High-Resolution and Accurate Mass

High-resolution and accurate mass measurements are closely related to each other because the obtainable mass accuracy also depends on sufficiently resolved peaks. Nevertheless, they should not be confused, as performing a measurement at high resolution alone does not equally imply measuring the accurate mass. 

---

Notwithstanding the efforts of groups attempting to precisely define acceptable practices and requirements, [20] confusion still exists on the differences between definition of the terms high resolution and accurate mass. [21] High resolution, as in any context, implies the ability to make confident measurements of small differences. In mass spectrometry, high resolution implies the ability to measure the small differences deriving from different elemental compositions, which have the... 

Discuss the difference in meanings of the terms high resolution and accurate mass . 

In order to successfully interpret a mass spectrum, we have to know about the isotopic masses and their relation to the atomic weights of the elements, about isotopic abundances and the isotopic patterns resulting therefrom and finally, about high-resolution and accurate mass measurements. These issues are closely related to each other, offer a wealth of analytical information, and are valid for any type of mass spectrometer and any ionization method employed. (The kinetic aspect of isotopic substitution are discussed in Chap. 2.9.)... 

With few exceptions, magnetic sector instruments are comparatively large devices capable of high resolution and accurate mass determination, and suited for a wide variety of ionization methods. Double-focusing sector instruments are the choice of MS laboratories with a large chemical diversity of samples. In recent years, there is a tendency to substitute these machines by TOE or by Fourier transform ion cyclotron resonance (FT-ICR) instruments. 

Define the tasks the mass spectrometer is to be acquired for i) Need for one or several ionization methods ii) GC-MS or LC-MS required iii) High resolution and accurate mass desirable iv) Is tandem MS an issue v) What sensitivity is needed vi) Is quantitative work important ... 

Cl in conjunction with a direct exposure probe is known as desorption chemical ionization (DCI). [30,89,90] In DCI, the analyte is applied from solution or suspension to the outside of a thin resistively heated wire loop or coil. Then, the analyte is directly exposed to the reagent gas plasma while being rapidly heated at rates of several hundred °C s and to temperatures up to about 1500 °C (Chap. 5.3.2 and Fig. 5.16). The actual shape of the wire, the method how exactly the sample is applied to it, and the heating rate are of importance for the analytical result. [91,92] The rapid heating of the sample plays an important role in promoting molecular species rather than pyrolysis products. [93] A laser can be used to effect extremely fast evaporation from the probe prior to CL [94] In case of nonavailability of a dedicated DCI probe, a field emitter on a field desorption probe (Chap. 8) might serve as a replacement. [30,95] Different from desorption electron ionization (DEI), DCI plays an important role. [92] DCI can be employed to detect arsenic compounds present in the marine and terrestrial environment [96], to determine the sequence distribution of P-hydroxyalkanoate units in bacterial copolyesters [97], to identify additives in polymer extracts [98] and more. [99] Provided appropriate experimental setup, high resolution and accurate mass measurements can also be achieved in DCI mode. [100]... 

Chapter 2 of this book describes the mass spectra produced by El ionization and how the spectral peak pattern relates to molecular structure. Chapter 3 describes in detail the use of high-resolution and accurate mass measurement to determine elemental compositions of ions as an aid to identifying unknown substances. Chapter 4 provides an introduction to some of the more modern techniques used to extend the usefulness of mass spectrometry into high molecular weight and more polar substances. 

The orbitrap analyzer itself does not provide for a mode of tandem MS operation. Instead, the steps of precursor ion isolation and dissociation are performed in a dedicated LIT as MSI (Chap. 4.8.4) prior to high-resolution and accurate mass analysis of the fragments by the orbitrap. Multiple-collision CID (Chap. 9.3) in the LIT of the LTQ-orbitrap instrument is sometimes not hard enough to achieve fragmentation of comparatively stable precursor ions, e.g., it can be insufficient to generate immonium ion fragments from protonated peptides. 

Double-focusing magnetic sector instruments represent the standard in FAB-MS, because they combine a suitable mass range with the ability to perform high-resolution and accurate mass measurements. Until the advent of ESI and MALDI, FAB-MS/MS on magnetic four-sector instruments was the method of choice for... 

One must also accept the challenge and responsibility of using accurate mass measurements correctly. Kondrat and Gross illustrate some of the prevalent misconceptions about the utility and appropriateness of accurate measurements, and illustrate the confusion that still exists on the differences between definition of the terms high resolution and accurate mass. Busch illustrates the use of and limitations of accurate mass measurements. Biemann s discussion drives the point home in much more detail. Biemaim lists three typical uses for accurate mass measurements elucidation of fragmentation pathways, confirmation of structure of the synthetic products, and elucidation of completely unknown structures. The demands for necessary... 

Due to the obtained purity of the urine extracts combined with the high resolution and accurate mass capability of the analytical instrumentation, 72 stimulants and metabolites, 21 narcotics and metabolites, 10 P2-agonists, and 31 P-blockers are, besides numerous additional compounds, simultaneously detected at minimum required performance levels. ... 

---