Direct Sampling MS and MS/MS for Real Time Air Monitoring
A number of investigators have shown the utility of direct sampling ion trap mass spectrometry (DSITMS) for on-line, real-time monitoring applications. While ion trap instrumentation continues to become smaller, more sensitive, and equipped with more high performance features (i.e., tandem MS, extended mass range, increased mass resolution), the development of improved sample introduction systems and tandem MS methods for target compound analysis are essential to making DSITMS techniques more widely used for these applications.
This work involves the characterization of two different sample introduction systems for real-time monitoring trace levels of CFCs in air. Gas standards of CF2Cl2 (CFC12), CFCl3 (CFC11), and CCl4 in a balance gas of air were obtained from commercial sources or generated with an Environics gas dilution system. A Teledyne model 3DQ ion trap mass spectrometer was employed for the analyses.
The first sample introduction system, termed the SIS/ORNL inlet, was developed by Scientific Instrument Services based on a design by researchers at Oak Ridge National Labs (1). This device mixes the air sample with helium, passes the resulting mixture through an open-split interface, a short section of 75um ID deactivated fused silica capillary, and then into the ion trap. Analysis is done in a continuous fashion, with the sample inlet being presented with either the sample or lab air as background. Results from 10 replicate analyses of a sample containing 50 ppbv CCl4 in air via MS/MS are shown in Figure 1. Detection limits were on the order of 50 ppbv in MS, selected ion monitoring (SIM), and MS/MS modes for CF2Cl2, CFCl3, and CCl4. Precisions were on the order of 5%. These results are comparable to those of other investigations using similar inlet systems (1-5). This system is easy to install and optimize, provides better than unit mass resolution, and gives minimal hysteresis and carryover.
The second sample introduction system employs a valve in the sample loop configuration. The valve is equipped with a 15 ml sample loop and is connected to the standard 3DQ transfer line with a 0.25 mm ID deactivated fused silica capillary. Analysis is done in a discrete fashion, with the contents of the sample loop flushed into the ion trap using zero-grade air. This air also serves as a buffer gas for the ion trap. Results from 10 replicate analyses of a sample containing 50 ppbv CCl4 in air via MS/MS are shown in Figure 2. Detection limits were on the order or 50 ppbv in MS, SIM, and MS/MS modes for CF2Cl2, CFCl3, and CCl4. Precisions were on the order of 5%. These detection limits are several orders of magnitude lower than the only other work reported in the literature using air as a buffer gas (6) and comparable to those obtained using the SIS/ORNL inlet. More importantly, they corroborate the results of Lammert and Wells and show that the ion trap can be operated using air as a buffer gas and provide unit mass resolution and comparable sensitivities for mass ranges less than 200 amu (7,8). On a more practical note, it should be stated that tuning the ion trap parameters to obtain these results is not straightforward, and that charge exchange and large amount of air present in the ion trap may yield unanticipated ion-molecule reactions.
- Wise et. al., Proceedings of the 38th ASMS Conference on Mass Spectometry, p. 1483.
- Berberich et. al., Proceedings of the 39th ASMS Conference on Mass Spectometry, p. 1279.
- Wise et. al., Proceedings of the 39th ASMS Conference on Mass Spectometry, p. 1205
- Thompson et. al., Proceedings of the 40th ASMS Conference on Mass Spectometry, p. 653.
- Wise et. al., Proceedings of the 42nd ASMS Conference on Mass Spectometry, p. 874.
- Cameron et. al., J. Am. Soc. Mass Spectrom., 1993, 4, p. 774.
- Lammert and Weils, Proceedings of the 42nd ASMS Conference on Mass Spectometry, in press.
- Lammert and Weils, Rapid Commun. Mass Spectrom., in press.