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Restek Capillary Column Installation Guide - Section II: In-depth Installation Information

Restek Capillary Column Installation Guide - Section II: In-depth Installation Information

The following section provides in-depth information on instrument preparation procedures for installing and operating fused silica and stainless steel capillary columns.

  1. Instrument preparation
    1. Gas purification
    2. Carrier gas selection
    3. Using hydrogen as a carrier gas
    4. Flow regulated/back pressure pneumatic systems
    5. Head pressure regulated pneumatic systems
    6. Injector maintenance
    7. Protection against dirty samples
    8. Replacing critical seals
    9. Changing septa
    10. Setting gas flow rates
  2. Column Mounting and Installation
    1. Installation preparation
    2. Inlet installation
    3. Establishing flow
    4. Good operating practice
    5. Outlet installation
    6. Leak-checking techniques
  3. Setting Optimum Flow Rates
  4. Confirming Installation Integrity
    1. Dead volume peak shape test
    2. Solvent peak shape test
  5. Conditioning
  6. Test Mixtures


I. Instrument Preparation


Gas Purification
Make sure the carrier gas is clean. The carrier gas MUST contain less than 1ppm of oxygen, moisture, or any other trace contaminants to prevent column degradation, decreased column lifetime, and increased stationary phase bleed. Contaminants such as trace hydrocarbons cause ghost peaks to appear during temperature programming and compromise the validity of analytical data. Make-up gas should also be contaminant-free or baseline fluctuations and excessive detector noise can occur. Detector gases, such as hydrogen and compressed air, should be free of water and hydrocarbons or excessive baseline noise can result.

We highly recommend installing a High Capacity Indicating Oxygen Trap on both the carrier and make-up gas lines. Because moisture reacts with most oxygen traps, we suggest installing a Moisture Trap in front of the oxygen trap. (Install purifiers as closely as possible to the GC's bulkhead fitting, not system-wide. If purifiers are installed system-wide, a leaky fitting downstream of the trap could allow oxygen and moisture to enter the gas stream and degrade column performance.) The Moisture Trap can also be used on the FID air line or the ECD make-up gas line to eliminate noisy, rolling baselines when operating at high detector sensitivities. If hydrocarbon contamination is suspected, install a Hydrocarbon Trap between the moisture and oxygen traps. To prevent spontaneous breakage, coil the line leading to and from the purifiers to relieve strain and isolate instrument vibrations.

A note on equipment: Because oxygen, moisture, and elastomeric contaminants can migrate through rubber or elastomeric diaphragms and enter the carrier gas, all regulators should be equipped with stainless steel diaphragms.
Traps shown:

A. Moisture Trap:
Super-Clean Ultra-High Capacity Moisture Filter (cat.# 22028)

B. Hydrocarbon Trap:
Super-Clean Ultra-High Capacity Hydrocarbon Filter (cat.# 22030)

C. High Capacity Indicating Oxygen Trap:
Super-Clean Ultra-High Capacity Oxygen Filter (cat.# 22029)

Carrier Gas Selection

A fast carrier gas that exhibits a flat van Deemter profile is essential in obtaining optimum capillary column performance. Because capillary columns average over 30 meters in length (compared to 2 meters for packed columns), a carrier gas that minimizes the effect of dead time is important. In addition, because capillary columns are head pressure controlled, not flow-controlled like most packed columns, the carrier gas flow decreases by 40 percent when programming from ambient to 300°C. Therefore, a carrier gas that retains high efficiency over a wide range of flow rates is essential for obtaining good resolution throughout a temperature-programmed analysis.

Figure A shows the van Deemter profile for hydrogen, helium, and nitrogen carrier gases. The curves were generated by plotting the Height Equivalent to a Theoretical Plate (H.E.T.P., the length of the column divided by the total number of theoretical plates) against the column's average linear velocity. The lowest point on the curve indicates the carrier gas velocity at which the highest column efficiency is reached.




Hydrogen is the fastest carrier gas (uopt: 40cm/sec.) and exhibits the flattest van Deemter profile. Helium is the next best choice (uopt: 20cm/sec.). The head pressures at optimum flow rates are similar for hydrogen and helium because hydrogen has half the viscosity and double the linear velocity of helium. Nitrogen's performance is inferior for capillary columns and is usually not recommended because of the slow optimum linear velocity (uopt: 12cm/sec.) and steep van Deemter profile.

Figure B compares hydrogen to helium in an isothermal analysis using optimum linear velocities. The resolution is similar, but the analysis time is reduced by 50 percent when using hydrogen. However, helium does improve the separation of very low boiling or early eluting compounds since they allow more interaction with the stationary phase. Temperature programming usually provides similar analysis times between hydrogen and helium since the elution of most compounds strongly depends on the oven temperature. Therefore, the savings in analysis times are not as noticeable as when using isothermal oven conditions.

Figure C illustrates that hydrogen is only slightly faster than helium when both carrier gases are operated under the same temperature-programmed conditions. Also, note that helium improves the resolution of the early eluting compounds (peaks 1 & 2).

Exert Caution when using Hydrogen as a Carrier Gas

Hydrogen is explosive when concentrations exceed 4 percent in air and should only be used by individuals who have received proper training and understand the potential hazards. Proper safety precautions should be taken to prevent an explosion in the oven chamber. Some gas chromatographs are designed with spring loaded doors, perforated or corrugated metal oven chambers, and back pressure/flow controlled pneumatics which minimize the hazards when using hydrogen carrier gas. Additional precautions include:

  • Frequently checking for leaks using a thermal conductivity leak detector (cat.# 22655).
  • Minimizing the amount of carrier gas that could be expelled in the oven chamber if a leak were to occur by installing a needle valve, restrictor, or flow controller prior to the carrier inlet bulkhead fitting (only necessary for head pressure controlled systems).
  • Purging an inert gas (N2) into the oven chamber to displace oxygen and prevent an explosive atmosphere from forming.

Hydrogen is expelled from both the split vent and septum purge when it is used as a carrier gas. Because of hydrogen's fast diffusivity, an explosion in a laboratory setting is highly unlikely. However, a spark from static electricity can ignite the hydrogen exiting from a septum purge or split vent which could cause a flame. Precautions to minimize the problems with hydrogen exiting the split vent or septum purge include:

  • Plumbing the exit lines to a hood or venting the escaping gas outside.
  • Plumbing the lines to exit into a vial of water.
  • Plumbing the exit lines to a position where analysts could not get burned if inadvertent ignition occurred.

Flow Regulated/Back Pressure Pneumatic Systems

Figure D illustrates a flow regulated/back pressure system commonly used for split/splitless inlets (HP-5890, Varian 3300, 3500, & 3600 GCs). A flow controller positioned upstream of the injector controls the total amount of carrier gas that is expelled from the split vent, septum purge, and column (knob labeled "Total Flow" on HP 5890 GCs). The back pressure regulator (knob labeled "Column Head Pressure" on HP 5890 GCs) stops or reduces the flow from exiting the split vent until the desired column head pressure is reached. The flow controller provides the increase of pressure necessary to meet the requirements of the back pressure regulator. It is the back pressure regulator, located downstream of the split point, that actually controls the capillary column flow rate. One of the primary benefits of a flow controlled/back pressure regulated system is that adjustments to the capillary column flow rate (via head pressure changes) do not affect the amount of carrier gas exiting the splitter vent. Once the desired split vent flow rate is achieved, the flow controller setting should not have to be changed when installing different columns.




Flow regulated/back pressure systems prevent a drastic carrier gas loss that can happen if an inlet fitting or column leak occurs. Leaks are indicated by a failure to obtain the proper operating pressure with the capillary column. A common mistake is to increase the total system flow by turning the flow controller up when a proper head pressure cannot be obtained rather than checking for inlet leaks.

Head Pressure Regulated Pneumatic Systems

Figure E illustrates a head pressure regulated inlet system used in some split/splitless inlet systems (PE Autosystem, 8500, and Shimadzu 9A and 14A GCs). A single-stage pressure regulator is used to control the flow rate in the capillary column by increasing or decreasing the upstream inlet pressure. The split vent and septum purge flow rates are controlled by a needle valve or variable restrictor located downstream of the pressure regulator. Head pressure systems require adjustment of the needle valve controlling the septum purge or split vent every time a change is made in the column's head pressure.




We recommend placing a throttling valve (needle valve or restrictor) on the carrier gas inlet bulkhead fitting of pressure-regulated systems to prevent a catastrophic carrier gas loss should an inlet leak occur. If several GCs are attached to a common carrier gas source, a leak in one GC could drain the carrier gas from all other GCs, causing a loss of flow and subsequent damage to all capillary columns in the entire system. To prevent this from happening, limit the flow of carrier gas to each gas chromatograph (via a throttling valve) until it matches the flow requirements of the inlet system. This can be detected when the column's head pressure starts to decrease if the throttling valve is closed any further.

Injector Maintenance

Perform injector maintenance prior to installing a capillary column. Periodic maintenance is required after installation, depending on the number of injections and the cleanliness of the samples. Maintenance includes cleaning and deactivating inlet sleeves, replacing critical inlet seals, and replacing the septum. Review the instrument manual inlet diagram prior to disassembly.

Cleaning and Deactivating Injector Sleeves
Don't install a new Restek column with a dirty injector sleeve! For optimum column performance, the inlet sleeve needs to be free of septum particles, sample residue, and ferrule fragments. Use deactivated inlet sleeves when analyzing samples with active functional groups or compounds prone to decomposition or adsorption onto untreated glass surfaces.

If the sleeve is deactivated and not excessively dirty, cleaning with organic solvents will not affect the integrity of the deactivation layer. First, use non-swelling organic solvents such as methanol or isopropyl alcohol to remove septum particles that adhere to the sleeve wall. Next, use solvents such as pentane, methylene chloride, or toluene to remove sample residue. Nylon tube brushes and pipe cleaners (cat.# 20108) are ideal for cleaning sleeves. Do not use laboratory detergents, acids, or bases to clean sleeves because they will remove the deactivation layer and require sleeve resilanization. Sleeves that are very dirty or contain pyrolyzed residue can be difficult to clean. Heating borosilicate or quartz sleeves overnight in a muffle furnace at 550°C will remove most contaminants. Etching with a 1:1:1 mixture of hydrofluoric acid, sulfuric acid, and deionized water for ten seconds is also very effective. However, heating to 550°C or using the acid-etch procedure will remove the deactivation layer and require sleeve resilanization. Caution - Exert extreme caution when using hydrofluoric acid. Hydrofluoric acid can cause severe burns and nerve damage if it is ingested, inhaled, or brought into contact with the skin. Only properly trained professionals equipped with the appropriate safety devices should attempt to handle strong acids.

Most new sleeves received from instrument manufacturers are not deactivated. Before optimum column performance can be achieved, sleeves must be deactivated. Restek has developed a unique, high temperature silanization procedure for sleeve deactivation that ensures a highly inert surface. These sleeves can be purchased from Restek at a cost below many instrument manufacturer's cost.

Refer to Restek's Chromatography Products Catalog to obtain the part number for your specific instrument. For more information on inlet sleeves, request a copy of Restek's bulletin Operating Hints for Split/Splitless Injectors or Guide to Direct/On-column Flash Vaporization Injection.

Protection Against Dirty Samples

Sleeve packing materials such as fused silica wool (cat.# 20790), glass wool (cat.# 20789), or fused silica beads (cat.# 20791) act as filters when analyzing samples containing high molecular weight residue or particulates. However, both wool and beads greatly increase the surface area that the sample contacts and can be a source of adsorption or breakdown. It is critical that the wool or beads be properly deactivated. If you plan to use wool, be careful inserting it into the sleeve because active sites can be created as the fibers break. We do not recommend using packings coated with stationary phases. Alternative sleeve designs that minimize sample interaction with non-volatile residue are also available.

Replacing Critical Seals

Replace the critical seal prior to installing an inlet sleeve (see instrument manual for seal location). Most capillary injection ports use a rubber o-ring or graphite ferrule to seal the sleeve inside the injection port body. The seal must fit tightly around the sleeve to prevent the carrier gas from leaking around the outside of the sleeve. If your GC uses a ferrule as the inlet seal, always pre-swage the ferrule to fit the sleeve before tightening it in the inlet (especially Varian inlets).



Changing Septa

Always use a high quality, low bleed septa. We recommend replacing the septum frequently to prevent leaks and fragmentation. Otherwise, multiple injections and continuous exposure to a hot injection port will decompose the septum, causing particles to fall into the sleeve. Septum particles are a potential source of ghost peaks, loss of inertness, and carrier gas flow occlusion as shown in Figure F. It is best to install a new septum at the end of an analytical sequence so that it can condition in the injector and reduce the incidence of ghost peaks. Always use clean forceps when handling septa to avoid contamination.

Setting Detector and Make-up Gas Flow Rates

Confirm that the make-up gas, detector fuel, and oxidant flow rates are set according to the instrument's specifications (Table I, below). Make-up gas flow rates set too low will cause tailing solvent peaks, baseline disturbances, decreased sensitivity, and detector noise. Some instruments do not have leak-tight detector cavities and require flow rate verification before the column is installed into the detector. However, for GCs with leak-tight detector cavities, it is usually easier to check detector and make-up gas flow rates after the column is installed.


Table I Typical FID Flow Rates


InstrumentH2 (ml/min.)Make-up (ml/min.)Air (ml/min.)
Hewlett-Packard 3020400
Varian 3020300
Shimadzu 30-6040500
PerkinElmer 45--450


II. Column Mounting and Installation


Do not remove the high-temperature string holding the fused silica capillary column inside the Restek cage. The high-temperature string is capable of withstanding more than 400°C and does not decompose during continuous operation at oven temperatures used in capillary chromatography.

When hanging the column on the oven support rod, be careful that fused silica tubing does not contact any metal parts. Stainless steel columns can be placed directly on the oven support rod. If there is not an oven support rod, one can be made by inserting a temperature resistant pegboard hook into the corrugated oven wall or by hanging a 1/16-inch "S" hook from the oven ceiling. Be careful not to damage the oven thermocouple or interfere with the fan operation when installing homemade brackets.

Position the column so that it is midway between the injector and detector. This reduces thermal gradients and enhances retention time reproducibility. Uncoil one or two loops of tubing. When using fused silica columns, be careful not to scratch the column surface against the metal cross bars when removing loops. This abrasion of the polyimide coating could lead to spontaneous breakage.

Caution - When removing loops from 0.53mm ID columns, pull the tubing from the cage at the point with the widest gap between the metal crossbars. Avoid sharp bends that will break the tubing.

Choosing Ferrules

Graphite or Vespel/graphite ferrules are used to seal the column to the injector and detector in capillary gas chromatography. Both ferrule types have advantages and disadvantages. Graphite ferrules are the easiest to use, and they are leak-free, universal for most systems, and preferred by most beginning capillary chromatographers. Because graphite ferrules are soft, they easily conform to column outside diameters and different types of instrument fittings. However, they can flake or fragment upon removal, causing particles to lodge in the injector or detector sleeves, and they will not hold a seal under vacuum. Vespel/graphite ferrules are hard and they must match the column and fitting dimensions closely to seal properly. In addition, because Vespel/graphite ferrules can deform during initial heating, they need to be re-tightened or leakage will occur. Vespel/graphite ferrules do not fragment, can be reused many times, and are preferred by mass spectroscopists since they do not contaminate the ion source with particles and maintain their seal under vacuum. In all cases, it is best to choose a ferrule that fits snugly or is slightly larger than the capillary tubing OD (see table below). This minimizes the need for excessive torque to properly seal the ferrule to the column.


Nominal
Tubing ID
Nominal Tubing OD
MXT Fused Silica
   0.05mm --- ---    0.363mm    +/-0.012mm
   0.10mm    0.23mm    +/-0.0254mm    0.363mm    +/-0.012mm
   0.15mm    0.41mm    +/-0.0254mm    0.363mm    +/-0.012mm
   0.18mm    0.36mm    +/-0.0254mm    0.34mm    +/-0.01mm
   0.25mm    0.41mm    +/-0.0254mm    0.37mm    +/-0.04mm
   0.28mm    0.56mm    +/-0.0254mm --- ---
   0.32mm    0.41mm    +/-0.0254mm    0.45mm    +/-0.04mm
   0.53mm    0.74mm    +/-0.0254mm    0.69mm    +/-0.05mm
   0.75mm    0.93mm    +/-0.0254mm --- ---



Installation Preparation

Cut each column end squarely, approximately 10 centimeters from the end seals. To obtain a square cut with fused silica columns, place the column end against the forefinger and score the polyimide layer lightly and rapidly with a sapphire scribe (cat.# 20115) or a ceramic scoring wafer (cat.# 20116). Score only one side of the column. Point the column end down to prevent polyimide or fused silica shards from falling inside, and quickly flick the column just above the score.



Proper and improper fused silica cuts.

Cut metal capillary tubing by scoring the tubing wall (without cutting completely through) with the edge of a sharp file or ceramic scoring wafer. Wipe any filings off the tubing and bend it away from the score. Once the score opens, bend the tubing in the opposite direction (toward the score) until it snaps into two pieces. If the hole is not round or there is a burr on the tubing, try the procedure again. The flat side of a ceramic scoring wafer can be used to polish or round the column end into a smooth conical shape. We do not recommend using high speed wheels or grinders to cut the metal tubing since they may introduce metal filings into the tubing or ruin the polymer near the cut from the high temperatures created.



Proper and improper MXT column cuts.

Next, install the nut and ferrule to the inlet in the manner described in the instrument manual. Use a pin vise and drill (cat.# 20103) to enlarge the ferrule ID if it does not slide easily onto the column. Prevent shards from falling into the column bore by pointing the column end down when installing the ferrule. Slide the connecting nut and ferrule approximately 20cm down the length of the column to make installation easier. Cut an additional 10cm from the column end after the nut and ferrule have been installed to remove any ferrule fragments that might have been forced into the column bore. Examine the quality of the cut with a small 10x pocket magnifier (cat.# 20124) and make sure that the cut is square. Jagged silica edges or exposed polyimide cause adsorption and tailing peaks, so it is very important that the column ends are cut uniformly. It may take several times, but once a square cut has been obtained, proceed with the installation. (Use an old column to practice making consistently square cuts.)

Inlet Installation

Consult the instrument manual to determine the correct insertion distance for the injector. It is important to install the column at the exact distance recommended by the injector manufacturer or poor peak symmetry and quantitation could occur. Lay the column end beside a ruler and position the nut and ferrule to the exact distance required for installation. Next, mark the insertion distance. This can be done one of three ways: with a pair of Slide-Lok Tweezers (cat.# 20101), with typewriter correction fluid, or by positioning a septum on the column to hold the nut and ferrule. If using a septum, slide it away from the nut after the fitting is tightened and leave it in the oven cavity while performing the analysis.

Gently insert the column end into the inlet fitting, making sure that the end is not crushed or scraped against the metal injection port fittings. While maintaining the correct distance, use a capillary wrench to tighten the nut approximately one-half turn past finger-tight until the column is held firmly. The ferrule is tight if the column cannot be pulled from the fitting while applying gentle pressure.


Various ways to mark the
insertion distance

Make sure the fused silica tubing is not sharply bent when installing the column (Figure G). The tubing should gently bend from the cage to the fitting in angles greater than 90° or in diameters greater than 15cm. Sharp bends weaken the fused silica and eventually cause spontaneous breakage during use. If the tubing cannot be positioned to avoid sharp bends, then repeat the installation process and uncoil the appropriate amount of tubing from the cage.



Establishing Flow

Turn the carrier gas on and set the column head pressure to the values indicated in Table II*. These values represent approximate head pressures and flow rates. The exact optimum pressures and flow rates for a particular column will be set at a later time.

Table II Approximate Column Head Pressure (He or H2 carrier gas)


length (m) 0.18mm ID 0.25mm ID 0.32mm ID 0.53mm ID
15 6psig 3psig 2psig
20 14psig
30 12psig 8psig 4psig
40 30psig
60 24psig 16psig 8psig
105 40psig 30psig 14psig
Septum Purge Flow: between 2 and 5 cc/min.

* If you are having difficulty establishing the appropriate column head pressure for back pressure regulated systems, then suspect septum or inlet ferrule leaks.

The split ratio is the amount of carrier gas exiting the split vent vs. the amount of carrier gas entering the capillary column. The split ratio should be adjusted so the sample amount reaching the column does not exceed the column's capacity. Typically, a split ratio of 50 to 1 is used. Table III lists common split vent flow rates found using hydrogen or helium carrier gases. Use the equation below to calculate the split ratio.

While the flow rate exiting the split vent is easy to measure with conventional bubble meters, the low flow rate exiting a capillary column can be difficult to measure. The following equation can be used to approximate the column flow rate.

where pi = 3.1459, column radius and length are in centimeters, and time is in minutes.

For example, a 30 meter x 0.53mm ID column operated at 20cm/sec. linear velocity with helium has a flow rate of 2.65cm3/min.

Table III Typical Split Vent Flow Rates (50 to 1 split ratio)

Carrier gas 0.18mm ID 0.25mm ID 0.32mm ID 0.53mm ID
helium 15cc/min. 35cc/min. 80cc/min. 125cc/min.
hydrogen 30cc/min. 70cc/min. 160cc/min. 250cc/min.

Safety Tip: Always use a split vent trap when injecting hazardous or carcinogenic chemicals into a split/splitless inlet system.

Good Operating Practice

Operating a column without carrier gas flow causes irreparable damage to the stationary phase. Confirm flow by inserting the column outlet into a vial of solvent such as acetone or isopropyl alcohol prior to installing it into the detector. The appearance of bubbles at the column outlet confirms carrier gas flow. Allow the column to purge with carrier gas for fifteen minutes before installing the column outlet into the detector to remove any room air that may have diffused inside the column.

Outlet Installation

Install the nut and ferrule to the detector in the manner described in the instrument manual. Gently insert the column end into the outlet fitting making sure that it is not crushed or scraped against the metal detector parts. Regardless of the GC manufacturer, a higher degree of inertness and better peak symmetry results if the column end can be installed 1 to 3mm from the detector jet orifice. Be careful not to push the column beyond the jet orifice or the column end will burn in the flame. Some jets are too narrow to insert the column close to the jet orifice. If this is the case, pull the column end approximately 2mm away from the narrowed point to prevent flow occlusion or loss of inertness. While maintaining the correct insertion distance, use a capillary wrench to tighten the nut approximately one-half turn past finger-tight until the column is held firmly. The ferrule is tight when the column cannot be pulled from the fitting while applying gentle pressure.

Note — Be cautious when using stainless steel or aluminum-clad columns in gas chromatographs or GC/MS systems with electrically energized detector jets or orifices. These columns will conduct electricity and cause a short if the end of the column is installed too far into the energized detector. Always turn off the electrometer with Varian, PerkinElmer, and Shimadzu FIDs (since the detector is not grounded) when installing stainless steel or aluminum clad columns.

Leak-Checking Techniques

The best way to leak-check a capillary column system is to use a thermal conductivity leak detector (cat.# 22655)*. These portable devices detect minute traces of helium or hydrogen carrier gas without contaminating the system. Leaks in mass spectrometers can easily be determined by monitoring for Mass 28 (N2) or 32 (O2).*

Never use liquid leak detectors that contain soaps or surfactants in capillary chromatography. Leaks draw these materials inside the system and contaminate the column, making high sensitivity operation difficult. In addition, liquid leak detectors can cause permanent damage to the capillary column by depolymerizing the silicone stationary phase.

Once the system is leak-free, set the injector and detector temperatures approximately 20°C above the final operating temperature of the analysis or at the column's maximum operating temperature. Then light or turn on the detector. Caution: Do NOT exceed the maximum operating temperature of the column.

* Spraying argon gas and monitoring for Mass 40 is also effective for mass spectrometers.


III. Setting Optimum Flow Rates

The most accurate and reproducible way to set the capillary column flow is by injecting a non-retained substance (see table IV) to determine the linear velocity (dead volume time) and adjusting the head pressure until the linear velocity is at its optimum value. Measuring the flow rate at the column outlet is not recommended because it does not account for column-to-column variations. Relying on head-pressure readings is not recommended due to instrument and column variations. Exact flow rate values for a particular column can only be determined after the linear velocity is set at its optimum value.

Because most capillary columns are operated in a pressure (not flow) controlled mode, the temperature at which the linear velocity is set is critical. To obtain the optimum performance, linear velocity should always be set at the operating temperature for an isothermal analysis. For a temperature-programmed analysis, the column's linear velocity should be optimized at an oven temperature where a hard to separate peak pair elutes. If there are no critical peak pairs, raise the oven temperature to the temperature reached midway through the programmed run. Always document which non-retained compound was used and the temperature at which the linear velocity was set in order to easily reproduce the analysis.

To set dead time, inject 2.0µl of a non-retained substance that is compatible with the detector (Table IV). Accurately mark the injection starting time and peak elution time with an electronic integrator.

Table IV


Detector Type Recommended Dead Volume Compound
FID/TCD CH4
NPD acetonitrile vapors
ECD methylene chloride vapors or air
ELCD dichlorodifluoromethane vapors
MS O2 or N2 (air)
PID ethylene or acetylene

The compounds listed above may be slightly retained on thick film phases (1.0 to 7.0µm) giving erroneous dead volume times. However, they are reproducible for similar column types on subsequent analyses.

Adjust the column head pressure until the correct dead time is obtained for the appropriate column length and carrier gas (Table V). Once the dead volume time has been finalized, check the split vent and septa purge flow to make sure they did not change significantly. (Head pressure regulated capillary systems require adjustment of the split vent flow if the pressure changed significantly. Back pressure regulated capillary systems should not require adjustment.)

The values in Table V were obtained using the formula for average linear velocity (u). The optimum u is 40cm/sec. for hydrogen, 20cm/sec. for helium, and 12cm/sec. for nitrogen*. Insert the appropriate values in the equation below to obtain the required dead volume time for column lengths not listed.

Table V Dead Volume Times (for commonly used capillary columns)


length (m) hydrogen helium
15 0.63 min./TCD 1.25 min.
30 1.25 min. 2.5 min.
60 2.5 min. 5.0 min.
105 4.38 min. 8.75 min.

* Nitrogen is not recommended as a carrier gas for most capillary columns because inadequate resolution and longer analysis times result.


IV. Confirming Installation Integrity

We highly recommend using the dead volume peak shape test and the solvent peak shape test to confirm installation integrity.

Dead Volume Peak Shape Test
Examine the dead volume peak. A sharp, narrow peak that shows no sign of tailing indicates an unobstructed sample pathway and correct installation (Figure H). Tailing peaks indicate improper column installation, gross contamination of the splitter sleeve, a cracked splitter sleeve, improper sweeping of the column end by make-up gas, a crushed column end, or a column that has degraded. The cause of a tailing non-retained peak must be corrected before using the column analytically.

Figure H: Dead volume peak shape test

Tailing peaks indicate
improper installation

 

Symmetrical Peaks indicate
proper installation

 


Solvent Peak Shape Test
The solvent peak shape test is an additional indicator of proper column installation in the inlet and outlet. Since compounds used to set the dead volume are usually gases at room temperature (methane), they are not extremely sensitive indicators of system or installation problems. A 1µl injection of a liquid solvent, such as methylene chloride, expands to over 500µl of gas volume, making any potential installation or system problem readily apparent. A tailing solvent peak is a sensitive indicator of broken, undeactivated, or contaminated inlet sleeves. Tailing solvents also indicate problems with inadequate make-up gas or improper column insertion into the detector.

To perform the test, inject 1µl of a solvent in the split mode at 40°C isothermal and examine the peak shape (Figure I). The solvent peak should be symmetrical and show minimal tailing. If tailing appears, suspect an installation or system problem. The cause of a tailing solvent peak must be corrected before using the column analytically.



V. Conditioning

Before conditioning a column at an elevated temperature, make sure there is proper flow, there are no leaks present, and there is an ample supply of oxygen-free carrier gas for the conditioning period. Conditioning at elevated temperatures without flow permanently damages or destroys the performance of the capillary column. Conditioning with an oxygen leak present causes the column to exhibit permanent high bleed and destroys its utility at high operating temperatures.

To condition the column, set the GC oven at 40°C, hold fifteen minutes, then program at 10°C/min. to the maximum operating temperature (see the test chromatogram included with the column). Alternatively, the column can be conditioned 25°C below the maximum operating temperature if it is going to be used at relatively low temperatures. Hold the column at this temperature for two hours or until the baseline stabilizes. Overnight conditioning is not necessary with Restek capillary columns operated at moderate detector sensitivities (approx. 8 x 10-11 AFS). Overnight conditioning is necessary when the column is going to be operated at high detector sensitivities (<4 x 10-11 AFS) and at oven temperatures close to the maximum operating temperature (see Figure J). Extra conditioning may be required if operating the column at high sensitivity (<1 x 10-11 AFS) or using thick films (>1µm). The column should not be installed in very sensitive or hard-to-clean detectors such as ECDs, NPDs, FPDs, PIDs, ELCDs, or Mass Spectrometers during the initial conditioning period. This practice is particularly important with very thick film columns (>3µm) which produce more stationary phase bleed. (Before conditioning thick film columns, cap the detector.) The Crossbond procedure used by Restek produces columns with very low bleed levels. If your column is experiencing higher bleed than shown on the test chromatogram, contact us immediately at 800-356-1688 (ext. 4).

Figure J: Overnight conditioning reduces column bleed


VI. Test Mixtures

Restek tests every column with a stringent test mix to determine that the column and GC systems are performing correctly. It is good analytical practice to run the test mixture before analyzing samples to assess system problems or chemical incompatibilities that may be present. It is also good practice to inject the test mix weekly to monitor column performance and to indicate when maintenance procedures are needed. Refer to the "Column Maintenance and Rinsing" section in this guide.

Inject a column test mixture according to the test chromatogram conditions. Review the test chromatogram to determine peak identities for your specific column. Carefully compare Restek's test chromatogram and your analytical run, noting changes in peak shapes. In general, tailing hydrocarbon and Fatty Acid Methyl Ester (FAME) peaks indicate dead volume or contamination in the inlet or detector. Check the inlet and outlet sleeves for ferrule or septa fragments and reinstall the column. Excessively tailing solvent peaks and tailing or adsorbed peaks such as 2,3-butanediol, octanol, 2-ethylhexanoic acid, or dicyclohexylamine indicate the need for cleaning and redeactivating the split/splitless sleeve or that there is a problem with the make-up gas system. Figure K shows the Grob mix run on a relatively non-polar stationary phase.

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