Mono-isotopic mass

A mixture of peptide calibration standards—e.g., des-Arg1 -bradykimn (mono-isotopic mass 904.4681), angiotensin 1 (monoisotopic mass 1296.6853), ACTH (1-17 clip) (monoisotopic mass 2093.0867), ACTH (18-39 clip) (monoisotopic mass 2465.1989)—is made to a concentration of 2 pmol/pL each in 0.1% TFA in water. This solution is then mixed 1 1 with matrix solution to give the final peptide-calibration mixture. 

What are the differences between nominal mass, exact mass, accurate mass, mono-isotopic mass, molecular mass, and isobaric mass The nominal mass of a compound, ion, or fragment is calculated using the masses of the elements rounded to the nearest whole number (e.g., C = 12, H = 1, O = 16). For example, cholesterol with an empirical formula C27H4gO has a nominal molecular mass of 386 Da. It is common to see molecular mass referred to as molecular weight (MW). 

Mono-isotopic mass is the calculated mass of a compound based on the accurate masses of the most common stable isotopes of the elanents present. For the elements commonly found in organic molecules the mono-isotopic mass is composed of the isotopes of lowest mass, e.g., 12.0000 for carbon and 15.9949 for oxygen. The mono-isotopic mass is usually the most relevant measure of mass, because of the ability of instruments to observe the masses of the individual isotopes. 

Calculated Mono-Isotopic Masses of Sodiated Fragment Ions of DjAj Isomers... 

The exact mass of the most abundant isotope of an element is termed monoisotopic mass [6]. The monoisotopic mass of a molecule is the sum of the monoisotopic masses of the elements in its empirical formula. As mentioned before, the mono- isotopic mass is not necessarily the naturally occurring isotope of lowest mass. However, for the common elements in organic mass spectrometry the monoisotopic mass is obtained using the mass of the lowest mass isotope of that element, because this is also the most abundant isotope of the respective element (Chap. 3.1.5.1). 

State-of-the-art TOF-SIMS instruments feature surface sensitivities well below one ppm of a mono layer, mass resolutions well above 10,000, mass accuracies in the ppm range, and lateral and depth resolutions below 100 nm and 1 nm, respectively. They can be applied to a wide variety of materials, all kinds of sample geometries, and to both conductors and insulators without requiring any sample preparation or pretreatment. TOF-SIMS combines high lateral and depth resolution with the extreme sensitivity and variety of information supplied by mass spectrometry (all elements, isotopes, molecules). This combination makes TOF-SIMS a unique technique for surface and thin film analysis, supplying information which is inaccessible by any other surface analytical technique, for example EDX, AES, or XPS. 

If the isotopic distribution is very broad and/or there are elements encountered that have a lowest mass isotope of very low abundance, recognition of the mono-isotopic peak would become rather uncertain. However, there are ways to cope with that situation. 

As we have already seen, the isotopic mass also is the exact mass of an isotope. The isotopic mass is very close but not equal to the nominal mass of that isotope (Table 3.1). Accordingly, the calculated exact mass of a molecule or of a mono-isotopic ion equals its monoisotopic mass (Chap. 3.1.4). The isotope C represents the only exception from non-integer isotopic masses, because the unified atomic mass [u] is defined as of the mass of one atom of nuclide C. 

Gaseous sample introduction into an ICP-MS presents different problems. Owing to its extremely sensitive nature, Dean et al. [13] introduced the sample as the gaseous hydride by a flow-injection approach. This was reasonably effective because lower volumes of samples and reagents were in use. They utibzed nitric acid as a carrier stream to prevent the formation of argon chloride species in the plasma. Argon chloride has the same mass as arsenic which is mono-isotopic, and this severely bmits arsenic determination. An additional problem was that the sensitivity was extremely dependent on the purity of reagents. 

Isotopes of a chemical element are nuclides with the same number of protons (Z) but a different number of neutrons N) in the atomic nucleus. Isotopes of a chemical element (e.g., H and of hydrogen Cl and Cl of chlorine or Fe, Fe, Fe and Fe of iron, respectively) have the same number of protons (Z) and possess the same chemical properties, but differ in the number of neutrons (N) and thus in the mass number (A). With increasing Z, the number of neutrons in a stable atomic nucleus is higher than the number of protons. For mono-isotopic elements. 

One example is the high-resolution ESI-FT-ICR MS analysis of a mixture of Verapamil and peptide A. The following MS data were obtained for the mono-isotopic molecular ion and the isotopic molecular ions of peptide A using an external calibration (calculated mass mass error in ppm) (829.5393, 0.1 ppm), (830.5423, 1.3ppm), (831.5450, l.Oppm), and (832.5476,1.2ppm). Similar results were obtained for the Verapamil (data not shown). 

Conventional ICP-MS has great sensitivity but suffers from serious interferences. Chlorine interference leads to the formation of " °Ar Cl, which has the same mass/charge ratio as the mono-isotopic As (w/z = 75). Hence, HCl and HCIO4 should not be used for preservation or dissolution if ICP-MS is to be used. There may also be significant interference in samples with naturally high Cl/As ratios. A chlorine concentration of 1,000 mg gives an arsenic signal equivalent to —3-10 p,g L. ... 

Since stable isotope dilution depends on the mass spectrometric determination of isotope ratios, the method is generally restricted to elements which have at least two stable isotopes. However, in the case of mono isotopic cesium, the long-lived fission product Cs136 or Cs137 can be used as a tracer for the analysis of this element. In all, some 68 elements can be... 

Quadrupole ICP-MS instruments are considerably cheaper and easier to operate, but the detection limits are generally inferior compared to SF-ICP-MS. Furthermore, as the quadrupole mass filter provides a resolution of only 300, polyatomic interferences are likely, especially in the mass range 40-80 amu. A well-known interference appears on As (ICP-MS Interferenz Tabelle 1995). The As isotope is a mono-isotope, which... 

The method is particularly useful in determinmg the abundances of REE at low concentrations, although four of the REE (Pr, Tb, Ho and Tm) are mono-isotopic and cannot be analysed by this method. The main disadvantage is that even with automated mass spectrometry the method is time-consuming and expensive and so is normally reserved for measurements which can be used to calibrate other more rapid methods. 

The electrospray ionisation mass spectrometric analyses were performed using a Finnigan LCQ ion trap mass spectrometer. The samples were dissolved in methanol or chloroform methanol system (10 1 v/v) and such solutions were introduced to the ESI source by continuous infosion by means of the instrument syringe pmnp with the rate of 3 iL/min. The ESI source was operated at 4.25 kV and the capillary heater was set to 200°C. For ESI-MS" experiments mass selected mono-isotopic parent ions were isolated in the trap and collisionally activated with 33% ejection RF-amplitude at standard He pressure. The experiments were performed in the positive and negative-ion mode. 

Molecular mass is the average mass of a compound obtained when an accounting is made for all isotopes of the elements present based on their relative abundances, e.g., C = 12.0108 Da, O = 15.9994 Da. If the instrument used cannot resolve the individual isotopes, the observed peaks include all isotopes present. Molecular mass is sometimes referred to as average mass. For cholesterol the molecular (average) mass is 386.6616 Da. (Note the difference between this number and the mono-isotopic exact mass value, 386.3549 Da, as described above.) Molecular mass does have a dimension as it is an absolute value unit, based on 1/12 of the mass of the isotope (in lUPAC units), i.e., 1.6605 x 10- kg. If, however, the mass of an analyte is considered as a ratio with respect to the mass of C, then the dimensions cancel out and the resulting dimensionless number is the relative molecular mass. These two terms are equivalent in everyday usage. 

An overview of the uranium and plutonium certified reference materials that are used for safeguards and fissile material control for bulk and particle analysis was presented at a conference organized by the European Joint Research Centre (JRC) (Jakopic et al. 2012). There are three main types of CRMs single isotope standards for isotope dilution measur ents CRMs for isotope ratio calibration and mass discrimination correction and reference materials for age dating. As far as uranium is concerned, there are (almost pure) mono-isotopic standards and... 

The possibility that a particular element with a value of Z may have varying values of A - Z, the number of neutrons, gives rise to the phenomenon of isotopy. Atoms having the same value of Z but different values of A-Z (i.e. the number of neutrons) are isotopes of that atom. Isotopes of any particular element have exactly the same chemical properties, but their physical properties vary slightly because they are dependent upon the atomic mass. A minority of elements, such as the fluorine atom, are mono-isotopic in that their nuclei are unique. Examples of isotopy are shown in Table 1.2. 

It has been pointed out that routine accurate mass measurements are conducted at resolutions which are normally too low to separate isobaric isotopologs. Unfortunately, multiple isotopic compositions under the same signal tend to distort the peak shape (Fig. 3.31) [64], This effect causes problems when elemental compositions have to be determined from such multi-isotopolog peaks, e.g., if the mono-isotopic peak is too weak as in case of many transition metals. Generally, the observed decrease in mass accuracy is not dramatic and is somewhat counterbalanced by the information derived from the isotopic pattern. However, it can be observed that mass accuracy decreases by about 50% on such unresolved signals. 

Multi-element IDMS is available when using ICP-MS. For transient signals, however, the multi-element capability is reduced as two isotopes have to be measured for each element. In principle all elements of the periodic system having two natural isotopes can be analysed by IDMS as long as mass spectrometric methods for isotope ratio determinations are available. Moreover even mono-isotopic elements can be determined, if they also have a long-lived radionuclide isotope such as iodine-129. 

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