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Gas chromatography can be used to separate, detect, and measure compounds that can be volatilized while remaining thermally stable. Gases also can be introduced into a GC. The technique is widely used for many applications, including alternative energy, cannabis, chemical, consumer products, environmental, food, forensics, petroleum, and pharmaceutical testing. Gas chromatography helps scientists conduct research, ensure product quality, determine product safety, and more.
Common carrier gases for gas chromatography are helium, hydrogen, and nitrogen. Argon also is an option but is rarely used. The graph below shows the van Deemter curves for He, H2, and N2. A van Deemter curve plots the relationship between carrier gas linear velocity (u) on the X-axis and the height equivalent to a theoretical plate (HETP or H) on the Y-axis. The optimum linear velocity for a carrier gas occurs at the minimum H value on its van Deemter curve. At this velocity, band broadening effects from longitudinal diffusion, eddy diffusion, and resistance to mass transfer are minimized, resulting in sharper chromatographic peaks.
Helium is by far the most popular GC carrier gas because it is inert and achieves good separation in a reasonable amount of time. Hydrogen exhibits similar chromatographic efficiency to helium but does so at a higher linear velocity. Additionally, its van Deemter curve is flatter as velocity increases, which makes hydrogen the most popular carrier gas for fast applications. Nitrogen yields the sharpest peaks but requires the lowest linear velocity to achieve, and compared to helium and hydrogen, peak broadening degrades more rapidly as velocity increases. This generally results in longer analysis times when using nitrogen unless the resolution requirements for peaks of interest can still be achieved despite the broadening at higher velocities.
There are other factors to consider when using hydrogen or nitrogen instead of helium, especially when using a mass spectrometer. Learn more about alternative GC carrier gases.
There are ways to conserve helium, such as performing a helium audit and using GC tools like Agilent Gas Saver and the optional helium conservation module for 8890, 8860, 8850, and 7890 GCs. You can also consider switching to hydrogen or nitrogen as an alternative. If you change carrier gas, method parameters will have to be adjusted to compensate for the different properties. Learn how to handle the hassles of the helium shortage and calculate how much you can save if you choose to conserve helium.
Large-volume injection, or LVI, can be used in gas chromatography to improve the detection of trace components in a sample. It also can reduce the amount of sample preparation (pre-concentration) because the analytes of interest are focused in the GC inlet liner. Agilent GC systems can perform LVI using either the programmable temperature vaporizing (PTV) inlet or the Multimode inlet (MMI). Learn more about LVI in this tutorial.
Backflush reverses the column flow in a GC system after the last compound of interest has been transferred to a primary analytical column (pre-column backflush), a secondary analytical column (mid-column backflush) or eluted (post-column backflush). This removes high-boiling components that can increase background, shift retention times, and result in frequent maintenance. Backflush can also reduce cycle time, increase throughput, improve data quality, and extend column life by eliminating the need to remove high boilers with a “bake out” step. Your lab will analyze more samples at lower costs.
Agilent Capillary Flow Technology modules facilitate fast and flexible implementation of backflush in our 8890 and 7890A GC systems. The Intuvo 9000 GC can be quickly and easily configured for backflush using standard Flow Chip options. The 990 Micro GC offers channels with integrated backflush capabilities.
There are several ways to implement fast GC. One is to increase the heating and cool-down rates of your GC system.
You also can attain faster gas chromatography by switching from helium to hydrogen carrier gas.
A simple yet often overlooked approach for reducing analysis time is to shorten the column. Many GC methods have excess resolution. Using a column that is half the length results in run times that are twice as fast, but the chromatographic resolution is only reduced by 1.41 (square root of 2).
Agilent provides a free method translation software tool that calculates adjustments to head pressures, oven temperature program rates, and relative run times so you can quickly implement new parameters to speed up a current GC method while ensuring that relative retention order is maintained, i.e., peaks elute in the same order.
Watch this webinar to learn more.
Multidimensional GC is a technique that uses more than one column with different stationary phases to analyze complex samples where there may be co-eluting peaks. The columns are part of the same flow path, which increases the GC resolving power – peaks that cannot be fully separated on the first column can be separated on the second column. The ability to resolve overlapping peaks without having to perform a second injection leads to greater accuracy, more complete sample characterization, and higher throughput.
2D-GC is a type of multidimensional GC where only a section of the chromatographic run is diverted to a second column for further separation. This is also known as heart cutting and is accomplished through the use of a Deans switch or a valve in the instrument.
GCxGC, also referred to as comprehensive 2D-GC, is a special type of multidimensional GC that utilizes a modulator to collect eluent from the end of the first column and inject it onto a second column to separate otherwise co-eluting components. It differs from other multidimensional techniques in that the entire sample – not just a fraction – undergoes separation in both columns. This results in a significant increase in resolving power (peak capacity/theoretical plates) and can give additional compositional insights for complex samples.
The main performance differences are found in the 6th-generation EPC, oven, and FID. The 8850 GC offers an integrated touch screen and remote access through the browser interface. It also incorporates instrument intelligence with on-board help and many more diagnostic tests compared to the 6850 GC.
8850 GC | 6850 GC | |
EPC pressure control (0-150 psi) | ±0.001 psi | ±0.01 psi |
Oven cool-down (350 °C to 50 °C, 22 °C ambient) | < 2.5 minutes | 5.2 minutes |
Maximum temperature ramp | 300 °C/min | 240 °C/min |
Oven temperature ramps | 32 plus 33 plateaus | 6 plus 7 plateaus |
FID sensitivity | <1.2 pg C/s | <5pg C/s |
FID maximum data acquisition rate | 1,000 Hz | 200 Hz |
User interfaces | Integrated touch screen, browser interface for remote access and control, CDS | Six-button, two-line display with detachable handheld controller, CDS |
Diagnostics |
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* Self-guided
**Continuous, automated test
Agilent offers standard, mass spec, Ultra Inert (UI), and Ultra Low Bleed (Q) GC columns.
Standard columns are good for general applications, but as detectors grow more sensitive, any background noise becomes more apparent. With a mass spectrometer (MS), there are several factors that can create background noise in your chromatogram, and one such factor is column bleed.
Mass spec columns have about 50% less column bleed than the standard versions, making
them ideal for use with sensitive detectors, like an MS.
Agilent Ultra Inert GC columns go through an extra step to make them more inert to active compounds, reducing the chance of chromatographic issues such as peak tailing, ghost peaks, poor resolution, and signal loss.
Agilent Ultra Low Bleed columns (Q) combine Ultra Inert surface deactivation with Ultra Low-Bleed chemistry to deliver exceptional signal-to-noise ratios and mass spectral integrity for consistent and reliable column performance. Q columns are ideal for GC/TQ and GC/TOF use and trace level applications.
The most used stationary phases in GC are 1, 5, 624, and Wax. These columns span a range from non-polar to polar and it is good practice to have these columns on hand. The most ubiquitous columns are a 5 or 5ms phase and are a good starting point when selecting a column chemistry. Specifically for mass spec, a DB-5ms UI or DB-5Q are great general-purpose columns.
If you are analyzing active compounds, especially at trace levels, you need to ensure that the entire GC flow path is deactivated. Our Inert Flow Path split/splitless inlet and Ultra Inert consumables (inlet liners, columns, gold seals, gold-plated flexible metal ferrules) contribute to an inert flow path that gives improved performance for sensitive analytes at very low concentration levels, extending the range and confidence of quantification and detection.
This can be caused by several things, such as impurities in your carrier gas supply or sample solvent, dirty delivery tubing, contaminants in the syringe or inlet components, or carryover from a previous injection. Be sure you are using high-purity solvents and gases, as well as inert liners and clean septa. Always use a filter in your gas line before the GC and be sure to change it when needed. You can consider using backflush to eliminate carryover. If you are using an autosampler, use sufficient solvent washes to avoid cross-contamination from sample to sample.
Some common causes of ignition issues in a GC flame ionization detector are:
Read our troubleshooting article that addresses each of the issues above.
A leak in your split/splitless inlet can manifest itself in different ways. With a large leak, the inlet might not be able to reach its pressure setpoint. For smaller leaks, various diagnostic tests may fail or there could be chromatographic issues like poor retention time or peak area reproducibility, higher than normal background, baseline drift, peak tailing, and more. To help troubleshoot a possible leak, read this informative article, access our 8890 and 7890 troubleshooting guides, or consider these Agilent University courses for the 8890 and 7890 GC platforms.
Column bleed is the normal background signal caused by slow degradation of the stationary phase over time. All columns bleed and the extent to which they do is influenced by the phase type, temperature, and film thickness.
Bleed is typically pretty low and does not interfere with the chromatography. High bleed is when something causes the stationary phase to break down faster and more significantly than usual. This results in a high, rising background at elevated temperatures (starting at about 30 °C before the upper temperature limit) and can make accurate quantitation difficult, especially for low-concentration analytes. It’s not great for your mass spec if you’re using one and your column will bleed to death much sooner than it should, resulting in higher costs.
Excessive column bleed can be caused by things like a leak that introduces oxygen into the column, conditioning the column above the maximum temperature limit, operating at high temperatures with insufficient gas flow rates, or the presence of inorganic acids and bases in your sample.
You can keep bleed at a manageable level and extend your column life by:
It also helps to use low-bleed columns with high thermal stability, such as Agilent Ultra Inert GC Columns. In addition, backflush can help minimize excessive column bleed by eliminating the need to “bake out” high-boiling compounds at elevated temperatures.
For additional information, watch this video and our on-demand webinar.
Peak fronting is when a GC peak displays excessive asymmetry with a leading edge. A normal peak is almost symmetrical. Fronting can be caused by one or more of the following.
Column overload. If the component concentration is too high, you can dilute the sample, ensure that the correct volume has been injected, check the syringe size, decease the injection volume, or increase the split ratio.
Improper injection technique. This is mainly seen when using manual injection where the injection is too slow, or the plunger depression is erratic. The best solution is to use an autosampler but if that is not an option, check that the syringe plunger is moving freely. The sample should be pulled entirely into the syringe body (no solution left in the needle) and moving freely and fast when plunger is depressed.
Reverse solvent effect. This occurs when a sample component is highly soluble in the injection solvent. Changing the solvent or using a retention gap can address this.
Mixed sample solvent with large differences in polarity or boiling point. The analyte has different solubility in different solvents. Change to a single solvent or use a retention gap.
Improper column installation. Remove and re-install the GC column.
Column contamination. You can bake out the column for 1-2 hours. Do not exceed the recommended maximum column temperature as recommended by the vendor or you can damage the column. Alternatively, you can trim the column by removing 0.5 to 1 m from the front of the column. If this is a recurring issue, you can consider implementing backflush.
Tailing is when a gas chromatography peak has excessive asymmetry with a trailing edge. A normal peak is almost symmetrical. This phenomenon can be due to several factors.
Split ratio is too low. Increase the split ratio.
Column contamination. You can bake out the column for 1-2 hours. Do not exceed the recommended maximum column temperature as recommended by the vendor or you can damage the column. Alternatively, you can trim the column by removing 0.5 to 1 m from the front of the column. If this is a recurring issue, you can consider implementing backflush.
Column activity. This is irreversible and cannot be fixed. The GC column must be replaced.
Improper injection technique. This is mainly seen when using manual injection where the injection is too slow, or the plunger depression is erratic. The best solution is to use an autosampler but if that is not an option, check that the syringe plunger is moving freely. The sample should be pulled entirely into the syringe body (no solution left in the needle) and moving freely and fast when plunger is depressed.
Improper column installation. This may include poor column cutting. Remove and re-install the column.
Solvent effect violation for splitless or on-column injections. Lower the oven starting temperature.
Mixed sample solvent with large differences in polarity or boiling point. The analyte has different solubility in different solvents. Change to a single solvent or use a retention gap.
Peak splitting and broadening are a result of the analyte being deposited incorrectly onto the column. It is a single-compound effect, not near co-eluting peaks. Causes and solutions are listed below.
Improper injection technique. This is mainly seen when using manual injection where the injection is too slow, or the plunger depression is erratic. The best solution is to use an autosampler but if that is not an option, check that the syringe plunger is moving freely. The sample should be pulled entirely into the syringe body (no solution left in the needle) and moving freely and fast when plunger is depressed.
Improper column installation. This may include poor column cutting. Remove and re-install the column.
Inlet temperature is too low. Increase the injector temperature to ensure fast transfer of analyte to the column.
Inlet temperature is too high. This may cause analyte degradation. Reduce the injector temperature or change to a cool injection technique.
Mixed sample solvent with large differences in polarity or boiling point. The analyte has different solubility in different solvents. Change to a single solvent or use a retention gap.
Poor sample focusing. Use a retention gap to focus the sample on the analytical column.
A flame ionization detector (FID) typically uses three types of gases to maintain a stable flame and signal output: hydrogen as fuel, air as oxidizer, and either helium or (typically) nitrogen as the makeup gas. Most detectors use a makeup gas to increase the total flow rate through the detector body, which sweeps peaks out of the detector quickly, avoiding mixing of components and loss of resolution. This is particularly important with capillary columns because the column flow rates are so small.
A general rule is to set the hydrogen flow to 30-35 mL/min, the carrier gas plus makeup (total inert gas) to 30-35 mL/min or a 1:1 ratio to the total inert gas, and the air to 400 mL/minute.
Makeup gas, as with all other gases, should be filtered to reduce the chance of impurities affecting the results.
Most modern GC methods either ramp the oven temperature at a set rate throughout the run or use multi-step programs with different ramp rates. Older methods commonly used isothermal oven temperatures before instruments were able to perform temperature programming; however, some modern methods, such as blood alcohol content, still use this approach.
For temperature ramps, the initial temperature should be low enough to separate early eluting compounds of interest and the final temperature should be hot enough to ensure the highest-boiling compounds are eluted, while not exceeding the maximum temperature limit for the column. The rate should be low enough to resolve all compounds of interest. If your method results in more than sufficient resolution, the temperature ramp can be increased to shorten the run time and increase throughput.
Because default or standard inlet conditions are suitable for 80-90% of all samples, the following conditions are a good place to start when developing a new method.
The recommended temperature for detectors is as follows.
There are many factors, such as geometry, volume and inner diameter, deactivation, and packing, that determine which inlet liner is best suited for your application. The type of injection being performed also will impact your choice.
For split injections, the inlet should vaporize and mix the sample quickly. A split taper inlet with glass wool is good for this as the increased surface area enhances vaporization and mixing of the sample while also wiping the syringe needle to improve repeatability.
A single taper liner with glass wool is good for most splitless injections. The taper at the bottom of the inlet limits the interaction between sample analytes and the gold seal while helping focus the sample to the column head. The wool assists in vaporization and can trap non-volatile sample components that could contaminate the GC column.
For other injection techniques and liner options, see our inlet selection guide poster and GC inlet liner FAQs.
Watch this brief video for more information about proper inlet liner selection.
Graphite ferrules are soft and can be re-used. They do not shrink when heated so re-tightening is not required. They are often used to connect GC columns to the inlet and detector. However, because they are porous to oxygen, they should not be used with air-sensitive columns or detectors (ECD, mass spectrometer). Graphite ferrules can shed particles that can enter or block the column, so it is recommended to trim the column after inserting it through the ferrule. If overtightened, they also can contaminate the bottom of the inlet or the detector jet. These ferrules are rated to 450 °C.
Vespel ferrules are made from polyimide, which is harder than graphite so greater torque is required to attain a good seal. They are impermeable to oxygen so they can be used to connect to a mass spec or oxygen-sensitive detectors but also will shrink when heated and cooled so frequent re-tightening is needed. These are best suited to isothermal applications below the recommended maximum temperature limit of 280 °C or connections outside the GC oven. Under these conditions, Vespel ferrules can be re-used. However, if they are used above the maximum operating temperature, they can stick to the outer coating of capillary columns and possibly even inside the fitting.
Because each material has specific benefits, graphite/Vespel ferrules are most commonly used to make fused silica column connections in a GC oven. They do tend to shrink with temperature cycling, so they need to be re-tightened several times (after cooldown) within the first 10 or so temperature programmed runs. These ferrules can be used up to 350 °C.
Learn more about choosing the right ferrule for your GC application.
A split injection is when the sample introduced into the GC inlet (split/splitless, multimode, PTV) is split, with only a small fraction being transferred to the capillary column. Most of the sample is directed out the split vent. This type of injection is used for concentrated samples.
The split ratio determines the amount of sample that enters the column. A higher ratio means less sample goes to the column. A split ratio that is too low can result in poor peak shape and column overload. If it’s set too high, sensitivity will suffer, and it wastes carrier gas.
The minimum recommended split ratio for different columns is shown below.
Column internal diameter (mm) | Split ratio |
0.10 | 1:50-1:75 |
0.18-0.25 | 1:10-1:20 |
0.32 | 1:8-1:15 |
0.53 | 1:2-1:5 |
This unique interface is one of the intelligent features of the Intuvo 9000, 8890, 8860 and 8850 gas chromatograph systems. It enables the user to connect to the GC from any computer or mobile device on the same network by leveraging the internet browser already present on most devices. No additional software, including a chromatography data system, is needed. To access the GC, its IP address is simply entered into the address bar of the internet browser. You can then remotely view instrument status, run diagnostics, check maintenance logs, or view service videos. A PIN can be defined through the GC touch screen that prompts a user for the four- to eight-digit code when attempting to access the instrument through the browser interface.
It is important to note that this functionality does not require an internet connection. Whether you use Google, Chrome, Firefox, Safari, or any other browser software, it is simply a tool used to connect your device to the GC through your lab network.
A Smart Key is a device that is included with GC columns. It plugs into the 8890 (front panel) or Intuvo 9000 (oven compartment) and stores information about that specific column, such as age, temperature limits, serial number, and usage. It also includes default parameters for configuration that helps automate method setup and reduce the possibility of manual entry errors. You can keep track of column information in the Early Maintenance Feedback (EMF) screens in the GC touchscreen, Agilent data system, or browser interface.
In addition, a Smart Key is included with every Intuvo Flow Chip to enable automatic system configuration and help set specific method parameters.
Peak evaluation is an automated and customizable tool for the comparison of chromatographic data to a previously collected reference chromatogram. It allows the user to specify analytes of interest from their separation and define acceptable criteria for the method and analysis they are running. Throughout a sequence, peak evaluation can monitor the user-specified analytes of interest for changes in peak retention time, area, height, width, symmetry, resolution, and relative related attributes. When peak evaluation is enabled, the 8890, 8850 or Intuvo 9000 GC examines the data internally and compares it to the user-defined reference chromatogram after each run.
If any user-specified criteria fail, the system will alert the user and can suggest maintenance procedures or guide the user to the onboard troubleshooting feature to help determine the cause of the failure. The user can also elect to stop a sequence from continuing onto the next sample upon failure if desired, thus allowing the opportunity to perform any maintenance or troubleshooting steps and prevent the loss of samples.
Watch these short peak evaluation videos and read the white paper to learn more.
Blank runs are used to trace the source of artificially introduced contamination. They are critical to accurate quantitative analysis and are often required by regulatory agencies as part of the quality control process.
The 8890, 8850 and Intuvo 9000 GC systems evaluate blank run data and identifies problems such as baseline excursions, unexpected peaks, and elevated baseline from the column stationary phase. It then raises a “not ready” notification if the blank isn’t truly blank.
A screen, accessible from the browser interface, lets you accept defaults based on Agilent recommendations or tailor blank analysis to your needs. You can also select what should happen if blank analysis fails (warn and continue, pause, or abort).
Watch this brief video to learn how to set up blank evaluation.
The 8890, 8850 or Intuvo 9000 GC system will automatically evaluate detector checkout samples, providing a written summary report in the diagnostic section.
Early Maintenance Feedback (EMF) is a feature on Agilent GC systems that tracks the usage and performance of various instrument components. When a component reaches a predefined threshold, the system alerts the user to perform routine maintenance. This can prevent potential issues with chromatography or hardware that could lead to unexpected downtime and costly repairs.
EMF settings can be configured and modified to match specific maintenance schedules and requirements. This ensures that the alerts are tailored to usage patterns.
If you receive an EMF alert, replace any required parts, clean components, or perform other tasks as needed then manually reset the counter(s). If you are using an intelligent GC system (8890, 8860, 8850, or Intuvo 9000), follow the guided instructions provided by the system to carry out the maintenance. Once complete, the counter(s) will reset automatically.
Learn more about clearing EMF counters to zero.
Agilent intelligent GC systems also can create trend plots that allow users to confidently set EMF counters at optimal values for specific workflows.