Unit mass resolution

The following conclusions may be drawn from these facts concerning the assessment of quadrupole and ion trap instruments  

1) The formula R = ml Am does not give any meaningful figures for quadrupole and ion trap instruments and therefore cannot be used without specifying the mass used for calculating R. 

3) The visible optical resolution and peak width is the same in the upper and lower mass ranges. It can easily be seen that the signals corresponding to whole numbers are well separated. This corresponds in practice to the typically used resolution for quadrupole and ion trap instruments. 

4) The mass resolution, which is constant over the whole mass range, is set up by the manufacturer in the electronics of the instrument and is the same for all types and manufacturers. The peak width is chosen in such a way that the distance between two neighbouring nominal mass signals corresponds to one mass unit (1 u = 1000 mu, the unit Da is used alternatively). High mass resolution, as in a magnetic sector or Orbitrap instrument, is not possible for quadrupole and ion trap instruments within the framework of the scan technique used. 

5) The mass range of quadrupole and ion trap instruments varies but still has no effect on the mass resolution. 

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Since the microchannel plate collector records the arrival times of all ions, the resolution depends on the resolution of the TOP instrument and on the response time of the microchannel plate. A microchannel plate with a pore size of 10 pm or less has a very fast response time of less than 2 nsec. The TOP instrument with microchannel plate detector is capable of unit mass resolution beyond m/z 3000. 

A newer hybrid system available commercially is the magnetic sector—TOF hybrid (38). The precursor ions can be selected with better than unit—mass resolution by msl and the product ion ions detected at high sensitivity by the TOF ms2 (39). 

Our SIMS data is taken using a VGQ8 quadrupole with unit mass resolution between 0-300 amu and a differentially pumped argon ion gun used in a defocused mode. A current of l-2x 10 A in a 0.S cm spot area is used. 

Quadrupoles are low-resolution MS instruments frequently used for molecular weight determination. QMS provides unit-mass resolution, sufficient dynamic range, good quantitation capabilities, and easy sample introduction without severe vacuum restrictions. The limited mass range (up to 4000 Da) generally does not pose problems in polymer/additive analysis. Some limitations of QMS in polymer research are ... 

Orbital trapping mass spectrometers achieve resolutions of up to 105 and would be the next choice after ToF mass spectrometers if resolving powers above 104 are required. In addition to mass resolution, the selectivity of an MS can be critical to distinguish between co-eluting and not mass-resolved compounds. For example, typical triple-quad mass spectrometers usually cannot achieve better than unit-mass resolution. However, special operation modes like neutral loss scans and precursor ion scans can filter out compounds of interest even if neither LC separations nor MS scans would be sufficient to resolve these compounds (note that this is a filtering step). 

One of the more powerful techniques is a new software tool called mass defect filtering.176 185-188 A mass defect can be defined as the difference between the exact mass and nominal mass of a compound.189 Typically, drug-like molecules (and their metabolites) will have mass defects that differ from those of endogenous matrix materials. While a mass spectrometer that has unit mass resolution cannot differentiate a test compound from an isobaric matrix compound, a high mass resolution MS may be able to differentiate many isobaric matrix compounds from test compounds. 

Ion trapping devices are sensitive to overload because of the detrimental effects of coulombic repulsion on ion trajectories. The maximum number of ions that can be stored in a QTT is about 10 -10, but it reduces to about 10 -10 if unit mass resolution in an RF scan is desired. Axial modulation, a sub-type of resonant ejection, allows to increase the number of ions stored in the QIT by one order of magnitude while maintaining unit mass resolution. [160,161] During the RF scan, the modulation voltage with a fixed amplitude and frequency is applied between the end caps. Its frequency is chosen slightly below V2 of the fundamental RF frequency, because for Pz < 1, e.g., = 0.98, we have z = (0 + 0.98/2) = 0.49 x... 

Mass resolution Unit mass resolution Unit mass resolution > 10,000 1000-10,000 > 10,000... 

With soft ionization techniques such as MALDI, ions of m/z 200000 can be routinely detected. The mass range is mainly limited by the fact that with the detector the response decreases with increasing m/z of the ions. The mass resolution of a TOF mass analyzer is relatively poor (unit mass resolution and less) and is affected by factors that create a distribution in the flight time of ions with the same m/z. The simplest way to increase the mass resolution is to increase the length of flight tube or to reduce the kinetic energy spread of the ions leaving the source. 

In tandem MS mode, because the product ions are recorded with the same TOF mass analyzers as in full scan mode, the same high resolution and mass accuracy is obtained. Isolation of the precursor ion can be performed either at unit mass resolution or at 2-3 m/z units for multiply charged ions. Accurate mass measurements of the elemental composition of product ions greatly facilitate spectra interpretation and the main applications are peptide analysis and metabolite identification using electrospray iomzation [68]. In TOF mass analyzers accurate mass determination can be affected by various parameters such as (i) ion intensities, (ii) room temperature or (iii) detector dead time. Interestingly, the mass spectrum can be recalibrated post-acquisition using the mass of a known ion (lock mass). The lock mass can be a cluster ion in full scan mode or the residual precursor ion in the product ion mode. For LC-MS analysis a dual spray (LockSpray) source has been described, which allows the continuous introduction of a reference analyte into the mass spectrometer for improved accurate mass measurements [69]. The versatile precursor ion scan, another specific feature of the triple quadrupole, is maintained in the QqTOF instrument. However, in pre-... 

Several highly characteristic peaks are observed in Fig. 2. The compositions of till major peaks are given in Table 2. These are based on their exact mass determination, which is accurate to within 0.01 amu. This is possible in a TOFSIMS instrument because of its higher mass resolution as compared with quadrupole instruments. Therefore, peak identities can be determined with higher certainty than in quadrupole instruments which have only unit mass resolution. 

Gu, M., Wang, Y., Zhao, X. G., and Gu, Z. M. (2006). Accurate mass filtering of ion chromatograms for metabolite identification using a unit mass resolution liquid chromatography/ mass spectrometry system. Rapid Commun. Mass Spectrom. 20 764-770. 

Increasing the resolution also allows the separation and differentiation of endogenous compounds in biological matrices from a drug of interest or its metabolites. A comparison of mass spectra of sulfacetamide obtained using a quadmpole mass spectrometer (unit mass resolution) with that obtained using a time-of-flight mass spectrometer (R = 10,000) is shown in Fig. 4.9. While at unit mass resolution the... 

IS. ISS provides an atomic identification only (see Figure 3(b)) since the binary collision carries no direct chemical information though chemical effects can influence ion yields through changes in neutralization rates. In principle, all elements can be detected from a measurement of Ej/Eq, but in practice, elemental resolution is a limiting factor (6,12). He+ primary beams are most commonly used and since the best resolution is obtained for (see Eq. (1)), unit mass resolution can... 

Laser desorption Fourier transform mass spectrometry (LD-FTMS) results from a series of peptides and polymers are presented. Successful production of molecular ions of peptides with masses up to 2000 amu is demonstrated. The amount of structurally useful fragmentation diminishes rapidly with increasing mass. Preliminary results of laser photodissociation experiments in an attempt to increase the available structural information are also presented. The synthetic biopolymer poly(phenylalanine) is used as a model for higher molecular weight peptides and produces ions approaching m/z 4000. Current instrument resolution limits are demonstrated utilizing a polyethylene-glycol) polymer, with unit mass resolution obtainable to almost 4000 amu. 

They are still the workhorses of coupled mass spectrometric applications, as they are relatively simple to run and service, relatively inexpensive (for a mass spectrometer), and provide unit mass resolution and scanning speeds up to approximately 10,000 amu/s. This even allows for simultaneous scan/ selected ion monitoring (SIM) operation, in which one part of the data acquisition time is used to scan an entire spectrum, whereas the other part is used to record the intensities of selected ions, thus providing both qualitative information and sensitive quantitation. They are thus suitable for many GC-MS and liquid chromatography-mass spectrometry (LC-MS) applications. In contrast to GC-MS with electron impact (El) ionization, however, LC-MS provides only limited structural information as a consequence of the soft ionization techniques commonly used with LC-MS instruments [electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI)]. Because of this limitation, other types of mass spectrometers are increasingly gaining in importance for LC-MS. 

The skimmed and collimated cluster beam passes into the quadrupole mass spectrometer (Extrel, C-50), which has a mass range of 0-1200 amu with unit mass resolution. The mass spectrometer chamber is pumped by a turbomolecular pump (360Is-1). The pressure in the mass spectrometer chamber (P3), when the beam is in operation, is always less than 1 x 10 6 torr. This is necessary to ensure that the contributions from reactions of the cluster ions with the background gas are not significant. The distance of the nozzle from the ion source varies in the range 20.5-22.5 cm, depending on the nozzle to skimmer distance. 

A mass range of 450 amu with unit mass resolution was obtained. Limits of detection for trace analysis in air of methyl salicylate (1.24 ppb) and for nitrobenzene (629 ppt) were achieved. Isolation and collision-induced dissociation efficiencies in MS/MS experiments were greater than 50%. 

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