Mass accuracy specification

In LC-MS, specific ionisation conditions can be required for different types of species. This means that in LC-MS studies on extractable additives, it is necessary to use a range of experimental conditions to cover detection of all types of potential species. Depending on instrument type, it is also possible to isolate ions in complex matrices and obtain positive identifications by further unique fragmentation of these ions (by MS-MS or MSn). Quantitative methods based on this secondary ionisation can be employed. The mass accuracy of LC-MS detection systems continues to improve. Accurate mass measurement improves the certainty of identification. Advanced systems are typically offering 1-2 ppm (mass dependent) mass accuracy. 

Note The numerous ionization methods and mass analyzers in use have created a demand for a large number of calibration compounds to suit their specific needs. Therefore, mass calibration compounds will occasionally be addressed later in the chapters on ionization methods. It is also not possible to specify a general level of mass accuracy with external calibration. Depending on the type of mass analyzer and on the frequency of recalibration, mass accuracy can be as high as 1 mmu or as low as 0.5 u. 

In tandem MS mode, because the product ions are recorded with the same TOF mass analyzers as in full scan mode, the same high resolution and mass accuracy is obtained. Isolation of the precursor ion can be performed either at unit mass resolution or at 2-3 m/z units for multiply charged ions. Accurate mass measurements of the elemental composition of product ions greatly facilitate spectra interpretation and the main applications are peptide analysis and metabolite identification using electrospray iomzation [68]. In TOF mass analyzers accurate mass determination can be affected by various parameters such as (i) ion intensities, (ii) room temperature or (iii) detector dead time. Interestingly, the mass spectrum can be recalibrated post-acquisition using the mass of a known ion (lock mass). The lock mass can be a cluster ion in full scan mode or the residual precursor ion in the product ion mode. For LC-MS analysis a dual spray (LockSpray) source has been described, which allows the continuous introduction of a reference analyte into the mass spectrometer for improved accurate mass measurements [69]. The versatile precursor ion scan, another specific feature of the triple quadrupole, is maintained in the QqTOF instrument. However, in pre-... 

These techniques still are not sufficient for true specificity. As mass increases from 200 to 500, the number of formulas mathematically possible for that mass increases rapidly, and a mass accuracy yielding a single, unambiguous formula quickly becomes impossible (Kind and Fiehn, 2006 Kind et al., 2007). To reduce the possibilities, the atom types and abundance required to produce the observed isotope... 

The quadrupole/time-of-flight analyzer (QToF) has become a key option in the qualitative and quantitative analytical arena. Instruments with resolving power of 20000 (50% valley definition) can provide <5ppm mass accuracy for parent and product ion identification and for 20mda mass selection windows quantitation. While the triple quadra-pole retains the lead in sensitivity for quantification, the QToF has a decided edge on specificity (Micromass, 1999) and qualitative analysis. 

The mass spectrometer is theoretically capable of analyzing any gas in a mixture of gases with a speed, specificity, sensitivity, and accuracy that cannot be matched by other known means of analysis. However, the initial cost of the instrument and accessory equipment has thus far precluded its widespread use. Fowler and Hugh Jones described a mass spectrometer specifically for respiratory physiological studies. Extensive use has been made of this instrument (W6-W8). 

A more general approach that can be employed for all mass analyzers [as time-of-flight (TOF), ion cyclotron resonance (Fourier trans-form-mass spectroscopy, FT-MS), Q-TOF] is based upon measuring the full width half-maximum, as shown in Fig. 2.2. In the same figure, the definition of mass accuracy is also reported. This parameter reflects on the specificity of mass measurements. In fact, on one hand it allows to determine the accurate mass of a selected ion (and, consequently, its elemental composition). On the other hand it allows to operate in accurate mass mode in order to identify species of interest present in complex matrices on the basis of their elemental composition. 

We used the instrument installation mass resolution and accuracy specifications as a guideline to define a good spectrum for protein identification. A good... 

The process of method validation (i.e., evaluation of the assay) affects the quality of the quantitative data directly [9 A Guide to Analytical Method Validation, Waters Corporation]. Through method validation, it is assured that the method developed is acceptable. Issues involved in the validation of a mass spectrometry method for quantitative analysis are similar to those in any other analytical technique. The validation involves undertaking a series of studies to demonstrate the limit of detection G OD) limit of quantitation (LOQ) linear range specificity within-day precision and accuracy and day-to-day precision and accuracy, specificity, and robustness of the method. All of these parameters must be determined with those commonly accepted good laboratory practices criteria that are applicable in the vafidation of analytical methods. 

With modern QTOF and Orbitrap mass spectrometers, mass resolution of 10" 10 with a mass accuracy within 5 ppm is routinely achieved. The advances in HRMS have not only provided accurate MS measurement but also signifieantly improved the specificity of LC—MS-based metabolite identification approaches in recent years (Tiller et al., 2008 Zhang et al., 2008a). For example, specificity of background subtraction in high-resolution full scanning is significantly improved. Current HRMS measures m/z values with precision at... 

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