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Chapter 20: 20.9 (page 534)

Why do double-focusing mass spectrometers give narrower peaks and higher resolutions than single-focusing instrument?

Short Answer

Expert verified

The double-focusing mass spectrometers give narrower peaks and higher resolutions than single-focusing instrument because the kinetic energy spread is minimized by the addition of electrostatic analyzer.

Step by step solution

01

Concept Introduction

A mass spectrometer that uses an electric field to select ions from an ion beam that have the same kinetic energy and a wedge-shaped magnetic field to separate the total ion beam into discrete ion beams based on the ions' mass-to-charge ratio.

02

Concept Explanation

The resolution of a single focusing mass spectrometer is limited by the initial kinetic energy spread of the sample molecules. This spread is minimized in a double focusing instrument by accelerating the sample through an electrostatic analyzer, which limits the range of kinetic energies of ion being introduced into the magnetic sector analyzer.

Significantly narrower peaks are the result.

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Most popular questions from this chapter

Identify the ions responsible for the peaks in the mass spectrum shown in Figure 20-22b.

Measuring the approximate mass of an ion without using a standard can be accomplished via the following variant of the peak-matching technique described in Problem20-14. The peak-matching technique is used to alternately cause the P+ion and the (P+1)+ions to reach the detector. It is assumed that the difference in mass between P+and (P+1)+is due to a single C13replacing a C12atom.

(a) If the accelerating voltage for P+1+is labeled V2and that for P+is V1, derive a relationship that

relates the ratio V2/V1to the mass of P+.

(b) If V2/V1=0.987753, calculate the mass of the P+ion.

Calculate the accelerating voltage that would be

What mass differences can just be resolved at m values of 100,1000,2000,3000and 5000if the mass spectrometer has a resolution of

(a) 500(b) 1000(c) 3000(d) 5000?

In a magnetic sector (single-focusing) mass spectrometer, it might be reasonable under some circumstances to monitor one m/zvalue, to then monitor a second m/z, and to repeat this pattern in a cyclic manner.

Rapidly switching between two accelerating voltages while keeping all other conditions constant is called

peak matching.

(a) Derive a general expression that relates the ratio of the accelerating voltages to the ratio of the

corresponding m/zvalues.

(b) Use this equation to calculate m/zof an unknown peak if m/zof the ion used as a standard, CF3+is 69.00and the ratio of Vunknown/Vstandardis 0.965035.

(c) Based on your answer in part (b), and the assumption that the unknown is an organic compound that has a mass of 143, draw some conclusions about your answer in part (b), and about the compound.
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