A basic idea of MS instrumentation is important to understand the scope of MS experiments. All MSs have three main components: an ion source (typically at atmospheric pressure), a mass analyzer, and a detector (latter two components are at high vacuum). Ions are transported from one component to another using ion guides, which constrain ions using radio-frequency (RF) fields while allowing background gas to be pumped away. Mass analyzers are broadly classified into low-and high-resolution analyzers (LR-MS and HR-MS, respectively) based on the smallest difference in m/z between two peaks that can be reliably distinguished. LR-MS, such as quadrupole mass analyzers (QMA/QMS), is ideal for quick identification of m/z and abundances of different chemical components in a mixture but offer a resolution of 1000-3000. HR-MS is ideal for distinguishing the various chemical species that make up complex mixtures such as atmospheric particles, including isomers and isotopologues. HR-MS that can separate ions with mass differences of 0.001 Da and resolving powers of 50,000-100,000 or higher are preferred in the analysis of atmospheric particles and require the use of sophisticated and expensive instrumentation. Time-of-flight (TOF), Orbitrap and Fourier transform ion cyclotron resonance (FT-ICR) are commonly applied high resolution techniques. Both commercial and custom-built MS benefit from the use of ion traps, designed to capture and store ions using electromagnetic fields. Ions can be trapped by a RF electric fields (Paul traps) or by a combination of electric and magnetic fields (Penning trap). FT-ICR MS, for example, uses a Penning trap while TOF analyzers are often coupled with Paul traps. Trapped ions are cooled by multiple collisions with buffer gases such as helium to reduce their internal energy and limit any fragmentation. Carefully controlled collisions between ions and buffer gas (Collision Induced Dissociation (CID)) can lead to fragmentation, where analysis of the fragment masses offers insight into the structure of the ions. Mass selection, where an ion of a specific m/z is isolated, prior to, and after, ion trapping enables the study of cluster/size specific effects in laboratory MS experiments. For example, isolated and trapped ions can be exposed to different reactive vapors and any resulting changes in m/z can be detected, providing a way to study the kinetics and thermodynamics of gas phase reactions. Coupling MS with other analytical techniques such as chromatography, ion mobility spectrometry (IMS) and spectroscopy offers enhanced characterization, including information on the size, chemical behaviour and structure of the analyte. A discussion on the various instruments used in field studies of atmospheric particle is outside of the scope of this review but will be briefly discussed here. The Aerosol MS (AMS), the Aerosol TOF MS (ATOFMS), and the Atmospheric Pressure Interface TOF MS (APiTOF) are often used to examine ambient particles. The AMS can measure the mass of an ensemble of particles having a particular aerodynamic diameter, while the ATOFMS measures the mass of single particles. Both instruments apply MS on particles and provide insight into the chemical constitution of the sampled particles, but are not sensitive to particles with diameters of several tens of nanometers. The APiTOF, shown in Figure , is used to probe the mass and composition of nucleation mode particles and is sensitive to ambient ions and clusters. An ionization source and a mobility analyzer are often included in the sampling region of the APiTOF the sampled particles are directed into the MS using ion guides and are ultimately detected by a TOF analyzer.