Identical masses

Zone Melting. A similar material balance may be made for a zone of mass moving a short distance in such a manner that the mass dm of sohd is frozen out and an identical mass melts into the zone (Fig. 4). For the first zone pass, it is assumed that the rod is initially at uniform composition i to obtain the following (1,4,8) ... 

The properties used to identify a substance must be intensive that is, they must be independent of amount The fact that a sample weighs 4.02 g or has a volume of 229 mL tells us nothing about its identity mass and volume are extensive properties that is, they depend on amount. Beyond that substances may be identified on the basis of their—... 

Singly and doubly interlocked [2]catenanes can exist as topological stereoisomers (see p. 144 for a discussion of diastereomers). Catenanes 35 and 36 are such stereoisomers and would be expected to have identical mass spectra. Analysis showed that 35 is more constrained and cannot readily accommodate an excess of energy during the mass spectrometry ionization process and, hence, breaks more easily. 

It must be underlined that independently of the MS equipment characteristics, no information about stereo-chemistry can be obtained. In fact, cis and trans isomers of the corresponding carotenoid showed identical mass spectra, as did carotenoids with epoxide groups at 5,6 and 5,8 positions. In addition, special care should be taken in assigning carotenoid molecular masses to avoid confusion due to the various ions that may be formed depending on measurement conditions. 

Instruments are calibrated with substances of known relative molecular mass and very accurate mass measurements can be made with this type of analyser. Sensitivity is very high and there is virtually no upper limit to the working mass range. The performance of TOF instruments can be improved further by the incorporation of an electrostatic reflector (reflectron) which ensures that all ions of identical mass reach the detector simultaneously by correcting for any differences in their kinetic energies. 

As expected, the incorporation of pendant unsaturation in the resists greatly enhances sensitivity as demonstrated by a comparison of the contrast curves for poly(N-aiiyl maleimide-VBC) and the structurally similar poiy(N-ethyl maleimide-VBC) (Figure 4). Both polymers have similar molecular weights and nearly identical mass absorption coefficients but the allyl-containing copolymer is 5X faster. 

The importance of a good chromatographic separation prior to the mass spectrometric analysis has to be underlined in face of the large number of possible surfactant-derived isobaric compounds to be encountered during the microbial degradation of LAS containing traces of DATS. For example, Cn-LAS and methylated C8-DATSC yield the same molecular ion [M — H] at m/z 311 identical masses of the [M — H] ions (m/z 355) are calculated for methylated Cn-SPC and dimethylated C8-DATSdC. 

Although the company officials and plant personnel cooperated fully in acquiring wastewater samples for analysis, they provided little information concerning identities of reactants of company products. Specific ring substitution patterns are not easily derived from mass spectrometrlc data and are included here only for cases where the literature specifies the Isomer or in cases where we have purchased standards and observed identical mass spectra and gas chromatographic retention times. 

For routine analysis, it is suggested that mass determination of an HPLC purified product is sufficient to be relatively assured that the correct product has been made. Of course, this approach will neither determine if the correct amino acids have been incorporated into an incorrect sequence nor determine if a substitution of amino acids of identical mass has occurred. However, the numerous checkpoints for automated peptide synthesis (bar codes, printouts, etc.) should greatly reduce the probability of this occurring. Should access to mass spectrometric analysis not be available, amino acid analysis is preferable. 

A time-of-flight spectrometer can be used as a mass analyzer, an ion kinetic energy analyzer, and an ion reaction time analyzer. We will consider here only what factors affect the resolution of the system in mass analysis.74 The same consideration can easily be extended to find the resolution in other analyses. There are at least two kinds of mass resolution. One refers to the ability of the system to separate two ion species of nearly equal masses in the same mass spectrum. This is related to the sharpness of the mass lines, or the full width at half maximum (FWHM) of the mass lines. The other refers to the ability of the system to distinguish two ion species of nearly identical masses, but not necessarily in the same mass spectrum. This latter mass resolution is related to the sharpness of reference points in the mass lines such as the onset flight times of the ion species, and the overall long-term stability of the system. This latter resolution determined also how accurately the instrument can measure the mass of ion species. Although this latter resolution is more closely related to ion kinetic energy analysis and is as important as the former one, we will consider here only the former kind, or the conventional kind, of mass resolution. 

Unlike with GC-MS, quality criteria for identification of drug residues by LC-MS have not been yet defined within the European Union, but this is currently under review. Criteria for GC-MS stipulate the measurement of preferably at least four diagnostic ions. However, this is not always possible with LC-MS because most compounds will only produce an M ion in positive mode or a M ion in negative mode, with little fragmentation when using thermospray (TSP), electrospray (ESP), or atmospheric pressure chemical ionization (APCI). Even where the ions and ratios are in agreement, there will be still possibility of misidentification. For this reason, mass spectra data are often interpreted with additional supporting data such as the LC retention times, as, for example, in the LC-MS analysis of sulfadimethoxine and sulfadoxine that present identical mass spectra (24). 

How Acids with the Same Normality Affect a Metal. Pour identical amounts (10 ml) of 1 V hydrochloric, sulphuric, and acetic acid solutions into three test tubes. Take three identical pieces of magnesium (not over 0.2 g) or identical masses (not over 0.2 g) of basic copper carbonate and simultaneously introduce them into the tubes with the acids. Note in which acid the magnesium (or carbonate) dissolves more rapidly. Why ... 

Fig. 6. An example showing the use of gas chromatography and mass spectrometry (GC-MS) for identifying reaction products on zeolites. Styrene was first reacted for ca. 30 min on activated zeolite HY at 298 K in a sealed glass tube the sample was extracted using toluene as solvent, and the extracts were then analyzed with GC-MS. The total ion chromatogram of the extracts (a) shows three major peaks eluting at 13.83, 13.93, and 14.31 min, respectively. The peak at 14.31 min was readily identified as the linear dimer. The peaks at 13.83 and 13.93 min show equal ion intensity and nearly identical mass spectra (the mass spectrum of the 13.93-min peak is shown in (b)), and these were assigned to the cis and trans isomers of the cyclic dimer.

The mass spectra [17] give a grossly simplified picture of the complex mixture which can result from a ring-opening polymerization. Knotted isomers can have almost identical mass spectra, dominated by the parent ion. With the reversibility of metathesis, the polyolefin mixture near equilibrium is expected to contain a large variety of knotted and linked species. 

A reasonable match of the experimental spectrum to one in the computer library is not proof of molecular structure—it is just a clue.7 You must be able to explain all major peaks (and even minor peaks at high m/z) in the spectrum in terms of the proposed structure, and you should obtain a matching spectrum from an authentic sample before reaching a conclusion. The authentic sample must have the same chromatographic retention time as the proposed unknown. Many isomers produce nearly identical mass spectra. 

The anomeric phenyl glycosides (5) and (6) afford practically identical mass spectra. A characteristic feature of the mass spectra of the phenyl glycosides is the strong increase of intensity of the series A fragments (m/e 219, 187, 155) and of the E (m/e 111) fragment. These ions are formed by elimination of the phenoxyl grouping as phenol or as the C H60 radical the latter is rather stable, due to interaction of the odd electron with the benzene ring. At the same time, peaks of the series B, C, D, and E (except E ) become less intense, or even disappear completely from the mass spectrum. 

An ion containing a less abundant combination of isotopes, also included under P.I.D., is not classified separately because identification is usually more simple from the more abundant isotopic combination. The mass number and relative abundance of isotopic ions can be calculated from the accompanying table. It might be argued that classification of these could be useful where the more abundant isotopic combination is obscured by another ion of nominally identical mass. This, however, will be an unusual circumstance and can be overcome by careful use of the table or by the use of exact empirical structure determination through high resolution techniques. 

All four parent 1,5-, 1,6-, 1,7-, and 1,8-naphthyridines afford essentially identical mass spectra (see Fig. I).49 The three most abundant fragment ions in their spectra are found at (a) mje 103, the ion resulting from direct expulsion of HCN from the molecular ion,... 

At the same location on Earth, two objects with identical masses have identical weights that is, the objects experience an identical pull of the earth s gravity. Thus, the mass of an object can be measured by comparing the weight of the object to the weight of a reference standard of known mass. Much of the confusion... 

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