Resolution: unit

Bidirectional transmission. Configuration parameters such as span, engineering units, resolution, and so on, must be communicated to the smart transmitter. 

Resolution Quadrupole instruments are not capable of achieving the high resolution that is common with double-focusing magnetic-sector instruments. In GC/MS analyses, a compromise is struck between sensitivity (ion transmission) and mass resolution. In the quadrupole instrument, the resolution is set to the lowest possible value commensurate with resolving peaks differing by 1 Dalton (unit resolution). 

Rapid scanning mass spectrometers providing unit resolution are routinely used as chroaatographic detectors. Ion separation is accomplished using either a magnetic sector, quadrupole filter or ion trap device. Ions can also be separated by time-of-flight or ion cyclotron resonance mass analyzers but these devices are not widely used with chromatograidiic inlets and will not be discussed here [20]. 

Most mass spectrometers will resolve ions with unit resolution up to at least 2000 Da, and so monoisotopic atomic masses are used in these cases. Above 2000 Da, the resolution should be checked and, if it is insufficient to resolve adjacent isotopes, then average atomic masses can be used in calculations. 

MS11 capabilities. However, ions may then subsequently be detected at unit resolution using an electron multiplier or, alternatively, focused in a C-Trap (Figure 5.2) and then transferred and detected at high resolution using the Orbitrap. In our experience with the LTQ-Orbitrap, ions may be measured with a resolution of approximately 60,000 with online LC/MS in the full scan mode. 

Quadrupole analyzers generally are operated at so-called unit resolution normally restricting their use to typical low resolution (LR) applications. [111,112] At unit resolution adjacent peaks are just separated from each other over the entire m/z range, i.e., R = 20 at m/z 20, R = 200 at m/z 200, and R = 2000 at m/z 2000 (Fig. 4.37). 

The resolution as adjusted by the U/V ratio cannot arbitrarily be increased, but is ultimately limited by the mechanical accuracy with which the rods are constructed and supported ( 10pm). [Ill] Above an m/z value characteristic of each quadrupole assembly, any further improvement of resolution can only be achieved at the cost of significantly reduced transmission. High-performance quadrupoles allowing for about 10-fold unit resolution have only recently been developed. [112]... 

Fig. 4.37. El mass spectrum of perfluorotributylamine (mass calibrant FC43) to demonstrate unit resolution of a quadmpole analyzer. The expanded views a-c show peaks separated to almost identical degree.

With external ion sources it became feasible to interface any ionization method to the QIT mass analyzer. [171] However, commercial QITs are chiefly offered for two fields of applications i) GC-MS systems with El and Cl, because they are either inexpensive or capable of MS/MS to improve selectivity of the analysis (Chap. 12) and ii) instruments equipped with atmospheric pressure ionization (API) methods (Chap. 11) offering higher mass range, and some 5-fold unit resolution to resolve isotopic patterns of multiply charged ions (Fig. 4.47). [149,162,172,173]... 

The overall separation potential of an electromigration technique can be expressed by the peak capacity ( ), which is defined as the maximum number of peaks that can be separated within a given separation time, usually coincident with the time interval between the first and last detected peak in the electropherogram, while retaining unit resolution for all adjacent peak pairs ... 

Fractionating power makes it possible to establish the relative increment in particle diameter or mass that can be separated with unit resolution. It can be demonstrated (by substituting Equations 12.14, 12.20, and 12.21b or c into Equation 12.22a or b, respectively) that F (or F ) can also by expressed by... 

Often an expression such as unit resolution with 15% valley is used. This means that the bottom of the valley between two adjacent peaks of identical height comes to 15 % of the height of the peak or, put another way, at 7.5 % of its peak height the line width DM measured across an individual peak equals 1 amu (atomic mass unit) see in this context the schematic drawing in Fig. 4.10. 

Fig. 3 Upper traces, apparent peak absorptance vs AxDop/Axres, the Doppler width per unit resolution. Each trace is identified by the actual peak absorptance. Lower traces, percentage error incurred when (Ax op + Axr2es)1/2 is used to approximate Axobs for an absorption line vs A cDop/Ajcres. The curves are labeled with the appropriate equivalent width per unit Doppler width as EQW/DOPW.

In this chapter we focus primarily on calibration of LC-MS where the mass spectrometer is operating at unit resolution, resolution that is sufficient to separate two peaks one mass unit apart. This kind of low-resolution mass filter covers almost 90 percent of the instruments commonly used for qualitative and/or quantitative analysis of small molecules. Batch-to-batch qualification testing of the instrument is also described. For the calibration of high-resolution mass spectrometers such as magnetic sector, TOF, or FTICR coupled with liquid chromatography, readers are referred to specific publications. 

Peak Width. Peak width depends on the mass resolution. A resolution of 1 mass unit is sufficient to distinguish ions in most qualitative/quantitative small molecule applications. A typical definition of unit resolution is when the peak width at half-height is about 0.6 to 0.8 mass unit. The profile scan of ions on a typical benchtop LC-MS has a bandwidth of about 1 mass unit (Figure 13.1). 

It is usually desirable to have a consistent peak width over the entire mass range. If the dc is held constant, the peak width varies over the mass range and increases as the mass increases (Figure 13.7). Adjusting the slope of the operating line increase the resolution. The resolution normally obtained is not sufficient to deduce the elemental analysis. Usually, quadrupole mass spectrometers are low-resolution instruments and operate at unit resolution. 

Figure 13.8. Loss of resolution at high masses in a quadrupole mass filter (a) unit resolution (b) low resolution.

There are two important categories of magnetic-deflection mass spectrometers low (unit) resolution and high resolution. Low-resolution instruments can be defined arbitrarily as the instruments that separate unit masses up to m/z 2000[R = 2000/(2000 — 1999) = 2000]. A high-resolution instrument with R = 20,000 can distinguish between C16H2602 and C15H24N02 ... 

In general, the quadrupole instruments do not achieve the mass range and the high resolution of sector instruments. However, the mass range and resolution are adequate for unit-resolution mass spectrometry, and the rapid scan and sensitivity make them especially suitable for use with capillary gas chromatography (Fig. 2.46). 

So far, we have discussed the mass spectrum in terms of unit resolutions The unit mass of the molecular ion of CyHyNO (Fig. 2.1) is m/z 121—that is, the sum of the unit masses of the most abundant isotopes ... 

For most of the Problems in this text, the unit-resolution molecular ion, used in conjunction with IR and NMR, will suffice for determining the molecular formula by browsing in Appendix A. For several more difficult Problems, the high-resolution formula masses— for use with Appendix A (see Section 2.4.2)—have been supplied. 

Resolving Power (RP) A measurement of how effectively a mass analyzer can distinguish between two peaks at different, but similar m/z. Mathematically, the formula M/ AM is used, where M is the m/z value for one of the peaks and AM is the spacing, in unified atomic mass units, between the peaks. Most commonly, AM is the mass resolution, either via the 10% valley or FWHM definitions (see below). (Note that the definition used will affect the resolving power calculated.) Resolving power of 500-1000 approximately corresponds to unit resolution (e.g., at m/z 700 and FWHM resolution of 0.7, RP = 1000). 

Unit Resolution Setting the resolution to produce a peak 1 mass unit wide at the base. For a Gaussian-shaped peak, the FWHM width for unit resolution is about 0.7 u. 

Transmitted parent ions Q1 at unit resolution Q3 at enhanced resolution... 

Figure 1.12. Calibration curves lor loratadine (SCH 29851) and desloratadine (SCH 34117) obtained under unit- and enhanced-resolution conditions. The precision and accuracy under both unit- and enhanced-resolution conditions met the assay acceptance criteria, correlation coefficients at enhanced resolution (0.993) were lower than those obtained at unit resolution (0.999). The lower correlation coefficients under enhanced-resolution conditions might have resulted from a slight mass window shift during the long overnight 17-h run. (Reprinted with permission from Yang et al., 2002.)...

---