrolysis analysis, a small quantity of the material is scraped off from the recyclate and the virgin material granulates.

Instrumentation

The experimental setup comprises three primary components: a pyrolyzer, a gas chromatograph, and a drift tube IMS as a detector, equipped with driver and data acquisition electronics. In simple terms, the recyclate sample is pyrolyzed and the resulting gaseous pyrolysate is separated by a GC before eluting into the IMS for detection.

A scheme of the experimental setup with all components is shown in Fig. 1. Each component is described in detail below.

Fig. 1
figure 1

The experimental setup comprises a CDS 6200 pyrolyzer, a self-built GC with a 50:1 split valve, and a self-built, miniaturized drift tube IMS with self-built data acquisition

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Pyrolyzer

A commercially available pyrolyzer (CDS 6200, CDS Analytical, Oxford, PA, USA) is utilized for the analysis of recyclate or virgin material samples. The samples, which comprise approximately 200 µg of the respective material, are introduced into constricted quartz sample tubes (type 6201–3004, CDS Analytical, Oxford, PA, USA). Prior to loading a quartz tube with a sample, the tube is cleaned in the pyrolyzer by heating it to 900 °C and holding it for 20 s while the pyrolysis chamber is flushed with nitrogen. During this time, internal valves guide the nitrogen flow directly to the waste. Despite the extensive options offered by the CDS 6200, only drying and single-shot analysis with a flash pyrolysis are used for this study without any trapping. Each sample is first dried for 60 s at 100 °C before the flash pyrolysis is initiated, with the heating coil then ramping up at maximum ramping speed of about 1000 °C/s to the pyrolysis temperature of 800 °C, which is held for 30 s.

Gas Chromatograph

A heated transfer line (maintaining a temperature of 200 °C at all times, regulated by the pyrolyzer) links the pyrolyzer to the downstream GC via a self-built split valve. The split valve is equipped with a backpressure valve (9000AMBF900, Parker Hannifin GmbH, Bielefeld, Germany) at one outlet, which allows a split flow to be adjusted to approximately 50:1 for this particular application. The split valve is continuously maintained at 200 °C. The GC comprises a 5 m long separation column with an inner diameter (ID) of 250 µm and a cross-bond diphenyl/dimethyl polysiloxane phase of 1 µm film thickness (Rtx Volatiles, Restek GmbH, Bad Homburg, Germany). This column is installed in a self-built, miniaturized GC oven previously described elsewhere [36]. Again, nitrogen 5.5 is used as carrier gas, with a pressure of 500 mbar at the split valve. Temperatures of all interfaces (not being part of the pyrolyzer) and the GC oven are controlled by proportional-integral-derivative (PID) temperature controllers (CN7523, OMEGA Engineering GmbH, Deckenpfronn, Germany) equipped with type K thermocouples and additional solid-state relays. During each run, the temperature of the GC is maintained at 40 °C for a period of two minutes prior to an increase to 100 °C for a duration of 6 min. A 2 cm long PEEK capillary with an inner diameter of 1 mm is used to transfer the eluate from the GC to the IMS.

Ion Mobility Spectrometer

The miniaturized drift tube IMS used as a GC detector in this work is also self-built, including all the driver electronics and data acquisition. A detailed description of both can be found elsewhere [37, 38]. The operating parameters are summarized in Table 1. As mentioned above, the choice of an appropriate ionization source can be beneficial for suppressing the background signal, here PE. Thus, we operate the IMS with a tritium ionization source and a field switching ion gate to inject ions into the drift region [39]. As the development of all self-built components, also all measurements related to the IMS were performed at Leibniz University Hannover, Institute of Electrical Engineering and Measurement Technology, Department of Sensors and Measurement Technology, Hannover, Germany.

Table 1 Operating parameters of the experimental setup
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A list of all operating parameters of the experimental setup is given in Table 1.

Results and discussion

When using an IMS with tritium ionization source as GC detector, a continuous signal of the reactant ion peak is measured as long as no compounds elute from the GC. In turn, product ions form when compounds elute from the GC and if such compounds can be ionized by chemical gas phase reactions with reactant ions. This leads to a decrease in the amplitude of the reactant ion peak and an additional peak of the respective ion species from the eluted compound in the ion mobility spectrum. At higher concentrations of analytes in the reaction region, clustering can occur, which can also be observed in the ion mobility spectrum, e.g. as dimer peaks. Therefore, the reactant ion signal relates to the sum of compounds in the reaction region, and thus, the reactant ion signal over time yields the gas chromatogram. Such a decrease in the reactant ion signal due to eluted compounds is referred to as a chromatographic peak X, which may consist of a monomer signal XM and dimer signal XD. For better comparability, the captured data were pretreated to compensate possible deviations caused by instrument variations, such as effects of temperature and pressure.

Stacking all the recorded ion mobility spectra of a single GC run adds an additional retention time axis and results in a topographic plot, as shown in Fig. 1. For better visualization, the peak amplitude information is color coded and truncated at 8 pA giving the topographic plot shown in Fig. 2. Here, instead of drift time the inverse ion mobility \(1/{K}_{0}\sim {t}_{d}\) is plotted over the GC retention time \({t}_{R}\).

Fig. 2
figure 2

Topographic plots of Py-GC-IMS data of A virgin PE, B virgin PET, C dog food bag foil, and D PE recyclate with 25 wt.% dog food bag foil giving 3.3 wt.% PET in PE. Labeled chromatographic peaks are Peak A at \({t}_{R}\) = 46 s (\(1/{K}_{0}\)= 0.53 \(Vs/c{m}^{2}\)), Peak B at \({t}_{R}\) = 57 s (\(1/{K}_{0}\) = 0.53 \(Vs/c{m}^{2}\)), Peak C at \({t}_{R}\) = 75 s (\(1/{K}_{0}({C}_{M})\) = 0.58 \(Vs/c{m}^{2}\) and \(1/{K}_{0}({C}_{D})\) = 0.68 \(Vs/c{m}^{2}\)), Peak D at \({t}_{R}\) = 268 s (\(1/{K}_{0}\) = 0.67 \(Vs/c{m}^{2}\)), Peak E at \({t}_{R}\) = 350 s (\(1/{K}_{0}({E}_{M})\) = 0.60 \(Vs/c{m}^{2}\) and \(1/{K}_{0}({E}_{D})\) = 0.79 \(Vs/c{m}^{2}\)), Peak F at \({t}_{R}\) = 412 s (\(1/{K}_{0}({F}_{M})\) = 0.65 \(Vs/c{m}^{2}\) and \(1/{K}_{0}({F}_{D})\) = 0.85 \(Vs/c{m}^{2}\)). All operating parameters are listed in Table 1. Sample weight 200 µg. Color scale is limited to 8 pA for better visualization

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Figure 2A shows the topographic plot resulting from the pyrolysis of approximately 200 µg of virgin PE material. As desired, only a few peaks appear, of which the peaks A and B are considered for further investigation. Compared to the pyrolysis of the same amount of virgin PET, as shown in Fig. 2B, the signal caused by PE being the background matrix for the later detection of PET in PE is negligible.

The topographic plot of the dog food bag foil, as shown in Fig. 2C, clearly show, that these foils contain PE and PET, as the topographic plot contains the PET and PE related signals from Fig. 2A, in particular the peaks A and B, and Fig. 2B. Further to the signals related to PET and PE, the topographic plot of the dog food bag foil contains various signals in the lower inverse mobility and lower retention time region, which are most likely volatile compounds (VOCs). VOCs are likely to evaporate during melting in the mechanical recycling process, as they are not present in the final recyclate, see Fig. 2D. This correlates well with the fact that the dog food bag foil has still a noticeable odor of dog food it previously contained, but the recyclate does not. The amount of 13.1 wt.% (standard deviation σ = 1.1, n = 3) PET in the dog food bag foils was determined by Py-GC–MS at IKK. Therefore, the PET content in the recyclates tested is 3.3 wt.%, 2.6 wt.%, 2.0 wt.%, 1.3 wt.% for the respective recyclate mixing ratios of virgin PE and dog food bag foil of 75/25, 80/20, 85/15 and 90/10 wt.%/wt.%.

Considering the results of all PET containing samples (Fig. 2B–D), it is worth mentioning, that most of the compounds reach the IMS in rather high concentrations, since monomer and dimer peaks (two peaks with different \({1/K}_{0}\) but same \({t}_{R}\)) can be observed for almost all significant peaks in the topographic plots. The drift tube IMS response is strongly non-linear [29]. Typical response curves for monomers and dimers can be found in the literature, where linearization approaches are also reported [40,41,42]. With increasing concentration, the monomer signals start to decrease to zero after reaching a maximum, while the dimer signals typically flatten at higher concentrations, so that the change in signal response becomes small compared to the change in analyte concentration.

Figure 3 shows the chromatographic peak intensities of the peaks A–F, which comprise of the monomer (subscript M) and dimer peak (subscript D) intensities on the inverse mobility axis and depend on the composition of the analyzed recyclate. As mentioned above, the chromatographic peak intensity can be derived from the decrease of the reactant ion peak giving the total product ion current at a certain retention time including both, the monomer and dimer ions, and thus giving the concentration of the compound that relates to the chromatographic peak. As also mentioned above, gas phase ion chemistry plays a significant role in IMS response, and thus, some alkenes, alkanes and dienes, which are dominant pyrolysis products of PE, may have limited response without dimer formation [30, 33, 35]. Looking at peaks A and B, which clearly relate to PE, the strong non-linear IMS response with low sensitivity for higher concentrations leads to an almost constant signal intensity despite an increasing PET concentration.

Fig. 3
figure 3

Decrease in reactant ion peak amplitude representing the chromatographic peaks A–F at selected retention times for different PET contents (wt.%) in the PE recyclates. All measurement parameters are listed in Table 1. Standard deviation of the PET content (σ = 1.1%, n = 3) is given as x error bars, standard deviation of the IMS signal (σ = 0.11 pA, n = 80) is given as y error bars, but hidden by the data points

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Peak C clearly correlates with increasing amount of dog food bag foil and most likely originates from the PET it contains. From the 3.3 wt.% in the recyclate (Fig. 2D), up to the 13.1 wt.% PET in the dog food bag foil (Fig. 2C) increasing intensity is visible. Even the modest range of 1.3 wt.% to 3.3 wt.% PET in the recyclate samples leads to a visible change of the reactant ion peak intensity, and thus, a change of the chromatic peak intensity of 3 pA.

The signal of peak D correlates with a compound that is not part of the virgin PET or PE, but is part of the dog food bag foil, as it is present in the topographic plot of the pure dog food bag foil (Fig. 2C) and in all topographic plots of the tested recyclates. The signal does not significantly vary within the investigated PET content range. However, it is observed to be higher in the pure dog food bag foil samples. This indicates that a change in concentration only results in a minor change in the IMS response, and that these concentrations probably belong to the flat part of the IMS response curve with respect to the compound related to peak D. As mentioned above, the dog food bag foils are colored due to the product design and labeling of the bags, but the different colors of the dog food bag foil samples do not show a significant change in the topographic plots.

Peaks E and F demonstrate the most pronounced response to the PET concentration, exhibiting a change of 5.6 pA and 7.2 pA, respectively, within the range of 1.3 wt.% to 3.3 wt.% PET in the recyclate samples. Therefore, the chromatographic peaks C, E, and F can be employed as reliable indicators of PET contamination in PE.

All peaks considered here are pyrolysis products of the respective samples. While the pyrolysis products of PE are mainly alkenes, alkanes and dienes, the list of observed pyrolysis products of PET described in the literature is much longer [43,44,45]. However, this feasibility study shows that IMS gives a highly sensitive response to compounds that correlate with PET contamination in PE recyclates. As the investigations were limited to available material with a PET content of 1.3 wt.% to 3.3 wt.%, but pyrolysate compounds still form dimers in the IMS, it is likely that much lower levels of PET can be detected with this experimental setup at much higher sensitivity as known from IMS for small concentrations.

The dimer formation observed in our measurements is not an issue. As mentioned, it merely demonstrates the high sensitivity of the setup. However, if higher PET concentration have to be quantified, which is difficult due to the non-linear response curve of IMS for higher concentrations, less sample can be pyrolyzed or higher GC split ratios can be employed to reduce the analyte concentration reaching the GC and IMS, thereby decreasing dimer formation. More complex mixtures will also not present any issues, provided that the GC is capable of separating the individual constituents of the pyrolysate, which will then reach the IMS in a sequential manner. Nevertheless, co-eluting compounds can sometimes form mixed dimers consisting of different molecules if co-eluting in high concentrations. Further details on the formation of dimers and mixed dimers in GC-IMS can be found in prior work [36].

All measurements were conducted with a pyrolysis temperature of 800 °C. This temperature was selected as a starting point, and it was found to be effective in yielding reasonable results. As this work represents the feasibility of the experimental setup and not a dedicated method development, no further optimization of the pyrolysis temperature has been carried out.

It should be also noted that the homogeneity of the recyclate samples is very limited due to the mechanical recycling process and the properties of PE and PET. Scraping a sample from the recyclate granulate of 3–4 mm in diameter may result in different PET concentrations in the sample than in the recyclate. The same applies to other contaminants, such as print colors. Thus, the primary uncertainty pertains to the sample. To counteract this, a relatively large sample of approximately 200 µg was used for each measurement, as analysis of a smaller sample may yield less dimer formation but considerable variation in the respective concentrations. Nevertheless, the total amount of PET entering the IMS should be considered as an estimate. However, for the lowest recyclate mixing ratio of 90/10 (wt.%/wt.%) of virgin PE and dog food bag foil, the initial 200 µg sample contains approximately 1.3 wt.% PET (equivalent to 2.6 µg). Before the pyrolysate enters the GC, a split valve (50:1) removes approximately 98% of the pyrolysate to prevent overloading the GC and contamination of the IMS, which may result in memory effects. Due to the single gas outlet of the IMS located at the reaction region, as shown in Fig. 1, further dilution of the GC eluent with the drift gas from the IMS needs to be considered. Thus, approximately a ratio of 100:3 results. This implies that a total of only 1.6 ng of PET pyrolysis products enters the IMS during a measurement, yet still results in dimer formation within the IMS, and condensation effects at any point in the setup that further reduce PET constituents entering the IMS are still neglected, since they also would cause memory effects, which have not been observed during all the measurements. Nevertheless, the results demonstrate the remarkable sensitivity of the IMS for this application.

Conclusion

In this work, a Py-GC-IMS with self-built, miniaturized GC-IMS is evaluated for fast detection of PET impurities in PE recyclates towards in-process quality monitoring. Virgin PE and PET as well as dog food bag foils as recyclate feedstock are characterized to identify the origin of certain peaks in the topographic plot of the analyzed recyclates. Since many pyrolysate compounds show dimer formation in the measurements, high concentrations reach the IMS. Thus, the IMS signals merely change when changing the recyclate composition due to the non-linear response curve saturating at high concentrations. Based on this, it is likely that much lower concentrations can be detected by the system. This is a promising result for operators of mechanical recycling plants would like to monitor the PET contamination and thus PE recyclate quality. In addition, the system can give an indication of the olfactory quality by detecting VOCs.

As an outlook, although a complex commercial pyrolyzer is used in this work, only drying and single shot pyrolysis are applied, which can be realized with less effort. Considering the level of miniaturization of the drift tube IMS and the GC, the whole system can be further miniaturized towards a small benchtop device.