Phthalates are widely used in many industries, such as plastics, consumer goods, children’s toys, food packaging, and cosmetics. Companies must comply with regulations for the analysis of phthalates and report the accurate level of different phthalates’ concentrations in their products.

Traditional sample preparation techniques, such as solvent extraction and filtration, are cumbersome, time-consuming, and suffer from analyst-to-analyst variability. Many polymers also often contain a large number of plasticizers (several tens of percent), which co-elute with the phthalates. This so-called matrix interference can lead to either false positives or false negatives. Besides, matrix interference makes an accurate determination of the co-eluting phthalates problematic. So the traditional techniques also suffer from poor accuracy in differentiating the co-eluded compounds.

Nowadays, companies and testing laboratories integrated the modern-day pyrolysis GC/MS system into their analytical protocols for the analysis of regulated phthalates. The modern-day pyrolysis system comprised of a multi-functional micro-furnace pyrolyzer, which enables researchers to perform multiple analytical techniques, both isothermal and temperature programmed. Thermal Desorption(TD) is one of the modes of operations of the micro-furnace that is being used to thermally extract and differentiate phthalates in polymeric mixtures. Thermal Desorption eliminates any solvent extraction or sample pretreatment and differentiates the co-eluded compounds using the unique quantitative ions associated with regulated phthalates.

Experimental: A sheet of PVC containing DINCH at 40% and six restricted phthalates at 0.1% each was analyzed. Small pieces (~20mg) sampled from several different locations were dissolved in 1 mL of THF (20 mg/mL). 10 µL of the solution was placed in an inert sample cup, and the solvent evaporated, leaving a thin film of the sample on the surface of the cup. In this experiment, two modes of operations of the micro-furnace were used, Evolved Gas Analysis (thermal analysis) and Thermal Desorption (separation analysis). To identify the optimal temperature zone for Thermal Desorption, Evolved Gas Analysis was first performed.

Results: The EGA thermogram of the PVC sheet is shown in Fig. 1. It contains peaks originating from the plasticizers, HCl (thermal decomposition of PVC), and aromatic compounds (which are attributed to the thermal decomposition of polyenes upon the dehydro-chlorination of PVC). Characteristic ions for DINCH, HCl, and the phthalates of interest are used to define the optimal thermal desorption temperature zone: 100-320ºC. 

Two different calibration methods (absolute calibration and standard addition) were evaluated to determine which method most effectively reduces or eliminates the matrix effects caused by the presence of a large number of plasticizers in the sample. A calibration standard (Ph-Mix) containing 0.1% of each phthalate was used. Identification was based on the retention time of each phthalate’s characteristic ion and the common m/z149 ion. Quantitation was based on the peak area of the characteristic ion.

No DINCH interferences were observed for DBP, BBP, and DEHP, and both methods gave similar results – Fig.2. However, DNOP, DINP, and DIDP co-elute with the DINCH peak envelop, which potentially will degrade the accuracy of the phthalate concentration determination. Specifically, if there is no DINCH in the sample, the retention time of DNOP shifts and the peak width at half-height is double what it is when the sample contains DINCH.

In addition, the peak height is 60% lower when DINCH is present; consequently, when the absolute calibration method is used, the concentrations of DNOP, DINP, and DIDP are higher than the true value of 0.1%. On the other hand, the concentration of the phthalates obtained using the standard addition method are very close to the true value of 0.1%. The results show that standard addition is the preferred method when quantitating phthalates in PVC using TD-GC/MS. Standard addition minimizes the interference when high concentrations of plasticizers which co-elute with the phthalates of interest are present. [2]

To learn more about the micro-furnace pyrolyzer, its modes of operations, and how it can be used for your analytical protocols, simply connect with us.  Also to download our Rubber & Plastics Materials Characterization E-booklet, click here.

  References:

1. Yuzawa, et al., Anal. Sci., 25 (2009) 1057
2. This technical note was developed by Frontier Laboratories Ltd. 4-16-20 Saikon, Koriyama, Fukushima, 963-8862 JAPAN. www.frontier-lab.com