Resolving-power requirements

The requirements for the analysis of the iso-a-acids are diverse and, to a large extent, dictate the resolving power required of the chromatography. The requirement to separate cis/trans pairs of the iso-a-acids is often driven by research projects. Hughes (16) used a multicomponent mobile phase and a NovaPak C18 cartridge to obtain resolution of the five iso-a-acids in beer within 7 minutes (Fig. 3). The separation of cw-isohumulone (often the major iso-a-acid in beer) from frans-isoadhumulone (most minor of the six iso-a-acids) usually proves to be difficult. Other workers have successfully resolved the six compounds, and micro HPLC has proved particularly effective (34). 

Resolving-power requirements at m/e 300 are approximately 10,GOO-17,000 for hydrocarbon/oxygen compound pairs, 37,000 for nitrogen/ hydrocarbon isotope pairs, 88,000 for hydrocarbon/sulfur compound pairs, etc. These requirements increase linearly with increasing molecular weight and are twice as large at m/e 600 than at m/e 300. 

Table I. Resolving Power Required to Separate Members of Possible Multiplets at m/e 300 and 301...

With respect to mass spectral matching, the criteria for identification vary depending on the technique used for mass spectral data acquisition (see summary of requirements in Table 8). It is interesting to note that while the FDA does not rule out the use of exact mass measurements, it views these data as problematical as there are no generally accepted specific standards for their use. The problem here is that it is difficult to be definitive about the resolving power required, particularly, when analytes have masses greater than m/z 500. Clearly the resolving power and accuracy must be sufficient to exclude all reasonable alternative elemental compositions and they recommend that if exact mass measurements are to be used then multiple structurally specific ions should be measured. 

Analysis in flowing solutions, as performed in particular with high performance liquid chromatography (HPLC) and flow injection analysis, (FIA) has developed rapidly over the last decade and now plays an important function in most analytical laboratories throughout the world. There is little doubt, however, that even HPLC lacks the resolving power required to solve analytical problems in complex matrices with minimal sample preparation. Often, the resolving power of the detection method is called upon to assist in the solution of these problems. This is particularly true with electrochemical detection (ED) systems which offer a certain degree of selectivity based on differences in oxidation or reduction potentials of the species to be determined. In recent years, the advent of chemically modified electrodes (CMEs) has provided a stimulus to further improve both the sensitivity and selectivity of ED systems used in HPLC and FIA. 

Interference filters provide the best wavelength selectivity of any filters available. It is not possible to provide the necessary resolving power required for more complex spectral isolation. The filters therefore are primarily useful for simple systems where passage of a spectral band will meet spectral isolation requirements. 

Example 3.1 Calculate the resolving power required to resolve two singly charged ions of elemental compositions C3H7COOH and C3H7CONH3. 

Calculate the resolving power required to separate the molecular ions of (a) CsHeS and C5H12D, (b) C12H24 and C6H4N2O4, and (c) CigHigO and C18H20N. 

When dealing with the analysis of low analyte levels in complex matrices, the resolution of the MS detector is a key parameter for consistent and reliable mass assignment [64,65]. Thus, the mass resolving power required to prevent false negative results in LC—MS residue screening methods depends on factors such as the anal)he concentration, type of matrix, and sample preparation [66]. 

Note The resolving power required to fully resolve isotopic patterns is independent of the charge state of the ion. The minimum numerical value of R is always equal to the ion s mass number. 

As has been pointed out previously, HRMS can be used for the characterization and quantification of PCTs and toxaphene, but its application to routine analysis is difficult because of the high resolving power required, and the high cost of the instrumentation. EI-MS MS can be an excellent alternative to HRMS because of its high sensitivity and its lower cost. 

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