Ion-trap detectors

One example of normal-phase liquid chromatography coupled to gas chromatography is the determination of alkylated, oxygenated and nitrated polycyclic aromatic compounds (PACs) in urban air particulate extracts (97). Since such extracts are very complex, LC-GC is the best possible separation technique. A quartz microfibre filter retains the particulate material and supercritical fluid extraction (SPE) with CO2 and a toluene modifier extracts the organic components from the dust particles. The final extract is then dissolved in -hexane and analysed by NPLC. The transfer at 100 p.1 min of different fractions to the GC system by an on-column interface enabled many PACs to be detected by an ion-trap detector. A flame ionization detector (PID) and a 350 p.1 loop interface was used to quantify the identified compounds. The experimental conditions employed are shown in Table 13.2. 

Specificity is unsurpassed. Traditionally, MS was performed on very large and expensive high-resolution sector instruments operated by experienced specialists. The introduction of low-resolution (1 amu), low-cost, bench-top mass spectrometers in the early 1980s provided analysts with a robust analytical tool with a more universal range of application. Two types of bench-top mass spectrometers have predominated the quadrupole or mass-selective detector (MSD) and the ion-trap detector (ITD). These instruments do not have to be operated by specialists and can be utilized routinely by residue analysts after limited training. The MSD is normally operated in the SIM mode to increase detection sensitivity, whereas the ITD is more suited to operate in the full-scan mode, as little or no increase in sensitivity is gained by using SIM. Both MSDs and ITDs are widely used in many laboratories for pesticide residue analyses, and the preferred choice of instrument can only be made after assessment of the performance for a particular application. 

Fieiure 9-1 Schematic view of (A) the bench-top quadrupole mass spectrometer (Hewlett-Packard) and (B) the ion trap detector (Finnigan MAT). 

Moving belt (MB) Atmospheric pressure (API) Ion trap detector (ITD)... 

As in the case in the analysis of food samples, the introduction of relatively inexpensive MS detectors for GC has had a substantial impact on the determination of methylxanthines by GC. For example, in 1990, Benchekroun published a paper in which a GC-MS method for the quantitation of tri-, di-, and monmethylxanthines and uric acid from hepatocyte incubation media was described.55 The method described allows for the measurement of the concentration of 14 methylxanthines and methyluric acid metabolites of methylxanthines. In other studies, GC-MS has also been used. Two examples from the recent literature are studies by Simek and Lartigue-Mattei, respectively.58 57 In the first case, GC-MS using an ion trap detector was used to provide confirmatory data to support a microbore HPLC technique. TMS derivatives of the compounds of interest were formed and separated on a 25 m DB-% column directly coupled to the ion trap detector. In the second example, allopurinol, oxypurinol, hypoxanthine, and xanthine were assayed simultaneously using GC-MS. 

Simek, P., Jegorov, A., and Dusbabek, F., Determination of purine bases and nucleosides by conventional and microbore high performance chromatography and gas chromatography with an ion trap detector, J. Chromatogr., 679,1951,1994. 

Eichelberger JW, Bellar TA, Donnelly JP, et al. 1990. Determination of volatile organics in drinking water with USEPA method 524.2 and the ion trap detector. J Chromatogr Sci 28 460-467. 

When operated as a specific detector the ion-trap detector is more sensitive still but not to the extent that would be expected from the performance of other mass spectrometers operated in this mode in view of the large number of ions monitored in full scan mode there is little more sensitivity to be gained by spending a little extra time scanning a narrow mass range, and the detection limit in this mode is in the region of l-2pg. 

The power of the system to overcome the problems associated with coeluting compounds is demonstrated in conjunction with the use of deuterated (or13C-labelled compounds) as internal standards. Such techniques could not be used in conventional gas chromatography as the deuterated compounds often co-elute, making quantification difficult if not impossible. With the ion-trap detector, however, it is easily possible to differentiate between the ions arising from the different compounds and the intensities of these ions could then be used for quantification of the compounds involved. The application of such techniques can be shown by... 

The signals from masses 292 and 326 characteristic of tetra- and pentachlorobiphenyl are shown in Fig. 1.6 (b,c). The specific detection mode of the ion-trap detector can be used to improve detection limits. This detector can monitor specific masses that are characteristic of compounds of interest. The detector records the signal for only those masses and ignores all others. Interference from other compounds is virtually eliminated with the Finnigan MAT 700 detector—up to 16 different groups of masses can be monitored or a mass range of up to 40 masses can be handled. With this flexibility it is possible to monitor only the masses of interest and to improve detection limits. 

Kelly, P.E. Ion trap detector literature reference list. Finnigan MAT IDT 21. 

Campbell, C. The ion trap detector for gas Chromatography technology and application. Finnigan MAT IDT 15. 

Yost, R.A., McClennan, W. and Menzzelaar, H.L.C. Enhanced full scan sensitivity and dynamic range in Finnigan MAT ion trap detector with automatic gain control of software. Finnigan MAT IDT 22. 

Richards, J.M. and Bradford, D.C. Development of a Curie Point pyrolyser inlet from the Finnigan MAT ion-trap detector. Finnigan MAT IDT 25. 

Bishop, P. The ion trap detector, universal and specific detection in one detector. Finnigan MAT IDT 28. 

Todd, J., Mylchreest, I., Berry, T. and Games, D. Supercritical chromatography mass spectrometry with an ion-trap detector. Finnigan MAT IDT 46. 

Eichelberger, J.W. and Slivon, L.E. Existence of self chemical ionization in the ion-trap detector. Finnigan MAT IDT 48. 

Gas chromatographic analyses of China White (Fentanyl) with the ion-trap detector. Finnigan MAT Application Data Sheet ADS 13. 

Ion-trap detector Perkin-Elmer Corporation Analytical Instruments Division 761 Main Avenue... 

Despite the difficulties of on-line automation, the need to develop such systems is considerable. The increase in the number of different compounds that must be determined and the number of samples required for a meaningful survey or laboratory study make it essential to improve the quality and throughput of samples. There are a number of stages in fully automating trace organic analysis. Autosampler LC or GC-data systems as GC-MS or GC-ion trap detector (ITD) are well established and require no further elaboration here [191, 203, 495]. 

---