Mass analyzers accuracy/precision

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... 

Mass spectroscopy (MS) methods can be used to analyze complex mixtures of proteins and peptides in minutes and with mass accuracy several orders of magnitude better than that obtained from electrophoretic methods. For proteins with a known sequence, the mass measurement accuracies are generally sufficient to identify the products resistant to proteolysis precisely and to define compact domains within proteins effectively. 

Elemental mass spectrometry has undergone a major expansion in the past 15-20 years. Many new a, elopments in sample introduction systems, ionization sources, and mass analyzers have been realized. A vast array of hybrid combinations of these has resulted from specific analytical needs such as improved detection limits, precision, accuracy, elemental coverage, ease of use, throughput, and sample size. As can be seen from most of the other chapters in this volume, however, the mass analyzers used to date have primarily been magnetic sector and quadrupole mass spectrometers. Ion trapping devices, be they quadrupole ion (Paul) [1] traps or Fourier transform ion cyclotron resonance (Penning) traps, have been used quite sparingly and most work to date has concentrated on proof of principal experiments rather that actual applications. 

A method based on ICP-MS using an SF mass analyzer with multicollector unit was employed for the precise determination of 87Sr-86Sr in 11 wines [96]. To avoid the 87Sr-87Rb isobaric interference, Sr was separated from Rb and the Sr-rich fraction was introduced into the plasma. Precision values of 0.002-0.003 percent were routinely obtained. To check the accuracy, a CRM namely, the NIST SRM 987, was analyzed, measurements being within +0.02 percent of the target value. 

The dynamic range of a mass spectrometer is defined as the range over which a linear response is observed for an analyte as a function of analyte concentration. It is a critical instrument performance parameter, particularly for quantitative applications, because it defines the concentration range over which analytes can be determined without sample dilution or preconcentration, which effects the accuracy and precision of an analytical method. Dynamic range is limited by physiochemical processes, such as sample preparation and ionization, and instrumental design, such as the type of mass analyzer used and the ion detection scheme. 

The development of ionization techniques and mass analyzers has enabled mass spectrometrists to determine the accurate mass of small molecules as well as biomolecules that are present at low levels. The modem quadrupole time of flight (Q-TOF), Fourier transform ion cyclotron resonance MS (FT-ICR MS), and Orbitrap instruments allow determination of the mass of an ion with accuracies and precisions beyond the decimal point. Mass accuracy measurement, typically measured and reported as parts per million (ppm), is essential for elemental composition assignment. 

Mass accuracy is affected mainly by the precision of machining and assembly of the electrodes of mass analyzers as well as the stability of their electronic systems. Imperfections in the construction of mass analyzers originate from mechanical errors during manufacturing and assembly of components. It is difficult to ensure an overall machining and assembly error of <10 pm even for a small mass analyzer, such as IT. Assuming one dimension of a mass analyzer is roughly 6 cm, a 10 pm error corresponds to 0.017% relative error. The... 

In a scenario that occurs often in a protein biochemistry lab setting, a researcher has isolated a protein of unknown function, or he/she has overexpressed a protein in Escherichia coli and wishes to characterize it. A common practice today is to submit a small amount of the protein to a core mass spectrometry laboratory for a molecular weight measurement. Using either ESI or MALDl, a molecular weight with a precision and accuracy of 0.05% or better can be measured. This, of course, depends heavily on the purity of the protein sample, the relative size of the protein, the presence or absence of post-translational modifications (PTM) (e.g., phosphorylation, glycosylation, etc.), the resolution of the mass analyzer, and so on. Primary structure information (i.e., amino acid... 

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