Basic Capillary GC Theory and
Practical Troubleshooting
Part 4, Troubleshooting
Katherine K. Stenerson
Supelco, 595 North Harrison Road, Bellefonte, PA 16823
T404077
HCE
GC Troubleshooting: Topics
• Basic Troubleshooting Strategy
• Preventing Problems
-
Gas Purification
Injection Technique
Liner Selection and Care
Column Installation
Guard Columns
• Identifying Common Problems
• Recommended Reading
• Discussion
Troubleshooting Strategy
• Have appropriate equipment and supplies on hand.
• Establish a systematic approach.
• Know what to look for first.
• Record what you did to correct the problem.
Troubleshooting Strategy
• Suggested equipment to have on hand for
troubleshooting:
-
Electronic Leak Detector
Flow Meter
“Test” Column
Replacement Accessories (Syringes, Ferrules, Septa, Liners)
Replacement Purifiers
Troubleshooting Strategy
• Isolate the source of the problem:
Run
Reference
Standard
Problem
was sample
related
OK
Not OK
Check operating
parameters
OK
Not OK
Install
Test Column
Correct the
parameter
OK
Problem
was column
related
Not OK
Problem was
in the inlet or with
the carrier gas
Not OK
Switch
Detector
OK
Problem was
in the Original
detector
Troubleshooting Strategy
• Approaching the problem…
- Check first to see if a “fix” for the problem is already known.
- Check the Supelco Capillary GC Troubleshooting Guide
(Bulletin 853.)
- Check the instrument maintenance record.
- Talk to others in your lab.
Troubleshooting Strategy
• Five major sources of chromatographic problems:
-
Operator Error
The Sample
The Column
The Gas Flow System (both internal and external to the GC)
The GC Electrical System
Troubleshooting Strategy
• When reviewing method parameters, consider these
questions:
- Should I be doing split or splitless injection?
- Is my starting temperature low enough to allow sufficient
sample focusing?
- For splitless injections, is my splitter opening at the appropriate
time?
- Is my column flow set to give me maximum efficiency at the
most critical point?
- Are heated zones (injectors, detectors, interfaces) set
appropriately?
- Am I using the appropriate liner type?
Preventing Problems
• The best way to solve problems is to prevent them!
- Install and maintain proper purification for all gases in the GC
system.
- Maintain the injector by periodically inspecting and changing
the liner, septa, and seal.
- Use the proper injection technique - this includes using the
right liner for the job.
- Install the column at the recommended insertion distances.
- When necessary, use a guard column to protect the analytical
column.
Gas Purification
• Carrier Gas
- At minimum, remove hydrocarbons, water, and oxygen.
• Hydrogen (FID)
- At minimum, remove hydrocarbons.
• Air (FID)
- At minimum, remove water and hydrocarbons.
• Nitrogen make-up (FID, ECD)
- At minimum, remove hydrocarbons.
• P-5 make-up (ECD)
- At minimum remove hydrocarbons, halocarbons, and oxygen.
Gas Purification
• Acceptable purity levels for chromatography grade
gases:
Impurity / Maximum Concentration
Gas
Helium
Nitrogen
Air
Hydrogen
Argon/
Methane
O2
<1.0 ppm
<1.0 ppm
20-22%
<1.0 ppm
H2O
<1.0 ppm
<1.0 ppm
<1.0 ppm
<1.0 ppm
CO2
<1.0 ppm
<1.0 ppm
<1.0 ppm
<1.0 ppm
CO
<1.0 ppm
<1.0 ppm
<1.0 ppm
<1.0 ppm
Total
Hydrocarbons
<1.0 ppm
<1.0 ppm
<1.0 ppm
<1.0 ppm
<1.0 ppm
<1.0 ppm
<1.0 ppm
<1.0 ppm
<1.0 ppm
Gas Purification
• Suggested gas purifiers:
Carrier
Hydrocarbons
Water
Oxygen
Supelcarb HC
Supelpure HC
Mol Sieve 5A
OMI-2
H2
Air
Mol Sieve 5A
N2 makeup
P-5
P-5
OMI-2
OMI-2
Gas Purification
• What are some signs that my purifiers need to be
changed?
• Hydrocarbon Traps
- Noise in the baseline (FID)
- Increase in background peaks
on tune (MSD)
- Higher than normal baseline
reading on FID
- Extra peaks visible in run
• Molecular Sieve 5A
- Increase in column bleed
- Water visible in MS
background
- Poor peak shapes for gaseous
VOCs (purge and trap)
- Extra peaks visible in run
- OMI™-2 color change
Injector Maintenance
• Change (as needed):
- Septa
- Liner and O-ring
- Seal and washer
• Inspect the Inlet Periodically
- Look for contamination in the liner
- Look for residue on the seal
Injection Technique
• It is important to choose the injection technique that
is appropriate for your analysis. In Capillary GC, the
techniques used are:
-
Split
Splitless
Direct
On-column
Injection Technique
• Split Injection
- A vaporizing type injection designed to limit the amount of
sample reaching the capillary column.
- Sample is split and a small portion flows to the column while
the bulk is typically vented through the split vent port.
- Split injection can be used in an isothermal or temperature
programmed analysis.
Injection Technique
• Splitless Injection
- Sample is introduced into a heated injection port operating in a
nonsplitting mode.
- Sample vaporizes and sample cloud is mixed with carrier gas
and transferred into the column.
- Sample condenses on head of the capillary column due to the
oven temperature being 10-20ºC below matrix solvent boiling
point.
- After 1.5 to 2 injector volumes have entered the column, split
vent is opened and inlet purged.
Injection Technique
• The volume of a splitless liner is important:
Typical Splitless Injection Liner Volumes
Liner Length
Liner ID
Liner Volume
78.5mm
78.5mm
78.5mm
4.0mm
2.0mm
1.0mm
986µL
246µL
62µL
Injection Technique
• Solvent expansion volumes of 1µL injection at
specified temperatures and pressures:
200°C Inlet
Head pressure
Solvent
300°C Inlet Temp.
Head pressure
BP (°C)
10
20
30
10
20
30
77
236
168
131
286
204
58
Hexane
Isooctane
68.7
99.2
177
140
126
100
98
77
214
170
153
121
119
94
Methanol
y
chloride
64.5
40.1
570
360
406
257
315
200
691
437
492
311
382
241
MTBE
Water
55
100
194
1279
138
910
108
706
235
1548
167
1102
130
855
Ethyl acetate
Injection Technique
• Direct Injection
- A vaporizing type injection typically used with wide bore
capillary columns in a converted packed column GC.
- Sample is injected into a heated injection sleeve, vaporized,
and transported directly to the column in the carrier gas flow.
Similar to a packed column flash vaporization injector.
- Analyses can be isothermal or temperature programmed.
Injection Technique
• Cold On-Column Injection
- A non-vaporizing type injection in which the liquid sample is
directly deposited at the inlet of the capillary columns.
- All analyses are temperature programmed analyses since a
liquid sample is deposited in the column.
- No liners are typically required.
- Special syringes are required.
- 0.53mm ID fused silica created to allow insertion of 26 gauge
needle into column.
Injection Technique
• Injection speed can have an effect.
Rapid, smooth injection
0
2
4
6
8
10
Time (min)
Slow, “choppy” injection
0
2
4
6
Time (min)
8
10
Liner Selection
• Some liners used for split injection:
Cup
Baffle (Varian)
Split/splitless, wool packed
Liner Selection
• Some liners used for splitless injection:
2 mm ID, straight
Dual-tapered
Single-tapered
Liner Selection
• Packed liners, PROs and CONs:
• PROs
- Packing liners helps aid in the
vaporization process
- Packing liners can help
improve reproducibility of area
counts by minimizing droplets
reaching the head of the
column
- Packing can act as a particle
trap
• CONs
- The packing does act as a
short packed column and can
influence results
- Can cause discrimination of
higher molecular weight
compounds
- Can cause adsorption &
sample degradation
Liner Selection
• The ID of the liner can affect sensitivity:
Splitless injection,
2mm vs. 4mm ID liner
Liner Care
• If you must clean a liner….
- Handle liners with gloves or forceps.
- Use clean compressed gas and/or a fine brush to remove
particles.
- Rinse liner in an appropriate solvent and dry with clean
compressed gas.
- Use mineral acid and/or detergent only if absolutely necessary.
Be sure to deactivate the liner after after this process.
- If repacking with glass wool, make sure it has been
deactivated.
Liner Care
• The results of using undeactivated glass wool in
4mm ID liner used for pesticide analysis:
5
2
1.
2.
3.
4.
5.
6.
Deactivated
glass wool
18
6
3 4
1
20
4,4’-DDE
Endrin
4,4’-DDD
Endrin aldehyde
4,4’-DDT
Endrin ketone
22
Time (min)
24
26
2
1
Undeactivated
glass wool
3
5
6
4
18
20
22
Time (min)
24
26
Column Installation
• Installing the column too low in the inlet can result in
peak tailing.
This column was
installed too low
in the inlet.
0
2
4
6
8
Time (min)
10
12
14
Guard Columns
• Choose a guard column that has been deactivated.
• Usually, the ID of the guard matches the analytical
column.
• A 5-10 meter length is normally used.
• Connect with either a GlasSeal™ or butt connector.
Common Problems
1. Poor Peak Shapes (either tailing, fronting, or just
generally ugly.)
2. Nonlinearity
3. Baseline Noise and /or Drift
4. Ghost Peaks
5. Missing Peaks / Poor Response
6. Insufficient Resolution
Poor Peak Shape
• In Gas-liquid chromatography, fronting may indicate
column overload.
||
• Tailing may indicate activity in the system or
improper column installation.
||
Poor Peak Shape
• Generally ugly peaks, such as α,α-dimethylphenethylamine, can be caused by a variety of problems.
Nonlinearity
• The most common causes are:
-
Column overload
Detector overload
Standards preparation
Poor peak shape resulting in improper integration
Nonlinearity and Column Overload
• An Example of Column Overload:
Fronting
due to
overload
6.0
7.0
An Example of Column Overload:
• Preventing column overload:
-
Inject a smaller amount and/or increase split ratio.
Use a thicker film column.
Use a column with a wider ID.
Decrease upper limit of calibration range.
Use a column of slightly different polarity.
Nonlinearity and Poor Peak Shape
• An example of poor peak shape affecting linearity:
- The poor peak shape of benzoic acid here is caused by
solubility problems with the 5% phenyl methylpolysiloxane
phase.
Baseline Noise and Drift
• Common causes:
-
Column bleed
Septa bleed
Dirty detector
Contaminants in carrier gas / carrier gas purity
Column Bleed
• Results from the normal degradation of the
stationary phase.
• All columns bleed to some extent.
• Bleed increases with temperature.
• The amount of bleed will increase in the presence of
oxygen.
Column Bleed
• A Typical Bleed Profile:
2
Bleed measured as the
difference between 1 and 2.
1
4
Column Bleed
• Column bleed and an MSD:
- Visible as baseline rise in the TIC.
- Check spectra for key bleed ions:
• Equity-1: 73, 207, 281
• Equity-5: 207, 281
• Equity-1701: 207, 269
• SPB™-624: 207, 269
• Make sure interface temp. is < column max. temp.
Column Bleed
• Common Bleed Ions
CH3
CH3
CH3
CH3
CH3
Si
Si
CH3
O
O
+ Si
Si
O
O
CH3
CH3
Si
CH3
Si
CH3
O
CH3
O
CH3
Si +
Si +
CH3
207: D3
CH3
O
284: D4
CH3
73: TMS
Column Bleed
• Equity-1: MS Spectra of Bleed
Abundance
Scan 8569 (43.681 min): EQ12506A.D
207
7000
D3
6500
6000
5500
5000
4500
TMS
4000
D4
3500
3000
281
2500
73
2000
1500
179
1000 45
500
0
96
133
239
115
161
299 327 355
373 401
447
40 60 80 100120140160180200220240260280300320340360380400420440
m/z-->
Column Bleed
• Equity-5: MS Spectra of Bleed
Abundance
Scan 4393 (22.148 min): 0326003.D
207
13000
12500
12000
11500
207
11000
10500
10000
9500
9000
8500
8000
7500
7000
6500
6000
5500
5000
281
4500
73
4000
3500
3000
2500
281
73
2000
1500
96
133
253
1000
177
500
0
115
51
156
331
355
311
405 429
40 60 80 100120140160180200220240260280300320340360380400420
m/z-->
Column Bleed
• SPB™-624: MS Spectra of Bleed
Abundance
Scan 8581 (33.704 min): 1022006.D
207
24000
22000
207
20000
18000
16000
14000
269
12000
269
10000
8000
6000
44
253
4000
73
191
133
147
2000
59
0
40
m/z-->
96
60
119
177
163
223239
283
322 343
298
80 100 120 140 160 180 200 220 240 260 280 300 320 340
Column Bleed
• Equity-1701: MS Spectra of Bleed
Abundance
Scan 9126 (84.849 min): 0215001.D
207
14000
13000
207
12000
11000
10000
9000
8000
7000
6000
269
5000
4000
269
3000
96
2000
1000
51
73
133
115
177
159
240
309
343
384
438
0
40 60 80 100120140160180200220240260280300320340360380400420440
m/z-->
Septa Bleed
• Septa Bleed: MS Spectra
Abundance
Scan 604 (7.460 min): 1201001.D
73
44000
42000
40000
38000
73
36000
34000
32000
30000
28000
147
26000
24000
22000
20000
281
147
18000
16000
281
14000
12000
10000
8000
6000
4000
327
45
207
2000
131
95 115
0
40
m/z-->
60
163
191
223
251
297
343
415
399
383
80 100120140160180200220240260280300320340360380400420
Column & Septa Bleed
• Minimize bleed!
- Sufficiently purge column with carrier gas before ramping it up
in temperature.
- Make sure carrier gas is scrubbed for water and oxygen.
- Check integrity of all fittings leading to the column.
- Do not heat the column above its maximum temp.
- Precondition the column prior to use.
- Use a high quality, high temperature septa and ferrules.
Baseline Noise and Drift
• Effect of carrier gas purity on baseline noise:
GCMS baseline comparisons
12000
11500
11000
10500
10000
9500
9000
8500
H2 carrier from tank
8000
7500
7000
6500
6000
5500
5000
4500
4000
H2 carrier from a generator
3500
3000
2500
2000
1500
1000
500
0
1
Ghost Peaks
• Possible causes:
- Residue in the inlet liner and at the head of the column
- Contaminated syringe / and or wash solutions on an
autosampler
- Sample carryover
- Contaminated carrier gas
Ghost Peaks
• If pieces of septa get into an inlet liner...
2200000
2000000
1800000
1600000
1400000
1200000
1000000
800000
600000
400000
200000
0
Response test mix, before
Ghost Peaks
…even a simple analysis can be ruined.
3500000
3000000
2500000
2000000
1500000
1000000
500000
0
Time-->
Response test mix, after
Missing Peaks / Poor Response
• Sample decomposition
-
Activity in the inlet or column
Injection port temperature too high
Sample not stable enough for GC
Standards not stable
• Coelution
• Insufficient run time / final temperature
• Sample not volatile enough for GC
• Improper column installation
Missing Peaks / Poor Response
• Nasty samples can damage a column by creating
active sites.
250000
200000
150000
100000
50000
0
4-nitrophenol
300000
2,4-dinitrophenol
350000
2-methyl-3,5-dinitrophenol
400000
pentachlorophenol
Before Sample Injection
450000
Missing Peaks / Poor Response
• Responses of some acidic compounds were affected.
700000
600000
500000
400000
300000
200000
100000
0
Time-->
Pentachlorophenol should be here
800000
After Sample Injection
2-methyl-3,5-dinitrophenol
900000
2,4-DNP & 4-NP should be here
Abundance
Missing Peaks / Poor Response
• Response can also be affected by the position of the
column in the inlet.
2,800,000
2600000
8 mm above top of ferrule
2400000
2200000
2000000
1800000
1600000
1400000
1200000
1000000
800000
600,000
400000
200000
0
Missing Peaks / Poor Response
• Here, the column was not inserted far enough.
2,200,000
2000000
1800000
1600000
1400000
1200000
1000000
800000
600000
400000
200,000
0
5 mm above top of ferrule
Missing Peaks / Poor Response
• Here, the column was inserted too far:
1,800,000
1600000
1400000
1200000
1000000
800000
600000
400000
200,000
0
20 mm above top of ferrule
Insufficient Resolution
• Column
- Longer columns increase resolution
- Smaller ID columns increase resolution
- A different phase altogether may be needed
• Conditions
- Carrier gas flow too fast or slow
- Oven ramp rate too fast
- Wrong starting or ending temperature
Supelco Bulletins
741:
783:
853:
875:
895:
897:
898:
899:
916:
918:
The Supelco Guide to Leak-Free Connections
Cleaning Flame Ionization Detectors
Capillary Troubleshooting Guide
Supelco Capillary GC Selection Guide
Installation and Maintenance Instructions for 0.25 mm
and 0.32 mm ID Fused Silica Capillary Columns
Installation and Maintenance Instructions for 0.53 mm
ID Fused Silica Capillary Columns
Gas Management Systems for GC
Capillary GC Inlet Sleeve Selection Guide
Purge and Trap System Guide
Selecting Purifiers for Gas Chromatography
Supelco Service
• Supelco Technical Service
- phone: 1-800-359-3041 (US only), 814-359-3041
- email: [email protected]
• Supelco Customer Service
- phone: 1-800-247-6628 (US only), 814-359-3441
- email: [email protected]
• Sigma-Aldrich Website
- www.sigma-aldrich.com
Discussion