Charge/mass measurements

Mass analyzer Quantity Measured MassjMass-to- Resolution charge at 1,000Daj ranges charge Mass measurement accuracy Dynamic range Operating pressure (Torr) Cost... 

This last m/z value is easy to measure accurately, and, if its relationship to the true mass is known (n = 10), then the true mass can be measured very accurately. The multicharged ions have typical m/z values of <3000 Da, which means that conventional quadrupole or magnetic-sector analyzers can be used for mass measurement. Actually, the spectrum consists of a series of multicharged protonated molecular ions [M + nWY for each component present in the sample. Each ion in the series differs by plus and minus one charge from adjacent ions ([M + uH] + n -an integer series for example, 1, 2, 3,. .., etc.). Mathematical transformation of the spectrum produces a true molecular mass profile of the sample (Figure 40.5). 

When multicharged ions are formed, the simple rule of thumb used widely in mass spectrometry that m/z = m because, usually, z = 1 no longer applies for z > 1 then m/z < m, and the apparent mass of an ion is much smaller than its true mass. Accurate mass measurement is much easier at low mass than at high, and the small m/z values, corresponding to high mass with multiple charges, yield accurate values for the high mass. 

Equation (8) shows us how to calculate the charge/mass ratio for the electron if r is measured and both v and B are known. 

Hendricks (H5) and Cho (C2, Method No. 1) both measured particle charge by measuring the voltage pulse on an oscilloscope due to the passage of a charged particle through a drift tube detector. The particle mass was... 

The error results were analyzed against final system pressure and volume of inert charge. The measured moles of CO2 were accurate at worst to within about 3% just using the Ideal Gas Law to calculate the result (no mass spectra were taken). The data were independent of the volume of the charge. 

Example ESI on a magnetic sector instrument set to R = 20,000 allows for the flail resolution of isotopic peaks in case of medium-molecular weight proteins (Fig. 11.21). This enables the direct determination of the charge state of the ions from the spacing of the isotopic peaks, i.e., 1 1h- for the lysozyme ion due to the average spaces of Am = 0.091 u and 13h- for the myoglobin ion due to Am = 0.077 u. In this particular case, the lysozyme [M+llH]" ion serves as a mass reference for the accurate mass measurement of the unknown" [M+13H] ion. [103]... 

The classical electrostatic balance is the Millikan condenser, which was first used to measure the charge on the electron in the famous Millikan oil-drop experiment. In principle, the device can be used to levitate a small charged mass by using the electrical field generated by two flat plates to... 

Figure 5A, B shows the isotopic distribution, of protonated bosentan (C27H30N5O6S, Mr 552.6) with a mass resolution of 0.5 and 0.1 at FWHM, respectively. It is worthwhile to observe the mass shift of the most abundant ion from m/z 552.2006 to m/z 552.1911. This value does not change with a mass resolving power of 15 000 (Fig. 1.5C) or even 500000 (Fig. 1.5D). Accurate mass measurements are essential to obtain the elemental composition of unknown compounds or for confirmatory analysis. An important aspect in the calculation of the exact mass of a charged ion is to count for the loss of the electron for the protonated molecule [M+H]+. The mass of the electron is about 2000 times lower than of the proton and corresponds to 9.10956 x 10 kg. The exact mass of protonated bosentan without counting the electron loss is 552.1917 units, while it is 552.1911 units with counting the loss of the electron. This represents an error of about 1 ppm.

In tandem MS mode, because the product ions are recorded with the same TOF mass analyzers as in full scan mode, the same high resolution and mass accuracy is obtained. Isolation of the precursor ion can be performed either at unit mass resolution or at 2-3 m/z units for multiply charged ions. Accurate mass measurements of the elemental composition of product ions greatly facilitate spectra interpretation and the main applications are peptide analysis and metabolite identification using electrospray iomzation [68]. In TOF mass analyzers accurate mass determination can be affected by various parameters such as (i) ion intensities, (ii) room temperature or (iii) detector dead time. Interestingly, the mass spectrum can be recalibrated post-acquisition using the mass of a known ion (lock mass). The lock mass can be a cluster ion in full scan mode or the residual precursor ion in the product ion mode. For LC-MS analysis a dual spray (LockSpray) source has been described, which allows the continuous introduction of a reference analyte into the mass spectrometer for improved accurate mass measurements [69]. The versatile precursor ion scan, another specific feature of the triple quadrupole, is maintained in the QqTOF instrument. However, in pre-... 

The results obtained with NaCl at 25°C and with KCl at 25°, 35° and 45°C in Eastman Kodak 398-3 cellulose acetate are listed in Table I. When examining the data it should be remembered that the fixed charge capacity measured here is that effective in electro-kinetic properties of the membrane it is not a quantity of analytical chemistry. Nevertheless, because NaCl and KCl are very similar in their electrochemical properties, one would expect the apparent number of moles of fixed charges per unit mass of dry... 

An instrument that measures the isotopic mass ratio of a gas by bombarding the sample in an electron beam, such that the molecular ions generated can be deflected in their trajectories through a magnetic field in accordance to their charge/mass ratios. These devices are extremely accurate and reliable, and many stable isotope experiments can be analyzed by converting the isotopi-caUy substituted metabolite into carbon dioxide, water, or molecular nitrogen prior to I RMS measurements. 

For each mass value, a signal quantity at the detector is recorded as counts per second, but may also be in another form. Since there are generally many mass values acquired, there is a ion intensity for each mass. This can be displayed as a bar chart of m/z ratio versus signal intensity. This chart is known as a mass spectrum. Spectra come in many variants. Full scan mass spectra show a range of masses with individual values at regular intervals of 1 or even 0.1 Da. SRM measures only the precursor ion mass and its product ion. Some mass spectra are processed data that convert m/z to mass. This is often seen in protein analysis where the multiply charged mass spectra are converted to a spectrum of just mass, simplifying the spectra interpretation. 

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