RESEARCH PROJECTS  

H OME R ESEARCH PROJECTS P EOPLE P UBLICATIONS B OOK PUBLISHED C ONTACT US

    Pulse Introduction Membrane Extraction (PIME) for on-site analysis of VOCs in water.

    Continuous Non-Methane Organic Carbon (C-NMOC) monitoring.

    Microtrap Mass Spectrometry (MTMS).

    Microtrap - G.C for monitoring VOCs in air.

    Silicon Micromachined sensor device.


MICROTRAP MASS SPECTROMETRY
 

MICROTRAP MASS SPECTROMETRY (MTMS) FOR CONTINUOUS ON-LINE MONITORING OF AIR EMISSIONS

Background | Mass Spectrometry | Acknowledgement | References

BACKGROUND


Continuous on-line monitoring of air emission offers the advantage of obtaining real-time information about a chemical process or an environmental emission. Regulatory concerns and public fear about emissions of hazardous materials from incinerators and other emission sources are prompting the development of continues emission monitors (CEM) for organic, inorganic and metal emissions.

Compared to traditional field sampling followed by laboratory analysis, a CEM requires sampling, sample conditioning, and analysis to be done on-line. The instrument has to be automated, rugged and should be able to deal with complex matrices containing potential interference. Since sample handing and storage are eliminated, these techniques produce more accurate results.

Mass spectrometers (MS) have several attractive features such as
  • fast response time,
  • high sensitivity
    • and
  • high resolution spectra containing molecular weight and structural information
. These characteristics are highly desirable in air monitoring where low concentrations are encountered in a complex air matrix. The availability of different ionization techniques and the MS-MS capabilities of modern iontrap mass spectrometers make MS a powerful analytical tool. There is significant potential for employing mass spectrometers as a CEM where the desired mass range can be scanned within seconds. One of the challenges in implementing mass spectrometry as a CEM is that the organics are present in trace concentration (ppm to ppt levels) while background gases such as H2O and CO2 may be present at percent levels. Moisture in particular is a source of serious interference in on-line mass spectrometry. This is particularly true for combustion sources such as incinerator where moisture concentration may be as high as 30% by volume.

Consequently, on-line mass spectrometry for emission monitoring requires an effective sampling interface to eliminate the moisture and other gases such as CO2, H2O and CH4 prior to the entrance into the ionization chamber.

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MICROTRAP MASS SPECTROMETRY


The development of a microtrap interface for on-line mass spectrometry is presented for continuous monitoring of VOCs in air emissions. The system is shown in figure 1.

As the sample containing VOCs and background gases are passed through the sampling system, the microtrap selectively traps the organics but allows the rest to go through. Thus, the microtrap serves as a separator as well as an on-line preconcentrator.

The organics are injected into the mass spectrometer via rapid thermal desorption of the microtrap. Continuous (or near-continuous) monitoring is achieved making a series of desorptions while the air continuously flows through the system. Corresponding to each desorption a mass spectrum of the sample components is obtained.

The microtrap is configured with a gas sampling valve as shown. First, the air stream passes through the microtrap. The organics were trapped by the sorbent and the matrix gases were vented. When the valve was switched to the injection position, the helium stream passed through the microtrap and into the mass spectrometer. The flow direction of He was reversed to backflush the microtrap. The microtrap was then heated.

The TIC trace and the associated mass spectra as a function of time for a standard containing 1.2 ppm tetrachloroethane and 1.7 ppm toluene (along with 3% moisture, 8% CO 2 and 120 ppm CH4) are shown in figure 2.
Peak A corresponds to a point in time right after the valve was switched to the inject positions. This peak was mainly due to residual background gases remaining in the microtrap. The mass spectra at this point showed high intensity of m/z 18, 28 and 44 from H2O, N2 and CO 2 respectively. A few second delay was appropriate to flush out all the background gases from the microtrap.

The peak B was generated from the thermal desorption from the microtrap. The main components here were the organics. Practically none of the background species are seen here. High precision was obtained. The relative standard deviation (RSD) was 4.6% based on five consecutive measurements. The sensitivity was 2000 more with the microtrap interface as compared to direct introduction.

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ACKNOWLEDGEMENT


This work was funded in part from a grant from US EPA at Research Triangle Park, NC.

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REFERENCES

  1. A microtrap interface for continuous on-line mass spectrometric monitoring of VOCs in air emissions. S. Mitra, C. Feng, L. Zhang, W. Ho and G. McAllister. J. of Mass Spectrometry, 34, 478 (1999).





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