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DEVELOPMENT OF MICROTRAP-GC FOR CONTINUOUS MONITROING OF ORGANIC EMISISONS IN AIR EMISSIONS
Background |
Development |
Performance |
Advantages |
References
BACKGROUND
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.
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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.
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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.
ADVANTAGES OF MICROTRAP
- Simple instrumentation for fast on-line analysis.
- Detection limits at ppb levels.
- No interference from moisture.
- No cryogenic concentration is necessary.
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REFERENCES
- S. Mitra and Y. Chen, J. of Chromatography. 648, 415 (1993).
- S. Mitra and A. Lai, J. of Chromatogr. Sci. 33, 285(1995).
- 3. S. Mitra, Y. Xu, W. Chen, and A. Lai., Journal of Chromatography A,
727, 111-118 (1996).
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Pulse Introduction Membrane Extraction
(PIME) for on-site analysis of VOCs in water.