Commercially available foods and beverages are exposed to a variety of substances during production and storage processes. The materials in contact with foods leach into the products and could have an impact on consumer health. Leaching is exacerbated when the plastic is exposed to heat. Increasing concern about food contact materials has led to a heightened need for manufacturers and processors to conduct contamination analysis.
This article first presents a method to detect polyethylene terephthalate (PET) and, next, a technique to identify phthalate esters. Each is a chemical used to manufacture plastics commonly found in food and beverage packaging materials. By using these analytical procedures, it is possible to identify the source of contamination and take appropriate countermeasures.
To identify PET, we demonstrate qualitative analysis of assumed resins in a food packaging material by using a pyrolysis-GC/MS method, including analysis of trace contaminants and contaminants in multilayer films, which are difficult to analyze using a Fourier transform infrared (FTIR) spectrometer.
Then, we demonstrate how to identify specific phthalate esters by confirming their molecular weight using the solvent mediated chemical ionization (SMCI) method. This technique is an effective alternative to using electron ionization, in which mass spectra are similar, which makes identification difficult.
Analyzing Resins in Food Packaging Material Using Pyrolysis-GC/MS
The FTIR and energy dispersive X-ray fluorescence (EDXRF) spectrometers are commonly used in identifying contaminants by instrumental analysis. However, these methods have limitations when analyzing trace impurities and contaminants in multilayer films. A different approach that enables the qualitative analysis of resin materials and additives contained in trace organic contaminants involves thermal methods: pyrolysis-GC/MS and thermal extraction-GC/MS.
Here we present an analysis of the resins in a food packaging material using the pyrolysis-GC/MS method, assuming food contamination. A Shimadzu OPTIC-4 multimode inlet for GC/MS was used in the analysis by pyrolysis-GC/MS. Because the OPTIC-4 enables high-speed heating (60°C/s) to a maximum temperature of 600°C, diverse sample injection modes are available and simple pyrolysis was possible.
Sample and analysis conditions. A commercially available food packaging material was used as the real sample material. The sample material was cut with a knife to obtain a sample weighing approximately 0.2 mg, which was inserted into the difficult matrix introduction (DMI) microvial of the OPTIC-4 and then set in the DMI insert liner.
Qualitative analysis of resin material. Figure 1 shows the obtained pyrogram (total ion chromatogram obtained by pyrolysis-GC/MS). According to a reference containing pyrolysis data on resins, this is a distinctive pyrogram of polyethylene (PE), in which hydrocarbon species are arranged at equal intervals. Therefore, it could be inferred that the foreign matter in this experiment contains PE as the base material.
In addition to the peaks seen in the pyrogram of PE, three distinctive peaks—(a) to (c) —are also detected in the pyrogram of the real sample. Compound identification of these peaks was carried out using the NIST Research Library and the above-mentioned reference. As a result, it was found that (b) is caprolactam, a compound characteristically seen as a pyrolysis product of polyamide (PA), and (a) and (c) were identified respectively as 4-(vinyloxycarbonyl) benzoic acid and benzoic acid, which are compounds characteristically seen as pyrolysis products of PET.
To identify contaminants in food products, the resins contained in an assumed foreign matter sample were analyzed by the pyrolysis-GC/MS method in an OPTIC-4 multimode inlet. As a result, qualitative analysis of the composite resin was possible from the pyrogram and pyrolysis products. Thus, this experiment demonstrates the possibility of qualitative analysis of resin materials using the pyrolysis-GC/MS method, including analysis of trace contaminants and contaminants in multilayer films, which are difficult to analyze using FTIR. This analysis technique makes it possible to identify the source of contamination and take appropriate countermeasures.
Identification of Phthalate Esters Using the SMCI Method
During production and storage processes, commercially available foods and beverages come into contact with a variety of substances, such as phthalate esters, which are used as plasticizers for polyvinyl chloride. Phthalate esters present a health concern because of their connection with endocrine disruption effects, developmental toxicity, reproductive toxicity, and tissue damage.
Phthalate esters share the same basic structure, and their mass spectra are similar when the electron ionization (EI) method is used, which can make the identification of target phthalate esters difficult. Conventionally, in such cases, the molecular weight is confirmed via the positive chemical ionization (PCI) method, using methane, isobutane, and other flammable, high pressure gases. In contrast, if the use of flammable, high pressure gases is problematic, the molecular weight can be confirmed via the SMCI method using organic solvents.
Here, we present the results of an analysis of phthalate esters using the SMCI method.
Samples and analytical conditions. A standard solution of phthalate esters was prepared to a concentration of 1.0 ng/mL. The solution was measured using the EI and SMCI methods.
EI and SMCI mass spectra. When a similarity search was performed from the EI mass spectrum for Di-n-octyl phthalate, phthalate esters with different molecular weights but with a high degree of similarity were identified. Because compound identification using only the EI mass spectrum was difficult, the number of candidate compounds was narrowed down by confirming the molecular weights using the SMCI mass spectrum.
Additionally, Figure 3 shows the mass spectra for typical phthalate esters using the EI method and SMCI method, respectively, and Table 1 shows the capability of confirmation of molecular derived ions.
The molecular ions for many of the phthalate esters cannot be confirmed using the EI method. In contrast, using the SMCI method, the protonated molecular ions for all the phthalate esters can be confirmed, which provides strong support for compound identification.
For many phthalate esters, confirmation of molecular weight from the EI mass spectrum is difficult. However, pseudo molecular ions can be confirmed using the SMCI method. Accordingly, even if the use of a flammable, high pressure gas is problematic, it is evident that the SMCI method is effective for the confirmation of molecular weights.
Dr. Kuhn is a chromatographer and marketing manager for food and consumer products at Shimadzu Scientific Instruments. Reach him at [email protected].