Open split interface

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. 

The open-split interface dilutes eluted analytes with additional makeup gas and then splits the mixture into two fractions. An inert, fused-silica restrictor routes a portion of the sample-carrier gas mixture from the interface into the MS ionization chamber, whereas the remainder is vented to atmosphere. An open-split interface can affect the shapes and areas of peaks because solutes make contact with several items including the interface liner, the outer surfaces of the fused-silica column, and the restrictor, which can adsorb trace-level analytes if they are not properly deactivated (21). 

The second issue is to interface two detectors to a single GC. Since both the MS and the matrix isolation interface are destructive detectors, the sample is split in a 1 1 effluent splitter and half the sample is routed to each detector via a specially-designed open split interface (18). 

Volatiles isolated by the purge-and-trap method were analyzed by GC-MS using a Varian 3400 gas chromatograph coupled to a Finnigan MAT 8230 high resolution mass spectrometer equipped with an open split interface. Mass spectra were obtained by electron ionization at 70 eV and an ion source temperature of 250°C. The filament emission current was 1 milliampere and spectra were recorded on a Finnigan MAT SS 300 Data system. 

Another option is an open split interface which sucks the helium out before the sample goes on to the ionisation chamber. These can concentrate the analytes which gives a sensitivity advantage. 

Figure 5.4. Various forms of GC/MS interfaces (a) direct coupling of capillary columns with MS (b) open-split interface (c) jet-separator interface (d) molecnlar-effnsion interface.

GC is an ideal separation technique for small thermally stable volatile molecules. Capillary columns provide high-resolution separation of complex mixtures. These columns, owing to their low gas flow requirements, can be coupled directly to mass spectrometry. A variety of interfaces, such as an open-split interface, a jet-separator interface, and a molecular-effusion interface, are used to handle larger carrier gas flows of packed GC columns. 

Ether extracts were analyzed via GC/MS. A Varian Model 3700 gas chromatograph was used with a 0.32 mm id x 15 m fused silica column coated with a 1 micron film of DB-5. The following oven conditions were employed 5 min at 60 C then 5 C/min to 230 C and a final hold of 10 min. The column effluent was passed through an open split interface into a Finnigan model 705 Ion Trap Mass spectrometer. Identifications were achieved by comparison of the generated spectra to those of the NBS Library Compilation or to published spectra. Relative concentrations of the products were determined using the Ion Trap quantitation program. 

Open split interface to reduce gas flow into IRMS... 

H2, N2, and CO, are then separated via a 5 A packed GC molecular sieve column (or similar). The gases then enter the IRMS for analysis via an open split interface. Only a small portion of the analyte gases (e.g., 0.3%) enters the IRMS. Hydrogen and oxygen isotope values can be measured from a single analysis [1,9,32,33]. 

H2O. The analyte gases then pass to the IRMS via an open split interface [1]. 

The hyphenated techniques CGC - MS. CGC -FTIR, and CGC-AED are generally used as stand-alone units. Due to the nondestructive character of FTIR, CGC-FTIR-MS units (Fig. 39) are possible and have been commercialized. The software then allows simultaneous recording of the infrared and mass spectra of the eluting compounds. In principle. CGC-FTIR-MS-AED is also jxrssible if an open split interface is applied for the CGC- FTIR-MS combination and the split-line is directed into the AED detector. The fundamental aspects of CGC-MS, CGC-FTIR and CGC-AED are discussed in [58], [59], and [60]. 

In addition, the open-split interface also offers an automatic peak dilution capability. Due to the transfer of the analyte gases in a helium stream more than one interface can be coupled in parallel for alternate use to an isotope ratio MS via separate needle valves. 

Figure 2.208 Open split interface to IRMS, effected by moving the transfer capillary to IRMS from the column inlet to the He sample flow region. CThermo Fisher Scientific.)...

In this application the GC is coupled to the mass spectrometer by an open split interface. A restrictor limits the carrier gas flow. Open coupling was chosen because the sensitivity of the ion trap GC-MS makes the concentration of large quantities of air superfluous. Open coupling dilutes the moisture, which may be contained in the sample, to an acceptable level so that cryofocusing can be used without additional drying. 

Transferline Open split interface Ramp 1 Final temperature Temperature 8°C/min 200°C 250 °C SGE type GMCC/90, mounted in the transfer line to the MS restrictor capillary 0.05 mm ID, adjusted to 2.5% transmission... 

Open split interface High split ratio... 

A gas chromatograph equipped with a methylsilicone WCOT column is interfaced to a fast scanning mass spectrometer which is suitable for capillary column GC/MS analyses. The sample is injected either through a capillary splitter port or a cool on column injector capable of introducing a small sample size without overloading the column. The capillary column is interfaced directly to the mass spectrometer or by way of an open split interface or other appropriate device. 

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