Biodegradable polymers are increasingly being used as an alternative to conventional commercial polymers since, unlike synthetic polymers, they do not persist as long in the environment thus making them an ecofriendly alternative. The raw materials used for making biodegradable polymers include renewable resources like chitin, cellulose, starch and non-renewables such as poly(butylene adipate-co-terephthalate). The addition of additives to biodegradable polymers enhances their functional capabilities by altering their physical properties like flexibility, toughness, or barrier properties.
However, such additives percolate into their surroundings, effecting products like foodstuffs that are in contact with them. Therefore, it is vital to characterize the additives used in food packaging so good manufacturing standards are maintained and safety conditions are met. Characterization of additives is done via extensive extractions to separate additives from the polymer matrix. This article describes the identification of additives used in bioplastics through ambient ionization mass spectrometry. The analysis of each of the samples was completed within a few seconds and a broad range of additives used in various types of biodegradable polymers were examined.
The ambient source for the study was a PerkinElmer AxION® Direct Sample Analysis™ (DSA™) source. The DSA was positioned on the PerkinElmer AxION 2 Time-of-Flight (TOF) mass spectrometer and the mass spectrum was recorded with the DSA controller software.
Polymer pieces measuring 1cmX1cm were deposited in the DSA source at the front of the mass spectrometer and analyzed for a period of 5 seconds. The bioplastic samples that were analyzed were used as salad containers. These samples included commercial poly (L-lactic acid) (PLLA). The compositions of the sampled PLLA were 2wt.% butylated hydroxytoluene (BHT), 0.95wt.% alpha-tocopherol and 0.5wt.% BHT. These samples were analyzed in positive ion polarity across the mass range of m/z 60-560.
A calibration solution was analyzed prior to each sample and was used to internally calibrate each data file. The AxION EC ID™ software was used for analyzing measured ions of unknown composition. Selected databases were searched with the help of the AxION EC ID software to spot elemental composition matches. The software also provided a ranked summary of these matches based on analyte mass accuracy and isotope ratio information. Possible compound structures were identified by the software. The web version of the Pubchem database was referred to in this study.
Salad containers made of commercially available PLLA were studied using the DSA. Figure 1 depicts the spectrum, which indicated the presence of polymer fragment ions of lactic acid. The presence of other ions was also indicated by the spectrum. These ions were detected with the help of precise mass information and the AxION EC ID software.
Figure 1. Spectra of commercially available PLLA polymer using the DSA/TOF. The fragments of the polymer (monomer, dimers) were identified using accurate mass. An ion at m/z 338.3424 was also observed.
One such ion observed at m/z 338.3424 (Fig. 1) was identified as likely to be the protonated molecule of erucylamide, which has the elemental composition C22H43NO (Fig. 2). Erucylamide is often added as a slip agent, antifogging or lubricant in plastic films.
Figure 2. AxION EC ID software: The elemental composition of m/z 338.3425 was identified as C22H43NO. The software also provides a list of possible structures for the given elemental composition and one of the listed structures was erucylamide, an additive often used in bioplastics.
The PLLA plastic with the BHT additive at 2% by weight exhibited the dominance of BHT M+ ion with its fragments in the spectrum, as illustrated in Figure 3. Apart from the BHT ion another major ion that corresponds to the accurate mass of erucylamide was also spotted in the spectrum.
Figure 3. Spectra for PLLA containing 2 wt.% BHT. BHT along with its fragments was identified (mass accuracy < 5 ppm). A dominant ion corresponding to the accurate mass of erucylamide was observed at m/z 338.3410.
The PLLA biopolymer containing a-tocopherol as an additive exhibited a prominent peak in the [M+H]+ ion of the antioxidant as well as a fragment ion that corresponds to a-tocopherol, as shown in Figure 4.
Figure 4. Spectra for PLLA biopolymer containing 0.95% α-tocopherol and its fragment ion at m/z 165.0911 were observed. A contaminant peak for erucylamide was also observed at m/z 338.3429.
Erucylamide was found in these spectra, indicating that it was used as an additive in the biopolymer. Ecoflex®, a petroleum-based biodegradable polymer, containing carbon black and the BHT additive, was analyzed by the DSA source. Ecoflex® is a polymer made of poly(butylene adipate-co-terephthalate). The resulting spectrum, shown in Figure 5, determined the presence of BHT and adipic acid, a polymer degradant. The identification of the elemental composition and the likely structure of the unknown compound of having m/z 201.1126 was done with AxION EC ID software. The elemental composition of the compound was predicted by the software as C10H16O. Precise identification of the structure is not within the scope of this study.
Figure 5. Spectra of Ecoflex biopolymer containing carbon black and BHT. The spectra shows presence of BHT, adipic acid, along with an unidentified peak at m/z 201.1126 and its dimer at 401.2185.
Known additives in biodegradable polymers were rapidly identified by exposing a small piece of the material for a few seconds to the ambient ionization DSA source, thereby eliminating the need for complex and time consuming extraction procedures. Unknown analytes detected in the biodegradable polymers were identified using accurate mass and isotope profile information along with AxION EC ID software, a powerful database search tool.
This information has been sourced, reviewed and adapted from materials provided by PerkinElmer.
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