Trap Selection, Fast GC, and Troubleshooting Strategy for Purge & Trap GC Volatiles

Prezentace | 2014 | MerckInstrumentace

Purge and Trap, Spotřební materiál

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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

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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

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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

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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)

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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

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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

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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

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Macropore

Mesopore

Micropore

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

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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)

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

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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.

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

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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.

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

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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?

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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

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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

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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

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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

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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

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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

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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

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Fast GC is manipulating a number of column and instrument parameters to provide faster analysis

times while maintaining resolution.

Fast GC for Volatiles
Conventional GC (30 m x 0.25 mm I.D., 1.4 µm), 30:1 Split

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17 minute run; poor responses for the gases.

17 min.

Fast GC for Volatiles
Fast GC (20 m x 0.18 mm I.D., 1.0 µm), 30:1 Split

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9.5 minute run; still poor responses for the gases.

9.5

min.

Fast GC for Volatiles
Fast GC (20 m x 0.18 mm I.D., 1.0 µm), 100:1 Split

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9 minute run; much improved responses and peak shapes for the gases.

9 min.

Fast GC for Volatiles
Modified Column Parameters and Instrument Conditions

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Conventional GC

Fast GC (30:1 split)

Fast GC (100:1 split)

desorption
process

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)

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)

carrier gas

helium, 1.1 mL/min

helium, 1.2 mL/min

helium, 1.5 mL/min

injection

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).

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

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Fast GC for Volatiles
Peak IDs

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• 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

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

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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

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Troubleshooting Strategy
Overview / Flow

Supelco Technical Service strategy successfully used to help many customers over many years.

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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

Was any

operating

parameter

changed?

Yes

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

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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)

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$background image$

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])

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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

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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

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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

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Supelco site

Bellefonte, Pennsylvania (USA)

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We welcome your suggestions on how we can improve the course content.

Fluka site

Buchs (Switzerland)

Thank You!

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