Monoisotopic peak

Natural Isotopic Abundances. The relative abundances of natural isotopes produce peaks one or more mass units larger than the parent ion (Table 7.75a). For a compound C H O N, a formula allows one to calculate the percent of the heavy isotope contributions from a monoisotopic peak, Pto the Pm + 1 peak ... 

Isotope peaks can be very informative in GC/MS analysis. Generally for interpretation, one focuses on the monoisotopic peak. The monoisotopic... 

A comparison between the theoretical isotope pattern of CgsH OsgLi, for the lithiated 18-mer of 3 (see inset in Figure 6), with that experimentally obtained indicates that they are very similar. Furthermore, the experimentally observed monoisotopic peak at m/z 1907.9 is very similar to that expected (m/z 1908.0) for this oligomer. 

If there are two bromine or four chlorine atoms contained in the empirical formula, the isotopic peaks become more intensive than the monoisotopic peak, because the second isotope is of much higher abundance than in case of the isotope. 

It has been pointed out that routine accurate mass measurements are conducted at resolutions which are too low to separate isobaric isotopic compositions in most cases. Unfortunately, coverage of multiple isotopic compositions under the same signal distort the peak shape. This effect causes a loss of mass accuracy when elemental compositions have to be determined from such multicomponent peaks, e.g., if the monoisotopic peak is too weak as the case with many transition metals. The observed decrease in mass accuracy is not dramatic and the loss of mass accuracy is counterbalanced by the information derived from the isotopic pattern. However, it can be observed that mass accuracy decreases, e.g., from 2-3 mmu on monoisotopic peaks to about 4—7 mmu on multicomponent signals. 

When G2-OH is mixed with a fourfold molar excess of Cu + ions the spectrum in Fig. 9b results. These data indicate that each G2-OH can sorb at least four Cu + ions. Moreover, the separation between adjacent copper adducts is 62.5, which indicates that the oxidation state of Cu inside dendrimer during the MALDI-MS experiments is -1-1. Reflectron-mode MS also confirms this assignment the mass differences between the monoisotopic peaks of protonated dendrimers, singlecopper adducts, and double-copper adducts are 61.96 and 61.93, respectively, which is consistent with the assignment of the adduct ions as [Mis + Cu(l)]+ and [Mis + 2Cu(I)-H] +. We speculate that the presence of Cu+ is a consequence of the photochemical reduction of Cu + during ionization. Such photoreduction in MALDI MS measurements has been observed previously when polymers or peptides are used as ligands for Cu + [117,118]. 

This means that for a protein of mass >10 kDa there will be a confusing array of peaks in the mass spectrum and it may be difficult to pick out the relatively minor "monoisotopic" peak that arises from molecules containing only 12C, 1H, 14N, leO, and 32S. In fact, the peak representing the most abundant mass will be a few mass units higher than the monoisotopic peak.215 216 (see Study Question 15). New computer programs have been devised to assist in the analysis. Use of 13C and 15N-depleted nutrients also extends the applicability of mass spectrometry.217... 

Figure 3. Reflector MALX)I MS was performed on an aliquot of the chymotryptic peak fraction (Figure 2). The monoisotopic peak could be determined via an internal standard to an accuracy of 40 ppm.

In which case (low or high MW) is the monoisotopic peak intensity higher than isotopic peak intensity ... 

Monoisotopic peak is the more intense in low molecular weight compounds. When the number of carbon atoms increase, molecules bearing one or more 13C atoms becomes common, and therefore these peaks are more intense. At high MW, there is a negligible probability of not bearing at least one 13C atom therefore the monoisotopic peak is not observable in these spectra. 

A common question asked of the mass spectrometrist is how close in miz can two peaks at mJz = 1 000 or mIz = 20,000 be and still be distinguished with confidence The detailed answer is not as simple as one would like because it is dependent on the sample and instrument resolution. First, there are different definitions that exist for the resolving power of a mass spectrometer. A common definition, R = ml Am at mass m, is used in this chapter. The value of Amk is the measurement of the mass peak at full width at half maximum (FWHM). If the FWHM for the C monoisotopic peak for a 1000-amu peptide is 0.5 amu, then R = 2000, and for a 20,000-amu protein with the same FWHM, R = 40,000. For the preceding example, the isotope peaks would be distinguishable. In other words, for miz up to 10,000 amu, where z = 1, it is possible to obtain monoisotopic information with a mass spectrometer capable of resolution between 20,000 to 30,000. At high mass (>10,000 amu), however, most mass spectrometers can only acquire average molecular weight data, not monoisotopic data. The Fourier transform mass spectrometer is capable ofR= 1,000,000 at miz = 1000. 

Figure 10.30 The difference between low-resolution and high-resolution MS. The nominal mass of the molecule C101H145N34O44 = 2537 Da. Atthe low resolution of 200, a single peak is measured with an average mass atthe peak maximum = 2539.5 Da. A high-resolution instrument (resolution = 2500) separates the monoisotopic peaks and permits measurement of exact masses. [Courtesy of Professor Gary Siuzdak, Scripps Research Institute Center for Mass Spectrometry (www.masspec.scripps.edu). The website has a number of tutorials on MS.]...

Algorithms that perform peak detection usually take into consideration the probable isotopic distribution when looking for the relevant monoisotopic masses. For example, Breen et al. [52] use a Poisson model to calculate the isotope distribution in order to select the monoisotopic peaks. These algorithms should also be able to separate overlapping isotopic patterns. 

A potential drawback for top-down analyses is that, as the molecular weight of a protein increases, it becomes less likely that the monoisotopic peak will be observed. For proteins > 15 kDa, the monoisotopic peak is 1% abundance and cannot be detected [131,132], It has been shown also that natural variation in C versus abundance can shift the most abundant isotopic peak of carbonic anhy-drase by 1 Da [131]. These phenomena place a limit on the accurate determination of protein molecular mass and, consequently, PTM assignment for example, is a 2 Da shift due to either abundance and deamination, or disulfide cleavage ... 

Marshall and co-workers showed that by isolating proteins from E. coli grown on C-depleted glucose and N-depleted ammonium sulfate, it is possible to reduce significantly the mass shift and width of the isotope distribution [133]. For 99.99% and 99.99% N, the upper mass limit for a relative abundance of 1% of monoisotopic peak is 100 kDa. 

The monoisotopic mass is meaningful for low-mass compounds because the elemental composition can be determined from a well-defined isotopic pattern of the molecular ion. The nominal and monoisotopic masses can both be correlated with the most abundant peak in the isotopic cluster. As the mass of a compound, however, increases, the isotopic pattern becomes more synunetrical and extends over many masses [21]. Also, the monoisotopic peak becomes difficult to identify. For high-mass compounds (e.g., proteins and oligonucleotides), the molecular ion profile measured coalesces and becomes a single asymmetric peak. For such compounds, the average mass value is accepted as the molecular mass. 

Deisotoping. MALDI-TOF-MS instruments produce high-resolution data in reflec-tron mode. A peptide can be resolved into several isotopic peaks, determined by the number of isotopes of carbon, nitrogen, and others, that it contains. The isotope peak intensities are highly correlated across samples thus, for comparative analysis it is necessary to use a single monoisotopic peak for each peptide. Furthermore, for the identification and characterization of peptides using MS/... 

MS, the monoisotopic masses are used. The process of identifying monoisotopic peaks is called deisotoping. A Poisson model of an isotopic peak cluster can be used for deisotoping [37]. The method is able to identify monoisotopic peaks even in cases where masses of peptides overlap. 

---