Peak matching

Therefore, for accurate mass measurement, a standard mass peak (M,) is selected, and the accelerating voltage (V) is changed until the sample ion peak (M ) exactly coincides with the position of Mj. This technique is called peak matching, and the ratio between the original and new voltages (VA ) multiplied by mass (Mj) gives the unknown mass, M . 

Peak matching can be done on quadrupole and magnetic-sector mass spectrometers, but only the latter, particularly as double-focusing instruments, have sufficiently high resolution for the technique to be useful at high mass. 

By high-resolution mass spectrometry, ions of known mass from a standard substance can be separated from ions of unknown mass derived from a sample substance. By measuring the unknown mass relative to the known ones through interpolation or peak matching, the unknown can be measured. An accurate mass can be used to obtain an elemental composition for an ion. If the latter is the molecular ion, the composition is the molecular formula. 

Constituents were identified by peak matching against standards in the NIST 95 Computer Library. The relative amounts of constituents were calculated by integrating all peaks with areas greater than 1%. 

Finally, the instrument can be operated in the peak-matching mode, which provides optimum mass resolving power and mass accuracy. Here the magnetic field strength is kept constant and the electric sector and acceleration voltages are scanned over a relatively small m/z range. This mode of operation is suitable when two ions that are very close in mass need to be separated or when the elemental composition of a molecule is to be determined at high resolution. 

A mass accuracy better than 1 ppm is routinely obtained by the better instruments when they are operated in a peak matching mode. An interlaboratory study of mass accuracy of different instruments and operating modes can be found in Reference 231. 

As a final test for comparing and contrasting mass spectra, an experienced and cautious investigator will always make a visual comparison. If there are a large number of data files, however, this process is tedious and time-consuming. Therefore, the new macros described below may be used to streamline the decisions about which putative peak matches require visual inspection of the actual mass spectra. 

Just because the retention times match does not necessarily mean that you have identified the peak. To be sure, change the stationary phase and inject again. If a sample peak is again resolved from the others (and it is the same size), and if the retention times of the same standard peak match the resolved peak, you can be sure that you have identified the peak and your quantitation in step 5 is valid. 

Mass range up to 7000 Da. Exact mass measurements are usually done by peak matching. The accuracy of the mass is the same as obtained in El, Cl. Relatively low sensitivity. Molecular ions often absent High mass range. Sample amount very low (picomoles or less). Mass accuracy (0.1 to 0.01%) is normally not as high as for other mass spectrometry methods. 

CH2N2 with 100% 3deld. Those samples were dissolved in CH2CI2 in concentrations 1.4mg/mL and injected for GC-FID and GC-MS analysis. The identification of methylated betulinic acid in extracts was done with use Wiley and NBS peak matching library search system. Authentic standard of the betuhnic acid and data reported in the literature were also used for further identification as described. 

Before starting the automated calibration, a number of the peak match parameters need to be set. These parameters determine the limits within which the acquired data must lie for the software to recognize the calibration masses and result in a successful calibration. Some of these parameters are ... 

Peak maximum standard deviation or maximum difference between the predicted and actual mass. During calibration the difference between the measured mass in the acquired calibration file and the true mass in the reference file is taken for each pair of matched peaks. If this value exceeds the set value, the calibration will fail. Reducing the value of the standard deviation gives a more stringent limit, while increasing the standard deviation means that the requirement is easier to meet, but this may allow incorrect peak matching. 

Once a full instrument calibration is in place, it is not always necessary to repeat the full calibration procedure when the instrument is next used. Instead of a full calibration, a calibration verification can be performed by infusing the calibration solution and setting all peak matching parameters to the values that were used... 

Detection of additional but non-diagnostic ion High-resolution mass spectrometry Peak match of molecular ion... 

Accurate mass determination using techniques such as peak matching is a reliable way to obtain the empirical formula. Knowledge of the accurate mass to four or five decimal places (see Table 16.1) allows the elemental composition of the compound to be obtained. Before the use of computer algorithms, empirical formula determination was made with the help of Beynon tables. Currently, data systems propose the most probable empirical composition for a given mass. The types of elements suspected to be in the compound can be input into the algorithm to reduce calculation time. 

Identification of the different types of ions observed in a mass spectrum through peak-matching and metastable ion analysis allows the determination of molecular structure. Several newer mass spectrometric techniques Mass analysed ion kinetic energy (MIKE) or reversed Nier-Johnson geometry) can also be used in spectral interpretation. These techniques are described in specialised monographs. 

Partition coefficient, 6 Peak asymmetry, 8 Peak matching, 297 PEEK, 46... 

Fig. 39 Aggregation of colloids coated with long DNA strands above a sticky surface leads to flying carpets, floating 2D crystals, (a) Confocal microscopy image of a flying carpet (scale bar is 10 pm), (b) Pair correlation function of the structure depicted in (a). The observed peaks match those expected for a perfect hexagonal crystal (black triangles). Reproduced with permission from [165]...

Zhang, H., Zhu, M., Ma, L., He, H., Humphreys, W. G., and Sanders, M. (2006). Combining MDF and peak match techniques for comprehensive and selective detection of drug metabolites in vivo. In Proceedings of the 54th ASMS Conference on Mass Spectrometry and Allied Topics, Seattle, WA. 

In studies of toxaphene in Lakes Superior [46] and Ontario [51,67], two persistent congeners, B8-1413 and B9-1979, were determined in air and six other congeners in water. B8-1413 and B9-1979 were of particular interest because they were among the most prominent chlorobornanes in Great Lakes fish [74,81,82], It should be noted that although peaks matching the retention... 

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