How is Mass Spectrometry Used for Air Monitoring?

A large number of investigators have shown that Direct Sampling Ion Trap Mass Spectrometry (DSITMS) is a beneficial method for a range of real-time and online monitoring applications.

As technologies develop further, the ion trap instrumentation is becoming progressively more sensitive and compact. It is also being supplied with more high-performance features such as increased mass resolution, extended mass range, and tandem MS.

It is critical that enhanced sample introduction systems and tandem MS methods for target compound analysis are created to enable the frequent use of DSITMS methods in these applications.

Experiment

The current developmental work comprises of the characterization of two alternative sample introduction systems to allow for the live monitoring of trace levels of CFCs in air.

Gas standards of CF₂Cl₂ (CFC₁₂), CFCl₃ (CFC₁₁) and CCl₄ in a balance gas of air were produced utilizing an Environics gas dilution system or commercial sources. The investigations were then carried out by utilizing a Teledyne model 3DQ ion trap mass spectrometer.

A group of researchers at Oak Ridge National Labs ⁽¹⁾ produced a design which was employed by Scientific Instrument Services to create the first sample introduction system that is known as the SIS/ORNL inlet.

This device combines the air sample with helium, where the mixture created travels through an open-split interface and into the ion trap. The open-split interface is a small section of 75 µm ID deactivated fused silica capillary.

The sample inlet is presented with the laboratory air or the sample as a background in order to perform continuous analysis.

Results

The recorded limits of detection were of the order of 50 ppbv in Selected Ion Monitoring (SIM), MS, and MS/MS modes for CF₂Cl₂, CFCl₃, and CCl_4. In all of these, the precisions were of the order of 5%.

It was found that these results favorably compared with those of different investigations that comprised of similar inlet systems ^(1-5).

It should be established that the system employed here, which can be simply optimized and installed, provides an enhanced unit mass resolution and allows only minimal carryover and hysteresis.

The valve utilized in the sample loop configuration is also used in the second sample introduction system. It is supplied with a 15 ml sample loop and is attached to the standard 3DQ transfer line utilizing a 0.25 mm ID deactivated fused silica capillary.

The analysis was discretely performed., Zero-grade air was used to act as a buffer gas for the ion trap, and to flush the contents of the sample loop into the ion trap.

The detection limits observed were of the order or 50 ppbv in SIM, MS, and MS/MS modes for CF₂Cl₂, CFCl₃, and CCl_4. In all of which, precisions were of the order of 5%.

These detection limits are significantly lower in contrast with an alternative study found in literature which was the only other study that utilized air as a buffer gas ⁽⁶⁾.

The detection limits found are in line with the results of Lammert and Wells. They establish the use of the ion trap by utilizing air as a buffer gas. It also provides unit mass resolution and similar sensitivities for mass ranges less than 200 amu ^(7,8).

The tuning procedure needed for the ion trap parameters is complex. Unexpected ion-molecule reactions can be caused by this process due to the large amount of air present in the ion trap and the charge exchange.

Environics Mixer for Mass Spectrometry

The S4040 system is engineered for the preparation of calibration standards employed in Mass Spectrometry or Gas Chromatography (GC).

Environics systems can generate gas concentrations from percent to ppb levels at the necessary flow rates for single or multi-point calibration.

In every application, Environics mixers enable the customer to utilize cost-effective bulk tanks or single-gas cylinders to replace a high number of costly blended cylinders.

References and Further Reading

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.