A review of laser-induced breakdown spectroscopy for plastic analysis

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Highlights

  • The recent progress in plastic analysis using LIBS technology is presented.

  • LIBS instruments, analytical chemistry methods, and applications for plastic analysis are reviewed.

  • Advantages, problems (and their solutions), and future perspective for plastic analysis are commend.

Abstract

With the extensive use of plastic in our daily lives, the increasing requirements for disposal and quality control of plastic products raise more concerns. Laser-induced breakdown spectroscopy (LIBS) has the advantages of requiring little or no sample pretreatment and being high-speed and minimally invasive. The combination of LIBS and analytical chemistry methods has become an efficient method of plastic analysis. This review provides a comprehensive overview of LIBS instruments used for plastic analysis, discrimination among different types of plastics, and the qualitative and quantitative analysis of plastic. Furthermore, applications of plastic are reviewed, including in toys, food containers, e-waste, etc. Finally, limitations and the potential developing trend for the topic are proposed.

Introduction

Plastic is composed of a polymer and varying additives that give such substances their chemical and physical characteristics [1]. With the extensive use of plastic products, plastic pollution has become a pressing issue for environmental protection [2], [3]. Currently, manual sorting, Inductively coupled plasma (ICP), Raman spectroscopy, Near-infrared (NIR) spectroscopy, X-ray fluorescence spectrometry, and Atomic absorption spectroscopy (AAS) are frequently used methods for identifying plastic. Manual sorting is a traditional method based on differences in physical properties [4], but it presents some shortcomings such as being error-prone, high-cost, and time-consuming [4], [5]; ICP, which provides efficient atomization, excitation and ionization, has advantages for the analysis of liquid samples with high accuracy [6], while it presents time-consuming steps for sample dissolution prior to the plastic analysis [7]; Raman spectroscopy evaluated as a spectroscopic method can be used to detect molecule bands [8], despite the signal of the Raman spectrum being very weak for plastic analysis [9]; NIR spectroscopy is the most widely used method and meets the sorting requirements for plastic identification [2], [10], however, it cannot be used for colored or opaque plastic (and plastic thicker than 20 microns) analysis [11]; X-ray fluorescence spectrometry provides another sorting technique, with the only advantage of being able to identify polyvinyl chloride by detecting chlorine atoms [2], [12]; AAS has the advantage of higher sensitivity [13], [14], however, it must be crushed and dissolved in liquid, which is tedious and time-consuming [15]. There is a pressing need for developing a newer, faster, and more efficient method for the recycling and quality control of plastic products [16].

Laser-induced breakdown spectroscopy (LIBS) has been developed as a new powerful technology for analytical chemistry [17]. Element-specific lines are emitted from the decay processes of the plasma, detected by a spectrometer, and analyzed by monitoring their positions and intensities to carry out qualitative and quantitative measurements [17]. LIBS has many advantages in the analysis of solid samples including the ability to analyze samples with little or no preparation [18], with a simple method for setting up an apparatus; it is also minimally invasive [19] and high speed [7]. LIBS has been widely used in the metallurgy industry [20], [21], [22], [23], environmental monitoring [24], [25], [26], archeology [27], [28], [29], space exploration [30], [31], [32], [33], geological applications [34], [35], [36], [37], biomedical detection [38], [39], [40], etc. In recent years, LIBS has become a viable tool for the fast detection of colored and polluted plastics in laboratory and field investigations [18], [19], [41]. This review aims to present a discrimination, qualitative and quantitative analysis of plastic and its applications in the plastic field, following a concise description of LIBS instruments used in plastic analysis.

Section snippets

LIBS instruments

LIBS instruments that are used for plastic analysis, as shown in Fig. 1, generally consist of a pulsed laser for generating a plasma, optic systems to concentrate and gather the light, a spectrometer that contains a detector to record the spectrum, extra components that include an object holder and a computer to analyze the spectrum [17]. As listed in Table 1, the most prevalent laser used in plastic analysis is the Nd:YAG (neodymium-doped yttrium aluminum garnet) laser operated at the

Discrimination of plastic

Plastic types studied in published literature include acrylonitrile-butadiene-styrene (ABS), polycarbonate (PC), polystyrene (PS), styrene butadiene (SB), high-density polyethylene (HDPE), polyethylene (PE), polytetrafluoroethylene (PTFE), polyamide (PA), low-density polyethylene (LDPE), polyphenylene oxide (PPO), polyoxymethylene (POM), polyethylene terephthalate (PET), polymathy methacrylate (PMMA), high impact polystyrene (HIPS), polypropylene (PP), thermoplastic polyester (TPO),

Material identification

Material identification is the first stage in the recycling and reuse of plastic waste. Analytical chemistry methods are used for exacting the differences in LIBS plastic spectra, with chemometric methods being popular methods [16], [42], [48], [54], [56], [61], [67], [68].

Selecting suitable spectral variables can improve classification accuracy because analytically useless parts of LIBS plastic spectra are eliminated. Artificial neural networks (ANN) were used for classifying eight types of

Quantitative analysis of plastic

The ultimate goal of any analysis technique is quantitative analysis [19]. Due to the limitation of LIBS instruments in plastic analysis, including the matrix effect and self-absorption effect, the precision of quantitative plastic analysis using a LIBS instrument is relatively lower than other instruments. LIBS quantitative analysis includes calibration and calibration-free method [74].

LIBS applications in the plastic field

In recent years, an increasing number of plastic products have been studied for realizing recycling and quality control of plastic products, with a few studies applied to field applications using On-line LIBS instrument. LIBS was used with an autofocusing unit to quantify the concentration of the toxic elements (Cd, Cr, Hg, Pb, Sb, and Br) in real end-of-life waste plastic pieces, with the use of single- and double-pulse lasers studied to improve sensitivity and limits of detection [7]. LIBS,

Conclusions and perspective

Recent progress for LIBS in plastic analysis was presented in this review, including LIBS instruments used for plastic analysis, discrimination of plastic, qualitative and quantitative analysis of plastic, and LIBS applications in the plastic field. Due to the specific advantages of LIBS, it has become a promising method in fast detection of toxic elements in toys and the recycling of e-waste plastic and food containers.

Material identification using an LIBS instrument is a focus study in

Acknowledgements

The authors are grateful for the financial support from National Major Scientific Instruments and Equipment Development Special Funds (No. 2016YFF0103303 and No. 2011YQ03011302) and National Natural Science Foundation of China (Grant No. 61505066).

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