Capillary GC Troubleshooting: a Practical Approach Katherine Stenerson Gas Separations R&D Supelco, Supelco Park, Bellefonte, PA 16823 1999 Sigma-Aldrich Co.
Outline Basic Troubleshooting Strategy Preventing Problems 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.
Suggested Equipment to Have on Hand for Troubleshooting: Electronic Leak Detector Flow Meter Test Column Replacement Accessories (Syringes, Ferrules, Septa) Replacement Purifiers
Five Major Sources of Chromatographic Problems: ❶ Operator Error ❷ The Sample ❸ The Column ❹ The GC Electrical System ❺ The Gas Flow System (both internal and external to the GC)
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 back in the instrument maintenance record. Talk to others in your lab.
Isolate the Source of the Problem Run Ref. Std. OK Check operating parameters Not OK Correct the parameter OK Not OK Problem was sample related Install Test Column OK Problem was Column related Not OK Cap off Detector OK Not OK Problem is in the Inlet or with the carrier gas Problem is in the Detector
When reviewing operating method parameters consider the following:? Is my starting temp. 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?
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 (H/P.) Use the proper injection technique-this includes using the right liner for the job. 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 (especially halogencontaining), oxygen.
Acceptable Purity Levels for Chromatography Grade Gases Impurity / Maximum Concentration Gas O2 H2O CO2 CO Total Hydrocarbons Helium <1.0 ppm <1.0 ppm <1.0 ppm <1.0 ppm <1.0 ppm Nitrogen <1.0 ppm <1.0 ppm <1.0 ppm <1.0 ppm <1.0 ppm Air 20-22% <1.0 ppm <1.0 ppm <1.0 ppm <1.0 ppm Hydrogen <1.0 ppm <1.0 ppm <1.0 ppm <1.0 ppm <1.0 ppm Argon/ Methane <1.0 ppm <1.0 ppm <1.0 ppm <1.0 ppm <1.0 ppm
P-5 Suggested purifiers Hydrocarbons Water Oxygen Carrier Supelcarb HC Supelpure HC Mole Sieve 5A OMI -2 H 2 Air Mole Sieve 5A N 2 makeup P-5 OMI -2 OMI -2
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): 1. Liner and O-ring * 2. Seal and washer * * H/P GCs Inspect the inlet periodically -Look for contamination in the liner -Look for residue on the seal
Using the right liner and injection technique can also prevent problems: Split Injection used for concentrated samples high flow of carrier gas through liner during injection should use liner designed for split injection Splitless Injection used for trace analysis low flow of carrier gas through liner during injection inertness and internal volume of liner used are critical
Some liners used for split injection Cup (unpacked) Cup (wool packed) Split/splitless- wool packed
Some liners used for splitless injection 2 mm ID, straight Dual-tapered Single-tapered
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.
Using a Guard Column Choose a guard column which 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
1. Poor Peak Shapes Fronting can indicate column overload. Tailing can indicate activity in the system or improper column installation.
1. Poor Peak Shapes (cont.) Generally ugly peaks, such as a,adimethylphenethylamine, can be caused by a variety of problems.
2. Nonlinearity The most common causes are: Column overload Detector overload Standards preparation Poor peak shape resulting in improper integration
An Example of Overload: Peaks have broad bases, fronting on last few visible in run.
Preventing column overload for splitless injections: Inject a smaller amount / use a 1 ul syringe. 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.
An example of poor peak shape affecting linearity: Benzoic acid is typically of poor shape when doing splitless injections.
3. Common causes of baseline noise / drift. Column bleed Dirty detector Contaminants in carrier gas / carrier gas purity
Effect of carrier gas purity on baseline noise: 12000 11500 11000 10500 10000 9500 9000 8500 8000 7500 7000 6500 6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0 1 H2 carrier from tank H2 carrier from a generator
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.
A Typical Bleed Profile: 2 Bleed measured as the difference between 1 and 2. 1 4
Column Bleed on an MSD Visible as baseline rise in the TIC. Check spectra for key bleed ions: PTE -5: 207, 281 SPB -1: 73, 207, 281 SPB -624: 207, 269 Make sure interface temp. is < column max. temp.
Ion 207 corresponds to a fragment known as D3: CH 3 CH 3 CH 3 O Si Si O O + Si CH 3 CH 3
MS spectra of bleed from a PTE -5 Column Abundance 6000 5500 Scan 2895 (27.487 min): 1118015.D 207 207 5000 4500 4000 3500 3000 2500 281 2000 1500 73 96 133 281 1000 191 253 500 50 115 162 223 327 355 415 0 40 60 80 100120140160180200220240260280300320340360380400 m/z-->
MS spectra of bleed from an SPB -1 Column Abundance 11500 11000 10500 10000 9500 9000 8500 8000 7500 7000 73 73 Scan 1941 (28.793 min): 0122002.D 6500 6000 5500 5000 44 207 4500 4000 3500 3000 207 281 2500 2000 1500 147 281 341 1000 96 119 179 500 253 429 313 401 0 40 60 80 100120140160180200220240260280300320340360380400420 m/z-->
MS spectra of bleed from an SPB -624 Column Abundance 24000 Scan 8581 (33.704 min): 1022006.D 207 22000 20000 207 18000 16000 14000 12000 269 10000 269 8000 6000 44 253 4000 133 191 73 96 147 177 2000 119 283 322 343 59 163 223 239 298 0 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 m/z-->
MS Spectra of Septa Bleed Abundance 44000 42000 40000 73 Scan 604 (7.460 min): 1201001.D 38000 36000 34000 73 32000 30000 28000 26000 24000 147 22000 20000 18000 16000 14000 147 281 281 12000 10000 8000 m/z--> 6000 4000 2000 0 45 40 60 80 100120140160180200220240260280300320340360380400420 327 207 399 415 131 95 191 115 163 223 251 297 343 383
Prevent column bleed! Sufficiently purge column with carrier before ramping it up in temperature. Make sure carrier gas is filtered 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.
4. Ghost Peaks 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
If pieces of septa get into an inlet liner... 2200000 2000000 1800000 1600000 1400000 Response test mix, before 1200000 1000000 800000 600000 400000 200000 0
even a simple analysis can be ruined. 3500000 3000000 2500000 2000000 Response test mix, after 1500000 1000000 500000 0 Time-->
5. 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
2-methyl-3,5-dinitrophenol Nasty samples can damage a column by creating active sites: 450000 Before Sample Injection 400000 350000 300000 250000 200000 2,4-dinitrophenol 4-nitrophenol pentachlorophenol 150000 100000 50000 0
Pentachlorophenol should be here Here, the responses of some acids were affected: Abundance After sample injection 900000 800000 700000 600000 500000 400000 300000 200000 2,4-DNP & 4-NP should be here 2-methyl-3,5-dinitrophenol 100000 0 Time-->
Response can also be affected by the position of the column in the inlet: 2800000 2600000 2400000 2200000 2000000 1800000 8 mm above top of ferrule 1600000 1400000 1200000 1000000 800000 600000 400000 200000 0
Here, the column was not inserted far enough: 2200000 2000000 1800000 1600000 1400000 5 mm above top of ferrule 1200000 1000000 800000 600000 400000 200000 0
Here, the column was inserted too far: 1800000 1600000 1400000 1200000 20 mm above top of ferrule 1000000 800000 600000 400000 200000 0
6. Insufficient Resolution Wrong column Longer columns increase resolution. Smaller ID columns increase resolution. A different phase altogether may be needed. Wrong Conditions Carrier gas flow too fast or slow. Oven ramp rate too fast.
Recommended Reading Supelco Bulletins 1. 741: The Supelco Guide to Leak-Free Connections 2. 783: Cleaning Flame Ionization Detectors 3. 853: Capillary Troubleshooting Guide 4. 875: Supelco Capillary GC Selection Guide 5. 895: Installation and Maintenance Instructions for.25 mm and.32 mm ID Fused Silica Capillary Columns 6. 897: Installation and Maintenance Instructions for.53 mm ID Fused Silica Capillary Columns 7. 898: Gas Management Systems for GC 8. 899: Capillary GC Inlet Sleeve Selection Guide 9. 916: Purge and Trap System Guide 10. 918: Selecting Purifiers for Gas Chromatography
Help is just a phone call or mouse click away! Supelco Supelco Technical Service phone: 1-800-359-3041 email: techservice@supelco.sial.com Supelco Supelco Customer Service phone: 1-800-247-6628 email: supelco@sial.com Sigma-Aldrich Website www.sigma-aldrich.com