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Guide to GC Column Selection and Optimizing Separations

  • 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.

diagram-article-GNAR1724A-01.jpg
 

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

figure-article-GNAR1724A-01.jpg
 

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).
For additional help search our chromatogram database at www.restek.com or
 
equation-article-GNAR1724A-01.jpg
 

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

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

Figure 2: 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, 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

DB-Wax, Wax 52 CB

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-bDEXm, Rt-bDEXsm,
Rt-bDEXse, Rt-bDEXsp,
Rt-bDEXsa, Rt-bDEXcst,
Rt-gDEXsa

Chiral compounds

Foods, Flavors, & Fragrances

Rt-2560

cis/trans FAMEs

HP-88

SPB-2560

FAMEWAX

Marine oils

Select FAME

Omegawax

Rxi-65TG

Triglycerides

Rxi-PAH

Polycyclic aromatic hydrocarbons (PAHs)

Agilent Select PAH

Petroleum & Petrochemical

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

Rtx-G27 w/IntegraGuard

Organic volatile impurities (USP <467>)

Rtx-G43 w/IntegraGuard

Organic volatile impurities (USP <467>)

Rxi-624Sil MS

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-5Sil MS

Semivolatiles

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

 

Rxi-17Sil MS

Polycyclic aromatic hydrocarbons (PAHs)

DB-17ms,VF-17ms, CP-Sil 24 CB

OPTIMA 17 MS

BPX50

ZB-50

Rtx-Mineral Oil

DIN EN ISO 9377-2

Select Mineral Oil

 
equation-article-GNAR1724A-02.jpg
 

Select Column Film Thickness and Column ID Based on Retention Factor

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

Figure 3: Characteristics and recommended applications based on film thickness.
 
figure-article-GNAR1724A-03b.jpg
 

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

Figure 4: Characteristics and recommended applications based on column inner diameter.
 
figure-article-GNAR1724A-04b.jpg
 

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.

  Film Thickness (df)

Column ID

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

 
equation-article-GNAR1724A-03.jpg
 

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

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

Figure 5: Characteristics and recommended applications based on column length.
 
figure-article-GNAR1724A-05b.jpg
 

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.

 

Column Inner Diameter (mm)

Characteristic

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

1.3

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.

figure-article-GNAR1724A-06.jpg

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.

 

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

Causes

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

 

Causes

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

Causes

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

 

Causes

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

 

Causes

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

Causes

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

Causes

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)

 

 

 

 

Causes

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

 

 

 

 

 

Causes

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

Causes

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

 

Causes

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 carryover

  • 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.

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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).

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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!

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

 

GNAR1724B-UNV

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