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OCR for page 167
CHAPTER V
Instrumentation in
Chemistry
All scientific knowledge is rooted in our
abilities to observe and measure the world
around us. Thus, science benefits enor-
mously when more sensitive measuring
techniques come on the scene. This is the
situation in chemistry today.
The discussions to follow will identify a
number of powerful instrumental methods
that are now the everyday tools of re-
search chemists. We will focus on the
capabilities of today's instruments and on
how much they have changed over the
last decade or two.
~67
OCR for page 168
Al
Hi,
A Laser Flashlight
A laser flashlight? Sounds like something out of Buck Rogers or Star
Trek! What would that be? Well, to bring this down to Earth, let's first
think about what a laser is and then how to make it into a flashlight.
Lasers are very special light sources. They put out pencil-sha~p beams
of pure color end so intense they can be used to cut patterns in steel. Also,
they can be focused so sharply they are better than a surgeon's knife in
mending the retina in your eye. Finally, they can give light pulses as short
as a millionth of a millionth of a second! That's what's called a picosecond. With
shutter speeds that fast, chemists can now `'photograph'' the fastest chemical
changes known.
And how do lasers work? It all begins with a whole bunch of atoms or molecules
all ready to emit light of exactly the same color. Atoms and molecules usually
absorb light, not emit it, so somehow we've not to DUmD them UD in energy SO Hat
~ ,&. ~ &. ~ ~
they're more inclined to emit than absorb. This is called a "population inversion.'
Once we've got a population inversion, there are some tricky things to do with mirrors
to make a laser out of it but we don't have to go into everything.
So how do we pump up the molecules to get that population inversion? One good way
to do it is to use electrical energy, like we do In a fluorescent light. That's what you might
use to light up a dark closet to look for your missing sneaker. And it works fine as long as
you have a long enough extension cord. But think of looking for your jacl: ~ the back of
your car when you have a flat out on a dark highway. That's where a flashlight comes In
handy.
In a flashlight, the energy comes Tom a chemical reaction. That's what the batteries are
all about. Could we use a chemical reaction to pump a laser? If so, it would be a "chemical
laser,'' our laser flashlight. But that would require a chemical reaction that produces a
population inversion. Trying to find out whether there are such reactions led chemists to
the discovery of the first chemical laser. O[course, it's hardly news that chemical reactions
can emit light. Candles do it all the fume. And thirds about a firefly—he (or she) Carl do it
without an extension cord. These emissions show that a reaction follows special
pathways, and when energy Is released, these preferred pathways might be a super
~ ~ way to get a population inversion.
Mar W '599~
._
. _
'it
'~_- ~
._
168
I,
The suspense, though, was that chemical lasers were not discovered
by looking at bit flames or by copying the firefly. Chemical lasers
were discovered to operate best In the infiared, where the human
Or eye is blind. This spectral region is where molecular vibrations
` ~ can cause molecules to absorb or emit light. From these lasers
cat . . .
` we reamed that quite a few reactions prefer reaction path-
.~ Sways that put most of the available energy into vibrational
`~' motions of the final products. Why this happens still isn't
"I clear, but we're working on it. In the meantime, we have a
whole bunch of fine chemical lasers. They can be very
efficient, which has made at least one chemical laser a
candidate to be the match to lift the nuclear fire of nuclear
fusion. If that worked, chemical lasers would help us get '~clean" nuclear energy for the
rest of tune. Chemical lasers can also be very intense, as shown with the fluonne-hydrogen
flame laser. What good is that? Well? that gets us back to Buck Rogers, Star Trek. and Star
Wars. If you want a laser out in space, you'll either need a chemical laser or a mighty long
extension cord.
So wraths next? We're still after that firefly.
OCR for page 169
V-A. INSTRUMENTATION FOR STUDY OF CHEMICAL REACTIONS
V-A. Instrumentation for Study of Chemical Reactions
Section IV-A indicated how the chemist's use of the most modern instrumenta-
tion is making it possible to investigate even the fastest chemical processes in
intimate detail. We are witnessing a quantum jump ahead in our understanding of
the factors that control the rates of chemical reactions. Among the tools respon-
sible for this rapid advance are lasers, computers, molecular beams, synchrotrons,
and, on the horizon, free-electron lasers. We will consider each in turn.
LASERS
Chemical lasers have been discussed in "A Laser Flashlight" on page 168. For a
laser to work, it needs a "population inversion" in which there are more molecules
that have enough energy to emit light than there are molecules ready to absorb
light. To maintain such an inversion, energy must be injected somehow. Some
energy-releasing reactions do this (resulting in chemical lasers), but the energy can
be injected in other ways. The simplest way is through irradiation with a
conventional light source. However, electrical energy input is probably the most
convenient way to establish a
population inversion. The ap-
paratus need not be much dif-
ferent from a fluorescent light
fixture.
Whatever the manner of en-
ergy input (the "pumping"
method), the special qualities of
laser light arise from "stimu-
lated emission," which can be
regarded as the inverse of light absorption. A photon of light of the exact energy
needed to excite a molecule from one energy level to another, higher level can
stimulate emission of a second photon from a molecule already in the higher level. The
second photon that is thus produced turns out to be perfectly in-phase ("coherent")
with the electromagnetic wave of the first photon that started it all. This coherence
gives lasers their distinctive character. It accounts, for example, for the pencil-
sharpness that permitted us to reflect a laser `'searchlight" beam offa mirror placed on
the Moon by the Apollo astronauts.
The remaining feature of a laser is a set of accurately focussed mirrors that cause
any stimulated emission to go back and forth many times through the population
inversion. These mirrors are called an optical cavity; they permit and cause the
buildup of the special qualities of laser light.
Lasers bring to mind a brilliant beam of light cutting through a sheet of steed or
shining deep into space. But to a scientist, the beauty of the laser lies in its ability
to deliver light of extremely high intensity, extremely high power, extremely high
spectral purity, and/or extremely short duration. For a given experiment, laser
design is dictated by the one of these features of greatest value to the experiment
at hand, and usually at some sacrifice in the others. Some of this trade-off is
PUMP I NO
LIGHT
hv~
coo o
Font
ax' 0 0 0
o
E2
LIGHT ABSORPTION CAN ESTABLISH
A POPULATION INVERSION
169
OCR for page 170
170
MIRROR
F LUORESCENT M I RROR
Dl SCHARGE
Z=~=~
~ ..
~ AM I TG.NT ~
_ _ HIGH
—VO LTAGE
~t ~ ~
A LASER NEF.D NOT BE; COMPLICATED
INSTRUMENTATION IN CHEMISTRY
imposed by the Uncertainty
Principle. This fundamental
premise of quantum mechanics
states that the duration of a
light pulse is related to and
limits the spectral purity.
Thus, the Uncertainty Princi-
ple tells us that if a pulse is as
short as one picosecond (10-~2
seconds), there will be an uncertainty in the frequency (the color) at least as large
as 5 cm-. With this much frequency spread, most information is lost about
molecular rotations of gaseous molecules. On the other hand, if a line width of 0.005
cm~~ is needed to detect individual rotational states, then the molecule of interest
must be examined by a light pulse at least as long as one nanosecond (10-9
seconds). This limitation depaves us of time information about species or events
with shorter lifetimes than the nanosecond probe.
PULSE DURATION ~ > SPECTRAL PURITY
,-
ONE
MICROSECOND = .000 OO1 SECOND
ONE
NANOSECOND S .000 OOO OO 1 SECOND
ONE
PICOSECOND = .000 OOO OOO OO 1 SECOND
ONE
FEMTOSECOND - .000 OOO OOO OOO OO 1 SECOND
- .ooooo5 cm l
.005 cm l
< - 5 cm
5OOO cm l
PULSE DURATION LIMITS FREQUEN CY ACCURACY
AND V ICE VERSA
Developments in the Last Decade
There were three crucially important developments in laser technology that took
place during the 1970s and they are having a great impact on chemistry. First,
several types of tunable lasers were developed and became commercially available.
A "tunable laser" is one whose color (wavelength) can be selected according to
need. The wider the range of the spectrum that lasers can work over, the more
valuable they are as a research tool. The most important of these was the dye laser,
which gave continuous color tuning throughout the visible region of the spectrum
and a bit beyond into the near-infrared and near-ultraviolet. Dyes are chemical
compounds whose intense color causes them to absorb light efficiently so that they
can then emit coherent laser light. Second, the invention of efficient ultraviolet
lasers gave scientists access to the photochem~cally important ultraviolet region at
wavelengths shorter than 300 mm. These include "excimer lasers'' that are based
upon light emitted from molecules formed from electronically excited reactants. An
example is the krypton fluonde laser. Krypton is an inert gas that does not form
bonds in its ground state. After one of its valence electrons is excited, however, the
OCR for page 171
V-A. INSTRUMENTATIO1V FOR STUDY OF CHEMICAL REACTIONS
resulting krypton atom has the
chemistry of rubidium. Thus,
the molecule formed between
Kr and F has the bond strength
and stability of RbF. This is a
desirable factor in building up
concentration to reach a popu-
lation inversion, so that it can
emit laser light. The third de-
velopment was the discovery
of methods of laser operation
that gave short-duration light
puIses~ne picosecond or
Kr electron
Krypton ' coll~S'°~
(ground state)
5s O
4p
Is
3d ~ ~ 3) ~ fin
-17~1
Kr.
Krypton
(excited state)
Hi,
~0
}fir (gas) + F2 (gas) ~ (KrF)~(gas) + F(gas)
Pb~gas) ~ F2(gas) - > RbF(gas) + F(gas3
Rb
Rubidium
(ground state)
0
AFTER EXCITATION KRYPTON REACTS LIKE RUBlI)IUM
THAT MAKES EXCIMER LASERS POSSIBLE
less.
In 1970, the tunable dye laser did not exist except as a laboratory curiosity. In the
early 1980s, almost every chemistry research laboratory had more than one tunable
laser source. Tunable lasers can now be conveniently operated over the wavelength
range from 4 microns in the infrared (40,000 A) to 1,600 A in the ultraviolet beyond
the wavelength at which air becomes opaque (i.e., into the range called the
"vacuum ultraviolets. Already in the state-of-the-art stage are lasers that extend
the wavelength range to beyond 20 microns (200,000 A) in the infrared and to less
than 1,000 A in the vacuum ultraviolet.
Chemical Applications
Table V-A-l lists many chemical applications of lasers. It is important to note
that most of the more powerful lasers are not continuously tunable; they have only
particular output wavelengths. They are most useful in the study of solid matenals,
which will usually absorb a wide range of wavelengths of light. For most chemical
applications, tunable sources are critically important, and these lasers are often
TABLE V-A-1 Some Research Areas Utilizing the Laser
Area Research Application
Solar energy research, photosynthesis
Uranium, plutonium isotope purification
Trace element analysis, environmental
monitoring
Probing flames, explosions
Monitoring industrial processes
Cell discrimination and separation
Photochemistry within biological cells
Gas-phase deactivations, Chemical reactions
Laser Used
-
Excimer, dye
Excimer, dye, TEA CO2
Continuous ion, color center
Photochemistry
Isotope separation
Atomic absorption,
fluorescence
Combustion diagnostics
Atmospheric gas analysis
Biological cell sorting
Cell bleaching
Microsecond kinetics
(1-100) x 10-6 see
Nanosecond kinetics
10-6 to 10-9 see
Picosecond kinetics
10-9 to 10-'2 see
Subpicosecond kinetics
<10-'2 see
Excited state lifetimes, very fast reactions
Fast electronic state deactivation, coherence
decay in liquids
Vibrational deactivation in solids and liquids
Solid-state, dye
Semiconductor diode
Ion laser
Dye laser
Flashlamp dye, TEA CO2,
chemical
Solid-state, excimer
Ion, solid-state
Ion, solid-state
OCR for page 172
172
INSTRUMENTATION IN CHEMISTRY
excited with another powerful, single-frequency laser. Having the best suited laser
system is essential for work at many of today's most exciting chemical research
frontiers.
COMPUTERS
The use of computers by chemists has paralleled the tremendous computer
development of the last three decades. The size of this growth is reflected in the
number of industrial installations of the largest IBM computers over this same time
penod. In the mid-1950s, there were 20 or 30 such machines (IBM 701s). By the
mid-1960s, the much more
powerful 7094 and 360 systems
numbered about 350. Today,
there are perhaps 1,700 indus-
tnal installations of IBM 3033s.
This numerical growth has
been accompanied by a phe-
nomenal increase in computer
power.
The extent to which chemis-
try has benefited from this
growth can be seen by compar-
ing two landmark calculations.
For polyatomic molecules, the
first theoretical calculations
based upon the Schroedinger
Wave Equation without any
simplifying assumptions (an ab
initio calculation) appeared in
the 1960s. Of special impor-
tance was the study of rotation
around the carbon-carbon bond
of ethane, C2H6. As the hydrogen atoms at one end rotate past the hydrogen atom
at the other end, the energy rises to a maximum. To learn the height of this internal
rotation barrier, theoretical cal-
TABLE V-A-2 Relative Computing Speeds of
Computer Levels
2000
,_ 1 500
<5:
I:
cat
z
~ Too
lo:
-
o
~ 500
:~
MA I NFRAME COMPUTERS
_
_
t
~ 1 1 1 1 1 1 1 1
t I I I
1 960 1 970 1 980
YEAR
INDUSTRIAL USE OF LARGE COMPUTERS
Computing
Superminicomputers
Mainframes
Supercomputers
Example
DEC VAX 11/780
IBM 3033
CRAY IS
(1)
10-15
80-120
culations ("self-consistent field"
method) were based upon a ba-
Relative SiS set of 16 functions.
Speed This can be contrasted with a
recent and similar study of deca-
methy! ferrocene, ECs(CH31512Fe.
This calculation used a basis set
of 501 functions. Since such stud-
ies require computing effort proportional to the fourth power of the number of basis
functions, the decamethy! ferrocene computation involves (501/1614 or one million
times more computation than the ethane problem!
\
OCR for page 173
V-A. INSTRUMENTATION FOR STUDY OF CHEMICAL REACTIONS
Superminicomputers
This level of computer has become a workhorse in chemistry. Instruments like the
DEC VAX 11/780 are comparable to the worId's largest mainframe computers
available in the late 1960s. They have revolutionized computing in chemistry because
of their substantial capacity,
high speed, and lowered cost,
which is now in the range
$300,000 to $600,000.
The last 20 years have also
seen three important develop-
ment phases for the use of com-
puters in chemical expenments.
In the first, computerization
phase, advances in both hard-
ware and software greatly im-
proved our ability to accumulate
measurements (data acquisi-
tion). Then an automation phase increased the possibilities for experiment control
through continuous morutonug of cntical parameters. F~nady, a "knowledge engineer-
ing" phase ushered in an era in which computers perform high-level tasks to interpret
collected information.
An excellent example is the Fourier Transform aIgonthm which permits us to record
spectral data over a long time period, thereby to achieve high speck resolution.
Because this allows detection of quite weak signals, this aIgonthm is now routinely
used to record ~3C NMR sign~s and to transform infrared inte~erograms. Because of
the success of these instruments, the Fourier Transform aigonthm is now being
incorporated into ad sorts of equipment: electrochemical, microwave, ion cyclotron
resonance, dielectnc, and solid-state NMR instrumentation.
\ ASH
C_H
me,
ETHANE
1 6 FUNCTIONS
CH3` C ~ _CH~
- ~ -C
CH3 C - C C—CH3
HA
~ ,
HI ~
CH3—Cx —C ACHE
ARC . '-Can
CH , CH3
3
(50 1)4 _ 1 0
DECAMETHYL FERROCENE
50 I FUNCTIONS
A MILLION-FOLD ADVANCE IN TWO DECADES
Mainframe and Supercomputers
Some needs for computation in chemistry can be met only with the greater
capacity and capability of the largest scientific computers (Cray/M and X-MP or
CYBER 205), coupled with specialized resources such as software libranes and
graphics systems. This is most notably true for electronic structure studies for
many atom molecules beginning with the complete Schroedinger Equation and
without approximations tab initio calculations).
Another area that will benefit from supercomputers is computational biochem-
istry. Most dynamical simulation procedures applicable to biological molecules
require calculation of the simultaneous motions of many atoms. A conventional
LOO-picosecond molecular dynamics simulation of a small protein in water would
require about 100 hours on a DEC VAX Il/780 or 10 hours on an IBM 3033.
Calculations of the rate constant for a simple activated process require a sequence
of dynamical simulations to determine the free energy barner, and additional
simulations to determine the nonequilibrium contributions; the times can now
173
OCR for page 174
174
INSTRUMENTATION IN CHEMISTRY
reach 1,000 hours on a DEC VAX 11/780. More complicated processes or longer
simulations become impossible without the much higher speeds of supercomputers.
MOLECULAR BEAMS
The advances of vacuum technology over the last three decades have made it
possible to reduce the pressure in an experimental apparatus to a point at which
molecular collisions become quite improbable (e.g., at pressures below 10-9 torr).
Under these conditions, molecules that enter the vacuum chamber stream to the
opposite chamber wall without deflection. Such a situation is called a "molecular
beam." This provides a special opportunity to study chemical reactions. The most
obvious application is to cause two such molecular beams to intersect. When a
molecular collision does occur, it is almost always in this intersection zone. If the
collision causes a chemical reaction, the product fragments leave the reaction zone
with energies and directions that provide information about the reactive collision.
By measuring the spatial distribution and fragment energies, we can learn intimate
details about single-collision chemistry.
Capabilities
A typical, crossed molecular beam apparatus can contain as many as eight
differentially pumped regions provided by various high-speed and ultrahigh-vacuum
pumping equipment. It may be necessary to maintain a pressure differential from one
atmosphere of pressure behind the nozzle of the molecular beam source to 10-i ' torr
at the innermost ionization chamber of the detector. What is glibly ceded the
"detector" is likely to be an extremely sensitive mass spectrometer with which to
measure the velocity and angular distributions of products. By replacing one of the
beams by a high-power laser, molecular beam systems are now giving new kinds of
information on the dynamics and mechanism of primary photochem~cal processes.
In the past 5 years, molecular beam experiments have played a crucial role in
advancing our fundamental understandings of elementary chemical reactions at the
microscopic level. These advances provide deeper insights with which to build our
explanations of macroscopic chemical phenomena from the information gathered in
microscopic experiments. The pervasive importance of these deeper insights was
recognized in the award of the 1986 Nobel Prize in Chemistry to those responsible for
bringing molecular beams into chemistry.
SYNCHROTRON LIGHT SOURCES
Characteristics of Synchrotron Sources
The most intense, currently available source of tunable radiation in the extreme
ultraviolet and X-ray region is synchrotron radiation, which is produced when
energetic electrons are deflected in a magnetic field. That happens, of course, all the
time in a synchrotron, which is an instrument that accelerates electrons to very high
energies for particle physics studies. To reach these high energies, the electrons
must be "recycled" through the accelerating zone many, many times. "Recycling"
requires bending their trajectories through four successive 90-degree turns. At each
OCR for page 175
V-A. INSTRUMENTATION FOR STUDY OF CHEMICAL REACTIONS
of these turns, the acceleration needed to change direction causes intensive
radiation over the entire spectral range from the far-infrared to the X-rav range.
This has, in the past, been looked on as an imtating energy loss.
Now, however, synchrotrons are running out of things to do in high-energy
physics. Hence, attention has turned from synchrotrons as accelerators (with
radiation seen as undesired energy loss) toward snychrotrons as sources of light.
Devices are placed inside the accelerator that increase the number of sharp bends
in the electron trajectories to increase these radiative properties. These devices are
descriptively called "wigglers" or "undulators." They show potential for intensity
increases by several powers of 10 over the already bright radiation emitted by an
ordinary synchrotron. Principal current use of tunable synchrotron radiation fails in
the X-ray energy range, 1-100 keV.
— ~.~ ~ —^'D— '
Applications of Synchrotron Sources in Chemistry
Extended X-ray Absorption Fine Structure (EXAFS) has been one of the more
fruitful applications of synchrotron radiation to solid substances. When one of an
atom's inner-sheD electrons is excited by an X-ray photon, the atom emits light that is
then diffracted by neighboring atoms. The result is a diffraction pattern that contains
information about the interatom-
ic spacings of these neighbors. ~ ° ~
Much attention has been di- ~ BW5
FREE ELECTRON .
C 10 . "' i; LASERS . ~
r~£ 1 o1
o
1 0
LO
Con 15
10
ol 4
OCR for page 176
176
INSTRUMENTATION IN CHEMISTRY
stimulated emission can occur to produce laser light. Such a device is called a
Free-Electron Laser (FEL).
Potential Capabilities
Experience to date indicates that high-efficiency wavelength tunability and high
average and peak power will all be forthcoming over a wavelength range extending
from microwave frequencies through the infrared and visible to the vacuum
ultraviolet spectral ranges. Average bnghtnesses several powers of 10 greater than
those provided by conventional tunable lasers or synchrotron sources might be
possible, particularly in the ultraviolet. An FEL has been operated at Los Alamos
National Laboratory, based on a linear accelerator 2 or 3 meters long. Once a
second, the device provides a train of pulses of tunable infrared radiation
currently, in the 9- to 11-micron wavelength range, with 30-picosecond pulses, peak
power of 5 megawatts, and 50-nanosecond spacing between pulses. Such perform-
ance extended over the mid-infrared spectral region (4 to 50 microns) would open
the way to many novel applications in chemistry. Examples are vibrational
relaxation, multiphoton excitation, nonlinear processes in the infrared region, fast
chemical kinetics, infrared study of adsorbed molecules, and light-catalyzed
chemical reactions. As the wavelength is moved through the visible and toward the
ultraviolet, a variety of novel chemical applications could be explored in photo-
chemistry and fast chemical kinetics, as well as multiple photon and other nonlinear
processes.
SUPPLEMENTARY READING
Chemical & Engineering News
"Laser Vaporization of Graphite Gives Sta-
ble 60-Carbon Molecules" by R.M. Baum
(C.&E.N. stab, vol. 63, pp. 20-22, Dec.
23, 1985.
"Imaging Method Provides Mass Transport'
(C.&E.N. staff, vol. 63, p. 29, Sept. 23,
1985.
'~Computers Gaining Fiery Hold in Chemical
Labs" by P. Zurer (C.& E.N. staff), vol.
63, pp. 21-31, Aug. 19, 1985.
"Supercomputers Helping Scientists Crack
Massive Problems Faster" by R. Dagani,
, .
vol. 63, pp. 7-14, Aug. 12, 1985.
"Spectroscopic Methods Useful in Inorganic
Labs" (C.&E.N. staff), vol. 63, pp. 33-39,
Jan. 14, 1985.
"Technique Allows High Resolution Spec-
troscopy of Molecular Ions" by R.M.
Baum (C.&E.N. staff), vol. 62, pp. 34-35,
Feb. 20, 1984.
"Extreme Vacuum Ultraviolet Light Source
Developed" by R.M. Baum (C.&E.N.
stab, vol. 61, pp. 28-29, Feb. 7, 1983.
"Synchrotron Radiation" by K. O. Hodgson
and S. Doniach, vol. 56, pp. 26-27, Aug.
21, 1978.
OCR for page 177
The Ant That Doesn't Like Licorice
While touring through the Costa Rican jungle recently I stumbled on a terribly wide
path completely devoid of plant life. The path must have been 6 feet wide, and as I
strolled along it I tried to keep out of the way of the native ants who were bustling past
me. Each of the ones going the other way was carrying a big piece of leaf overhead; the
whole bunch looked like a fleet of Chinese junks sailing along.
Suddenly, I was oversalted by this very attractive native ant. `'Hi there!" I
introduced myself, '`My name is Red Ant. What's your name?" Blushing, she
answered,"My last name is Formicidae, but they call me
L`eafeutter.'' "Say, that's a pretty name. Why do they call
you that?" Giggling, she said, `'Everyone knows why
it's because that's my business." She gracefully
pointed an antenna at a pitifill-looking tree up the path.
"See that?" she asked, "My sisters and I did that.
We cut every one of the leaves off that tree in only 5 ~~_
days. Enough to feed the whole family for 2 months.'' ~
She turned to leave. "Don't go!" I exclaimed, "I'll Go' ~ , _
get you a leaf from this tree right here." Reaching ~ ~~'
toward a lush tree that all the other ants were passing by,
pulled oiT a leaf and presented it to Leafcutter. "Pew!" she ~~''~'~' Am'
said, holding her nose, "Take it away—~ hate licorice." Sure enough,
the leaf ~ held smelled just like liconce. I wondered what was wrong with licorice
Leafcutter explained `'I'm not sure why, but Mama doesn't like it when
we bung leaves that smell like that into the anthill." I was still
poled so I asked her if she'd show me her home.
Leafeutter lived in this gorgeous anthill along with her 5
Anion sisters, 500 brothers, her Mama, and, believe it or
not, a Angus! Her lazy brothers never lifted a feeler to
bring in even one leaf—all they seemed to do was amuse ~
Mama. And guess what? The ants didn't even eat all ~7:
~~'
those leaves they brought home at Soothe fungus did!
Apparently, the ants don't have the right enzymes to
metabolize carbohydrates. But the fungus thrives off those leaves,
and in gratitude to the ants for supplying them, it coIlveIts their carbohydrates into
delicious sugars that the ant family lives on. '`Mama says we're symbiotic," Lealcutter
exploded.
Scientists have also taken an interest in Lealcutter and her family. They have
concentrated on the leaves that Leaicutter doesn't like, trying to find out what protects
these leaves over the others. Using liquid chromatography, they've extracted 10 to 15
milligrams of about 50 different compounds from great piles of the rejected leaves. Then
they've worked to purify and identify these compounds. NMR studies have shown that
every one of those trees that L~eaicutter dislikes contain compounds with molecular
structures like that of carophyllene oxide, the compound that gives Sconce its flavor.
They've adso got this notion that it's the fungus that gets sick on those leaves. And
when the fungus gets sick there's no sugar for the ant family. So it looks as though the
liconce-flavored trees have learned to synthesize their own fungicide to protect
themselves from the Leatcutters The next step for those scientists will be to try to
synthesize some similar compounds to combat harmful fungi elsewhere. The next step
for me is Into a cozy little anthill with Leaicutter we're getting hitched in the spring.
177
OCR for page 192
192
INSTRUMENTATION IN CHEMISTRY
first few layers. Two or more complementary methods used together can greatly
enhance the significance of any single measurement used alone.
TABLE V-C-1 Instrumentation Relevant to Chemistry on Surfaces
. .
Method
Electron energy loss EELS
spectroscopy
Bombard or
Irradiate
Acronym with:
Electrons,
1-10 eV
Physical Basis
Information Obtained
Molecular structure,
surface bonding of
adsorbed molecules
Molecular structure of
adsorbed molecules
Energy of surface
binding
Vibrational excitation of
surface molecules
Infrared spectroscopy IRS
Thermal desorption
Auger spectroscopy
Low-energy electron
diffraction
Secondary ion mass
spectroscopy
TDS
Auger
LEED
SIMS
Heat
Infrared light Vibrational excitation of
surface molecules
Thermally induced
desorpiion of
adsorbates
Electron emission from
surface atoms
Back-scattenng,
diffraction
Ejection of surface
Electrons,
2-3 keV
Electrons,
10-300 eV
Ions, 1-20
keV
atoms as ions
Surface composition
Atomic surface
structure
Surface composition
Electrons are useful surface probes because their energies, and hence, their
wavelengths, can be accurately controlled with their accelerating voltage. At low
energy, near 25 electron volts, the wavelength of an electron is close to the atomic
spacings in a metal, so a beam of such electrons reflected from the surface will show
diffraction ejects. Thus, low-energy electron diffraction (LEED) can play the same
role in determining bond distances and bond angles in surface chemistry as X-ray
diffraction plays in the structural chemistry of solids. LEED reveals the atomic
structure of clean surfaces, as well as any regulanty in the packing of atoms and
molecules adsorbed on the surface.
In the Auger (pronounced "Ohjay'') effect, high-energy electrons (2,000-3,000
eV) striking an atom cause the atom to eject a secondary electron from an inner
shell. The energy of the ejected electron is determined by the energy levels of the
atom it came from, so measurement of the electron energy identifies the atom.
Since the bombarding electrons do not penetrate deeply, these secondary electrons
reveal, with high sensitivity, the composition of the first few surface layers. This
information can be important because surface impurities and i'Te~ulanties can
. , , ~
,~ ~ _ _ ~ _
dominate surface chemistry. Hence, the combination of Auger and LEED is used
routinely to verify the cleanliness and perfection of the surface under study.
Electron energy loss spectroscopy (EELS) is of particular value because it
detects the resonant vibrational frequencies of atoms and molecules bound to the
surface. Chemists routinely use such vibrational frequencies for gaseous molecules
to decide which atoms are hooked to which, how strong the bonds are, and their
molecular geometry (see Infrared Spectroscopy later in this section). In EELS, an
electron beam of known energy is bounced off the metallic surface into an energy
analyzer. If the electrons hit an area where a molecule is adsorbed, the molecule
can be left vibrating in one of its characteristic motions. The energy needed to do
this, determined by the frequency of the motion, is taken away from the kinetic
OCR for page 193
V<. INSTRUMENTATION AND THE NATIONAL WELL-BEING
energy of the electron. The
measurement of these electron
energy losses of the reflected
beam gives a vibrational spec-
trum of the adsorbed mole-
cules.
Ion scattering from surfaces
has been used for surface com-
positior~ analysis with great
sensitivity, 109 atoms/cm2. In
secondary ion mass spectros-
copy (SIMS), neutral and ion-
ized atoms and molecular frag-
ments are ejected by bombard-
ment with high-energy (1-20
keV) inert gas ions. Ton scat-
tenng spectroscopy determines the surface composition by the energy change of
inert gas ions upon surface scattering. Ton etching removes atoms from surfaces
layer by layer. The combined use of ion etching and electron spectroscopy yields
a depth profile analysis of the chemical composition in the near-surface region. This
combination of instrumental methods is called "dynamic SIMS."
The availability of high-intensity laser sources is now awakening the develop-
ment of a new set of surface-sensitive techniques. Surface infrared spectroscopy,
laser Raman spectroscopy, and second harmonic generation surface spectroscopy
all provide information about the surface chemical bonds of adsorbed atoms and
molecules. All of these emerging surface science techniques will permit us to watch
chemical reactions as they occur on well-charactenzed and clean surfaces. This is
an important development in chemistry because surfaces provide the two-dimen-
sional reaction domain that accounts for heterogeneous catalysis.
reflected
electrons
E2-E1-1200
\
incident Of
electrons p'
/
;~ ~,~
ASH \ He
C
/ \
~ ~ ~ ~ M
E, /
EELS Vibrations of Surface Molecules
The Energy Loss }dentifies the Vibrational Mode
SURFACE ANALYSIS
Any sensitive measurement technique can be used as an analytical tool. This is
the case in the surface sciences. Every one of the capabilities listed in Table V-C-1
can be put to analytical use in the pursuit of questions that may be only remotely
connected to the surface sciences. As an example, a state-of-the-art laser micro-
probe device designed to desorb (remove) molecules from a solid surface can be
used to detect the presence of a pesticide on the leaf of a plant. Such a capability
was quite impossible only 10 years ago; today it permits us to contemplate tracking
the amount, stability, weathering, and chemistry of a pesticide in field use. Of
course, the analytical technique may just as well be concerned with monitonug or
clarifying chemical changes that take place on a surface or with a surface. Many of
these analytical studies relate to catalysis. In Section IV-C, examples were given of
the use of EELS to determine the molecular structures that exist on a catalyst
surface as it functions. Such applications have given rise to surface analysis, a new
subdivision of analytical chemistry.
193
OCR for page 194
194
INSTRUMENTATION IN CHEMISTRY
The elective sampling depth is a most important feature of any surface analytical
technique. Sampling depth is important, because the measuring technique must be
appropriate to the phenomenon under study. For example, bonding to the surface,
wettability, and catalysis in-
OOO LATERS valve only a few atomic layers,
whereas surface hardening
treatments involve 10 to 1,000
atomic layers. Typical sam-
pling depths for the primary
surface analytical techniques
are one or two atomic layers
for low-energy ion scattering, 5
A depth for SIMS, 20 A for the
Auger technique, and 100 ~ for
ion etching coupled with
SIMS. Laser mass spectrome-
try, the Raman microprobe,
and scanning electron micros-
copy (SEM) reach from 1,000
to 10,000 A (i.e.' to one
micron). l he shallower the
sampling depth of the tech-
nique, the more finely it is able
to define the surface composi-
tion of a sample.
A major challenge in the de-
velopment of surface analytical
instrumentation is the reinforcement of its quantitative dimension. Most of the
examples given have been concerned with what is there. We must also be able to
determine how much. Another important problem is the development of micro-
probes which can provide both chemical and positional information about surface
species. Currently, Auger and ion microorobes are useful in this respect for
_
it, ,,, ,,,, ~ ,,, ~ ~
CATA LTSI S ~
////
WETTAB I L I TT
_~
URFACE BOND I NG
cn
::~
E EECTR I CA ~ CONDUCT I V I TTl
l
~/~<
//////././//, . <~//,//~/.~,///////////////////////~/////~
PASSIVATION, SURFACE TREATMENTS I
~7~s~//////~
_OPT I CA L AB SORPT ~ ON //D
~,/,~,~i'~'~6,Y~/,~,~Y,~,~,.,^~/,~,,Y/~/i~//~//////~/y~y~///~/~
CORRO S I ON
~_~ ~~ ~~__~
- Low
ILM STRUCTURES ~
~'~///~////~///~/~///////~/////////////////////////////~/////~///////////////~/~
VISUAL EFFECTS, COLOR
ISS SIMS ESCA SIMS LASER SEM
(STATIC ) AUGER (DYNAMIC ) MS RAMAN
HOW DEEP IS THE SURFACE?
IT DEPENDS ON WHAT YOU CARE ABOUT
At. . .. ..
mapping elemental composition, as in revealing both the presence and location of
the trace contaminants phosphorus and lead in silicon chips. However, they are not
yet able to detect and map large organic molecules such as carcinogens or
therapeutic drugs. Characterization of small particles is another important chal-
lenge for surface analysis; this is particularly important in environmental monitor-
ing where the analysis of carcinogenic hydrocarbons on atmospheric dust and other
particulates is a current problem.
CHROMATOGRAPHY
Chromatography separates molecules or ions by dividing species between a
moving phase and a stationary phase. A liquid or a gas flowing continuously
through a tube (called a "column") provides the moving phase. The stationary
phase can be either small solid particles packed in the tube or, for a small-diameter
OCR for page 195
V-C. INSTRUMENTATION AND THE NATIONAL WELL-BEING
tube (a capilIary), the walls of the tube itself. If a pulse or a squirt of soluble
substance enters the tube at one end, that substance will have some tendency to
stick to the stationary surface, becoming adsorbed. However, the continuing flow
of fresh solvent keeps acting to redissolve this adsorbed matenal, moving it
forward in the tube. How fast the process of adsorbing and desorbing takes place
depends sensitively on the composition and structure of the substance. Conse-
quently, different substances that entered the tube together in the same pulse will
move at different speeds through the tube, so they will exit at different times.
This separation technique takes advantage of small differences in properties such
as solubility, absorbability, volatility, stereochemistry, and ion exchange, so that
understanding the fundamental chemistry of these interactions is basic to progress
in the field. Liquid chromatography has shown an impressive growth since 1970.
The current $400 million annual sales are mainly by U.S. manufacturers. This
growth has come through innovations such as high pressure and moving phases of
changing composition ("gradient moving phases") to give greater speed and
resolution. "Bonded-molecule" stationary phases are chemically designed to
increase selectivity and to extend the useful lifetime of a column. Detection also has
improved with electrochemical, fluorometric, and mass spectrometric detectors,
reaching sensitivities as low as 1o-~2 grams. Although gas chromatography is a
more mature field by perhaps a decade, important advances continue to appear.
High-speed separations can now be accomplished in a few tenths of a second;
portable instruments the size of a matchbox are in use outside of the laboratory. A
complex mixture can be separated into literally thousands of components, using
fused-silica capillary columns that are a direct spin-off from optical fiber technology
for communications. It is even possible to separate compounds that differ only in
isotopic composition.
, . _
High-Performance Liquid Chromatography (HPLC)
Dunng the 1970s, theoretical understandings of the complex flow and mass
transfer phenomena involved in chromatographic separation helped perfect column
design. During this same period, small-diameter (3-10-micron) silica particles with
controlled porosity were introduced. Synthetic advances in silica chemistry led to
the tailoring of particle diameter, pore diameter, and pore size distribution. Today,
15-cm columns with efficiencies exceeding 10,000 distillation steps ("theoretical
plates") are routine.
Still another major advance of the 1970s was the introduction of chemically
bonded phases in which surfaces of porous silica are covalently coated with organic
molecules containing silicon (organosilanes). Especially important is the use of
hydrocarbon attachments (such as n-octy} and n-octadecyI) to make the surface
look like an organic solvent. Then, the mobile liquid phase is typically an
organic-aqueous mixture. This is called reversed phase chromatography (RPEC),
and it currently provides well over 50 percent of all HPEC separations. It is
especially well suited to substances that are at least partially soluble in water
(drugs, biochemicals, aromatics, etc.~.
Finally, the microprocessor/computer is playing an increasing role. "Smart"
HPEC instruments are under development to program the performance. New
195
OCR for page 196
196
INSTRUMENTATION lN CHEMISTRY
detectors of greater sensitivity and selectivity are on the horizon. In particular,
laser spectroscopy promises to yield highly sensitive devices for subpicogram
detection (less than 1o-~2 grams).
Because of performance improvements, HPEC is having a major impact on
diverse fields of biochemistry, biomedicine, pharmaceutical development, environ-
mental monitonng, and forensic science. Today, peptide analysis and isolation
requires HPEC because of its separating power and speed. Analysis of amino acids
in protein/peptide sequencing is conventionally accomplished by RPEC. In clinical
analysis, therapeutic drug monitoring can be accomplished by HPEC. The analysis
of catecholamines (important as "neurotransmitters") is typically accomplished by
RPEC with electrochemical detection. Isoenzyme analysis, which is important, for
example, in assessment of heart damage after an attack, can be rapidly accom-
plished by HPEC. The analysis of polar and high-molecular-weight organic species
in waste streams in sewage or factory treatment can be performed by HPEC, while
the separation and analysis of phenols in water supplies by RPEC is recommended.
Analysis of narcotics, inks, paints, and blood represent only a few of the forensic
applications in police laboratories.
Capillary Chromatography
This version of chromatography uses an open capillary tube with a thin liquid
layer on its inner wall. It began with capillary gas chromatography (GC), but the
fragility of glass as an inert matenal for GC capillary columns discouraged many
potential users. Now we have flexible, fused-silica capilIanes with a polymer
overcoat, a spin-off of fiber optics technology. These advances in capillary column
technology led to intensive commercialization dunng the 1970s. Today's capillary
columns exhibit efficiencies between ]05 and 106 distillation steps ("theoretical
plates") and are capable of separating literally hundreds of components within a
narrow boiling point range. Direct introduction of samples at the nanogram (IO-9
grams) levels has been developed, and much effort has been directed at perfection
of gas-phase ionization detectors. Combined advances in the column and detector
areas now make trace analytical determinations below 10-~2-gram levels practical
by capillary gas chromatography.
Of particular note is the combination of capillary GC with powerful identification
methods such as mass spectrometry and Fourier Transform infrared spectroscopy,
as mentioned in Section IV-C. The combined techniques are now routinely capable
of identifying numerous compounds of interest that are present in complex
mixtures in only nanogram quantities. They have been used in identification of new
biologically important molecules, as well as in drug metabolism studies, forensic
applications, and identifications of trace environmental pollutants.
Every fluid has a characteristic temperature and pressure above which its gas and
liquid phases become indistinguishable. Above these critical conditions, the
"supercritical" fluid displays exceptionally low viscosity, and it can become a
much better solvent. Consequently, the use of supercntical fluids in capillary
chromatography has recently emerged as a promising approach to the analysis of
complex nonvolatile mixtures. As the solute diffusion coefficients and viscosities of
supercritical fluids are more favorable than those of normal liquids, chromato-
OCR for page 197
V-C. INSTRUMENTATION AND THE NATIONAL WELL-BEING
graphic performance is substantially enhanced. Furthermore, the optical transpar-
ency of supercritical fluids makes them attractive for certain optical detection
techniques.
Field-Flow Fractionation (FFF)
Chromatography becomes more difficult to apply as molecular size grows, and it
becomes ineffective in separating macromolecules and colloidal particles in the size
range 0.01 to 1 micron in diameter. A recent innovation, field-flow fractionation,
may fill this need. In FFF, a liquid sample flows through a thin (0.~-0.3 mm),
nbbon-like flow channel. A temperature difference or electric field is maintained
across the ribbon. Each constituent in the sample distributes itself in a way that is
determined by its diffusional properties and its response to the applied thermal or
electncal field. Since flow through the channel is fastest near the middle of the
ribbon, substances that are pulled close to the ribbon wall move more slowly than
substances that reside near the middle of the flow channel. Separations are thus
achieved. A useful aspect of this technique is that the strength of the applied field
can be varied in a deliberate and programmed way by a computer during the course
of the separation.
Such thermal gradients are effective in separating most synthetic polymers.
The mass range of molecules and particles to which FFF has been applied
extends from molecular weights of 1,000 up to 10~8, that is, up to particle sizes
of about 100-micron diameters. FFF appears to be applicable to nearly any
complex molecular or particulate material within that vast range. Applications
have so far included macromolecules and particles of biological and biomedical
relevance (proteins, viruses, subcellular particles, liposomes, artificial blood,
and whole celIs), of industrial importance (both nonpolar and water-soluble
polymers, coal liquid residues, emulsions, and colloidal silica), and of environ-
mental significance (waterborne colloids and the tiny particles called fly ash in
smoke plumes).
INFRARED SPECTROSCOPY
A molecule can be pictorially, but accurately, viewed as a collection of wooden
balls held together in a fixed geometry by springs. The masses of the balls are
proportional to the atomic masses, and the strengths of the springs are proportional
to the strengths of the chemical bonds. Such a "ball-and-spring" mode! will have
resonant vibrational frequencies in which the wooden balls move back and forth ire
regular patterns. These frequencies are determined by the masses, the spnug
constants, and the geometry. A
molecule is exactly the same.
If measured, the resonant fre-
quencies give direct informa-
tion about the molecular archi-
tecture.
Consider, for example, the
water molecule. This bent,
Resonant Vibrational Motions of H2O
it'd \~
/~/ All
or ~
'0 'A
%~` it\\
3587 cm~ 1 35(30 cm~ 1 1600 cm~
Vibrational Frequencies Reveal Bond Strengths
and Bond Angles
197
OCR for page 198
198
At
A:
o
a:
INSTRUMENTATION IN CHEMISTRY
tnatomic molecule has three resonant vibrations. In one of these, the two bonds
stretch back and forth in phase, and in another, the two bonds both stretch, but this
time out of phase. In the third characteristic vibration, the bond angle alternately
opens and closes.
Such molecular vibrations do not break bonds, so they require little energy.
Absorption of light is one way to excite these vibrations, but photons of
appropriate energy are in the infrared spectral region, far beyond the visual
sensitivity of the human eye. A typical molecular vibration, such as the bending
motion of the water molecule, has a frequency of 4.S x 10~3 vibrations per
second. This unwieldy number is usually brought into reasonable magnitude by
dividing it by the speed of light, which changes the dimensions to I/cm or cm-
("reciprocal centimeters".
4.8 X 10~3 vibrations/sec
_ =
3 x 10~° cm/see
1,600 cm-~.
Infrared vibrational frequencies are always expressed in reciprocal centimeters
(cm-~) (sometimes called "wave numbers". The measurement of these molec-
ular frequencies is called vibrational spectroscopy or infrared spectroscopy.
These vibrational frequencies are so characteristic that they furnish a distinctive
and easily measured "fingerprint" for each molecule. This spectral fingerprint,
once measured for a particular molecule, can be used to determine whether that
molecule is present in a sample and, if so, how much. The vibrational frequencies
also reveal the molecular structure and bond strengths in the molecule, so they can
be used to learn about the molecular architecture. When an unknown compound is
under study, the infrared spectrum provides one of
the easiest ways to decide what the compound is
likely to be.
Because infrared spectroscopy is so informa-
tive, it has become one of the routine diagnostic
tools of chemistry. A large, research-oriented
chemistry department might operate 5 to 10 infra-
red spectrometers with capabilities ranging from
rugged, low-resolution instrument for instruction
in an advanced first-year chemistry course, to
high-resolution Fourier Transform Infrared Spec-
trometers (FTIR), suited to molecular structure
determination and specialized research use.
DIFLUOROPROPENE 1R
SOLID KRYPTON, ~ 2
Mel
cis
and
gauche
em_
JE - ~£ PBOlOLTSIS (hi
me_
~1 ~
gaucaet l
~_J ~
By- --or- ~
cis1
. I I I' I t
1400 1200 1000
V(cm l)
_~
·"E. (hi)—BE=RE (hi)
~""""'B~"~\~"'~"'~
FTIR DIFFERENCE SPECTROSCOPY
SHOWS ROTAMER INTERCONVERSION
Computer-Aided Spectrometers
Modern research infrared spectrometers incor-
porate computers to permit programmed opera-
tion, data collection, and data manipulation. The
major impact of computers, however, has been
their influence on the performance of Fourier
Transform interferometers. The perfection of the
OCR for page 199
V<. INSTRUMENTATION AND THE NATIONAL WELL-BEING
Fourier Transform algorithm (program), plus the reduction in accompanying
computer costs, brought the interferometer from a trouble-plagued, research-
only instrument to a routine, high-performance workhorse. A notable capability
brought by the computer is the ease and accuracy with which one spectrum can
be subtracted from another to emphasize small changes. This is called a
difference spectrum. One important application relates to infrared spectra of
biological samples in which evidence of a chemical change associated with a
certain specific biological function can be completely covered up by the heavy
infrared spectrum of the inactive substrate in which the sample is located.
The digitized data permit precise spectral subtraction so that the back-
ground spectrum can be virtually eliminated to reveal the spectral changes of
interest.
Another vivid display of the value of the difference capability is provided by
photolysis of molecules suspended in a cryogenic solid ('`matrix isolations. If
the digitized spectrum before photolysis is subtracted from the spectrum after
photolysis, only the features that change are seen. Any molecule that is being
consumed presents its spectral features downward, while spectral features of the
growing product extend upward. This has been used, for example, to distinguish
the two forms (cis- and gauche-) of the 2,3,-difluoropropene in the cluttered
spectrum of a complex mixture. A laser tuned to an absorption frequency of one
of the "rotamers," say, the cis- form, is used to irradiate the cold sample.
Absorption of this light adds enough energy to the absorbing cis- molecule to
permit it to convert to the gauche- form. Then in the difference spectrum, the cis-
molecule spectrum appears as a negative spectrum and the gauche- molecule
spectrum as a positive one. Absorptions due to other molecules do not change,
so they simply do not appear at all.
Applications
The coupling of FTIR with gas chromatographic separations in a variety of
analytical uses has been dis-
cussed. Also, as noted earlier,
infrared spectroscopy is a spe-
cially effective method for
monitoring and studying at-
mospheric chemistry. This is
because gaseous molecules of Laser magnetic
low molecular weight are im-
portar~t, including formaide-
hyde, nitric acid, sulfur diox-
ide, acetaldehyde, ozone, ox-
ides of chlorine and nitrogen,
nitrous oxide, carbon dioxide,
and the Freons. These sub-
stances are influential partici-
pants in photochemical smog
production, acid rain, strato-
TABLE V-C-2 Additional Instrumental
Techniques in Modern Chemistry
Instrument
Information Obtained
Reaction rates of gaseous
molecular ions
Precise molecular
structures, gaseous free
radicals
Vibrational spectrum
Lifetimes9 electronically
excited molecules
Stereo confol~llations
Laser-activated cell sorter
Automated analysis of
protein sequence
Automated synthesis of
designed DNA segments
Molecular structure, gases
Tracking radiotracers
Ion cyclotron
resonance
resonance
Laser Raman
Fluorimeter
Circular dichroism
Flow cytometer
Protein sequencer
Oligonucleotide
synthesizer
Electron diffraction
Scintillation counter
199
OCR for page 200
200
INSTRUMENTATION IN CHEMISTRY
spheric disturbance of the ozone layer, and the "greenhouse effect.'' Infrared
spectroscopy shows where they are and how much is there.
OTHER INSTRUMENTATION
In Sections V-A, V-B, and V-C, there has been detailed discussion of over a
dozen different classes of instrumentation which are important in defining and
advancing the current frontiers of chemistry. By no means, however, is the list
all-inclusive. Table V-C-2 lists additional types of equipment and what kinds of
chemical information each one provides. The length of this table is only one more
signal of the crucial importance of instrumentation in modern chemistry.
SUPPLEMENTARY READING
Chemical & Engineering News
"Instrumentation '86~0ptical Spectroscopy"
(C.& E.N. staff), vol. 64, pp. 3442, Mar. 24,
1986.
"Instrumentation 'chromatography" (C.
& E.N. staff), vol. 64, pp. 52-68, Mar. 24,
1986.
"Instrumentation '8~Mass Spectrometry"
(C.& E.N. staff), vol. 64, pp. 70-72, Mar.
24, 1986.
"Low Cost FTIR Microscopy Units Gain
Wider Use in Microanalysis" (C.& E.N.
stab, vol. 63, pp. 15-16, Dec. 9, 1985.
"pity Chromatography" by Parikh and P.
Cuatrecasas, vol. 63, pp. 17-31, Aug. 26,
1985.
"GC Detector Uses Gold Catalyst for Oxi-
dation Reactions" by W. Worthy (C.&
Em. staff), vol. 63, pp. 42-44, June 24,
1985.
'~X-Ray Technique May Provide New Way
to Study Surfaces, Films" by W. Worthy
(C.& E.N. stab, vol. 63, pp. 28-30, April
8, 1985.
"Centrifugal Force Speeds Up Countercur-
rent Chromatography" by S.C. Stinson
(C.& E.N. staid, vol. 62, pp. 35-37, Nov.
26, 1984.
Science
"A New Dimension in Gas Chromatogra-
phy" by T.H. Maugh II (Science stair),
vol. 227, pp. 1570-1571, Mar. 29, 1985.
"Ion Beams for Compositional Analysis"
(SIMS), by A.L. Robinson (Science stab),
vol. 227, pp. 1571-1572, Mar. 29, 1985.
OCR for page 201
CHAPTER VI
The Risk/Benefit Equation
in Chemistry
OCR for page 202
-I ~-
~o',~
~ f — ~ ~
—~<~ ~~
OF <~lnvestigat~ng Smog Soup ~c ~ . I,=,
~0
~ o'N`o ~ ~
Air pollution Is a visible reminder of the price we sometimes pay for progress.
Emissions from thousands of sources pour into the atmosphere a myriad of molecules
that react and re-react to forth a "smog soup." We are already aware of some of the
potential dangers of leaving these processes unstudied and unchecked: respiratory
ailments, acid rain, and the greenhouse eject. Sur~ns~ngly, you and ~ are the principal
cuipnts in generating much of this unpleasant brew~veryt~me we start our cars or
switch on our air conditioning or central heating! Transportation, heating, cooling, and
lighting account for about two-thirds of U.S. energy use, almost all derived from
combustion of petroleum and coal.
Pinpointing cause and effect relationships begins, inevitably, with the identification
and measurement of what is up there, tiny molecules at parts-per-billion concentrations
in the mixing bowl of the sky. Finding out what substances are there, how they are
reacting, where they came from, and what can be done about them are all matters of
chemistry. The first two questions require accurate analysis of trace pollutants.
Physical and analytic chemists have successfully applied to such detective work their
most sensitive techniques. An example is the Fourier Transform Infrared Spectrome-
ter. This sophisticated device can look through a mile or so of city air and identify all the
chemical substances present and tell us their concentrations down to the parts-per-
billion level. Recognizing a substance at such a low concentration is comparable to
asking a machine to recognize you in a crowd at a rock concert attended by the entire
U.S. population.
~ . . .
How does this superb device work? "Infrared" means light just beyond the red end
of the rainbow visible to the human eye. Hence infrared light is invisible, though we can
tell it is there by the warmth felt under an infrared lamp. But molecules can '~see"
inhered light. Every polyatomic molecule absorbs infrared "colors" that are uniquely
characteristic of its molecular structure. Thus each molecular substance has an infrared
absorption "fingerpr~nt"~i~erent from any other substance. By examining these
finge~pnnts, chemists cart identify the molecules that are present.
Boa An example of what can be done is the measurement of formaldehyde and
nitric acid as trace constituents in Los Angeles smog. Unequivocal detection,
~ using almost a m~le-long path through the polluted air, revealed the growth
_ ~ \ dunug the day of these two bad actors and tied their production to
photochemical processes initiated by sunlight. Continuing experiments led
~'J \ to detailed characterization of the simultaneous and interacting concentra-
@_: ~ tions of ozone, peroxyacetyl nitrate (PAN), formic acid, formaldehyde,
and nitric acid ir1 the atmosphere. These detections removed an obstacle
to the complete understanding of how unburned gasoline and oxides of
iitrogeI1 leaving our exhaust pipe end up as eye and lung irritants
in the atmosphere. This advance doesn't eliminate smog
=~ soup, but it is a bin steD toward that desirable end
~ n' ~ ~ _ ~
i1~ =~w
202
Representative terms from entire chapter:
mass spectrometry
