WATER POLLUTION CONTROL RESEARCH SERIES
DAST 11 15080
OIL TAGGING SYSTEM STUDY
Summary
by
RESEARCH DIVISION, ENVIRONMENTAL
AND APPLIED SCIENCES CENTER
MELPAR
An American-Standard Company
7700 Arlington Boulevard
Falls Church, Virginia 22046
for the
FEDERAL WATER POLLUTION CONTROL ADMINISTRATION
DEPARTMENT of the INTERIOR
Contract No. 14-12-500
MELPAR Ref. 9059
October 1969
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WATER POLLUTION CONTROL RESEARCH SERIES
The Water Pollution Control Research Reports describe the results
and progress in the control and abatement of pollution of our
Nation’s waters. They provide a central source of information
on the research, development, and demonstration activities of
the Federal Water Pollution Control Administration, Department
of the Interior, through in-house research and grants and con-
tracts with Federal, State, and local agencies, research institu-
tions, and industrial organizations.
Water Pollution Control Research Reports will be distributed to
requesters as supplies permit. Requests should be sent to the
Planning and Resources Office, Office of Research and Develop-
ment, Federal Water Pollution Control Administration, Department
of the Interior, Washington, D.C. 20242.
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OIL TAGGING SYSTEM STUDY
Summary
by
Research Division, Environmental and Applied Sciences Center
MELPAR
An American-Standard Company
7700 Arlington Boulevard
Falls Church, Virginia 22046
for the
FEDERAL WATER POLLUTION CONTROL ADMINISTRATION
DEPARTMENT OF THE INTERIOR
Contract Number 14-12-500
October 1969
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FWPCA Review Notice
This report has been reviewed by the Federal
Water Pollution Control Administration and
approved for publication. Approval does not
signify that the contents necessarily reflect
the views and policies of the Federal Water
Pollution Control Administration, nor does
mention of trade names or commercial pro-
ducts constitute endorsement or recommenda-
tion for use.
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OIL TAGGifiG SYSTEM STUDY
J. Horowitz, G. D. Gumtz, R. G. Nemchin, T. P. Meloy
ABSTRACT
Several methods of identifying the source of oil pollution are critically
examined. These methods are grouped into two categories: passive tagging
and active tagging. Passive tagging assumes that oils are so chemically diverse
that their contents constitute a stable chemical fingerprint that can be unequiv-
ocally disclosed in the laboratory. Active tagging requires that an inexpensive,
coded material be added to oil; this material must be chemically and physically
stable in both oil and oil slicks; it must also be readily identifiable by available
analytical techniques; and it must have no adverse effect on the oil’s subsequent
use. Three methods of passive tagging (trace metals, sulfur-isotope ratios,
and paper chromatography) and three methods of active tagging (halogenated
polycyclic aromatics, organometallics, and coded microspheroids) have been
examined. Passive tags cannot be recommended because the passive tags are
quite likely to mingle, to evaporate, to be dissolved, or to be oxidized; even if
these processes do not occur, they can create formidable forensic problems
for the prosecution and telling counter-arguments for the defense. Since active
tags are designed to be stable and identifiable, they are satisfactory for the job;
and the three types of active tags reviewed show promise and merit.
This report was submitted in partial fulfillment of Contract Number
14-12-500 between the Federal Water Pollution Control Administration and
Melpar, an American-Standard Company.
Key Words
Oil Pollution
Oil Tagging
Analytical Techniques
Surveillance
Enforcement
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CONTENTS
Page
Abstract
General 1
Weathering of Oil in a Marine Environment 1
Identification of Oil Slicks 2
Passive Tagging 3
Special Cases of Passive Tagging 8
Active Tagging 10
Halogenated Aromatics 11
Organometallics 14
Radioactive Tags 15
Particulate Tags 16
Addition of Active Tags to Petroleum 18
Costs of Active Tagging 20
Tagging Codes 21
Tagging System 22
Conclusions 22
References 24
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GENERAL
According to Federal Water Pollution Control Administration estimates,
there are approximately 7000 oil spills in U.S. waters annually. This figure
does not tell the whole story; the true figure may be several thousand more.
The problem of oil pollution is patently serious, and grows more serious with
time. One of the great problems in combatting oil pollution is to identify the
pollutor: Oil pollution is illegal under at least six statutes of the U.S. Code.
While small amounts of oil pollution can come from thousands of sources, a
great deal of oil pollution apparently comes from a few types of sources:
tankers, cargo ships and barges, offshore wells, storage tanks, and pipelines.
Waterborne sources account for over half the pollution. While the problem is
large, it is not infinite. There are 14,000 offshore wells, and 2,000 tankers
making about 10,000 calls a year on U.S. ports. And only 400 of the tankers
fly the U.S. flag. The problem is: How to identify the source of the oil dis-
charge, cheaply and conclusively?
The most direct approach to tracing the source of pollution is to compare
the slick with the oil carried by the suspect source. This approach assumes
that the slick will not differ significantly from the true source.
WEATHERING OF OIL 114 A MARINE ENVIRONMENT
Oil, however, is no simple compound or mixture of compounds and does
not maintain its original integrity in a marine environment. Crude and fuel oils
show enormous variation in chemical content, both qualitatively and quantitatively.
Naturally, variations in chemical content are accompanied by variations in phys-
ical properties. Perhaps the most important event in the history of an oil slick
is the evaporation of the light fractions. While the rate of evaporation will be
influenced by many variables, after several days at sea the slick will consist
almost entirely of compounds whose boiling point is above 370°C (approximately
70O0F), viz, compounds containing at least 20 carbon atoms, and probably of
complex chemical structure. Most oil spilled on calm water spreads rapidly to
form a film about 2 cm thick. Although many variables affect its subsequent
thinning, its final thickness will be 10-100 microns on pure water, but approx-
imately 1 mm on heavily contaminated water. 2 Thin films offer a larger sur-
face (and hence, are more easily acted upon by the environment) than thick films.
While evaporation undoubtedly accounts for most of the volumetric loss
in the slick, solution in sea water may also play a significant role. R. A. Dean’
has argued that, while the lightest hydrocarbons have finite solubilities in water,
some heavier petroleum constituents, while undoubtedly less soluble, may still
go into solution, when considering huge volumes of sea water. Dean has con-
tended that if a solubility on the order of one part per million is assumed, the
whole cargo of the Torrey Canyon could have been dissolved in a patch of sea
20 miles square and 500 feet deep. In this connection, the presence of
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surfactants in natural crudes (surfactants such as porphyr ins and metallo-
porphyrins) assumes a new significance. Moreover, trace metals in petroleum
are often in water-soluble form, or become water-soluble as the slick weathers.
While the slick is spreading, evaporating, and being leached of its water-
soluble components, other weathering processes act upon the residue: cata-
lytic autoxidation, photochemical oxidation, polymerization, and microbial
oxidation. While it is known that oil changes its character because of
oxidation, solar radiation, and microbial action, there is little information
about the specific mechanisms, reaction times, and reaction products. The
unsaturated, alkyl-aromatic, and alicyclic hydrocarbons in petroleum are
probably oxidized to acids via peroxide, hydroperoxide, alcohol, and ketone
intermediates. In spite of the absorption of the shorter wavelengths of the
sun’s rays by the atmosphere—which reduces the ultraviolet light (UV) of less
than 4000 A to no more than 5% of the total—there remains e 4 nough energy in
the residual radiation to break a variety of chemical bonds; in general, aro-
matic hydrocarbons are more stable than aliphatic hydrocarbons to UV and
visible radiation. Ellis & Wells 5 have reported that solar UV causes oils to
become colored and gummy in the presence of oxygen, and that the rate of gum
formation triples for every 20°F rise in temperature. Woisky & Chapman 6
have shown that UV caused porphyrins to disintegrate rapidly in the presence
of oxygen. Oil slicks are also attacked by oxidizing microflora, which are
common in both fresh and salt water. While many of the compounds in petro-
leum are undoubtedly metabolized, the metabolic pathways for large and com-
plex hydrocarbons are not well established, 8 and the kinetics of oxidation
(even for the smaller hydrocarbons) seem to be rather slow. It is well to bear
in mind, however, that the species distribution of saprophytic microorganisms
varies widely with latitude, and that diversified enzymatic activity is much
more common in cold-water than in warm-water bacteria. The question of
microbial oxidation and assimilation of petroleum compounds is further com-
plicated by the anomalous phenomena of co-oxidation and oxidation of nonj. 0
assimilable substrates, which have been reviewed by McKenna & Kallio.
IDENTIFICATION OF OIL SLICKS
There are two general approaches to identification, which are sometimes
called active tagging and passive tagging. In passive tagging nothing is added
to the oil; the slick itself must provide all the evidence for its own identification.
Active tagging involves adding some known and readily identifiable material to
the oil. The added material must satisfy several criteria: It must be soluble
or dispersible in oil and insoluble and nondispersible in water; it must be easily
detectable in extreme dilution commensurate with a high coding capability; it
must be chemically and physically stable in both spilled and unspiled oil; and
it must not interfere with the commercial uses of petroleum. One additional
criterion: It must not be too expensive. Two figures of speech are helpful in
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distinguishing active tagging from passive tagging: Passive tagging amounts
to a “fingerprint,” and active tagging amounts to a chemical or physical
“license plate.”
PASSIVE TAGGING
When oil is spilled, it may be irradiated; mixed with complicated
aqueous solutions (sea water, polluted fresh water, or brackish swamp water);
exposed to oxygen and air pollutants; and intimately associated with a variety
of aquatic microbes; depending on local conditions, it may have been partially
evaporated, emulsified, photolyz ed, autoxidized, polymer ized, enzymatically
oxidized, and may have undergone ionic exchange. What part of the fingerprint
can be stable to all these conditions? Heavy and highly branched alkanes are
excellent candidates; so are the heavy and complex hydrocarbons of mixed
structure that are common in lubricating oil. However, the heavy branched
alkanes are (in general) exceedingly rare in oil, because the concentration of
branched-chain paraff ins in crude oil is a rapidly decelerating function of
molecular weight. The mixed-structure hydrocarbons common in lubricating
oils permit an almost iiifinite number of structures, and the 1 elihood of posi-
tively identifying any one of them becomes vanishingly small. After all,
even though several hundred compounds have been identified in crude oil, the
identified compounds “probably account for well under 50% of the constituents
of a crude petroleum. ,,12 should be apparent that the hydrocarbons in slicks
of crude and heavy oils will contain an even higher percentage of unidentified
structures. One must therefore conclude that although heavy, complex hydro-
carbons are likely to be stable constituents of oil slicks, they will not serve as
passive tags because they are exceedingly rare, exceedingly difficult to iden-
tify, or both.
Trace metal content (especially vanadium and nickel) and the sulfur
isotopic ratios in oil have been suggested as fingerprints that could be used to
identify the source of oil slicks.
Many inorganics have been reported in petroleum ash; reports of 37
thorganics were found, and the list could no doubt be extended. Many of these
elements are rarely found, others are frequently found in low concentrations;
only a few are frequently found in petroleum ash in concent rations above
0. 1%, and, of these few, special importance attaches to vanadium and nickel.
These two metals are important because of the problems they cause
industry. Like most elements of the first transition series, both are dehy-
drogenation catalysts (nickel is 4 to 10 times more active than vanadium),
and both are commonly found in petroleum as volatile compounds. For this
reason, they are often found in distillate fractions, their concentration in-
creasing with the depth of the cut. Some of these volatile organometallic
compounds alter the selectivity of cracking catalysts, increasing the
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formation of coke and hydrogen and decreasing the yield of gasoline. Non-
volatile vanadium, which concentrates in the residuum from which heavy fuel
oil is produced, is extremely corrosive to fire-clay bricks used in furnaces;
the vanadium oxides form low-melting eutectics with clay, producing hard,
glassy slags that are extremely difficult to remove. 13, 14
Both vanadium and nickel are comm 1 nly found in petroleum as com-
pounds of widely varying molecular weight. Often, these compounds are
water-soluble inorganics. 16 Of the organic compounds, special importance
attaches to the nickel and vana.dyl metalloporphyrins because these compounds
are rather stable constituents of petroleum and have great geochemical sig-
nificance. 17 It is well to remember, however, that only a small fraction of
nickel and vanadium in petroleum is usually found in porphyrins. While vana-
dyl and nickel porphyrins are rather stable compounds, they are also inter-
facially active and tend to concentrate at hydrocarbon-water interfaces. In
light of this property, several workers have used water sprays to extract
metallopor hyrins from oil; this method is thoroughly established. 16 Wolsky
& Chapman have also shown that when porphyrins are irradiated by UV in
the presence of oxygen, they are rapidly decomposed. Furthermore, while
the metalloporphyrins in oil usually have reduced side chains, oxidizing con-
ditions usually result in poly-carboxylic poryhyrin structuije s which have a
very marked affinity for water interfaces. lo Indeed, Falk has demonstrated
an almost linear relationship between the number of carboxyl groups on a
porphyrin and the porphyrin’s mobility in a solvent system of lutidine and water.
In view of the fact that both the vanadium and the nickel in petroleum
are often volatile and water-soluble, it is difficult to understand why they have
been seriously recommended as passive tags. Nevertheless, they have been
recommended off and on since 1955, if not before. Since all the recommended
methods require sampling of oil slicks—which will have been partially evapo-
rated, thereby concentrating the involatiles—it may be pointless to compare
the absolute metallic contents of the slick with that of the unspiled oil. Rather,
the ratio of metals (in particular, the ratio of vanadium to nickel) must be
assumed constant. This -may be an invalid assumption, since (in general)
nickel compounds in petroleum are usually reported to be more volatile and
more water-soluble than vanadium compounds. The arguments advanced for
vanadium and nickel tags seem to ignore these facts:
Vanadium and nickel are held in the oils as co-ordinated
complexes with porphyrins, and are not affected by con-
tact with sea water. . °
or from a more recent proposal:
The method simply entails measuring the vanadium and
nickel contents of the unknown oil pollutant. There is a
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very specific amount of these metals associated with
every crude -- in essence constituting an individual
metallic trait. There are other chemical and physical
properties for the various crudes, but these are sub-
ject to change during weathering/aging [ sic]. The
metallic components, however, are stable and are not
affected by weathering. By simply comparing the
metallic content of the spill against [ sic] that of the
suspected source, it is possible to limit the probable
source to a specific vessel, pipeline or storage
facility. 21
Others have proposed analyzing even larger metallic suites, for example,
vanadium, sodium, manganese, and cobalt; this method was proclaimed to offer:
definite promise for the trace—element characterization
matching [ sicj of an actual oil-slick sample with one
sample of fuel oil -- out of a number of suspect samples....
The various characterizing trace elements, occurring in
the original crude oils largely as essentially non-volatile
metal porphyrins, remain in the residue left when the
volatile components of the crude evaporate (as from an
oil slick), and when the light ends are distilled off in
refining. Thus NAA [ neutron activation analysis]
matching of the levels of these trace elements ——
between an oil slick and its polluting source -- may,
for the first time, provide a method of identification
of the source of oil pollution of waters that wi l 2 aid in
the enforcement of present oil pollution laws.
While the idea of using vanadium and nickel to identify oil slicks has, at
the very least, the merit of being based on some firm fact—for vanadium and
nickel are very commonly found in oils, and they are often found (in small part)
as metalloporphyrins, of which they are by far the two most stable 18 —these
other metals, especially sodium, are very poorly advised. Sodium, for example,
is common in petroleum but it is also the most common cationic metal in sea
water; 23 it is therefore readily available to participate in exchange and replace-
ment reactions with the oil slick. The petroporphyrin nucleus is an obvious
target for exchange reactions with sodium ions from the sea; unfortunately,
these sodium porphyrins are not very stable in the presence of other cations.
Hodgson 18 and his colleagues have brilliantly summarized the evidence: It is
thoroughly established that alkali metal ions in porphyrin molecules are readily
replaced by other cations. They are alkali metal, large divalent metals, and
finally, small divalent metals. The metalloporphyrins and complexes of por-
phyrins incorporating monovalent cations show rapid exchange. It has been
known since the late 1940’s that both the mono- and di-salts of porphyrins with
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alkali metals are very unstable and break down in the presence of water. While
other petroleum compounds of sodium may be more stable than sodium porphyrin,
it is well to remember that one general characteristic of sodium compounds is
their water solubiity, and water-soluble compounds are not likely to remain in
an oil slick for long.
Quite aside from the physical and chemical stability of metals in petro-
leum, there is the problem of distinguishing two or more samples that may be
very similar. While the vanadium/nickel ratio of Kuwaiti petroleum is quite
different from the corresponding ratio in Venezuelan petroleum, the U.S. does
not import equal amounts of petroleum or petroleum products from these two
countries. For example, about 80% of the residual fuel oil imported into the
U.S. comes from Venezuela and the Netherlands Antilles, and over 90% of all
the residual fuel oil that is imported by the U. S. is received in PAD District 1
(roughly, the New England, Middle Atlantic, and Southeastern States). 24
Imported residual fuel oil is far more likely to be spilled in an East Coast harbor
than in a Pacific Coast harbor, and the oil is very probably a South American
export. Similar considerations apply to other petroleum products. Sumatran
crude oil, for example, is imported almost exclusively into the Pacific region;
Venezuela and Canada together account for about two-thirds of the crude oil
imported into the U.S. Exactly the same argument also applies to oil produced
within the U.S. Texas, Louisiana, California, and Oklahoma together produce
about 80% of U .S. crude oil; much of this oil comes from such giant fields as
Wilmington, East Texas, and Kelly-Snyder. It is therefore likely that a spill
of US, crude oil is a spill of oil from Texas, Louisiana, California, or Okla-
homa, and it is quite likely that this oil came from one of the major oil fields
in those states. The more specific the argument gets, the greater the likelihood
becomes. For example, most of the crude oil that is processed by the great
refineries of Eastern Pennsylvania and New Jersey is sent via tankers and
barges from Texas and Louisiana; it therefore follows that a spill of crude oil
on the Delaware River almost certainly involves Texas or Louisiana crude, and
very probably involves a crude oil from one of the large oil fields in those states.
How can vanadium/nickel ratios be used to distinguish one Wilmington crude
from another, or one Venezuelan residual fuel oil from another? The economic
realities are not especially relevant to a pure chemical argument, but in a dis-
cussion of the probability of oil spills, they should not be entirely forgotten either.
Keeping this discussion of economic realities in the back of our mind,
one can proceed to the use of sulfur isotopes as passive tags. This discussion
is heavily based on the work of Thode and his colleagues 2528 at McMaster
University (in Hamilton, Ontario), who have been studying sulfur-isotope ratios
in a variety of geologically important materials (including petroleum) since the
late 1940’s. In this discussion, as in the discussion of metallic suites in petro-
leum, the work of scientists, whose investigations were conceived and published
without oil tagging in mind, has been chosen.
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Isotope fractionation is an established and important geochemical process:
Oceanic carbonate is rich in 0-18 with respect to ocean water, plants are enriched
in C-12 with respect to atmospheric carbon dioxide, and sea sulfate is rich in
S-34 with respect to the sulfur found in meteorites and basic igneous rocks.
Although there are nine sulfur isotopes, only four of them (S-32, -33, -34, and
-36) occur in natural sulfur; all four are stable, but they are not equally dis-
tributed: S-32 predominates (95.0%), and S—34 (4.22%) accounts for most of the
remainder. The ratio of these two most common isotopes is often standardized
with respect to meteorites because this ratio is virtually invariant; in the Earth’s
mantle, however, the content of S-34 varies by approximately 100%. The 34/32
ratio has been studied in many materials, including petroleum and sea water:
In sea water the ratio is +20 ±2%. while in petroleum it varies from -8% to
+33%—indeed, it varies from -8% to +28% in petroleums from Utah alone:
And the apparent significance of this phenomenon has not been lost on proponents
of passive tagging.
The fundamental fact in the geochemistry of sulfur isotopes is that S-34
tends to sulfate and S-32 tends to sulfide: S-32 is the more readily reduced
isotope and S—34 is the more readily oxidized. 34-sulfide plus 32-sulfate tends
to 32-sulfide plus 34-sulfate. In short, the heavier isotope is more readily
oxidized, and the lighter isotope is more readily reduced.
While all the sulfur compounds in petroleum have not yet been identified,
all the identified sulfur compounds are reduced: thiols (mercaptans), disulfides,
sulfides, and thiophenes. 15 The ultimate source of this sulfur is not definitely
established, but many authorities have argued that it must usually be oceanic
sulfate, reduced to sulfide by such bacteria as Desulfovibrio desulfuricans ;
Davis 7 has summarized the evidence well. These bacteria preferentially
metabolize the lighter mass isotope, viz, S-32, and it has been sown by
several investigators that this preference is usually around 15%; the 34/32
ratio in petroleum should therefore be about 15% lower in petroleum than in the
sulfates of ancient oceans. The group at McMaster University has shown that
this is usually the case. This group has shown that, while the absolute sulfur
content of petroleum declines with time, the sulfur-isotope ratio stays remark-
ably constant. They have also shown that the content of sulfur isotopes in
Devonian petroleums extending from the 49th Parallel to the Arctic is nearly
constant, and have argued that the most reasonable explanation for this phe-
nomenon is a large sea, which provided the large reservoir of sulfur of uniform
isotopic content and constant oil-forming conditions. They were therefore
pleased to announce that the isotope ratios in Ordovic Ian and Silurian oil of
Texas and Ontario, although separated by several thousand miles, were quite
similar.
What is the conclusion to be drawn from this evidence? It is, quite
simply, that S-isotope ratios are just another general index of geological age.
Geochemists have several such indices (e. g., ratio of complex cyclic structures
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to simple chain structures, not to mention radiocarbon dating). It is difficult
to see, however, how any such index can be of much use in identifying oil spills.
For example, the United States’ largest foreign source of oil (Venezuela) is a
major shipper of Tertiary crude; so is Louisiana, the second most productive
oil state. The second largest foreign source of oil (Canada) sends large quan-
tities of pre-Mississippian crude to U.S. East Coast refineries; so does Texas,
the largest oil-producing state. Miocene oil is common in Louisiana; it is also
common in Indonesia. Are S-isotope ratios sufficiently precise to distinguish
Miocene from Eocene? —if not, oil from Fresno, California, cannot be dis—
tinguished from the oil of Sanga Sanga, Borneo, or Plaquemines Parish,
Louisiana. 24 ’ 3 °
There is another objective, however, that may be even more important.
The sulfur in oil is reduced, but when oil is spilled at sea, the sulfur is put in
an oxidizing environment. Because of kinetic isotope fractionation, the S-34
will be more readily oxidized than the S-32, other things being equal. The
logic of this process leads to the conclusion that the more the spill is oxidized,
the lower the 34/32 ratio will be; a week-old oil spill should have a lower
isotope ratio than a day-old spill from the same source. If the fractionation
effect were strong enough, the delta-value might go from highly positive to very
negative. This combination of geochemical fact, oxidation kinetics, and
petroleum-trade statistics leads to the conclusion that sulfur-isotope ratios
could not very well be used to prove a violation of oil-pollution laws.
SPECIAL CASES OF PASSIVE TAGGING
While no easily identified passive tag that is unaffected by marine environ-
ments is known, several kinds of passive tags can be used profitably as pre-
suinptive tests on the identity of oil spills under special conditions. In general,
the requirements on passive tags should not be different from the requirements
on active tags: Both kinds of tags should be unchanged by weathering and micro-
bial oxidation, soluble or dispersible in oil, insoluble and nondispersible in
water, and involatile. However, there are special circumstances in which some
of the requirements on passive tags may be relaxed. Where oil pollution is in-
frequent, where there are few possible sources of pollution, and where weather—
ing may be assumed to be inconsequential or insignificant (because of the slick’s
freshness), several methods of chemical analysis—such as spectrophotometry,
gas chromotography, fractional distillation, pyrolysis, mass spectrometry, and
comprehensive physical measurement, i. e., density, viscosity, and refractive
index—supplemented by other independent evidence—may identify oil slicks by
passive tags in the spilled oil. Chemical analysis is useful when it does not
have to bear the whole burden of proof. A combination of analytical techniques
(such as gas-liquid chromatography, controlled pyrolysis, mass spectrometry,
and computer calculations) cannot be relied upon to yield a “fingerprint” of the
slick. 3 ’ If the spill has not been extensively weathered and if there are not
many possible sources of pollution, the combination of analytical techniques
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is wasteful: Where other independent evidence is convincing, extensive chemical
analysis is not required. However, where there are many possible sources of
pollution, where pollution is frequent, and where the spill may have been weath-
ered for days or weeks (or mixed with other spills, some of which may have
been weathered), it seems highly improbable that any combination of analytical
techniques could stand up in court to a carefully reasoned chemical defense.
The problem of mixed and continuous pollution is not unknown. According
to Ludwig and his colleagues, 32,33 the beaches in Southern California are
commonly polluted by a mixture of oil and asphalt from natural seepage, liberally
supplemented by tanker and drilling wastes. Similar conditions frequently per-
tain along the Gulf Coast. A senior scientist at the Woods Hole Oceanographic
Institution was recently reported 34 to have said that he could not sail anywhere
in the Atlantic—even a thousand miles from land—without finding oil. The meth-
ods discussed caxmot be recommended for such areas as Southern California,
nor for areas that are frequently afflicted with oil pollution.
To begin, all the methods of passive tagging already discussed can be
used (although they are more expensive than several other methods) as long as
there is plenty of independent evidence that the oil slicks are fresh and have not
been mixed with other oil pollution. Other simpler methods of identification
were published in the U.S. and England in the early 1950’s; these methods make
use of the simple techniques of paper chromatography which will be used as an
example in the following paragraphs.
Schuldiner 35 reported (in 1951) a simple method of spot chromatography
that had produced evidence for prosecuting and convicting oil polluters in
Baltimore, Maryland, and the upper Chesapeake Bay area. He sampled the oil
slick, then boarded suspected ships and sampled bunker fuel oils, bilges, loose
oil on the decks or sides of the vessel, on fuel intake valves, and on adjacent
docks. The samples from the slick and the ship were strictly treated as com-
parison samples. Small spots of the samples were applied to blotting paper,
allowed to develop completely (i. e., to stop spreading), and were then examined
under ordinary and UV illumination. He found that various crude, semi-refined,
and refined petroleum products gave characteristic spot chromatograms, and
that the chromatograms from known crudes invariably gave identical radial
fluorescence patterns. While Schuldiner’s chromatograms can be effective,
they are not perfect: They entail at least three troublesome problems. First,
confluent multiple spills will give chromatograms that are characteristic of the
mixture, not of the components of the pollution; it is very doubtful that the chem-
ical mixture can be resolved into its legal components, or that a given slick can
be proved not to be a mixture unless the circumstantial evidence is overwhelming.
(This objection applies with equal force to all other methods of passive tagging.)
Second, any but the freshest slicks may have become viscous and emulsified,
may have experienced considerable evaporation, and their asphaltenes and other
heavy components may have agglutinated into a discrete particulate phase; 36 the
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chromatograms of the slick and the source are likely to differ unless the slick
is fresh, and the size of the difference will largely depend on the slick’s age.
Moreover, if the slick is very viscous or emulsified, it will have to be filtered,
extracted, heated, and/or dissolved in a non-fluorescing solvent in order to be
chromatogrammed; while this treatment may not be damaging, it is not helpful
to the prosecution’s case either. Third, it is not an unmixed blessing that iden-
tical chromatograms are invariably produced by crudes from a given area. If
all (or even many) Venezuelan crudes give identical chromatograms, and if two
or more tankers carrying Venezuelan crude have recently been in the area of a
spill, it is not clear how one ship rather than the other could be blamed.
Another simple chromatographic method (this time, paper-strip chroma-
tography) was independently published in 1953 by Herd, 37 who claimed that he
had used this method to identify fuel oil that had been spilled in the harbor of
Wellington, New Zealand. While Herd was apparently ignorant of Schuldiner’s
work (in any event, he did not cite it), his claims are strikingly similar, and he
asserted that his method “gives visual evidence of complete identity or of non-
identity and, if necessary, the strips can be used as evidence in a court of law.”
All the reservations that apply to Schuldiner’s methods apply just as strongly to
Herd’s. One further reservation merits special attention: Herd’s methods
“identify” only the ether-soluble portion of the slick. Although the ether-insoluble
fraction of a slick is admittedly difficult to work with in the analytical laboratory,
it can provide valuable information (particularly on naturally occurring micro-
solids); in any event, it does not seem prudent to ignore it altogether because it
can be a sizable fraction of the slick. Ludwig & Carter 32 have reported that 12
crude oils contained (on the average) 7. 9% ether-insoluble material, that some
crudes contained as much as 15.5% ether-insolubles, and that one film of oil
collected at a beach near Santa Barbara contained 6.5% ether-insolubles.
ACTIVE TAGGING
There are reasons to have serious reservations about the validity and
efficacy of passive tagging. Identification based on passive characteristics
seems to be especially worrisome because the passive tags are quite likely to
mingle, to evaporate, to be dissolved, or to be oxidized at sea; furthermore,
even if these processes do not occur, they create formidable forensic problems
for the prosecution, and telling counter-arguments for the defense.
If the intrinsic components of the oil slick do not lend themselves to
reliable identification, the possibility of introducing additives (active tags)
that will suit the purposes must be considered. What is required of an active
tag? Ideally, it should satisfy ten criteria.
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1. It must be an unusual material that is never found in the natural
environment and is found only in the petroleum to which it has been added.
2. It must be compatible with (i.e., chemically unreactive and physically
stable) and soluble or dispersible in the oil.
3. It must be both insoluble and nondispersible in water.
4. It must be relatively involatile.
5. It must be stable to chemical, photochemical, and microbial
degradation in the oil-slick environment.
6. It must be easily detectable in extremely small quantities by readily
available analytical techniques.
7. It must have the ability to be modified so that the material can be
coded, and when added to a particular oil shipment provide a unique code or
“license plate” for that carrier or transporter.
8. It must remain with the oil during the course of its history following
a spill.
9. It must in no way interfere with the end-use applications for that oil,
nor complicate further processing and refining.
10. It must be available in quantity at economical prices.
While few tags will be ideal, many candidate tags will satisfy these
criteria to some degree.
Although many types of tags have been investigated, there are two types
of oil-soluble tags and particulate tags that have merit. The problem of radio-
active tags has also been considered.
HALOGE NAT ED AROMATIC S
Over the past several decades, many studies have been performed to
identify and characterize the constituents of petroleum. Although many types
of aliphatic, aromatic, and heterocyclic compounds have been identified, there
is no evidence of halogenated, polynuclear aromatics in crude oils. This class
of compounds has many other characteristics that are worthy of attention.
First, polynuclear hydrocarbons are (in comparison with aliphatic hydro-
carbons) quite stable to oxidation. 38 This stability is due, in part, to the
resonance and bond-dissociation energies of the aromatic system, whose
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resonance energies increase markedly as the number of condensed rings
increases. 39 Halogenation of aromatic rings has, in common with nitration,
sulfonation, and acylation, the great advantage of stabilizing the ring against
further attack; 4 ° it also increases the involatiity of the parent hydrocarbon,
but (in general) does not affect water-insolubility or oil-solubiity.
An important feature of halogenated polynuclear aromatics is that there
are many different ways to substitute F, Cl, Br, or I (or their combinations) on
condensed aromatic nuclei, thereby providing a large number of specific tagging
compounds. Moreover, there are many isomers of each parent hydrocarbon.
Depending on the parent hydrocarbon, a large number of mono-, di-, and poly-
substituted isomers can be produced with halogens, singly or, a fortiori , in
combination. The simplest possible example, naphthalene, has two products
of monosubstitution (C 10 H 7 X), ten products of monosubstitution (C 10 H 7 X), ten
products of disubstitution (C 10 H 6 X 2 ), and 14 of the mixed type (C 10 H 6 XY), where
X and Y are any two halogens. There are 42 known and fully identified poly-
chloronaphthalenes; it is obvious that larger condensed systems (say of six or
more rings) offer even more structural isomers for selective halogenation.
Even the monochloronaphthalenes have boiling points above 250°C (482°F), and
further halogenation raises the boiling point correspondingly; the vapor pressures
similarly decline with increasing polycyclic condensation. The stability of over-
crowded halogen compounds (e. g., octachloronaphthalene) has also been demon-
strated. 41 In view of the various polycyclic nuclei (and the several structural
arrangements that are possible for each) that may be halogenated, and the
numerous possibilities for halogenation (involving single or several halogens), it
is clear that a large family of coded compounds that can be readily distinguished
from natural petroleum compounds can be synthesized.
It is important for active tags to be identifiable in the slick even after
long exposure to the elements and weathering, which ordinarily increases the
concentration of asphaltenes in the lick. Petroleum asphaltenes are polycy ic,
comprising mostly aromatic rings 4 in a pericondensed sheet configuration.
In general, these asphaltenes might present a considerable background problem.
However, the analytical technique suggested for the detection of halogenated
active tags should minimize this problem, viz, gas chromatography using an
electron-capture detector.
The electron-capture detector is a substance-specific device that is
sensitive to certain types of molecules containing électrophiic atoms or groups
(e.g., halogens, carbonyls, nitro groups, etc.) and certain condensed-ring
aromatics, metals, and the like. 4 53 It has a very low sensitivity for hydro-
carbons other than aromatics. Furthermore (and very important), the
detector is extremely sensitive to halogen substituents: It can reliably detect
halogenated hydrocarbons in quantities as low as io to io_6 ppm; 55 under
optimized conditions, a projected sensitivity of o 10 ppm has been reported.
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Electron-capture gas chromatography has been used successfully to determine
parts-per-billion concentrations of chlorinated insecticides in waste water
samples, 56, 57 and chlorinated hydrocarbons in air samples. 58 Since gas
chromatography has been used extensively to separate and characterize the
components of petroleum and its products, 59, 60 it can be used with reliability
and precision to separate the components of an oil-spill sample. Coupled with
the extreme sensitivity afforded by the electron-capture detector, gas chroma-
tography can be used to ascertain the presence of an active tag by virtue of the
tag’s structure and substituents. Detection and identification of the tag can be
expedited by reference to a catalog of retention times of the several tagging
compounds on a standardized chromatographic column. When an oil sample is
brought in for analysis, the retention times of the various chromatographic
peaks of the sample can quickly bp compared with the catalog, and the proper
component peaks identified. The correlation between chemical structure and
column retention times (or elution values) is well documented. 61, 62 Halogenated
alipliatics, 63, heterocyclic compounds, and unhalogenated polynuclear aro-
matics (as well as inorganic halogens) should not interfere because their retention
times (and their detectability by the electron-capture device) will not correspond
to the tag’s behavior. However, if several peaks should occur near (or overlap)
the tag’s peak, these particular fractions can be collected and rechromatographed
under operating conditions that will give a better separation.
Although nothing is known experimentally about the stability of halogenated
polycycles in an oil-spill environment, there are excellent theoretical reasons
for believing that these compounds will be stable. In general, aromatic structures
are stable to oxidation at ambient temperatures and pressures; halogenation fur-
ther protects the aromatic nucleus from chemical attack. There are excellent
grounds for believing that halogenated tags will be much less reactive than the
rather inert condensed aromatics that are naturally present in oil, often in
large quantities. Since condensed aromatic compounds are not easily attacked
by microorganisms, and since halogenated aromatics are even more stable than
the corresponding parent hydrocarbons, there is also excellent reason for be-
lieving that the tags will be stable to microbial attack. All of this, however, is
theoretical conjecture, not proven experimental fact.
Because of the great sensitivity of the electron-capture detector to halo-
genated compounds, it should be evident that very little of the tagging material
is required to tag a large volume of oil. For example, the largest tankers now
on the drawing board will hold 500,000 tons of oil. How much tag must be added
to identify such immense cargoes? The theoretical limit of detec tion by electron-
capture gas chromatography is io12. Therefore, 0. 016 ounce of tag is
(theoretically) sufficient, if properly mixed, to tag a half million tons of oil.
But why push the limit of detectability? Why not allow a safety factor of three
orders of magnitude? Why not, indeed, because then only 1 pound of tag (about
1 pint) will safely do the job. And “safely” is very safe: One gram of oil slick
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(about 1/4 ounce) will contain one nanogram of tag, and one nanogram of a halo-
genated aromatic substance is extremely easy to detect by electron—capture gas
chromatography.
It is not without interest that electron-capture gas chromatography is
several orders of magnitude more sensitive than emission spectroscopy 65 and
approximately two to three orders of magnitude more sensitive than ordinary
neutron-activation analysis 66 ’ 67 (i. e., activation that does not use very fast
neutron fluxes). By combining gas chromatography with mass spectroscopy,
the tags can be absolutely identified. 68,69 Just as important, because of the
sensitivity of the electron-capture detector, the tag can be used in extreme
dilution, and can therefore be added to the oil in amounts which will not interfere
with further processing or uses of the oil.
ORGANOMETALLICS
Halogenated aromatic molecules are not the only attractive cl 8 ss of oil-
soluble tags. In 1962, a group at the National Bureau of Standards, in collab-
oration with the American Petroleum Institute, developed a number of stable,
oil-soluble organic compounds (mostly organometallics) for use as analytical
standards in the spectrographic analysis of petroleum products. The trace
elements include Al, Ba, B, Cd, Ca, Cr, Co, Cu, Fe, Pb, Li, Mg, Mn, Hg,
Ni, P, K, Si, Ag, Na, Sr, Sn, V, and Zn; as cyclohexanebutyrates and/or
ethylhexanoates, these metals make spectrographic standards that have the
following properties: Each is reproducible, stable, nonvolatile, oil-soluble,
and compatible with all the other spectrographic standards in this series. Many
of these metals can be detected spectrographically in extreme dilution (parts per
million or less), so that one gram could be used to tag a ton or more of petroleum.
The 28 compounds that were selected as standards were chosen from an initial
list of over 150 candidates; many of the candidates that were rejected as spectro-
graphic standards may still be useful as active tags for oil spills: One of the
principal reasons for rejection was extreme chemical purity (e. g., the metal
naphthenates), and this requirement might be somewhat relaxed for oil-spill
tags. These 28 standard samples, prepared in solid form for easy weighing and
handling, may be purchased for $2.00 a gram from the National Bureau of Stan-
dards. Obviously, in mass production (with some relaxation of the requirements
for extreme chemical purity), the price might be significantly reduced.
Even though all this work on spectrographic standards has made the use
of certain organometallics an attractive possibility, there are still several prob—
lems and doubts that must be resolved. First among these is the problem of
solubiity. Only a few of the Bureau of Standard’s organometallic standards
could be dissolved in oil, without heat or other solvents. If the standard were
dissolved in a solubiizing agent (such as a free carboxylic acid, a diketone, or
an ester) and then diluted with the base oil, however, the resultant solutions had
optimum stability and highest concentrations of trace metal. The solutions were
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judged satisfactorily stable if a solution of at least 500 ppm of the trace metal
of interest remained “clear and devoid of visible change during several weeks at
room temperature.” An excellent solubilizer and stabilizer was found to be an
equimolar mixture of bis (2-ethylhexyl)amine and bis(2-ethylhexyl)dithiocarbamic
acid in an equal volume of xylene. It is not clear, however, what the highest
stable concentration of metals might be. A related question is more important
still: What happens to the analytical standard once the solubilizers and stabilizers
have evaporated, as is likely after a slick has aged on the sea for several days?
Will the standards gel or precipitate?
A second major problem is background: Most crude oils contain traces
of inorganics (especially metals), and some crudes have more than 0. 5% ash;
there are several excellent reviews of the literature on this topic. 13, 14 Many
refined petroleum products also contain metals, either deliberately added (e. g.,
components of additives to impede corrosion and oxidation or to improve lubri-
cating properties), or adventitiously present (e. g., volatile organometallics in
the crude that were not removed during refining). How can this background be
distinguished from the active tag? Analysis of petroleum shipments for the
presence of background interferents is obviously not the answer: — It is much
too costly, for one thing, and for another there is no guarantee that the metallic
suites in the oil slicks will be identical to those in the unspilled oil. Indeed,
there is every reason to believe that the suites will be drastically different.
A better solution is to separate the tag from the slick (by chromatography,
for example) before attempting further identification. Since the tag is a chemi-
cally stable “known,” and since the organic moiety of the tag (e. g., cyclo-
hexanebutyrate) should be rare in petroleum, there is no reason to believe that
background suites will invalidate this method of tagging. However, separating
and concentrating the tag from each slick sample adds to the time and cost of
identification; since there are several thousand oil spills in U.S. waters each
year, the problem of time and cost is not inconsiderable.
RADIOACTIVE TAGS
Radioactive isotopes have been examined for possible use in a tagging
system, and eliminated for two broad reasons. First, there are not enough
isotopes with the right half-life to make the system work and second the use of
radioactive material in public places is objectionable for fear of contamination.
Ideally, a radioactive tag has a lifetime of 90 days, a reasonable cost,
is easily detected, and readily available. Lifetime of all tags should be about
the same so that the relative abundance of isotopes does not vary with time.
Thus, a minimum tagging system would have 30 or more tags available with
half-lives between 70 and 110 days. Of 1700 isotopes examined, 15 have half-
lives of 70 to 110 days and, of the 15, a majority were either too expensive or
too difficult to detect.
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Politically it is not wise to put radioactive material in oil when there is
any possibility that the radioactive material might escape into the air or water.
Recent experiences in Japan with the visit of nuclear submarines show how
sensitive the public is about radioactive contamination of water.
PARTICULATE TAGS
Before discussing active particulate tags, it is well to say a little about
the particulate material that is naturally foi.md in petroleum. The basic reference
on this subject is a 1937 paper by J. M. Sanders, 71 to whom all more recent
researchers are heavily indebted. In general, the particulate matter in petro-
leum consists of spores, pollens, fragments of tissue, hairs, microalgae, bits
of feathers, etc. In addition, much of the particulate matter is in the form of
asphaltenes, which are identified as suspended solids, colloidal particles, or
solute molecules, depending on the investigator and his techniques. 43 Although
these particles can convey a wealth of information to the careful investigator,
their relatively small number and wide variety prohibit their use as particulate
passive tags; furthermore, an oil spill may entrain poll 2 ens, spores, and other
biological matter during its exposure to the elements.
Adding well-characterized particles to oils is a promising technique. In
the first place, these particles can be easily manufactured under current tech-
nology; second, they can be added to oil in small, measured quantities; third,
there is good evidence that, if the need arises, they can be isolated from the oil
matrix in quantities sufficient for very specific identification. Active particulate
tags share all the advantages of active oil-soluble tags (viz, specificity and small
amounts of tag for proper identification), and offer several other advantages
that might make them more suitable. Foremost among these advantages is the
ease with which such particles may be produced by one basic technique: grinding
and spheroidLzing of solids produced from an appropriate liquid melt.
Rather homogeneous solid material can be obtained by (1) solidification
of a liquid melt, (2) coprecipitation from solution, (3) efficient mixing and S inter-
ing of suitable solids, and (4) deposition from a vapor phase; almost any such
material may then be spheroidized. It is recommended that particulate tags be
prepared as microspheroids for several reasons: (1) Their small surface area
per unit volume and lack of areas of large surface curvature make them relatively
insusceptible to chemical reactions; (2) these same qualities make it rather un-
likely that they will clog or impact upon passage through small apertures, or
adhere to interfaces; (3) microspheroids are much more readily sized (e. g., by
sieving, sedimentation, and microscopic methods) than any other shape; and (4)
the technology for producing them is eminently feasible, and so is the technology
for characterizing them. The flowability of microspheroids makes them ideal
for unit operations where mixing is necessarily of importance; flowability is
obviously important where trace solids are to be added to a very large amount
of liquid. All these points also apply to simplifying the preconcentration and
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isolation required in any analysis of suspensoids. Finally, small particles
(10-50 microns) are not rare in nature; but very smooth microspheroids of
this size are much less common.
The most common process for making microspheroids is liquefaction of
a particle followed by solidification in a dilute, slowly moving gaseous ‘ suspension.”
Other common techniques are: tabling to produce Lucite spheroids; deposition
from liquid solution for certain resins; and mechanical action, as when polishing
compounds used in ball mills produce ceramic or glass spheres. There are also
several methods for making microspheroids from metals.
Many materials have been made into microspheroids: soft and hard
glasses 7375 (which may be “doped’ with a wide variety of metallic salts),
metals, 76, 77 ceramics, nitrides, carbides, celluloses, starches, polystyrene,
phenolic resins, and many other organics. 78 Many suitable microspheroids are
already commercially available; others can be expeditiously made by custom
synthesis. Their costs range from a fraction of a cent a pound (for some com-
mercially available materials) to $1,000 a pound (for custom synthesized rare
metal microspheroids).
Exact methods for analysis and identification of oil spills that contain
particulate tags depend on many factors. A thumbnail sketch of what, at this
time, appears to be a reasonable procedure follows:
a. Microspheroids of about 10 microns diameter would be produced
by spheroidizing a suitable, finely ground and sized solid.
b. These particles would be characterized in preliminary tests to
ensure proper quality control over such critical factors as particle diameter,
density, content of trace additives, etc.; the data would then be carefully cataloged.
c. The particles would then be metered into the specified petroleum
(preferably as it passes through a pipeline or as it is pumped into a tanker). In 4
order to have, on the average, io particles per liter of oil, only about 5 x 10
gram of particles needs to be added to each ton of oil; this amounts to 50 grams
for a 100,000—ton tanker.
d. Upon spillage, a representative sample would be taken from the
slick; the sample need not be much more than a few litei s.
e. The particles in about a liter of spilled oil would then be precon-
centrated as follows: (1) centrifugation and washing with a light hydrocarbon,
(2) washing with more digestive liquids (perhaps acids or bases), (3) a terminal
filtration and wash on a Millipore filter, and (4) transfer of the solids to suitable
microscopic mounts.
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1. Analysis (as in step b.) to identify the particles. These tests
might include microscopy (for size and shape, color, phosphoresence, fluores-
cence), microanalysis (for density, hardness, trace analysis), or micro-
spectrometry (light absorption, fluor imetry, reflectance). M icrospectro-
fluorimetry could provide a characteristic spectrograph of just one or of a
number of particles; such a trace would, indeed, be a “fingerprint.”
g. The data from the preceding step would be compared with the data
catalog (see step b.).
h. In the case of a multiple, confluent spill, the individual offenders
could be identified readily, provided that only one type of tag had been added to
each oil. To determine how much of the spill was due to each offender would
involve characterizing a larger, representative sample of microparticles;
based on the number density of each type of tag, proportional liabilities could
be estimated.
Microspheroidal tags permit of a very large number of identifiable forms.
Considering only particle sizes of 10-50 microns with an error of ±2. 0 microns
for a given size, there are at least 11 distinct and well-defined sizes. ‘ For
density, given a range of 0.9 - 1.50 grams/mi and an error of ±2%, we have,
again, at least 11 possible characterizations. 80 The product of 11 sizes and
ii densities is 121 distinct particle types. In addition to these types, particles
may contain trace amounts of chemically detectable materials, which may be
characterized qualitatively or quantitatively. A series of microtests could
easily determine whether or not 20 or so readily identifiable trace chemicals
are present in the particle. 81-85 With a very simple binary code (in which each
of the trace chemicals is merely determined as “present” or absent”), 20 trace
chemicals permits 220 (1, 048, 576) possible particle characterizations. In
combination with the 121 basic particle types, trace-chemical binary coding
gives well over 100 million types of identifiable particulate tags.
ADDITION OF ACTWE TAGS TO PETROLEUM
The metering of small amounts of liquids has been developed by the
chemical, pharmaceutical, and processing industries; ideally, the tags would
be injected into the oil as it passes through a pipeline or as it is pumped into a
tanker). For the particulate tags we recommend (2-50 micron spheroids), the
same techniques will suffice. Addition of a tracer material to petroleum in-
volves two similar and related criteria: First, the tracer should be in a homo-
geneous phase, and, second, this carrier phase should be injected uniformly
into the petroleum so that the resulting tagged dispersion (or solution) is also
uniform. There is ample evidence that relatively stable dispersions of particles
in a petroleum-based liquid are easily produced. It is important that this sta-
bility be maintained when the carrier fluid is injected into the petroleum.
Surfactants are critical to stability. 86 In crude oil, enough surfactants (such as
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porphyrins) are usually present to assure stability (at least for the dilute sus-
pensions being considered); in refined products, it may be necessary to select
the particulate tag with great care so that the appropriate surface properties
(hydrophobic ity and oleophilicity) pertain. Highly polar materials (e.g., ionic
salts) actually stabilize suspensions when the particles are larger than about
1 micron; the only important exceptions are metal sols, which, however, are
stabilized by organometallic molecules.
According to Koelmans, “. . . a [ disperse] system is stable when a cross-
ing of the potential barrier between the particles by thermal motion alone is very
improbable”; 87 this potential barrier is about 15 kT for a modestly charged
double-layer and is much greater than the usual thermal energies (about 1 kT).
This energy difference also applies to particles approaching an interface:
Thermal energies cannot overcome the surface potential barrier. Very con-
centrated suspensions (20-90 weight %) may be stabilized by adding small quan-
tities of dissimilar particles; 88 this stabilization technique may be of value in
prescribing carrier suspensions.
Once a stable carrier suspension has been produced, it can be metered
into petroleum without further problems. Many metering pumps are available
at nominal cost; some of these (through mechanical action on a disposable dia-
phragm, bladder, or flexible tube) do not even contact the metered fluid. Since
the volume of the metered suspension or solution may be increased almost at
will, pump capacities are not of special importance. Sufficiently stable particle
suspensions can be produced and then metered into the petroleum which is to be
tagged; since this latter step represents a very large dilution, the resulting sus-
pension is expected to be very stable.
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COSTS OF ACTiVE TAGGING
The materials cost for particle tagging should be very small. The
matrix of the particles will probably be a quite common plastic or glass, and
would cost a few cents to a few dollars a pound. Assuming the cost of par-
ticulate tagging material is $4. 53 a pound and one milligram is added per
metric ton of oil, then the per ton material cost is one thousandth of a cent
per ton. The trace materials imbedded in this matrix would cost about the
same but would be used in almost negligible quantities. The capital outlay
for equipment is more expensive. For instance, laboratory apparatus to pro-
duce the microparticles would nm in the neighborhood of $5, 000. A micro-
spectrofluorimeter with a lens system for single-particle measurements might
cost $30, 000; however, spectrographic analyses of the desired type may be
purchased commercially for about $50 a sample. High quality microscopic
equipment would be absolutely necessary; it is available for $4, 000 to $20, 000.
Halogenated, polynuclear aromatic hydrocarbons vary enormously in
price. While many halogenated benzenes can be bought for under $4 a pound,
the halogenated polynuclear compounds can be quite expensive. For example,
9-bromoanthracene costs $1,000 a pound, 2-fluoronaphthalene costs $360 a
pound, but 1, 3, 6, 8-tetrachioropyrene costs only $30 a pound. Even the most
expensive of these compounds, however, goes a long way in a tagging system.
Approximately 0. 001 lb of tag in 50, 000 tons of oil can be detected by electron-
capture gas chromatography; even assuming the worst (the outrageous price
of $5000 a pound for tagging material), $5 worth of tag would identify 50,000
tons of oil—so that the materials cost for tagging one ton of oil is 0. 010.
Obviously, many halogenat -i tags can be had for a fraction of this extreme
price. Excellent electron-capt.re gas chromatographs can be had for $5, 000
or less, so that the capital outlay for detection equipment is not a large
expense.
The organometallic spectrographic standards that the National Bureau
of Standards designed specifically for analysis of petroleum products are
available from the Bureau for $2 a gram (roughly $900 a pound). These tags
are detectable in dilutions of i0 , so that one gram of tag would identify
about one ton of oil. The ureau’ s spectrographic standards are exacting in
the extreme, and these standards are not mass produced. By relaxing the
requirements for such extreme chemical purity and increasing the production
schedule, it seems probable that significant cost savings could be effected.
Analytical instrumentation for spectrography is widely available commercially
at prices beginning at several thousand dollars, and are standard equipment in
most analytical laboratories.
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Since the amount of tagging material that will be added to a ton of
oil—whether the tags be particulate or liquid—is about one part in a billion,
the cost of the tagging material is low per ton of oil. Costs of adding the
tagging material should be in the same order of magnitude. Including all the
quality control, analysis, and service, the actual cost of adding a “license
plate” should be less than a few cents per ton of oil.
Feasibility studies and demonstration of the system’s efficacy should
be run before the operational tagging begins. However, the economics and
technology are such that the tagging of oil can begin now.
TAGGING CODES
Apart from the type of material used for tagging, “codes” and “tagging
profiles” will be involved in the system. Oil shipped into one port will be
mixed upon unloading with oil from other tankers and hence the stored oil with
its mixture of tagging codes will have its own unique tagging profile at a given
time. Oil subsequently loaded onto a tanker will be re-tagged by the unique
code assigned to the tanker. Thus, the re-tagged oil will have a new tagging
profile whose dominant code will be that of the tanker that last tagged the oil.
Thus, any oil or oil slick from a tanker will have an underlying tag noise with
a superimposed tanker code. To identify the source—last jurisdiction—of the
oil not only must the individual tags be identified but also the relative abundance
of each tag.
When an unknown batch of oil is recovered for source identification and
analysis is begun, two immediate questions arise: Is this oil spilled from a
tanker? Who is the parent corporation? The first question arises because
many ships, as opposed to tankers, spill oil, and the second arises because
oil is within a corporate jurisdiction much longer than in a tanker jurisdiction.
To speed up analysis a special tag would be used to identify all oil ever trans-
ported by tanker. Since there are a few very large oil companies, special
tags could be assigned to each of the large companies, and smaller companies
and independents could be grouped and given a special tag for their group.
This would be in addition to the unique tag or code of tags assigned to a tanker.
Thus, a quick analysis could answer the question as to whether the oil was
ever shipped by tanker and which is the overall company in charge of the oil.
If an independent tanker carried the oil this would be readily seen.
It would be very wise in setting up this tagging system to require each
ship to tag its fuel oil at a bunkering station. This would eliminate confusion
of tanker slicks and ship-fuel oil slicks.
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TAGGII [ G SYSTEM
Each oil company and each ship would have its own tagging code
assigned. Depending on the system the code could be a unique tag or a code
of tags. When a tanker loads an oil cargo, two taggings would take place:
one by the loading station and one by the tanker.
Each tanker would have its pre-mixed and prepared code carried in
sealed bottles. When loading is to begin, the coding bottle would be placed
in the code metering pump and the coded fluid metered into the pipeline ahead
of the pumps loading the tanker. The natural turbulence of the process would
distribute the tagged material throughout the oil. Simultaneously the loading
station would tag the oil with its own tag. A sample of the tagged oil would
be taken for quality control and for subsequent comparisons with spills.
At each unloading the oil from the tanker would be sampled and then
tagged by the shore facility. The sample would be analyzed for quality con-
trol and for the record in case of a spill claim. Tagging during unloading
would cancel the tanker profile and eliminate the possibility of false claims
against the tanker for oil subsequently spilled.
Tampering with the tagging system would be difficult because tagging
is difficult to do except during loading and unloading in that thorough mixing
is necessary. Furthermore, since a variety of tags are involved, it will be
difficult to alter a batch of oil without its being obvious. Sophisticated analysis
of the tags can be used to show that some of the tags are spurious. In other
words, different batches of the same tag can have subtle differences that can
be detected.
CONC LUS IONS
Two approaches to identifying oil spills: Passive tagging and active
tagging have been examined. Passive tagging is a convenient circumlocution
for no tagging at all: Nothing is added to the oil; the slick itself must provide
all the evidence for its identification. Active tagging involves adding some
known and readily identifiable material to the oil. The added material must
satisfy several criteria: It must be soluble or dispersible in oil and insoluble
and nondispersible in water; it must be chemically and physically stable in
both spilled and unspilled oil; it must not interfere with the refining or with
the commercial uses of petroleum; and it must not be too expensive. Two
figures of speech are helpful in distinguishing active tagging from passive
tagging: Passive tagging amounts to a “fingerprint,” active tagging amounts
to a chemical or physical “license plate.”
22
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Passive tagging does not seem promising, for the fingerprint is likely
to be altered or to disappear as the slick is exposed to the abundant sources
of chemical and physical energy in the sea and sky. The easily identifiable
passive tags of petroleum (e. g., isotope ratios and trace metals) cannot be
assumed to be stable in the slick—indeed, there is little doubt that they are
unstable—and the more stable passive tags in the slick (e. g., heavy aromatic
and alkylaromatic hydrocarbons) are prohibitively difficult to separate and
identify.
Active tagging, however, offers several attractive possibilites: Making
a good license plate seems much easier than finding a good fingerprint. Two
types of tags: Oil-soluble tags (organometallics and halogenated aromatic
polycycles) and particulate tags (coded microspheroids) have been examined.
Several promising organometallics have been developed by the National Bureau
of Standards in conjunction with the American Petroleum Institute; these com-
pounds sell for $2 a gram, they are detectable in parts per million, and they
seem to have all the properties of good active tags (compatibility with oil,
solubility in water, chemical and physical stability, and ease of identification).
The halogenated polycycles have never been used for analytical tracing in
petroleum; consequently, no direct experience recommends them. However,
they should have many desirable properties, and chemical theory recommends
them. Their cost can range from a few dollars to a few thousand dollars a
pound, but they are very readily detectable in parts per billion by electron-
capture gas chromatography.
Although the feasibility of particulate tags has not been demonstrated
in oil spills, coded microspheroids appear to have exceptional promise:
Many are already commercially available at very low prices; an astronomic
variety of coded microspheroids can be cheaply and easily made by custom
synthesis; and all the particulate tags can be quickly separated from samples
of slick and readily identified by available analytical techniques.
23
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