Over the past few months I have received a number of questions regarding the identification and possible sources of contamination observed in GC-MS systems (GC/MSD, EI). Contamination can be sourced to the instrument itself or to materials used in the preparation of samples. We shall review a few of the most common types of contamination observed and how you can use the MS detector data to identify them in this article.
Air Leaks / Contamination: The GC-MS detector operates under vacuum conditions so air must be kept to a minimum detectable level.
- Common Sources: Improper fitting connections; improper fitting materials/age; damaged or worn vacuum seals.
- Identification: High background noise levels (poor S/N of stds); Peaks associated with air (i.e. m/z 18, 28, 32, 44).
Acetone Cleaning Fluid Contamination: Often used to clean the metal parts of the source, acetone may appear in the signal. Look for peaks at m/z 43 & 58.
Dimethylpolysiloxane Contamination/Bleed: Silicone from the various seals and septa may appear and depending on the chemical source, are often detected at m/z 147, 207,221, 281,295,355 & 429. *Always use high quality seals & inject blanks often to check for contamination.
Plasticizer Contamination: Many plastics are used in the seals found in the GC-MS instrument. These plastics may eventually bleed into the system and be detected (due to normal wear, heat stress or even poor sample preparation). It is critical to identify which ones result from worn out seals vs. use of plastics as part of improper sample preparation procedures (e.g. improper handling techniques, glove material, plastic sample tubes, washing glassware with soaps etc). One of the most common peaks observed will be m/z 149 (Phthalate).
Diffusion Pump Oil Fluid Contamination: Turning off the carrier gas flow may allow for some of the diffusion pump oil to back-stream. *Maintain carrier gas flow when this system is ON. If diffusion pump oil has entered the source, you should observe a strong signal peak (e.g. m/z 262, 446 for the oil).
Foreline Pump Oil Fluid Contamination: Look up the specific chemical composition of the oil used to obtain applicable m/z values to check for (e.g. hydrocarbons, m/z 69 ...).
*Avoid GC-MS contamination and trouble by first receiving the proper training to operate, use and maintain the GC-MS system. This includes making sure the entire system is fully maintained, seals changed, performance monitored regularly, insuring all of the vacuum pump oils are changed on a regular schedule and always use high quality replacement parts. Maintenance is something that most users can perform themselves, especially if they have completed a formal hands-on training class using their own GC-MS system (Most manufacturers offer basic on-site Maintenance classes).