Title: Method and apparatus for concentrating samples for analysis
Abstract: A method for analyzing a gas sample is described. The method comprises providing a gas sample, increasing pressure applied to the gas sample to compress the sample to a smaller volume and provide a pneumatically focused gas sample, and analyzing the pneumatically focused gas sample using any of a variety of analytical techniques. Pneumatic Focusing generally means increasing the pressure of the sample, column or cell to a pressure of from about 100 psi to about 15,000 psi, more typically from about 200 psi to about 2,000 psi. Examples including gas chromatography and absorption spectroscopy are illustrated herein. Numerous other examples could be given. The method is well suited for analyzing gaseous samples, such as ambient air samples, both continuously, and remotely, using computer control. Continuously sampling ambient air provides a method for real-time monitoring of air quality. Continuous monitoring allows for pollutant exposure and allows for the identification of emission sources. The method is also well suited for analysis of breath exhalations from respiring organisms useful in metabolic studies or disease diagnosis.
Patent Number: 6,952,945 Issued on 10/11/2005 to O'Brien
| Inventors:
|
O'Brien; Robert (Clackamas, OR)
|
| Assignee:
|
The State of Oregon Acting By and Through The State Board of Higher Education (Portland, OR)
|
| Appl. No.:
|
770942 |
| Filed:
|
January 25, 2001 |
| Current U.S. Class: |
73/23.35; 73/23.2 |
| Intern'l Class: |
G01N 030/02; G01N 030/90 |
| Field of Search: |
73/2335,232,234,864.83
210/634
342/90
356/73
435/13
436/89
702/24
|
References Cited [Referenced By]
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| Foreign Patent Documents |
| WO 99/2286/8 | May., 1999 | WO.
| |
Primary Examiner: Williams; Hezron
Assistant Examiner: Frank; Rodney
Attorney, Agent or Firm: Klarquist Sparkman, LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application claims priority from U.S. provisional application No. 60/177,923,
filed on Jan. 25, 2000, which is incorporated herein by referenced.
Claims
1. A method for analyzing a gas sample, comprising:
providing a gas sample or converting a sample to a gas sample;
increasing pressure applied to the sample to compress the sample to a smaller
volume and provide a pneumatically focused gas sample; and
analyzing the pneumatically focused gas sample by gas chromatography.
2. The method according to claim 1 where the sample is pneumatically focused
concurrently with or prior to reaching a separatory column.
3. The method according to claim 1 where increasing the pressure to pneumatically
focus the gas sample comprises increasing the pressure of the sample to a pressure
of from about 100 psi to about 15,000 psi.
4. The method according to claim 1 where increasing the pressure to pneumatically
focus the gas sample comprises increasing the pressure of the sample to a pressure
of from about 200 psi to about 2,000 psi.
5. The method according to claim 1 where increasing the pressure to pneumatically
focus the gas sample comprises increasing the pressure of the sample to a pressure
of from about 300 psi to about 700 psi.
6. The method according to claim 1 where increasing the pressure to pneumatically
focus the gas sample is accomplished using a gas selected from the group consisting
of hydrogen, helium, nitrogen, argon, carbon dioxide, air, or mixtures thereof.
7. The method according to claim 1 where increasing the pressure to pneumatically
focus the gas sample is accomplished using a focusing carrier gas containing an
internal standard.
8. The method according to claim 1 where methane in the sample is used as an
internal standard.
9. The method according to claim 1 where the gas sample is pneumatically focused
using a carrier gas or a compressor at a first pressure and further comprising
rapidly decreasing or increasing pressure between a first and second pressure.
10. The method according to claim 1 where analyzing the pneumatically focused
sample comprises cooling a head portion of the column prior to injecting the pneumatically
focused sample onto the column.
11. The method according to claim 1 where analyzing the pneumatically focused
sample comprises heating the column subsequent to injecting the pneumatically focused
sample onto the column.
12. The method according to claim 1 where analyzing the pneumatically focused
sample includes eluting a pneumatically focused sample with a first carrier gas,
and then eluting the column with a second carrier gas.
13. The method according to claim 1 and further comprising continuously analyzing
pneumatically focused samples.
14. The method according to claim 1 and further comprising averaging individual
chromatograms of pneumatically focused samples.
15. The method according to claim 14 where peak locations determined from the
average are used to integrate peak areas in individual chromatograms contributing
to the average.
16. The method according to claim 1 where analytes from the pneumatically focused
sample are determined by a detector selected from the consisting of FID, IR, FTIR,
NDIR, ECD, TCD NPD, FPD, UV/Visibie detector and combinations thereof.
17. The method according to claim 1 where the pneumatically focused sample is
parallel or serially injected onto plural parallel or serial separatory columns.
18. The method according to claim 17 where the pneumatically focused sample is
analyzed by 2-dimensional chromatography.
19. The method according to claim 17 where the pneumatically focused sample is
analyzed by comprehensive chromatography.
20. The method of claim 1 where the providing the sample, the increasing pressure
on the sample, and the analyzing the sample are automated.
21. The method according to claim 20 where the providing the sample, the increasing
pressure on the sample, and the analyzing the sample are computer controlled.
22. The method according to claim 1 where portions of the pneumatically focused
sample are fed to separate columns upstream of separate, plural detectors.
23. The method according to claim 22 where the detectors are connected in series.
24. The method according to claim 22 where the plural detector are connected
in parallel.
25. The method according to claim 1 where the pneumatically focused sample is
fed to plural separatory columns.
26. The method according to claim 25 where the separatory columns are connected
in parallel.
27. The method according to claim 1 where the gas sample is provided as a pre-stored
gaseous sample.
28. The method according to claim 1 where the gas sample includes a material
selected from the group of air toxics, VOCs, OVOCs, metabolites, anesthetics, and
combinations thereof.
29. The method according to claim 1 where the gas sample is collected at a boundary
of a site for fence-line monitoring of analytes.
30. The method according to claim 1 where providing the gaseous sample comprises
providing the sample to a column within a period of less than one minute.
31. The method according to claim 30 and providing the sample to a column within
a period of less than about 1 second.
32. The method according to claim 31 and providing the sample to a column within
a period of less than about 1 millisecond.
33. The method according to claim 1 and further comprising determining the directional
distribution of pollution sources.
34. The method according to claim 1 and further comprising using a Gaussian Plume
model to determine source location, emission rate, or both.
35. The method according to claim 1 and further comprising determining analyte
source location by triangulation.
36. The method according to claim 1 and further comprising removing materials
from the gaseous sample prior to pneumatically focusing the sample.
37. The method according to claim 36 where materials removed from the sample
are selected from the group consisting of water vapor, aerosols, ozone, NO
2,
and combinations thereof.
38. The method according to claim 36 where the materials are removed by filtering,
absorption, vortexing, and combinations thereof.
39. The method according to claim 1 further comprising condensing water vapor
in the gaseous sample by pneumatic focusing.
40. The method according to claim 39 where the condensed water vapor is removed
prior to analyzing the gaseous sample using an analytical device.
41. The method according to claim 40 where the condensed water vapor contains
water-soluble analytes, and such water-soluble analytes are collected for continuous
or discontinuous subsequent analysis.
42. The method according to claim 7 where methane is added to the focusing-carrier gas.
43. The method according to claim 1 where the pneumatically focused sample is
separated into aqueous and gaseous components which are separately analyzed.
44. The method according to claim 1 where the pneumatically focused sample
is a gas; and
is subsequently cryogenically liquefied.
45. The method according to claim 1 wherein pneumatic focusing is used to make
eddy correlation measurements to quantify fluxes.
46. The method according to claim 4 where increasing the pressure to pneumatically
focus the gas sample comprises increasing the pressure of the sample to a pressure
of from about 300 psi to about 1,500 psi.
47. The method according to claim 1 where portions of the pneumatically focused
sample are fed to separate columns upstream of a single detector.
48. The method according to claim 47 where the separate columns are connected
in parallel.
49. The method according to claim 1 where analyzing the pneumatically focused
gas sample by gas chromatography comprises analyzing the sample using a packed
capillary column.
50. The method according to claim 2 where the separatory column comprises a packed
capillary column.
51. The method according to claim 17 where at least one of the columns comprises
a packed capillary column.
52. The method according to claim 22 where at least one of the separate columns
comprises a packed capillary column.
53. The method according to claim 25 where at least one of the separatory columns
comprises a packed capillary column.
54. The method according to claim 26 where at least one of the separatory columns
comprises a packed capillary column.
55. The method according to claim 47 where at least one of the separate columns
comprises a packed capillary column.
56. A method for analyzing VOCs, comprising:
compressing a gas sample comprising VOCS, to a smaller volume in a sample collection
tube by increasing pressure applied to the sample using a carrier-pneumatic focusing
gas to provide a pneumatically focused sample;
separating VOC components of the pneumatically focused sample on a gas chromatographic
column; and
detecting the separated VOC components to provide an analysis of the VOC content
of the sample.
57. The method according to claim 56 where increasing pressure applied to the
sample comprises increasing the pressure to a pressure of about 100 psi to about
15,000 psi.
58. The method according to claim 57 where increasing pressure applied to the
sample comprises increasing the pressure to a pressure of about 200 psi to about
2,000 psi.
59. The method according to claim 56 where the gas chromatographic column comprises
a packed capillary column.
60. The method according to claim 56 where detecting the separated VOC components
comprises detecting the components using an FID detector.
61. The method according to claim 56 where the method is automated.
62. The method according to claim 61 where the method is computer controlled.
63. The method according to claim 1 further comprising controlling a flow rate
of a carrier gas through a gas chromatographic column using a valve downstream
of the column.
64. The method according to claim 1 further comprising controlling a flow rate
of a carrier gas through a gas chromatographic column using a valve downstream
of a detector.
65. The method according to claim 1 where the gas sample is an air sample.
66. The method according to claim 1 where the gas sample is a breath sample.
67. The method according to claim 1 where providing a gas sample comprises continuously
providing an air sample for pollution analysis.
68. The method according to claim 1 where providing a gas sample comprises continuously
providing a breath sample for analysis.
69. The method according to claim 1 where the gas sample is an exhalation from
a respiratory organism.
70. The method according to claim 1 where the sample is a water sample.
Description
FIELD
This application concerns an apparatus for concentration and analysis of samples,
particularly gas samples, and methods for monitoring/analyzing samples using the
apparatus. Certain disclosed embodiments of the method and apparatus are particularly
useful for concentrating and analyzing ambient air samples for continuous, real-time
air pollution analysis to measure human exposure to pollutants or to locate and
quantify emissions sources.
BACKGROUND
Volatile organic compounds (VOCs) as described by the United States Environmental
Protection Agency (EPA) include components of fuels, solvents, and chemical feedstocks
commonly used for internal combustion engine fuel, power and heat generation, cleaning,
chemical, pharmaceutical, agricultural, semiconductor and other industries. VOCs
are highly regulated in the U.S. and elsewhere in the world because they contribute
to photochemical smog formation. A subset of VOC compounds includes those compounds
designated by the EPA as toxic chemicals, including those compounds designated
as Air Toxics. "Air Toxics" are harmful to breathe. As such they are regulated
by the EPA in ambient and indoor air, and by OSHA in the workplace.
Atmospheric VOCs and/or Air Toxics are currently measured under USEPA
guidance at regular times and places as part of the Photochemical Assessment Monitoring
Stations (PAMS). These VOCs may be measured according to EPA Method TO-14A using
samples collected in special canisters. Another method for measuring Air Toxics
or VOCs uses active sampling into sorbent tubes using EPA Method TO-17. In either
case the canisters or sorbent tubes are then transported to a gas chromatography
laboratory for analysis using (for instance) thermal desorption of the adsorbent
cartridges, or flushing or pumping from the canisters. This is followed by cryogenic
or some other type of cooling. Detailed instructions on these procedures are freely
available from the USEPA, which publishes the TO-xx methods. Gas chromatography
methods for air analysis are recently summarized in an extensive review article
written by Detlev Helmig, entitled "Air Analysis by Gas Chromatography," Journal
of Chromatography A, 843:129-146 (1999).
Harmful or toxic chemicals based upon organic chemicals typically have a
carbon skeleton and usually are derived from petroleum. The simplest members of
this wide range of compounds are hydrocarbons (HC's), compounds containing only
the elements carbon and hydrogen. Hydrocarbons consist of alkanes (all single bonds),
alkenes (at least one carbon/carbon double bond), alkynes (at least one carbon/carbon
triple bond), and aromatics, which contain conjugated carbon/carbon double bonds,
and are derivatives of benzene, C
6H
6. These bonding functionalities
may exist in combination with one another, making an individual hydrocarbon belong
to more than one class. There is no strict upper limit to the molecular weight,
molecular size, or carbon number of such compounds. As the carbon number increases,
the compounds have decreasing vapor pressure and, if present in the atmosphere
at all, are increasingly present in suspended particulate matter rather than as
gases. Table I provides exemplary members of each HC family.
| TABLE I |
| Examples of Hydrocarbons Classified as VOCs |
| Alkanes |
Alkenes |
Alkynes |
Aromatics |
| 1. |
Methane (CH4) |
1. |
Ethene (C2H4) |
1. |
Ethyne (C2H2) |
1. |
Benzene |
| |
|
|
|
|
|
|
(C6H6) |
| 2. |
Ethane (C2H6) |
2. |
Propene |
2. |
Propyne (C3H4) |
2. |
Methylbenzene |
| |
|
|
(C3H6) |
|
|
|
(C7H8), i.e., |
| |
|
|
|
|
|
|
toluene |
| 3. |
Propane (C3H8) |
3. |
Butene (C4H8), |
3. |
Butyne (C4H6), |
3. |
Ethylbenzene |
| |
|
|
which exists in |
|
which exists in |
|
(C8H10) |
| |
|
|
isometric forms |
|
isomeric forms |
| 4. |
Butane |
4. |
Butadiene |
|
|
4. |
Dimethylbenzene |
| |
(C4H10), which |
|
(C4H6) |
|
|
|
(C8H10), |
| |
exists in |
|
|
|
|
|
i.e., xylene, |
| |
isomeric forms |
|
|
|
|
|
which exists in |
| |
|
|
|
|
|
|
isomeric forms |
| |
|
|
|
|
|
5. |
Naphthalene |
| |
|
|
|
|
|
|
(C10H8) |
Other VOC compounds include carbon, hydrogen, and at least one other element,
especially including (but not limited to) the elements oxygen, sulfur, nitrogen,
phosphorus, and the halogens, such as fluorine, chlorine, bromine and iodine. Such
compounds are used in the chemical, electronics, agricultural, and many other industries
as solvents, pesticides, drugs, and so forth. Compounds containing the elements
C, H, and O are sometimes called oxygenated volatile organic compounds, OVOCs.
Table II provides a few exemplary members of this extended VOC family.
| TABLE II |
| Examples of VOCs other than Pure Hydrocarbons |
| Oxygen- |
|
|
| Containing |
Sulfur- |
| OVOCs |
Containing |
Halogen-Containing |
| 1. |
Aldehydes |
1. |
Sulfides |
1. |
Chlorocarbons |
| |
|
|
|
|
(e.g., CHCL3, CH2Cl2, |
| |
|
|
|
|
CH3Cl, CH3CCl3, C2Cl4 |
| 2. |
Ketones |
2. |
Sulfates |
2. |
Halons (e.g., CH3Br, CH3I) |
| 3. |
Acids |
3. |
Mercaptans |
3. |
Chlorofluorocarbons |
| |
|
|
|
|
(e.g., CCl2F2, |
| |
|
|
|
|
CClF3, CCl4, CF4) |
| 4. |
Ethers |
4. |
Thiols |
| 5. |
Alcohols |
A partial listing of chemical compounds found in the atmosphere is Chemical Compounds
in the Atmosphere, (1978) Academic Press, T. E. Graedel. This book lists many hundreds
of such compounds known when it was published more than 20 years ago.
A major source of atmospheric hydrocarbons is automobile gasoline, which typically
contains hydrocarbons having carbon numbers greater than 3. Methane, natural gas,
is widespread and relatively constant in the atmosphere at concentrations of about
1.8 ppm by volume. Natural gas is about 95% methane and 5% ethane. Propane makes
up the bulk of liquefied petroleum gas (LPG). Gasoline and diesel fuel and their
resulting combustion byproducts together contain more than 200 individual hydrocarbons.
See Fraser et al., "Air Quality Model Data Evaluation for Organics. 4. C2-C36 Non-aromatic
Hydrocarbons," Environ. Sci. Technol., 31:2356-2367 (1997). Since these compounds,
along with oxides of nitrogen also produced in combustion, react chemically in
the atmosphere to produce smog, there is worldwide interest in controlling their
atmospheric emission, and in measuring their individual (speciated) concentrations.
Air Toxics are compounds directly harmful to human health, and the EPA has many
regulations dealing with their emission and atmospheric concentration. Efficient
measuring of ambient concentrations is highly important. All ambient gaseous compounds
also appear in human breath since they are inhaled. In addition, metabolic processes
add additional volatile compounds to exhaled breath, such as ethanol, acetone,
isoprene, pentane and others. Study of metabolic processes of respiratory organisms
and diagnosis of disease would benefit greatly from automated VOC analysis in exhaled
air. Chromatographic analysis of anesthesia environments such as hospitals has
been reviewed by A Uyanik in Journal of Chromatography B 693 (1997) 1-9. From this
review it is clear that a sensitive, inexpensive, compact gas chromatograph would
be a useful tool for operating rooms and associated environments.
Sick Building Syndrome involves poorly characterized human diseases and ailments
associated with outgassing of toxic materials in the indoor environment. Sources
of such toxic materials can include carpets, drapes, particle board, etc. Harmful
fungi and bacteria which can thrive in moist or poorly ventilated environments
often emit characteristic VOC or OVOC compounds (e.g. heptanol) which, although
they may not be toxic themselves, can serve as indicators of the presence and abundance
of such harmful organisms.
Chemical synthesis or process streams, clean rooms and other industrial
areas require automated, sensitive gas analysis procedures which may be routinely
implemented for reasonable costs. Other areas which would benefit from highly sensitive
analytical air analysis methods would be those areas dealing with naturally occurring
and artificially applied pheromones for insect attraction and/or control.
In sampling trace level VOCs, air toxics, metabolites or other analytes in the
atmosphere, in breath, or other gaseous environments, the concentration of target
analytes often is below the detection limit of a particular analytical technique.
Such analysis is often termed trace gas analysis. A wide range of concentrations
may be present, for instance from 1 ppmV (1 part per million by volume) down to
1 pptV—a range of one million. For instance, in gas chromatography, a flame
ionization detector cannot detect many VOCs of ambient atmospheres or in breath
samples unless they are concentrated. Two concentration methods are commonly employed:
(a) cryogenic focusing/concentration and (b) adsorbent focusing/concentration.
In each method an air sample of the desired volume is passed through an accumulation
chamber, which consists of:
- (a) a 'U-tube' immersed in a cryogenic liquid, such as liquid oxygen
or air, or which is otherwise cooled sufficiently that some or all of the target
analytes condense to liquids or solids within the U-tube trap, also referred to
herein as a cryotrap. Most of the air sample does not condense and therefore passes
through the trap; or
- (b) a sorbent-filled trap, which absorbs or adsorbs some or all of the
target analytes, allowing most of the sample to pass through. Such traps can operate
at ambient temperature or below.
Either procedure concentrates the desired analytes to a concentration much
higher than their original concentration in the air sample. After the desired air
volume has passed through the trap, yielding sufficient analyte, the trap is heated
to transfer the concentrated analytes into a chromatographic column or other analytical device.
Both of these procedures are commonly used in the field of atmospheric analysis,
air pollution, etc. However, each has drawbacks, which makes them less amenable
to automating an air-monitoring instrument, especially for field use. In the case
of cryogenic focusing, the cryogenic liquid must be stored on site and pumped as
needed for cryogenic focusing. Although electrically cooled devices are available,
such devices typically cannot obtain sufficiently low temperatures to collect all
of the VOCs that can be condensed by cryogenic focusing. Another problem with cryofocusing
is the large amount of atmospheric materials, particularly water and carbon dioxide,
which are trapped along with desired analytes, unless separately removed before
the cryotrap. Yet another problem with cryofocusing is that such instruments typically
reside in laboratories to which samples must be transported in special containers.
Although such transport has been extensively studied, there remains the possibility
of sample modification so that spurious compounds may either be added to or subtracted
from transported and/or stored samples. For sorbent-filled traps, the sorbent material
must adsorb and desorb a wide range of potential analytes because the target analyte
volatilities vary greatly. A strongly absorbent material may collect all analytes,
but temperatures high enough to cause desorption of the least volatile analytes
may cause decomposition of analytes or the absorbent collecting material itself.
A less absorbent material may sorb and desorb the heavier analytes, but not collect
the more volatile analytes, which therefore are not completely collected. Another
problem with sorption is the tendency for the material to desorb over a period
of time when heated. This can require refocusing with cryogens or other methods
during analysis. Sorbent and cryofocusing can be used in combination. A final problem
with adsorbents is possible chemical reaction or decomposition of the target analytes
during collection, transport or storage of the adsorbent cartridges, or the presence
of artifacts acquired on the adsorbents before or after sampling. Such artifacts
are not uncommon in atmospheric sampling and often lead to spurious conclusions
about atmospheric trace-gas composition. Ambient air sampling and breath analysis
would benefit greatly from in-situ, continuous, real time analytical instrumentation.
Such instrumentation is not widely available nor currently practical.
Gas chromatography methods for air analysis are recently summarized in an extensive
review article written by Detlev Helmig, entitled "Air Analysis by Gas Chromatography,"
Journal of Chromatography A, 843:129-146 (1999), which is incorporated herein by
reference. Helmig's review substantiates the conclusion that only two primary methods
are known for concentrating analytes in an ambient air sample, cryofocusing and
absorbent traps. These methods are poorly amenable to developing remotely operated,
continuous sampling methods for ambient air although such methods have been reported.
For instance J P Greenberg, B Lee, D Helmig and P R Zimmerman have described a
"Fully automated gas chromatograph-flame ionization detector system for the in
situ determination of atmospheric non-methane hydrocarbons at low parts per trillion
concentration" in Journal of Chromatography A 676 (1994) pp. 389-98. This system
was designed to (1) rapidly trap air samples of up to 4 liters volume to allow
for sub-parts per trillion detection limits, (2) eliminate interferences from ambient
ozone, water vapor and carbon dioxide, and (3) reduce to negligible levels any
contamination in the analytical systems, and (4) allow for continuous unattended
operation. This instrument used cryogenic sample freeze-out and was successfully
employed for measurements in the state of Hawaii. However, it apparently has seen
limited additional use since that time, probably because of its cost, complexity
and use of cryogenic fluids.
Other pertinent areas include breath analysis. For instance, U.S. Pat. No.
5,293,875, "In-vivo Measurement of End-tidal Carbon Monoxide Concentration Apparatus
and Methods" describes a noninvasive device and methods for measuring the end-tidal
carbon monoxide concentration in a patient's breath, particularly newborn and premature
infants. The patient's breath is monitored. An average carbon monoxide concentration
is determined based on an average of discrete samples in a given time period. An
easy to use microcontroller-based device containing a carbon dioxide detector,
a carbon monoxide detector and a pump for use in a hospital, home, physician's
office or clinic by persons not requiring high skill and training is described.
KD Oliver and 7 co-authors of Mantech Environmental, the USEPA, XonTech and Varian
Chromatographic Systems have described a "Technique for Monitoring Toxic VOCs in
Air: Sorbent Preconcentration, Closed-Cycle Cooler Cryofocusing and GC/MS analysis"
in Environmental Science and Technology 30 (1996) 1939-1945. This powerful but
very complex, automated system usually is attended by various operators and has
seen only intermittent field use, perhaps due to operational expense and complexity.
Air pollution is increasingly regulated throughout the world. Knowing the source
of pollution emissions is essential to this regulatory process so that regulation
can be efficient and cost effective. One method for determining air pollution sources
is source characterization. That is, individual sources are surveyed either by
direct measurements of emissions or by apportionation by generic emission factors.
Usually local, regional, or national pollution control agencies maintain emission
inventories and issue emission permits. Such emission inventories are widely viewed
as unreliable. Once emission factors for a variety of pollutant species, including
VOCs, are available, individual measurements of atmospheric VOCs at any site can
be assigned quantitatively to the major sources by mathematical processes referred
as Source Apportionment or Chemical Element Balances. Efficient, cost-effective
measurements of ambient VOCs, Air Toxics, and other pollutant concentrations will
allow this source apportionment procedure to be carried out more efficiently. Beyond
source apportionment, recently developed computer programs (program UNMIX developed
by Dr. Ronald Henry of the University of Southern California) now allow sources
to be determined from ambient VOC measurements without any direct source information.
(See ScienceNewsOnline Jun. 28, 1997 and the USC Chronicle Sep. 1, 1997, included
herein) As Dr. Henry describes it, these programs allow the ambient air data to
analyze itself. This extremely powerful new mathematical technique would benefit
greatly from low-cost, and therefore frequent measurements of VOCs and other such
compounds in polluted air.
In addition to the organic compounds discussed above, there is a need for the
determination of various inorganic atmospheric constituents. A few examples are
NO, NO
2, SO
2, H
2S, O
3, CO, etc. Many
of these have specific instrumental methods and measurement devices devoted specifically
to their determination, for instance in automobile testing as well as in ambient
air. A more general method involves measurement of one or more of such species
(including VOC and OVOC compounds discussed above) by light absorption. This may
occur typically in the ultraviolet, visible, or infrared. When species are present
at very low concentrations, often long path lengths are used. This may involve
meters or kilometers through the open atmosphere, or reflected paths in a localized
instrument. Examples of such techniques are differential optical absorption spectroscopy
(DOAS) and Fourier transform infrared spectroscopy (FTIR). Such instruments may
determine one or many atmospheric components simultaneously using light at various
suitable wavelengths.
Despite these previously developed techniques and inventions, there still
is a need for an apparatus and method for continuous, and remote if desired, concentration
and analysis of gaseous samples. Such a method and apparatus, if available, would
allow automation of methods for analyzing analytes in a gaseous sample, such as
air-pollution analysis, clinical breath analysis, metabolic studies, process streams,
clean rooms, etc.
SUMMARY
The disclosed embodiments address the problems and shortcomings associated with
the prior methods and apparatuses described in the Background section, and provide
many advantages relative to prior methods and apparatuses directed to potentially
continuous spectrometric or GC analysis of gaseous samples. For example, Pneumatic
Focusing as described herein operates very rapidly as pressurization and transit
of the sample through a chromatographic column are inherently very fast due to
the high pressure driving the analysis. The speed of the analysis can be adjusted
by adjusting a Pneumatic Focusing valve, which controls the column flow rate. All
features can be controlled by a computer to optimize the most important parameters.
Hence, the present technology allows for the development of portable, compact,
fast, multi-detector, multi-column instruments that can be used, if desired, for
continuously obtaining and analyzing a pneumatically focused gas sample.
The method does not require cryofocusing, or sorbent-trap focusing, as with prior
methods, although it should be appreciated that the present invention can be practiced
in combination with cryofocusing and/or sorbent-trap focusing of analytes in laboratory
or field use. For example, cryofocusing a sample after it has been pneumatically
focused might provide better resolution than is achieved by practicing either method
separately, particularly for the more volatile analytes being analyzed. In a chromatographic
system, a sample is separated into components which then must be delivered to a
suitable analytical device (such as a FID, an ECD, etc) for detection and quantification.
Pneumatic Focusing is advantageous for concentrating such samples before injection
into the chromatographic column. Pneumatic Focusing is equally applicable for direct
introduction of a sample into an analytical device, such as a UV-VIS or IR absorption
cell, in which case a chromatographic column need not be employed. One chief advantage
and application of Pneumatic Focusing is it's applicability to trace gas measurements.
Atmospheric trace gases range in concentration from methane (1.8 ppm in the global
troposphere) down in concentration to a host of species at the ppt (0.000001 ppm)
level in clean air. A similar concentration range is undoubtedly present in exhaled
breath. Many such breath components are present in inhaled air, but a variety of
exhaled metabolites are of real interest because of diagnostic information they
could provide. Important metabolites and disease markers may be present at very
low concentrations and may be difficult to distinguish from compounds already present
in inhaled air.
Pneumatically focused chromatography represents a superior approach
to prior measurements, such as those described above concerning EPA measurements.
This is because Pneumatic Focusing is more easily automated for laboratory analysis
of such VOCs or Air Toxics from canisters or sorbent tubes, or most especially,
to real-time, continuous, in-the-field sampling of these gases wherein the problems
and artifacts associated with sample collection, transport and storage are mitigated
or eliminated altogether. The advantage of Pneumatic Focusing is that it is simpler,
more easily automated, less prone to artifacts, more easily calibrated and can
provide more extensive measurements of atmospheric VOCs, Air Toxics, breath components,
etc. at less cost than with present methods.
One embodiment of the present invention concerns a method for analyzing a gas
sample. The method comprises providing a gas sample, increasing pressure applied
to the gas sample to compress the sample to a smaller volume and provide a pneumatically
focused gas sample, and thereafter analyzing the pneumatically focused gas sample,
such as by using a gas chromatograph or spectrometric cell. Typically, the gas
sample is pneumatically focused prior to or concurrently with reaching a separatory
column or spectrometric cell. The method is well suited for analyzing ambient air
samples, both continuously, and can be, but does not necessarily have to be, run
remotely using computer control and telemetric data transfer. Continuously sampling
ambient air provides a method for real-time monitoring of indoor or outdoor air
quality or for in-situ clinical analysis of breath samples from subjects or patients.
As used herein, Pneumatic Focusing generally means increasing the pressure of
a gaseous sample from a starting pressure (e.g. atmospheric pressure) to a pressure
of from about 100 psi to about 15,000 psi, more typically from about 200 psi to
about 2,000 psi, with working embodiments having been practiced primarily at Pneumatic
Focusing pressures of from about 250 psi to about 500 psi in the case of gas chromatography
and from 150 psi to 2,000 psi in the case of absorption spectroscopy. Pneumatic
Focusing can be carried out with a sample originating as a gas, in which case the
sample may be focused (pressurized) in a sample cell or concurrently as it is introduced
to a chromatographic column or spectrometric cell. Pneumatic Focusing may also
be carried out with a liquid sample vaporized at an effective vaporization temperature
upon introduction into a gas chromatograph or heated spectrometric cell. In either
case high pressure in the sampling or analytical environment will serve to focus
(concentrate) the sample for better detectability of the target analytes. One goal
of Pneumatic Focusing is to allow introduction of large quantities of analytes
into analytical devices. Another goal is the removal of undesired condensable vapors,
such as water vapor. When used with gas chromatography we call this procedure Pneumatic
Focusing Gas Chromatography (PFGC). The method also can comprise reducing the pressure
of the carrier gas, such as to pressures below about 100 psi, simultaneously with
or subsequent to the pneumatically focused sample being injected onto a separatory
column so that the chromatography occurs at more normally employed pressures. In
the case of spectroscopy, Pneumatic Focusing can mean continuously or discretely
increasing the pressure of a gaseous sample either in, or before entrance into,
a spectrometric cell so that absorbances are adjusted to an optimum level for enhanced
signal-to-noise ratio and improved sensitivity. Pneumatic Focusing also can comprise
suddenly increasing or decreasing the pressure between a higher and a lower pressure
for observation of transient absorptions that are not observable at constant high
or low pressure. In one working embodiment in a uv/visible light absorption cell,
pressure was abruptly increased from ambient (˜15 psi) to pressures ranging
from 150 to 2000 psi. Pressure was also abruptly dropped within the same range
of pressures. Transient absorbances occurring during these pressure transients
can be useful in measuring concentrations of trace absorbers or in studying nucleation
processes or in measuring concentrations of nucleating aerosols. The apparatus
where Pneumatic Focusing (or defocusing) is carried out can be either heated or
cooled from ambient temperatures to prevent or enhance such aerosol nucleation,
or to enhance or retard adsorption or absorption to the surfaces of the apparatus.
The region of 16 device in which Pneumatic Focusing is carried out may include
but is not limited to chromatographic columns, sample loops for chromatographic
columns, spectrometric light absorption cells, electromagnetic waveguides, such
as optical or infrared waveguides, etc.
Condensable vapors (such as water vapor which may interfere with an analysis)
may be removed in a prefocusing prechamber if desired before the sample is introduced
to the light absorption chamber or chromatographic column. Such vapors may be either
discarded or analyzed separately by automated transfer to additional analytical devices.
Spectrometric measurements are normally interpreted in terms of the
Beer-Lambert Law I=I
oe-
acl or alternately I=I
o10-
a'cl
where a is the absorption coefficient, c is the absorber's concentration
and l is the path length. Using this law, previously measured and recorded absorption
coefficients, a measured path length, and an experimentally measured absorbance
Io/I, it is common practice to determine the concentration c of an analyte. Thus
absorption responds to the product of concentration and path length.
In carrying out Spectrometric Pneumatic Focusing (SPF) it is possible to control
with a combination of temperature and pressure the disposition of various condensable
or adsorbable vapors in a confined sample or in a continuous sample stream. When
pressure of a gaseous mixture is increased, as in Pneumatic Focusing, the absorptivity
of target analytes may change in ways not obvious from a consideration of Beers
Law. For instance, in preliminary investigations of Pneumatic Focusing Spectroscopy
we have observed all of the following when pressure was gradually increased or decreased:
1. The absorbance increases linearly with pressure due to increasing concentration
as expected from Beer's Law.
2. The absorbance increases proportional to the square root of the pressure ratio
due to dimerization of the target analyte to produce a nonabsorbing dimer.
3. The absorbance increases proportional to the square of the pressure ratio
due
to absorption of dimers or collision complexes.
4. The absorbance increases and remains constant due to condensation of the target
analyte to a liquid which is removed from the view of the absorbed light beam.
5. The absorbance may increase, decrease, or otherwise behave erratically due
to phenomena not currently understood.
6. Continuous oscillations in cell transmission which were wavelength dependent.
When the Pneumatic Focusing pressure was increased or decreased suddenly additional
phenomena have been observed, some of which may be due to nucleation and/or growth
of light scattering aerosols.
Heating and/or cooling a separatory column subsequent to injecting fluetuations
the pneumatically focused sample also could be advantageous. For example, the method
may involve cooling a head portion of the column prior to injecting the pneumatically
focused sample onto the column and heating the column subsequent to injecting the
pneumatically focused sample onto the column.
In chromatography plural eluting gasses can be used to elute the pneumatically
focused sample. For example, the method may involve eluting a pneumatically focused
sample with a first carrier gas, and then eluting the column with a second carrier
gas. And, either the first or second gas (usually the second) may be a supercritical
fluid. See S Pentoney et al., "Combined Gas and Supercritical Fluid Chromatography
for the High Resolution Separation of Volatile and Nonvolatile Compounds," Journal
of Chromatographic Science, 5:93-98 (1987). It could also be advantageous after
Pneumatic Focusing to drop the column pressure to lower values and then gradually
increase it again, gradually switching from a non-supercritical to a supercritical
fluid for better elution of analytes through the column. Such approach could in
some instances obviate the need for temperature programming of the column with
resultant reduction in power requirements to facilitate portability and field operation.
The method can involve continuous sampling. This embodiment provides a considerable
amount of data. This collection of data allows individual chromatograms, collected
over time, to be averaged. This can, for example, provide a well-defined, stable
baseline so that analyte peaks are easier to discern, identify and quantify, thereby
increasing sensitivity. Once peak locations are established on a low-noise, averaged
chromatogram, these peak locations may be used to unambiguously identify the position
of individual low-intensity peaks on the individual chromatograms which formed
the average. This can improve the sensitivity and lower the limit of detection.
The method can be practiced with various detectors. A working embodiment of the
invention used a flame ionization detector. However, most commercially available
detectors can be used in combination with a system for pneumatically focusing a
gas sample as described herein. Additionally, the invention can be practiced using
plural separatory columns connected in series, or in parallel. The method can be
used in combination with other techniques currently known or hereafter developed
for focusing analytes in a gas sample. For example, Pneumatic Focusing can be done
in combination with cryofocusing, absorbent focusing, or both. Pneumatic Focusing
can also be carried out in a suitably designed spectrometric cell which also serves
as the sample loop (injection volume) for a chromatographic or other system so
that spectroscopic properties of the sample may be determined prior to separation
and analysis chromatographically or by other means. Such approach would be beneficial,
for instance, in determining the total hydrocarbon content of a VOC sample by application
(without limitation) of non-dispersive infrared analysis to the CH bond region
of the spectrum. Gasses may be passed at high pressure through a spectrometric
cell after separation and elution from a separatory column as well.
A gas chromatograph and gaseous sample analysis system also is disclosed. One
embodiment
of the system comprised: a sample loop for receiving a first volume of a gaseous
sample; a separatory column fluidly connected to and downstream of the sample loop;
an inline pressure-increasing valve downstream of the separatory column, which
increases system pressure to pneumatically focus the gaseous sample and adjust
flow rate through the system. Flow rate can be adjusted as desired to be substantially
about the same linear flow rate through the system as prior to increasing the pressure
with the inline valve, less than the linear flow rate through the system prior
to increasing the pressure with the inline valve, or greater than the linear flow
rate through the system prior to increasing the pressure with the inline valve;
and a detector downstream of the pressure increasing valve for detecting analytes.
The system can further comprise plural sample collection coils, and plural separatory
columns, connected either in parallel or in series with appropriate switching,
heart-cutting, 2-dimensional chromatographing or other manipulation of the analytes
during separation and analysis. In one working embodiment, a single air sample
distributed into two separate sample loops was simultaneously injected into two
differ
