Full mass spectrometric scanning

Figure 3.32 Full-mass spectra at peak maxima of the constant-neutral-loss TIC trace shown in Figure 3.31(a), obtained after (a) 13.25 and (b) 15.9 min, from the LC-MS analysis of a mixture of atrazine and its degradation products. Reprinted from J. Chro-matogr., A, 915, Steen, R. J. C. A., Bobeldijk, I. and Brinkman, U. A. Th., Screening for transformation products of pesticides using tandem mass spectrometric scan modes , 129-137, Copyright (2001), with permission from Elsevier Science.

A further common problem the analyst faces when integrating SPC into analytical procedures for the determination of LAS is the scarcity of available reference compounds, thereby complicating their determination. Therefore, the identification of the analytes has to be performed by comparison of retention time and absolute peak area ratio between the deprotonated molecular ion and the fragment ion, relative to the ratio obtained from the authentic standard ( 20%). Retention times of SPC, for which no standards were available, can be determined once by mass spectrometric identification in full-scan mode. 

Traditional methodologies for structural identification of trace level impurities in drng substances/products usually involve fractionation of each impurities using a scaled-np analytical chromatographic method, followed by off-line spectroscopic analysis. Coupling of HPLC separation and electrospray mass spectrometry allows on-line acquisition of full scan mass spectra and generation of tandem mass spectrometric data. LC/ESI MS has revolntionized trace analysis for qnalitative and quantitative studies in pharmaceutical analysis. 

When working with non-radiolabeled drugs the major challenge is to find metabolites in the biological matrices. Because the enzymes responsible for metabolism are quite well characterized metabolic changes can partially be predicted. For example hydroxylation of the parent drug is in many cases the principal metabolic pathway. From a mass spectrometric point of view it results in an increase of 16 units in the mass spectrum. In the full-scan mode an extracted ion current profile can be used to screen for potential metabolites. In a second step a product ion spectrum is recorded for structural interpretation. Ideally, one would like to obtain relative molecular mass information and the corresponding product ion spectrum in the same LC-MS run. This information can be obtained by data dependant acquisition (DDA), as illustrated in Fig. 1.39. 

Current detection limits are < 1 ng for full scan mass spectra and < 1 pg for multiple ion monitoring mass spectrometry. Compound identifications are based on the comparisons with authentic standards, GC retention time, literature mass spectra and the interpretation of mass spectrometric fragmentation patterns. The MS methods used for the various markers and studies are listed in Table 1. 

Mullen W, Boitier A, Stewart AJ, Crozier A. 2004. Flavonoid metabolites in human plasma and urine after the consumption of red onions Analysis by liquid chromatography with photodiode array and full scan tandem mass spectrometric detection. J Chromatogr A 1058 163-168. 

It is now possible to move on to discussion of general parameters of a mass spectrometer that are relevant to the selectivity-sensitivity compromise that is ubiquitous in analytical chemistry. Full scan acquisition mode refers to operation of a mass analyzer so as to acquire the entire spectrum of ions (or a major portion thereof) introduced into the analyzer. In the case of a pure compound, full scan mode delivers the maximum chemical information that the mass spectrometric experiment (MS or MS/MS) can... 

Figure 6.7 Sketch illustrating the importance of the number of mass spectrometric data points (either full spectra or SIM or MRM) acquired across a chromatographic peak, for adequate representation of the shape of the peak in order to calculate accurate peak areas and/or heights. Usually 10 scans (or SIM or MRM acquisition windows) are considered necessary to provide an adequate representation of the peak centroid (and thus retention time) and peak area.

The basis for mass spectrometric fingerprinting is full-scan spectra within a mass range that covers the molecular weight (MW) of the compounds of interest. Different ionization techniques may be used prior to the mass spectrometric detection. ESI, atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI) will produce different... 

Eor direct characterization of disubstituted piperidine alkaloids in extracts of L. inflata, tandem mass spectrometric method was developed using electrospray ionization. Analysis was performed in positive ion mode on a triple quadropole LC/MS system. The identification and structural elucidation of the alkaloids were performed by comparing their changes in molecular mass (AM), full-scan MS-MS spectra with those of lobeline, lobelanine, norlobelanine, and lobelanidine. These alkaloids and ten other derivatives were identified in the plant extracts. 

Spots of samples separated on TLC are two-dimensional in their shapes. Several bands of samples can be run in adjacent lanes on a TLC plate, and scanning in one-dimensional axis through the center axis of each lane can provide mass spectrometric information about the compounds separated. However, high performance TLC and many other forms of TLC use two-dimensional development or circular development methods. A full two-dimensional imaging scan is necessary to discern the location of sample spots on the chromatogram, and to determine the degree of spot overlap if any. There have been far fewer reports of two-dimensional TLC/MS than for one-dimensional scanning... 

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