Cone voltage

Ions can be induced to fragment by increasing an electric potential known as a cone voltage, which speeds them. Accelerating the ions causes them to collide more energetically with neutral molecules, a process that causes them to fragment (collision-induced decomposition). 

Esi-ms measurements were performed on a Agilent LC M,SD system with the following operational parameters capillary voltage 4.0 kV, cone voltage, 50 V and solvent flow (methanol - water, 50% v/v) 0.3 inL/min. All esi mass specttal data in the positive ion mode were acquired and processed using HP Chem.Station software. The concenttation of aluminum was 0.5 mmol dm while that of mfx were varied in the interval 0.5-1.0 mmol dm. The pH values were pH 4.0, 6.0, 7.2 and 8.5. The specttum obtained at A1 to mfx concenttation ratio 1 2 and pH 4.0 is shown in Fig. 1. 

The Effect of Mobile-Phase Additives and Cone-Voltage 147... 

The Use of Cone-Voltage Fragmentation in Conjunction with High-Accuracy Mass Measurements and LC-MS for Metabolite Identification... 

The ionization techniques most widely used for LC-MS, however, are termed soft ionization in that they produce primarily molecular species with little fragmentation. It is unlikely that the molecular weight alone will allow a structural assignment to be made and it is therefore desirable to be able to generate structural information from such techniques. There are two ways in which this may be done, one of which, the so-called cone-voltage or in-source fragmentation, is associated specifically with the ionization techniques of electiospray and APCl and is discussed later in Section 4.7.4. The other, termed mass spectrometry-mass spectrometry (MS-MS) or tandem mass spectrometry, is applicable to all forms of ionization, provided that appropriate hardware is available, and is described here. 

Further experimental work involving cone voltage studies may provide further confirmatory evidence but the most likely explanation is that the mass spectrum of the component with retention time 4.65 min is that shown in Figure 3.17, while the mass spectrum of the second component is that obtained by background subtraction, and is shown in Figure 3.22. 

Electrospray is a soft-ionization method prodncing intact molecular species and structural information is not usually available. Electrospray sources are capable of producing structural information from cone-voltage fragmentation but these spectra are not always easily interpretable. Experimentally, the best solution is to use a mass spectrometer capable of MS-MS operation but this has not inconsequential financial implications. 

In addition to the formation of these ions of direct analytical utility, APCI leads to the formation of ion clusters involving solvent molecules. Since these tend to make interpretation more difficult, they need to be removed and this may be accomplished either by the use of a curtain gas or by cone-voltage fragmentation (see Section 4.7.4 above) which is also applicable to APCI. 

Structural information is not usually available unless cone-voltage fragmentation or MS-MS is used. 

To understand the circumstances in which particular features of mass spectrometry, such as high-resolution measurements, MS-MS and cone-voltage fragmentation, selected-ion monitoring and selected-decomposition monitoring, may be nsed to address particular analytical problems. 

Factors may be classified as quantitative when they take particular values, e.g. concentration or temperature, or qualitative when their presence or absence is of interest. As mentioned previously, for an LC-MS experiment the factors could include the composition of the mobile phase employed, its pH and flow rate [3], the nature and concentration of any mobile-phase additive, e.g. buffer or ion-pair reagent, the make-up of the solution in which the sample is injected [4], the ionization technique, spray voltage for electrospray, nebulizer temperature for APCI, nebulizing gas pressure, mass spectrometer source temperature, cone voltage in the mass spectrometer source, and the nature and pressure of gas in the collision cell if MS-MS is employed. For quantification, the assessment of results is likely to be on the basis of the selectivity and sensitivity of the analysis, i.e. the chromatographic separation and the maximum production of molecular species or product ions if MS-MS is employed. 

What is meant by the term cone-voltage fragmentation and what information may it provide ... 

It has been shown [18] that when high cone-voltages were used in conjunction with negative-ion electrospray, phosphopeptides produce diagnostic ions at m/z 63 (P02 ) and m/z 79 (P03 ). LC-MS analysis of a trypsin digest of bovine... 

APCI and ESI spectra were obtained by using a range of cone-voltages, with the major ions observed being detailed in Table 5.12. With the exception of benomyl, under APCI conditions all compounds produced abundant molecular... 

While it is true, therefore, that a general method for analysis could be developed and used successfully for a number of applications based on previous knowledge, for those in which optimum performance is required it is advisable to carry out preliminary experiments in which the variables of ionization technique, polarity, cone-voltage, etc., as well as those of sample isolation, are investigated (see Section 5.1 above). 

The use of cone-voltage fragmentation (CVF) to obtain structural information was discussed earlier in Section 4.7.4 and the comment was made there that because this has the potential to bring about fragmentation of aU ions in the source of the mass spectrometer the spectra were often difficult to interpret. When the analyte under consideration is of a low molecular weight, its spectrum is likely to consist only of a few ions and CVF is of more practical value. 

Figure 5.50 Accurate-mass cone-voltage spectrum of Glyburide determined from in vitro incubation samples. Reprinted with permission from Zhang, H., Henion, J., Yang, Y. and Spooner, N., Anal. Chem., 72, 3342-3348 (2000). Copyright (2000) American Chemical Society.

Cone-voltage fragmentation Fragmentation of ions, commonly produced by APCl or electrospray ionization, effected by the application of a voltage within the source of the mass spectrometer. 

Fragmentor voltage Another term for cone-voltage fragmentation. 

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