Low-resolution mass

Hernandez, C. V. and Rutledge, D. N. (1994). Characterization of cocoa masses Low resolution pulse NMR study of the effect of geographical origin and roasting on fluidification. Food Chem. 49, 83-93. 

Only unit mass (low) resolution is required to discriminate between different elements, because isotopes of different elements differ by 1 u as can be seen in Appendix... 

Only unit mass (low) resolution is required to discriminate between different elements, because isotopes of different elements differ by 1 unit mass as can be seen in Appendix lO.A. There are only a few isotopic ovalaps between elements, so one can usually find an isotope to measure for any given element. In fact, there is only one element that cannot be definitely identified by ICP-MS, the element indium. One of the problems at the end of this chapter asks you to explain why. The abundance of each isotope is a quantitative measure of that element s concentration in the original sample. The isotope patterns for the elements are shown in Figure 10.38. 

An important operating characteristic of a mass spectrometer is its resolution (i.e., how well it separates ions of different mass). Low-resolution mass spectrometry refers to instruments capable of distinguishing among ions of different nominal mass [i.e., ions that differ by one or more atomic mass units (amu)]. High-resolution mass spectrometry refers to instruments capable of distinguishing among ions that differ in precise mass by as little as 0.0001 amu. 

The AutoSpec -TOF hybrid mass spectrometer combines the advantages of a magnetic/electric-sector instrument with those of time-of-flight to give a versatile instrument capable of MS or MS/MS at high or low resolution. 

A simple mass spectrometer of low resolution (many quadrupoles, magnetic sectors, time-of-flight) cannot easily be used for accurate mass measurement and, usually, a double-focusing magnetic/electric-sector or Fourier-transform ion cyclotron resonance instrument is needed. 

Commercial Instruments Because of the proliferation of applications of mass spectrometry in organic and analytical chemistry, there are instruments marketed by numerous companies today. Some of the popular ones are the low resolution, single focusing model 21-490 and the high resolution, double focusing models, 21-492 and 21-110 of Du Pont de Nemours Co, and the double focusing model MS-9 of Associated Electrical Industries... 

In this core, concentrations of PCBs (determined as Aroclor 1254 and 1260, by high resolution gas chromatography, electron capture detection and high resolution gas chromatography-low resolution mass spectrometry) were <30 ng and those of total DDT (p,p DDT + p,p DDD + p,p DDE) <5 ng g Campesan et al. (21) in 11 sediment samples from Valle di Brenta, determined by GC-ECD the following mean concentrations (ng gd.w.) ... 

Low-resolution devices are those that can separate and measure m/z ratios to the nearest integer value and have a numerical resolution of up to around 1000. As such, they can separate (resolve), for example, ions at m/z 28 and 29, i.e. they allow the analyst to differentiate between CO+ and CHO, or C2H4+ and C2H5+. Using these types of instrnment, we need only consider the masses of the isotopes as integers, e.g. = 12 Da, = 1 Da, = 14 Da and = 16 Da. 

The resolution of most mass spectrometers (the ability to separate ions of similar m/z values - see Section 3.3 above) in routine use is sufficient to allow the separation of the ions containing the individual isotopes if low-molecular-weight compounds (<1000 Da) are being studied. This is illustrated in Figure 4.13 which shows the molecular-ion region of a compound having the molecular formula C35H48N8O11S determined with a mass spectrometer resolution of 1500. The masses of the isotopes present in this molecule are shown in Table 4.2. 

Low-resolution mass spectrometer A spectrometer which is capable of measuring the m/z ratio of an ion to the nearest integer value. 

Magnetic sector A low-resolution mass analyser in which the variation of a magnetic field is used to bring ions of different m/z ratios to a detector. 

Figure 2. Negative FAB mass spectra of maitotoxin. The numbers denote the mass number at the centroid of each peak. A A survey scan at a low resolution (R=300). B. Resolution enhanced spectrum (R=3000) for ion clusters at around m/z 3300. C. Resolution enhanced spectrum (R=3000) for ion clusters at around miz 3400.

Fig. 6.—One of the Molecular-ion Clusters Obtained from a Sample of Deuteropermethy-lated. Cyclic /3(l- 2)-Glucans (see Section VI,S). [All high-mass samples give unresolved clusters of this type if the mass spectrometer is operated at low resolution. The peak is 6 mass units wide at half height. The mass is assigned by using the mass marker, which gives marks every 4 mass units, as shown. The center of the peak corresponds to the chemical molecular weight of an [M + NKJ species.]...

Low resolution MS yields specificity comparable to that of high resolution MS, if a relatively pure sample is delivered to the ion source. Either high resolution GC or additional sample purification is required. To obtain sufficient specificity, it is necessary to demonstrate that the intensities of the major peaks in the mass spectrum are in the correct proportions. Usually 10 to 50 ng of sample is required to establish identity unambiguously. Use of preparative GC for purification of nitrosamines detected by the TEA ( ) is readily adaptable to any nitrosamine present in a complex mixture and requires a minimum of analytical method development when new types of samples are examined. 

P.C. Jurs, B.R. Kowalski and T.L. Isenhour, Computerized learning machines applied to chemical problems. Molecular formula determination from low resolution mass spectrometry. Anal. Chem., 41 (1969)21-27. 

Specificity is unsurpassed. Traditionally, MS was performed on very large and expensive high-resolution sector instruments operated by experienced specialists. The introduction of low-resolution (1 amu), low-cost, bench-top mass spectrometers in the early 1980s provided analysts with a robust analytical tool with a more universal range of application. Two types of bench-top mass spectrometers have predominated the quadrupole or mass-selective detector (MSD) and the ion-trap detector (ITD). These instruments do not have to be operated by specialists and can be utilized routinely by residue analysts after limited training. The MSD is normally operated in the SIM mode to increase detection sensitivity, whereas the ITD is more suited to operate in the full-scan mode, as little or no increase in sensitivity is gained by using SIM. Both MSDs and ITDs are widely used in many laboratories for pesticide residue analyses, and the preferred choice of instrument can only be made after assessment of the performance for a particular application. 

Speleothem frequency distributions have provided a useful tool for broad comparisons, but they suffer from the problem of biased sampling strategies and low resolution at times of known abrupt change. The increased precision afforded by mass-spectrometric techniques will result in fewer studies using this approach to assess of growth frequency and, more often, records of continuous deposition and growth rate studies will be graphically illustrated. 

In other experiments, PVC/plasticiser extracts (n-hexane) were separated by SEC using THF or chloroform as the mobile phase. Similarly, PE film was immersed in THF for several hours and the extract was concentrated by a factor of 20 prior to injection into a SEC system [51]. However, use of extraction techniques followed by injection into a SEC system for separation of low-MW additives is not the most obvious analytical approach in view of the relatively low resolution of conventional SEC in the low molecular mass range. For this purpose efficient column packing materials with small pore sizes are to be used. 

The corresponding liquid-phase chemistry can be used to promote ion formation by appropriate choice of solvent and pH, salt addition to form M.Na+ or M.NH4+, and postcolumn addition of reagents. The primary applications of ESI-MS are in the biopolymer field. The phenomenon of routine multiple charging is exclusive to electrospray, which makes it a very valuable technique in the fine chemical and biochemical field, because mass spectrometers can analyse high-molecular-mass samples without any need to extend their mass range, and without any loss of sensitivity. However, with ESI, molecules are not always produced with a distribution of charge states [137], Nevertheless, this phenomenon somehow complicates the determination of the true mass of the unknown. With conventional low-resolution mass spectrometers, the true mass of the macromolecule is determined by an indirect and iterative computational method. 

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 ... 

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