Gas chromatographic-ion trap mass spectrometric analysis of volatile organic compounds by ion-molecule reactions using the electron-deficient reagent ion CCl3(+)

Multiplexing Quadrupole and Ion Trap Operation Modes on a "Brick" Miniature Mass Spectrometer

Although a quadruple mass analyzer and an ion trap mass analyzer have complementary analytical features, they usually have different geometries, operational modes, and electronic control systems. As a continuous effort to extend its coverage, both quadrupole and ion trap operation modes were realized on a "brick" miniature mass spectrometer with a single mass analyzer. In the quadrupole operation mode, low-mass ions ranging from 31 to 502 Th can be analyzed. On the other hand, the ion trap mode can be utilized to cover ions with higher mass to charge ratios (up to 922 Th), as well as performing tandem mass spectrometry. To realize the multiplexing of both operation modes, a printed circuit board (PCB)-based multi-electrode quadrupole-ion trap mass analyzer was designed and integrated in the system. To cover both volatile and non-volatile molecules, two ionization sources were also implemented, including a nano electrospray ionization source and an in-vacuum plasma ionization source. Performances of the instrument operated in these two modes were characterized, such as mass resolution, sensitivity, and mass range. Results demonstrate that the combination of the quadrupole and ion trap operation modes can provide new capabilities when solving analytical problems.

A membrane introduction mass spectrometer utilizing ion-molecule reactions for the on-line speciation and quantitation of volatile organic molecules

RATIONALE

The ability of membrane introduction mass spectrometry to quantitatively resolve low molecular weight volatile organic compounds (VOCs) such as benzene, toluene, ethylbenzene and xylene (BTEX) using electron ionization (EI) can be compromised by isobaric interferences. This work focuses on reducing isobaric interferences with ion-molecule reactions in a portable quadrupole ion trap mass spectrometer for the analysis of VOCs.

METHODS

EI was used to produce reagent ions from precursors (chloroform, methyl iodide, trichloroethylene or chlorobenzene) that were continually infused into the helium acceptor phase upstream of the membrane introduction mass spectrometry (MIMS) sampling interface. The reagent ions were selectively stored in the ion trap, and then allowed to react with target VOC analytes in air samples via ion-molecule reactions within the trap storage volume. A variety of reaction times were examined (50-5000 ms), and the resulting product ions were analyzed in positive ion mode.

RESULTS

The detection limits achieved were comparable with those obtained using EI (low ppbv), and in some cases better than for EI coupled with tandem mass spectrometry (MS/MS). For the VOCs studied, isobaric interferences were greatly reduced or eliminated using chloroform as a reagent gas. The predominant ionization mechanism was via adduct formation, although charge transfer and hydride abstractions were also observed. An internal standard was shown to be effective at correcting for signal changes due to consumption of reagent ions when complex mixtures were sampled.

CONCLUSIONS

Ion-molecule reactions were exploited to eliminate isobaric interferences that are often encountered in direct, real-time analysis strategies for atmospheric VOC mixtures. The use of a continuously infused internal standard will improve quantitative results in field applications where analyte concentration and sample complexity may be wide ranging.

Multiple mass analysis using an ion trap array (ITA) mass analyzer

A novel ion trap array (ITA) mass analyzer with six ion trapping and analyzing channels was investigated. It is capable of analyzing multiple samples simultaneously. The ITA was built with several planar electrodes made of stainless steel and 12 identical parallel zirconia ceramic substrates plated with conductive metal layers. Each two of the opposing ceramic electrode plates formed a boundary of an ion trap channel and six identical ion trapping and analyzing channels were placed in parallel without physical electrode between any two adjacent channels. The electric field distribution inside each channel was studied with simulation. The new design took the advantage of high precision machining attributable to the rigidity of ceramic, and the convenience of surface patterning technique. The ITA system was tested by using a two-channel electrospray ionization source, a multichannel simultaneous quadruple ion guide, and two detectors. The simultaneous analysis of two different samples with two adjacent ITA channels was achieved and independent mass spectra were obtained. For each channel, the mass resolution was tested. Additional ion trap functions such as mass-selected ion isolation and collision-induced dissociation (CID) were also tested. The results show that one ITA is well suited for multiple simultaneous mass analyses.

Determination of hazardous volatile organic compounds in the Hoffmann list by ion-molecule reaction mass spectrometry

RATIONALE

Off-line gas or liquid chromatographic mass spectrometry techniques are the most widely used method for analysis of hazardous, carcinogenic volatile organic compounds (VOCs) in mainstream cigarette smoke. However, these conventional techniques can lead to modification of VOCs during sample preparation due to the high reactivity of VOCs. Thus, the development of on-line mass spectrometric methods for analysis of VOCs is desirable to circumvent this problem.

METHODS

The accurate identification of VOCs is a critical step in the analysis of cigarette smoke. Here, we use ion-molecule reaction mass spectrometry (IMR-MS) to study the behavior of standard VOCs in the Hoffmann list during this analytical procedure, and then to profile the VOCs in mainstream cigarette smoke using this on-line mass spectrometric method.

RESULTS

We first discuss and summarize the charge transfer (CT) ionization and further fragmentation of 20 standard VOCs in the Hoffmann list with the ion reagents Hg(+), Xe(+), and Kr(+). The IMR-MS instrument was then connected to a Borgwaldt-RM20H rotary smoking machine in order to study VOCs in mainstream cigarette smoke on-line. Using this procedure, more than 20 VOCs were identified by IMR-MS by comparison with experimental results obtained on standard VOCs.

CONCLUSIONS

The IMR-MS technique can potentially result in reduced molecular fragmentation during analysis of VOCs. However, significant fragmentation still occurs during IMR-MS when the ionization energy (IE) of the ion reagent is much higher than the IE of the VOC, given that excess energy is stored in the newly formed ion during CT ionization. Given that IMR-MS cannot distinguish between isobaric compounds or isomers, we summarize the possible overlapping mass peaks from these isobaric species that may be present in analyses of VOCs. Selection of the ion reagent for IMR-MS should be based on the need to ensure CT ionization of the analytes, as well as avoiding their severe fragmentation.