Molecular sieve abstract
A molecular sieve trap is disclosed for selective trapping of substances
in a gas stream, the trap being particularly useful in systems for
detecting nitrogen compounds such as N-nitrosamines or nitrogen
oxides in a sample. The trap comprises a cartridge containing a
packing of a granular adsorbent molecular sieve material with a
carefully controlled pore size of about ten angstroms and a preference
for adsorbing polar substances. The packing readily passes NO and
NO.sub.2 gases in a gas stream directed through the trap while trapping
and retaining larger and/or more polar molecules such as those of
double-bonded carbon compounds and sulfur compounds which could
otherwise interfere with subsequent measurement of the NO or NO.sub.x
content of the gas stream. An N-nitroso compound detection system
incorporating the molecular sieve trap is described wherein the
molecular sieve trap is interposed between a pyrolyzer and a chemiluminescent
NO detector to selectively trap substances in the reactor effluent.
Also disclosed is a NO.sub.x detection system having a molecular
sieve trap in a sample inlet line of a NO.sub.x detector.
Molecular sieve claims
What is claimed is:
1. Apparatus for selectively trapping substances in a gas stream
comprising a cartridge having an inlet at one end for receiving
a gas stream, an outlet at the end opposite said inlet end, and
a shell defining a chamber between said inlet and said outlet, said
chamber containing a packing of a granular molecular sieve material
permeable to the flow therethrough of NO gas and adapted to trap
and hold substances therein which might otherwise interfere with
measurement of the NO gas which has passed through said packing.
2. Apparatus as in claim 1 wherein said molecular sieve material
has a nominal pore diameter of about 10 angstroms.
3. Apparatus as in claim 1 or 2 wherein said molecular sieve material
is a crystalline sodium alumino-silicate material.
4. Apparatus as in claim 1 wherein said chamber contains, in addition
to said packing of granular molecular sieve material, a second packing
of a drying agent between said inlet and said packing of granular
molecular sieve material, said drying agent being permeable to the
flow therethrough of NO gas and adapted to remove water from the
gas stream.
5. Apparatus as in claim 4 wherein said drying agent is a granular
material selected from the group consisting of sodium sulfate, calcium
sulfate, phosphorous pentachloride, aluminum silicates, and combinations
thereof.
6. Apparatus as in claim 1 further including porous retainer means
adjacent to the inlet and outlet of said cartridge for holding said
packing in said chamber.
7. Apparatus as in claim 1 and 2 wherein said particles are of
60 to 80 mesh size.
8. Apparatus for measuring in a sample the amount of predetermined
nitrogen compounds comprising:
a chromatograph for receiving said sample and producing an effluent
including, at separate intervals of time, a first portion containing
said predetermined nitrogen compounds and a second portion without
said compounds;
a reactor for selectively liberating NO gas from the nitrogen compounds
of said first portion, thereby producing a reactor effluent containing
NO gas;
a NO detector for measuring the amount of NO gas liberated in said
reactor; and
a trap between said reactor and said detector for selectively trapping
substances from said reactor effluent, said trap including a packing
of a granular molecular sieve material permeable to the flow therethrough
of NO gas and effective to trap and hold substances whose presence
in said detector might interfere with measurement of the NO gas
liberated by said reactor.
9. Apparatus as in claim 8 wherein said predetermined nitrogen
compounds are N-nitroso compounds having the general formula ##STR2##
wherein R.sub.1 and R.sub.2 are the same or different organic radicals,
including those radicals which together with the non-nitroso N of
the depicted N--NO bond constitute a nitrogen heterocyclic radical;
and wherein said reactor is operable to selectively liberate NO
gas from said N-nitroso compounds by breaking the N--NO bonds of
said N-nitroso compounds.
10. Apparatus as in claim 9 wherein said reactor comprises a pyrolyzer
for heating the N-nitroso compounds with an amount of thermal energy
effective to break the N--NO bonds thereof.
11. Apparatus as in claim 8 wherein said molecular sieve material
comprises particles having pores with a nominal diameter of about
10 angstroms.
12. Apparatus as in claim 11 wherein said molecular sieve material
is a crystalline sodium alumino-silicate material.
13. Apparatus as in claim 8 wherein said particles are of 60 to
80 mesh size.
14. Apparatus as in claim 8 wherein said chromatograph comprises
a gas chromatograph including a column and means for driving a mixture
of said sample and a carrier gas through said column.
15. Apparatus as in claim 8 wherein said trap comprises a cartridge
having an inlet for receiving said reactor effluent, an outlet,
and a cylindrical shell defining a chamber between said inlet and
said outlet, said chamber containing said packing of molecular sieve
material and also containing, between said inlet and said packing
of molecular sieve material, a second packing of a drying agent
permeable to the flow therethrough of NO gas and adapted to remove
water from said reactor effluent.
16. Apparatus for measuring in a gaseous sample the concentration
of at least one oxide of nitrogen comprising:
a reaction chamber for permitting a chemiluminescent reaction therein
of a selected constituent of the sample with a reagent;
means for supplying said reagent and said gaseous sample to the
reaction chamber;
means for detecting the light produced in said chamber during a
chemiluminescent reaction; and a trap for selectively removing substances
from said gaseous sample prior to entry of said sample into said
reaction chamber, said trap comprising a packing of a granular molecular
sieve material permeable to the flow therethrough of NO and NO.sub.2
gases and effective to trap and hold substances whose presence in
said reaction chamber might interefere with measurement of NO or
NO.sub.2 gas in said sample.
17. Apparatus as in claim 16 wherein said molecular sieve material
comprises particles having pores with a nominal diameter of about
10 angstroms.
18. Apparatus as in claim 16 wherein said reagent is ozone and
said selected constituent of the sample is NO gas.
19. Apparatus as in claim 16 wherein said molecular sieve material
comprises particles of a crystalline sodium alumino-silicate material
of 60 to 80 mesh size.
20. Apparatus as in claim 19 wherein said trap further comprises
a packing of a drying agent permeable to the flow therethrough of
NO and NO.sub.2 gases and operable to remove water from said sample
prior to passage of said sample through said packing of molecular
sieve material.
21. Apparatus as in claim 16 further including an NO.sub.2 converter
between, and in fluid communication with, said trap and said reaction
chamber for converting NO.sub.2 gas in said sample to NO gas, thereby
to permit measurement of the total concentration of NO gas and NO.sub.2
gas of said gaseous sample.
Molecular sieve description
CROSS-REFERENCE TO RELATED PATENT
The subject matter of the present invention is related to that
disclosed in U.S. Pat. No. 3996002 "Method and Apparatus
for Measuring the N-nitroso Compound Content of a Sample",
issued Dec. 7 1976 to David H. Fine, assigned to the assignee
of the present invention. The material disclosed in that patent
is incorporated herein by reference thereto.
BACKGROUND OF THE INVENTION
The invention relates to apparatus for selectively trapping substances
in a gas stream, and to systems incorporating selective traps, particularly
systems for detecting nitrogen-containing compounds such as nitroso
compounds, amines, nitrofurans, nitrotoluenes, and nitrogen oxides
present in liquid or gaseous samples.
Among the nitrogen-containing compounds whose detection has become
of increasing interest are N-nitroso compounds or N-nitrosamines,
which have the general formula ##STR1## wherein R.sub.1 and R.sub.2
are the same or different organic radicals including those radicals
which together with the non-nitroso N of the depicted N--NO bond
constitute a nitrogen heterocyclic radical. Many of these compounds
are highly carcinogenic with respect to animals. Although their
effect on humans is current unknown, efforts to monitor the presence
of N-nitroso compounds have increased, particularly since these
compounds have been found in trace quantities in many materials
which are contacted or taken internally by humans, including artificial
food additives, tobacco smoke, beer, pesticides, and the atmosphere.
Considerable work has been conducted on detection and measurement
of N-nitroso compounds. One system which has been successfully used
to measure the N-nitrosamine content of a sample is described and
claimed in the above-referenced U.S. Pat. No. 3996002. The system
described therein includes (a) a liquid or gas chromotograph for
separating in time the N-nitroso compound portion of a sample from
portions containing compounds such as nitrates or nitrites; (b)
a non-catalytic pyrolyzer for selectively breaking the N--NO bond
in the N-nitroso compounds to selectively liberate nitric oxide
(NO) gas; and (c) a NO detector for measuring the amount of NO gas
liberated.
In some embodiments of the above-noted system, a cold trap is included
between the pyrolyzer and NO detector for eliminating from the effluent
of the pyrolyzer certain trace contaminants, carrier gases, and
the pyrolysis products associated with certain solvents. A typical
cold trap used in N-nitrosamine detection comprises a conduit immersed
in a bath of a cryogenic medium and having an inlet and outlet for
throughflow of pyrolyzer effluent. The cryogenic medium-e.g., mixtures
such as dry ice and acetone, or liquid nitrogen and pentane, or
liquid nitrogen and isooctane-is selected to maintain a temperature
sufficiently low to condense gases in the pyrolyzer effluent whose
presence in the NO detector could interfere with accurate detection
of NO. This temperature must, however, be sufficiently above the
freezing point of NO (about - 162.degree. C.) to permit all of the
NO gas to pass through the cold trap and into the NO detector. The
condensation of potentially interfering gases and retention of their
condensate in the cold trap prevents their erroneous contribution
to the N-nitroso compound content signal of the detector. This trapping
thus helps to provide accurate detection of N-nitroso compounds
and also permits the use of solvent programming and/or the use a
greater variety of carrier gases in the system.
While the foregoing patented system has proven very effective in
measuring the concentration of N-nitrosamines and other nitrogen-containing
compounds in liquid or gaseous samples, the cold traps used as part
of the system present certain drawbacks. For example, the traps
use cryogenic agents which are relatively expensive and require
careful handling. The cold traps also require considerable upkeep
such as the periodic removal of condensate and the addition of cryogenic
agent to maintain desired cold trap temperatures. The cold traps
inherently do not remove gases with freezing points lower than the
temperature maintained in the traps. Also, they may not remove all
of the gases with higher freezing points because during flow of
the gas through the trap there may be insufficient contact between
some of the gas and the chilled environment of the trap. In addition,
if particulate matter such as dust or dirt is present in the pyrolyzer
effluent, it may pass through a cold trap, decreasing detection
sensitivity and necessitating frequent cleaning of optical components
of the NO detector. Moreover, the cold traps do not readily provide
an indication when their performance as trapping devices deteriorates.
Accordingly, it is an object of the invention to provide an improved
trap for selectively removing substances from a gas stream.
It is also an object of the invention to provide a system for measuring
the content of certain nitrogen-containing compounds of a sample.
It is an object of the invention to provide a trap which, in an
apparatus which determines nitrogen compound content by selective
liberation in a reactor of NO gas from a sample followed by measurement
of the NO gas liberated, an improved trap for removing from the
reactor effluent certain substances which could interfere with accurate
measurement of the NO gas liberated.
It is a particular object of the invention to provide an improved
system for detecting N-nitroso compounds and having a trap according
to the preceding objects which is also inexpensive, convenient to
use, and requires little or no maintenance.
It is also an object of the invention to provide an improved system
for detecting oxides of nitrogen in a gas stream.
SUMMARY OF THE INVENTION
The invention concerns a molecular sieve trap for the selective
trapping of substances in a gas stream, and systems for detecting
nitrogen compounds which include the molecular sieve trap as an
essential component.
The trap of the invention comprises a cartridge which contains
a packing of a granular adsorbent material such as a zeolite or
molecular sieve material having a precisely controlled pore size
and a selected particle size. A preferred packing material comprises
60 to 80 mesh size particles of Type 13X Union Carbide molecular
sieve material, a synthetic, crystalline, activated alumino-silicate
having a precisely controlled pore size of about ten angstroms.
When a gas stream containing NO and/or NO.sub.2 gas is directed
through the trap, the trap operates as a selective filter to readily
pass these gases through the packing while trapping and retaining
certain substances in the gas stream which could otherwise interfere
with subsequent measurement of NO or NO.sub.2 gas.
In accordance with one preferred embodiment of the invention, a
NO.sub.x detection system is provided incorporating a molecular
sieve trap in a sample inlet line of a chemiluminescent analyzer.
The trap acts as a filter to readily pass NO and NO.sub.2 gases
in a gaseous sample for subsequent detection while trapping substances
which could interfere with accurate operation of the analyzer by
mimicking the presence of NO and NO.sub.2 and thus indicating erroneously
high levels of these gases.
In accordance with another embodiment of the invention, a nitrogen
compound detection system is provided which incorporates a molecular
sieve trap upstream of a NO detector. A preferred detection system
includes a chromotograph for receiving a liquid or gaseous sample
and separating it into an effluent portion containing nitrogen compounds
to be detected and an effluent portion without such compounds; a
reactor such as a pyrolyzer for selectively liberating NO gas from
the nitrogen compounds to be detected; an NO detector for measuring
the amount of NO gas liberated; and a molecular sieve trap located
between the reactor and the NO detector for passing NO gas in the
reactor effluent while trapping substances including double-bonded
carbon compounds, sulfur compounds such as hydrogen sulfide, vinyl
halides, and dust and dirt particles. The removal of such substances
from the reactor effluent prevents them from erroneously contributing
to the output signal of the NO detector. For example, during operation
of a preferred system for detecting N-nitroso compounds which comprises,
in seris, a gas chromatograph, a pyrolyzer, a molecular sieve trap
and a chemiluminescent NO detector, the trapped compounds are unavailable
to participate in chemiluminescent reactions which could mimic the
behavior of NO gas. False NO detector output signals are thereby
avoided. Also, background noise level is reduced, increasing the
sensitivity of the system and thus permitting its use in the detection
of very low levels of N-nitroso compounds.
In addition to facilitating accurate measurement of nitrogen compounds
such as N-nitroso compounds in a sample, the molecular sieve trap
of the invention is considerably less expensive than cold traps
used in prior art detection systems and is easy to install and replace.
Trap maintenance is minimal and is simplified in that during operation
of a system including the trap, the output signal of the NO detector
provides a clear indication when effectiveness of the molecular
sieve trap diminishes.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of a molecular sieve trap according
to a preferred embodiment of the invention.
FIG. 2 is a sectional view of a molecular sieve trap according
to another embodiment of the invention.
FIG. 3 is a block diagram of a system for detecting nitrogen compounds
according to a preferred embodiment of the invention.
FIG. 4 is a block diagram of a system for detecting N-nitrosamines
and certain other nitrogen compounds of a sample according to a
preferred embodiment of the invention.
FIGS. 5 and 6 are chromatograms obtained in tests conducted on
samples containing several known N-nitroso compounds and which illustrate
results obtained using a detection system with a molecular sieve
trap according to the present invention (FIG. 5) and a prior art
N-nitroso compound detection system including a cold trap (FIG.
6).
FIGS. 7-9 are chromatograms obtained in tests conducted on samples
collected from an industrial plant and which illustrate results
obtained using: a prior art detection system with no trap (FIG.
7), a prior art detection system with a cold trap (FIG. 8), and
a detection system with a molecular sieve trap according to the
present invention (FIG. 9).
FIG. 10 is a block diagram of a NO.sub.x detection system according
to a preferred embodiment of the invention.
FIG. 11 is a plot of output signal versus time from a test of a
known sample in a NO.sub.x detection system of the invention.
FIG. 12 is a plot of output signal versus time from a test of a
gas sample from an automobile exhaust in a NO.sub.x detection system
of the invention .
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
FIG. 1 shows a cross-sectional view of a molecular sieve trap 20
according to the invention. The trap is uniquely suited for passing
NO and NO.sub.2 gases in a gas stream directed through the trap
while selectively trapping substances including double-bonded carbon
compounds such as ethylene, sulfur compounds such as hydrogen sulfide,
and vinyl halides such as vinyl chloride. Removal of these substances
from a gas stream facilitates accurate measurement of its NO and
NO.sub.2 contents.
The trap preferably comprises a cartridge 20 including a generally
cylindrical, open-ended shell 21 an inlet end cap 22 and an outlet
end cap 24. The shell 21 and the caps 22 and 24 are fabricated from
nylon or other suitable material and when connected together define
a cavity 28 for holding one or more packings. In one preferred arrangement
as shown in FIG. 1 the upstream portion of the cavity contains a
packing 30 while the downstream portion of the cavity adjacent to
the outlet end cap 24 is left unfilled. To prevent escape of the
packing 30 from the cavity 28 during handling or use of the trap
20 a porous retainer or plug 33 is provided between the inlet end
cap 22 and the packing 30 and a similar plug 34 is provided at
the downstream end of the packing 30.
The packing 30 within the cavity 28 comprises a dry granular adsorbent
material with a unique sieve-like selectivity such that it passes
NO and NO.sub.2 gases contained in a gas stream directed through
the trap 20 while trapping and retaining certain other substances
with larger and/or more polar molecules. A preferred material for
the packing 30 is Type 13X Union Carbide Molecular Sieve adsorbent,
available from Union Carbide Corporation's National Specialty Gas
Office, South Plainfield, N.J. The Type 13X material is a synthetic,
crystalline, activated sodium alumino-silicate having the unit cell
formula
and having uniformly sized pores of nominal diameter equal to ten
angstroms. This material, or another zeolite or molecular sieve
adsorbent having pores of about ten angstrom size, will exclude
from passage therethrough molecules having an effective diameter
greater than ten angstroms. In addition, it will trap certain other
substances based on a preference for retaining polar or polarizable
molecules.
The Type 13X material for the packing 30 may be obtained in the
form of powder, beads, or pellets which then are mechanically ground
and/or sieved as needed to obtain particles of a size suitable for
use in the trap 20. A currently preferred packing 30 for the trap
is Type 13X molecular sieve material of 60 to 80 mesh size. Particles
as small as about 600 mesh size have performed satisfactorily as
a packing 30 but are less preferred than the 60 to 80 mesh size
particles. Large particles, such as those with dimensions of one-sixteenth
inch or larger do not provide acceptable trapping selectivity, and
in fact have been found in tests to trap or retard passage of NO
gas.
In one trap found suitable for use as described in the present
invention, the portion of the cavity 28 filled by the packing 30
comprises a cylinder having a diameter of about three-fourths of
an inch and a length (between the plugs 33 and 34) of about one-half
inch.
The cavity 28 of the molecular sieve trap 20 may contain a single
packing 30 as shown in FIG. 1 in which case the unfilled portion
of the cavity between the packing 30 and the outlet end cap 24 serves
as a vacuum chamber reservoir during use of the trap in a system
for detecting nitrogen compounds such as N-nitrosamines. This reservoir
or unfilled cavity has proven effective in helping to maintain constant
vacuum levels in systems such as that shown in FIG. 4 wherein vacuum
is applied by a pump located downstream of the trap 20. This has
assisted in eliminating drift in the baseline level of output signals
of the system and in improving its detection sensitivity.
In an alternate trap configuration (FIG. 2) the packing 30 is positioned
in the downstream portion of the cavity 28 of a trap 31 and a second
packing 32 is provided between the packing 30 and the inlet end
cap 22. The packing 32 comprises a sorbent material for removing
water vapor from a gas stream prior to the passage of the gas stream
into the packing 30. Removal of the water prevents it from being
trapped in and clogging the pores of the molecular sieve material,
which would impair the effectiveness of the packing 30 as a selective
trap. The sorbent material of the packing 32 may be any material
which is capable of removing moisture from the gas stream without
trapping NO gas or reacting with substances in the gas stream. Suitable
materials are granular sodium sulfate, calcium sulfate, phosphorous
pentachloride, aluminum silicates, or combinations thereof.
To assure retention of the packing 32 within the cavity 28 of the
trap 31 porous retainers or plugs 35 and 36 similar to the plugs
33 and 34 are provided adjacent to the ends of the packing 32. One
suitable material for the plugs 33 34 35 and 36 is polyethylene
in the form of beads of 240 mesh size pressed to form a layer about
one-eighth of an inch thick. Plastic plugs are preferable to metal
screens as retainers because plastic plugs have only a minor effect
on the packings 30 and 32 during ultrasonic welding of the end caps
22 and 24 to the shell 21 to form the trap. Metal screens by contrast,
may vibrate during ultrasonic welding and abrade some of the packing
materials into dust, thus decreasing effectiveness of the trap.
Each of the end caps 22 and 24 includes a collar 40 at one end
which fits tightly within the open ends of the shell 21. To form
a sealed, vacuum-tight trap, the end caps are joined to the shell
by, for example, ultrasonically welding the annular end face 42
of each cap to one end of the shell 21. The welding locations and
the tight frictional fit between the collar 40 of each end cap and
the inner wall of the shell 21 isolates the packings 30 and 32 from
the welding sites, thus reducing the risk of contaminating these
packings during joining of the caps 22 and 24 to the shell 21. The
possibility of contamination is further minimized by appropriate
selection of materials for the caps 22 and 24 and the shell 21.
In general, these components may be made of any rigid material such
as plastic or metal, but nylon is preferred since it is readily
moldable and does not outgas during either ultrasonic welding of
the end caps and the shell or during subsequent use of the trap
20 or 31 to selectively remove substances from a gas stream.
Although two identical caps 22 and 24 are indicated in the traps
20 and 31 it may be desirable, particularly when the trap includes
two different packings, to alter the shape of one cap, or clearly
label it, to help assure proper orientation of the trap with respect
to an incoming gas stream during use--i.e., so that the gas stream
is directed through the moisture-removing packing 32 prior to contacting
the packing 30. Also, one of the end caps 22 and 24 may be formed
integrally with the shell 21 to reduce the number of parts and welding
operations required to form the trap. If an integral shell-and-cap
design is to be used with a trap configuration having two packings,
it may be desirable to eliminate the annular support 44 which projects
from the inner wall of the shell 21 in order to permit convenient
loading of the packings and the required plugs into the cavity 28
prior to welding of the shell 21 and the reamining cap.
To facilitate connection of the trap to a tube or line carrying
a gas stream whose NO or NO.sub.2 content is to be determined, the
end caps 22 and 24 include tubelike, elongated portions 50 and 52
respectively, which extend outwardly from the shell 21 of the assembled
trap 20 along a central axis 54 thereof. The elongated portions
terminate with closed ends 56 and 57 which prevent moisture and
other contaminants from contacting thg packings 30 and 32 during
storage or shipment of the trap. When it is desired to use the trap
for selective filtering of a gas stream, the closed ends of the
trap are removed as by severing. Then the resulting open-ended elongated
portions are attached by vacuum-tight fittings to a line carrying
a gas stream to be received through the inlet end cap 22 of the
trap and to another line for carrying the gaseous effluent of the
trap away through the outlet end cap 24 of the trap.
FIG. 3 shows a block diagram of a nitrogen compound detection system
60 incorporating the currently preferred molecular sieve trap 20
as an essential component. The system 60 comprises, in series arrangement,
a chromatograph 62 an NO reactor 64 a molecular sieve trap 20
and a NO detector 68. A sample to be tested for certain nitrogen
compounds such as N-nitroso compounds is directed in liquid or gaseous
form to the chromatograph 62 which serves to separate the sample
into different portions and to produce an effluent including, at
different intervals of time, a first portion containing the nitrogen
compounds to be detected and a second portion without these nitrogen
compounds. The NO reactor 64 receives the effluent of the chromatograph
62 and applies energy to the first portion thereof in an amount
sufficient to selectively release NO gas from the nitrogen compounds
to be detected. The NO reactor 64 may, for example, be a non-catalytic
pyrolyzer or may comprise a temperature-controlled reaction chamber
with a source of ultraviolet radiation therein. The gaseous products
of the NO reactor 64 are applied to the molecular sieve trap 20
which passes substantially all of the NO gas in these reactor products
to the NO detector 68 while retaining substances which might interfere
with proper operation of the NO detector 68. The effluent of the
trap 20 is then applied to the detector 68 which may be a chemiluminescent
detector, a mass spectrometer tuned to the mass of NO, an instrument
for measuring radiation absorption of NO gas, or any other suitable
NO detector. The NO detector 68 accurately measures the NO content
of the trap effluent, and in particular measures the concentration
of NO gas released from the first sample portion, and furnishes
a direct indication of the amount and identity of the nitrogen compounds
to be detected.
A preferred system for detecting nitrogen compounds, particularly
N-nitroso compounds, contained in a sample is shown in block diagram
form in FIG. 4. The system 70 comprises a gas chromatograph 72 arranged
in series with a pyrolyzer 74 a molecular sieve trap 75 and a
NO detector 76.
The gas chromatograph 72 which may be one of many known types
which provide a sample flow rate on the order of 4-200 cc/minute,
receives a sample in either gaseous or liquid form and produces
a timewise separation in the flow of certain portions of the sample
through the remaining components of the system. The sample is first
supplied to an injector 80 of the chromatograph 72 which also receives
a carrier gas such as helium or argon from a carrier gas reservoir
82. Then the carrier gas and sample are injected into a column 84
(if the sample is received as a liquid by the chromatograph 72
it is vaporized near the inlet of the column 84). The column 84
operates to separate the gaseous sample so that the effluent of
the chromatograph 72 includes, at different intervals of time,
a first portion containing N-nitroso compounds (hereinafter termed
the N-nitroso portion) and a second portion without N-nitroso compounds
(hereinafter termed the non-N-nitroso portion). Such separation
is necessary since otherwise certain nitrates, nitrites, or other
compounds in the sample might break down to produce NO gas in the
pyrolyzer 74 at the same time as NO is intentionally liberated therein
from N-nitroso compounds, which would hinder accurate detection
of the NO liberated by the N-nitroso compounds. The column 84 also
produces a timewise separation between different N-nitroso compounds
in the N-nitroso portion.
The pyrolyzer 74 which is positioned to receive the effluent of
the chromatograph 72 comprises means for selectively liberating
NO gas from the N-nitroso portion of the sample by applying an amount
of thermal energy sufficient to break the N-NO bonds of the N-nitroso
compounds without breaking other molecular bonds in any substantial
numbers. A preferred non-catalytic pyrolyzer, described in more
detail in the above-referenced U.S. Pat. No. 3996002 includes
a cylindrical tube with ceramic, chemically inert interior walls.
The pyrolyzer 74 also includes, or has associated therewith, a temperature
controller such as an electric heater which surrounds the tube and
is capable of maintaining a desired temperature such as about 300.degree.
C. to 500.degree. C. within the tube.
The gaseous effluent of the pyrolyzer 74 is applied to a molecular
sieve trap 75 such as the trap 20 containing a packing 30 (FIG.
1). Substantially all of the NO gases in the pyrolyzer effluent,
including, most importantly, the NO gas liberated from the N-nitroso
portion of the sample, pass through the trap 75 and into the NO
detector 76. NO.sub.2 gas also passes through the trap 75 but does
not interfere with detection of NO gas. However, certain compounds
having molecules which are larger and/or more polar than NO are
retained by the trap 75 so that they cannot interfere with subsequent
measurement of NO gas in the detector 76.
In the preferred N-nitroso compound detection system 70 illustrated
in FIG. 4 the detector 76 for measuring the amount of NO gas which
is produced by pyrolysis of the sample comprises a chemiluminescent
ozone reaction detector such as a Model 10-A NO--NO.sub.x analyzer
manufactured by Thermo Electron Corporation, Waltham, Massachusetts.
The detector 76 which typically is arranged to operate at a pressure
in the range of 0.5 -20 mm. Hg, includes a chemiluminescent reaction
chamber 90 for receiving and combining a stream of ozone and the
effluent of the molecular sieve trap 75. A chemiluminescent reaction
occurs in the chamber 90 between the ozone and any NO gas present.
The intensity of this reaction, as sensed and converted to an electrical
output signal by a photodetector 92 is directly indicative of the
quantity of NO gas input to the detector 76 and thus of the N-nitroso
compounds present in the sample.
Reference is made to FIGS. 5 and 6 which are chromatograms or plots
versus time of the output of the NO detector 76 obtained in tests
conducted to determine the effectiveness of the system of the present
invention in measuring the N-nitroso compound content of a sample.
Local peaks in these output signals indicate the presence of N-nitroso
compounds tested, and the areas under each peak are indicative of
the relative concentrations of these compounds. FIG. 5 shows test
results obtained when five microliters of a solution of acetone
containing 0.625 nanograms each of seven N-nitroso compounds was
input to the detection system 70 including a molecular sieve trap
with a single packing of 60 to 80 mesh Type 13X molecular sieve
material. The N-nitroso compound tested were the following: N-nitroso-dimethylamine
(NDMA), -diethylamine (NDEA), -dipropylamine (NDPA), -dibutylamine
(NDBA) -piperdine (NPIP), -pyrrolidine (NPYRR), and -morpholine
(NMOR). FIG. 6 shows results obtained when a similar sample was
input to a detection system with a cold trap substituted for the
molecular sieve trap 75.
The nearly identical plots of detector output versus time, both
clearly showing detection of the seven N-nitroso standards, demonstrate
that the molecular sieve trap 75 passes the NO gas input thereto
from the pyrolyzer 74 to permit accurate detection of N-nitroso
compounds.
FIGS. 7-9 show plots versus time of the output of a NO detector
obtained from tests of equal portions of a liquid sample input to
two prior art nitrogen compound detection systems and to the system
70 of the invention. The liquid sample was obtained by passing a
solvent through an air-sampling cartridge which had previously monitored
air in an industrial plant. FIG. 7 illustrates test results from
a prior art system differing from the system 70 in that it did not
include a trap between the pyrolyzer 74 and the NO detector 76;
FIG. 8 shows results from a prior art system including a cold trap;
and FIG. 9 shows test results from the system 70 including a molecular
sieve trap 75 with a single packing of 60 to 80 mesh Type 13X molecular
sieve material.
When no trap was used, the output of the NO detector 76 (FIG. 7)
showed numerous peaks. By contrast, when either a cold trap or the
molecular sieve trap of the present invention was included in the
nitrogen compound detection system, the output (FIGS. 8 and 9) of
the NO detector contained only a single peak. This single peak revealed
the presence of an N-nitrosamine and also indicated that the sample
in fact included but a single NO-producing compound. These plots
shows that the molecular sieve trap is effective (and so also is
the cold trap in this instance) in removing non-NO producing compounds
from the pyrolyzer effluent of a nitrogen compound detection system.
The data set forth above, and results of other tests, confirm the
utility of the molecular sieve trap for selectively trapping substances
in a gas stream, particularly in a system for detecting certain
nitrogen compounds such as N-nitroso compounds in a sample. This
trap, when packed with a suitable molecular sieve material, readily
passes NO and NO.sub.2 gas while retaining larger and/or more polar
molecules. The materials trapped include substantially all known
compounds whose presence in a NO detector could interfere with accurate
detection of NO gas such as by producing a chemiluminescent reaction
similar to that of NO gas. In addition, the molecular sieve trap
removes dust and dirt particles which may be present in a gas stream
such as the effluent of a pyrolyzer of a N-nitroso compound detection
system. This assists in keeping clean any light-transmitting elements
of the NO detector contacting the trap effluent such as the lens
of a photodetector in a chemiluminescent detector and helps to eliminate
noise in the electrical output of the detector. In tests conducted
to compare operation of the N-nitroso compound detection system
70 of the present invention with a prior art system differing only
in its inclusion of a cold trap instead of a molecular sieve trap,
the molecular sieve trap has been found to provide NO detector output
signals with higher signal-to-noise ratios. Thus a system with the
molecular sieve trap provides increased sensitivity, which permits
accurate detection of very low concentrations (a few parts per billion
) of N-nitroso compounds. Also, for detection of low levels of nitrogen
compounds, the molecular sieve trap simplifies sample preparation--for
example, its use in a N-nitroso compound detection system eliminates
the need to pre-concentrate certain samples, as by solvent extraction,
prior to their analysis.
In contrast to the cold traps used in systems for detecting N-nitroso
compounds, the molecular sieve trap requires virtually no maintenance.
Moreover, unlike cold traps, when the effectiveness of the molecular
sieve trap to selectively retain materials diminishes, this is clearly
indicated by a distinct spreading or broadening of the peaks in
the detector output signal. At the onset of evidence of band-spreading
or at predetermined intervals such as at the beginning of each 24-hour
period of use, the molecular sieve trap may be quickly and easily
uncoupled from the tube or other sample passage line extending between
the pyrolyzer and the NO detector of a nitrogen compound detection
system such as the system 70 of the invention, and a fresh sieve
trap may be substituted for the spent trap.
Another important use of the molecular sieve trap in detection
of nitrogen compounds is in combination with, or as part of, a detector
for measuring the concentration of oxides of nitrogen such as NO
and NO.sub.2 present in a gaseous sample. The trap, when positioned
in a sample inlet line of a NO.sub.x detector (NO.sub.x being defined
herein as the total amount of NO and NO.sub.2 gases in a sample),
acts as a filter to readily pass NO and NO.sub.2 while removing
compounds which could interfere with accurate measurements of the
NO.sub.x content of the sample.
FIG. 10 shows a simplified block diagram of a preferred NO.sub.x
detection system 100 incorporating a molecular sieve trap 102 in
a sample inlet line 104 of a chemiluminescent analyzer 106. The
trap 102 may contain a single packing of a sodium alumino-silicate
molecular sieve material such as Type 13x material, whose particles
have pores of nominal diameter equal to about 10 angstroms. Alternatively,
the trap may contain two packings, the additional packing being
asorbent material for removing water vapor from the gas stream as
described earlier in connection with FIG. 2 hereof. When a gas sample
containing NO and/or NO.sub.2 is directed into the trap 102 NO
and NO.sub.2 pass through the trap and to the analyzer 106 while
compounds of large molecular size or high polarity relative to NO
and NO.sub.2 --e.g. compounds such as vinyl chloride, hydrogen sulfide,
ethylene and other unsaturated hydrocarbons, and nitrosamines--are
retained in the trap 102. Thus these latter compounds are unavailable
to mimic NO or NO.sub.2 in a subsequent chemiluminescent reaction.
The chemiluminescent analyzer 106 includes a reaction chamber 108
to which ozone and a gas sample are furnished, e.g. through concentric
portions of tubes 110 and 112 respectively. A photodetector 114
is provided for monitoring the chemiluminescent reaction between
the ozone and the NO gas of the sample, and a signal processor 116
is connected to the photodetector 114 for processing its signals
to compute the NO.sub.x content of the sample.
To permit determination of both the NO and NO.sub.2 contents of
a gas sample, the chemiluminescent analyzer 106 includes a valve
118 and an NO.sub.2 converter 120 (which may be any of several commercially
available reactive or catalytic devices for converting NO.sub.2
gas to NO gas). For calculation of NO gas content, the valve 118
is positioned to channel the gaseous effluent of the trap 102 through
the conduit 122 directly to the reaction chamber 108. Since the
NO.sub.2 gas in the sample does not readily react with ozone in
the chamber 108 the output signal of the photodetector 114 for
this mode of operation is indicative of the NO content of the sample.
Alternatively, the valve 118 may be positioned so that the sample
is directed through a conduit 124 and through the NO.sub.2 converter
120 prior to its entry into the chamber 108. In this mode, any NO.sub.2
in the sample is converted to NO and the output signal of the photodetector
114 is indicative of the total NO.sub.x content of the sample. The
signal processor 116 may compute the NO.sub.2 content of the sample
by subtracting the NO content from the NO.sub.x content.
FIGS. 11 and 12 are output signals of the NO.sub.x and NO concentrations
measured in tests conducted on gas samples using a NO.sub.x detector
system similar to the system 100 shown in FIG. 10. The test system,
however, also included means for either connecting the molecular
sieve trap 102 to the inlet line 104 to filter the inlet flow of
a gas sample or disconnecting it therefrom to pass the sample unfiltered
to the analyzer 106. The molecular sieve trap employed in these
tests comprised a cylindrical cartridge containing a single packing
of 60 to 80 mesh Type 13x molecular sieve material. For each test
the system was first operated without the trap, then at the times
indicated in FIGS. 11 and 12 the trap was connected to thereafter
selectively filter the gas sample being analyzed. During both tests,
the valve 118 was switched at regular intervals to first direct
flow through the conduit 122 and then to direct flow through the
conduit 124 and the NO.sub.2 converter 120. As a result, the output
signals of FIGS. 11 and 12 exhibit an alternating succession of
peaks whose levels are indicative of the NO.sub.x content of the
gas sample and valleys whose levels are indicative of the NO content
of the sample. FIG. 11 illustrates test results obtained when a
sample containing small amounts of NO and NO.sub.2 was analyzed.
The similarity of the detector output readings before and after
the molecular sieve trap was connected to filter the sample indicates
that the trap passes NO and NO.sub.2 gases and thus does not alter
the NO.sub.x, NO, or NO.sub.2 content of the sample.
The detector output signals of FIG. 12 are results of tests on
a gas sample obtained from an automobile exhaust. In this signal
trace the spikes or sharp peaks obtained immediately after each
switch from measurement of NO to measurement of NO.sub.x are spurious
peaks and should be disregarded in reading the NO.sub.x content
of the sample. One important trend illustrated in the signal trace
is the large drop in the NO.sub.x content signal after activation
of the molecular sieve trap. This indicates that the trap removed
certain substances in the gas stream which, when unfiltered, had
erroneously contributed to the measurement of NO.sub.x by the analyzer.
For the sample tested, the difference in indicated NO.sub.x content
is about twenty percent, which demonstrates the significant improvement
in accuracy of NO.sub.x measurements obtainable when the molecular
sieve trap is employed. Also, for this particular sample, the improvement
is attributable primarily to more accurate measurement of NO.sub.2
content since the base level of NO (minimum values of the signal
trace of FIG. 12) changed only slightly upon activation of the trap
as a sample filter.
While the invention has been shown and described with respect to
preferred embodiments thereof, it is apparent that the molecular
sieve trap and the nitrogen compound detection system may be embodied
in other specific forms without departing from the spirit or essential
characteristics of the invention. The scope of the invention is
indicated by the appended claims, and all changes which come within
the range of equivalency of these claims are intended to be embraced
therein. |