Homework Assignment #4

Based on your readings of Cooks and McLuckey please answer the following.

1. The two scanning modes in ion trap mass spectrometry are the mass-selective mode and the resonant ejection mode. Explain each in terms of the stability diagrams shown in the article by Cooks et al.

2. Why is the quadrupole ion trap more efficient with “weak” ion sources (i.e. the number of ions that are produced per unit time is small) than with “strong” ion sources?

3. How does the bath gas help to trap ions that are not formed in the center of the quadrupole ion trap? Why is the bath gas needed to trap externally injected ions?

4. Explain using a stability diagram how MS/MS is performed in a quadrupole ion trap? How are precursor ions selected? How are they activated for collision induced dissociation? How is the product ion spectrum scanned?

5. It can be shown that when a_z=0, the maximum value of q_z that gives a stable ion trajectory is 0.908. In other words, the stability diagram intersects the a_z axis at q_z = 0.908. (i) Use the above value for q_z max to derive equation 1 in the article by McLuckey et al. Note that McLuckey’s equation assumes an optimum trap geometry, i.e. r_o²=2z₀². (ii) Calculate the minimum m/z in daltons that can be trapped with a commercial ion trap having r_o= 1.00 cm that is operated at a frequency of 1.1 MHz (this value is equal toΏ/2π) when an rf potential of 10 V is applied, (iii) What determined the highest m/z ion tahta can be trapped under conditions in part ii?

In Class Assignment

1. Why is it preferable to scan the magnetic filed strength rather than the acceleration potential in a magnetic sector mass spectrometer? What limits the highest m/z ion that can be analyzed with a magnetic/electric sector instrument?

2. The electric sector permits a small range of ion kinetic energies to be selected. Use a Figure to explain how this is done. Why does use of an electric sector result in a decrease in the overall ion signal intensity?

Use Figure2 to explain why mass resolution is dependent upon the distribution of ion kinetic energies. Why don’t all ions have the same kinetic energy (Hint: refer to first assignments.)

What is meant by the “direction focusing curve” and “velocity focusing curve” in Figure 3? What is the effect of decreasing the width of the intermediate slit in Figure 3? In a conventional mass spectrometer, an exit slit and channel plate detector would be placed after the magnetic sector. Show where these would be located in Figure 3. Alternatively, an array detector could be used to simultaneously monitor the signal intensities across an entire m/z range. Where would such detectors be located in Figure 3?