Alternative headspace sampling methodologies and detection techniques related to gas chromatography
Alternatieve headspacemethoden en detectietechnieken op het gebied van gaschromatografie
van Boxtel, Niels; R0300809
Summary In this work, several aspects around headspace (HS) sampling and detection hyphenated with gas chromatography (GC) were covered. Regarding HS sampling, the results of three studies were reported around the problems encountered with the analysis of aqueous samples and/or high boiling analytes with a high affinity for the matrix in chapters 2 to 4 in this manuscript, whilst the final chapter covers the development and characterization of a novel GC detector. In chapter 2, the use of the full evaporation technique (FET) for the analysis of high boiling analytes with a high affinity for apolar matrices was evaluated and compared with the conventional static HS (sHS) sampling approach. A FET method has been developed and validated for the analysis of typical high boiling analytes (bp. > 200 °C) including camphor, menthol, methyl salicylate and ethyl salicylate that are often used in various topical formulations. Data have shown that FET is an excellent approach to circumvent matrix effects that are often encountered with sHS methods. The method showed excellent recovery and repeatability during validation and was finally applied on commercial formulations such as Radosalil , ThermoCream , Vicks Vaporub and Reflexspray . In chapter 3, acetone acetals were employed as water scavengers for the analysis of aqueous samples using HS-GC. After optimization of the scavenging reaction conditions, the approach was used for FET analysis of various typical high boiling polar residual solvents that are miscible with water. The procedure enabled sample enrichment which provided a significant gain in sensitivity of the FET analysis of these analytes and it was finally applied on a cefotaxime sample for the quantification of residual N-methylpyrrolidone (NMP). During experiments it was revealed that the same procedure can be used for the quantitative derivatization of ethylene glycol (EG) in aqueous samples. The formation of the significantly more volatile 2,2-dimethyl-1,3-dioxolane (2,2-DD) enabled determining EG using sHS sampling. In chapter 4, a novel HS approach for the analysis of quaternary ammonium salts (QAS) in aqueous samples is presented in which the reported water scavenging method from chapter 3 is used for the removal of water and sample enrichment. Screening experiments revealed that QAS substituted with benzyl and methyl groups degrade to form benzyl chloride and chloromethane under the used experimental conditions. By using chloromethane and benzyl chloride for quantification of such QAS, matching calibration standards are not needed. This means that one kind can be used for the determination of other QAS that also yield chloromethane and benzyl chloride as reaction products. The methodology was used for the analysis of denatonium benzoate (DB) in EG based cooling liquids and the analysis of benzoxonium chloride (BZOCL) or benzethonium chloride (BZTCl) in mouth sprays. Finally in chapter 5, work around the development and characterization of a novel GC detector is presented. The detector used a micro cavity hollow cathode discharge (µCHCD) plasma as ion source combined with an either positively or negatively biased capture electrode. Typical advantages of the used low power µCHCD are the increased ionization efficiency compared to the hydrogen flame used in the flame ionization detector (FID) and operation at atmospheric pressure without the need for additional gasses. During development of the µCHCD ion source it was found that the detector geometry was of great influence on the obtained signal. By adapting to a geometry with a wide anode clearance angle, a sensitive response in the pg range was obtained for various analytes including carbon tetrachloride (CCl4) which is poorly detected with the FID. It was noticed that halogenated analytes were possibly subjected to a different ionization mechanism than the non-halogenated species. Halogenated ones most likely form negative ions in the electron rich plasma as these were most efficiently detected by a positively biased capture electrode.