Valley, definition

To use the formula in Equation 38.2, it is necessary to define at what stage the two peaks representing the two masses are actually separate (Figure 38.4). The depth of the valley between the two peaks serves this purpose, with valley definitions of 5, 10, or 50%. A 5% valley definition is a much stricter criterion of separation efficiency than the 50% definition. 

Resolution 10% valley definition, m/Am. Let two peaks of equal height in a mass spectrum at masses m and m/Am be separated by a valley that at its lowest point is just 10% of the height of either peak. For similar peaks at a mass exceeding m, let the height of the valley at its lowest point be more (by any amount) than 10% of either peak height. Then the resolution (10% valley definition) is m/Am. It is usually a function of m therefore, m/Am should be given for a number of values of m. 

Resolution (10% valley definition) Let two peaks of equal height in a mass spectrum at masses m and (m - Am) be separated by a valley that at its lowest point is 10% of the height of the peaks. 

Comment See comment for Ten percent valley definition above. 

Two neighboring peaks are assumed to be sufficiently separated when the valley separating their maxima has decreased to 10 % of their intensity. Hence, this is known as 10 % valley definition of resolution, Rio%- The 10 % valley conditions are fulfilled if the peak width at 5 % relative height equals the mass difference of the corresponding ions, because then the 5 % contribution of each peak to the same point of the m/z axis adds up to 10 % (Fig. 3.16). 

Mass spectrometry. (+)Fast atom bombardment (FAB) mass spectrometry was carried out with a JEOL JMS-SX/SX102A mass spectrometer. Dried samples were dissolved in methanol-water, mixed with (thio-) glycerol, and applied to a direct insertion probe. During the high resolution FAB-MS measurements, a resolving power of 10,000 (10% valley definition) was used. Cesium iodide, glycerol, or polyethylene oxide (MWav = 600) was used to calibrate the mass spectrometer. 

Figure 3.15 Demonstration of two definitions of mass resolution (R = m/4m) 10 % valley definition and peak width definition.

FWHM Full width at half-maximum. Mass resolution is often difficult to determine at or near the base of a peak due to baseline noise and peak overlap. It is more common to measure the width of the peak halfway to the peak maximum, where a clean measurement is possible. The most common alternative to FWHM was the 10% valley definition, in which the peak width at 10% of height was examined. This latter definition is common in the literature, especially for magnetic sector mass spectrometers, but is currently used much less frequently than FWHM. The choice of FWHM or 10% valley has an impact on the calculation of resolving power. 

The quadrupole/time-of-flight analyzer (QToF) has become a key option in the qualitative and quantitative analytical arena. Instruments with resolving power of 20000 (50% valley definition) can provide <5ppm mass accuracy for parent and product ion identification and for 20mda mass selection windows quantitation. While the triple quadra-pole retains the lead in sensitivity for quantification, the QToF has a decided edge on specificity (Micromass, 1999) and qualitative analysis. 

Figure 15.5. Resolution can be calculated using different definitions. The more stringent of these is the 10% valley definition, which is more common in magnetic sector MS.

A more convenient definition, commonly used with trapping and ToF analyzers, pertains to a single well-resolved peak where AMX is the peak width at a specified height x, usually half-maximum height (full-width at half-maximum height, FWHM) (Figure 5(b)). It should be noted that this FWHM definition of resolution equates to about twice that calculated from the 10% valley definition. 

From the theoretical point of view, the resolution parameters can be described as shown in Fig. 2.1. It follows that a relevant parameter is the valley existing between the two peaks. Usually, resolution data are related to 10% valley definition. 

The resolution present in different mass analyzers can be affected by different parameters and different definitions can be employed. Thus, in the case of a magnetic sector instrument the above 10% valley definition is usually employed, while in the case of a quadrupole mass filter the operating conditions are such to keep the AM constant through the entire mass range. Consequently, in the case of a quadrupole mass filter the resolution will be 1000 at m/z 1000 and 100 at mJz 100, while in the case of the magnetic sector the resolution will be, for example, 1000 at m/z 1000 and 10,000 at m/z 100. This parameter will be useful to evaluate and compare the performances of different mass analyzers. 

The sensitivity of this SFC-MS combination was evaluated with two standards methyl arachidate (326 Da) and tristearin (890 Da). The instrximent was tuned to "unit" resolution (10% valley definition) before the sensitivity measurements were made. Splitless injection of 800 pg of methyl arachidate produced a mass chromatogram of m/z 344, [M+NH4] , with a S/N of approximately 4. The scan range was m/z 100 to 500 with a 1 s scan cycle time. Splitless injection of 12 ng of tristearin produced a mass chromatogram of m/z 908,... 

Generally, the higher the resolution the better is the separation. But when are two peaks considered as being separated This is a question of definition and depends on the analyser. For TOF, Am is deflned as the full width at half maximum (FWHM), i.e. the width of the peak at half its height (Fig. 4.5). With this definition, it is possible to read Rs out of a single peak. Typical resolutions obtained for TOF instruments are Rs = 15,000 (FWHM). For other mass analysers, other definitions like the 70 % valley or 50 % valley are used (Fig. 4.6). For the 50 % valley definition, two peaks are considered separated if the minimum between them (the valley) is not more than 50 % of the peak height whereas for the 10% valley, the minimum between two peaks must not be more than 10% of the peak height. 

To obtain these data the FD ion currents were recorded on a homebuilt single focusing mass spectrometer of low resolution equipped with a FD source with micromanipulator . The FD ion source employed is schematically described in Fig. 12. The FD emitter (at + 8 kV) is positioned at a distance of 2 mm from the counter electrode (at — 4 kV). Only the first lens (at approximately + 2 kV) is used for the focusing of the ion beam whereas all other reflection plates are at ground potential. With an entrance slit width of 0.1 mm and an exit slit width of 0.5 mm, a resolution of about 300 (10% valley definition) is achieved. This experimental set-up considerably simplifies the operation of the FD mass spectrometer because it allows... 

A rapid sampling method for direct aerosol analysis using FD-mass spectrometer has been described . In a predominantly inorganic aerosol three alkali metal cations and a number of cluster ions of intact salts have been identified under the conditions of high mass resolution (R 15,000, 10 per cent valley definition) using photographic registration. 

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