Identification of antioxidant and ultraviolet light stabilizing additives in plastics by liquid chromatography/mass spectrometry

Abstract

The information obtained by absorbance detection when coupled with liquid chromatography is usually not specific enough to allow the qualitative ldentHication of compounds present in complex samples with any degree of certainty. With the use of mass spectrometric detection (LCIMS), characteristic mass spectral data can be obtained whkh greatly aid in eiucklating the Mentlty of unknown compounds. In this paper, ma88 spectrometric detection Is used in series with absorbance detectian to Identify or characterize antioxidants and UV light stabilizing addltives in plastic materlaIs which were separated by liquid chromatography. The two detectors were operated in series without any significant loss of chromatographic resolution. Nanogram quantities of additives could be detected by LCIMS. Lower detection limits were achieved by reconstructing mass chromatograms for selected ions. These studies demonstrate the feasibility of using LC/MS to Characterize additives In plastic materials. Chemical additives are frequently used to enhance the useful lifetime of polymeric materials. Antioxidants and ultraviolet (UV) light stabilizers are added to minimize potential polymer degradation by oxidative (1-3) or photochemical processes (3,4). Identification and quantitation of these compounds are necessary for polymer degradation studies and for quality-assurance tests. Due to the low concentration of additives in the polymer matrix (typically less than l%), a sensitive method of analysis must be employed. The high molecular weight and polarity of many antioxidants and UV light stabilizers often precludes the use of gas chromatography. High-performance liquid chromatography has been used to effectively separate these compounds (2,5-9). Identification of compounds present in real samples is based on comparison of the peaks' retention times or capacity factors (k') with those of known standards. Identification by this method not only is tedious and timeconsuming but also requires one to have all of the suspected compounds on hand so that tentative identifications can be made. Even so, erroneous identifications can often be made due to the relatively poor separating ability of HPLC (compared to capillary gas chromatography) and the chemical similarity of many compounds. The combination of liquid chromatography with mass spectrometric detection (LC/MS) can circumvent many of these problems. Many compounds, such as distearyl thiodipropionate (DSTDP), which do not absorb ultraviolet light at standard UV detection wavelengths (254 nm and 280 nm) will not be detected with a conventional UV absorbance detector. The mass spectrometer is a more universal detector. The information yielded by LC/MS greatly aids in elucidating the structure of unknown compounds. In our laboratory, ultraviolet absorbance and mass spectrometric detection were used in series to provide unique qualitative and quantitative information on plastic additives separated by liquid chromatography. Additional peak 0003-2700/85/0357-0672$01.50/0 broadening introduced by this detection arrangement was minimal. The system was used to characterize the additives present in several polymeric samples.