Mass reference spectrum

Assuming that the mass spectrometer has sufficient mass resolution, the computer can prepare accurate ma.ss data on the m/z values from an unknown substance. To prepare that data, the system must acquire the mass spectrum of a known reference substance for which accurate masses for its ions are already known, and the computer must have a stored table of these reference masses. The computer is programmed first to inspect the newly acquired data from the reference compound in comparison with its stored reference spectrum if all is well, the system then acquires data from the unknown substance. By comparison and interpolation techniques using the known reference... 

Any mass spectrometer requires mass calibration before use. However, the procedures to perform it properly and the number of calibration points needed may largely differ between different types of mass analyzers. Typically, several peaks of well-known m/z values evenly distributed over the mass range of interest are necessary. These are supplied from a well-known mass calibration compound or mass reference compound. Calibration is then performed by recording a mass spectrum of the calibration compound and subsequent correlation of experimental m/z values to the mass reference list. Usually, this conversion of the mass reference list to a calibration is accomplished by the mass spectrometer s data system. Thereby, the mass spectrum is recalibrated by interpolation of the m/z scale between the assigned calibration peaks to obtain the best match. The mass calibration obtained may then be stored in a calibration file and used for future measurements without the presence of a calibration compound. This procedure is termed external mass calibration. 

As the ion source pressure rises, ion-molecule reactions become possible, sample ions reacting with sample molecules. In the case of exact mass measurement, reaction can occur with the PFK mass reference 126). The observed reactions in the mass spectrum of ruthen-nium porphyrincarbonyl, yielding ions of the type [M-CO -h C, F2n] n = 1-4), illustrate this problem. Similarly, in the spectrum of Ni(PF3)4 ion-molecule reactions result in species such as Ni2(PF3) + n = 2-5) and Ni2(PF)2(PF3)m (m = 2-4) and, in the (CO)5CrC(CH3)OCH3 system, reactions of the following type are observed 127). 

Figure 10. Mass spectrum of one of the main components of Pinus montana resin in comparison with reference spectrum of isopimarate. Conditions as in Figure 9. (Reproduced with permission from ref. 3. Copyright 1985, Elsevier.)...

If the acquired spectrum looks like the reference spectrum and all of the peaks expected pass the criteria above, the calibration is acceptable. If the instrument has never been calibrated before or the previous calibration file has been misplaced, a more detailed calibration routine must be used. This calibration procedure requires the location of known peaks, starting with the lowest mass ions. When a peak has been located, a digital-to-analog conversion value is assigned to the exact mass of the ion. 

When each peak in the reference spectrum has been matched with a corresponding peak in the spectrum acquired, the mass difference is calculated for each pair of peaks (see Section 3.1.2). These mass differences are plotted as points on a graph each data point has the mass of the acquired peak as its x coordinate, and the mass difference above as its coordinate, and a smooth curve is drawn through the points (Figure 13.10) [5]. The polynomial order parameter controls the type of curve that is drawn and can be set to any value between 1 and 5 ... 

Figure 13.10. Plot of the mass difference between reference spectrum and acquired spectrum. MSI static calibration, 28 matches of 28 tested references, SD = 0.0465. (Courtesy of Micromass Quattro LC.)...

The most efficient silicon cells produced are based on p — n homojunctions and convert 23.1% of the energy in incident light set to simulate the global air mass (AM) 1.5 spectrum, an artificial reference spectrum used to standardize measurement of PV power, with an intensity of 1000 W/m2... 

The following table lists the most popular reference compounds for use under electron impact conditions in mass spectrometry. For accurate mass measurements, the reference compound is introduced and ionized concurrently with the sample and the reference peaks are resolved from sample peaks. Reference compounds should contain as few heteroatoms and isotopes as possible. This is to facilitate the assignment of reference masses and minimize the occurrence of unresolved multiplets within the reference spectrum.1 An approximate upper mass limit should assist in the selection of the appropriate reference.12... 

If the abundances of the entire ethanol contracted spectrum are lowered — keeping their relative heights the same as in the reference spectrum — a point is reached where none of the unknown ions is of lower abundance than the corresponding ethanol peaks. In the example shown in Figure 7, this occurs when the mass 46 ions become the same height. If a measurement reproducibility of 20% is assumed, two other ions (m/e 43 and 45) are found to agree with the abundances required by the ethanol reference spectrum because they fall within the prescribed tolerance window. In this example, the mixture was about 50% ethanol the excess intensities at m/e 31 and 32 were due to methanol and the excess intensity at m/e 44 was due to the additional presence of CO2. 

Figure 6. Comparison of SRC-II mass 132 MS/MS ("upper,) with the reference spectrum of 3-methylindole ("lower)...

One of the most serious limitations in the application of these powerful GC/MS and LC/MS systems is the accurate and efficient identification of this flood of unknown mass spectra. A variety of computer-assisted techniques have been proposed (2, 3, 4), which can be classified generally as "retrieval" or "interpretive programs (2). The former matches the unknown mass spectrum against a data base of reference spectra the ultimate limitation of this approach is the size of the data base, which currently contains the mass spectra of 33,000 different compounds (j>, 6), less than 1% of the number listed by Chemical Abstracts. If a satisfactorily-matching reference spectrum cannot be found by the retrieval program, an interpretive algorithm can be used to obtain partial or complete structure information, or to aid the human interpreter in this task (7-10). 

Fig. 10. Comparative overnight 13C spectra of an 200 pg sample of a secondary fungal metabolite fumonisin B (6) in D20. The data shown in the top trace were acquired with the sample dissolved in 140 pL of solvent in a 3 mm NMR probe. The data were acquired overnight at 125 MHz using a Nalorac MD-500-3, 3 mm micro-dual NMR probe. The s/n ratio of the experiment was 5.5 1. A ketone carbonyl was expected in the 200-230 ppm region of the spectrum based on mass spectrometric fragmentation data and was not observed in the 13C reference spectrum. In an effort to observe the carbonyl 13C resonance to complete the work, the sample was washed from the 3 mm NMR tube, concentrated to 20pL and transferred quantitatively to a 40 pL nano-cell. The overnight data acquisition was repeated using a Varian 500 MHz heteronuclear Nano-probe to afford the spectrum shown in the bottom trace. The s/n ratio in the bottom spectrum was 11 1, with the expected carbonyl resonance observed at 219.5 ppm. (Reprinted with permission from Ref. 20. Copyright 1995, American Chemical Society and American Society of Pharmacognosy.)...

The mass spectra of compounds of known structure may be collected and indexed according to molecular weight or molecular formula, as in the American Petroleum Institute scheme. Large collections of reference spectra can be indexed and a matching search carried out by computer (e.g. Hites and Biemann, 1968 Crawford and Morrison, 1968a). The approach is the least subjective of the ones described in this section, but it requires the reference spectrum to be available before an identification can be made. The method can be extremely useful where a relatively limited range of compounds is studied repetitive. ... 

These eriteria veiy mueh meet the ones reeently established in the EU guidelines [2], These eriteria ate based on the principle of identification points . One point is earned for each ion in the mass spectrum and/or for the precursor ion in a production MS-MS spectrum, and one-and-a-half point for a product ion in the MS-MS spectrum. For the confirmation of Group A (illegal) compounds four identification points are required, while for the confirmation of Group B (legal, but with maximum residue level, MRL) compounds three identification points are required. In addition, ion ratios must be within 20-30% of the ratios in the reference spectrum. 

The reverse search refers to the fact that the algorithm checks whether a peak from the reference spectrum is present in the unknown (and in the appropriate abundance) and not the other way around. In this way, the reverse search ignores peaks in the unknown that are not present in the reference. The mass library searches are, in fact, much more elaborate, and provide at the end a list of possible matches and for each of them a calculated percentage match. 

Figure 2. Reactivity of aluminum clusters toward D2. (a) Time-of-flight mass spectrum of bare Al and their deuterium addition products. Dashed trace—reference spectrum in which He only is pulsed into reactor, solid trace—Dj/He mixture is used, (b) Relative rate constants of Al (n = 2-14) toward chemisorption of Dj obtained from data such as that presented in (a). (From Cox et al. )...

Figure A3.5.2. The Ar" photofragment energy spectrum for the dissociation of ions at 752.5 nm. The upper scale gives the kinetic energy release in the centre-of-mass reference frame, both parallel and antiparallel to the ion beam velocity vector in the laboratory.

When peaks are incompletely separated identification may still be possible using a reverse search. The ability of an algorithm to match two or more components in the mass spectrum of a mixture is aided by requiring only that the peaks of the reference spectrum are present in the unknown spectrum rather than the other way round, as for a normal (or forward) search. The hit list of retrieved library spectra should then represent the compounds in the spectrum of the mixture provided that their spectra are present in the reference library. Subtracting the best-hit library spectrum from the mixture spectrum produces a residual spectrum that can then be matched against the other spectra in the hit list in a forward search. In a sequential process identification of the component spectra may be achieved. 

AM 1.5 G is a shorthand notation used by the PV community to denote the air-mass 1.5 global reference spectrum, estimated at 1000 W/m. ... 

For an unknown mass spectrum (A), in this case of a component (known to be 2-hydroxyxanthone) extracted from a plant, the system retrieves from a library of 39,000 reference spectra five references having a similarity index (SI) value of at least 5 per cent (as compared with spectrum A). First on the hit list is 2-hydroxyxanthone,of which the reference spectrum (B) has an Sl-value of 69.2 per cent. References 2,3 and 4 are isomers, of which the mass spectra are expected (on mass spectrometric grounds) to show much similarity with that of 2-hydroxyxanthone. Reference 5 is a different compound, which, however, has some structural featinres in common with hydroxy-xanthones. As to the Sl-values only an exact copy of a reference spectrum 3delds an SI value of 100 per cent. Two different spectra of a compoimd, recorded under different experimental conditions, however, will always exhibit systematic differences, which in some cases may be qxiite substantial. The similarity index used in this library search system takes account of such differences. ... 

Since the 1960s, mass spectrometry has played a pivotal role in the field of structure elucidation and identification of organic compounds. Over the years, a wealth of knowledge has been gained on reactions of gas-phase ions from the use of a variety of mass spectrometric techniques. Mass spectrometry can be used to identify unknown compounds or to perform de nova stmcture determination. The former is relatively easy if one knows accurate mass and a reference spectrum. The latter is much more difficult and requires detailed knowledge of the rules for interpretation of a mass spectrum. Providing this knowledge is the focus of this chapter. 

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