7.1.3 Trouble Shooting
A variety of problems will be experienced when using a GC-MS for prolonged time. A few of the most obvious are discussed below. Instrument manuals normally come with a trouble-shooting guide.
7.1.3.a. Leak Detection. Atmospheric leaks will occur from time to time. The most likely sources of these leaks are the column fittings and the door to the MS vacuum chamber. Leaks may be present if mass numbers corresponding to N2, O2, H2O, CO2, and Ar appear in the spectra. System leaks are easily checked by setting the instrument to constant monitoring mode and then spraying canned Freon at each fitting and watching for a detector response. A readily available leak detection agent is DustOff that contains difluoroethane (CAS #75-37-6).
7.1.3.b. Contamination of the GC-MS system. Unfortunately, all systems become contaminated with time. The key to minimizing time locating the source of contamination is to systematically isolate each system and therefore the source of contamination. A good practice is the following.
-First, look for the obvious. What was the last thing changed prior to the presence of contamination? Was a septum, liner, column, gas filter, or gas tank recently changed?
-Check each potential source for problems, especially the filters and liners. There have also been cases of contaminated 5-nine gas being delivered from suppliers and contaminated injector liners direct from the factory.
- Check the solvent for contamination by eliminating sample introduction and only running solvent"
-An easy way to isolate the injector and check for contamination, without taking it apart, is to cool the injector and conduct a temperature run without sample injection. If the contamination is not present when the injector is cooled, a contaminated injector is likely.
-Identify your contaminate with the spectra library. If your contamination is the analyte, then the contamination is likely to be on the “front-end” of the GC-MS system (syringe or injector liner). Hydrocarbon contamination from oils is possible and will be indicative when ions are present at 43, 57, 71, and 87 mass units. Siloxanes are indicative at mass units of 73 and 207 mass units. Phenyl degradation from column degradation will be present at 281 mass units. Phthalates are ubiquitous in the environment and will give an ion peak at 149 mass units.
7.1.3.c. Plugged Needle. As noted in section 7.1.2, needles frequently become plugged with pieces of septum. This is indicated when a sample is thought to be injected but no ions or peaks appear, including the solvent.
7.1.3.d. Broken Columns. Another explanation for a lack of detector response is a broken column. This is easily observed by cooling the oven and inspecting the column. Never allow the column to rub against a surface as it will wear off the protective coating of the column and promote a break in the column.
7.1.3.e. Low Sensitivity/High Gain on the EM. This is indicative of a worn out detector.
| Frank's Homepage |
©Dunnivant & Ginsbach, 2008