Optical Extraction of Single Microplastics Followed by Online Molecular and Elemental Characterization

TL;DR

A new method combines optical trapping, Raman spectroscopy, and ICP-MS to accurately detect and characterize microplastics in complex environments.

Contribution

A novel trimodal platform using optical extraction for single microplastic characterization is developed and demonstrated.

Findings

Optical trapping improved detection limits for microplastic size and mass by factors of 3.1 and 28, respectively.

The method enabled polymer identification and reduced background interference in SP ICP-MS.

The platform was successfully tested on microplastics in high carbon matrices and soil samples.

Abstract

The accurate characterization
of microplastics (MPs) in complex
matrices remains a major analytical challenge and requires advanced
methods, which decipher information on size and polymer identity at
single particle resolution. Single particle (SP) inductively coupled
plasma–mass spectrometry (ICP-MS) has emerged as an element-selective
method to detect individual particles in a one-by-one fashion and
can be used to detect and characterize MPs regarding carbon mass and
particle sizes. However, this technique has two relevant shortcomings.
First, its ability to pinpoint small MPs requires a low dissolved
C-background. Second, SP ICP-MS neither distinguishes different MP
polymer species from each other nor from other C-particulates (e.g.,
cells, black carbon). As such, the application of SP ICP-MS is significantly
limited when targeting MPs in complex matrices without a priori knowledge. Here, we present a novel trimodal analytical platform
that integrates optofluidic force induction (OF2i), single particle
Raman spectroscopy (SP Raman), and SP ICP-MS for multimodal online
MP characterization. The main objective was the development and demonstration
of an optical extraction mechanism, in which an optical trap employing
a weakly focused laser vortex beam was used to immobilize MPs from
a sample suspension. This provided two analytical opportunities, which
were demonstrated in conjunction with a SP Raman module and SP ICP-MS.
First, the optical trapping enabled the investigation of inelastically
scattered light of individual MPs via Raman spectroscopy and consequently,
the identification of polymer type. Second, the trapping enabled a
matrix exchange, reducing the background signal in SP ICP-MS. Both
the polymer identification and the reduction of background signal
resulted in improved detection and calibration capabilities in SP
ICP-MS. Polymer type identification via SP Raman provided the C-mass
fraction and polymer density in MPs, which are critical factors for
size estimations in SP ICP-MS. However, the reduced background and
improved size detection limit enabled the analysis of smaller MPs.
In a proof-of-concept, 5 μm polystyrene particles were dispersed
in a high carbon content matrix (1 g C/L) and analyzed via OF2i-SP
Raman-SP ICP-MS using the optical extraction mechanism. The mass and
size detection limits were improved by factors of 28 and 3.1, respectively,
and were determined to be 1.0 μm and 0.6 pg of carbon per particle.
In a second proof-of-concept, polyamide-6 (PA-6) MPs were spiked into
soil to simulate a complex terrestrial environment at the lab-scale.
Following the resuspension, PA-6 MPs were optically extracted and
analyzed using SP Raman and SP ICP-MS. The optical extraction of MPs
is a new concept, which enables isolation of specific polymer particles
for molecular and elemental single particle characterization. However,
the developed methods can also be expanded to characterize inorganic
(non-polymeric) nano- and microstructures.

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