Mass scale calibration

FIGURE 3.1 Mass scale calibration and the use of a lock mass. 

E2008, Standard Test Method for Volatility Rate by Thermogravimetry E2040, Standard Test Method for Mass Scale Calibration of Thermogravi-metric Analyzers... 

Modern SSIM spectrometers are equipped with data acquisition/processing stations, and the programs allow control of the spectrometer during acquisition and standard spectrum interpretation and quantification (mass scale calibration, peak locating. 

Computer-controlled establishment of the mass scale and correction for mass defect is of great practical importance, particularly in quadrupole instruments. Mass calibration is customarily achieved by introducing a known compound whose spectrum is recognized by the computer, followed by comparison to an authentic spectrum stored in the memory. A very attractive alternate method of mass scale calibration is to utilize the compound of interest itself to establish the mass scale (assuming that the mass spectrum of the compound is known). 

In principle, mass scale calibration is independent from the ionization technique. For many years we have routinely calibrated by the use of a combination of corona discharge and controlled cluster formation in the free-jet expansion. Corona discharge of the headspace of an aqueous ammonia solution creates NH4 (H20) with n = 0-4 after passage through the gas curtain. If the curtain gas flow is reduced but not cut off completely, one can tune the gas flow to allow a controlled amount of moisture into the free-jet expansion so that a complete series of NH4 (H20) ions is formed that extends all the way up to mIz 2000. The calibration mass series starts at miz 54 and one out of every live clusters can be used, giving a calibration separation of 90 on the miz scale. The relatively low abundance of deuterium and O, along with the higher abundance of 0 in the cluster ions, makes this caUbrant suitable for resolution adjustment on isotope peaks. 

Other instrumental advantages include its high sensitivity and a linear mass scale to m/z 10,000 at full sensitivity. The linearity of the mass scale means that it is necessary to calibrate the spectrometer using a single or sometimes two known mass standards. Some calibration is necessary because the start of the mass scale is subject to some instrumental zero offset. The digitized accumulation of spectra provides a better signal-to-noise ratio than can be obtained from one spectrum alone. 

Polynomial order = 4. Used for calibrations that include the lower end of the mass scale, with closely spaced reference peaks. This is suitable for calibrations with polyethylene and poly propylene glycols (see Section 13.3.3) that extend below 300 amu. 

Calibrants are required to calibrate the mass scale of any mass spectrometer, and it is important to find reference compounds that are compatible with a particular ion source. Calibrants commonly used in electron ionization (El) and chemical ionization (Cl), such as perfluorocarbons, are not applicable in the ESI mode. The right calibrants for LC-ESI-MS should (1) not give memory effects (2) not cause source contamination through the introduction of nonvolatile material (3) be applicable in both positive- and negative-ion mode. The main calibrants used or still in use to calibrate ESI-MS can be divided into the following categories polymers, perfluoroalkyl triazines, proteins, alkali metal salt clusters, polyethers, water clusters, and acetate salts. 

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Fig. 10. Mass spectra of from MALDIMS, and from ESIMS. For MALDIMS the mass scale was calibrated with myoglobin ions as an internal standard. For ESIMS the mass scale was calibrated externally using the doubly charged and singly charged ions of gramicidin S. The right hand panel of the ESIMS spectrum shows the deconvoluted spectrum, giving measured molecular weights for the mtgor species, which differ by one hexose. Deviations from the calculated values are shown in parentheses.

The stable carbon isotope ratios of dissolved inorganic carbon (DIC) and benthic foraminiferal calcite generally are determined with isotope ratio gas mass spectrometers calibrated via NBS 19 international standard to the VPDB (Vienna Pee Dee Belemnite) scale. All values are given in 8-notation versus VPDB with an overall precision of measurements including sample preparation usually better than +0.06 and +0.1%o for calcite and DIC carbon isotopes, respectively. Except one single-specimen based dataset (Hill et al. 2004), all stable isotope data from papers referred to in this overview are from species-specific multi-specimens analyses. The number of specimens used for a single analysis depended on size and weight of species but usually varied between 2 and 25. 

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