Evidence
#1:
Re:
Discussion of partition
coefficients in discussing biotoxicity
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Terms
I understand better and use more accurately:
- partition
coefficient: a ratio of the concentration of a particular solute
in two
different mediums; in environmental chemistry, it can be used to
express the lipophilic or lipophobic nature of the solute, as well as
the volatility of the solute.
- Kow :
partition coefficient between octanol and water, high values indicate
greater lipophilic nature
- Kaw : partition coefficient between air
and water
(a.k.a. Henry's Law constant: partial pressure of gas above water = Kaw x
concentration of gas in water), high values indicate
greater volatility
- Koa
:partition coefficient between octanol and air, high values
indicate
greater lipophilic nature
The
evidence below shows that I was still insecure in my understanding and
use of partition coefficients terms (Koa,Kaw, Kow) as they applied to the
exposure to and toxicity of substances in the environment. The
idea of a partition coefficient made sense to me mathematically before
(i.e. I knew partition meant ratio), but until Chem505, my ability to
connect these values to a substance's volatility (and thus mobility or
tendency to disperse widely) and bioaccumulation (i.e. increased
concentration in the fat tissues of organisms higher up the food chain)
was still tenuous.
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Baseline evidence:
- "Pre-exam" for Chem505: Question 2 and 4
JUNE 2008
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Later Evidence:
- Koa: "Post
pre-exam" for Chem505: Question 2 and 4
AUGUST 2008
- Kaw:
PIM #1: the Clausius-Clapeyron Equation,
ΔH, & Henry's Law Constant for PCB congener 56+60
JULY 2008
- Problem set #9:
Persistence, Bioaccumulation, Toxicity
AUGUST 2008
- Kow :PIM
#4: Hexabromocyclododecane (HBCD)--brominated flame retardant (BFR)
persistence, bioaccumulation, and toxicity (PBT)
AUGUST 2008
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"Pre-exam":
(click on image for
larger view)
An analysis of the baseline shows I was comfortable
with the mathematical concept of a partition, or ratio, and that I had
some idea of how this term was connected to lipid solubility. I
did not, however, have a concrete understanding of how this ratio was
connected to bioaccumulation.
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"Post pre-exam":
(click
on image for larger view)
I have included the post pre-exam for symmetry--it
shows that I did indeed strengthen my understanding of the partition
coefficient (if looking merely at pre-/ post-exam grades, my growth in
understanding was responsible for 50% of the total improvement in my
grade, i.e. 2 of the 4 point improvement).
PIM #1:
PCB (Congener 56+60): discussion of Henry's Law Constant, Kaw
JULY 2008
(click on image
below for larger image of highlighted section)
click here
for full
.pdf of original
In this
piece of evidence, the Clausius-Clapeyron equation was used to
determine the ΔH
from a set of temperature
and partial pressure data at a particular site in Michigan. In
the course of the assignment, I learned that Kaw and
ΔH have an inverse
relationship. Substances with low Kaw
and high ΔH are
not very volatile; substance with high Kaw
and low ΔH, are volatile. Volatile substance (those
with Kaw
and low ΔH)
are
much more likely to volatilize than remain dissolved in water because
the energy it takes to vaporize it is low).
Linking Kaw and ΔH
to the relative location
of the congeners' origin is a connection I had not made before. A
volatile substance will tend to have a non-local source; in contrast, a
non-volatile substance (or low volatility substance) will tend to have
a local source because it does not vaporize and travel far. Thus, by
looking at the air-water
partition coefficient, Kaw
, one can determine how mobile a substance is.
Those substance with high Kaw
values can migrate far away from their location of
origin.
PROBLEM SET #9: Persistence,
Bioaccumulation, Toxicity
AUGUST 2008
(click
on image below for larger image)
PIM #4: HBCD: discussion of Kow
AUGUST 2008
(click on image
below for larger image of highlighted
section)
click here
for full
.pdf of original
Back to top
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Evidence #2:
Re: Discussion of semiconductors in Diode
Lasers
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Terms
I understand better and use more accurately in explaining how diode
lasers work:
- p-doped semiconductor--a semiconductor that has a very small
percentage of its ions replaced with electron deficient atoms; this
produces positively charged "holes" in the valence band which can
act as charge carriers
- n-doped semiconductor--a semiconductor that has a very small
percentage of its ions replaced with atoms with more electrons than the
original semiconductor; this produces excess electrons that occupy the
conduction band and can act as charge carriers.
- conduction band--refers to a small range of energy
levels corresponding to the lowest energy antibonding orbitals
- valence band--refers
to a small range of energy levels corresonding to the highest energy
bonding orbitals
- pumping by electrical current--the
application of a voltage that produces an electrical current; this
electrical current maintains the inherent population inversion in a p-n
junction necessary for light amplification.
The terms above are fairly specific to
the field of semiconductors. I have grown in my ability to use
them accurately as I have increased in my understanding of how
semiconductors in diode laser work.
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Baseline evidence:
- Chem506
Class notes: introduction to semiconductors
AUGUST 2008
- Lasers Preview Question (Quiz #5 in Chem507):
FEBRUARY 2009
- Excerpt from Diode Laser Wiki (Chem507)
MARCH 2009
- E-mail to Dr. Michael Topp re: diode lasers
AUGUST 2009
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Later Evidence:
- Explanation of
chalk diagrams (drawn during conversation
with Dr. Michael Topp in Chem508)
and electronic image (e-mailed by Dr. Topp)
AUGUST 2009
- Follow-up e-mail to Dr. Michael Topp re: diode
lasers
AUGUST 2009
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As an Environmental Chemistry major,
I had some understanding of the mechanisms underlying lasers because of
my senior year (2001-2002) Physical Chemistry lab course where I
learned about how dye lasers worked. While diode lasers and LEDs
(light emitting diodes) had, by that point, become ubiquitous in
consumer applications (e.g. CD players, laser pointers), I did not
connect the way in which they worked with what I had learned in my
courses. I certainly don't recall having any substantive
exposure to the chemical concepts underlying semiconduction.
The first piece of baseline evidence shows where I first meaningfully
encountered terms related to semiconductors, and began my understanding
of the molecular and energetic properties that makes semiconduction
possible. I recorded the notes in large part
because I found the topic quite interesting, even though I didn't fully
understand it. In this evidence, you see my first exposure to the
terms n-doping and p-doping, as well as an introduction to the concept
of a band gap that separates a conduction band from a valence band and
the concept of applying a potential (voltage) to a semiconductor to
produce light or electricity. My understanding, however, was
still passive, and
I did not understand these terms well enough to
teach or explain the concept with much clarity or accuracy.
Chem506 notes on semiconductors:
(click
on image for larger view)
The second piece of baseline evidence shows what I
recalled about the properties and applications of lasers. I remembered their
coherent, monochromatic, colinear nature (a result of the amplification
produced during stimulated emission in an appropriate laser
cavity). I also recalled some applications. This evidence
shows the beginning of a shift where I had to produce evidence of my
understanding.
Laser
Preview Question:
(click
on image for larger view)
The third and fourth piece
of evidence (and excerpt from a diode laser wiki assignment in Chem506
and an e-mail exchange with Professor Topp in Chem508, respectively)
shows how I interpreted and synthesized outside resources as I began to
create my own understanding of how diode lasers worked.
In examining these documents, it is clear that I was still mixing up
ideas and producing a Frankenstein-like amalgam of conceptual
understanding. I had a difficult time differentiating between the
way in which non-diode, optically pumped lasers and diode, electrically
pumped lasers achieved and maintained population inversion. A
quote from the last piece of evidence summarizes my fluency in using
these terms: "a
bit labored and not completely accurate."
Excerpt from Diode Laser Wiki:
(click on
image to go to larger view)
E-mail to Dr. Topp:
(click on image to
go to larger view)
Specifically,
I did not realize that the p-n junction of a diode already produces an
artificially created population inversion because of the structure of
an
n-doped and p-doped semiconductor. An electrical current is
used to pump the diode laser to maintain the original pre-existing
population inversion, which allows for light amplication.
Without the current, the extra electrons from the n-conductor would
relax into the holes of the p-conductor and produce an electrically
neutral region (effectively quenching the ability to produce
light).
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The first piece of baseline evidence shows
how I have integrated semiconductor terminology into my lexicon.
Specifically, I have used my own phrasing while accurately using the
scientific terms--this shows I have progressed from merely parroting or
copying terminology and explanations from an external source to being
able to generate my own explanations. I am fairly comfortable that I
could explain or teach the underlying concept now (at this level).
Explanation
of the P-N
Junction (chalk and electronic diagram):
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P-doped and N-doped semiconductors
The p-n junction of a diode occurs at the boundary between a p-doped
semiconductor and an n-doped semiconductor. The n-doped semiconductor is
made by doping with atoms with an extra electron, which must occupy the
conduction band because the valence band is fully occupied. In
the p-doped semiconductor, there are "holes" or vacancies in the
valence band due to doping with electron deficient atoms.
The Natural
Population Inversion of a P-N Junction
The specific properties of each semiconductor (i.e. "holes" in the
valence band and extra electrons in the conduction band, respectively)
produce a natural population inversion when the two types of
semiconductors are brought together.
Pumping
As the
conduction band electrons relax and occupy the "holes," the
population inversion can only be maintained by applying an potential
that
removes electrons from the p-type semiconductor and replenishes the
electrons in the n-type semiconductor (i.e. applying a voltage that
produces a current). The application of this potential is only used to
pump the diode
laser to maintain the original pre-existing population inversion and
allow for continuous lasing action. Without the current, the
extra electrons from the n-conductor would relax into the holes of the
p-conductor and produce an electrically neutral region (effectively
quenching the ability to produce light).
Diode lasers,
compared to other lasers (e.g. dye lasers)
Similarities. Diode
laser and other lasers share general charateristics. Both must
maintain a population inversion to produce the signature amplification
of light that occurs with stimulated emission. Likewise, both
diode lasers and other lasers must have some type of resonant laser
cavity that aids in this process. For both types of lasers, the
relaxation of an electron across the band gap produces light emission.
Differences.
In diode lasers, the population inversion exists because of the nature
of the p-n junction (even before voltage is applied). Voltage is
applied (i.e. the diode is pumped with an electric current) only to
maintain this population inversion. In non-diode lasers, the
population inversion typically does NOT exist before the laser is
pumped. Furthermore, the resonant laser cavity in a diode laser
is often the gap within the p-n junction itself, whereas the resonant
laser cavity in a non-diode laser is typically a long columnar tube in
which the laser medium resides.
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The second piece of evidence shows positive feedback that
indicates my growth in understanding and ability to use terms related
to diodes and diode lasers.
Follow-up e-mail to Dr. Topp:
(click on image to
go to larger view)
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