Download The Mass Spectrophotometer

The Mass Spectrophotometer
And you!!
RECALL:
• When a charged particle is moving within a
magnetic field there is an interaction of the
moving charge with the interacting magnetic
field:
•
FB = qo
v
B
• Units:
N
c
m/s
T
Application
•
•
•
•
The mass spectrophotometer
Used to determine the masses of isotopes
Or the mass of a molecule
Or the different molecular masses in a sample
so you can have insight into the composition
of the sample
• Or to purify samples (like enrichment of
uranium...)
How does it work????
• Charged particles (with known charge) are
accelerated using a particle accelerator (do
you recall those from the last unit of study?)
• The charged particles pass into parallel plates
with E field of given magnitude
Diagram the following:
+
Detection
plate
+
+
+
-
What direction is the E field?
What direction is the force on the charge?
Where will the charged particle go?
+
Detection
plate
+
+
+
E
Fele
-
So... a B field is used to balance the Fele
What direction should the B field be so that it can
balance the Fele??????????(think 2nd RHR)
+
Detection
plate
Fb
+
+
+
F ele
-
IF YOU ARE GIVEN
• The V of the charged particle – then you can
calculate the magnitude of the B by setting
•
Fele
=
Fb
•
q E
=
q v B
• But for most of the problems you face – you
will be given B and use it to calculate the v
So... What path will the charged particle take once it
leaves the (balanced) E field region (where only the B
field acts on the charge)?
+
Detection
plate
+
+
+
-
F ele
r
Fb
So now we can calculate the V of the
charged particle...
• Set
Fele
qE
SO: V = E
B
=
Fb
q V B
Knowing: E, B, q, r, and calculated v
• We can calculate the mass of the charged
particle...
• Recall your studies of circular motion...
•
Fc
=
Fb
m v2
r
=
qvb
Hey guys...I have a great idea...
Let’s try a sample calculation
• An ion with a +2 charge is passed through an E
field with a magnitude of 5.1 x 104 N/C, and a
B field with a magnitude of .75 T. The
measured r of the ion’s trajectory is 15.2 cm.
What is the mass of the ion???
Just remember that:
• You can solve by setting Fc = FB
• But you will have 2 unknown variables
(m and v)
• So solve for v first by setting Fele = FB
DID you find:
• V = 6.8 x 104 m/s ?????????????
• And m = 5.37 x 10 -25 kg ???????
• GOOD JOB!!!!!!!!!!!!!!!!
• This type of problem might just be on the test –
• Just
saying..........................................................................................
.....................................................................................................
....................................................................................................
So... What path will the charged particle take if it
is more massive than the first particle?
• Larger r !!!
+
Detection
plate
+
+
+
+
-
F ele
Fb