What are the Differences between GC and HPLC?

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Gas Chromatograph

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High Performance Liquid Chromatograph

Gas chromatography and High Performance Liquid Chromatography are both separation techniques which have gained immense popularity in both academic and industrial laboratories. High Performance Liquid Chromatography has found favour in applications covering pharmaceuticals, foods, life sciences and polymers whereas Gas Chromatography has significant applications in petroleum and petrochemical industry, flavours and fragrances and, environmental air monitoring,. A general question that comes up in your mind is how our HPLC and GC are different from each other and what factors decide suitability of one over the other.

Mobile phase

As the name suggests High Performance Liquid Chromatography uses a liquid mobile phase and gas chromatography uses a gas as the carrier. Liquids are generally mixtures of solvents of compatible polarities whereas in gas chromatography the mobile phase is a single high purity gas.

Operating temperature

HPLC separations are mostly carried out at ambient temperatures whereas Gas Chromatography separations are carried out at elevated temperatures which can be held at a constant value (isothermal) or variable as decided by the temperature program. Newer packings have extended temperature limits of High Performance Liquid Chromatography operation as well.

Nature of compounds

Gas chromatography separations are mainly carried out on compounds ranging in molecular weights up to a few hundreds. Such compounds separate on differences in their volatilities and remain stable at high temperatures. On the other hand compounds separated on HPLC have higher molecular weights ranging from a few hundreds to several millions for large polymers and biomolecules. Such compounds can be analysed at room temperature only because at elevated temperatures they tend to degrade.

Column Dimensions

Liquids used as carrier in HPLC generally have higher viscosity in comparison to gases used in Gas Chromatography. This results in increased column back pressures in HPLC. It is for this reason that High Performance Liquid Chromatography columns are much shorter and have wider diameters in comparison to GC columns which can be much longer and narrower. Increased column length improves resolution between closely spaced peaks. As the trend is towards faster analysis columns used for HPLC are as short as 1 cm in length.

Column Packings

Column packings offer greater resistance to flow of liquids in comparison to gases. Gases also have a higher permeability of solid supports than liquids. The retention mode of HPLC columns depends on polarity differences or molecular sizes whereas GC separations are based on differences in volatilities of compounds

Detection principles

HPLC detection is commonly based on nondestructive detection such as UV, RI, photodiode array detectors, conductivity and laser detection. On the other hand Gas Chromatography detection is based largely on destructive principles such as a FID, NPD and FPD. Mass spectrometry detectors common to both LC and GC are destructive in nature.

Cost of operation

HPLC solvents are costly in comparison to gases used for GC analysis. In addition to cost of solvents the maintenance cost of High Performance Liquid Chromatography systems is also higher due to high pressures developed in pumps and columns. GC analysis in comparison is less costly and there is lesser maintenance cost.

Both HPLC and GC are versatile and have contributed to to the ever increasing scope of applications. Tandem techniques such as LC – MS – MS and GC – MS – MS have expanded the limits of detection to new frontiers of detection and automation has also contributed to increased laboratory throughputs.

Please share your experiences and offer your valued comments.

About Dr. Deepak Bhanot

Dr Deepak Bhanot is a seasoned professional having nearly 30 years expertise beginning from sales and product support of analytical instruments. After completing his graduation and post graduation from Delhi University and IIT Delhi he went on to Loughborough University of Technology, UK for doctorate research in analytical chemistry. His mission is to develop training programs on analytical techniques and share his experiences with broad spectrum of users ranging from professionals engaged in analytical development and research as well as young enthusiasts fresh from academics who wish to embark upon a career in analytical industry.

Comments

  1. Girish Kulkarni says:

    Dear Dr. Deepak Bhanot

    You have wonderfully explained the differences between GC & HPLC. I used to read your articles with great Interest & wants you to add articles on Advanced gas chromatography & its techniques. I wish if you can throw some light on operation of switching valves, Column back flush, column used in this techniques ( Ex..RGA, NGA..etc…) in Petroleum Industry.

    Thanks for all your articles………

    Regards
    Girish Kulkarni

    • Dear Girish,
      Many thanks for your encouraging comments.The advanced level programme on GC is in the pipeline and should be available later this year and will cover some of the topics suggested by you.

  2. ANJI REDDY KALAKOTA says:

    Hello Deepak Sir..

    Thanks a lot for sharing your valuable knowledge to who wants to learn about Chromatography operations like me.. And with your information now (not only me) I have known the difference between GC & HPLC.
    Thank You very much….

    Best Regards
    K.Anjireddy

  3. Dr. Jaigopal Sharma says:

    Thanks for en lighting. I want to analysis Amino Acid profile of fish feed. Could you please suggest best instrument for the same.
    Regards

  4. Many thanks for sharing ur knowledge and get information about the distinguish between GC and HPLC clearly.

  5. Perrine Chua. says:

    Dear Dr. Deepak Bhanot,

    Good day. Thanks for the wonderful explanation regarding the difference of both methods. Would like to clarify, in you context you did mentioned about volatilizes is the focus of GC. In this point of view, juices extraction will not be suit well with GC method? Would you mind to share with me some articles regarding GC or LC for extract crude. Thanks.

    • Dr. Saurabh Arora says:

      Hi, GC can be used for the volatile components in a juice though the juice itself is not volatile. what are you trying to analyse in the juice?

  6. mike toyoshima says:

    Hello Dr. Deepak Bhanot,
    Thank you for the clear and detailed explanation of HPLC (liquid) versus GC (gas). I am an old-time Medical Technologist that started when atomic absorption with a graphic furnace was advanced.
    I am interested in knowing if, for a given molecule/analyte, there is a preferred method of measurement. You mentioned variables such as operating temperature, nature of the compound (analyte) measured, detection by either conservative methods (UV, RI, PhotoDiodeArray) or destructive methods (FID, NPD, FPD – what do these acronyms stand for?).
    So, if we are trying to compare the values for a given compound/analyte (perhaps Cefazolin) and HPLC is the preferred SEPARATION method, versus GC: 1) because of the nature of the compound, is there a preferred method (LC versus GC); 2) if there is, and for example it is LC, is there a preferred detection method (UV, RI, photodiode array detectors, conductivity and laser detection)? Or, for the amount of measurand (for example, therapeutic level of cefazolin in a patient serum sample in a mg/L range), are the standards/calibrators so exact (perhaps gravimetrically measured), that the exact detection method (PDA or tandem MS) does NOT make a large difference in the reported value? Or, perhaps because of the nature of the compound, ONLY LC is used, and ONLY with a tandem mass spec detection, as an example.
    My goal is to compare – in this case – serum/plasma antibiotic levels (but it could be any compound) and want to know HOW much difference the type of chromatography (GC or LC) or the final detection method (UV, RI, PDA) really makes in reporting the value. Perhaps based on your description of the SIZE of the compound, let’s say LC is the preferred method. Then, for different labs that use MS or PDA or UV as the final detector, will there be a significant difference in their values, or will they be comparable? I do not have access to any proficiency surveys, but my guess is that based on the compound, there are a limited number of commonly used ways to separate and detect that compound, and, I am hoping the detection method is NOT a large variable. Further, for DIFFERENT manufacturers that use the same separation/detection method, do the INSTRUMENTS make a large difference in the final value? Again, a proficiency would likely show this clearly.
    I hope to learn more from you about how comparable assayed values are for the same compound, using different separation/detection methods.
    Again, my sincerest thanks for an enlightening article.
    mike toyoshima

    • Hi Mike
      First of all thanks for the encouraging comments.The acronyms referred by you relate to detectors for Gas Chromatography.
      FID- Flame ionization detector, NPD-Nitrogen Phosphorus detector.FPD- Flame photometric detector.The query raised by you is mainly a method development issue. You will be fortunate if you come across a standard operating procedure based on a validated method using a technique with the right choice of technique combined with a suitable column and detector. In absence of any such method you can develop your own in-house method depending on availability of resources. However, for proficiency level testing in different laboratories there has to be the same method of analysis using identical set of conditions

  7. Magnificent web site. A lot of useful info here.

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