Mass axis calibration

Da, we would need a resolution of (28.0061 - 27.9949) = 0.0112 Da in 28 Da. To convert this resolution to ppm, divide 0.0112 by 28 and multiply by 1 x 10 a resolution of 400 ppm is required. The resolving power needed would be 27.9949/0.00112 = 2500. Mass accuracy describes how well the top of the MS peak can be defined and kept stable and its position identified on the mass axis. Mass axis calibration is usually established by a separate portion of the tuning algorithm, which identifies the centroids of several mass peaks of precisely known mass in the spectrum of a suitable calibration compound and adjusts tuning parameters to place these at the appropriate positions on the mass axis. Like resolution, it can be expressed as a percentage or ppm value of the calibration mass values. 

The series of well-defined sulfur marker peaks provides the data points on which the computer system performs a high-order polynomial fit to construct a mass axis calibration curve. The latter can then be used to convert experimentally measured intensity-time profiles to intensity-m/z plots (mass spectra). A caveat that should not be ignored is the formation of heteronudear species by gas-phase reactions of sulfur with other inorganic substances [13]. Even though none of these het-eronuclear species may exist in the solid state or in solution, their presence in the gas phase complicates mass spectra. 

Sample extraction and HTpSPE followed the procedure presented in Section 10.3.1.1 for fruit and vegetables. For pL-FlA-TOFMS analysis, acetonitrile/water (1/1, v/v) was applied including 0.05% formic acid at a capillary flow rate of 2 j,L/min. The eluent included 5% mass calibration solution to perform the mass axis calibration of the system. High-resolution mass spectra were recorded in the full-scan mode, and the quantitation of pesticides was done using the signal-to-noise ratios of the exact mass signals, normalized to TDCPP, when pure solvent standards were applied for comparison [27]. 

Sensor balancing at the mass axis (often erroneously referred to as calibration) is done today in a very easy fashion vi/ith the softvi/are (e.g. SQX, Transpector-Ware) and can be observed directly in the saeen. Naturally, not only the arrangement along the mass axis will be determined here, but also the shape of the lines, i.e. resolution and sensitivity (see Section 4.5). 

ESMS was perfonned with a Fisons VG Quattro outfitted with a Hsons Electrospray Source. Samples were dissolved in 1.0 mL of 50% methanol-1% acetic acid, then diluted 1 10 with 50% acetonitrile-1.0 mM anmumium acetate to give 25 pmol/pL. A 10 pL aliquot of each sample was injected into a 10 pL/min stream of 50% acetonitrile-1.0 mM ammonium acetate. Data was processed using Fisons MassLynx Software. MALDI-MS was performed with a Vestec Benchtop lit linear dme-of-flight mass spectrometer, opmted in the linear mode with an N2 laser (337 nm). Samples were dissolved in 1.0 mL of 25% acetonitrile-0.1% TFA, then diluted 3 100 to give 5-10 pmol/pL. A 0.5 pL aliquot of each sample solution was added to 0.5 pL of matrix [a-cyano-4-hydroxycinnamic aci saturated solution in 50% acetonitrile-2% TFA]. Samples were dried at ambient temperature and pressure. Each spectrum was the sum of ion intensity from 10-50 larer pulses. Tlie mass axis was calibrated externally. 

Periodic calibration of the thermobalance will prevent errors on the mass axis of the recorder. Many investigators calibrate the instrument before each run by adding a known weight to the sample container. 

Mass Spectrometer Tuning. A three-step process was used to tune the quadrupole mass spectrometer prior to its use as a detector for SFC. In the first step, perfluorotributylamine (Pfaltz Bauer Inc., Stamford, Conn.) was ionized by electron ionization and used to calibrate the mass axis. In the second step, methane was introduced into the Cl source and the reagent ion profiles were optimized. In the third step, the mass resolution was adjusted for improved sensitivity. This was accomplished by introducing a volatile brominated compound, such as 2-bromopentane, into the Cl source. The mass spectrometer s resolving power was reduced such that the peaks... 

A reference compound is needed to determine the molecular mass. The mass of an unknown is computed by comparing its signal on the mass axis with that of the known mass reference peaks. A reference compound is also required to calibrate the data system and to tune and performance-check the instrument. A calibration standard has the following desirable characteristics (1) it should yield a sufficient number of regularly spaced abundant ions across the entire scan range (2) those reference ions should have negative mass defects to prevent overlap with the nsnal compounds containing C, H, N, and O and (3) it should be readily available, chemically inert, and sufficiently volatile. 

Apart from the use of uniform (or almost uniform) standards, other methods for determining the BB function have been developed. For example, by assuming a uniform and Gaussian BB function with a linear molar mass calibration, it is possible to use the mass and molar mass chromatograms for simultaneously estimating the standard deviation of the BB function and the calibration coefficients.Alternatively, if the shape of the MMD is known (e.g., it is a Poisson distribution on a linear molar mass axis), then the BB function can be estimated from the difference between the (mass or molar mass) chromatogram and its theoretical prediction in the absence of BB. Finally, the BB function can be theoretically predicted from a representative fractionation model. " Unfortunately, however, this approach is so far unfeasible due to the difficulty in determining the associated physicochemical parameters. 

In the past few years, some advances in mass accuracy have made LC-TOF MS a useful tool for molecular identification of unknown compounds. One of the improvements consists in the design of the double sprayer with reference solutions, which corrects the instrument drift by continuous calibration of the mass axis, with an improved mass accuracy to less than 3 ppm. In conclusion, the advantages with respect to LC-MS (SRM) instruments are (a) a large number of targets can be screened at the same time without loss of sensitivity (b) unknown peaks can be identified on the basis of accurate mass and isotopic profile evaluation and, (c) data... 

This short sununary leads to the conclusion that the mass axis, customarily drawn as the ordinate, is much better defined than the temperature abscissa. As in DTA, careful calibration of temperature is necessary. The most reliable should be a temperature calibration within the equipment under conditions close to the actual measurement conditions. 

A common source of error in ICP-MS is that negative masses are produced for unknown solutions. This occurs when the calibration line crosses the vertical axis at a value significantly above zero (Fig. 9.6(b)), which is often the result of high counts from the instrument blank, usually caused by an uncorrected interference. Any unknown samples which have a CPS below this value will then produce negative masses. One solution to this is to computationally force the line through the origin, or to remove outliers from the calibration line, but it is much better to identify the true cause of the problem (i.e., find the interference, if this is the source), and to rerun the samples. 

So far, the concepts of exact mass, mass accuracy and resolution have been introduced without considering the means by which accurate mass measurements can be realized. The key to this problem is mass calibration. Resolution alone can separate ions of different m/z value, but it does not automatically include the information where on the m/z axis the respective signals precisely are located. 

The magnetic scan is synchronised with the x-axis of a recorder and calibrated to appear as mass number (strictly m/e). The amplified current from the ion collector gives the relative abundance of ions on the y-axis. The signals are usually pre-processed by a computer that assigns a relative abundance of 100% to the strongest peak (base peak). 

Polymer analysis searches for M distribution. In both ThFFF and SEC, this is achieved by the calibration procedure that allows one to transform the retention time axis and the signal axis, respectively, of the fractogram or chromatogram into molar mass distribution [3]. In SEC, calibration has to be executed on each new column and repeatedly checked during its current employment... 

Clusters, produced in a supersonic expansion, are ionized by laser. Delayed pulsed extraction is used to send the ions towards a lm time-of-flight mass spectrometer perpendicular to the jet axis. During the delay time between ionization and extraction, the ions spot size increases, due to their kinetic energy. The result is a broader mass peak with a width that is related to the kinetic energy released after the ionization and can be deduced after calibration of the experiment. 

To obtain the calibration curve, the amount ratio of unlabeled compound/labeled compound (x-axis) is plotted against the ratio of peak area of the mass trace of the unlabeled... 

For polymer systems without UV activity the combination of a RI detector with a density (D) detector can be used. The working principle of the density detector is based on the mechanical oscillator method. Since this detector yields a signal for every polymer, provided that its density is different from the density of the mobile phase, this detector can be regarded as universal [29,30,36]. The separation of mixtures of polystyrene and polybutadiene by SEC with dual den-sity-RI detection is presented in Figs. 7 and 8. In a first set of experiments, the response factors of both polymers in both detectors have to be determined. Then from the intensity of each slice of the elution curves in both detectors, the mass distribution of both polymers across the elution volume axis can be calculated. As can be seen in Fig. 7, a separation into the component peaks is obtained due to the fact that the molar masses of PS and PB are sufficiently different. For both components the individual elution profiles can be determined and using corresponding calibration curves for PS and PB the individual MMDs can be calculated. The same information can be extracted from an experiment where the molar masses of the components are similar and SEC separation does not work (see Fig. 8). Again the individual mass distributions are obtained and the MMDs for PS and PB can be determined. 

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