Samsung RT34M Uživatelský manuál stáhnout pdf (Strana 33)

Dushyant Patel LC-MS/MS Method

S.K.P.C.P.E.R., Kherva 21 M .Pharm. Thesis

mass-selected ions are then transmitted (through the second and third rod assemblies) to

the ion detection system.

¾ When the system is operated as a tandem mass spectrometer, as before, the sample is

ionized in the ion source and is mass analyzed by the first rod assembly. In this case,

however, mass-selected ions exiting the first rod assembly are allowed to collide with an

inert gas in the second rod assembly and fragment to produce a new set of ion products.

(The second rod assembly is surrounded by a chamber called the collision cell. The

collision cell can be pressurized with an inert gas.) The secondary ion products then

undergo a further mass analysis in the third rod assembly and selected ions are detected.

In this mode, two stages of analysis are performed. In the first stage, ion production

occurs in the ion source, and the first mass analyzer carries out mass analysis. In the

second stage of analysis, formation of new ionic species occurs in the collision cell, and

the second mass analyzer performs mass analysis. Two stages of mass analysis yield far

greater chemical specificity than a single stage can achieve, because two discrete but

directly related sets of masses can be selected and determined.

¾ Systems can be used to elucidate the structures of pure organic compounds and the

structures of the components within mixtures. Each ionic fragment of a molecule formed

in the ion source and separated in a first stage of mass analysis can be further fragmented

and then further separated in a second stage of mass analysis to build up an entire

structure for the molecule, piece by piece. Thus, all pathways for the formation and

fragmentation of each ion in the mass spectrum can be investigated.

¾ The two stages of mass analysis, with resultant reduction of chemical noise in the final

mass spectrum, make analysis very selective and sensitive.

¾ Each sequence of single or triple-stage mass analysis of the ions is called a scan. The

system uses several different scan modes and different scan types to filter, fragment, or

transmit ions in the mass analyzer. The ability to vary the scan mode and scan type, as

well as the ionization and ion polarity modes, affords the user great flexibility in the

instrumentation for solving complex analytical problems.

Ion polarity modes:

¾ Two ion polarity modes: Positive or negative.

¾ Both positively charged and negatively charged ions are formed in the ionization source

of the mass spectrometer. The mass spectrometer can control whether positive ions or

negative ions are transmitted to the mass analyzer for mass analysis by changing the

polarity of the potentials applied to the ionization source and ion guides. The ion guides

are located between the ionization source and the mass analyzer.

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Samsung RT34M Uživatelský manuál Strana 33