Mass spectrometry in the operating room
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Mass spectrometry in the operating room
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[post_content] => Practice Passage (Question 1-4)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
One technique used to differentiate between cancerous and normal tissue is running samples through a mass spectrometer. Chemists have discovered that the ratios of certain lipids from cell membranes can uniquely identify various biological tissues, including tumors. A library of lipid spectrometer signatures can be gathered and referenced to determine if a tissue has cancer cells or not, but such techniques require special preparation as spectroscopy requires ionized particles for analysis.
Chemists recently have found however, that the smoke produced from electrosurgical knives happens to be comprised of the exact type of ionized particles needed for mass spectrometry. These ionized particles move through the spectrometer past a magnetic field, and depending on the charge of the particles, and the direction of the field, this magnetic field would cause a semi-circular deflection of the ionized particles’ path. The radius of this deflection is directly dependent on the m/z ratio, which is the ratio of a particles mass to its charge. Thus, simply by analyzing smoke as surgeons remove a tumor, a mass spectrometer can identify whether or not the cut tissue matches a profile of cancer, which can inform the surgeon whether or not an incision is adequate for the accurate removal of cancer.
Figure 1. Charged particle with velocity V passing through magnetic field B
Citation: Sci Transl Med 17 July 2013: Intraoperative Tissue Identification Using Rapid Evaporative Ionization Mass Spectrometry Júlia Balog, László Sasi-Szabó, James Kinross, Matthew R. Lewis, Laura J. Muirhead,Kirill Veselkov, Reza Mirnezami, Balázs Dezső, László Damjanovich, Ara Darzi,Jeremy K. Nicholson and Zoltán Takáts Vol. 5, Issue 194, p. 194ra93
[post_title] => Mass spectrometry in the operating room
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[question] => As seen in Figure 1, a positively ionized particle with velocity V enters a magnetic field moving right, with the field going into the screen. Given this arrangement, what direction would the particle initially deflect toward as it moves through the field?
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[answer] => 1
[description] => Reason for the Correct Answer:
You can determine the direction of deflection using the right hand rule.
In the right hand rule, the direction of velocity should match the direction your fingers point. The direction of the magnetic field should be the same as the direction your palm is facing.
If your fingers point right, and your palm points into the page, your thumb will indicate the direction of the particle deflection, which should be upwards.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Up the screen
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[each_answer] => B. Out of the screen
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[each_answer] => C. Into the screen
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[each_answer] => D. Down the screen
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[1] => Array
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[quiz_unique_key] => 3873426850
[question] => A particle with a velocity of 1×106 m/s is placed within a mass spectrometer. Upon entering into the machine, it experiences an acceleration of 4×1012 m/s2. The mass of the particle is 4×10-27 kg and it carries a charge of 2×10-19 C. What is the magnitude of the magnetic field?
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[answer] => 4
[description] => Reason for the Correct Answer:
The magnetic force can be found either from the equation Fb=ma or Fb= qVB.
Since we know the mass and acceleration of the particle, we can solve for force, and then substitute it into the second equation.
F=ma
F=(4×10-27)(4×1012) = 1.6 x 10-14
By substituting in force into the magnetic force equation, we can solve for the magnitude of the magnetic field.
1.6 x 10-14 = qVB
= (2×10-19)(1×106)B
B = 0.08
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[answers] => Array
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[0] => Array
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[each_answer] => A. 8 T
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[each_answer] => B. 0.8 T
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[each_answer] => C. 0.008 T
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[each_answer] => D. 0.08 T
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[quiz_unique_key] => 83407773
[question] => Which of these is true of the mass spectrometer?
I. The magnetic field applied to the ionized particles acts as a centripetal force upon the particles.
II. More massive molecules passed through the mass spectrometer will have a larger radius of the deflection.
III. A larger charge on the molecule would result in a larger radius of deflection within the field.
[value] => Array
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[answer] => 1
[description] => Reason for the Correct Answer:
The magnetic field does indeed act as a centripetal force upon the ionized particle. This is why it travels in a circle with a specific radius, though in a mass spectrometer, the particles are absorbed by a detector instead of simply circling in the field.
The radius of the circle which the ionized particle travels in is dependent upon the m/z ratio, where a larger mass (m) a larger radius.
Since the radius is dependent on the m/z ratio, and the z represents charge, it turns out that the particles with lesser charge will have a larger radius of deflection. Therefore the surgeon is mistaken for claim III.
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[answers] => Array
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[0] => Array
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[each_answer] => A. I and II
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[each_answer] => B. I, II and III
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[each_answer] => C. I and III
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[each_answer] => D. III only
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[quiz_unique_key] => 2261298308
[question] => As an ionized particle passes through a magnetic field, how will the kinetic energy of the particle be affected?
[value] => Array
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[answer] => 4
[description] => Reason for the Correct Answer:
Kinetic energy is found from the formula KE= ½ mv^2. Therefore a change in kinetic energy must involve a change in velocity or mass.
The mass of the particles will not be changing as they pass through the magnetic field.
Because the magnetic force always acts perpendicularly to the particle, it never experiences a change in speed, despite experiencing a constant acceleration.
Since the velocity and mass are constant, the kinetic energy will not change.
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[0] => Array
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[each_answer] => A. Its kinetic energy will decrease as it passes through the field.
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[each_answer] => B. Its kinetic energy will increase as it passes through the field.
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[each_answer] => C. Its kinetic energy will change depending on the charge of the particle.
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[each_answer] => D. Its kinetic energy will stay the same as it passes through the field.
)
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[quiz_unique_key] => 2377279144
[question] => The identification of cancerous tissue versus normal tissue using mass spec is dependent upon the lipid profile of the tissue. What aspect of these different lipids is being analyzed when using mass spec?
[value] => Array
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[answer] => 4
[description] => Reason for the Correct Answer:
The absorption of electromagnetic radiation is what infrared spectrometry measures.
The measurement of the magnetic resonance of atomic nuclei is what NMR measures
Mass spectrometry specifically measures the molecular weight of particles as they pass through a magnetic field.
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[answers] => Array
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[0] => Array
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[each_answer] => A. The various electromagnetic radiation frequencies absorbed by the lipids
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[each_answer] => B. The variation of infrared absorption of the lipids
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[each_answer] => C. The magnetic resonance of various atomic nuclei within lipids
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[each_answer] => D. The molecular weight of the lipids
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