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Note 78: A New Solution to Eliminate MS Down-Time With No-Tool-Changing of Analytical GC Columns

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By Hans T. Cornet and Hans T. Cornet, SGT Middelburg B.V., Arnesteinweg 23A tel. : +31 118 623095, P.O. Box 8027, NL-4330 EA Middelburg, Netherlands www.sgt-nl.com fax : +31 118 639536, E-mail: sgtnl@wxs.nl

Presented at PittCon 99, Orlando, FL, March 1999.

INTRODUCTION

The Mass Spectrometric Detector (MS Detector) is a unique GC detector because it operates in a vacuum. Unlike for conventional GC detectors and differentially pumped mass spectrometers, changing the GC column on singly pumped benchtop MS usually requires GC/MS shut down procedure that allows the MS to cool down and vent. After column installation, pump down procedure is applied and it takes more than four hours (usually overnight) until MS is in a stable vacuum and ready for analysis.

SGT's new and patented MS-Interface Kit makes the GC column exchange user-friendly and totally eliminates switching-off benchtop MS.

Description

In order to achieve a connection between the analytical capillary column and the MS detector, avoiding the necessity of switching-off the MS detector when columns and/or injector liners need to be changed, the SGT MS-Interface Kit (M.I.K.), as shown in Picture 1 and 2, is designed to interface the column and theMS detector.

The capillary column is no longer physically connected to the MS detector but to the M.I.K. The M.I.K. is connected to the MS detector using uncoated fused-silica tubing (column extension) of a specific inner diameter. The connection between the capillary column and the column extension is made using a glass press-fit connector.

But!

Press-Fit Connectors have a bad namend a questionable reputation. The mean two reasons of leaking connections and fused-silica sticking in regular press-fit connections are badly cutted columns (irregular column surfaces e.d.) and the fact that too much or too little forced is used to make a proper, leaktight connection in a press-fit connector.

How are these two problems solved in the MS-Interface Kit?

Problem 1 : Bad Column Cuts

The problem of the bad and irregular column cuts is solved by using the SGT Shortix™ and Shortix™ Plus Capillary Column Cutters for making proper and clean cuts.

Shortix™ and Shortix™ Plus

Problem 2 : Amount of Force for Proper Connection

The problem of the amount of force used to make a proper column/press-fit connection is solved by using spring pressure at the top of the press-fit connector. When a column is pushed into the press-fit in the M.I.K., the special spring at the top of the press-fit regulates the amount of force needed to make the connection. It is now practically impossible to use too much, or too little force. A leaktight connection now can be made by every user of the M.I.K Therefore the SGT MS-Interface has been made 'SELF-COMPENSATING'

Working Principle of MIK Column Installed

Via a SS-Column Guide the analytical capillary column is connected to the SC-Glass Press-Fit inside the M.I.K. From the Press-Fit a piece of uncoated fused-silica is connected into the MS-Detector. The force needed to make a proper, leaktight connection of the column into the Press-Fit is regulated by the 'Self-Compensating Spring' at the top of the Press-Fit. This spring precisely regulates the force with which the column is connected into the Press-Fit. A repeatable, clean, leaktight and No-Tool-Needed connection is the result

Working Principle of MIK Column NOT Installed

By pushing the Piston, the column and SS-Column Guide can be removed from the MS-Interface Connector, leaving a clean and re-usable Press-Fit. By switching a valve, clean carriergas enters the MIK through the Shield-Gas side inlet while the Piston closes the MS-Interface, protecting it from air. The MS-Detector stays On-Line because of the incoming Shield-Gas. When a new column is installed, the Shield-Gas is taken off the system and a new analysis can be performed SECONDS AFTER INSTALLATION OF THE COLUMN!

Experimental

All analysis were performed by using an HP 5973 Mass Selective Detector coupled to an HP 6890 Series Gas Chromatograph. The GC was equipped with split/splitless injector with Electronic Pressure Control (EPC). A major advantage of EPC in conjunction with the MSD is the constant flow mode. The column head pressure is adjusted to track the temperature of the GC oven so that the volume of carrier gas reaching the ion source is constant. This constant source pressure then results in improved reproducibility of the mass spectra produced by the MSD. This and other features of EPC are described in detail in several publications1 . Injection was performed in splitless mode using a HP 7673 automatic sampler. The instrument configuration and analytical conditions are summarized in Table 1.

Table 1. Experimental Conditions For Analysis of Polynuclear Aromatic Hydrocarbons (PAH)

  Gas Chromatograph
Column: 30m x 0.25mm x 0.25µm HP-5MS
Inlet: Split/splitless
Injection: 1µl; HP 7673 Automatic Liquid Sampler (ALS)
Inlet temp: 325°C
Inlet press: 9.44 psi at 60°C; 39 cm/sec with He carrier, 1.1 ml/min, Constant Flow Mode
MSD Transfer Line: 320°C
Temperature Program: 60°C (2 min), 25°C/min to 160°C (0 min) ,3°C/min to 300°C (0 min), 25°C/min to 325°C (1 min)
MS Quad: 150°C
MS Source: 230°C
Scan: 10 - 280
EM Voltage: 1506
Test mixture: 16 EPA Polyaromatic hydrocarbons 610-M diluted in cyclohexan for final concentrations in the range from 20 - 1ng/µl

Regular GC/MSD Leak-Check

There are two ways to check for leaks in an GC column when it is hooked up to a MSD. For gross helium leaks, a helium detector can be used on its high sensitivity setting2. Air leaks into the connection will not register on the helium leak detector, but will be seen by the MS. Once there are no detectable helium leaks, the MSD can be used to check for small leaks. By going into the diagnostics/vacuum control, the edit MSD parameter's screen can be displayed. By clicking on more parameters, acquisition masses can be changed to 69, 18 and 28. With the PFTBA valve open, a spectrum can be obtained in the scan range from 10 -80 a.m.u. If ions at m/z 28 and 32 for N2 and O2 are present with no corresponding m/z =18 for H2O then there is a leak in a system. Figure 2 shows a PFTBA spectrum from a leak-free system (example with direct GC column connection).

Figure 1. Analysis of 16 PAH's with new GC column connected to the mass spectrometer through an interface that is sealed with column nut and ferrule (direct connection) is shown.

Figure 2. A spectrum of PFTBA from a leak free system. Notice that m/z=28 is 1.49 % of m/z = 69 and the water (m/z = 18) is only 0.33%

Comparison of PAH Analysis With and Without MS-Interface

Figure 3 shows that peak shape and responses are identical with and without MS-Interface Kit and due to the additional 1m transfer line retention times are shifted for 20-30 sec. Recently introduced Retention Time Locking (RTL) can be useful in reproducing the original retention time of a compound after column maintenance. 3,4

Figure 3. Analysis of PAH's With and Without MS-Interface Kit (M.I.K.)

Influence of Column Change On Air Diffusion in MS

Following example demonstrate how easy and quick column can be changed when M.I.K. is already installed. There is no need to vent the MSD when column has to be replaced (maintained). Figure 4 shows the leak at the moment when the column is removed from the new M.I.K.

Just 10 sec later when new column is in place, the vacuum has been already back and Air & Water check test passed (figure 5).

Figure 4. A spectrum of PFTBA From a System With a Leak. There is no GC column in place!

Figure 5. A spectrum of PFTBA from a leak free system just 10 seconds later when the column is already connected! Notice that m/z=28 is 2.13% of m/z=69 and the water (m/z) is 0.96%.

As can be seen from overlaid chromatograms (n=11) shown on figure 6 and 7, analysis of 16 EPA Polyaromatic hydrocarbons is showing even better baseline stability when using the new SGT M.I.K. compared to direct GC column connection. This example clearly demonstrates advantages of quick GC column exchange without switching-off the Mass Spectrometric Detector as the vacuum system is stable within a minutes and MSD is ready for multiple analysis.

Figure 6. Analysis of 16 EPA PAH's By GC/MSD Using Standard Direct Column Connection

Figure 7. Analysis of 16 EPA PAH's By GC/MSD Using M.I.K

Conclusions

The use of the MS-Interface Kit offers some important advantages for benchtop MS users. Using an MS-Interface Kit a GC column can be exchanged within the minutes instead of waiting for hours to cool-down, vent and pump down the MS. The MS-Interface Kit not only provides quick column exchange,it also allows the MS to remain hot in stable vacuum and be ready for routine analysis. After installation, a MS-Interface Kit does not require any ferrules, column nuts or tools for column installations . Using the MS-Interface increases baseline stability.