Continuous ion source

In ToF-MS, the ion source is pulsed to create packets of ions. In the conventional procedure, the system waits for all the ions in a packet to reach the detector before injecting the next packet of ions. Complications arise when ToF-MS is coupled to a continuous ion source. Such coupling is therefore often accomplished by the orthogonal extraction approach, in which a segment of the ion stream is accelerated orthogonally by a push-out pulse. However, in this process, up to 95 % of the information contained in the ion steam is lost. Recently, Hadamard transform time-of-flight mass spectrometry (HT-ToFMS) was developed to couple continuous ion... 

TI is a very precise and accurate method in stable isotope ratio measurements and quantification of inorganic elements, for example, by isotope dilution mass spectrometry [8]. Because TI is a continuous ion source, it could be coupled to any analyzer that is suitable for such sources. However, because the strength of TI lies in the quantitative precision and accuracy, sector analyzers are preferred to ensure maximum quality. 

Continuous ion sources, such as ESI, can be connected to the TOF analyzers through orthogonal acceleration (oa-TOF) [205-209]. In oa-TOF, the ions generated by the source enter the TOF analyzer perpendicular to its main axis (Fig. 2.11). The acceleration potential is initially set to zero and the start pulse is generated instantaneously as the potential is raised and the ions are accelerated into the field-free flight tube. 

Pulsed ion sources for axial injection such as MALDI and SIMS, and earlier plasma desorption are suitable for TOF analyzers. For continuous ion sources such as ESI the oa configuration is suitable. 

The compatibility is excellent with continuous ion sources such as ESI, dynamic SIMS, CF-FAB, ICP, El, Cl, etc. Sector instalments are not well-suited for pulsed ionization methods, although there are examples where MALDI sources have been utilized [225-229]. Sector instruments are usually larger and more expensive than other mass analyzers, such as TOFs, quadrupole filters, and traps. 

Like sector analyzers, quadrupole analyzers are well suited for continuous ion sources such as ESI, but are not well-suited for pulsed ionization methods. Quadmpole mass spectrometers are generally substantially cheaper and smaller than sector instruments and Qq-TOFs. They are very often used in combination with GC and LC, and single or triple quadmpole mass filters are very common benchtop instruments for routine measurements. 

Quadmpole ion traps were originally coupled to continuous ion sources but also work well with pulsed ion sources. The cylindrical QIT is a compact device with a diameter and length of 5 cm and the linear QIT is the size of a quadmpole mass filter, that is, 20 cm long. As stand-alone instruments they are nowadays of benchtop size and, together with quadmpole mass filters, are considered to be standard low-cost devices, which are commonly coupled to LC systems. 

In principle, it would be possible to perform multistage mass spectrometry like in an ICR analyzer although with no gas CID would of course not be possible, but other dissociation methods could be employed. There might, however, be technical issues. At the time of writing, fragmentation is performed in the linear QIT preceeding the orbitrap in Thermo Fischer Scientific s instrument. Both pulsed and continuous ion sources can be employed. There are several ion sources that can be employed with Thermo Fischer Scientific s orbitrap. 

Coles, J. Guilhaus, M. Orthogonal Acceleration - a New Direction for TOF-MS Fast, Sensitive Mass Analysis for Continuous Ion Sources. Trends Anal. Chem. 1993,12, 203-213. 

Use of Continuous Ion Sources with Time-of-Flight Mass Spectrometry... 

Although some early research did focus on introducing a pulsed external ion beam from a plasma jet into the acceleration region of the TOF-MS [12], it has not been until recently that continuous ion sources such as plasmas, electrospray devices, and other atmospheric-pressure discharges have been coupled with TOF-MS. Unfortunately, the inherently pulsed nature of the spectrometer can cause this marriage to be quite inefficient. The primary challenge in using the TOF-MS with... 

A plasma source was coupled to a TOF-MS as early as the 1960s, when workers at Bendix [12] used such an arrangement to analyze the chemical species in a plasma jet. The instrument utilized a pulsed supersonic inlet probe similar to that found in current inductively coupled plasma mass spectrometry (ICP-MS) quadru-pole instruments and employed a TOF-MS that was oriented at a 90° angle to the input ion beam. More importantly, however, it used a pulsed extraction field to extract ions from the plasma source and accelerate them into the flight tube. It is this concept of injecting discrete ion bunches into the TOF-MS analyzer that has been almost ubiquitously employed by workers using continuous ion sources [17,18]. 

Ion transport from a continuous ion source to the extraction/acceleration region of the TOF-MS presents many of the challenges also associated with other mass spectrometers. Specifically, a means of efficiently focusing incoming ions into a beam of well-defined energy and spatial characteristics is required in order to achieve optimal performance for any mass analyzer. 

A TOF mass analyser requires a pulsed ion introduction. In an electrospray-TOF combination, the duty cycle is an important issue. A significant improvement in the duty cycle can be achieved in an ion-trap-TOF hybrid instmment the ions from a continuous ion source are accumulated in the ion trap between two ion introduction events. An ion-trap-TOF hybrid instrument was first described by the group of Lubman [68-69]. The system consists of an atmospheric-pressure ion source with a vacuum interface, a set of Einzel lenses, an ion-trap device, and a reflectron time-of-flight mass analyser. The system was applied for fast analysis in combination with a variety of separation techniques [70]. 

Unlike many mass spectrometers, which allow the use of several different ionization methods on the same instrument, MALDI-TOF instruments are generally single purpose, being devoted exclusively to MALDI. In practice, El and ESI methods are also used with TOP mass spectrometers. However, the continuous nature of these sources causes a natural mismatch to TOF-MS, which is a pulsed technique. The traditional way to couple a continuous ion source to a TOF-MS is to dynamically deflect the ion beam past a narrow slit, thus generating a pulsed ion beam at the exit side of the siit. However, this results in low sensitivity as more than 99% of the ion beam is discarded. 

Most continuous ion sources are suited to oa-TOF-MS it can also be used effectively as the final mass analyzer in hybrid instruments which combine two mass spectrometers in an approach called tandem mass spectrometry or MS/MS. 

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