Calibration compounds

Nevertheless, this method was successfully applied by Gulyaeva et al. for the log P and log D determination of 15 P-sympatholytic drugs [56]. Another study by Welerowicz and Buszewski compared the HpophiHcity values of P-blockers obtained with a column made of a monoHthic-silica Cjg with a conventional porous silica particles Cjg as reference material [27]. A modified method was used for evaluating logP with two main differences (i) logfeg was considered rather than retention times, and (ii) benzene and butyl-benzene were used as calibration compounds. 

For chemical systems of interest, photolysis produces intermediates, such as radicals or biradicals, whose energetics relative to the reactants are unknown. The energetics of the intermediate can be established by comparison of the acoustic wave generated by the non-radiative decay to create the intermediate, producing thermal energy , with that of a reference or calibration compound whose excited-state decay converts the entire photon energy into heat, / (ref). The ratio of acoustic wave amplitudes, a, represents the fraction of the photon energy that is converted into heat. 

We can illustrate the application of PAC to a simple photochemical reaction. Acetone is readily excited to its singlet excited state which rapidly undergoes efficient intersystem crossing to its triplet state. The triplet state decays in solution primarily by radiationless decay. The PAC experimental waveforms obtained from the photoexcitation of acetone in air and argon-saturated cyclohexane are shown in Fig. 1. In addition, the waveform obtained from the calibration compound 2-hydroxybenzophenone is also shown. 

The calibration constant K strongly depends on the instrumental specifications, geometry of the calorimeter (Kd), and the solvent thermoelastic properties (x ) It can be determined through a comparative assay made under the same conditions as the main experiment but using a photoacoustic calibrant instead of the sample compound. The calibrants are substances that have known values of (pm from independent measurements, or more conveniently, substances that dissipate all of the absorbed energy as heat (e/>nr = 1), like ferrocene [286] or ort/w-hydroxybcnzophenone [287]. Note that the computation of K is not really needed, because a direct comparison of the signals obtained with sample and calibration compounds allows it to be eliminated from the calculations. 

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. 

Note The numerous ionization methods and mass analyzers in use have created a demand for a large number of calibration compounds to suit their specific needs. Therefore, mass calibration compounds will occasionally be addressed later in the chapters on ionization methods. It is also not possible to specify a general level of mass accuracy with external calibration. Depending on the type of mass analyzer and on the frequency of recalibration, mass accuracy can be as high as 1 mmu or as low as 0.5 u. 

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. 

Brinded, K.A. Tiller, P.R. Lane, S.J. Triton X-100 As a Reference Compound for Ammonia High-Resolution CI-MS and as a Tuning and Calibration Compound for Thermospray. Rapid Commun. Mass Spectrom. 1993, 7, 1059-1061. 

To obtain absolute concentrations of metabolites, calibration techniques are necessary. For this purpose an external calibration compound of known concentration can be measured to which the metabolite signals are referenced. Another possibility is the use of spectral signals from a tissue compound with known concentration serving as internal reference. 

Table I. Influence of Solvent and Temperature on the Calibration Constant with Benzil as Calibrating Compound...

Table II. Influence of the Molecular Weight of the Calibrating Compound ...

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. 

Briefly, the method involves determining the capacity factors (retention time corrected for an unretained substance) for a suitable set of reference substances (having known K(k values) using RP-HPLC. The relationship between the capacity factors and Kol for the reference or calibration compounds is determined from regression analysis of a log-log plot of the two properties. The capacity factors of compounds having unknown Koc values then are determined using the identical experimental conditions, and Koc values then are calculated from the regression expression. 

This sort of correction should be helpful for improving specific mass measurements for GC/FTMS analyses where it is possible to add a calibrant compound to the mixture, or where some component is known to be present. Unlike the method where a static... 

For linear calibrations, compounds may be quantified using the average response factor or the average calibration factor (Equation 2, Appendix 22). The measure of linearity is the RSD, calculated according to Equation 3, Appendix 22. 

Prepare the syringes two syringes are filled with solvent and the others with sample solutions (lignins or calibrating compound). All are inserted into the head. 

M. Moini, B.L. Jones, R.M. Rogers, L. Jiang, Sodium trifiuoroacetate as a tune/calibration compound for positive- and negative-ion ESI-MS in the mass range of 100-4000Da, J. Am. Soc. Mass Spectrom., 9 (1998) 977. 

The determination of differences in isotopic ratios requires very precise measurements. The combustion step for the sample preparation is usually carried out immediately prior to the injection into the MS. There exist instruments which associate in line a gas chromatograph, a tubular combustion oven, containing copper oxide heated to 800 °C, and a low-resolution MS equipped with several Faraday detectors, each collecting the signal corresponding to a specific mass. A calibration compound is co-injected with the product to be studied. 

Assays requiring an internal calibration depend on the near identical behaviour of the calibration compound and the analyte. This is easily achieved by using an analogue of the analyte which has some protons in its stmcture replaced by deuterons. Provided that the deuterons are in such positions that they are stable to the analytical procedure and not back exchanged for proton, and do not affect the physical properties of the molecule (for instance changing the proton affinity) then the substitution of three or more protons will shift the deuterium labelled response clear from the isotopic peaks of the analyte due to natural and contributions. Such compounds act as excellent internal standards. Suitable labelling also enables the deuterium labelled compound to act as a carrier to improve recovery of the analyte at low levels (the so called stable isotope carrier effect [32]). 

The mass spectrum of calibration compound, FC-43, as shown in Figure 15.15, is well-known to mass spectrometry practitioners because it is seen frequently during... 

Entry of a regulated sample stream, e.g., calibration compound GC interface LC interface... 

The most common use of batch inlets is to introduce a controlled flow of compounds for calibrating the mass scale. Frequently used calibration compounds include perfluorotributylamine (FC-43, heptacosa) and perfluorokerosene (PFK) both are effective with electron ionization (El) but give limited responses in chemical ionization (Cl). If used for calibration in Cl, or to provide lock masses (Section 3.1.1), the concentration of the reagent gas must be reduced. This reducation somewhat compromises the effectiveness of the Cl process. 

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