Chromatography Forum

Hi all,

I am very new to this forum and I just started learning GC-MS. I have one important question regarding a GC-MS project. I have to measure hydrocarbons in air sample. For that, I am using ORBO3 (activated coconut charcoal tubes) tubes. Air from the cylinders are forced through this tubes for certain period of time and charcoal is desorbed to collect hydrocarbons. Based on referring some documents, I am thinking to use carbon disulfide as a desorbing solvent. This solvent after desorption of charcoal has to be screened by GC-MS by full scan technique, identify compounds by library search.

However, I don't have any idea about what internal standard should I use (may be naphthalene?) and at what concentration of internal standard should I use (may be 1ppm?). If I use certain concentration of internal standard, how can I quantitate compounds (based on internal standard concentration?). If I have to quantitate based on internal standard can you please explain me how should I quantitate in general based on internal standard? Do I have to use any other compounds for other than internal standard in to charcoal dissolved carbon disulfide solvent?

Can any one suggest what column should I use?

Can you please help me with this project.

Thank you very much,
Srikanth.S

hydrocarbons are nonpolar so you want to use a nonpolar column such as db-1 or if you are looking at aromatics a db-5. How heavy are your hydrocarbons? If they are very light you may need a thick phase column to have sufficient resolution.

Internal standard the general guidelines are structuraly similar to your analytes, elutes under similar conditions to your analytes but does not coelute, and concentration I would aim for the middle of your analyte concentration curve.

For desorption solvent you need a solvent that does not cover up your analytes. If you are looking at fairly heavy hydrocarbons your can use carbon disulfide (which will show up on the MS and you should program the solvent delay so that the MSD comes on after it is eluted so you don't cause undo wear on the filaments and detector). CS2 isn't a very popular solvent for GC/MS. Most people use hydrocarbon solvents like Pentane, isopentane, hexane, heptane, isoctane depending on how early your first analyte elutes. If you are looking at very volatile analytes you may want to pick a very high boiling solvent and use that and have the MSD shut off after your analtes elute and while the solvent elutes.

Hi thank you for the response. Those guidelines are very helpful. As I said I am learning GC-MS, I have following questions:

i. Is there any way to quantitate compounds observed with out running any specific calibrants? Because I will collect data in scan mode, I want to know whether I can spike any standard/internal standard in to the sample and quantitate compounds observed(by library search) based on standard/internal standard response?

ii. As this is like an R&D work, I don't have any idea what compounds to expect. Are there any common internal standards which I can use with out any knowledge on compounds? I might collect data from 1amu to 300 amu.

Thank you,
Srikanth.S

In fact you don't necessarily need internal standard for your project, external standard calibration will suffice.
BTW - what you are going to do is widely described and normalized.
Just google for OSHA methods for hydrocarbons, solvents etc. and you will get complete procedures.

Hi Thank you for the suggestions. Sure, I will follow them. Because I want to do a full scan and find all possible compounds. I don't want to use a specific set of standards. So, in order to quantitate peaks obtained in full scan, can I use any standard/internal standard to see both spike recovery and to get approximate concentrations of analytes observed.

Thank you,
Srikanth.S

Hi Thank you for the suggestions. Sure, I will follow them. Because I want to do a full scan and find all possible compounds. I don't want to use a specific set of standards. So, in order to quantitate peaks obtained in full scan, can I use any standard/internal standard to see both spike recovery and to get approximate concentrations of analytes observed.

Thank you,
Srikanth.S

For accurate quantitation you will have to have a standard for each analyte to generate response factors. To get an estimated quantitation you can use an internal standard and assume it has a similar response factor to the analytes of interest.

Normally however, when using MS detection you quantitate using one specific m/z for each analyte. You can quantitate using total ion chromatogram on some software and it will function similar to what you would get with an FID.

As for solvents, on a DB5 column, Hexane will elute only a minute or less after CS2 which will mean that any analyte lighter than Hexane will probably be missed and Hexane will be difficult to see. If you are looking for the light compounds then maybe Methanol would be a better solvent. Methanol has a mass of less than 35, so you can scan from 35-300m/z and it won't interfere too much with your analysis. You don't want to scan below 32 or you will be picking up all of the water, nitrogen and oxygen in the background and have a very noisy chromatogram as you can never get an MS system totally leak free. Most applications begin scanning at 35m/z or 45m/z which will also avoid Argon and Carbon Dioxide interference.

The best internal standard to use would be a labeled hydrocarbon in the middle of the range of expected analytes. If the analytes are C3-C30, look for something like Dodecane-dx where some number of hydrogens have been replaced by Deuterium or possibly a C13 labeled hydrocarbon that will shift the mass several units above the unlabeled analyte. These will not be common in nature and should not only be unique analytes to use as a reference but also respond with a response factor near the analytes of interest.

Hi James,
Thanks for the reply. Your explanation is very helpful. We are thinking to use Carbon disulfide because it looks like carbondisulfide is the most common charcoal desorption solvent. So, do I have to scan from 76-300 m/z? and I cannot see any compounds below 76 mass (have to set up solvent delay).

Is it OK if I use duterated naphthalene (naphathalene-d8) as internal standard? If I use internal standard for quantification. I have to assume relative response factor as '1' and calculate unknown compounds concentrations? As you said my plan is to use peaks in TIC and do a library search for determining unknown compounds (with out analyzing mass spectrum). I am thinking to prepare a 0.5ppm internal standard and spike it into extracted samples. Then using area of internal standard obtained to calculate concentrations of unknown compounds (using areas of unknown compounds obtained). Please let me know if this approach is acceptable and let me know if you have any suggestions.

Thank you,
Srikanth.S

... Please let me know if this approach is acceptable and let me know if you have any suggestions.
...

It all depends on intended purpose of your work which we don't know. To make it simple: if you are doing it "for fun" it is acceptable. If you would like to officialy asses workplace exposure to hydrocarbons (and other harmful substances) your simplified method is definitely unacceptable.

BTW - for the begining you don't need standards, analitycal grade reagents will be sufficient. You need to determine desorption rates for each compound because they differ sometimes substantially. And to make it not so simple, desorption rates change when other substances are present, there is interaction between them.
And don't entirely trust your aquired spectra. Take retention times into account as well, you need standards for serious analitical work.

Hi,

Thanks for the straightforward reply. Sure, I will consider using standards after prefeasibility work. I am actually searching for standards mix. Let me know if you know any vendors selling hydrocarbons standards mix.

Thank you,
Srikanth.S

Check Restek. On their website, if you go under reference standards, to environmental standards, to compounds & classes, they have a list of hydrocarbons. They can also make custom mixes.

Check out this analytical lab's webpage about air testing. They seem to have some good reference materials, hopefully it will help you.
http://www.caslab.com/Air-Testing/

Hi thank you for sharing this website. Both suggestions are helpful...

Hi James,
Thanks for the reply. Your explanation is very helpful. We are thinking to use Carbon disulfide because it looks like carbondisulfide is the most common charcoal desorption solvent. So, do I have to scan from 76-300 m/z? and I cannot see any compounds below 76 mass (have to set up solvent delay).

Is it OK if I use duterated naphthalene (naphathalene-d8) as internal standard? If I use internal standard for quantification. I have to assume relative response factor as '1' and calculate unknown compounds concentrations? As you said my plan is to use peaks in TIC and do a library search for determining unknown compounds (with out analyzing mass spectrum). I am thinking to prepare a 0.5ppm internal standard and spike it into extracted samples. Then using area of internal standard obtained to calculate concentrations of unknown compounds (using areas of unknown compounds obtained). Please let me know if this approach is acceptable and let me know if you have any suggestions.

Thank you,
Srikanth.S

You can scan lower than 76 so long as you begin scanning after the carbon disulfide peak elutes from the column. If you are interested in compounds that elute before CS2, then you would need to begin scanning early, then turn the filament off while the solvent elutes, then back on after elution. If you have a high vacuum gauge you can tell when the solvent elutes normally from a spike in pressure and that will give you an idea of when the filament needs to be off or on.

For our analysis of heavier compounds we inject with Dichloromethane solvent but scan range is 35-550 beginning as soon as possible after the solvent elutes.

For the lightest hydrocarbons a good way to analyze would be thermal desorption of the charcoal traps. Some companies make inlets that will hold charcoal tubes the size of normal inlet liners and you rapidly heat the tubes in the inlet so you get all of the adsorbed analytes without having a solvent peak to deal with.

Hi thank you for the reply. Those are very helpful tips. The GC-MS we have don't have thermal desorption capability. the inlet should be specialized for incorporating thermal desorption tubes? Have a nice week.

Thank you,
Srikanth.S

I would recommend dichloromethane (methylene chloride) instead of CS2. You use CS2 with flame ionization detectors because it is essentially invisible to the FID. Dichloromethane for the GC-MS is clean, elutes very quickly and doesn't interfere with hydrocarbon masses. One of the main issues with CS2 is that it sticks to everything in the GC-MS, so you get it bleeding out over a very long time.

You need to scan from 35 amu through probably 400 amu. Since the hydrocarbons you are looking for are volatile in air you can probably get by with only scanning to 300 amu; this will increase your scan rate significantly.

In total ion mode (scan mode) the MS has a pretty consistent relative response factor for hydrocarbons. You can probably get by with a limited mixture and quantify the peaks you find against this limited mixture, as long as you do it with the total ion chromatogram. If you try to do this with extracted ions then you will need a much more complete calibration.

Hi,
Thank you for the suggestion. I referred some OSHA and NIOSH methods, they are using Carbon Disulfide as extraction solvent. I too heard that Carbon disulphide will deposit in the lines. If I use methylene disulphide as solvent can it extract efficiently hydrocarbons from charcoal? I am using ORBO32 charcoal tubes for trapping hydrocarbons. I will try to switch to methylene chloride and see extraction recovery. Incase if I have to use carbondisulphide, how can I rinse carbondisulphide thoroughly between injections and what wash solvent can I use?

Please let me know.
Thank you,
Srikanth.S