GC-MS interfacing

Samples sufficiently volatile for gas chromatography are readily handled by mass spectrometry. The two instruments are chiefly incompatible in the vast difference in operating pressures 760mmHg at the column exit and 10 -10 mm Hg in the analyser of the mass spectrometer. There are two solutions the vacuum system can be designed to accommodate a substantial fraction of the column effluent or a molecular separator can be employed to enrich sample relative to carrier gas effecting pressure reduction prior to transmission to the ion source. 

The predicted ideal path for minimum sample loss, when handling labile 

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Direct GC/MS interface. An interface in which all the effluent from a gas chromatograph passes into the mass spectrometer ion source during an analysis, without any splitting of the effluent. 

GC/MS interface. An interface between a gas chromatograph and a mass spectrometer that provides continuous introduction of effluent gas from a gas chromatograph to a mass spectrometer ion source. 

Separator GC/MS interface. An interface in which the effluent from the gas chromatograph is enriched in the ratio of sample to carrier gas. Separator, molecular separator, and enricher are synonymous terms. A separator should generally be defined as an effusion separator, a jet separator, or a membrane separator. 

GC/MS interface An interface between a GC and a MS that provides a continuous introduction of effluent gas to the MS ion source. 

A good way to test the operation of the GC and the GC-MS interface is to run a standard sample. For this purpose the so-called Grob test mixture [226] is suitable. 

GC-MS has been reviewed [203,204] with particular attention being paid to additive analysis [243] GC-MS interfaces were reported by Oehme [227]. For GC-REMPI-ToFMS, see Section 7.5. Various pertinent monographs are available [227,228,244,245]. 

With the advent of capillary GC, [50-54] the need for separators and the concomitant risk of suppression of certain components vanished. Capillary columns are operated at flow rates in the order of 1 ml min and therefore can be directly interfaced to EI/CI ion sources. [48,49] Thus, a modem GC-MS interface basically consists of a heated (glass) line bridging the distance between GC oven and ion source. On the ion source block, an entrance port often opposite to the direct probe is reserved for that purpose (Chap. 5.2.1). The interface should be operated at the highest temperature employed in the actual GC separation or at the highest temperature the column can tolerate (200-300 °C). Keeping the transfer line at lower temperature causes condensation of eluting components to the end of the column. 

Culea et al. reported a quantitative GC-MS analysis of procaine and some neurotransmitters in rat brain tissue [94], Procaine was extracted fi om brain homogenates by the ultrasonication method of Sundlof et al. [95], and was determined in its underivatized form on a 24 m glass capillary column coated with Silar IOC (temperature programmed from 120°C to 225°C at 12°C/min with pyrene as the internal standard). It was found necessary to wash the injector liner and the GC-MS interface stainless steel tubing with 1 1 0.1 M KOH-methanol so that the interface tubing could be coated with a film of OV-17 (from acetone solution), and to condition the apparatus by injecting bis-(trimethylsilyl)-acetamide and triethylamine. 

Two principal GC-MS interfaces are available for open-tubular GC columns. The so-called direct interface provides the highest possible detector sensitivity, whereas the open-split interface offers the least possible interference with chromatographic separation. With the direct interface, the column exit is routed from the GC oven through a heated transfer line directly into the ionization chamber. As long as the vacuum-pumping system can remove the carrier gas and maintain a sufficiently low pressure, the MS detector will function. Also, little chance exists for adsorptive loses of solute because the analytes contact only the GC column. 

A poorly designed GC/MS interface can easily compromise the performance of both the capillary GC system and the mass spectrometer. The interface should provide an inert transfer surface, yield full sample transmission, and maintain the chromatographic performance of the column. 

Figure 11.7. Open split GC/MS interface. Courtesy of Hewlett-Packard.

GC/MS interfaces constructed of all glass or glass-lined materials are required. Glass can be deactivated by silanizing with dichlorodimethylsilane. Inserting a fused silica column directly into the MS source is recommended care must be taken not to expose the end of the column to the electron beam. 

Whilst the object of this chapter has been to show the extent and type of HPLC technique that is used today in today s environmental laboratories, there are a number of less routine techniques that may or may not have an impact on routine environmental monitoring. One of the most potentially important of these is the use of LC-MS. The problems associated with using LC-MS for trace analysis are twofold one is the usual LC-MS problem of interfacing the second is that of sensitivity of detector. The interfacing problem may well continue to have partial (compared with GC-MS interfacing) solutions such as FAB, and thermospray, etc. However, even given the advances arising from electrospray interfaces the answer may well be to move away from LC-MS to supercritical fluids and SFC-MS. 

Chemical Analysis. Gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC) techniques were used to analyze 4-chlorophenol and its oxidation intermediates. For GC-MS analysis, the samples were acetylated in pyridine. The samples were first evaporated to dryness. Then 200 xL of pyridine and 200 (xL of acetic anhydride were added to the dry residue. The samples were heated at 65 °C for 2-3 h to ensure the complete acetylation reaction, and then gently evaporated to dryness in a nitrogen stream. Finally, the residue was redissolved in 0.1 mL of hexane for GC analysis. A GC (HP model 5890) equipped with mass selective detector (HP model 5971) and SPB-5 capillary column (Supelco Co., PA., 25- X 0.2-mm i.d. X 0.33-p.m film thickness) was used. To separate different intermediate products, various oven-temperature programs were performed. The GC-MS interface line was maintained at 300 °C. The mass-... 

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