Real Time Air Monitoring with Direct Sampling Ion Trap Mass Spectrometry

Many investigators have demonstrated that direct sampling ion trap mass spectrometry (DSITMS) can be used for on-line, real-time monitoring applications. The ion trap instrumentation is becoming smaller, more sensitive, and is being equipped with more high performance features (i.e., extended mass range, tandem MS, increased mass resolution). Therefore, it is essential that enhanced sample introduction systems and tandem MS methods for target compound analysis have to be developed to make DSITMS techniques more commonly used for these applications.

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Experiment

This developmental work involves characterizing two different sample introduction systems to enable real-time monitoring of trace levels of CFCs in air. Gas standards of CF₂Cl₂ (CFC₁₂), CFCl₃ (CFC₁₁), and CCl₄ in a balance gas of air were generated with an Environics gas dilution system or obtained from commercial sources. The analyses were performed using a Teledyne model 3DQ ion trap mass spectrometer.

A design developed by researchers at Oak Ridge National Labs ⁽¹⁾ was used by Scientific Instrument Services to develop the first sample introduction system, termed the SIS/ORNL inlet. This device blends helium with the air sample, makes the resulting mixture to pass through an open-split interface, a short section of 75 µm ID deactivated fused silica capillary, and then into the ion trap. Analysis is performed non-stop with the sample inlet being presented with either the lab air or sample as background.

Results

Figure 1 shows the results from 10 replicate analyses of a sample consisting of 50 ppbv CCl₄ in air via MS/MS. Detection limits recorded were of the order of 50 ppbv in MS, selected ion monitoring (SIM), and MS/MS modes for CF₂Cl₂, CFCl₃, and CCl₄. Precisions were of the order of 5%. These results compare favorably with those of other investigations employing similar inlet systems ^(1-5). This system can be easily installed and optimized, and offers better than unit mass resolution, and allows only minimal hysteresis and carryover.

A valve in the sample loop configuration is used in the second sample introduction system. The valve is provided with a 15 ml sample loop, and is joined to the standard 3DQ transfer line using a 0.25 mm ID deactivated fused silica capillary. Analysis is performed discretely, and zero-grade air is employed to flush the contents of the sample loop into the ion trap. The air also doubles up as a buffer gas for the ion trap.

Figure 2 shows the results from 10 replicate analyses of a sample containing 50 ppbv CCl₄ in air via MS/MS. Detection limits recorded were of the order or 50 ppbv in MS, SIM, and MS/MS modes for CF₂Cl₂, CFCl₃, and CCl₄. Precisions were of the order of 5%. These detection limits are considerably lower than the only other work reported in the literature using air as a buffer gas ⁽⁶⁾ but comparable to those obtained using the SIS/ORNL inlet. Specifically, the detection limits corroborate the results of Lammert and Wells, and demonstrate the use of the ion trap by employing air as a buffer gas and provide unit mass resolution and comparable sensitivities for mass ranges less than 200 amu ^(7,8). However, tuning the ion trap parameters to achieve these results is not simple, since unanticipated ion-molecule reactions can result due to the charge exchange and large amount of air present in the ion trap.

References

1. Wise et. al., Proceedings of the 38th ASMS Conference on Mass Spectometry, p. 1483.
2. Berberich et. al., Proceedings of the 39th ASMS Conference on Mass Spectometry, p. 1279.
3. Wise et. al., Proceedings of the 39th ASMS Conference on Mass Spectometry, p. 1205.
4.Thompson et. al., Proceedings of the 40th ASMS Conference on Mass Spectometry, p. 653.
5. Wise et. al., Proceedings of the 42nd ASMS Conference on Mass Spectometry, p. 874.
6. Cameron et. al., J. Am. Soc. Mass Spectrom., 1993, 4, p. 774.
7. Lammert and Weils, Proceedings of the 42nd ASMS Conference on Mass Spectometry, in press.
8. Lammert and Weils, Rapid Commun. Mass Spectrom., in press.

This information has been sourced, reviewed and adapted from materials provided by Environics, Inc.

For more information on this source, please visit Environics, Inc.