Utilization of a Helium Discharge Detector for Quantitation of Organohalogen Compounds

W&M ScholarWorks Dissertations, Theses, and Masters Projects Theses, Dissertations, & Master Projects 1989 Utilization of a Helium Discharge Detector for Quantitation of Organohalogen Compounds David andrew Ryan College of William & Mary - Arts & Sciences Follow this and additional works at: https://scholarworks.wm.edu/etd Part of the Analytical Chemistry Commons, and the Organic Chemistry Commons Recommended Citation Ryan, David andrew, "Utilization of a Helium Discharge Detector for Quantitation of Organohalogen Compounds" (1989). Dissertations, Theses, and Masters Projects. Paper 1539625518. https://dx.doi.org/doi:10.21220/s2-ktg6-qg96 This Thesis is brought to you for free and open access by the Theses, Dissertations, & Master Projects at W&M ScholarWorks. It has been accepted for inclusion in Dissertations, Theses, and Masters Projects by an authorized administrator of W&M ScholarWorks. For more information, please contact [email protected]. Utilization of a Helium Discharge Detector for Quantitation of Organohalogen Compounds A Thesis Presented to The Faculty of the Department of Chemistry The College of William and Mary in Virginia In Partial Fulfillment Of the Requirements for the Degree of Master of Arts by David Andrew Ryan 1989 Approval Sheet This thesis is submitted in partial fulfillment of the requirements for the degree of Master of Arts David Andrew Ryan Approved, August 1989 G iry Hollis Richard L. Q&y W. Rice Table of Contents List of Figures List of Tables A bstract Chapter I Halogen Selective Detectors for Gas Chromatography Chapter II Helium Discharge Detector 1. System Description 2. Excitation Process Chapter III Analytical Merits of the Helium Discharge Detector 1. Limits of Detection and Linear Response 16 2. Selectivity 3. Relative Response Chapter IV Application of the GC-HDD System to the Determination of Organohalogen Compounds in Water 1. Introduction 2. Experimental Chapter V Summary and Conclusions Literature Cited List of Figures Figure Page 1. Schematic Diagram of the Helium Discharge Assembly 1 0 2. Schematic Diagram of the GC-HDD System 1 2 3. Calibration Curve for 1,1,1-Trichloroethane 20 4. Chromatogram for Cl Selectivity 25 5. Chromatogram for Br Selectivity at 827 nm and 889 nm 29 6. Chromatogram from Cl Calibration Curve 37 7. Cl Calibration Curve 3 8 8. Br Calibration Curve 3 9 9. Flow Chart of Water Extraction Procedure 50 10. Chromatogram of Williamsburg Water Sample 53 11. Plot of Haloform Cone. vs. Hypochlorite Cone. 58 i v List of Tables Table Page 1. Overview of Operating Parameters 14 2. Physical Constants for Organohalogen Compounds 1 8 3. Limits of Detection for Halogen Compounds 22 4. Selectivity Ratios for Cl and Br 27 5. Detector Response for Br/CI Compounds 32 6. Cl/Br Ratios for Bromochloro-Compounds 34 7. Recovery and Limits of Detection for Haloforms in Water 52 8. Haloform Concentrations in Drinking Water 55 9. Haloform Concentrations from the Chlorination/ Bromination of Humic Acid 57 v Abstract The analytical merits of a Helium Discharge Detector (HDD) for gas chromatography, including limits of detection, linear response range, selectivity, and relative response, were evaluated for various brominated/chlorinated organic compounds. Limits of detection as low as 3 pg for chloroform were obtained, and the response of the HDD was determined to be dependent solely on the mole amount of halogen present. Application of the HDD to the quantitation of trace organohalogens in water samples was also investigated. QUANTITATION OF ORGANOHALOGEN COMPOUNDS Chapter I Halogen Selective Detectors for Gas Chromatography There are many different detectors available for use with GC. The decision of which type of detector is most applicable to a particular task is usually based on a variety of factors. After eliminating the detectors that cannot perform the necessary tasks, the pros and cons of the remaining systems can be evaluated to choose one that is most compatible with current needs. There are certain attributes which are considered very desirable for a detector that should be considered for evaluation. A list of criteria that should be considered for an appropriate detector includes the fo llo w in g : 1) adequate sensitivity (which is dependent on user objectives), 2) good stability and reproducibility, 3) a linear response to analyte concentrations over several orders of magnitude, 4) operational over a temperature range from room temperature to 400° C, 5) a short response time to the analyte that is independent of column flow rate, 6) high reliability and ease of use, 7) similarity in response to all solutes or high predictability and selective response toward one or more classes of solutes, and 8) non-destructive to the solute if possible.1 There are currently no GC detectors which meet all of these criteria yet a detector should meet the criteria, considered most important for a particular application. 2 Halogenated compounds constitute the largest fraction of priority pollutants as designated by the U.S. Environmental Protection Agency (EPA), with 76 out of the 114 designated compounds containing fluorine, chlorine, bromine or some combination of these.2 Thus, the need to develop versatile and sensitive detectors for these species is highly justified. There are currently a number of different detectors available which provide varying degrees of selective detection for halogenated compounds. The most common detector for halogenated compounds is .the electron capture detector (ECD). The detector is not truly element selective with respect to detecting only halogenated species. The ECD responds selectively to highly electrophilic species containing halogen, oxygen, and nitrogen. The detector functions by having the carrier gas pass over a beta-emitter such as Nickel 63, which causes the carrier gas to ionize and produce a constant current. The number of ions decreases in the presence of molecules that "capture" electrons, thus the detector measures a decrease in the current from the removal of electrons by electrophilic species. One reason that the ECD is such a popular detector is because of its high sensitivity, with detection limits in the subpicogram range for some halogenated compounds.3 The ECD is also non-destructive to analyte species, which is advantageous with respect to options such as interfacing to a mass spectrometer. Two drawbacks of this 3 detector, however, are that the linear response only spans about two orders of magnitude and the response is very dependent on the compound structure. For example, the limit of detection for dichloromethane is 4500 pg as opposed to 9 pg for trichloromethane.4 The response to isomeric species can vary greatly as well. For example, the signal observed from 1 ng of 2- chlorobiphenyl is approximately ten times less than for 4- chlorobiphenyl.5 Thus, the need for accurate response factors is required for quantitative purposes. Another element selective detector that has gained acceptance is the Flame Photometric Detector (FPD). Compounds pass through a H 2 /O 2 flame, from which excited atoms and simple molecules are produced. The FPD is widely used for the detection of phosphorus and sulphur by monitoring emission from PO and S 2 respectively.6 The FPD can also be used for the detection of halogenated compounds by inserting an indium pellet near the flame tip in the detector.7 The chlorine and bromine decomposition products react with the indium to give excited indium halogenides. As the halogenides return to the ground state, characteristic molecular band emission can be monitored at 359.9 nm for Cl and 372.7 nm for Br. The detection limit on a dual flame detector is 1x10_ 11 g/sec for chlorine compounds with a linear range over four orders of magnitude. The selectivity for Cl is between 1000:1 and 10000:1 relative to hydrocarbons and 100:1 relative to sulphur and 4 phosphorus compounds. Several problems are associated with the FPD. First of all, the stability of the signal is limited by deviations caused by flame flicker. A second disadvantage of the FPD is that large solvent injections can cause the flame to extinguish. One last drawback is that the instrument parameters need to be finely controlled because small deviations in the carrier or the flame gas flow rates can cause significant variations in the sensitivity of the d etec to r. A detector that has not found as much acceptance is the Microcoulometric Detector (MCD). In the MCD system analytes react with an electrochemically generated species. The response is based on the magnitude of the current required to reestablish an equilibrium for the electrochemical species. The MCD is selective toward sulphur, nitrogen and the halogens depending on the electrochemical reagent used. The major drawback of this detector, and the reason why it has not generated more appeal, is due to the poor detection limits which are at the microgram level.8 The Hall Electrolytic Conductivity Detector (HECD) has been widely used as an element selective detector for halogens. The HECD operates by passing the sample into a helium flow, which then proceeds into a pyrolysis zone where the compounds are decomposed. The decomposition products come into contact with the liquid phase ( e.g., deionized water) and are absorbed, forming an electrolyte 5 solution. The response of the detector is based on the change in the conductivity of the liquid phase. The HECD is selective with respect to halogen, nitrogen and sulphur compounds. Although the HECD has a large linear response range, the limits of detection are far less than those achieved with the ECD. Typical limits of detection are at the nanogram level.3 The HECD response also appears to be dependent only on the gram amount of halogen present and independent of compound structure. One of the most promising detectors for element selective detection is the Microwave Induced Plasma (MIP). The first commercial instrument based on the MIP detector was recently announced at the 1989 Pittsburgh Conference by Hewlett-Packard.9 The plasma is generated by irradiating flowing helium contained in a quartz tube with microwave radiation.

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