Mass Spectral Data Acquisition

Separation device Interface/ion source Mass spectrometer 

Data-Dependent Acquisition The specificity of mass spectrometry data is enhanced further when a spectrum is acquired in the tandem MS (MS/MS) 

Peak Parking Another approach to acquiring complete information in a narrow peak is to extend the analysis time over peaks of interest. In this approach, termed peak parking or variable-flow chromatography, the column head pressure, and hence the flow rate, are reduced instantaneously [3,4], With this approach, it is possible to perform higher-resolution narrow mass scans and acquire MS/MS scans on all coeluting components in a single narrow peak. 

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Method principles should include the technique used for mass spectral data acquisition. 

With respect to mass spectral matching, the criteria for identification vary depending on the technique used for mass spectral data acquisition (see summary of requirements in Table 8). It is interesting to note that while the FDA does not rule out the use of exact mass measurements, it views these data as problematical as there are no generally accepted specific standards for their use. The problem here is that it is difficult to be definitive about the resolving power required, particularly, when analytes have masses greater than m/z 500. Clearly the resolving power and accuracy must be sufficient to exclude all reasonable alternative elemental compositions and they recommend that if exact mass measurements are to be used then multiple structurally specific ions should be measured. 

Acknowledgments The authors thank Union Carbide Chemicals and Plastics Co., Inc. for permission to publish this work. We gratefully acknowledge Dr. P. C. Price for mass spectral data acquisition and evaluation. 

Figure 3 (A) Single-column GC analysis result. (B) Selected incompletely resolved heartcuts from A are directed to a second column where improved separation is achieved. Here two heart-cuts are performed. (C) Illustration of multidimensional analysis GC/MS, where the second dimension is a mass spectrum recorded at the mass spectral data acquisition rate. The success of this approach depends on the uniqueness of the mass spectrum at each point in the chromatogram. (D) The comprehensive two-dimensional GC method ideally spreads out the components in a two-dimensional space, according to the characteristics of each of the columns. Each component has a characteristic D and D retention time in the plane.

Apart from the actual acquisition of the mass spectrum and its subsequent display or printout, the raw mass spectral data can be processed in other ways, many of which have been touched on in other chapters in thi.s book. Some of the more important aspects of this sort of data manipulation are explained in greater detail below. 

Huang, N. Siegel, M. M. Kruppa, G. H. Laukien, F. H. 1999. Automation of a fourier transform ion cyclotron resonance mass spectrometer for acquisition, analysis, and e-mailing of high-resolution exact-mass electrospray ionization mass spectral data. ./. Am. Soc. Mass Spectrom., 10, 1166-1173. 

Still Photography. The use of a still camera to record the mass spectral data displayed on the oscillographic screen is an alternate photographic data acquisition technique. The simplest method is to photograph the oscilloscope display for a fixed time during which the sample is stressed and/or fails. This technique does not permit resolution of changes in the mass spectra with time but provides a way to record qualitative information with readily available equipment. 

The authors of the papers cited below describe different applications of both ES and APCI mass spectrometry. The LODs reported in these papers differ and are also hard to compare because of different ways of acquiring mass spectral data. Some report LODs as the smallest total amount detected, for example picogramme or femtomol, whereas others express LODs as the concentration injected (pg pl"0- Furthermore, the solvent flow rates differ substantially, from 1 plmin" to lOOOplmin" Finally, both the mass span over which data are collected and the total time of acquiring the data differ considerably. Typically, at a solvent flow rate of 400 pi min" and using gradient LC separation (20 pi loop) with mass spectrometric detection in SIM mode, a LOD of 5-10 fmol pr is achieved. Use of very low solvent flow rates (1 pi min" and the acquisition of data over, say, 3 min may result in lower LODs. This approach is very useful when the amount of sample is limited. 

Busch et al. (76) first described a custom-built secondary ion mass spectrometer for the analysis of TLC chromatograms in 1985. The instrument has been through several revisions since the initial prototype was constructed, including changes in the size of the chromatogram that could be accommodated within the vacuum chamber, the accuracy of sample placement for acquisition of spatial images, and in the data system used to control the scanning experiment and process the mass spectral data. 

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