Restek
Resource Hub / Technical Literature Library / Guide to GC Column Selection and Optimizing Separations

Guide to GC Column Selection and Optimizing Separations

articleBanner
  • Learn how to choose the right column the first time.
  • Optimize separations for the best balance of resolution and speed.
  • Troubleshoot quickly and effectively based on chromatographic symptoms.

You can improve lab productivity by assuring that speed and resolution are optimized. One of the best ways to do this is to use the resolution equation (Figure 1) as the key to controlling your separations. This fundamental equation helps you choose the best column stationary phase, length, inner diameter (ID), and film thickness for your specific applications. Once you understand the basics of how resolution is related to column characteristics, optimizing your analysis for both separation and speed becomes easier. This GC column selection guide discusses the basics of separation and teaches you how to choose the right GC column!

Resolution is the goal of every chromatographer, but how much resolution is enough? Practically speaking, we need enough retention to get sharp symmetrical peaks that are baseline resolved from each other, but not so much retention, that retention times are too long, and peaks start to broaden. To achieve this goal, we must consider the column and non-column factors that affect our “perfect separation.” Only then can we work towards selecting the right column and optimizing GC separations and analysis speed. Now, let’s consider the separation factor (α), retention factor (k), and efficiency (N) in turn and how they can help you select the right column and optimize your separation.

select the right column and optimize your separation

 

Figure 1: The resolution equation and factors that affect it.

resolution equation and factors that affect it

 

Shortcut to Column Selection

  1. Look for application-specific stationary phases first; these columns are optimized for specific analyses and will provide the best resolution in the shortest time (Table III).
  2. If an application-specific column is not available, and you need to measure low concentrations or are using a mass spectrometer (MS), then choose an Rxi column. Rxi technology unites outstanding inertness, low bleed, and high reproducibility, resulting in high-performance GC columns that are ideal for trace analysis and MS work (Table II).
  3. For other methods, choose a general-purpose Rtx column (Table II).

Use Separation Factor (α) to Choose the Best Stationary Phase

resolution equation

Choosing the right stationary phase is the first step toward optimizing your GC separation. It is the most important decision you will make because the separation factor (α) has the greatest impact on resolution, and it is strongly affected by stationary phase polarity and selectivity.

Stationary phase polarity is determined by the type and amount of functional groups in the stationary phase. When choosing a column, consider the polarity of both the stationary phase and your target analytes. If the stationary phase and analyte polarities are similar, then the attractive forces are strong, and more retention will result. Greater retention often results in increased resolution. Stationary phase polarity strongly influences column selectivity and the separation factor, making it a useful consideration when selecting a column.

Stationary phase selectivity is defined by IUPAC as the extent to which other substances interfere with the determination of a given substance. Selectivity is directly related to stationary phase composition and how it interacts with target compounds through intermolecular forces (e.g., hydrogen bonding, dispersion, dipole-dipole interactions, and shape selectivity). As methyl groups in the stationary phase are replaced by different functionalities, such as phenyl or cyanopropyl pendant groups, compounds that are more soluble with those functional groups (e.g., aromatics or polar compounds, respectively) will inter- act more and be retained longer, often leading to better resolution and increased selectivity. In another example of the effect of stationary phase-analyte interactions, an Rtx-200 stationary phase is highly selective for analytes containing lone pair electrons, such as halogen, nitrogen, or carbonyl groups, due to interactions with the fluorine pendant group in this phase. Selectivity can be approximated using existing applications or retention indices (Table I), making these useful tools for comparing phases and deciding which is most appropriate for a specific analysis.

Due to their influence on the separation factor, polarity and selectivity are primary considerations when selecting a column. However, temperature limits must also be considered. In general, highly polar stationary phases have lower maximum operating temperatures, so choosing a column with the appropriate maximum operating temperature, as well as optimal polarity and selectivity for the type of compounds being analyzed, is crucial. Use Table II and Figure 2 to determine which general-purpose column is most appropriate based on the selectivity, polarity, and the temperature requirements of your analysis. See Table III for a list of specialty stationary phases designed for specific applications.

Tech Tip

In many cases, different GC oven temperature programs can change the elution order of sample analytes on the same column. Reconfirm elution orders if changing GC oven temperature programs.

Table I: Kovat’s retention indices for GC phases can be used to approximate selectivity.

Stationary Phase Benzene Butanol Pentanone Nitropropane
100% Dimethyl polysiloxane 651 651 667 705
5% Diphenyl/95% dimethyl polysiloxane 667 667 689 743
20% Diphenyl/80% dimethyl polysiloxane 711 704 740 820
6% Cyanopropylphenyl/94% dimethyl polysiloxane 689 729 739 816
35% Diphenyl/65% dimethyl polysiloxane 746 733 773 867
Trifluoropropylmethyl polysiloxane 738 758 884 980
Phenyl methyl polysiloxane 778 769 813 921
14% Cyanopropylphenyl/86% dimethyl polysiloxane 721 778 784 881
65% Diphenyl/35% dimethyl polysiloxane 794 779 825 938
50% Cyanopropylmethyl/50% phenylmethyl polysiloxane 847 937 958 958
Polyethylene glycol 963 1158 998 1230

 

Stationary Phase

STATIONARY PHASE

Figure 2: Polarity scale of common stationary phases.
Polarity scale of common stationary phases

 

Tech Tip

Any homologous series of compounds, that is, analytes from the same chemical class (e.g., all alcohols, all ketones, or all aldehydes, etc.) will elute in boiling point order on any stationary phase. However, when different compound classes are mixed together in one sample, intermolecular forces between the analytes and the stationary phase are the dominant separation mechanism, not boiling point.

Table II: Relative polarity and maximum temperature are important considerations when selecting a GC stationary phase.

Restek Phase Composition (USP Nomenclature) Restek’s Max Temps* Agilent Phenomenex
Rxi-1HT,
Rxi-1ms, Rtx-1
100% Dimethyl polysiloxane (G1, G2, G38) 400 °C
350 °C
HP-1/HP-1ms,
DB-1/DB-1ms, VF-1ms, CP Sil 5 CB, Ultra 1, DB-1ht, HP-1ms UI, DB-1ms UI
ZB-1, ZB-1MS,
ZB-1HT Inferno
Rxi-5HT,
Rtx-5ms,
Rxi-5ms, Rtx-5
5% Diphenyl/95% dimethyl polysiloxane (G27, G36) 400 °C
350 °C
HP-5/HP-5ms, DB-5, Ultra 2, DB-5ht,
VF-5ht, CP-Sil 8 CB
ZB-5, ZB-5HT Inferno, ZB-5ms
Rxi-5Sil MS 5% (1,4-bis(dimethylsiloxy) phenylene/95% dimethyl polysiloxane 350 °C DB-5ms UI,
DB-5ms,VF-5ms
ZB-5msi
Rxi-XLB Proprietary Phase 360 °C DB-XLB, VF-Xms MR1, ZB-XLB
Rtx-20 20% Diphenyl/80% dimethyl polysiloxane (G28, G32) 320 °C
Rtx-35 35% Diphenyl/65% dimethyl polysiloxane (G42) 320 °C HP-35, DB-35 ZB-35
Rxi-35Sil MS Proprietary Phase 360 °C DB-35ms, DB-35ms UI, VF-35ms MR2
Rtx-50 Phenyl methyl polysiloxane (G3) 320 °C
Rxi-17 50% Diphenyl/50% dimethyl polysiloxane 320 °C DB-17ms, VF-17ms,
CP Sil 24 CB
ZB-50
Rxi-17Sil MS Proprietary Phase 360 °C DB-17ms, VF-17ms,
CP Sil 24 CB
ZB-50
Rtx-65 65% Diphenyl/35% dimethyl polysiloxane (G17) 300 °C
Rxi-624Sil MS Proprietary Phase 320 °C DB-624 UI, VF-624ms, CP-Select 624 CB ZB-624
Rtx-1301,
Rtx-624
6% Cyanopropylphenyl/94% dimethyl polysiloxane (G43) 280 °C
240 °C
DB-1301, DB-624,
CP-1301, VF-1301ms, VF-624ms
ZB-624
Rtx-1701 14% Cyanopropylphenyl/86% dimethyl polysiloxane (G46) 280 °C DB-1701, VF-1701ms, CP Sil 19 CB,
VF-1701 Pesticides, DB-1701R
ZB-1701, ZB-1701P
Rtx-200 Trifluoropropyl methyl polysiloxane (G6) 360 °C DB-200, VF-200ms, DB-210
Rtx-200ms Trifluoropropyl methyl polysiloxane (G6) 340 °C DB-200, VF-200ms, DB-210
Rtx-225 50% Cyanopropyl methyl/50% phenylmethyl polysiloxane (G7, G19) 240 °C DB-225ms,
CP Sil 43 CB
Rtx-440 Proprietary Phase 340 °C RESTEK INNOVATION
Rtx-2330 90% Biscyanopropyl/10% cyanopropylphenyl polysiloxane (G48) 275 °C VF-23ms
Rt-2560 Biscyanopropyl polysiloxane 250 °C HP-88, CP Sil 88
Rtx-Wax Polyethylene glycol (G14, G15, G16, G20, G39) 250 °C CP-Wax 52 CB,
DB-Wax, DB-WAX UI
ZB-WAX
Stabilwax Polyethylene glycol (G14, G15, G16, G20, G39) 260 °C HP-INNOWax,
VF-WaxMS
ZB-WAXPlus

* Maximum operating temperatures may vary with column film thickness.

Table III:  Application-specific phases designed for particular analyses.

Restek Applications Agilent Supelco Macherey-Nagel SGE Phenomenex
Rtx-Volatile Amine Volatile amines CP-VolAmine
Rtx-5Amine Amines CP-Sil 8 CB OPTIMA 5 Amine
Rtx-35Amine Amines RESTEK INNOVATION
Stabilwax-DB Amines CAM, CP WAX 51 Carbowax Amine FS-CW 20 M-AM
Stabilwax-DA Free fatty acids HP-FFAP,
DB-FFAP,
VF-DA,
CP WAX58 CB,
CP-FFAP CB
Nukol PERMABOND FFAP,
OPTIMA FFAP,
OPTIMA FFAP Plus
BP-21 ZB-FFAP
Chiral Columns
Rt-βDEXm,
Rt-βDEXsm,
Rt-βDEXse,
Rt-βDEXsp,
Rt-βDEXsa,
Rt-βDEXcst,
Rt-γDEXsa
Chiral compounds
Foods, Flavors, & Fragrances
Rt-2560 cis/trans FAMEs HP-88 SPB-2560
FAMEWAX Marine oils DB-FATWAX UI Omegawax
Rtx-65TG Triglycerides
Rxi-PAH Polycyclic aromatic hydrocarbons (PAHs) Agilent Select PAH
Petroleum & Petrochemical
Rxi-LAO Linear alpha olefin impurities RESTEK INNOVATION
Rt-Alumina BOND/CFC Chlorinated fluorocarbons (CFCs) RESTEK INNOVATION
Rt-Alumina BOND/MAPD Trace analysis of methyl acetylene, propadiene, and acetylene Select Al₂0₃ MAPD        
Rtx-DHA Detailed hydrocarbon analysis HP-PONA, DB-Petro, CP Sil PONA CB Petrocol DH BP1PONA
Rtx-2887,
MXT-2887
Hydrocarbons (ASTM D2887) DB-2887 Petrocol 2887,
Petrocol EX2887
D3606 Ethanol (ASTM D3606) RESTEK INNOVATION
Rt-TCEP Aromatics and oxygenates in gasoline CP-TCEP TCEP
MXT-1HT SimDist Simulated distillation DB-HT-SimDis,
CP-SimDist,
CP-SimDist Ultimetal
BPX1 ZB-1XT SimDist
Rtx-Biodiesel TG,
MXT-Biodiesel TG
Triglycerides in biodiesel Biodiesel,
Select Biodiesel
OPTIMA Biodiesel ZB-Bioethanol
Clinical/Forensic
Rtx-BAC Plus 1 Blood alcohol testing DB-ALC1 ZB-BAC1
Rtx-BAC Plus 2 Blood alcohol testing DB-ALC2 ZB-BAC2
Pharmaceutical
Rxi-624Sil MS (G43) Organic volatile impurities (USP <467>) DB-624,
VF-624ms,
CP-Select 624 CB
OPTIMA 624 LB BP624 ZB-624
Rtx-5 (G27) Organic volatile impurities (USP <467>) HP-5, DB-5,
CP Sil 8 CB
SPB-5 OPTIMA 5 BP5 ZB-5
Stabilwax (G16) Organic volatile impurities (USP <467>) HP-INNOWax,
CP Wax 52 CB,
VF-WAX MS
Supelcowax-10 OPTIMA WAXplus ZB-WAXplus
Environmental
Rxi-SVOCms Semivolatiles, PAHs DB-UI 8270D       ZB-SemiVolatiles
Rxi-5Sil MS Semivolatiles, PAHs DB-5ms,
DB-5msUI,
VF-5ms,
CP-Sil 8 CB
SLB-5ms OPTIMA 5MS Accent BPX5 ZB-5msi
Rtx-VMS Volatiles (EPA Methods 8260, 624, 524) RESTEK INNOVATION
Rxi-624Sil MS Volatiles (EPA Methods 624) DB-624,
VF-624ms,
CP-Select 624 CB
OPTIMA 624 LB BP624 ZB-624
Rtx-502.2 Volatiles (EPA Methods 8010, 8020, 502.2, 601, 602) DB-502.2 VOCOL
Rtx-Volatiles Volatiles (EPA Methods 8010, 8020, 502.2, 601, 602) VOCOL
Rtx-VRX Volatiles (EPA Methods 8010, 8020, 502.2, 601, 602) DB-VRX
Rtx-CLPesticides Organochlorine pesticides RESTEK INNOVATION
Rtx-CLPesticides2 Organochlorine pesticides RESTEK INNOVATION
Rtx-1614 Brominated flame retardants RESTEK INNOVATION
Rtx-PCB Polychlorinated biphenyl (PCB) congeners RESTEK INNOVATION
Rxi-XLB Polychlorinated biphenyl (PCB) congeners DB-XLB,VF-XMS MR1, ZB-XLB
Rtx-OPPesticides Organophosphorus pesticides RESTEK INNOVATION
Rtx-OPPesticides2 Organophosphorus pesticides RESTEK INNOVATION
Rtx-Dioxin2 Dioxins and furans RESTEK INNOVATION
Rtx-Mineral Oil DIN EN ISO 9377-2 Select Mineral Oil

Select Column Film Thickness and Column ID Based on Retention Factor

resolution equation

Once you have chosen the stationary phase, you need to determine which column film thickness and inner diameter combination will give the retention factor (k) needed for optimal resolution and speed. Retention factor is sometimes referred to as “capacity factor,” which should not be confused with sample loading capacity.

The retention factor (k) of a column is based on the time an analyte spends in the stationary phase rela- tive to the time it spends in the carrier gas. As a general rule, the thicker the film and the smaller the inner diameter, the more an analyte will be retained. Note that as temperature increases k decreases, so at higher temperatures analytes stay in the carrier gas longer and are less retained.

In practice, if the value of k is too large, the peak will broaden, which can reduce resolution by causing peaks to overlap or coelute. Narrow, symmetrical peaks are important to maximizing resolution, so the goal is to select a column with a sufficient retention factor, such that resolution occurs, and peak shape does not suffer. Once the proper stationary phase is selected, column film thickness, column inner diam- eter, and elution temperature should be optimized to produce an acceptable retention factor.

Tech Tip

The sample loading capacity of the column must also be considered; if the mass of the target analyte exceeds the sample loading capacity of the column, loss of resolution, poor reproducibility, and fronting peaks will result. A larger ID column with thicker film is recommended for higher concentration samples, such as purity analysis, to minimize sample overload.

Film Thickness

Film thickness (μm) has a direct effect on both the retention of each sample component and the maximum operating temperature of the column. When analyzing extremely volatile compounds, a thick film column should be used to increase retention; more separation is achieved because the compounds spend more time in the stationary phase. If analyzing high molecular weight compounds, a thinner film column should be used as this reduces the length of time that the analytes stay in the column and minimizes phase bleed at higher elution temperatures. Use Figure 3 to select the best film thickness for your application. Note that as a general rule, the thicker the film, the lower the maximum temperature; exceeding the maximum temperature can result in column bleed and should be avoided.

Film Thickness

FILM THICKNESS

Figure 3: Characteristics and recommended applications based on film thickness.
 
Thin film characteristics

 

Tech Tip

Remember, when changing either film thickness and/or the temperature program, you must reconfirm peak identifications as elution order changes can occur.

Inner Diameter (ID)

Column ID does not have as great an effect on retention factor as film thickness does. However, when selecting column ID with retention factor (k) in mind, a general rule of thumb applies; smaller ID columns produce higher retention factors compared to larger ID columns. This is due to less available mobile phase (carrier gas) volume in the column. Because smaller ID columns produce higher k values, they are more suited towards complex sample analysis where a range of low to high molecular weight compounds may exist in the sample (Figure 4). Keep in mind that both ID and film thickness should be optimized together to produce the best resolution and peak shape.

Inner Diameter

INNER DIAMETER

Figure 4: Characteristics and recommended applications based on column inner diameter.
 
Inner diameter characteristics

 

Tech Tip

When choosing column ID, the injection technique is also important because the column ID may need to be selected based on whether a split, splitless, direct, cool on-column injection, or other sample transfer method is being used. For example, 0.53 mm ID columns are ideal for cool on-column injections since the syringe needle (26 gauge) will fit into the large column ID. In addition, the detector and its optimal flow rate must be considered. Some MS detectors can only operate under column flow rates of up to 1.5 mL/min; therefore, a 0.53 mm ID column, which requires higher flows for proper chromatography, is not an option for MS work.

Phase Ratio (β)

The relationship between column inner diameter and stationary phase film thickness is expressed as phase ratio (β). If a good separation has been achieved on a larger diameter column, and a faster anal- ysis is desired, this can often be accomplished by reducing the inner diameter of the column without sacrificing, and sometimes even improving, separation efficiency. To maintain a similar compound elution pattern when narrowing column inner diameter, film thickness must also be changed. By choosing a column with a similar phase ratio, it will be easier to translate your application to the new column. Phase ratios for common column dimensions are given in Table IV. As shown here, an analyst wanting to decrease analysis time could switch from a 0.32 mm x 0.50 µm column (β = 160) to a 0.25 mm x 0.25 μm column (β = 250) and obtain a very similar separation upon proper method translation. Importantly, column inner diameter and stationary phase film thickness show a combined effect when it comes to sample loading capacity, which is decreased as column inner diameter and film thickness are reduced. It may be necessary to inject a lower sample amount in this case.

Table IV: Phase ratio (β)* values for common column dimensions. To maintain similar separations, choose columns with similar phase ratios when changing to a column with a different inner diameter or film thickness.

Column ID Film Thickness (df)
0.10 µm 0.25 µm 0.50 µm 1.0 µm 1.5 µm 3.0 µm 5.0 µm
0.18 mm 450 180 90 45 30 15 9
0.25 mm 625 250 125 63 42 21 13
0.32 mm 800 320 160 80 53 27 16
0.53 mm 1325 530 265 128 88 43 27

*Phase ratio (β) = radius/2df (Note: Convert variables to the same units prior to calculation.)

Consider Efficiency when Choosing Column Length, Column ID, and Carrier Gas

resolution equation

Column Length

Capillary GC columns are made in various lengths, typically 10, 15, 30, 60, and 105 meters, depending on the inner diameter. Longer columns provide more resolving power than shorter columns of the same inner diameter, but they also increase analysis time and should be used only for applications demanding the utmost in separation power. Column length should only be considered once the stationary phase has been determined. This is because the separation factor has the greatest effect on resolution and it is maximized through proper stationary phase choice for the compounds of interest. Doubling the column length (e.g., 30 m to 60 m) increases resolution by approximately 40%, while analysis time can be twice as long. In addition, longer columns cost more. Conversely, if a separation can be performed on a shorter column (e.g., 15 m versus 30 m), then both analysis time and column cost will be less. Figure 5 summarizes the characteristics and general application parameters for a range of typical column lengths.

Length

LENGTH

Figure 5: Characteristics and recommended applications based on column length.
 
Length characteristics

 

Inner Diameter (ID)

Compared to larger ID columns, smaller ID columns generate more plates per meter and sharper peaks, leading to better separation efficiencies. When more complex samples need to be analyzed, smaller ID columns can produce better separation of closely eluting peaks than larger ID columns. However, sample loading capacities are lower for smaller ID columns. Smaller ID columns, especially those at 0.18 mm and less, demand highly efficient injection techniques so that the column efficiency is not lost at the point of sample introduction. Column characteristics based on ID are presented in Table V.

Generally speaking, a 0.25 mm column will produce the most efficient sample analysis while simultaneously considering analysis time and sample loading capacity. For these reasons, in combination with its relatively low outlet flow, it is also the best column choice for GC-MS work.

Table V: General column characteristics based on ID.

Characteristic Column Inner Diameter (mm)
0.10 0.15 0.18 0.25 0.32 0.53
Nitrogen flow (mL/min) 0.2 0.3 0.3 0.4 0.6 0.9
Helium flow (mL/min) 0.6 0.8 1.0 1.4 1.8 3.0
Hydrogen flow (mL/min) 0.7 1.1 1.8 1.8 2.3 3.7
Sample loading capacity (ng) 2.5 10 20 50 125 500
Theoretical plates/meter 11,000 7000 6000 4000 3000 2000

Note: Flows listed are for maximum efficiency. Sample loading capacities are estimates only. Actual sample loading capacity varies with film thickness and analyte.

Tech Tip

When changing carrier gas flow rates, you must reconfirm peak identifications as elution order changes can occur.

Carrier Gas Type and Linear Velocity

Carrier gas choice and linear velocity significantly affect column separation efficiency, which is best illustrated using van Deemter plots (Figure 6). The optimum linear velocity for each gas is at the lowest point on the curve, where plate height (H) is minimized, and efficiency is maximized. As seen in Figure 6, the optimum linear velocities differ among common carrier gases.

Nitrogen provides the best efficiency; however, the steepness of its van Deemter plot on each side of optimum means that small changes in linear velocity can result in large negative changes in efficiency. Compared to nitrogen, helium has a wider range for optimal linear velocity, but offers slightly less efficiency. In addition, because of its optimum velocity being faster, analysis times with helium are about half those when using nitrogen, and there is only a small sacrifice in efficiency when velocity changes slightly. Of the three common carrier gases, hydrogen has the flattest van Deemter curve, which results in the shortest analysis times and the widest range of average linear velocity over which high efficiency is obtained.

Regardless of the type of gas used, the carrier gas head pressure is constant during column temperature programming, whereas the average linear velocity decreases during the run. For constant pressure work then, the optimal linear velocity should be set for the most critical separations. More common today, electronic pneumatic control (EPC) of carrier gas allows for constant flow or even constant linear velocity, which helps maintain high efficiency throughout a temperature programmed run.

Another consideration for carrier gas type that is important, even if not directly related to column efficiency, is whether a mass spectrometer (MS) is used as a vacuum-outlet detector for GC. In almost all cases, helium is the carrier gas of choice, not only for its chromatographic efficiency but also because it is easier to pump than hydrogen. Hydrogen can be reactive in MS sources, leading to undesirable spectrum changes for some compounds. Nitrogen is typically not a carrier gas option for GC-MS, as it severely reduces sensitivity.

Figure 6: Operating carrier gas at the optimum linear velocity will maximize efficiency at a given temperature. Red circles indicate optimum linear velocities for each carrier gas.

van Deemter Plot

GC Troubleshooting Tips

Basic Steps

Follow these basic troubleshooting steps to isolate problems related to the sample, injector, detector, and column. Check the obvious explanations first and change only one thing at a time until you identify and resolve the problem.

Check the Obvious:

  • Power supply
  • Electrical connections
  • Signal connections
  • Gas purity
  • Gas flows
  • Temperature settings
  • Syringe condition
  • Sample preparation
  • Analytical conditions


Identify the Cause:

  • Define the problem clearly; for example, “Over the last 4 days, only the phenols in my sample have been tailing.”
  • Review sample and maintenance records to identify trends in the data or problem indicators, such as area counts decreasing over time or injector maintenance not being performed as scheduled.
  • Use a logical sequence of steps to isolate possible causes.
 

Document Work and Verify System Performance:

  • Document all troubleshooting steps and results; this may help you identify and solve the next problem faster.
  • Always inject a test mix and compare it to previous data to ensure restored performance.

Example Troubleshooting Sequence

An analyst observed that no peaks appeared during a GC-FID analysis. The flowchart below shows a logical progression of steps that can be used to identify the cause and correct the problem.

decorative

 

Symptoms and Solutions

Good chromatography is critical to obtaining accurate, reproducible results. Coelutions, asymmetric peaks, baseline noise, and other issues are common challenges in the GC laboratory. These analytical problems and others can be overcome by troubleshooting your separations using the tips below.

Poor Resolution

Poor Resolution Cause Solutions
Non selective stationary phase
  • Choose appropriate stationary phase and column dimensions
Poor efficiency
  • Optimize carrier gas linear velocity and GC oven temperature program.
Sample overload
  • Adjust sample concentration or amount on column.
Incorrect analytical conditions used
  • Verify temperature program, flow rates, and column parameters.

 

Poor Retention Time Reproducibility

Poor Retention Time Reproducibility Cause Solutions
Leaks
  • Leak check injector and press-fit connections.
  • Replace critical seals (i.e., septa, O-rings, inlet disc, etc.).
Analyte adsorption
  • Maintain inlet liner and GC column.
  • Use properly deactivated liners, seals, and columns.
Resolution/integration issues
  • Avoid sample overload.
Incorrect column/oven temperature program
  • Verify column temperature and oven temperature program.
Incorrect or variable carrier gas flow rate/linear velocity
  • Verify the carrier gas flow and linear velocity.
  • Repair or replace parts if necessary.
Poor control of oven temperature programming
  • Confirm GC oven program falls within instrument manufacturer’s recommendation.
Incorrect oven equilibration time
  • Extend GC oven equilibration time.
If manual injection, delay between pushing start and actual injection
  • Use autosampler or standardize manual injection procedure.

 

Fronting Peaks

Fronting peaks Cause Solutions
Incompatible stationary phase
  • Choose appropriate stationary phase
Column overloading
  • Reduce amount injected, dilute sample.
  • Increase column inner diameter and/or film thickness.

 

Tailing Peaks

Tailing peaks Cause Solutions
Adsorption due to surface activity or contamination
  • Use properly cleaned and deactivated liner, seal, and column.
  • Trim inlet end of column.
  • Replace column if damaged.
Adsorption due to chemical composition of compound
  • Derivatize compound.
Leak in system
  • Check for leaks at all connections, replace critical seals if needed.
Installation issues
  • Minimize dead volume.
  • Verify that the column is cut properly (square).
  • Verify correct installation distances.

 

Split Peaks

Split peaks Cause Solutions
Mismatched solvent/stationary phase polarity
  • Adjust solvent or stationary phase to allow wetting.
Incomplete vaporization
  • Add surface area, such as wool, to the inlet liner to enhance vaporization.
  • Use proper injector temperature.
Sample loading capacity exceeded
  • Inject less sample (dilute, use split injection, reduce injection volume).
Fast autosampler injection into open liner
  • Use wool or slow injection speed.

 

Carryover/Ghost Peaks

Carryover/ghost peaks Cause Solutions
Contaminated syringe or rinse solvent
  • Replace rinse solvent.
  • Rinse or replace syringe.
Backflash (sample volume exceeds liner volume)
  • Inject a smaller amount.
  • Use a liner with a large internal diameter.
  • Increase head pressure (i.e., flowrate) to contain the vapor cloud.
  • Use slower injection rate.
  • Lower inlet temperature.
  • Increase split flow.
  • Use liner with packing.
  • Use pressure-pulse injection.
Last analysis ended too soon
  • Extend analysis time to allow all components and/or matrix interferences to elute.

 

High Bleed

High bleed Cause Solutions
Improper column conditioning
  • Increase conditioning time and/or temperature.
Contamination
  • Trim column and/or heat to maximum temperature to remove contaminants.
  • Replace carrier gas and/or detector gas filters.
  • Clean injector and detector.
Leak in system and oxidation of stationary phase
  • Check for oxygen leaks across the entire system and replace seals and/or filters.
  • Replace column.

 

Unstable Baseline (Spiking, Noise, Drift)

Unstable baseline Cause Solutions
Carrier gas leak or contamination
  • Leak check connections and replace seals if needed.
  • Replace carrier gas and/or detector gas filters.
Injector or detector contamination
  • Clean system and perform regular maintenance.
Column contamination or stationary phase bleed
  • Condition, trim, and rinse column.
Septum coring/bleed
  • Replace septum.
  • Inspect inlet liner for septa particles and replace liner if needed.
Loose cable or circuit board connections
  • Clean and repair electrical connections.
Variable carrier gas or detector gas flows
  • Verify flow rates are steady and reproducible; may need to replace or repair flow controller.
  • Leak check system.
Detector not ready
  • Allow enough time for detector temperatures and flows to equilibrate.

 

Response Variation

Response variation Cause Solutions
Sample issues
  • Check sample concentration.
  • Check sample preparation procedure.
  • Check sample decomposition/shelf life.
Syringe problems
  • Replace syringe.
  • Check autosampler operation.
Electronics
  • Verify signal settings and adjust if needed.
  • Repair or replace cables or boards.
Dirty or damaged detector
  • Perform detector maintenance or replace parts.
Flow/temperature settings wrong or variable
  • Verify steady flow rates and temperatures, then adjust settings and/or replace parts if needed.
Adsorption/reactivity
  • Remove contamination and use properly deactivated components.
Leaks
  • Check for leaks at all connections.
Change in sample introduction/injection method
  • Verify injection technique and change back to original technique.
  • Check that split ratio is correct.
  • Verify that purge time or splitless hold time is correct.

 

No Peaks

No peaks Cause Solutions
Injection problems
  • Plugged syringe; clean or replace syringe.
  • No sample; verify sample introduction.
  • Injecting into wrong injector; reset autosampler.
Broken column
  • Replace column.
Column installed into wrong injector or detector
  • Re-install column.
Detector problems
  • Signal not recorded; check detector cables and verify that detector is turned on.
  • Detector gas turned off or wrong flow rates; turn detector on and/or adjust flow rates.

 

Broad Peaks

Broad peaks Cause Solutions
High dead volume
  • Minimize dead volume in the GC system; verify proper column installation, proper connectors, proper liners, etc.
Low flow rates
  • Verify injector and detector flow rates and adjust if needed.
  • Verify make-up gas flow and adjust if needed.
Slow GC oven program
  • Increase GC oven programming rate.
Poor analyte/solvent focusing
  • Lower GC oven start temperature.
Column film is too thick
  • Reduce retention of compounds by decreasing film thickness and length.
Sample carryove
  • See Carryover/Ghost Peaks solutions.

 

Propel Method Development Forward with the Pro EZGC Chromatogram Modeler

This improved version of our popular Pro EZGC chromatogram modeler for polymer capillary columns is just as simple to use as the original, but it now offers advanced options for selecting phases, changing carrier gases and control parameters, further optimizing your results, and much more! Already a favorite of analysts around the world, the updated software helps you develop new methods or optimize existing ones more efficiently and effectively than ever before.

In just seconds, you can generate a customized, interactive model chromatogram that matches real-world chromatograms with exceptional accuracy. Zoom in, view chemical structures, and even overlay the mass spectra of coeluting compounds.

decorative

Pro EZGC Chromatogram Modeler

YOU NEED: To develop a method from scratch, including the column
and conditions.

YOU HAVE: An analyte list (and you may have a column in mind, too).

YOU GET: Customized, interactive model chromatograms that provide a specific phase, column dimensions and conditions. You can change columns, modify conditions, zoom in, view chemical structures, and even overlay mass spectra of coeluting compounds.

Watch our instructional videos and get started today at www.restek.com/proezgc

 

Modify Methods Quickly and with Confidence Using the EZGC Method Translator and Flow Calculator

The EZGC method translator and flow calculator tool makes it simple to switch carrier gases, change column dimensions or control parameters, or to optimize a method for speed or efficiency. Simply enter your method specifications, and the program will return a full set of calculated method conditions that will provide similar chromatography. Use the EZGC method translator and flow calculator tool to optimize your analysis for speed so you can increase sample throughput!

decorative

EZGC Method Translator and Flow Calculator

YOU NEED: To switch carrier gases, to change column dimensions or control parameters, or to optimize a method for speed or efficiency.

YOU HAVE: An existing method.

YOU GET: A full set of calculated method conditions that will provide similar chromatography. Results include oven program and run time as well as average velocity, flow rate, splitless valve time, and other control parameters—all in an easy-to-use, single-screen interface with seamless transfer between tools.

Start saving time today—develop, optimize, or translate methods quickly and with confidence using Restek’s EZGC online software suite! www.restek.com/ezgc-mtfc

GNAR1724C-UNV