7.1.2 Maintenance
7.1.2.a. Gas Filters: As noted in the previous section, ultra high purity gases are purified even further with resin filters (traps). These filters must be replaced periodically, usually after 5 to 10 tanks of gas depending on the size of the filter.
7.1.2.b. Septa. The interface where samples are introduced into the instrument is a silicone gum septum with a Teflon backing on the injector side of the septum. This allows the sharp needle of the syringe to be easily inserted into the injector chamber and the sample to be introduced. As more and more injections are made, the septa develops a slight perforation in it that will eventually leak carrier gas and allow the loss of sample during an injection. Therefore the septum must be replaced periodically, typically daily or just prior to a new run of standards and samples. Septa are relatively inexpensive so this is not a major cost issue.
7.1.2.c. Injection Syringes and Needles. Syringes can wear with time depending on the type of samples injected. Dirty samples will quickly clog the syringe by leaving residue in the barrel that interferes with the movement of the plunger. This can usually be avoided by numerous rinses between samples. However, it is sometimes necessary to disassemble the syringe and rinse with acid, polar organic solvent, and a nonpolar organic solvent. Injection needles can also plug with a piece of the septum. Most syringes come with a thin wire to remove this plug but this technique is rarely successful and the syringe is usually replaced. Syringes for manual injection are as inexpensive as $15, but autosampler syringes can easily cost $100.
7.1.2.d. Column Fitting. Columns are attached to the injector and detector ports with threaded nuts and ferrules, a soft hollow conical-shaped device that fits snugly around the column and fits into a receptor secured with a nut. As the nut is tightened, the ferrule is compressed around the column, preventing gas leaks. As the temperature is repeatedly raised and lowered, leaks can result from the cycling expansion and contraction, so the ferrule nuts need to be tightened periodically (weekly to monthly). Ferrules can be made of Teflon, ceramic, graphite, and composites of ceramic and graphite.
7.1.2.e. Glass Wool Plugs in the Injector Liner. Most injector liners have glass wool inserted into them to aid in the uniform mixing of the volatized solvent and analytes with the carrier gas. Over time (weeks to months) these liners accumulate pieces of the septum (referred to as septum worms) and nonvolatile components of the injected sample. Therefore the liners are routinely replaced when discoloration or evidence of cross contamination occurs. The frequency of replacement is directly related to the presence of nonvolatile components in the samples, and can range from weekly to yearly replacement cycles.
7.1.2.f. MS Tuning. The mass spectrometer, specifically the mass analyzer, must be calibrated with respect to mass, typically on a weekly basis. Some applications require daily tuning. Modern MS systems have an automated tuning sequence. For electron ionization systems, perfluorotribuylamine (PFTBA) is used. A small mass of volatilized PFTBA is introduced into the ionization chamber and the system automatically adjusts to correspond to its mass. Parameters such as repeller and accelerator voltages and gain on the EM are adjusted to achieve a given detector response. After this, the system can be tuned for any mass unit.
7.1.2.g. Ion Lens. The repulsion and acceleration lenses may accumulate nonvolatile residues when dirty samples are analyzed. Depending on the quality of a sample and the frequency of use, lenses will need to be taken out and rinsed with solvents, dried, and reassembled. The typical sign of a dirty lens is the need to apply higher than normal voltages to these lenses during the MS tuning procedure.
7.1.2.f. Gain on the Electron Multiplier. For a given mass of tuning compound (PFTBA), a specific counts per second of ions hitting the EM is expected. This is adjusted by increasing or decreasing the gain (potential) across the EM. As the EM ages, it may require an excess gain to be applied and the EM will need to be replaced.
7.1.2.h. Rotary Pump Oil. The rotary pump is lubricated with special grade oil. The gauge level should be checked monthly, and oil added if necessary. The oil should be replaced at least annually, sooner in high use situations and when dirty samples are analyzed. Many or most of the unionized analytes, contaminates, and solvents eventually reside in the rotary pump oil. Rotary pumps usually require semi-annual replacement due to oil leaks.
7.1.2.i. Analyte calibration. While not a normal part of maintenance, instruments are normally calibrated at least daily with analytes of interest.
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