Laser Direct IR Imaging Projects Microplastics Analysis

DARREN ROBEY, AGILENT TECHNOLOGIES INC.

Contamination of soil, air, and drinking water by microplastics is a growing focus of attention around the world. Environmental agencies are increasing the monitoring of waterways, and governmental bodies are seeking to protect these resources. Simultaneously, research institutions are trying to determine the extent and potential toxicological impacts of microplastic contamination. Optical technologies are playing a vital role in these studies.

Microplastic pollution has become a worldwide dilemma. Courtesy of Agilent.

Despite the enormous interest, the use of various analytical techniques and the lack of method standardization have resulted in a lack of consistency in data gathering. Therefore, it is difficult to compare studies to establish a broader understanding of the prevalence and impact of microplastics in the environment. A 2019 study, the first international cross-laboratory study of its kind, compared a range of common microplastic analysis techniques¹. While providing a useful comparison of the various methods, the authors said, “There were large discrepancies between the returned results, which point out an urgent need for standardization in microplastic analysis and method validation within the laboratories, including sample preparation and measurement, in order to ensure comparable and sound results.”

The options for analysis of microplastics fall into two broad groups of methodologies: spectroscopic techniques and techniques based on gas chromatography/mass spectrometry (GC/MS). Spectroscopic techniques include IR and Raman, whereas GC/MS may be combined with either pyrolysis (decomposition through heat) or thermoextraction and desorption. Each of these categories has its strengths and weaknesses in the study of microplastics. GC/MS techniques are favored when a determination of the total polymer mass within the plastic is needed. But spectroscopic techniques are useful, too, because they provide insights into the nature of the particles, including size and shape, in addition to polymer type.

Of the spectroscopic options, many scientists favor infrared microscopy — which uses a spectrophotometer and is based on the familiar Fourier transform infrared (FTIR) technique — for its ability to distinguish with relative ease between most common polymers. However, its broad applicability has been constrained by long analysis runtimes, especially for larger-scale studies and routine high-throughput testing, such as those required to support environmental regulations. Advancements in the field of infrared spectroscopy and microscopy, however, may well overcome these limitations.

Specificity with QCLs

A quantum cascade laser (QCL) is a semiconductor-based laser in which electrons cascade through a series of quantum wells and emit light. The most common application for QCLs is spectroscopic analysis of pollutants and gases in the atmosphere, although a wider range of applications are emerging for this modality. QCLs were first demonstrated at Bell Laboratories in 1994². However, it was not until 2002, when their use at room temperature became feasible in a wide variety of settings, that their application became more practical³.

In a QCL, electrons cascade or tunnel through a series of quantum wells formed by thin layers of semiconductor material. In a diode-based laser, electron-hole recombination across a semiconductor bandgap emits photons, and the photon wavelength is set by the chemistry of the materials used. In contrast to a diode-based laser, photon wavelength in a QCL is not determined by the semiconductor materials but rather by the thickness and distribution of the semiconductor layers⁴.

The Agilent 8700 LDIR (laser direct infrared) chemical imaging system. Courtesy of Agilent.

QCLs have been developed to operate from the mid-infrared (MIR) range right through to terahertz, but it is the information-rich fingerprint region of the MIR that is of particular interest in this area of spectroscopy. Unlike FTIR spectroscopy, in which a sample is exposed to the entire available wavelength range simultaneously, a QCL can be tuned to individual wavelengths, allowing alternative modes of operation not available with FTIR. In some ways, the QCL can be compared to a monochromator-based approach, since individual wavelengths are emitted independently. However, the QCL operates at speeds and wavelength accuracy that are orders of magnitude better in comparison to FTIR.

Evolutionary innovations in LDIR

The first application of laser direct infrared imaging (LDIR) to MIR spectroscopy used array-style detectors and tended to mimic the gathering of information by FTIR spectrometers while exploiting some of the advantages of the QCL. Especially notable in this regard was the elimination of the need for cryogenically cooled detectors.

In Agilent’s 8700 LDIR chemical imaging system, a QCL is combined with a single-point mercury cadmium telluride (MCT) detector (thermometrically cooled) and rapidly scanning optics. This combination facilitates two useful modes of action. In the first mode, the frequency is parked (i.e., a single wavelength is selected) while the optics move at high speed over the sample, reflecting the light back into the detector. In the second mode, the optics are parked at a single point over the sample while the QCL sweeps through the frequency range.

A schematic showing how quantum cascade laser technology works. MCT: mercury cadmium telluride; ATR: attenuated total reflectance. Courtesy of Agilent.

A key limitation of existing FTIR spectrometers, when applied to microplastics analysis, is that they need to collect the full spectrum for every pixel in the area surveyed. A typical FTIR spectrometer used for this purpose would be fitted with a focal plane array (FPA) detector. Even the largest arrays (128 × 128 pixels) would cover only an area of 700 × 700 µm. Acquisition time for a typical sample on a 10-mm-diameter filter could be more than 3 hours, while total data collected would be more than 30 GB, with processing times of up to 10 hours. Much of this data would be redundant because a full spectrum is collected for each pixel in the analysis area regardless of the presence or absence of a microplastics particle.

By combining a QCL with a single-point detector and rapidly scanning optics, a different approach can be taken. Using the first mode of action, the sample area can be rapidly scanned at a single wavelength. The information obtained from this sweep can be used not only to determine the location of particles but also to describe their size and shape. Once the particles are located, the second mode can be used. The objective can be moved over the analysis area to each particle and, using the parked objective/frequency sweep mode, obtain a full spectrum from each. This spectrum can then be immediately analyzed and the results reported.

Most importantly, spectra acquisition locations can be targeted to acquire data only at relevant points, eliminating redundant data and reducing data processing time. In addition, these workflows can be fully automated and total analysis times can be reduced from 10 or more hours to less than 1 hour.

In the future, analysis of the prevalence of microplastics in environmental and food samples will extend from research laboratories to routine analysis by regulatory bodies, water authorities, and the food and beverage industry. This could potentially support an improved regulatory approach, which would likely require accessible analytical techniques that could be used in a wide variety of settings in the field. A key change needed in current applications, to address a criticism made within many current studies that point out only a small number of samples are used from large bodies of water, is the ability to analyze more samples in a shorter time. This ability would lead to better information by reducing statistical variability.

MIR spectroscopy will remain a favored technique among scientists when information about not only the polymer type but also the number and physical characteristics of the particle is required. The ability to analyze more samples in a routine laboratory setting drives the need for robust, repeatable, fast, and simple-to-use analytical solutions.

Meet the author

Darren Robey is global product manager of the Infrared Imaging, Molecular Spectroscopy Group at Agilent Technologies Inc. He has worked with applied analytical techniques for more than 20 years in positions ranging from laboratory technician to product manager. Over the years, Robey has developed a keen interest in taking complex analytical techniques and developing systems and workflows to make them simple. Of special interest has been removing or limiting the potential for analyst error; email: darren.robey@agilent.com.

References

1. Y.K. Müller et al. (2020). Microplastic analysis — are we measuring the same? Results on the first global comparative study for the microplastic analysis in a water sample. Anal Bioanal Chem, Vol. 412, pp. 555-560.

2. J. Faist et al. (1994). Quantum cascade laser. Science, Vol. 264, Issue 5158, pp. 553-556.

3. M. Beck et al. (2002). Continuous wave operation of a mid-infrared semiconductor laser at room temperature. Science, Vol. 295, Issue 5553, pp. 301-305.

4. J. Faist et al. (2008). Progress in quantum cascade lasers. In Mid-Infrared Coherent Sources and Applications, M. Ebrahim- Zadeh and I.T. Sorokina, eds. NATO Science for Peace and Security Series B: Physics and Biophysics. Cham, Switzerland: Springer Nature.