| The gas chromatograph and
capillary column function as a complete system and not as
two individual parts. A problem or deficiency in any part
of the system usually will result in some type of
chromatographic difficulty. The same problem can be
caused by a number of different system deficiencies. A
logical and controlled troubleshooting procedure will
quickly and accurately identify the source of the
problem. This will result in the fastest, easiest and
most complete solution to the problem. Troubleshooting
is a skill that becomes easier with practice. Someone
equipped with the right tools and a rudimentary
understanding of capillary column gas chromatography, can
identify, locate and correct problems with minimal amount
of effort.
Flowmeter
A digital or manual model with a range of 10 to 500
mL/min is suitable.
New Syringe
A working syringe that has not been used for samples
should be available. Some problems may actually be
syringe or autosampler related.
Methane or Another Nonretained Compound
A non-retained compound is used to set and verify carrier
gas flow and to check out injector operation and setup.
New Septa, Ferrules and Injector Liners
These are used to replace parts that eventually become
defective, worn out or dirty.
Leak Detector
Electronic models are recommended. Liquid leak detection
fluids are satisfactory ,but care has to be exercised to
avoid possible contamination problems.
Column Test Mixture or Reference Sample
These are used to diagnose select system and column
problems. They are useful to compare current system
performance to past performance.
Checkout Column
This is a column that is not used for samples. The
performance and quality is known so that evaluation of
the system can be made. It helps to verify or eliminate
the previous column as the source of a problem.
Instrument Manuals
These are not a last resort. The manuals are a good
source of troubleshooting information special to a
particular model of gas chromatograph. Performance
specifications are often contained in the manuals.
Most performance problems can be placed within one of
eight areas. These are baseline disturbances, irregular
peak shapes or sizes, retention time shifts, loss of
separation or resolution, quantitation difficulties,
rapid column deterioration, ghost peaks and broad solvent
fronts. It is not uncommon to have more than one of these
problems occurring at the same time. Sometimes, it is
difficult to determine the actual nature of the problem.
This makes a logical and systematic approach to problem
solving very important.
It is important to realize that the following
comments and recommendations are generalizations and
simplifications. Every possible problem or
correction cannot be covered, nor can every detail be
mentioned. For more information, call Chrom Tech.
Spiking:
- Particulate matter passing through the detector.
Solution: Clean the detector per
the instruction manual.
- Loose connections on cables or circuit boards
(usually random spiking).
Solution: Clean and repair the
electrical connections as needed.
Noise:
- Contaminated injector and/or column.
Solution: Clean the injector.
Solvent rinse the column.
- The column is inserted into the flame of an FID,
NPD or FPD.
Solution: Reinstall the column.
- Air leak when using an ECD or TCD.
Solution: Find and repair the
leak.
- Incorrect combustion gases or flow rates when
using an FID, NPD or FPD.
Solution: Check and reset the
gases at their proper values.
- Physical defect in the detector.
Solution: Clean or replace parts
as necessary.
- Defective detector board.
Solution: Consult the
instruction manual or contact the GC
manufacturer.
Wander:
- Contaminated carrier gas if using isothermal
conditions.
Solution: Change the carrier gas
or use (change) carrier gas impurity traps.
- Contaminated gas chromatograph.
Solution: Clean the injector
and/or gas lines. Solvent rinse the column.
- Poor control of the carrier gas or detector gas
flows.
Solution: Clean, repair or
change the flow controller.
- Poor thermal control of the detector.
Solution: Consult the
instruction manual or contact the GC
manufacturer.
Drift (Upward):
- GC or column contamination.
Solution: Clean the injector.
Solvent rinse the column.
- Damaged stationary phase.
Solution: Replace the column.
Determine the cause of the damage (oxygen,
thermal or chemical) to prevent future problems.
Drift (Downward):
- Incomplete conditioning of the column.
Solution: Condition the column
until a stable baseline is obtained.
- Unequilibrated detector.
Solution: Allow the detector
enough time to equilibrate.
Offset:
- Injector or column contamination.
Solution: Clean the injector.
Solvent rinse the column.
- Column is inserted into the flame of an FID, NPD
or FPD.
Solution: Reinstall the column.
- Contaminated carrier or detector gases.
Solution: Change the gases or
install (change) impurity traps.
- Contaminated detector.
Solution: Clean the detector.
- Malfunctioning or improperly set recording
device.
Solution: Check the recorder
settings. Consult the instruction manual, or
contact the manufacturer.
No Peaks:
- Plugged syringe.
Solution: Clean the syringe or
use a new syringe.
- Broken column.
Solution: Replace or reinstall
the column.
- Injecting the sample into the wrong injector.
Solution: Use the correct
injector or move the column to the correct
injector.
- Column installed into the wrong detector.
Solution: Reinstall the column
into the correct detector.
- Integrator or recording device is connected to
the wrong detector or not connected at all.
Solution: Connect the integrator
to the correct detector.
- Detector gases improperly set or not on.
Solution: Check and reset the
detector gases.
- Very low or no carrier gas flow.
Solution:Immediately lower the
column temperature to 35-40°C. Measure and
verify the carrier gas flow rate. Check for
leaks.
All Peaks Reduced in Size:
- Partially plugged syringe.
Solution: Clean the syringe or
use a new syringe.
- Change in the injection technique.
Solution:Check the injection
technique and verify that it is the same as
before.
- Large leak in the injector (usually accompanied
by poor peak shapes).
Solution:Find and repair the
leak.
- Split ratio is too high.
Solution: Lower the split ratio.
- Too short of a purge activation time for
splitless injections.
Solution:Increase the purge
activation time.
- Very high septum purge flow.
Solution: Decrease the septum
purge flow.
- Too low of an injector temperature (especially
for high molecular weight or low volatility
compounds).
Solution: Increase the injector
temperature .
- Column temperature is not hot enough.
Solution: Increase the column
temperature or the upper tem perature value of
the column temperature program.
- Initial temperature of the column is too high for
splitless or on-column injections.
Solution: Decrease the initial
column temperature or use a higher boiling
solvent.
- High background signal caused by contamination,
excessive column bleed (damage) or autozero
problem.
Solution: Clean the GC. Solvent
rinse the column. Replace the bleeding column.
Check the autozero function and setting.
- Improperly operated detectors.
Solution: Consult the
instruction manual for the proper gas flows and
type and operating guidelines.
- Impurities in the detector gas.
Solution: Use impurity traps
and/or replace the contaminated gas.
- Detector-compound mismatch.
Solution: Make sure that the
detector will respond to the compounds being
analyzed.
- Excessive attenuated integrator signal.
Solution: Check and verify the
attenuation settings.
- Sample concentration or integrity problems.
Solution: Check the sample's
concentration or stability.
Select Peaks Reduced in Size:
- Column and/or liner activity or contamination, if
the reduction or loss is for active compounds
(e.g., amines, carboxylic acids, alcohols,
diols).
Solution: Clean or replace the
injector liner. Solvent rinse or replace the
column.
- Leak in the injector, if the reduction or loss is
the most volatile compounds.
Solution: Find and repair the
leak.
- Too high of an initial column temperature for
splitless or on-column injections.
Solution:Decrease the initial
column temperature or use a higher boiling
solvent.
- Mixed sample solvents for splitless or on-column
injections.
Solution: Use a single solvent
for sample injection.
- Decomposition or error in the sample.
Solution: Check and verify the
sample integrity and concentration.
Tailing Peaks:
- Active injector liner or column.
Solution: Clean or replace
liner. Replace the column if it is damaged.
- Contaminated injector liner or column.
Solution: Clean or replace the
injector liner. Solvent rinse the column.
- Dead volume caused by a poorly installed column,
liner or union.
Solution: Check and verify the
installation of each fitting. Re-install the
column, if necessary.
- Poorly cut column end.
Solution: Recut and reinstall
the column.
- Polarity mismatch of the stationary phase, solute
or solvent.
Solution: Change to a solvent or
phase that have a better polarity match.
- Cold spot in the flow path.
Solution: Check the flow path of
the sample for possible cold spots or zones.
- Solid debris in the liner or column.
Solution: Clean or replace the
liner. Cut the ends of the column until the
debris is removed.
- Poor injection technique (usually too slow of an
injection).
Solution: Change injection
technique.
- Too low of a split ratio.
Solution: Increase the split
ratio.
- Overloading on a PLOT column.
Solution: Decrease the amount of
sample reaching the column.
- Some compounds such as alcoholic amines, primary
and secondary amines, and carboxylic acids tail
on most columns.
Solution: Use a pH-modified
stationary phase. Derivatize the compounds. Some
peaks will always exhibit some tailing.
Rounded or Flat-Topped Peaks:
- Overloaded detector.
Solution: Decrease the amount of
sample reaching the detector.
- Exceeding the range of the integrator or
recording device (especially for computer
systems).
Solution: Reset the range or
attenuation levels on the recorder.
Split Peaks:
- Poor injection technique (jerky or erratic).
Solution: Change injection
technique (smooth and steady plunger depression).
- Poorly installed column in the injector.
Solution: Recut the column end
and reinstall in the injector.
- Column temperature fluctuations.
Solution: Check the oven
temperature or contact the GC manufacturer.
- Coelution of two or more compounds.
Solution: Check for any changes
in the operational parameters. Contamination or a
change in the sample will introduce additional
compounds to the injected sample. Check for these
possibilities.
- Mixed sample solvent for splitless or on-column
injections.
Solution: Use a single solvent
for sample injections.
Negative Peaks:
- All peaks are negative.
Solution: Check the polarity of
the recorder connections.
- Select peaks on a TCD.
Solution: Compound has greater
thermal conductivity than the carrier gas; a
negative peak is expected in this case.
- After a positive peak on an ECD.
Solution: Dirty or old ECD cell.
Clean or replace the ECD.
- Different column temperature.
Solution: Check and verify the
column temperature or temperature program.
- Different carrier gas flow rate or linear
velocity.
Solution: Check and verify the
carrier gas flow rate or linear velocity.
- Leak in the injector, especially the septum.
Solution: Find and repair the
leak. Change the septum.
- Contaminated column.
Solution: Solvent rinse the
column.
- Change in the sample solvent.
Solution: Use the same solvent
for all samples and standards.
- Contaminated column.
Solution: Solvent rinse the
column.
- Damaged stationary phase.
Solution: Replace the column.
Excessive bleed should be evident also.
- Different column temperature, carrier flow rate
or column.
Solution: Check and verify
temperature programs, flow rates and column
identity.
- Large changes in the sample concentration.
Solution: Adjust or compensate
for the concentration change.
- Improper injector operation.
Solution: Check the temperature,
split ratio, purge time and type of liner. Also
check for leaks.
-
- Injection technique.
Solution: Use a consistent
injection technique.
- Split discrimination.
Solution: Use a consistent
injection technique (volume, injector temperature
and split ratio).
- Using a different purge activation time for
splitless injection.
Solution: Use a consistent purge
activation time.
- Baseline disturbances.
Solution: See the section on
baseline disturbances
- Improper integrator or recorder settings.
Solution: Check and verify the
integrator and recorder settings.
- Inconsistent detector gas flows or temperatures.
Solution: Check and verify
detector operation.
- Column or liner activity
(adsorption).
Solution: Clean or replace the
injector liner. Solvent rinse or replace the
column.
- Exposure of the column to air (oxygen) at
elevated temperatures.
Solution: Find and repair any
leaks. Check the quality of the impurity traps
and carrier gas.
- Exceeding the upper temperature limit of the
column for prolonged periods.
Solution: Replace the column. Do
not exceed the upper tempera ture limits.
- Chemical damage.
Solution: Do not inject
inorganic acids or bases.
- Contamination of the column with high molecular
weight mate rials.
Solution: Use a sample
preparation technique to remove the problem
contaminants. Use a guard column.
- Column breakage.
Solution: Avoid abrading or
scratching the column. Avoid sharp turns or bends
in the
tubing.
- Contamination of the injector or column.
Solution: Clean the injector and
liner. Solvent rinse the column.
- Septum bleed.
Solution: Use a higher
temperature septum. Lower the injector
temperature. Condition septum before use.
- Previous run terminated too soon.
Solution: Use a higher
temperature to elute all of the sample
components. Prolong the run time to allow the
complete elution of the sample.
- Poorly installed column.
Solution: Recut and reinstall
the column.
- Leak in the injector.
Solution: Find and repair the
leak.
- Too low of a split ratio.
Solution: Use a higher split
ratio.
- Too low of an injector temperature.
Solution: Use a higher injector
temperature.
- Too long of a purge activation time for splitless
injections.
Solution: Use a shorter purge
activation time.
- Large injection volume.
Solution: Decrease the injection
size.
- Low column temperatures and high boiling solvent.
Solution: Use a higher initial
column temperature or a lower boiling solvent.
- High column temperatures and low boiling solvent.
Solution: Use a lower initial
column temperature or a higher boiling solvent. J
& W ADM100 Model Flowmeter
Flow rates are critical to efficient GC
operation. Make sure flow rates are correct by
using the J&W model ADM series of flowmeters.
They are based on "acoustic
displacement" technology. No bubbles, messy
liquids or breaking glassware to deal with. Ideal
for field or laboratory use. These flowmeters are
compatible with all noncorrosive gases. A
computer-optimized calibration incorporating a
NIST calibrated flow standard ensures the highest
available accuracy, making ISO9000 and GLP
compliance that much easier.
Hamilton Cemented Needle
Be sure to have a clean, working syringe.
Problems can sometimes be traced to the
autosamplers. Chrom Tech offers a complete line
of Hamilton Syringes.
Septa and Ferrules
J&W offers a complete line of silicone septa
and ferrules. Overused septa and ferrules are
prone to leaks, which can cause column bleed due
by allowing oxygen to be introduced. Particulates
from the overused septa and ferrules can also
cause problems when they contaminate the liner.
4 mm Splitless Liner
Pyrolyzed compounds can build up on liner walls.
This buildup causes clogging and sample
adsorption, which can result in a
nonrepresentative chromatogram.
Several manufacturers offer
"low bleed" columns. In some cases,
these are merely selected from the standard
production process, but in other cases the
columns are actually "synthesized" for
low bleed. In recent years, it has been
established that where functional groups (i.e.
phenyl) are inserted into the polysiloxane chain
as aryl inclusions, as opposed to being attached
to the chain as pendant groups, the resultant
phase possesses increased thermal and oxidative
resistance. Columns coated with such phases emit
lower levels of bleed signal and are capable of
going to higher temperatures. The increased
thermal resistance is apparent only at
temperatures above ca. 300 degrees. While some
users can reap the benefits of these
developments, others find little or no
improvement.. their bleed signals are still too
high.
True column bleed, of course, comes only from
the column. What the user perceives as bleed is
usually the total signal reaching the detector,
which is the summation of signal from the septum
(this gives a typical silicone mass spectrum),
the injector, and the detector, all of which is
usually blamed on the column.
It is good procedure to first check the
detector. Disconnect and remove the column, and
place an undrilled cap on the column attachment
fitting. Activate the detector, and note the
signal at 50 degrees. Increase the oven
temperature to 320 degrees, and again note the
signal. On a pristine detector, the FID signal
will increase by one to two picoamps. If the
increase exceeds this level, attention should be
directed to cleaning the detector, make-up gas
and hydrogen lines. Once the detector signal
falls to an acceptable level at 320 degrees,
attention should be directed to the injector. If
the injector liner is visibly soiled, the
injector should be cooled, dissembled and
interior cavities scrubbed with solvent and
natural bristle brushes or cotton swabs. After
assembling the injector, a "jumper
tube" (one to three meters of uncoated fused
silica or steel tubing) is then used to connect
the injector directly to the detector. The
injector heater should be energized, and the oven
set at 320 degrees. Any increase in
"bleed" signal over that observed with
the detector alone must come from the front end
of the instrument, and may originate with the
septum, the carrier gas line, in-line regulators,
valves, or flow controllers.
Wrap a new septum in aluminum foil, ensuring
that one face is smooth, and install this, smooth
side down. If the signal emanating from the
jumper tube is decreased, it indicates a need for
better quality septa. If the signal is still
high, materials entrained in the carrier gas may
have deposited in lines, valves, or regulators,
which should be dissembled and cleaned or
replaced.
When the combined signal from the injector and
detector falls to an acceptable level (one to two
picoamps @ 320 degrees on an FID), the user is
ready to install and reap the benefits of a true
low-bleed column. The bleed rate of conventional
columns is normally high enough to mask signal
from the injector and detector unless these
latter are heavily contaminated. With low bleed
columns, the signal from the injector and
detector assumes increased importance. This
spurious signal is not infrequently limiting, and
is usually (and incorrectly) perceived as
"column bleed".
Prof. Walt Jennings
Cofounder, J&W Scientific Incorporated
For additional information, see J. High Resol.
Chromatogr. 19:567(1995).
A Message from
Walt: Column Activity
The requirements for qualitative and quantitative validity that confront today's analysts demand
the
ultimate in analytical GC columns. Column quality
is reflected not only by column efficiency (usually measured in N/m), by the "delta bleed"
(signal at max temp versus signal at 50°), by the maximum usable temperature, and by the duplicability of
the "selectivity" (usually measured via the
retention indices of selected compounds), but also by the level of "column activity". Some of
these are inter-related.
Activity exercises negative effects on both
qualitative and quantitative analyses. Properly used, octanol and decanol are useful analytes to follow column
activity, but it is possible to manipulate their behavior. Alcohols tail because they undergo
hydrogen bonding with active sites on the column (or the stationary phase), and the strength of
hydrogen bonds varies inversely with temperature. Columns that exhibit unacceptable tailing at test
temperatures of 100-125° often generate needle sharp alcohol peaks at 150-180°, and while this
generates an impressive test chromatogram, it is unrealistic because these solutes elute at much
lower temperatures -- where H-bonding is stronger -- in
a real run.
The increased emphasis on low bleed columns has made many analysts more aware of this problem.
One method of producing a "low bleed"
column is lengthy exposure to excessive temperatures. Unfortunately, this usually results in a very
active column, but the reasons for this are not entirely clear. We recognize that deactivation treatments
are often more thermally labile than is the
stationary phase per se, but whether the end result is due
to the exposure of active silanols or to some other source of activity remains to be elucidated.
There is much to be said for the common polarity test mixture -- usually containing paraffin
hydrocarbons, esters, ketone(s), primary
alcohols, and one or two more demanding analytes -- e.g., diols, primary amines, and/or halogenated
phenols. The discriminating analyst sometimes adds a
specific test compound(s) that reflects his or her
particular requirements. If there is any possibility that
any of the analytes in the test mixture could interact
with each other on long term storage, some precautions are in order. Two or more solutions should be
prepared, each containing only analytes immune to interaction, and aliquots of these inert
solutions blended to produce the intact test solution just
prior to analysis.
The hydrocarbons are crucial to the test mixture; they serve as reference peaks. Paraffin
hydrocarbons have no active entities, and tailing
of a hydrocarbon peak is almost always indicative of a gas flow problem (too low a split ratio,
inadequate make-up gas, cracked injector liner, etc.). Only
if (or when!) the hydrocarbon peaks are sharp and symmetrical can the conformation of the peaks
produced by the more active analytes be
realistically evaluated. Columns that have been subjected to analysis of dirty samples and/or exposure to
excessively high temperatures for sustained
periods should be periodically re-tested. Unfortunately,
it is rarely possible to re-deactivate heat-damaged columns without adversely affecting some of their
other properties. In the next
"Message", we’ll examine some means of prolonging column life.
|
In This
Section... Troubleshooting
Tools
Eight Problem Categories
Baseline Disturbances
Irregular Peak Shapes or
Sizes
Retention Time Shifts
Loss of Separation or Resolution
Quantitation Difficulties
Rapid Column Deterioration
Ghost Peaks
Broad Solvent Front
A
Message from Walt
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