Mass calibration measurements

Abundances of lUPAC (the International Union of Pure and Applied Chemistry). Their most recent recommendations are tabulated on the inside front fly sheet. From this it is clear that there is still a wide variation in the reliability of the data. The most accurately quoted value is that for fluorine which is known to better than I part in 38 million the least accurate is for boron (1 part in 1500, i.e. 7 parts in [O ). Apart from boron all values are reliable to better than 5 parts in [O and the majority arc reliable to better than I part in 10. For some elements (such as boron) the rather large uncertainty arises not because of experimental error, since the use of mass-spcctrometric measurements has yielded results of very high precision, but because the natural variation in the relative abundance of the 2 isotopes °B and "B results in a range of values of at least 0.003 about the quoted value of 10.811. By contrast, there is no known variation in isotopic abundances for elements such as selenium and osmium, but calibrated mass-spcctrometric data are not available, and the existence of 6 and 7 stable isotopes respectively for these elements makes high precision difficult to obtain they are thus prime candidates for improvement. 

The value of dn/dC depends on the properties of the material, e.g., in case of proteins this coefficient is 0.188 ml/g35 and in case of glucose dissolved in water (used for calibration measurements as discussed later in this chapter) this coefficient is 0.069 ml/g36. In the second mode of operation, analytes bind to the sensor surface (e.g., mediated by a receptor layer). In this case a thin layer with thickness w and refractive index nw is formed by the adsorbed analytes. Because the value of nw (e.g., 1.45 for proteins) is usually different than the refractive index of the solution (e.g., 1.33 for water) that contains the analyte molecules, a phase change is induced. The average layer growth (Aw) on the sensor surface can be related to the mass change (Am) per surface area (A) ... 

Viscometry is used to measure average molar masses too [1-3]. It is an indirect method, since the measured quantity is the intrinsic viscosity (TV), which is related to the average molar masses calibration by a peculiar formula, called the Mark-Huwink-Sakurada equation [1-3]. 

Mass spectrometers measure the masses of positively charged ions striking a detector. This allows quantification of the sample by comparison with standard calibration gases for multicomponent mixtures. Mass spectrometers can analyze a sample point in less than 10 seconds. 

Qi HP, Taylor PDP, Berglund M, DeBievre P (1997b) Calibrated measurements of the isotopic composition and atomic weight of the natural Li isotopic reference material IRMM-016. Int J Mass Sped Ion Proc 171 263-268... 

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. 

Any mass spectrometer requires mass calibration before use. However, the procedures to perform it properly and the number of calibration points needed may largely differ between different types of mass analyzers. Typically, several peaks of well-known m/z values evenly distributed over the mass range of interest are necessary. These are supplied from a well-known mass calibration compound or mass reference compound. Calibration is then performed by recording a mass spectrum of the calibration compound and subsequent correlation of experimental m/z values to the mass reference list. Usually, this conversion of the mass reference list to a calibration is accomplished by the mass spectrometer s data system. Thereby, the mass spectrum is recalibrated by interpolation of the m/z scale between the assigned calibration peaks to obtain the best match. The mass calibration obtained may then be stored in a calibration file and used for future measurements without the presence of a calibration compound. This procedure is termed external mass calibration. 

If high-resolution measurements are performed in order to assign elemental compositions, internal mass calibration is almost always required. The calibration compound can be introduced from a second inlet system or be mixed with the analyte before the analysis. Mixing calibration compounds with the analyte requires some operational skills in order not to suppress the analyte by the reference or vice versa. Therefore, a separate inlet to introduce the calibration compound is advantageous. This can be achieved by introducing volatile standards such as PFK from a reference inlet system in electron ionization, by use of a dual-target probe in fast atom bombardment, or by use of a second sprayer in electrospray ionization. 

Example Cesium iodide is frequently used for mass calibration in fast atom bombardment (FAB) mass spectrometry (Chap. 9) because it yields cluster ions of the general formula [Cs(CsI)n] in positive-ion and [I(CsI)J in negative-ion mode. For the [Cs(CsI)io] cluster ion, m/z 2730.9 is calculated instead of the correct value m/z 2731.00405 by using only one decimal place instead of the exact values Mi33Cs = 132.905447 and M1271 = 126.904468. T e error of 0.104 u is acceptable for LR work, but definitely not acceptable if accurate mass measurements have to be performed. 

Fig. 6.34. LR- (a) and HR-EI (b) mass spectra of 2-(l-methylpropyl)-phenol. The elemental compositions as obtained from accurate mass measurement are directly attached to the corresponding peaks. Peaks with small-lettered labels belong to PFK and residual air used for internal mass calibration (Chap. 3.3).

Note PEGS ranging from PEG 300 to PEG 2000 are often used for mass calibration. They are particularly useful as internal reference (Chap. 3.3.5) for accurate mass measurements in positive-ion FAB-MS. 

An easy calibration strategy is possible in ICP-MS (in analogy to optical emission spectroscopy with an inductively coupled plasma source, ICP-OES) because aqueous standard solutions with well known analyte concentrations can be measured in a short time with good precision. Normally, internal standardization is applied in this calibration procedure, where an internal standard element of the same concentration is added to the standard solutions, the samples and the blank solution. The analytical procedure can then be optimized using the internal standard element. The internal standard element is commonly applied in ICP-MS and LA-ICP-MS to account for plasma instabilities, changes in sample transport, short and long term drifts of separation fields of the mass analyzer and other aspects which would lead to errors during mass spectrometric measurements. 

As a calibration procedure in ICP-MS via calibration curves, external calibration is usually applied whereby the blank solution is measured followed by a set of standard solutions with different analyte concentrations (at least three, and it is better to analyze more standard solutions in the same concentration range compared to the sample). After the mass spectrometric measurements of standard solutions, the calibration curve is created as a plot of ion intensities of analyte measured as a function of its concentration, and the linear regression line and the regression coefficient are calculated. As an example of an external calibration, the calibration curve of 239 Pu+ measured by ICP-SFMS with a shielded torch in the pgC1 range is illustrated in Figure 6.15. A regression... 

Mass Calibration The process by which the mass analyzer is calibrated such that a measured and displayed m/z is accurate. Well-characterized calibration compounds are utilized, and measured m/z values for these compounds are compared to theoretical m/z values. Calibrants commonly used include various polymeric species (such as polypropylene glyol, or PPGs poly tyrosine (poly-t)) or fluorinated species (perfluorokerosene or PFK) but can be any compound or mixture (Nal/KI) of compounds properly characterized for MS. 

Lock Mass Similar to internal calibration. The lock mass compound is monitored during analysis of the unknown, and the mass calibration is adjusted based on the comparison of the measured m/z and the theoretical m/z for the lock mass compound. If multiple lock mass compounds are used across the m/z range, the process effectively becomes internal calibration. Lock mass compound(s) can be introduced into the LC-MS source via a tee into the LC flow or sheath liquid inlet or dedicated sprayer. 

The LTQ-Orbitrap has resolution and mass accuracy performance close to that of the LTQ-FTICR. As shown in Table 5.3 (column 4), LTQ-Orbitrap accurate mass measurements, using external calibration, for a set of 30 pharmaceutical compounds resulted in less than 2.3 ppm error. The data were acquired with a 4-min, 1-mL/min-flow-rate, positive-mode LC-ESI-MS method where all measurements were performed within 5h from mass calibration. Mass accuracies below 2-3 ppm, and often below 1 ppm, can be routinely achieved in both the positive- and negative-ion mode (Table 5.3, columns 4 and 5). The long-term mass stability of the LTQ-Orbitrap is not as consistent as observed for the LTQ-FTICR-MS, and the Orbitrap requires more frequent mass calibration however, mass calibration is a routine procedure that can be accomplished within 5-10 min. Figure 5.7 displays a 70-h (external calibration) mass accuracy plot for three negative ions collected with a LTQ-Orbitrap where the observed accuracy is 2.5 ppm or better with little mass drift for each ion. Overall, for routine accurate mass measurements on the Orbitrap, once-a-week calibration (for the desired polarity) is required however, considering the ease of the process, more frequent external calibration is not a burden. 

Because frequency can be so precisely measured, the exact mass of an ion can be determined very accurately in the FTMS experiment (8). Typically, low parts-per-million accuracy can be achieved in the presence or even in the absence of an internal mass calibrant (13). In addition, a high degree of mass accuracy can be maintained for days without recalibration provided that the magnetic field remains stable. More detailed information on the theory of FTMS (1, 16, 28, 31-33) and the principles of Fourier transforms applied to spectroscopic techniques (9, 34) may be found in the literature. 

The mass calibration law gives mass measurement accuracy of ca. 2 ppm and a precision of ca. 1 ppm. Errors are still systematic, but the measurements are much less sensitive to space charge effects than those made with the cubic cell. One possible cause of the systematic errors may be magnetic field inhomogeneity caused by... 

Using a horizontal 4.7 Tesla cryoshimmed supercon magnet with a bore diameter of 15 cm, we have obtained two-parameter mass calibrations with absolute mass accuracies of better than 1.5 ppm without, and better than 0.4 ppm with an internal calibrant over a mass range from 18 to 502 amu (10). Absolute accuracies with an internal calibrant are slightly better because the calibrant and the unknown compound are measured under identical physical conditions, in particular with the exact same number of ions in the cell, resulting in identical space-charge shifts (2). 

Figure 30 displays a spectrum of lab air, neon, and argon. The Ar peak represents the leakage from the plasma chamber and thus can be used for mass calibration. The measured resolution ml Am at full width at half maximum (FWHM) is 10.8, and 4.3... 

Argyle et al. (2003, 2004) introduced a method to determine the average valence of Al203-supported VOx species under the conditions of propane ODH catalysis. First, calibration measurements were made the catalyst was reduced for various periods of time in H2 at 603 K, and then the amount of 02 required to fully restore the UV-vis spectra was measured by mass spectrometry. Spectra of the fully oxidized sample were recorded to generate the background. These relative reflectance spectra were converted by applying the Kubelka-Munk function and then the intensity in the range 1.5-1.9 eV was related to the extent of... 

Another interesting characteristic of the TOF analyser lies in its easy mass calibration with only two reference points. As in all the mass spectrometers, the TOF mass spectrometer requires a calibration equation to relate and convert the physical property that is measured to a mass value. For the TOF spectrometer, the physical property that is measured during an analysis is the flight time of the ions. As already mentioned, the flight time of an ion is related to its mass by the following equation ... 

The mass determination of ionic species (atomic or polyatomic ions) in mass spectrometry is always a comparative measurement, which means the mass of an ionic species is determined with respect to reference masses of elements (or substances) used for mass calibration. The reference mass is thus acquired from the mass unit (m = In = 1/12) of the mass of the neutral carbon isotope (m = 1.66 X 10 kg). A mass calibration is easy to perform in solid-state mass spectrometry if the sample contains carbon (using carbon cluster ions with whole masses, as discussed above). The so-called doublet method was apphed formerly, e.g., ions and doubly charged Mg + forming a doublet at the same nominal mass number 12 were considered, where they are slightly displaced with respect to one another. The doublet method is no longer of relevance in modern inorganic mass spectrometry. Orientation in the mass spectra can be carried out via the matrix, minor and trace elements after mass calibration and by comparing the measured isotopic pattern of elements with theoretical values. 

An internal mass calibration is generally needed to achieve mass measurement accuracy of 5 to lOppm with a Q-TOF MS analysis [62-64]. Internal calibration is based on mixing one or several internal standards or calibrants of known molecular weight with the analyte and then using the known masses to calibrate the mass measurements of unknowns that coexist in the sample mixture. 

Due to alterations in sample position and lag times in ion desorption when membranes are employed for MALDI analysis, the standards used for mass calibration must also be desorbed from the membrane [3,4]. The shot-to-shot reproducibility and accuracy of calibration was investigated by first calibrating the instrument with BSA blotted onto PE, then applying that calibration to other membrane-bound preparations of BSA on the multiple sample stage. Replicate measurements, each the sum of 20 laser shots, were typically within 98 Da (0.15%) standard deviation of each other and routinely within 26 Da (0.04%) of the accepted mass (n=8). 

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