Samsung RT34M Uživatelský manuál stáhnout pdf (Strana 40)

Dushyant Patel LC-MS/MS Method

S.K.P.C.P.E.R., Kherva 28 M .Pharm. Thesis

Profile data type:

¾ In the profile data type, you can see the shape of the peaks in the mass spectrum. Each

atomic mass unit is divided into many sampling intervals. The intensity of the ion current

is determined at each of the sampling intervals. The intensity at each sampling interval is

displayed with the intensities connected by a continuous line. In general, the profile scan

data type is used when you tune and calibrate the mass spectrometer so that you can

easily see and measure mass resolution.

Centroid data type:

¾ In the centroid data type, the mass spectrum is displayed as a bar graph. In this scan data

type, the intensities of each set of multiple sampling intervals are summed. This sum is

displayed versus the integral center of mass of the sampling intervals. In general, the

centroid scan data type is used for data acquisition because the scan speed is faster and

the disk space requirements are smaller. Data processing is also much faster for centroid

data.

4.1.2 TANDEM MASS SPECTROMETRY (MS/MS):

¾ All the mass spectrum tells us is the molecular weight, but it does not provide structure

information for the component of interest. Efforts to remove this limitation took the

form of the development of tandem mass spectrometric methods (MS/MS). Two

stages of mass analysis are required, one to select the precursor ion from other ions

generated in the ion source and one to analyze the product ions after collisions

therefore this approach is called tandem mass spectrometry (MS/MS)

10, 11

. The

schematic of tandem mass spectrometer is presented in Figure 4.4.

¾ In this technique ions are separated, identify and fragmented in a single instrument. A

single molecular ion (precursor) that is characteristic of a given analyte is pass through a

region where they are activated that causes them to fall apart to produce fragment ions

(product). This is usually done by colliding the ions with a neutral gas in a process called

collisional activation or collision-induced dissociation. It is a fast collision event, where

ion translational energy is converted into ion internal energy to obtain an ion in the

excited state, and slow unimolecular decomposition.

¾ The fragment ions are separated accordingly to their mass to charge ratio (m/z). The

resulting MS/MS spectrum consists only of product ions from selected precursor.

MS/MS can be performed in a number of single-stage instruments and a variety of two-

stage instruments. The most versatile and most widely used MS/MS configuration is the

triple quadrupole mass analyzer instrument as compared to other type of mass analyzer.

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