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DEVELOPMENT OF MICROTRAP-GC FOR CONTINUOUS MONITROING OF ORGANIC EMISISONS IN AIR EMISSIONS Background | Development | Performance | Advantages | References Continuous on-line monitoring offers several advantages and challenges over the conventional approach of sampling followed by laboratory analysis. These automated instruments are required to perform on-line sampling, sample conditioning and analysis. Near real-time information at a lower analytical cost is an advantage of this approach. Secondly, on-line analysis eliminates contamination and other errors that can be introduced during storage and transportation of the sample. The analytical results can be fed back for process control. Gas Chromatography has been used in continuous monitoring applications such as process stream analysis since the 1950s. A critical part of process GC is the sampling system that can condition, concentrate and inject sample into the GC column. Sample valves have been used as injectors of the process GC for continuous monitoring. These types of injectors are commercially available and are routinely used in industry. But the injection volume of a sample valve is limited and a typical sample volume for capillary column is about 100 ml. Thus a small detector response is generated, due to the small injection volume. If a large sample volume is injected using a sample valve, the peaks are broad and the resolution is poor. So the sample valve is inadequate to analyze trace level samples. Analysis of trace concentrations of sample at sub parts per million levels is usually done by sampling a large quantity of air and concentrating the pollutants. Sorbent traps are often used to extract analytes from a large sample. After sampling, the traps are transported to the laboratory, where the pollutants are desorbed and analyzed by GC or GC/MS. However, these techniques cannot be used in continuous, on-line analysis. BACK TO TOP | BACK TO RESEARCH PROJECTS DEVELOPMENT OF ON-LINE MICROTRAP SYSTEM Over the last few years we have reported the use of a micro-sorbent trap referred to as a microtrap as been used as concentration cum injection device for continuous monitoring of VOCs in gas stream. The microtrap is made by packing a capillary tubing with an adsorbent. The microtrap is placed in front of GC column instead of a conventional injection port. When organics pass through the microtrap, they are trapped and concentrated. The VOCs are released and injected into the GC column by rapid electrical heating. It can make reproducible injections. It is rugged, and exhibits long term stability. It has several advantages over an injection valve. Unlike an injection valve, it acts as an injection device as well as a sample preconcentrator. Thus much lower detection limits are obtained using on-line microtrap compared to a sample valve. This method has the potential advantage over valves in term of faster operation, smaller bandwidth and lower detection limit. The microtrap can be configured in different ways with a gas-sampling valve. In what has been referred to as sequential valve microtrap (SVM), a microtrap is connected in series with a gas sample valve. In this technique, a large volume injection (several milliliters) or several small volume (e. g., multiple 100 ml) injections are made by the sample valve. The analytes are trapped by the microtrap. Then the microtrap is heated to inject the analytes into the GC. The microtrap can be also be configured to make direct injections into a GC. This is referred as on-line microtrap (or OLMT). In sampling mode, a sample stream passes through the microtrap. The analytes are trapped by the microtrap, and the. Then the microtrap is heated and analytes are desorbed/injected into the GC column. Injection devices based on microtrap have shown advantages in continuous, on-line monitoring of VOCs in air low detection limits have been achieved and the device is simple and can be operated automatically. The instrumentation based on microtrap interface is shown in Fig. 1. BACK TO TOP | BACK TO RESEARCH PROJECTS PERFORMANCE OF THE MICROTRAP-G.C. Typical chromatograms of VOCs at ppb level in catalytic incinerator effluents using the .... are shown in Fig. 2. Detection limit defined as the concentration at three times the signal to noise ratio were determined for the three injection modes. The table lists the detection limits of benzene, TCE, toluene and ethylbenzene using valve, SVM and OLMT.
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