Trap Selection, Fast GC, and Troubleshooting Strategy for Purge & Trap GC Volatiles
© 2012 Sigma-Aldrich Co. All rights reserved.
sigma-aldrich.com/analytical
Trap Selection, Fast GC, and Troubleshooting
Strategy for Purge & Trap GC Volatiles
Updated: 18-Jun-2014
Agenda
Overview
Basic P&T Operation
Adsorbents
Trap Selection
Fast GC for Volatiles
Troubleshooting Strategy
Summary / Resources
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Overview
Why this Topic?
• Purge & trap (P&T) in combination
with gas chromatography (GC) is a
widely used analytic technique for
the analyses of volatiles, particularly
in the environmental laboratory
industry
• Many analysts performing this work
may not fully understand, nor
appreciate, the underlying
physical/chemical aspects
2
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Overview
Goals
• Gain a basic understanding of what
adsorbents and traps do
• Learn how to select the proper trap
for their applications
• Learn the Principles of Fast GC for
volatiles
• Learn how to employ a sound
troubleshooting strategy for use
when things go wrong
3
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Basic P&T Operation
Steps Prior to the Start of GC Analysis
The purge parameters are typically method-specified.
• Purge: helium is bubbled through the
sample, carrying volatile analytes to
the trap
• Dry-purge (optional): dry helium (by-
passing the sample) is passed
across the trap, removing any
residual moisture from the system
• Desorb pre-heat: the trap is heated
with no helium flow, causing
analytes to be thermally desorbed
from the adsorbent(s)
4
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Basic P&T Operation
Steps During GC Analysis
The desorb parameters are typically method-specified.
• Desorb: helium flow is added to the
trap in the opposite direction as the
purge mode, resulting in analytes
being swept onto the head of the GC
column
• Bake: the combination of helium
flow, heat, and time are used to
remove any non-desorbed
compounds from the trap, venting
them away from the GC, leaving the
trap ready for the next sample
5
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Adsorbents
Commonly Used in Purge Traps
Each adsorbent has specific advantages and drawbacks.
• Carbon adsorbents (Carbopack,
Carbotrap, Carboxen, Carbosieve,
activated charcoal) trap analytes
based on analyte size/shape
• Tenax (a porous polymer) traps
analytes predominantly based on
analyte size/shape
• Silica gel traps analytes
predominantly based on analyte
polarity
6
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Adsorbents
Particles and Pores
Pore: any cavity present on a solid surface with a depth:width ratio of ~10:1.
Macropore diameter >500 Å; Mesopore diameter 20 – 500 Å; Micropore diameter <20 Å
• Pore composition determines adsorption and
desorption characteristics
• Specialty carbons can be engineered with:
– More or less of any pore type to serve a specific
purpose
– Through pores or closed pores, which
influences microporous strength
– Tapered pores (from macro- to meso- to micro-)
which increases thermodynamic efficiency
7
sigma-aldrich.com/environmental
Macropore
Mesopore
Micropore
© 2012 Sigma-Aldrich Co. All rights reserved.
Adsorbents
Carbon
Advantages
• High desorption temperature (250 ºC)
– Rapid transfer to the GC, less band
broadening and improved
chromatography, especially the gases
– Better removal of large compounds
stuck in pores during the bake step
• Hydrophobic nature: less water trapped
and pushed over to the GC
• Can be dry-purged: to remove residual
moisture from the system
• Engineered pore composition:
application specific
8
sigma-aldrich.com/environmental
Drawbacks
• The stronger carbon adsorbents
(Carboxens and Carbosieves) have a
great affinity for methanol
– Fills pores, leaving none available for
gases (blow off back of trap)
– May displace gases (get adsorbed,
get displaced, blow off back of trap)
• Adsorbed methanol will be desorbed
during the desorption step, leaving the
trap ready for the next run (however, the
gases will be biased low for that run)
• Solution is to keep total amount of
methanol in samples, standards and
blanks <50
μL (<20 μL is desirable)
© 2012 Sigma-Aldrich Co. All rights reserved.
Adsorbents
Tenax
Advantages
• High desorption temperature (250 ºC)
• The ability to trap analytes with a wide
range of sizes/shapes (as small as
pentane; larger than trichlorobenzene)
• Well-studied (breakthrough volume data
for numerous analytes)
• Referenced in many methods and
numerous literature and easily
obtainable
9
sigma-aldrich.com/environmental
Drawbacks
• Not suitable for the gases and other
small molecules
• Oxidation of the polymer framework can
produce high background levels as
residual monomers left in the adsorbent
are released
– Detected as aromatic ring compounds
– Voids can be created, leading to
channeling
Tenax is a porous polymer based on 2,6-diphenylene-oxide.
© 2012 Sigma-Aldrich Co. All rights reserved.
Adsorbents
Silica Gel
Advantages
• Great affinity for polar analytes
(ketones) that are very water-soluble
and may not trap well on other
adsorbents
• Referenced in many methods and
numerous literature
• Easily obtainable
10
sigma-aldrich.com/environmental
Drawbacks
• Lower recommended desorption
temperature (180 ºC)
• Its great affinity for moisture: more water
is trapped and subsequently pushed
over to the GC
• Cannot be dry-purged without also
removing polar analytes
Unlike carbon and Tenax, silica gel traps analytes predominantly based on analyte polarity instead of
analyte size/shape.
© 2012 Sigma-Aldrich Co. All rights reserved.
Trap Selection
Adsorbent Bed Order (regardless of adsorbents)
Traps can also be designed without a strong adsorbent bed (small compounds blow off the back of
the trap during the purge mode).
• Sample comes into contact with the
weakest adsorbent first and the
strongest adsorbent last
– Larger compounds trap on the first bed
while the smaller compounds pass
through the first bed(s) and are
trapped on later bed(s)
– If a large compound gets onto a strong
adsorbent, it may become irreversibly
retained
• Traps works similar to a sieve
system
11
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Trap Selection
Which Trap is ‘Best’?
• No absolute ‘best’ trap for all
situations
• It really depends on
– The method being followed (the list of
analytes)
– Whether it is desirable to trap small
analytes (the gases)
– The hardware being used
– Is moisture management available?
– Is it being used or by-passed?
– If being used, is it functioning
properly?
12
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Trap Selection
Handling Moisture
Note: Traps that contain a silica gel bed cannot be dry-purged.
Note: Do not use a dry-purge with a properly operating moisture management.
• Water is a problem for volatile analyses
– Chromatographically interferes with the gases
– Damages quads in GC-MS systems
• Dry-Purge
– 1-2 minute will remove water to an acceptable
level (without loss of adsorbed analytes)
– 4-5 minute dry-purge will remove virtually all water
(with some loss of adsorbed analytes)
• Moisture Management
– Tubing section which grabs moisture as it passes
– Must be maintained for proper operation
13
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Trap Selection
Most Methods (dry-purge; no moisture management)
“K” Trap
• Supelco's most popular trap
• Able to adsorb/desorb small analytes (gases) to
less volatile analytes (1,3,5-trichlorobenzene)
“I” Trap
• Has a fourth bed in addition to the three beds in
the “K” trap
• The fourth bed is Carbopack C in front
– Weaker material than any bed in a “K” trap
– For more efficient adsorption/desorption of
larger analytes
14
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Trap Selection
Most Methods (moisture management; no dry-purge)
“C” Trap or “#10” Trap
• Great traps as long as moisture is being managed
by the concentrator
• Both traps have similarly arranged beds using a
combination of different materials
– Tenax polymer bed for analytes larger in size
than pentane (BTEX and halocarbons)
– Silica gel bed for polar analytes (ketones)
– Carbon bed for small analytes (gases)
– “C” trap has activated charcoal
– “#10” trap has a stronger carbon
“K” Trap or “I” Trap
• Will also work
15
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Trap Selection
Medium-Level Soil Methods
100
μL methanol extract added to 5 mL water
“E” Trap or “D” Trap
• This amount of methanol is not a problem
• Drawbacks are lower (180 ºC) desorption
temperature (peaks not as sharp) and increased
moisture transferred to the GC
• Try “D” trap if increased moisture is an issue (no
silica gel bed)
“K” Trap or “I” Trap
• Will also work
16
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Trap Selection
BTEX-Only Methods
“J” Trap
• Designed specifically for BTEX analysis
• Will not adsorb small compounds (methanol,
gases, MTBE), which allows for a shorter
analytical run
“K” Trap or “I” Trap
• Will also work
17
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Trap Selection
BTEX+MTBE and/or GRO methods
“M” Trap or “L” Trap
• Developed for underground storage tank (UST)
and leaking underground fuel tank (LUFT) testing
• Methanol and the gases will not be trapped
– Use “M” trap for BTEX, MTBE and analytes as
small as n-pentane
– Use “L” trap for BTEX and MTBE
“K” Trap or “I” Trap
• Will also work
18
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Fast GC for Volatiles
The Principles of Fast GC
• Decrease analysis time by using:
1. Shorter column
2. Quicker oven temperature ramp rate
3. Higher carrier gas linear velocity
But these changes also decrease resolution!
• Offset the decrease in resolution by also using:
4. Narrow I.D. column
5. Hydrogen carrier gas
6. Low film thickness
19
sigma-aldrich.com/environmental
Fast GC is manipulating a number of column and instrument parameters to provide faster analysis
times while maintaining resolution.
© 2012 Sigma-Aldrich Co. All rights reserved.
Fast GC for Volatiles
Conventional GC (30 m x 0.25 mm I.D., 1.4 µm), 30:1 Split
20
sigma-aldrich.com/environmental
17 minute run; poor responses for the gases.
17 min.
© 2012 Sigma-Aldrich Co. All rights reserved.
Fast GC for Volatiles
Fast GC (20 m x 0.18 mm I.D., 1.0 µm), 30:1 Split
21
sigma-aldrich.com/environmental
9.5 minute run; still poor responses for the gases.
9.5
min.
© 2012 Sigma-Aldrich Co. All rights reserved.
Fast GC for Volatiles
Fast GC (20 m x 0.18 mm I.D., 1.0 µm), 100:1 Split
22
sigma-aldrich.com/environmental
9 minute run; much improved responses and peak shapes for the gases.
9 min.
© 2012 Sigma-Aldrich Co. All rights reserved.
Fast GC for Volatiles
Modified Column Parameters and Instrument Conditions
23
sigma-aldrich.com/environmental
Conventional GC
Fast GC (30:1 split)
Fast GC (100:1 split)
desorption
process
40 mL/min at 210 ºC
for 2 min
40 mL/min at 210 ºC
for 2 min
150 mL/min at 210 ºC
for 2 min
column
SPB-624, 30 m x 0.25
mm I.D., 1.4 µm
(24255)
SPB-624, 20 m x 0.18
mm I.D., 1.0 µm
(28662-U)
SPB-624, 20 m x 0.18
mm I.D., 1.0 µm
(28662-U)
oven
40 ºC (2 min), 7 ºC/min
to 135 ºC, 30 ºC/min to
230 ºC (3 min)
40 ºC (1 min), 11
ºC/min to 125 ºC, 35
ºC/min to 230 ºC (2
min)
40 ºC (1 min), 11
ºC/min to 125 ºC, 35
ºC/min to 230 ºC (2
min)
carrier gas
helium, 1.1 mL/min
helium, 1.2 mL/min
helium, 1.5 mL/min
injection
30:1 split
30:1 split
100:1 split
Common mistake is not increasing the split when switching to a narrow I.D. column.
Sensitivity maintained because very sharp/narrow peaks are produced (greater signal-to-noise).
© 2012 Sigma-Aldrich Co. All rights reserved.
Fast GC for Volatiles
Identical Column Parameters and Instrument Conditions
• sample/matrix: each analyte at 50 ppb in 5 mL water
• purge trap: VOCARB 3000 “K” (24940-U)
• purge: 40 mL/min at 25 ºC for 11 min
• dry purge: 2 min
• desorption pre-heat: 205 ºC
• bake.: 260 ºC for 10 min
• transfer line/valve temp.: 110 ºC
• inj. temp.: 150 ºC
• detector: MS (interface at 200 ºC), 35-400 m/z
• liner: 0.75 mm I.D., direct (SPME) type, straight design
24
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Fast GC for Volatiles
Peak IDs
25
sigma-aldrich.com/environmental
• 1. Chloromethane
• 2. Vinyl Chloride
• 3. Bromomethane
• 4. Chloroethane
• 5. Trichlorofluoromethane
• 6. 1,1-Dichloroethene
• 7. Methylene chloride
• 8. trans-1,2-Dichloroethene
• 9. 1,1-Dichloroethane
• 10. Chloroform
• 11. Dibromofluoromethane
•
(surr.)
• 12. 1,1,1-Trichloroethane
• 13. Carbon tetrachloride
• 14. 1,2-Dichloroethane-d4
•
(surr.)
• 15. Benzene
• 16. 1,2-Dichloroethane
• 17. Fluorobenzene (I.S.)
• 18. Trichloroethene
• 19. 1,2-Dichloropropane
• 20. Bromodichloromethane
• 21. 2-Chloroethyl vinyl ether
• 22. cis-1,3-Dichloropropene
• 23. Toluene-d8 (surr.)
• 24. Toluene
• 25. trans-1,3-Dichloropropene
• 26. 1,1,2-Trichloroethane
• 27. Tetrachloroethene
• 28. Dibromochloromethane
• 29. Chlorobenzene-d5 (I.S.)
• 30. Chlorobenzene
• 31. Ethylbenzene
• 32. Bromoform
• 33. 4-Bromofluorobenzene
•
(surr.)
• 34. 1,1,2,2-Tetrachloroethane
• 35. 1,3-Dichlorobenzene
• 36. 1,4-Dichlorobenzene-d4
•
(I.S.)
• 37. 1,4-Dichlorobenzene
• 38. 1,2-Dichlorobenzene
© 2012 Sigma-Aldrich Co. All rights reserved.
Fast GC for Volatiles
Taking Advantage of a <10 Minute GC Run
Example equipment that allows P&T to catch up with improvements in GC speed.
• Ballistic Dry-Purge and Bake
– EST: Encon Evolution
– Tekmar: Stratum PTC
– Tekmar: Velocity XPT
• Two Concentrators per GC
– EST: Centurion WS
– EST: PT2
– OI: PT Express
26
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Troubleshooting Strategy
Best Practices
Documentation
• Use maintenance log books
• May save weeks of troubleshooting
Make one change at a time
• To uncover root cause
• Multiple changes may offset each other
Keep a ‘good’ trap
• Remove and store the trap as a reference for when issues occur at a later
time
• Replace the caps, place in original shipping container, label properly, and
protect from vibration
27
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Troubleshooting Strategy
Overview / Flow
Supelco Technical Service strategy successfully used to help many customers over many years.
28
sigma-aldrich.com/environmental
Install ‘good’ trap
(working when removed)
and evaluate
Working?
Incorrect
Parameter
Moisture
Issue
Trap
Issue
System
Issue
Re-install trap used
before troubleshooting
and evaluate
Working?
Yes
No
Yes
Was any
operating
parameter
changed?
No
Yes
No
© 2012 Sigma-Aldrich Co. All rights reserved.
Troubleshooting Strategy
Potential Cause: Moisture Issue
Perform preventative maintenance on moisture management
or find/eliminate active site and/or cold spot.
• As samples are purged, moisture is carried into the
concentrator
• If active sites or cold spots develop, moisture will
accumulate, causing
– Low/no response of the more water-soluble analytes
– Decreased I.S. response throughout the tune window
• Opening the system allowed the moisture to escape
– When the non-working trap was removed
– Also why the non-working trap now seems to work
29
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Troubleshooting Strategy
Potential Cause: Trap Issue
Note: This procedure is only for use with a Supelco carbon trap (H, I, J, K, L, or M). These traps can easily
handle this temperature. The reason that Supelco does not list this on the data sheet is that many customers
use PTFE ferrules (maximum temperature is 300 ºC) and we do not want to be responsible for melted PTFE
stuck in instruments. Customers must verify that their instrument can handle this temperature.
Try an elevated bake-out
• Make sure that a PTFE ferrule is NOT installed on the
trap (use a VESPEL or graphite ferrule able to withstand
400 ºC or higher)
• Bake trap with 40 mL/min helium at 375 ºC for 4 hours
• Cool the trap down and discard the ferrule
• Replace with a new ferrule (whatever normally used)
• Evaluate performance (if it works, great; if not, it is a ‘bad’
trap)
30
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Troubleshooting Strategy
Potential Cause: Incorrect Parameter
• Autosampler, concentrator, GC inlet, column dimension,
and detector combinations are quite large
• Impossible to write up what each individual parameter
should be (depends on hardware being used)
• Recommend contacting a Technical Service (have
complete list of all operating parameters handy)
– Concentrator manufacturers (CDS, EST, O.I., Tekmar)
– Trap manufacturers (CDS, EST, O.I., Supelco, Tekmar)
– System manufacturers (Agilent, PerkinElmer, Shimadzu,
Thermo, Varian)
– Third-party (MDL [www.ecsmdl.com])
31
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Troubleshooting Strategy
Potential Cause: System Issue
A drastic loss of response indicates where the issue resides.
Focus further troubleshooting in this area.
Make 50 and 200 ng/
μL standards (with I.S./surr.) in
methanol and perform the following five runs
1. Inject 0.5
μL (200 ng/μL) into the GC (if accessible)
2. Inject 2
μL (50 ng/μL) into the top of the trap; step to
desorb pre-heat
3. Inject 2
μL (50 ng/μL) into the top of the trap; dry-purge 5
minutes then step to desorb pre-heat
4. Inject 2
μL (50 ng/μL) into 5 mL water; set-up manually on
concentrator (do not use autosampler)
5. Inject 2
μL (50 ng/μL) into 5 mL water; set-up manually on
the autosampler
32
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Troubleshooting Strategy
Troubleshooting Worksheet (T596001)
• Should work for any combination of:
– Trap type
– Sample matrix
– Concentrator make/model
– Moisture reduction strategy
– GC conditions
• Fill out and use for phone, fax, or email
correspondence with any of the
Technical Services
33
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
Summary / Resources
Summary
• Select the proper trap for the application
• “K” trap will work for most applications
• Employ Fast GC to increase throughput
• Use a sound troubleshooting strategy
Web Site (sigma-aldrich.com/environmental)
• Sections for organics, metals, wet
chemistry, sample collection,
chromatograms, and newsletters
• From sample collection through sample
preparation to analysis, we have you
covered
34
sigma-aldrich.com/environmental
© 2012 Sigma-Aldrich Co. All rights reserved.
35
Supelco site
Bellefonte, Pennsylvania (USA)
© 2012 Sigma-Aldrich Co. All rights reserved.
36
sigma-aldrich.com/food
We welcome your suggestions on how we can improve the course content.
Fluka site
Buchs (Switzerland)
© 2012 Sigma-Aldrich Co. All rights reserved.
37
Thank You!
Podobná PDF
Klíčová slova
Klíčová slova
Klíčová slova