Mass accuracy limits

TOF mass spectrometers are very robust and usable with a wide variety of ion sources and inlet systems. Having only simple electrostatic and no magnetic fields, their construction, maintenance, and calibration are usually straightforward. There is no upper theoretical mass limitation all ions can be made to proceed from source to detector. In practice, there is a mass limitation in that it becomes increasingly difficult to discriminate between times of arrival at the detector as the m/z value becomes large. This effect, coupled with the spread in arrival times for any one m/z value, means that discrimination between unit masses becomes difficult at about m/z 3000. At m/z 50,000, overlap of 50 mass units is more typical i.e., mass accuracy is no better than about 50-100 mass... 

Atomic masses calculated in this manner, using data obtained with a mass spectrometer can in principle be precise to seven or eight significant figures. The accuracy of tabulated atomic masses is limited mostly by variations in natural abundances. Sulfur is an interesting case in point. It consists largely of two isotopes, fiS and fgS. The abundance of sulfur-34 varies from about 4.18% in sulfur deposits in Texas and Louisiana to 4.34% in volcanic sulfur from Italy. This leads to an uncertainty of 0.006 amu in the atomic mass of sulfur. 

Tables 6.27 and 6.31 show the main characteristics of ToF-MS. ToF-MS shows an optimum combination of resolution and sensitivity. ToF-MS instruments provide up to 40000 spectra s-1, a mass range exceeding 100000 (in principle unlimited), a resolution of 5000, and peak widths as short as 200 ms. This is better than quadruples and most ion traps can handle. Unlike the quadrupole-type instrument, the detector is detecting every introduced ion (high duty factor). This leads to a 20- to 100-times increase in sensitivity, compared to QMS used in scan mode. The mass range increases quadratically with the time range that is recorded. Only the ion source and detector impose the limits on the mass range. Mass accuracy in ToF-MS is sufficient to gain access to the elemental composition of a molecule. A single point is sufficient for the mass calibration of the instrument. ToF mass spectra are commonly calibrated using two known species, aluminium (27 Da) and coronene (300 Da). ToF is well established in combination with quite different ion sources like in SIMS, MALDI and ESI.

Lasers have advanced the analytical use of mass spectrometers to characterise additives in polymers, and routine application of MALDI is no longer limited to high molecular masses only. MALDI can now clearly produce isotopically resolved mass spectra of small molecules (<800 Da) in an L-ToF instrument, which can be used successfully for the characterisation of molecules of different chemical classes. High mass resolution with an improvement of mass accuracy to... 

The use of CIEF in combination with FTICR has been demonstrated in an analysis of the E. coli proteome (Jensen et al., 1999). For these experiments, E. coli was grown in a medium depleted of rare isotopes in order to increase the mass measurement accuracy. The high abundance isotopes are present at approximately 98.89% 12C, 99.63% 14N and 99.985% H. For peptides, the presence of rare isotopes does not significantly change the spectra but with undigested proteins, mass accuracy can be limited by the broadened distribution of ions of any given protein due to the incorporation... 

As noted with the chemotaxonomic studies, the limited resolving power and mass accuracy of MALDI-TOF complicates identification of unknown proteins. If the greatly improved resolving power and accuracy of MALDI-FTMS can be used to monitor overexpressed proteins, it could have significant advantages. Figure 13.12 is a MALDI-FTMS spectrum of E. coli whole cells that have been genetically altered to produce the soluble core domain mammalian cytochrome b5 protein, which consists of 98 amino acids. 

Mass analyzer Resolving power (xlO3) Mass accuracy (ppm) m z range (upper limit) (xlO3) Acquisition speed (Hz) Linear dynamic range Price... 

We first consider the continuous flow stirred tank reactor (CFSTR). A schematic presentation of the continuous flow stirred tank reactor is given in Figure 1. It is assumed that no mass transfer limitations exist regarding the supply of ozone. The accuracy of this assumption depends on the way ozone is supplied to the system and the reaction rate constants of the components involved. 

The sensitivity, compactness, automation, and low prices of ion trap instruments made them very popular in biological MS P Limitations of ion traps include low resolution and mass accuracy at high m/z. In addition, in MS/MS mode, the lower end of the fragment mass range cannot be visualized. Recent developments in the linear geometry of ion traps are aimed at improving on those limitations. 

Note Modem FT-ICR mass spectrometers offer ultrahigh resolving power (R= 10 -10 ) [193,194] and highest mass accuracy (Am = 10" -10 u, cf. examples in Chaps. 3.3.2 and 3.4.1), attomol detection limits (with nanoESI or MALDI sources), high mass range and MS capabilities. [195]... 

The disadvantages of this concept are clear i) mass accuracy and resolution are limited to = number of half cycles) ii) the electric signal for ion detection... 

The limited resolution and mass accuracy of the early MALDI-TOF instruments made the combination of MALDI with magnetic sector instruments (Chap. 4.3) desirable, [148,149] but this set-up suffered from low shot-to-shot reproducibility and poor sensitivity getting a full scan spectrum required thousands of laser shots while scanning the magnet. Even though eutectic matrix mixtures were introduced to circumvent such problems, [90,91] the MALDI-magnetic sector combination never became established. 

Quadrupole Energy and spatial distribution of ions produced in the ion source is not critical Low cost and easy to couple to LC Tandem MS experiments available in triple quadrupole or Q-TOF systems for sub-structure information and/or quantitative analysis Vacuum system demands are minimum Low resolution and low accuracy in mass measurement except in Q-TOF systems Mass range limited to approximately 4000... 

In terms of overall sensitivity (one molecule detection), mass resolution (in excess of 10 ), mass accuracy (<2ppm), scan speed, and MS capabilities, PT-ICR analyzers remain a very promising technique for the on-line coupling with CE. However, its technical demands, in terms of vacuum technology, and its high price currently limits its use mostly to fundamental studies. 

In 1974, Comarisov and Marshall60 developed Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). This technique allows mass spectrometric measurements at ultrahigh mass resolution (R = 100000-1000000), which is higher than that of any other type of mass spectrometer and has the highest mass accuracy at attomole detection limits. FTICR-MS is applied today together with soft ionization techniques, such as nano ESI (electrospray ionization) or MALDI (matrix assisted laser/desorption ionization) sources. 

Fourier transform ICR mass spectrometers together with any type of ion source, such as nanoESI, MALDI (or also an inductively coupled plasma ion source) permit mass spectrometric measurements to be performed at ultrahigh mass resolution (R = m/hm = 105—106) with a very low detection limit and the highest possible mass accuracy (Am = 10 3—10 4 Da). In addition, a high mass range is possible and FTICR-MS can be applied for MS/MS experiments.48 A comparison of different separation systems used in inorganic mass spectrometry is presented in Table 3.1. 

The capillary LC/MS-based approach for peptide mapping performed by Arnott and colleagues features miniaturized sampleloading procedures, which are routinely amenable to small quantities of peptides. The reliable characterization of protein/peptide mixtures in conjunction with the widely used 2-DGE methods offers a powerful fingerprinting approach in the pharmaceutical industry. Low femtomole detection limits (typically <50 femtomole) with a mass accuracy of +0.5Da provide unique advantages for protein identification. Liberal parameters for mass range and unmatched masses are used for the initial protein search, whereas more conservative parameters are used to reduce the number of matches and to improve the confidence in the search. 

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