Exponential mass calibration

Figure 2.244 Exponential mass calibration of the magnetic sector analyser.

A low-resolution mass measurement is a simple procedure and can be performed with most of the mass spectrometric systems discussed in Chapter 3. The instrument is set up at a resolving power (RP) above 1000. At this low resolving power, the molecular ions of most organic compounds that differ by a unit mass are well separated. The mass spectrometer is first mass-calibrated with an external calibration procedure. During the calibration scan, the computer stores the peak centroid (the center of gravity) time and the area of each peak. The mass of the ion is related exponentially to the peak centroid time ... 

What is more important, this calibration model is not derived from either the exponential or Russell s laws as is commonly perceived (and originally presented) [15, 32], Rather, it only requires the mass spectrometer response be linear [Eqs. (5.27) and (5.28)]. It is the interpretation of the slope and the intercept that can lead to the reliance on the exponential mass bias correction or even erroneous results. Consider, for example, the substitution of Eq. (5.34) in Eq. (5.32) ... 

Like the traditional mass bias correction approaches, the double-spike method also relies on the choice of the mass bias model. The original formalism of the double spikes employed the linear mass bias law and, although double-spike calibration equations adapted for the exponential mass bias discrimination are available, linear models are still often used owing to their simplicity (see, for example, [50-52]). The caveat here is that erroneous results can be obtained when a linear correction is applied to data that do not follow such behavior. This is illustrated below. 

Calibration of the measured isotope amount ratio, 787/86, is achieved by admixing known amounts of strontium that is enriched in Sr and Sr [24-27]. The isotopic composition of the double-spike strontium has to be known. Matrix matching does not have to be achieved per se. If the amount ratio of the non-radiogenic strontium isotopes, say N( Sr)/N( Sr), is well known and does not vary significantly in Nature, it can be used to calibrate the measured ratio 787/86 without any admixing of the double spike [28]. In both of its variations, this procedure involves the selection of an appropriate mass bias correction model, such as the exponential law (see below). 

Those users of LC/MS with some experience in mass spectrometry tend to polish the ion source and ion optics before introducing an important series of samples. Of course, maximum signal intensities in API LC/MS ionization techniques will be achieved when all residues are completely eliminated from the ion source surfaces. The electrical fields that are necessary to form and/or transport the ionic analytes before they reach the mass analyzer are influenced by the degree of contamination with non-volatile sample constituents. On the other hand, experienced LC/MS users know that the long-term stability of an LC/MS system will be increased if the spray chamber is intentionally slightly contaminated. This is of particular importance when one wishes or has to quantify by means of external calibration. With the most commonly used ion sources it is sufficient to inject the sample matrix 10-20 times in order to achieve an almost constant analyte response after an initial exponential drop. 

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