Chapter 20

Question 1

20-1. How do gaseous and desorption sources differ? What are the advantages of each?

Answer 1

With gaseious ionization sources, the sample is first volatilized (by heating if necessary) and then transmitted to the ionization area for ionization. In a desorption source, a probe is used and ionization takes place directly from the condensed phase.

The advantage of desorption ionization is that it can be applied to high molecular weight and thermally unstable samples. The advantage of gaseous ionization sources are their simplicity and speed (no need to use probe and wait for probed area to be pumped out).

Question 2

20·2. How do the spectra for electron-impact, field ionization, and chemical ionization sources differ from one another?

Answer 2

The most fragementation and thus the most complex spectra are encountered with electron impact ionization. Field ionization produces the simplest spectra. Chemical and electron impact ionization result in higher sensitivities than does field ionization.

Question 3

20-5. When a magnetic sector instrument was operated with an accelerating voltage of 3.00 x 10 3 V, a field of 0.126 T was required to focus the CH4, on the detector. (a) What range of field strengths would be required to scan the mass range between 16 and 250, for singly charged ions, if the accelerating voltage is held constant? (b) What range of accelerating voltages would be required to scan the mass range between 16 and 250, for singly charged ions, if the field strength is held constant?

Answer 3

Question 4

*20-6 Calculate the accelerating voltage that would be required to direct singly charged ions of mass 7,500 through an instrument that is identical to the one described in Example 20-4.

Answer 4

Here m= 7500 g/mol (1 mol/6.023 x1023 ions) (1kg/1000g) = 1.246 x 10-23kg/ion V=B2r2ez/2m= (0.240 Vs/m2)2 (0.127m)2(1.60x10-19 C/ion) / (2x1.246 x 10-23kg/ion) = 5.96 V

Question 5

20-8. On page 288 a qualitative discussion described how a positive ion would behave in the xz plane (positive dc potential plane) of a quadrupole mass filter. Construct a similar argument for the behavior of positive ions in the yz plane (negative dc potential plane).

Answer 5

The presence of a negative dc voltage in the yz plane causes positive ions to move toward the rods where they are annihilated. In the presence of an added ac voltage, this movement is inhibited during the positive half of the cycle with the lighter ions being more affected than the heavier ions. Thus the yz plane acts as a low-pass filter removing heavier ions.

Question 6

20-10. Discuss the differences between quadrupole ion-trap mass spectrometers and Fourier transform ICR mass spectrometers.

Answer 6

A quadrupole ion trap is similar to a linear quadrupole filter except it as a spherical 3-dimensional configuration. By a combination of fields, ions are temporarily stored within the trap. They are then released sequentially by increasing the radio frequency voltage applied to the ring electrode. The ejected ions then strike a detector. A plot of detector signal vs. the radio frequency voltage, related to the m/z value, is the mass spectrum. In an FT ICR instrument, ions are trapped in a cell by an electric trapping voltage and a magnetic field. Each ion assumes a circular motion in a plane perpendicular to the direction of the field. The cyclotron frequency depends on the inverse of the m/z value. In modern instruments a radio frequency pulse that increases linearly in frequency is employed. A time domain image current is generated after termination of the pulse. Fourier transformation of the time decay signal yields the mass spectrum.

Question 7

20-13 Calculate the ratio of the (M + 2)⁺ to M⁺ and the (M + 4)⁺ to M⁺ peak heights for (a) C₁₀H₆Br₂ (b) C₃H₇CIBr, (c) C₆H₄Cl₂

Answer 7

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