Headspace analysis monomers

HS-GC methods have equally been used for chromatographic analysis of residual volatile substances in PS [219]. In particular, various methods have been described for the determination of styrene monomer in PS by solution headspace analysis [204,220]. Residual styrene monomer in PS granules can be determined in about 100 min in DMF solution using n-butylbenzene as an internal standard for this monomer solid headspace sampling is considerably less suitable as over 20 h are required to reach equilibrium [204]. Shanks [221] has determined residual styrene and butadiene in polymers with an analytical sensitivity of 0.05 to 5 ppm by SHS analysis of polymer solutions. The method development for determination of residual styrene monomer in PS samples and of residual solvent (toluene) in a printed laminated plastic film by HS-GC was illustrated [207], Less volatile monomers such as styrene (b.p. 145 °C) and 2-ethylhexyl acrylate (b.p. 214 °C) may not be determined using headspace techniques with the same sensitivities realised for more volatile monomers. Steichen [216] has reported a 600-fold increase in headspace sensitivity for the analysis of residual 2-ethylhexyl acrylate by adding water to the solution in dimethylacetamide. 

Headspace analysis involves examination of the vapours derived from a sample by warming in a pressurized partially filled and sealed container. After equilibration under controlled conditions, the proportions of volatile sample components in the vapours of the headspace are representative of those in the bulk sample. The system, which is usually automated to ensure satisfactory reproducibility, consists of a thermostatically heated compartment in which batches of samples can be equilibrated, and a means of introducing small volumes of the headspace vapours under positive pressure into the carrier-gas stream for injection into the chromatograph (Figure 4.25). The technique is particularly useful for samples that are mixtures of volatile and non-volatile components such as residual monomers in polymers, flavours and perfumes, and solvents or alcohol in blood samples. Sensitivity can be improved by combining headspace analysis with thermal desorption whereby the sample vapours are first passed through an adsorption tube to pre-concentrate them prior to analysis. 

In another investigation, ethylene oxide in polyvinylchloride was determined by dissolving 65 mg of sample in 1 ml of dimethylacetamide [189]. Headspace analysis was conducted on a glass column packed with Porapak T under isothermal conditions. The solvent was removed by back-flushing. An external standard was used for calibration. A vinylchloride monomer was also detected in this analysis (Figure 4.3). 

In headspace analysis, the plastic is placed in a vial (at a raised temperature) and the volatiles formed are stripped by a flow of carrier gas. The stripped volatiles are trapped in a suitable sorbent (e.g., using a solid-phase microextraction device) and subsequently thermally desorbed into a gas chromatograph. Process gas chromatographs are used in industrial analysis of volatiles in plastics. An example of this technique is the determination of residual vinyl chloride monomer in plastics in the range of 5-50 g per kg. With direct injection of a polymer solution, there is a danger of side-effects (a loss of reactive monomers due to polymerization in the injection port or an increase in its content due to depolymerization at a high injection temperature). 

There are several injection methods currently used in GC monomer determinations. However, all of these techniques fall into two categories direct injection and injection of the vapour above the polymer, i.e., headspace analysis or polymer solution. Direct injection of a polymer, or its precipitates, is perhaps the most common method. This technique requires both an inert material in the injection port to trap the polymer, and a narrow window of acceptance on the injection port temperature. High temperatures lead to polymer decomposition while low temperatures prevent fast volatilisation of the monomer. In addition, the sample viscosity must be low enough to allow handling with a syringe. 

Headspace GLC analysis is a method used to monitor a vapour over a polymeric matrix. It is a very effective technique, but may require more time and effort than direct injection. This method can be performed manually, when a vial containing the monomer is heated, an equilibrium is established, for volatile compounds between the sample and the headspace above it. Because no dissolution step is required, sample viscosity problems and loss of response due to dilution are eliminated. Automated headspace analysis units are available from instrument manufacturers, as well as multiple extraction systems. Any analytically useful headspace method must obey Henry s law ... 

The more volatile monomers vinyl chloride, butadiene, and acrylonitrile can be determined by dissolution of the polymer and analysis of the equilibrated headspace above the polymer solution. By this method it was possible to determine vinyl chloride and bntadiene at the 0.05 ppm level and acrylonitrile down to 0.5 ppm. The injection of water into polymer solutions containing styrene and 2-ethylhexyl acrylate monomers prior to headspace analysis greatly enhanced the detection capability for these monomers making it possible to determine styrene down to 1 ppm and 2-ethylhexyl acrylate at 5 ppm. Incorporation of polymer into the calibration standards compensates for the effect which the polymer matrix has upon the equilibrium partitioning of the monomer between the solution and head space. The relative precision and error in the determination of these monomers near the quantitation limit was found to be less than 7%. 

Method for Quantitative Determination of Methyl Methacrylate Monomer in Bone Surgery by Headspace Analysis of Blood... 

Some of the applications of headspace analysis include the determination of vinyl chloride and other impurities in PVC, styrene monomer in PS, methyl methacrylate monomer in polyacrylates, ethylene in polyethylene, acrylonitrile monomer in ABS terpolymers, epichlorohydrin in epoxy resins, and residual solvents in polymers (see next). 

A further apparatus for carrying out solid polymer headspace analysis of PS for styrene monomer and aromatic volatiles has been described by Crompton and co-workers [1,2]. In this apparatus the polymer is heated to 300 °C in the absence of solvents, prior to its examination by GC. The apparatus is illustrated in Figure 5.3. 

Shanks has determined residual butadiene and styrene in polymers with an analytical sensitivity of 0.05 to 5 ppm by analysis of the equilibrated headspace over polymer solutions and determined acrylonitrile, alpha-methyl styrene and styrene monomers by headspace analysis over heated solid polymer samples. 

Headspace analysis involves chromatographing the vapors derived from a sample by warming it in a partially fQled vial sealed with a septum cap. After equilibration under controlled conditions, the proportions of volatile sample components in the headspace above the sample are representative of those in the bulk sample. The headspace vapors, which are under slight positive pressure, are sampled by a modified and automated injection system or gas s)uinge, and injected onto the column (Fig. 3(a)). The procedure is useful for mixtures of volatile and nonvolatile components, such as residual monomers in pol)uners, alcohol or solvents in blood samples (Fig. 3(b)), and flavors and perfumes in manufactured products, as it simplifies the chromatograms and protects the column from contamination by nonvolatile substances. 

SHS-GC is amongst the best methods of choice for volatile monomer analysis and effectively gives both isolation and preconcentration of the analyte in one step. Headspace methods are used extensively for... 

Vinyl chloride monomer (VCM) is the main substrate for the manufacture of polymers used as packaging materials for food. Since VCM is considered by lARC to be a human carcinogen, monomer levels in PVC food packaging materials are strictly controlled. To ensure a safe product, the residual content of VCM in the finished material or article is limited to one mg per kg in the final product (Council Directive 78/142/EEC). Furthermore, VCM should not be detectable in foodstuflfs. Commission Directives 80/766/EEC and 81/432/ EEC give the method of analysis for official control of the VCM level in food packaging materials and in foods - gas-phase chromatography using the headspace method, after dissolution or suspension of samples in N,N-dimethylacetamide. Both residual monomer content of the polymer and... 

Monomers are either gaseous or relatively volatile liquids and so GC and GC-MS based techniques are used to determine them in both the rubber compound and the food simulant/food product. To simplify the analysis, a static headspace sampler is often used to isolate the monomer from the sample matrix an extraction procedure often presenting chromatographic problems with the extraction solvent obscuring the analyte. 

Several SP materials have been used for the extraction of FRs from aqueous samples, plasma and milk (Table 31.7). Similar materials have been used for all FRs. Typical SP materials include Ci8 and Cg bonded to porous silica, highly cross-linked poly(styrene divinylbenzene) (PS-DVB), and graphitized carbon black (GCB). It is also possible to use XAD-2 resin for extraction of various FRs, pesticides, and plastic additives from large volumes of water (100 1). The analytes can then be either eluted from the resin by acetone hexane mixture, or Soxhlet extracted with acetone and hexane. For a specific determination of diphenyl phosphate in water and urine, molecularly imprinted polymers have been used in the solid phase extraction. The imprinted polymer was prepared using 2-vinylpyridine as the functional monomer, ethylene glycol dimethacrylate as the cross linker, and a structural analog of the analyte as the template molecule. Elution was done with methanol triethylamine as solvent. Also solid phase microextraction (SPME) has been applied in the analysis of PBDEs in water samples. The extraction has been done from a headspace of a heated water sample (100°C) using polydimethylsiloxane (PDMS) or polyacryl (PA) as the fiber material. ... 

In static headspace sampling (SHS), which relies totally on volatilisation to separate analytes from a sample matrix, important factors are related to diffusion and surface area. Precise thermal conditions are needed to determine occluded solvents, residual monomers, and additives in polymers and their composites. In particular, for accurate quantitative analysis in a static headspace experiment, the tem-perature/pressure conditions of the sample vessel are critical. In SHS-GC an aliquot of the headspace vapour in thermodynamic equilibrium with a solid sample (of known weight) is transferred to a GC or GC-MS for separation and identification. HS-GC is characterised by a relatively long thermostating time (up to 25 min) and short analysis time (2 min). With... 

PVC compositions are typically analyzed as an indication of correct formulation or to deconstruct competitive compounds. Occasionally, analysis is required by specification, such as the level (usually zero detectable) of vinyl chloride monomer (VCM). The latter is carried out by headspace gas chromatography (GC) per ASTM D 3749 for resins, and ASTM D 4443 for compounds. Such analyses are routinely done by suppliers. The formulator in the field will usually send samples to a testing laboratory. This is also the case with trace analysis, often for levels of lead, cadmium, arsenic, or mercury. 

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