Low resolution mass spectrometers

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

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. 

GC-MS using high-resolution capillary columns and low-resolution mass spectrometers has been a popular analytical technique in environmental organic geochemistry [507,572,573]. However, additional analytical techniques have been used very recently to extend the capabilities available for the determination of molecular markers. Such advances are discussed in the next few paragraphs, which show the alternative approaches to increase GC-MS sensitivity and specificity ... 

A low resolution mass spectrometer used for routine analysis by EPA Methods 8260 and 8270 consists of three main parts an ion source, a mass analyzer (both kept under vacuum conditions), and a detector. Although a variety of GC/MS instruments with different designs are available, the most common in the environmental laboratory are those with electron impact ion sources and quadrupole mass analyzers as they are best suited for the coupling with gas chromatography columns. These stable and reliable detectors with a relatively broad linear dynamic range became the workhorses of the environmental laboratory. 

The low resolution mass spectrometers used in EPA Methods 8260 and 8270 are not as sensitive as some of the selective chromatographic detectors (for example, the ECD) and for this reason are not capable of reaching the low detection limits that may be required for some DQOs. The mass spectrometer scans a large number of ion masses in a short period of time (for example, in EPA Method 8270, a mass range of 35-500 is scanned in 1 second) and dwells only briefly on each detected mass. In such full scan mode, the sensitivity of detection is traded for a wide range of detected ions. It is also affected by the background spectra (an equivalent of the electrical signal noise). 

A 47 peak was observed in the mass spectrum obtained when an irradiated mixture of N20 and OF2 was distilled at 132° K. This peak may be N2F+ from N2F2 or COF+ from COF2 these two ions are indistinguishable in a low resolution mass spectrometer. The latter compound has been observed to result from attack by F20 or its radiolysis products on the Kel-F grease used to lubricate the O-ring seals in our metering valves it is usually accompanied by C02 at close to the same distillation temperature. In the experiment under discussion, however, any C02+ would be indistinguishable from N20+. 

Sulfur tetrafluoride was not studied in detail because of its low purity. The amount of impurities that are difficult to remove from this material probably exceeds the overall radiation yield. The major impurity in commercially available SF4 is SOF2. Since one sulfur atom is equivalent to two oxygen atoms in a low resolution mass spectrometer, some confusion results in interpreting the mass spectra, and the system was not studied in detail. 

Low resolution mass spectrometers essentially have resolution of the order of 1000 (i.e. they are able to... 

For example, a compound having a molecular ion at miz = 60 using a low-resolution mass spectrometer could have the following molecular formulas ... 

The mass spectrometric analyses initially reported in this communication were performed with a double focusing low resolution mass spectrometer Model 270, Perkin-Elmer Corp., Norwalk, Conn., coupled through a Watson-Biemann type of molecular separator with a gas chromatographic system. (Commercial sources and trade names are provided for identification only. Their mention does not constitute endorsement by the Public Health Service or by the U. S. Dept, of Health, Education, and Welfare.)... 

MS-based assays that are aimed to characterize the specificity of catalytic NRPS domains and tailoring enzymes for carrier protein substrates can be done on high-resolution mass spectrometers or, for small substrate T domains (<20 kDa),105 on low-resolution mass spectrometers. For investigation of T domain substrate tolerance, the native T domain substrates of a catalytic NRPS domain or tailoring enzyme are exchanged by different T domains, for example, from different NRPS systems. In addition, the tolerance of T domain order can be tested for C domains and aminoacyl transferases by reverse-ordered tethering of native biosynthetic substrates to the native T domains and MS detection of the reaction product on the assayed upstream active site.93... 

A mass spectrometry detector (MSD) which consists of a low resolution mass spectrometer, cf. Chapter 16) can be placed to the outlet of the column. A fragmentation spectrum of each eluted compound is obtained. From the total ionic current... 

This is another method to find the molecular formula of the analyte. A low-resolution mass spectrometer will suffice. As explained below, the principle is based upon the isotopic composition of elements. 

All the mass spectra shown in this text were determined with a low-resolution mass spectrometer. Such spectrometers give the nominal molecular mass of a fragment— the mass to the nearest whole number. High-resolution mass spectrometers can determine the exact molecular mass of a fragment to a precision of 0.0001 amu. If we know the exact molecular mass of the molecular ion, we can determine the compound s molecular formula. For example, as the following listing shows, many compounds have a nominal molecular mass of 122 amu, but each of them has a different exact molecular mass. 

Ion microprobe analyzers are more sophisticated (and more expensive) instruments based on a beam of primary ions focused to a diameter of 2(X) nm to 1 pm. This beam can be moved across a surface (rastered) for about, 3(K) pm in both the. v and y directions, A. microscope is provided lo permit visual adjustment of the beam position. Mass analysis is performed with a double-focusing spectrometer. In some instruments, the primary ion beam passes through an additional low-resolution mass spectrometer so that only a single type of priniilry ion bombards the sample. The ion mi-croprobc veisioii of SIMS permits detailed studies of solid surfaces. 

For several decades, mass spectrometers were used primarily to determine atomic masses and isotopic ratios. Now they are applied to a large variety of chemical problems and low resolution mass spectrometers are used for routine chemical analysis. For exanq>le, a modem mass spectrometer can easily distinguish between species such as and... 

The masa spectrum of SK+F 47 tO-J (SK+F standard SJB- + f06-226-A) was obtained by direct Insertion of the solid Into an Hitachi Perkln-Elmer RMU-6E low resolution mass spectrometer. The results are presented in tabular form In Table I and as a bar graph in Figure 7. E. White and G, Roberts provided the following observations and assignments ... 

All of the spectra that we have described so far have been determined on what are called low-resolution mass spectrometers. These spectrometers, as we noted earlier, measure m/z values to the nearest whole-number mass unit. Many laboratories are equipped with this type of mass spectrometer. 

Figure 10.1 is a schematic of a typical low resolution mass spectrometer of the type commonly used with GC, Because of its small size, it is often referred to as a bench-top MS. 

Our discussion thus far has assumed that a mass spectrometer can distinguish between ions that differ in mass by one amu, where the m/z values are measured to the nearest whole number. In the jargon of MS, these are called low-resolution mass spectrometers. Much more expensive instruments, called high-resolution mass spectrometers, can measure the m/z ratios to 0.001 or 0.0001 amu and provide very accurate measurements of molar masses. These data may then be used to determine elemental composition, which is extremely valuable information when you are trying to assign a structure to an unknown compound. 

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