High-Resolution Mass Analyzers

Reconstructions of pathways of deep-water masses in the North Atlantic during the last 60,000 years have been performed by analyzing high resolution records of benthic foraminifera Cibicides wuellerstorfi as this species best reflects changes in the chemistry of bottom waters (Duplessy et al. 1988 Samtheim et al. 2001). The initial 5 C-signature of North Atlantic Deep Water (NADW) is 1.3-1.5%c. As... 

Positive ions are obtained from a sample by placing it in contact with the filament, which can be done by directing a gas or vapor over the hot filament but usually the sample is placed directly onto a cold filament, which is then inserted into the instrument and heated. The positive ions are accelerated from the filament by a negative electrode and then passed into a mass analyzer, where their m/z values are measured (Figure 7.1). The use of a suppressor grid in the ion source assembly reduces background ion effects to a very low level. Many types of mass analyzer could be used, but since very high resolutions are normally not needed and the masses involved are quite low, the mass analyzer can be a simple quadrupole. 

Commercial mass analyzers are based almost entirely on quadrupoles, magnetic sectors (with or without an added electric sector for high-resolution work), and time-of-flight (TOE) configurations or a combination of these. There are also ion traps and ion cyclotron resonance instruments. These are discussed as single use and combined (hybrid) use. 

Peaks are analyzed separately by their retention times, absorption, and fluorescence properties. RCCs show absorbance maxima near A.500 and 316 nm. For FCCs, UV-Vis specna show two prominent bands near 361 and 320 mn and a luminescence maximum at 436 mn and NCCs show UV-Vis spectra with absorbance maxima near 320 and 210 nm. Nevertheless, as none of these approaches is suitable for elucidating structures, it is necessary to apply additional MS and NMR analyses to fully characterize snuctural features. Electron spray ionization (ESI) and high-resolution EAB mass spectroscopy have been applied to elucidate the molecular formulae of colorless compounds. ... 

The nature of water residue analysis has changed dramatically over the past 60 years. Advances in SPE media have made the isolation of hard to extract residues more attainable. The use of specific and sensitive instrumentation as in HPLC/MS/MS has even precluded the need for extraction in many cases, since the samples can be directly analyzed as they are received from the Held. The future holds even more improvements in sensitivity with rugged new API interfaces and high-resolution mass spectrometers that will dramatically increase the specificity of detection and ease of analysis. 

The ability to detect small genetic changes becomes more difficult as mass increases. There is further an upper mass range where analysis is impractical. For low-resolution instruments this limit is around a 100 mer. Thus the mass has to be minimized or a high-resolution instrument employed. Alternatively, the smaller the piece of DNA analyzed, the more it chemically resembles a primer or nucleotide monomer thus separation of the two during cleanup is difficult to do. If the primers and nucleotides are not removed, they can provide a massive background on MS analysis or inhibit ionization of the PCR product by preferential ionization. Thus for practical reasons it is extremely difficult to employ a PCR product below a 40 to 50mer for direct ESI MS or ESI MS-MS analysis. 

Mass spectrometry has become one of the most important tools for analyzing proteins in complex biological samples. The ability to separate proteins and peptides in high resolution has made possible the simultaneous identification of hundreds of proteins within samples (for reviews, see Gingras et al., 2005 Hamdan and Righetti, 2005 Siuzdak, 2006). Proteins can be analyzed for their presence or compared between samples for their relative expression level. One cell population treated with a drug candidate, for instance, can be compared by mass spec to another sample as control to assess the affect of the drug on expression levels of certain proteins. 

For high-throughput analysis, it is important to increase the specihcity of each bioanalytical method. The enhancement of chromatographic resolution presents various limitations. Better selectivity can be obtained with TOF mass analyzers that routinely provide more than 5000 resolution (full width at half-mass or FWHM). The enhanced selectivity of a TOF MS is very attractive for problems such as matrix suppression and metabolite interference. In one report of quantitative analysis using SRM, TOF appeared less sensitive than triple quadrupole methods but exhibited comparable dynamic range with acceptable precision and accuracy.102... 

A systematic investigation of the free amino acids of the Leguminosae led to the isolation of a novel ninhydrin-positive compound from the leaves of Derris elliptica Benth. (Papilionidae) (93). This substance was analyzed as C6H,3N04 (microanalysis and high resolution mass spectrometry) and was shown to be an amino alcohol. The absence of a carbonyl in the 1R, the loss of 31 mass units in the mass spectrum, and a positive periodate cleavage reaction were best embodied into a dihydroxydihydroxymethylpyrrolidine structure. The relative simplicity of the NMR spectra (three peaks in the 13C spectrum four spin-system in the H spectrum) pointed out a symmetrical structure. Inasmuch as the material was optically active ([a]D 56.4, c = 7, H20), meso structures were ruled out, and the 2R, 3R, 4R, 5R relative configuration was retained (93). This structure (53) was further confirmed by an X-ray determination (94). 

The speed of the analyzer highly depends on the mode of operation. The scan speed affects both the resolution and the mass accuracy, so if high quality data is needed, a... 

The FTICR analyzer is relatively slow. In a low resolution mode (<25,000 FWHM) scans can be performed in substantially less than a second. A high resolution scan is more time demanding, and more than 1 s is often required for mass resolving powers of 100,000 or more. 

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