Chapter 20
Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 12, 2018 at 05:27:06 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Trace Analysis of Organic Compounds in Groundwater On-Column Preconcentration and Thermal Gradient Microbore Liquid Chromatography with Dual-Wavelength Absorbance Detection Leslie K. Moore and Robert E. Synovec Department of Chemistry, Center for Process Analytical Chemistry, University of Washington, BG-10, Seattle, WA 98195 Thermal gradient microbore liquid chromatography with dual wavelength absorbance detection is evaluated as a tunable on-site analyzer. Gradient programming allows the selectivity of the analyzer to be adjusted thus potentially identifying chemical interferences in a process or an environmental upset. The dynamic range for typical microbore liquid chromatography analytes is extended by concentrating the sample on-column. Analytes are concentrated 500 fold from 500 µL injection volumes without introducing appreciable band broadening following gradient elution. The relative change in capacity factor with temperature is found to be slightly higher for highly retained peaks, thus temperature programming allows analysis of highly retained compounds such as high molecular weight polycyclic aromatic hydrocarbons (PAHs). The effects of high temperature on band broadening was evaluated. For a given capacity factor, temperature improved chromatographic efficiency over room temperature, isocratic elution. In the last decade there has been increasing interest in on-site chemical analysis for process and environmental monitoring (J). On-site analysis and feedback control enables efficient utilization of resources and minimizes waste. To date, a number of in-situ spectroscopic and electrochemical methods have been developed for continuous real time monitoring (2). Available sensors are adequate for well characterized systems, however they lack the broad selectivity necessary to characterize constituents of an unforeseen process upset. Many devices also lack the flexibility to monitor extreme changes in the concentration of expected analytes. It would be useful to have an analyzer that responds to many different compounds and a wide range of concentrations. Furthermore, because a majority of process and environmental problems are aqueous based, it is of interest to develop an on-site analyzer for aqueous samples. Gradient microbore liquid chromatography (/-iLC) with dual wavelength absorbance detection has been proposed as an on-site tunable liquid analyzer (3). Chromatography provides greater selectivity than single sensors and has the potential to identify interferences in a process or an environmental upset.
0097-6156/92/0508-0243$06.00/0 © 1992 American Chemical Society
Breen and Dellarco; Pollution Prevention in Industrial Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
244
POLLUTION PREVENTION IN INDUSTRIAL PROCESSES
Microbore chromatography is well suited for remote analysis because the system requires a minimal amount of solvent preparation and storage. Thermal gradient microbore liquid chromatography ( T G - ^ L C ) optimizes analyzer selectivity, and solves the problem of applying gradients to small volume systems (4). It is much easier to control the temperature of the column than it is to uniformly mix two mobile phases with the precision necessary for optimal separation. Both mobile phase and temperature gradients improve selectivity by changing elution conditions with time. However separations at elevated temperatures are potentially more efficient than mobile phase gradient separations as increased temperature facilitates mass transfer of the analyte to the stationary phase (5). A dual wavelength absorbance detector with a fiber optic based in-situ flow cell is incorporated to optimize detection. The 1.2 pL flow cell and the column are both heated, so the analytes are separated and detected at the same temperature. The detector simultaneously measures the absorbance at two wavelength regions. The first is a wavelength region of interest for measuring the absorbance of analytes. The second is a reference wavelength region where little or no absorbance is expected. The detector produces two signals. The difference signal is the difference between the intensities at the two wavelength regions, and the sum signal is the sum of the intensities at the two wavelength regions. The difference signal corrects for baseline drift due to the temperature gradient and minimizes correlated noises associated with source fluctuations and changes in ambient temperature (6). The sum signal possesses information regarding column temperature changes. Two dual wavelength detectors are used in this study. A position sensitive detector (PSD) based dual wavelength detector has been described previously (7,8). The difference signal from this detector stabilizes baseline drift 15 fold over a conventional single wavelength absorbance detector. Typically, the difference signal also has low baseline noise characteristics of 1.1 χ 10~ A U (3 χ root mean square noise level). A second dual wavelength detector is a two diode detector. The two diode detector is operationally the same as the PSD detector, except there are two diodes instead of one continuous photoelectric surface. Other multiwavelength detectors such as photodiode array detectors and charge coupling devices could also be used to correct for refractive index aberrations due to temperature by employing the two wavelength subtraction technique. 4
The dynamic range of the analyzer is extended by preconcentrating dilute so lutions before the chromatographic separation. On-line concentration techniques include liquid-liquid (9) and membrane extraction (10), however solid phase ex traction (SPE) is commonly employed because it is a rapid way to extract and concentrate analytes from relatively clean matrices (11). A S P E cartridge often consists of the same packing material as the analytical column, however the mo bile phase conditions are chosen so the analytes are highly retained. It is clear that the broadening associated with eluting the analytes from a S P E cartridge could be eliminated by extracting and separating the analytes on one column. Guinebault and Broquaire have demonstrated on-column preconcentration and separation on analytical sized columns (12,13) and Slais has shown large volumes can be concentrated onto microbore columns (14). In this work we optimize the parameters associated with on-column preconcentration for microbore columns. We will also investigate the potential of thermal gradient elution in conjunction with dual wavelength absorbance detection for on-site analysis. Sample volumes of 500 μL will be injected on a microbore column enabling trace analysis of dinitrophenylhydrazine (DNPH) derivatized aldehydes and ketones in air samples,
Breen and Dellarco; Pollution Prevention in Industrial Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
20. MOORE AND SYNOVEC
Trace Analysis of Organic Compounds
245
and PAHs in water samples. The benefits of temperature programming in terms of increased mass transfer and improved chromatographic efficiency will also be reported.
Experimental Chromatographic System: A schematic diagram of the chromatographic system is illustrated in Figure 1. A syringe pump (ISCO, LC-500, Lincoln, NE) was utilized to deliver the mobile phase at a flow of 50 /iL/min in all cases. Injections of 1 / / L were accomplished with a microbore injection valve (Rheodyne 7520 Cotati, C A . ) . The 500 / / L injections were introduced with a six port injection valve (Rheodyne 7125 Cotati, C A ) and a 500 pL loop made in-house. The aldehyde and ketone samples were run on a low pressure (
