Ion storage techniques and time-of-flight mass spectrometry in physical forensic science
Effective investigative analysis requires proper selection of sample-collection procedures, preservation, and analysis methods. To achieve these objectives it is essential to tailor the collection and analysis methods to the application requirements, which are constrained by different parameters such as analysis time, sample concentration, matrix interferences, and analyte stability (e.g., surface activity (''stickiness'') and chemical reactivity). In addition, method optimization must be accomplished without compromising sample integrity. Maintaining sample integrity requires minimizing and characterizing contamination as well as reducing sample degradation and loss to prevent both false positive and negative detection, respectively. When specific constraints are defined, depending on target chemical(s) and application scenarios, it is sometimes necessary to modify or develop new equipment and methods to best satisfy the application requirements. For this work, we are interested in real-time monitoring of airborne chemicals, which are commonly referred to as volatile organic compounds (VOC). Although there are a number of techniques for remote analysis of VOCs, many of these applications do not provide the specificity and sensitivity needed in real-time application scenarios. For example, spectroscopic techniques are capable of providing low part-per-billion volume/volume (ppb v/v) detection if a compound is distributed over a large area, however, they provide limited compound structural information. A different approach using air monitoring mass spectrometry can provide more specific chemical information in terms of compound molecular weight and structural information, however, preconcentration is required to achieve low ppb detection. Operation of the trap as a pulsed extraction eliminates many of the limitations that accompany operating the trap using mass selective axial ejection. As pulsed extraction source, the IS can be operated under high ion/ion and ion/neutral collision conditions without regard to perturbing ion secular motion. This permits overfilling of the IS and the use of air as a collisional damping gas with little effect on resolution and mass accuracy. A subsequent advantage of using air as a collisional damping gas is that it enhances ionization efficiency by providing a mechanism for chemical ionization. In addition to ion storage, the ion trap can be operated to deliver an ion packet with a spatial and kinetic energy distribution well suited for time-of-flight analysis. This mode of operation enhances sensitivity and resolution permitting the TOF-MS analyzer to be used effectively for real-time air monitoring. Among the different changes to trap operation needed for optimal operation, those modifications that minimize dispersion of the ion packet during extraction have the greatest effect on performance. Together these modifications, which include (1) phase synchronization, (2) RF clamping and (3) bipolar extraction, permit resolution of up to 1600 m/{Delta}m at FWHM and detection into the low ppb range for VOCs. Sensitivity for semi-VOCs is significantly lower and is attributed to adsorption loss in the IS envelop. We are currently investigating solutions to this limitation.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15006378
- Report Number(s):
- UCRL-JC-139401; TRN: US200409%%60
- Resource Relation:
- Conference: 2000 IEEE/EIA International Frequency Control Symposium and Exhibition, Kansas City, MO (US), 06/06/2000--06/09/2000; Other Information: PBD: 27 Jun 2000
- Country of Publication:
- United States
- Language:
- English
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