Received: 6 June 2022 Revised: 1 July 2022 Accepted: 1 July 2022

DOI: 10.1002/rcm.9351

RESEARCH ARTICLE

Thermogravimetry combined with electrospray and

atmospheric pressure chemical ionization mass spectrometry
for characterization of synthetic polymers

Hsing-Jung Lin1 | Siou-Sian Jhang1 | Jui-Hung Hung1 | Yao-Sheng Zhang1 |

Heng-Liang Wu2,3 | Jentaie Shiea1,4,5

1
Department of Chemistry, National Sun
Yat-Sen University, Kaohsiung, Taiwan Rationale: Thermogravimetry (TG) combined with electrospray and atmospheric
2
Center for Condensed Matter Sciences, chemical ionization (ESI+APCI) mass spectrometry (MS) was developed to rapidly
National Taiwan University, Taipei, Taiwan
3
characterize thermal decomposition products of synthetic polymers and plastic
Center of Atomic Initiative for New Materials,
National Taiwan University, Taipei, Taiwan products. The ESI-based TG-MS method is useful for characterizing thermally labile,
4
Department of Medicinal and Applied nonvolatile, and polar compounds over an extensive mass range; and the APCI-based
Chemistry, Kaohsiung Medical University,
TG-MS counterpart is useful for characterizing volatile and nonpolar compounds.
Kaohsiung, Taiwan
5
Rapid Screening Research Center for
Both polar and nonpolar compounds can be simultaneously detected by ESI+APCI-
Toxicology and Biomedicine, National Sun Yat- based TG–MS.
Sen University, Kaohsiung, Taiwan
Methods: Analytes with different volatility were produced from TG operated at
Correspondence different temperatures, which were delivered through a heated stainless-steel tube
J. Shiea, Department of Chemistry, National
Sun Yat-Sen University, No. 70, Lienhai Rd.,
to the ESI+APCI source where they reacted with the primary charged species
Kaohsiung 80424, Taiwan. generated from electrospray and atmospheric pressure chemical ionization
Email: jetea@mail.nsysu.edu.tw
(ESI+APCI) of solvent and nitrogen. The analyte ions were then detected by an ion
Funding information trap mass spectrometer.
Rapid Screening Research Center for
Toxicology and Biomedicine, National Sun Yat- Results: A semi-volatile PEG 600 standard was used as the sample and protonated
Sen University; NSYSU-KMU Joint Research and sodiated molecular ions together with adduct ions including [(PEG)n + 15]+,
Project, Grant/Award Number: NSYSUKMU
111-I03; Ministry of Science and Technology
[(PEG)n + 18]+, and [(PEG)n + 29]+ were detected by TG-ESI+APCI-MS. The
of Taiwan, Grant/Award Number: technique was further utilized to characterize thermal decomposition products of
109-2113-M-110-007-MY3
nonvolatile polypropylene glycol (PPG) and polystyrene (PS) standards, as well as a
PS-made water cup and coffee cup lid. The characteristic fragments of PPG and PS
with mass differences of 58 and 104 between respective ion peaks were detected at
the maximum decomposition temperature (Tmax).
Conclusions: The information obtained from the TG–ESI+APCI-MS analysis is useful
in rapidly distinguishing different types of polymers and their products. In addition,
the signals of the additives in the polymer products, including antioxidants and
plasticizers, were also detected before the TG temperature reached Tmax.

1 | I N T RO DU CT I O N specificity, AIMS has been applied in many fields such as food safety,
antidrug, antiterrorism forensic science, environmental science, and
1–3
Ambient ionization mass spectrometry (AIMS) allows the direct molecular imaging. Electrospray ionization (ESI)4 and atmospheric
characterization of molecules in various raw samples in open air with pressure chemical ionization (APCI)5 are the two main ionization
minimal or no sample pretreatment. For its high sensitivity and mechanisms involved in AIMS. ESI-based AIMS utilizes the charged

Rapid Commun Mass Spectrom. 2022;36:e9351. wileyonlinelibrary.com/journal/rcm © 2022 John Wiley & Sons Ltd. 1 of 10
https://doi.org/10.1002/rcm.9351
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2 of 10 LIN ET AL.

species generated from an ESI emitter to desorb and ionize polar a probe for ionizing the analytes generated by IR irradiation has also
components in samples. Representative ESI-based ambient ionization been reported.19 The translocation behavior and metabolism of isotianil
6
techniques include desorption electrospray ionization (DESI), in tomato plants over time were studied using this technique.19
7
electrospray-assisted laser desorption ionization (ELDI), fused- Since AIMS enables operation in the open air, it is rational to couple
droplet electrospray ionization (FD-ESI),8 extractive electrospray it with some analytical methods for obtaining compositional information
ionization (EESI),9 and thermal desorption electrospray ionization of the sample without vacuum limitations, such as pyrolysis and
10
(TD-ESI). APCI-based AIMS techniques have distinct mechanisms of thermogravimetric analysis. Thermogravimetry (TG) is a well-established
ion formation via ion–molecule reactions between analytes and analytical technique that measures the rate of mass loss of a material as
charged species generated by plasma or corona discharge APCI. a function of the temperature under atmospheric pressure.20 The
Examples of APCI-based ambient ionization techniques include information on mass loss of the sample (due to thermal decomposition
10
desorption atmospheric pressure chemical ionization (DAPCI), direct or oxidation for loss of volatiles or additives) is used to determine the
analysis in real time (DART),11 dielectric barrier discharge ionization thermal stability of the sample. This analytical technique is particularly
(DBDI),12 and low-temperature plasma (LTP).13–15 useful for studying polymeric materials, like thermoplastics, thermosets,
ESI- and APCI-based AIMS have been successfully deployed for elastomers, composites, films, fibers, coatings, and paints.21 In addition,
direct analysis of a wide range of samples. Generally speaking, the use of TG in conjunction with other analytical techniques such as
ESI-based AIMS is useful for characterizing thermally labile, gas chromatography (GC) and Fourier-transform infrared (FTIR)
nonvolatile, and polar compounds over an extensive mass range, while spectroscopy helps to interpret the mechanisms of thermally induced
APCI-based AIMS is useful for characterizing volatile and nonpolar reactions of the sample.22,23 The combination of TG and mass
16–18
compounds. Therefore, complementary information on a sample spectrometry (MS) can provide online information on gaseous species
with complicated composition can be obtained through ESI-MS and and mass loss ratios resulting from thermal decomposition of the
APCI-MS analyses of the sample. For this reason, ambient ionization sample. The first use of TG–MS was reported in the 1990s, where the
sources with various arrangements of ESI and APCI techniques have gases released from the sample were introduced into a chamber for
been reported. Among them, we reported the simultaneous detection photoionization.24 The analyte ions were then detected by a quadrupole
of polar and nonpolar compounds using a concentric ESI+APCI dual mass analyzer attached to the chamber. TG coupled with a single-
ionization source which can be operated in ESI-only, APCI-only, and photon ionization/time-of-flight mass spectrometer and atmospheric
ESI+APCI mode.16,17 The concentric arrangement of the ESI and pressure chemical ionization/quadrupole mass spectrometer were
APCI source was constructed by inserting a fused-silica capillary for developed later.25–28 Thereafter, mass spectrometers equipped with
ESI into a stainless-steel tube enclosed in a quartz tube. One end of different types of soft ionization approaches have been further used to
the fused-silica capillary was attached to one arm of a three-way tee, characterize the evolved gas during TG analysis.29–33
whereas methanol solution was pumped into the second arm of the In this study, TG coupled to an ion trap mass spectrometer
three-way tee by a syringe pump. A copper electrode was inserted equipped with a dual ESI and APCI ionization source was developed for
into the third arm of the three-way tee. The high voltage required to characterization of thermally decomposed products from synthetic
induce ESI from the methanol solution flowing out of the capillary polymers. The additives and thermal decomposition products of
was introduced to the solution through solution conduction. This was polymer standards and plastic products generated in the TG step were
simply done by applying a high voltage at the copper electrode. For delivered via a heating transfer tube into the reaction chamber (i.e., dual
APCI, a high alternating current (AC) voltage was applied to the ring ESI+APCI ionization source) to react with the primary ion species in the
electrode on the quartz tube to generate plasma through a nitrogen ESI and APCI plumes. The experimental results have demonstrated the
gas stream flowing between the quartz tube and stainless-steel tube. capability of using TG–ESI+APCI-MS to rapidly determine the chemical
In ESI-only mode, multiply charged analyte ions were formed through compositions of polymer standards and their products.
fused-droplet ESI mechanisms16; however, singly charged analyte ions
were formed via ion–molecule reactions (IMRs) between analytes and
charged solvent species such as H+, [H2O]nH+, and [MeOH]mH+. In 2 | M A T E R I A L S A N D M ET H O D S
APCI-only mode, singly charged radical ions or protonated ions were
formed via IMRs between analytes and charged species. In ESI+APCI 2.1 | Samples and chemical reagents
mode, the ionization mechanisms are IMRs by the interactions of
analytes with the charged species in the ESI and APCI plumes. The Synthetic polymer standards including polyethylene glycol 600 (PEG
technique allows simultaneous ionization and detection of both polar 600), polypropylene glycol 400 (PPG 400), and polypropylene glycol
and nonpolar compounds in the sample. 2000 (PPG 2000) were purchased from Alfa Aesar (Heysham,
Later, a cross-shaped tube with symmetric discharge electrodes Lancashire, UK). Testing samples including octadecyl 3,5-di-tert-butyl-
was developed to generate dielectric barrier discharge to produce 4-hydroxyhydrocinnamate (AO1076), tris(2,4-di-tert-butylphenyl)
reactive primary species from inert gas, while a coaxial capillary was phosphate (AO168), PPG (polypropylene glycol, Arcol®, Covestro,
placed at the center of the cross-shaped tube to generate an Taiwan) and PS (polystyrene) pellets (HIPS PH-88S, Chimei, Taiwan)
electrospray plume.16 A similar design of a dual ESI and APCI source in were purchased from local chemical suppliers (Wineich, Taiwan).
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LIN ET AL. 3 of 10

PS-made products including a water cup and coffee cup lid were (0.1–10 mg) and placed in a platinum crucible in the TGA. The flow rate
purchased from a local supermarket. Reagent-grade acetic acid and of the nitrogen gas in the TGA was fixed at 50 mL/min throughout the
HPLC-grade methanol were purchased from J. T. Baker (Phillipsburg, analysis. The temperature of the TGA was increased from 30 to 600 C
NJ, USA) and Merck (Darmstadt, Germany), respectively. Distilled at different heating rates (10–100 C/min). The gaseous pyrolysates
deionized water (purified through a Milli-Q plus apparatus; Millipore, from the TGA were continuously purged and delivered through a
Molsheim, France) was used to prepare the ESI solution. heated interface (300 C) to an ion trap mass analyzer equipped with an
ESI+APCI dual ionization source for further analysis. The TGA
combined with the ESI+APCI-MS is illustrated in Figure 1.
2.2 | Thermogravimetric analysis

Information about the maximum decomposition temperature (Tmax), the 2.3 | Heated interface to combine TG with MS
mass loss curve (TG curve), and the first derivative of the TG curve
(i.e., DTG curve) of the polymer samples was obtained via analysis TG was combined with MS through a heated interface. The interface
using a commercial thermogravimetric analyzer (TGA1, Mettler Toledo, was comprised of a stainless-steel tube, cartridge heater, and heat
Zurich, Switzerland). The solid or liquid sample was weighed protection shield (Figure 1). A stainless-steel tube (2.1 cm i.d., 30 cm

F I G U R E 1 (A) Schematic
illustration of desorption/
ionization of the analyte by TG–
ESI+APCI-MS: A – Stainless-steel
tube (500 μm i.d., 700 μm o.d.); B
– Ring electrode (3.2 mm, applied
with 7.0 kVpp of AC voltage); C –
Quartz tube (2 mm i.d. and 3 mm
o.d.); D – ESI capillary (100 μm i.
d., 375 μm o.d., the orifice of the
mass spectrometer was applied
with 4.5 kV of DC voltage); E –
Nitrogen gas flow (0.4 L/min); F –
Stainless-steel transfer tube
(300 C). (B) Photograph of the
TG–ESI+APCI-MS setup [Color
figure can be viewed at
wileyonlinelibrary.com]
 10970231, 2022, 18, Downloaded from https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/10.1002/rcm.9351 by National Chung Hsing, Wiley Online Library on [27/05/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
4 of 10 LIN ET AL.

long) was used to connect and transfer gaseous pyrolysates from the these conditions, a stable ESI plume and APCI plasma were
TGA to the mass spectrometer. A 180 W cartridge heater and k-type generated in the ionization source without arcing. This dual
thermocouple were used to generate heat and control the ionization source could be operated in ESI-only, APCI-only, or
temperature of the stainless-steel tube. The outermost layer of the ESI+APCI mode to ionize the gaseous pyrolysates from the TGA.
stainless-steel tube was covered with glass fibers and thermal The control of the ionization mode was achieved simply by switching
insulation foam. The temperature of the heated interface was on/off of the high-voltage power suppliers for ESI and plasma APCI.

maintained at 300 C throughout the experimental period. For ESI-only mode, the high voltage for ESI was turned on while the
voltage for plasma-APCI was turned off, and vice versa for APCI-only
mode. Both high-voltage power suppliers for ESI and APCI were
2.4 | Dual ESI+APCI ionization source combined turned on for ESI+APCI mode. An ion trap mass spectrometer
with the mass spectrometer (Esquire 6000 plus, Bruker Daltonics, Bremen, Germany) was used to
record the mass spectra with a scan range of m/z 100–1000
The dual ionization source was constructed by combining an ESI (100 ms/scan) in positive ion mode.
source with a plasma-APCI source (Figure 1A). A fused-silica capillary
(i.d. 100 μm, o.d. 375 μm; D) was inserted into a stainless-steel tube
(i.d. 500 μm, o.d. 700 μm; A) which was enclosed in a quartz tube 3 | RESULTS AND DISCUSSION
(i.d. 2 mm, o.d. 3 mm; C). The ESI solution was methanol/water
(1:1, v/v) with 0.1% acetic acid and the solution was delivered 3.1 | Analysis of PEG 600 by ESI-MS and TG–
through the fused-silica capillary at a flow rate of 150 μL/h by a ESI+APCI-MS
syringe pump (KDS100, Merck). A high direct current (DC) electric
field was generated between the ESI solution flowing out of the The PEG standard has thermally unstable C–O bonds in its structure
capillary and the orifice of the mass spectrometer to induce ESI. This and will decompose as the temperature reaches Tmax. Previously,
was simply done by applying 4.5 kV on the orifice of the mass structural information of volatile and semi-volatile PEGs has been
spectrometer. A high AC voltage (7.0 kVpp, 19 kHz) was applied to published according to the results from pyrolysis and GC/MS.34–36 In
the ring electrode (i.d. 3.2 mm; B) attached to the quartz tube to this study, TG combined with ESI+APCI-MS was applied to
generate plasma from a nitrogen gas stream (0.4 L/min) flowing characterize a PEG standard. A solution of PEG 600 (10 ppm in
between the quartz tube (C) and the stainless-steel tube (A). Under methanol) was used to test the performance of the TG–ESI+APCI-MS

F I G U R E 2 Mass spectra of PEG 600 recorded by (A) infusion ESI-MS; (B) TG–ESI-MS at 447 C (Tmax of PEG 600); (C) TG–APCI-MS at Tmax;
and (D) TG–ESI+APCI-MS at Tmax. ●: [M + H]+, ▲: [M + NH4]+, ▼: [M + Na]+, ■: [M + C2H5]+, ◇: [M + CH3]+ [Color figure can be viewed
at wileyonlinelibrary.com]
 10970231, 2022, 18, Downloaded from https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/10.1002/rcm.9351 by National Chung Hsing, Wiley Online Library on [27/05/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
LIN ET AL. 5 of 10

method. Figure 2A displays the mass spectrum of PEG 600 analyzed (Figure S2c). The results indicated that small pyrolytic PEG molecules
by traditional infusion ESI-MS at room temperature to confirm the were successfully ionized via interactions with the primary charged
composition of the PEG 600 standard. The PEG 600 standard (Mw: species in the plasma. Figure 2D shows the TG–ESI+APCI mass
617, Mn: 588) with a polydispersity of 1.04 contained a rather narrow spectrum of PEG 600 recorded at Tmax (447 C). As can be seen, the
distribution of PEG units. Actually, two series of protonated ions detected by ESI-MS, TG–ESI-MS and TG–APCI-MS were all
oligomeric PEG ions (MH+, ●, M: (PEG)n) with a mass difference of detected on the TG–ESI+APCI mass spectrum. The experimental
44 between neighboring ion peaks were detected (Figure 2A). They results indicated that the use of the dual ionization source could
featured 5–8 repeating PEG units (m/z 239–371) with lower intensity efficiently ionize not only low but also high molecular weight PEG
and 9–17 repeated PEG units (m/z 415–811) with higher intensity. A 600 oligomers generated in TG. TG–ESI+APCI-MS was then used to
series of sodiated PEG ions (MNa+, ▼) were also detected between investigate the molecules released from different synthetic polymers
m/z 481 and 745 (10–16 repeated PEG units) but the intensity of the throughout the entire study.
sodiated ions was lower than that of the protonated PEGs. These
results indicated that H+ and Na+ were added to the ether oxygen
atom of dihydroxy-terminated PEG molecules to form MH+ and 3.2 | Analysis of PPG400, PPG2000, and PPG
+
MNa ions during the ESI processes. using TG–ESI+APCI-MS
The composition of the pyrolytic products of PEG 600 at Tmax
was investigated using TG–ESI+APCI-MS. Nitrogen was used as the The thermal degradation products of PPGs were investigated using
TG reaction gas (50 mL/min), and the temperature for TG analysis TG–ESI+APCI-MS. PPGs are polypropylene glycol oligomers which
  
was increased from 30 C to 600 C at a heating rate of 20 C/min. The are obtained by ring-opening polymerization of propylene oxide.
evolved gaseous molecules from TG were delivered to the dual Figure 3 displays the results of TG analysis (Figure 3A), DTG curves
ionization source to react with the primary charged species in the ESI (Figure 3B), the TG–ESI+APCI mass spectrum of PPG 400 recorded at
and/or APCI plumes to generate analyte ions. The results of TG 200 C (Figure 3C), and TG–ESI+APCI mass spectra of PPG 400, PPG
analysis indicated the onset temperature of PEG 600 was 385 C 2000, and PPG recorded at Tmax (Figures 3D–3F). The results of TG
(Figure S1a), and DTG analysis showed the maximum decomposition and DTG indicated that decomposition of PPG 400 started at
temperature (Tmax) was 447 C (see Figure S1b). Figure 2B shows the approximately 150 C and reached Tmax at 309 C. The Tmax of PPG
TG–ESI mass spectrum of PEG 600 recorded at Tmax. The ions at m/z 400 is lower than that of PPG 2000 (372 C) and PPG (375 C). The
476–828 (Δm = 44) representing 10–17 repeated PEG units of TG–ESI+APCI mass spectrum of PPG 400 at 200 C (Figure 3C) was
ethylene glycol (▲) were detected. It was found that the mass of the dominated by the ions from two unknown volatile impurities at m/z
main ion peaks detected by TG–ESI-MS was higher than those 335 and 408. Further identification of the unknown compounds must
detected by ESI-MS by 17 (compare Figures 2A and 2B), indicating be conducted by accurate mass measurement, and tandem mass
+
that these ions were actually [M + 18] . Tandem mass spectrometric spectrometric and other spectroscopic analysis such as Fourier-
(MS/MS) analysis of these [M + 18]+ ions indicated that the main transform infrared (FTIR) spectroscopy. In addition, the ionization
collision-induced dissociation (CID) product ion was [M + 1]+, efficiency of polymer molecules maybe increased by adding
suggesting that an ammonia molecule was lost from the ion of cationization reagents to the ESI solution.
+
[M + 18] (data not shown). The most probable origin of the Figure 3D displays the TG–ESI+APCI mass spectrum of PPG
ammonia was the impurity in the nitrogen gas. However, the 400 recorded at Tmax (309 C). The predominant ions on the mass
possibility for the ammonia to be generated by the plasma-excited N2 spectrum include those at m/z 309–483 (Δm = 58) from MH+ (●)
gas cannot be ruled out. The protonated ion peaks of MH+ and those at m/z 326–558 (Δm = 58) from MNH4+ (▲). These ions
+
(m/z 239723, ●) with lower intensity than MNH4 (▲) ions were with a mass difference of 58 between peaks are from PPG oligomers
also detected. with 4–8 repeated propyl glycol units. For comparison, PPG 400 was
Figure 2C shows the TG–APCI mass spectrum of PEG also analyzed using traditional infusion ESI-MS at room temperature
600 recorded at Tmax (447 C). Three series of ions were detected: (Figure S2a). Two series of the PPG 400 ions were detected. The first
(1) the ions at m/z 179–487 (Δm = 44) represented the adducted ion series (MNH4+, ▲) included the ions at m/z 326–616, while the
PEG 600 ions containing 3–10 repeated PEG units with a C2H5 group second ion series (MNa+, ▼) included the ions at m/z 331–621. The
+
([M + 29] , ■); (2) the ions at m/z 165–342 (Δm = 44) represented ions detected by TG–ESI+APCI-MS at Tmax match well with one of
the adducted PEG ions containing 4–9 repeated units with a CH3 the ion series ([M + NH4]+, ▲) detected by infusion ESI-MS at room
group ([M + 15]+, ◇); and (3) the ions at m/z 151–503 (Δm = 44) temperature.
represented the protonated PEG ions containing 3–11 repeated units Figure 3E shows the TG–ESI+APCI mass spectrum of the PPG
(MH+, ●). In contrast to the ions in (3), the ions in (1) and (2) detected 2000 standard recorded at Tmax (372 C). No molecular PPG 2000
by TG–APCI-MS were not found on the TG–ESI mass spectrum ions were detected at Tmax. For comparison, the Figures S2a-b show
(compare Figures 2B and 2C). The intensity of the M + 15 and the infusion ESI mass spectrum of the PPG 2000 standard. The
M + 29 ion series was lower than that at Tmax (447 C) as the APCI predominant ions shown on the ESI mass spectrum were from
mass spectrum was recorded at a lower temperature – 350 C singly, doubly, and triply charged PPG 2000 molecular ions. Actually,
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6 of 10 LIN ET AL.

F I G U R E 3 TG and TG–ESI+APCI-MS results of PPG 400, PPG 2000, and PPG: (A) TG curves of PPG 400 ( ), PPG 2000 (—), and PPG
(……); (B) DTG curves of PPG 400 ( ), PPG 2000 (—), and PPG (……); (C–F) ESI+APCI mass spectra of PPG 400 recorded at (C) 200 C and
(D) 309 C (Tmax, PPG 400); (E) ESI+APCI mass spectrum of PPG 2000 recorded at 372 C (Tmax, PPG 2000); and (F) ESI+APCI mass spectrum of
PPG recorded at 375 C (Tmax, PPG). ●: [M + H]+, ▲: [M + NH4]+, ■: [M + 29]+, □: [M + 43]+ [Color figure can be viewed at
wileyonlinelibrary.com]

four ion series of PPG 2000-related fragment ions with 6–11 Figure 3F shows the TG–ESI+APCI mass spectrum of PPG
repeated propyl glycol units (Δm = 58) adducted with H, CH3, C2H5, recorded at Tmax (375 C). Again, several PPG-related fragment ion
and C3H7 were detected on the TG–ESI+APCI mass spectrum: series with a mass difference of 58 between ion peaks were detected:
(1) m/z 425–657 (MH+, ●); (2) m/z 381–671 ([M + 15]+, ■); (1) m/z 367–772 (MH+, ●); (2) m/z 382–672 ([M + 15]+, ■); (3) m/z
+ +
(3) m/z 395–685 ([M + 29] , ▲); and (4) m/z 409–641 ([M + 43] , 396–686 ([M + 29]+, ▲); and (4) m/z 409–640 ([M + 43]+, □). For
□). The results indicated that the molecules of PPG 2000 comparison, the Figure S2c shows the infusion ESI mass spectrum of
decomposed at Tmax; hence, no PPG 2000 molecular ions could be the PPG standard. No characteristic PPG ions were detected by
detected by TG–ESI+APCI-MS. However, the characteristic infusion ESI-MS. Even though no molecular ions of PPG were
fragment ion peaks from PPG 2000 were still detected by the mass detected by TG–ESI+APCI-MS, the characteristic peaks of PPG were
difference of 58 (the repeating unit of PPG) between peaks in each still detected by the mass difference of 58 between the ion peaks in
ion series (Figure 3E). the lower mass region (m/z 300–800) (Figure 3F). The results clearly
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LIN ET AL. 7 of 10

indicate that even though the molecular ions of PPG 2000 and PPG Figure 4A shows the TG and DTG results of the PS pellets. It was
were not detected, the TG–ESI+APCI mass spectrum recorded at found that the PS pellets started decomposing at around 400 C and
Tmax still provided information regarding the composition of PPG reached Tmax at 457 C. Figure 4B shows the TG–ESI+APCI mass
standards, useful for identification of the unknown polymers. spectrum of the PS pellets recorded at Tmax. The total ion current
(TIC) from the analysis of the PS pellet is also shown in Figure 4A. The
molecular ions of PS in the PS pellet were not detected, indicating
3.3 | Analysis of PS pellets and PS food packaging that PS was completely decomposed at Tmax. However, series of ion
using TG–ESI+APCI-MS peaks with a mass difference of 104 (the mass of the repeating unit of
PS) were detected on the mass spectrum, suggesting that the
Due to the advantages of high transparency, easy machining, and low sample was PS. Several thermal degradation mechanisms of PS
cost, PS has been used as the primary food packaging material. In this have been suggested including random cleavage, depolymerization,
study, PS pellets and PS-made food packaging including a water cup intramolecular hydrogen transfer, and intermolecular transfer
and coffee cup lid were subjected to TG–ESI+APCI-MS analysis. reactions, which dramatically reduce the detected mass range of PS

F I G U R E 4 TG and TG–ESI+APCI-MS results of PS pellet: (A) TG, DTG, and TIC; (B–C) ESI+APCI mass spectra of PS pellet recorded at
(B) 470 C and (C) 350 C [Color figure can be viewed at wileyonlinelibrary.com]
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8 of 10 LIN ET AL.

ions by increasing the number of active centers and generating small di(2-ethylhexyl) phthalate (DEHP, m/z 391) and diisononyl phthalate
polymer fragments. The formation of free radicals results in the (DINP, m/z 419), were detected as the prominent ions on the
scission of the polymer backbone to form styrene and free radicals TG–ESI+APCI mass spectrum (Figure 4C). The identities of DEHP and
propagating on the backbone.37,38 DINP were confirmed by the ions at m/z 391 and 419 on the MS/MS
To explore if the PS-related ions with higher mass could be spectrum (data not shown). It has to be noted that the use of
detected at lower temperature, the mass spectrum recorded at 350 C additives in plastics such as plasticizers or other antioxidants to
was examined (Figure 4C), and no obvious signals from styrene improve the chemical/physical properties of the plastic material has
could be seen (m/z 104). The results suggested that the PS pellet was brought a new problem to our living environment, since some of
not decomposed at 350 C. However, two volatile phthalates, these additives are environmental hormones. For examples,

F I G U R E 5 TG and TG–ESI+APCI-MS results of the PS-made water cup and PS-made coffee cup lid: (A) TG curves, (B) DTG curves, and (C–E)
TG–ESI+APCI mass spectra of (C) PS-made water cup at Tmax (442 C), (D) PS-made coffee cup lid at Tmax (468 C), and (E) PS-made coffee cup lid
at 350 C [Color figure can be viewed at wileyonlinelibrary.com]
 10970231, 2022, 18, Downloaded from https://analyticalsciencejournals.onlinelibrary.wiley.com/doi/10.1002/rcm.9351 by National Chung Hsing, Wiley Online Library on [27/05/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
LIN ET AL. 9 of 10

phthalates are present in some medicines, children's toys, stationery, PE ER RE VIEW

food-packages, cosmetics, and many commodities. Actually, the The peer review history for this article is available at https://publons.
restricted use of these additives, plasticizers, flame retardants, and com/publon/10.1002/rcm.9351.
heavy metals in plastics and electronic equipment has become an
important issue for global societies since the release of the RoHS DATA AVAILABILITY STAT EMEN T
(Restriction of Hazardous Substances) in 2005.39,40 In addition, two Data openly available in a public repository that issues datasets
unknown ion peaks were detected (m/z 548 and 647). They were with DOIs.
tentatively assigned as AO1076 and AO168 due to matching of
molecular weights and retention times with the standards through OR CID
GC/MS analysis (Figure S3). AO168 is a common antioxidant that is Jentaie Shiea https://orcid.org/0000-0002-7153-3029
usually used in combination with AO1076 to protect plastic samples
from oxidation during thermal treatment processes. RE FE RE NCE S
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