The purge and trap procedure commonly used for analysis of volatile
organic compounds in water or air can be significantly improved
using traps employing clays and modified clays as adsorbents. Examples
of clays include attapulgite clay, attapulgite clay modified with
tetrasodiumpyrophosphate, carboxymethylcellulose and the combination
thereof, kaolin clay, kaolin clay modified with tetrasodiumpyrophosphate,
carboxymethylcellulose or the combination thereof, and mixtures
of the above clays.
What is claimed is:
1. A method of sequential adsorption, storage, and desorption of
a multiplicity of organic compounds contained in a gas stream at
concentrations from about 0.5 parts per billion to about 100 parts
per million with retention of the relative proportions of each of
said organic compounds in each of the adsorption, storage, and desorption
cycles comprising flowing said gas stream through an adsorbent bed
comprising a clay to adsorb said organic compounds in amounts which
are the same proportionate portion as each are found in said gas
stream, storing the adsorbed organic compounds in the clay, desorbing
said organic compounds in substantially the same proportionate portion
as each are stored in the clay at a temperature between about 50.degree.
C. and about 600.degree. C. for a time sufficient to desorb organic
compounds, and removing residual organic compounds from the clay
by heating the clay in a flowing gas at a temperature of at least
450.degree. C. for a time sufficient to remove residual organic
compounds from the clay.
2. The method of claim 1 wherein the clay is selected from the
group consisting of attapulgite clay, attapulgite clay modified
with tetrasodiumpyrophosphate, carboxymethylcellulose, and the combination
thereof, kaolin ciay, kaolin clay modified with tetrasodiumpyrophosphate,
carboxymethylcellulose, and the combination thereof, and mixtures
3. The method of claim 1 further characterized in having a water
removing adsorbent placed upstream of the clay adsorbent bed.
4. The method of claim 1 further characterized in that the clay
is present in combination with another adsorbent bed containing
adsorbent selected from the group consisting of silicalite, dealuminated
zeolite Y, and fumed silica.
FIELD OF THE INVENTION
The present invention involves using clays such as attapulgite
clay and modified attapulgite clays as the adsorbent in purge and
trap sorbent tubes.
BACKGROUND OF THE INVENTION
With the heightened environmental concern regarding the presence
of contaminants in drinking water it has become necessary to analyze
water for volatile organic compounds. The purge and trap technique
is a general purpose method for the identification and simultaneous
measurement of purgable, volatile organic compounds in water that
have sufficiently high volatility and sufficiently low water solubility
to be efficiently removed from water. Among the volatile organic
compounds which can be determined by the purge and trap procedure
are benzene, bromobenzene, carbon tetrachloride, chloroform, cumene,
naphthalene, styrene, toluene, the xylenes, vinyl chloride, tetrachloroethylene,
hexachlorobutadiene, methylene dichloride and fluorodichloromethane.
An analogous technique also is used for the analysis of volatile
organic compounds in air.
In a typical purge and trap procedure, exemplified by EPA Method
524.2 volatile organic compounds and surrogates with low water
solubility are purged (extracted) from the sample by bubbling an
inert gas through the aqueous sample. Purged sample components are
trapped in a tube containing suitable sorbent materials. When purging
is complete, the sorbent tube is heated and backflushed with helium
to desorb the trapped sample components into a capillary gas chromatography
(GC) column interfaced to a mass spectrometer (MS). The column is
temperature programmed to separate the analytes which are then detected
with the MS. Compounds eluting from the GC column are identified
by comparing their measured mass spectra and retention times to
reference spectra and retention times in a database. Reference spectra
and retention times for analytes are obtained by the measurement
of calibration standards under the same conditions used for samples.
A concentration of each identified component is measured by relating
the MS response of the quantitation ion produced by that compound
to the MS response of the quantitation ion produced by a compound
that is used as an internal standard. Surrogate analytes, whose
concentrations are known in every sample, are measured with the
same internal standard calibration procedure.
The foregoing description was for the analysis of volatile organic
materials in aqueous systems where the purge and trap technique
is appropriate. However, it should be clear that an analogous procedure
may be utilized for the analysis of volatile organic materials in,
e.g., air analysis. The exposition within will be directed with
particularity to analysis of volatile organics in aqueous media
using the purge and trap procedure, but this is done solely for
clarity and ease of exposition. It needs to be clearly understood
that the subject matter is not restricted to such analyses and is
capable of significant expansion.
This application focuses on the sorbent tubes used in purge and
trap analysis. In particular, our goal is the development of an
improved sample concentration sorbent tube, superior to those presently
available, to enhance the purge and trap procedure itself, both
as to its methodology and its results.
The jet separator specified in, for example, EPA Method 524.2 for
analysis using a GC/MS system can cause losses of 50% or more for
small analytes, a condition alleviated somewhat by interfacing the
column directly to a MS ion source. Elimination of the jet separator
requires low column flow rates, which are not compatible with flow
rates in purge and trap systems. Another option for improving sensitivity
is the use of larger samples. Since both of these options have significant
disadvantages, a sorbent tube that is considerably more efficient
than those commonly used is needed.
Present adsorbents in the sorbent tubes used for purge and trap
methodology appear to be one or more of various charcoals or porous
carbons, organic polymers such as that of 26-diphenylene oxide
(e.g., Tenax.RTM.), and silica gels. Each individually and even
in combination suffer from distinct limitations and disadvantages.
One disadvantage is that of limited capacity, so that "saturation"
of the adsorbent is all too readily attained, leading to error in
analytic results. Each also suffers from a lack of thermal stability,
with temperatures of about 200.degree. C. or greater likely to lead
to irreversible impairment as an adsorbent. Each additionally suffers
from hysteresis or "memory" effects, i.e., complete desorption
of some components may be difficult with additional desorption occurring
during subsequent analyses using the same sorbent tube in a different
purge cycle. This is frequently referred to as "carryover."
Perhaps the most severe limitation of materials commonly used as
adsorbents in purge and trap methods is their very limited linearity.
That is, the adsorbents typically utilized by those in the art discriminate
among the various classes of organic materials which may be present,
and also may discriminate among the organic materials within a class.
Thus, a substantial proportion of analysis time must be spent in
calibrating sorbent tubes for their nonlinearity. Therefore, adsorbents
which are linear, or nearly so, with respect to the adsorption,
storage and desorption of a broad spectrum of organic components
over a wide dynamic concentration range are desired.
Although the present invention is simple, it is an extraordinarily
effective solution to the foregoing problems. The present invention
demonstrates that when the sorbent tubes of the purge and trap unit
contain clays such as attapulgite clay and/or modified attapulgite
clays as adsorbents, semi-volatile organic compounds found in contaminated
water can be effectively and efficiently adsorbed. Furthermore,
the capacity of these clays as adsorbents in sorbent tubes mandated
for use by the EPA is sufficient so that "breakthrough"
through saturation by components at high concentrations is rarely
a threat. Complete desorption of all components is readily attained
with avoidance of hysteresis effects since substantially higher
desorption and bakeout temperatures are available as compared to
the commonly-used sorbent tube materials thereby eliminating analytical
errors arising from materials remaining in a sorbent tube from prior
analyses. The resulting benefit is better run-to-run reproducibility
and a higher precision in measurement evidenced by a lower relative
standard error of deviation. Another benefit is longer sorbent tube
lifetime. That is, the attapulgite clay adsorbents can undergo more
adsorb-desorb cycles than those in the sorbent tubes commonly in
commercial use. But most importantly is that the sorbent tubes of
the present invention exhibit linearity in absorption, linearity
in storage, and linearity in desorption, and exhibit such linearity
over a dynamic range that easily spans four orders of magnitude.
Thus calibration becomes an infrequent occurrence. These cumulative
benefits are substantial and offer advantages over the prior art
sorbent tubes which are advantages in kind rather than advantages
SUMMARY OF THE INVENTION
The purpose of this invention is to make possible the analysis
of volatile organic compounds in water or air by a purge and trap
procedure using clays as adsorbent materials which show linearity
in adsorption, storage, and desorption of the organic compounds
over a wide dynamic concentration range, with a high capacity, and
little or no hysteresis. An embodiment comprises utilizing as a
trap in classical purge-and-trap procedures an adsorbent which is
a clay, a modified clay, or a combination thereof. In a specific
embodiment the clay is attapulgite, tetrasodiumpyrophosphate-modified
attapulgite, carboxymethylcellulose-modified attapulgite, or a mixture
BRIEF DESCRIPTION OF THE DRAWING
The FIGURE is a simplified side view of the sorbent tube of the
invention having a bed of an adsorbent to adsorb water vapor and
a bed of attapulgite clay. Additional pieces of apparatus that may
be employed in connection with the apparatus of the invention are
DESCRIPTION OF THE INVENTION
The present invention relates to the deployment of well-known materials
as adsorbents in a particular field of use. Although the materials
themselves have been recognized as adsorbents and binders for adsorbents
in other applications, they have not been used as adsorbents for
the purge and trap procedures used in the analysis of organic compounds
present in water and in air. Although the analysis of trace organic
components in aqueous streams by a combination of gas chromatography
and mass spectrometry has for some time suffered from the limitations
of, e.g., silica, porous carbon, and organic polymers as adsorbents,
few substitutes have been commercially offered. We now describe
an alternative which is extraordinarily effective, for it makes
possible a procedure with enhanced linearity, greater reproducibility
and precision, decreased carryover adversely affecting analytical
accuracy, less frequent calibration, and longer operational material
The present advancement in purge and trap procedures is the employment
of clays and modified clays as adsorbents to replace, partially
or preferably totally, the current common adsorbents. Perhaps the
most outstanding feature of the clays and modified clays of this
invention as adsorbents compared to the current state of the art
is heir linearity. That is, the clays and modified clays adsorb,
store and desorb a wide range of organic species in proportionately
equal amounts; there is a one-to-one correspondence between the
incoming organic species and the desorbed species. This linearity
is independent of the mix of organic materials up to the adsorbent
saturation point, and linearity is maintained over a dynamic range
of better than 10.sup.4 often from 0.5 ppb up to 100 ppm, with
linearity over this range of less than 5%. In comparison, current
sorbent tube materials, such as silica gel, carbon in its various
forms, and organic polymers such as polyethers, exhibit variable
linearity depending upon the batch of material used, the thermal
history of the adsorbent, and the mixture of the adsorbed species.
In part the current commercial materials exhibit nonlinearity because
their structures may change with temperature cycling, whereas the
clays and modified clays disclosed herein are quite stable to temperature
An associated benefit of the high thermal stability of the clays
and modified clays is that both a high desorption temperature and
an even higher bakeout temperature can be employed. High desorption
temperatures tend to promote a linear response and also promote
short desorption times, thereby reducing analysis time. Because
the desorption temperature is inversely proportional to the concentration
of material remaining on the adsorbent, a high bakeout temperature
tends to minimize memory and hysteresis effects; bakeout at, e.g.,
it is common to use 500.degree. C. which tends to remove all adsorbed
species from the sorbent tube in a reasonably short time.
The adsorbents which are used in the present advanced sorbent tubes
are clays that are neutral, slightly acidic, or slightly basic.
Strongly basic clays such as hydrotalcite are not successful in
the present invention. The preferred clay is attapulgite clay and
attapulgite clay modified with tetrasodiumpyrophosphate, carboxymethylcellulose
or the combination thereof. Attapulgite is a complex hydrated magnesium
aluminum silicate, a colloidally dimensioned mineral having an acicular
or lathlike structure; see U.S. Pat. No. 3049499 hereby incorporated
by reference. It is also expected that kaolin clay and kaolin clay
modified with tetrasodiumpyrophosphate, carboxymethylcellulose or
the combination thereof would be successful in the present invention.
The use of the tetrasodiumpyrophosphate and carboxymethylcellulose
modifiers aid in increasing the macroporisity of the clays. Different
clays may be used individually or as mixtures. Additional adsorbents
may be used in combination with the clays. Examples of other suitable
adsorbents include silicalite, dealuminated zeolite Y, and fumed
silica. When using dealuminated zeolite Y the degree of dealuminization
must be sufficient to cause he material to be mostly hydrophobic
and organophilic. The additional adsorbent is chosen to have a high
selectivity for the compound of interest. Due to the difference
in desorption temperatures of the additional adsorbents and the
clays, it is preferred that the additional adsorbent and the clays
be located in separate sub-beds. The complexity of two sub-beds
results in that embodiment being less preferred. Metal oxides and
metal ions such as lithium, sodium and/or potassium can be added
or ion exchanged with the silicalite, dealuminated zeolite Y, or
These clays are usually employed as a binder to hold together aggregates
of very small particles of zeolitic adsorbent, and the clays have
been previously considered to adversely affect the performance of
at least zeolitic adsorbents used in purge and trap sorbent tubes.
Contrary to prior beliefs, it is demonstrated here that the clays
or modified clays themselves are excellent adsorbents for volatile
The conditions under which the clay and/or modified clay-packed
sorbent tubes are used in the purge and trap procedure include an
adsorption cycle usually conducted at about ambient temperature.
Certainly it is possible to cool the adsorbent, but generally adsorption
is conducted without significant cooling of the sorbent tube. The
desorption cycle is preferably conducted at a temperature as high
as is feasible. High temperatures favor linear response, and since
the materials of our invention are structurally thermostable, desorption
temperatures of about 200.degree. C. to about 500.degree. C. are
recommended. However, desorption may be performed over the range
from about 50.degree. C. up to about 600.degree. C., even though
the aforementioned narrower temperature range encompasses the more
usual working conditions of about 250.degree. C. to about 400.degree.
C. Excellent linearity in desorption among the members of a broad
spectrum of organic materials is observed under these conditions.
The desorption temperature is held for a time sufficient to desorb
the components of interest. The desorption temperature may be held
for about a five minute time period, and typically, the desorption
is held for a period of about one to two minutes to allow the adsorbent
to become completely heated. Fast desorption is preferred, and it
is therefore preferred to hold the desorption temperature for less
than one minute.
Residual organic materials are removed from the clays by heating
the latter to what is commonly called a bakeout temperature. The
higher the bakeout temperature, the lower will be the residual organic
materials on the adsorbent, since the desorption rate is an exponential
function of the amount of organic material remaining on the adsorbent.
Usually the bakeout temperature is substantially greater than the
desorption temperature, and for the materials of the present invention
a bakeout temperature about 500.degree. C. or higher is common.
It is to be emphasized that high bakeout temperatures are integral
to the absence of hysteresis and a minimum bakeout temperature of
450.degree. C. is recommended. The maximum bakeout temperature will
depend on the thermal stability of the clay or modified clay utilized
in the practice of this invention, a property which a skilled artisan
can readily determine either from the prior art or by simple experimentation.
Generally the maximum bakeout temperature will be at least 700.degree.
C., although additional benefits are unlikely above a bakeout temperature
of 600.degree. C. The bakeout temperature is held for a time sufficient
to desorb the residual organic materials on the adsorbent. The bakeout
temperature is generally held for about 5 to about 10 minutes, but
can be as little as about 2 minutes depending upon the application.
The clays and modified clays used in the sorbent tubes of the present
invention can eliminate or greatly reduce the dry purge step mandated
by the EPA protocols, a step which is included there to reduce the
amount of water on the adsorbent. Using a molecular sieve highly
selective for water over organic compounds can remove the water
prior to its contact with the clay adsorbent for volatile organic
compounds. The water removing adsorbent can be included either as
part of a unitary adsorption tube, with the water removing adsorbent
placed prior to, or upstream of, the clay adsorbent, or as a separate
bed. Having the water removing adsorbent present as a separate bed
has the advantage of not only protecting the clay adsorbent for
volatile organic compounds from moisture so as to maximize its capacity
for organic materials, but it also permits the bed to be removed
from the desorption flow path, thereby keeping moisture from the
chromatographic instrument. On the other hand, having the water
removing adsorbent as the first bed in the adsorbent tube allows
hydrophilic adsorbents to be used, but during the desorption step
this water is sent into the chromatographic instrument. For most
chromatographic detectors this does not pose a problem.
Turning to the FIGURE, the sorbent tube apparatus of the invention
is shown as a vessel 2 having a gas fluid inlet 4 and a gas fluid
outlet 6. The vessel may be constructed of any suitable material
able to conduct gas at the flow rate, temperature, and pressure
of the particular application. The gas fluid inlet and outlet may
further be equipped with connectors so that the apparatus may be
readily attached to the chromatographic system. Furthermore, the
gas fluid inlet and outlet may contain a retainer to prevent the
solid contents of the vessel from being removed from the vessel.
One bed of attapulgite clay is shown as bed 10 in the FIGURE. The
FIGURE also contains the optional bed 8 containing an adsorbent
to remove water vapor.
20 g Minugel (attapulgite clay), 0.4 g carboxymethylcellulose,
0.2 g tetrasodiumpyrophosphate, and about 120 mL of water was mixed
together to form a gelatinous mass. The mass was air dried at 90.degree.
C. and calcined to 600.degree. C. at 1.degree. C./min. and held
for 1 hour. The resulting 20-50 mesh particles of clay were packed
into sorbent tubes.
Most of the compounds of EPA method 524.2 rev 4 were used to evaluate
adsorbents for purge and trap applications as 20 ppb solutions (5
mL total). The procedures mandated in the foregoing EPA method were
followed. 5 mL of the water standard were introduced into the sparge
tube and sparged with helium at a flow rate of 40 mL/min. for 6
minutes. A 30 second dry purge was then initiated, after which the
sample was desorbed into the gas chromatograph for 2 minutes.
Similarly, most of the compounds of Methods TO-14 Determination
of Volatile Organic Compounds (VOCs) in Ambient Air Using SUMMA.RTM.
Passivated Canister Sample and Gas Chromatographic Analysis, and
most of the compounds of Compendium Method TO-17 Compendium of
Methods for the Determination of Toxic Organic Compounds in Ambient
Air, Second Edition; Determination of Volatile Organic Compounds
in Ambient Air Using Active Sampling Onto Sorbent Tubes were evaluated
using sorbent tubes packed with the modified minugel clay described
Results are given in the table, with the first column reciting
the name of the organic compound tested, and the next three columns
indicating whether the component is detectable by methods 524.2-rev.4
TO-14 and TO-17 discussed above. The final column indicates whether
the organic compound was detected using the sorbent tubes packed
with the modified Minugel clay described above. The value listed
in the final column is thousands of area
counts per parts-per-billion of the component tested. The results
demonstrate the adsorption, storage, and desorption capability of