Mass spectrometry is an analytical process for obtaining the molecular weight, chemical composition and structural information of a compound or sample based on the mass-to-charge ratio of charged particles. In general, in mass spectrometry, a sample undergoes ionization to form charged particles as ions; these charged particles are then passed through electric and/or magnetic fields to separate them according to their mass-to-charge ratio. The separated ions are then measured at a detector.
Mass spectrometers generally need to be operated at high vacuum (e.g., 10−4 to 10−6 Torr) to limit the interactions between ions and gas molecules within the mass spectrometer which would otherwise degrade performance. One challenge in mass spectrometry is providing an efficient method of getting representative ions from the sample into such a mass spectrometer vacuum system. In some mass spectrometry systems, the ionization process occurs within the vacuum envelope, but this limits the types of samples that can be analyzed to gas phase samples and solid samples that exhibit low vapor pressure.
Atmospheric Pressure Ionization (API) ion sources have become increasingly important as they have greatly increased the types of samples that can be measured by mass spectrometers. These sources form the ions at, or about, atmospheric pressure, outside the mass spectrometer and the ions and charged particles are transferred to the high vacuum region of the mass spectrometer through the Atmospheric Pressure Ionization (API) interface that generally includes a small ion inlet orifice or capillary and a transfer region that may contain a number of electric fields and intermediate vacuum stages to manipulate the charged particles and successively reduce the pressure.
This has allowed mass spectrometers to be interfaced to a large number of ionization techniques increasing the types of samples that can be measured, whether in gas, solid, or even liquid form. Exemplary ion sources include, but are not limited to, Electrospray Ionization (ESI), Atmospheric Chemical Ionization (APCI), Atmospheric Pressure Photo Ionization (APPI), Matrix Assisted Laser Ionization, (MALDI), Direct Analysis in Real Time (DART) and Desorption Electrospray Ionization (DESI). These ion sources have allowed mass spectrometers to be coupled to widely used tools such as High Performance Liquid Chromatography.
Ion sources such as ESI and APCI provide charged particles from liquid solutions of sample and solvent. The solution, including the molecules of interest, is pumped through an orifice or a capillary and an electric potential is either placed on the capillary (ESI) or a needle close to the mass analyzer. Coaxial nebulization gas may assist the formation of a plume of highly charged droplets from the capillary at atmospheric pressure. Since the ionization occurs directly from solution at atmospheric pressure, the ions formed in this process can sometimes be strongly solvated. Prior to measurement, the solvent molecules associated with the ions are removed. So the API interface performs many functions; it desolvates the charged droplets to form gas phase ions, it transfers these ions into the mass spectrometer analyzer maintained at high vacuum and removes the great majority of the air, gas and solvent molecules that enter the API interface with the ions.
The efficiency with which the API interface performs these functions determines the overall sensitivity of the system and other performance factors. In many API interfaces the pressure is reduced from atmosphere to high vacuum in one or more intermediate vacuum stages. With conventional API interfaces the number of ions that are sampled, and hence the sensitivity, are limited by the size of the apertures between the various stages. The larger the apertures the greater the sensitivity, but the larger and more expensive the vacuum pumps required to maintain the intermediate stages at the required pressure.
Increasing the gas flow into the mass spectrometer also increases the problems of contamination as more of the solvent and surrounding environment is admitted to the API interface. Many conventional mass spectrometers have direct line-of-sight through the system so that contamination that enters the API interface can end up in the analyzer and detector regions, degrading their performance and which is difficult and time consuming.