Mass Spectrometry & Spectroscopy
Recent Advances in HRAM Mass Spectrometry Christian Klein, Agilent Technologies, Inc, Santa Clara, CA-95051, USA,
Christian_Klein@agilent.com
In the last 20 years, mass spectrometry has evolved from purely academic instrumentation to a technique now present in most analytical laboratories. Together with ongoing improvements in seamless software workfl ows and capabilities, increases in sensitivity and resolution are key drivers for this development. Herein, we describe enhancements made to the following aspects of a typical high-resolution, accurate mass (HRAM) spectrometer: ion source, ion transmission, instrument tuning (for sensitivity improvements), detector adjustments, ion optics, electronics, and detector acquisition speed (for increased resolution). In addition, we also consider whether an increase in selectivity can be obtained by ion mobility and quadrupole-based techniques.
Introduction
Thompson’s fi rst description of a mass spectrometer in 1913 [1] was eventually followed by the introduction of time-of-fl ight (TOF) instrumentation in the mid 60s (Bendix TOFs); however, 20 more years passed before a TOF instrument would become commercially available with broader acceptance. Magnetic sector instruments provided suffi cient resolution for accurate mass measurements but were lacking sensitivity [2] in scanning mode, as the resolution is in reverse proportion to the sensitivity. Fourier- transform ion-cyclotron resonance (FT-ICR) instruments with high resolution as well as sensitivity were an alternative to the magnetic sector instruments, but the new TOF instruments were also an appropriate answer, showing a resolving power (m/ Δm) approaching that of the magnetic sector instruments. However, with further improvements, including the refl ector (or refl ectron) and the now commonly adopted orthogonal acceleration (OA) design, TOF instrumentation has effectively replaced the once dominant magnetic sector instrumentation since the millennium.
Figure 1 shows a typical setup of a modern LC/Q-TOF. The first component of the mass spectrometer is the ion source, where analytes are ionised. The most typical ion source used is electrospray ionisation (ESI), but other techniques, such as atmospheric pressure chemical ionisation (APCI) or atmospheric pressure photo ionisation (APPI), are available. After the ionisation process, ions are guided through a capillary into the vacuum chamber. The next elements encountered in the ion optics are the quadrupole, the collision cell, the pulser, a reflector (reflectron) in the flight tube, and the detector. The separation of ions in the flight tube based on different arrival times at the detector is the fundamental principle of TOF instruments. The signal from the detector is then processed to show a final m/z spectrum. These elements are common to all TOF instruments but vary in the details and modifications offered by different vendors [3].
Other HRAM spectrometry instruments include FT-ICR and the electrostatic ion trap, with the latter taking a large portion of the FT-ICR market since its introduction.
Experimental Sensitivity
Perhaps surprisingly, the fi rst option for increasing sensitivity is made before the analyte enters the mass spectrometer. Users may select between standard fl ow, low/ micro-fl ow, or nanofl ow. The common understanding is that the lower the fl ow rate, the higher the sensitivity. The determining factor here is the size of the droplets: the smaller the droplet, the fewer Coulomb fi ssions are needed to reach the fi nal ionisation state. Here, different ionisation theories come into play - either the ion evaporation model (IEM) for small molecules, or the charge residue model (CRM), which is important for large molecules and proteins. An informed decision by the user has to be made. In particular, nanofl ow is a challenge for routine analysis, as leak-free control of the fl ow path is notoriously diffi cult to achieve and hard to troubleshoot [4].
Microfl ow, therefore, could be the answer in an MS-centric world, but historically
Figure 1: Schematics of a TOF instrument. Basic elements include the source, the quadrupole, collision cell, fl ight tune and detector
method development was performed on liquid chromatography (LC)-based systems, as UV or fl uorescence detection, which did not focus on the requirements of mass spectrometry. The same applies for mobile phase additives; for example, trifl uoroacetic acid (TFA), utilised for its ion-pairing capabilities, is suitable for chromatography but not mass spectrometry, due to a reduction in sensitivity by ion-supression [5]. Consequently, an ESI source is ideal, exhibiting the performance of a micro/nano-fl ow source, but at standard fl ow rates.
In 2007, the concept of the Agilent Jet Stream technology (AJS) was introduced, as an extension of the classic ESI source, and soon after was widely adopted by other vendors. The critical aspects of ESI are the applied voltage and temperature. Most ESI sources have an additional gas flow in the nebuliser, assisting in the desolvation of the liquid, as well as a drying gas, typically coming in a counterflow towards the nebuliser. The innovative aspect of Jet Stream technology was to introduce a third gas stream: the sheath gas. This extremely hot gas surrounds the outcoming liquid from the nebuliser, and leads to a thermal focusing, optimising the ionisation efficiency. This results in a source so efficient that it becomes independent of the liquid flow [6] (Figure 2), and sensitivity levels matching those of micro-flow rates can easily be achieved.
INTERNATIONAL LABMATE - FEBRUARY 2020
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