GC/FID vs GC/MS - VOCs, SVOCs, pesticides, herbicides

[Caffeinated]

Don't have any GC experience, need some advice on equipment purchase because I'm getting conflicting advice from sellers.

Our company wants to measure any potential contaminants, batch to batch in our final plastic product. We plan on doing TCLP Leaching protocol with the product then want to test the leachate (and also the solid product) for VOCs, SVOCs (and maybe pesticides/herbicides, but that's not as crucial). Some companies are telling us we should buy a GC/MS, but I'm seeing that GC/FID may be able to do the job. I'm particularly interested because it seems simpler, and much cheaper. We do not have anyone with GC experience in the company, so will have to get training to learn/run the machine.

Examples of the types of things we're looking for (and detection limit, in mg/L):

1,4-Dichlorobenzene - 0.5 mg/L
1,1-Dichloroethylene (1,1-dichloroethane) - 0.7 mg/L
1,2-Dichloroethane - 0.5 mg/L
Benzene - 0.5 mg/L
Carbon tetrachloride - 0.2 mg/L
Chlorobenzene - 2 mg/L
Chloroform- 3 mg/L
Methyl ethyl ketone (2-butanone) - 200 mg/L
Pyridine - 5 mg/L
Tetrachloroethylene 0.7 mg/L
Trichloroethylene - 0.5 mg/L
Vinyl chloride - 0.2 mg/L
2,4-Dinitrotoluene - 0.13
2,4,5-Trichlorophenol - 400
2,4,6-Trichlorophenol - 0.5
Benzol(a)pyrene/Benzo(a)pyrene - 0.001
Cresol - 200
Hexachlorobenzene - 3
Hexachlorobutadiene - 0.5
Hexachloroethane - 3
Nitrobenzene - 2
Pentachlorophenol - 0.9


Thanks for any help

[rb6banjo]

If I were in your situation, I'd for sure want a mass spectrometer. If you're not experienced in GCMS, you should definitely invest in a training class as well. That PAH will be difficult with only a flame detector. Some of the other too but definitely that one.

[Caffeinated]

Thanks for the advice - where can I find a list of compounds that a GC/FID can detect? We may not need to look for all these things.

And is it true that FID is cheaper? I can't find any quotes for it

[rb6banjo]

FID can detect most anything made of C, H, and O and it is probably the least expensive and most robust of all GC detectors. The only requirement of your analyte is that it must burn. Your problem will be the detection limit. The mass spectrometer gives you an added amount of selectivity in that you can run in a mode called "single-ion monitoring". Essentially, everything that has masses in the spectrum that are not your key ions are ignored. Your S/N ratio is generally much, much better when you operate this way. This disadvantage of SIM is that you don't see anything else while you're operating this way.

A mass spec. is much, much more powerful than a flame. There are only a few analytes that you might rather have a flame than a mass spec. (glycols, etc.). Just looking at your analyte list, you might have a difficult time coming up with a single GCMS method to do all of those in one shot.

[James_Ball]

The problem with the FID will be detection limits on the halogenated compounds especially Carbontetrachloride. For the low limits on the volatiles you are probably looking at Purge and Trap also.

It is possible to set up a system with a GC/MS that can have the volatiles and semivolatiles on the same system. I have one setup using two columns from two separate injection ports. One port has the purge and trap attached the other has an autoinjector for the SVOC analysis. The columns are attached to a Y connector just before the entrance to the Mass Spec. If you use a high temp Rxi-624SilMS column for the volatiles and a Rxi-5SilMS column for semivolatiles then you could run everything on that list including the pesticides and possibly the herbicides all on one system.

The same setup can be used with FID detection also, but normally with that the pesticides and herbicides are detected with the ECD instead of the FID because it is much more sensitive to the halogenated compounds.

Look up methods EPA8015 for compound lists for FID, method EPA8260 for volatiles and method EPA8270 for semivolatiles by GC/MS and method EPA8081 and EPA8151 for the Pest and Herbs by GC/ECD.

The methods can be found here https://www.epa.gov/hw-sw846/sw-846-compendium

The methods will list the compunds and their expected detection limits so you can decide what method you will need to achieve the desired results for your samples.

[Steve Reimer]

If you want to look at volatiles and semi-volatiles in a plastic product, rather than a leachate, you will definitely want an MS.
You will need to look very carefully at your cleanup procedure for the semi-vols too. Solvent extracts of some types of plastics can be very nasty.

[AICMM]

Caffeinated,

A GC/MS is probably more appropriate for several reasons, not the least that you get a second dimension of confirmation if you have a hit.

Several other comments.

The list you present also requires volatile configuration and semi-volatile configuration which means a column switch at a minimum. Therefore, you either need a quick swap or to wait overnight whilst you move from one to the other.

The volatiles side of this list also requires sample introduction (usually purge-and-trap) which is a chunk of change of its own.

Finally, TCLP for volatiles requires zero-headspace extractor (or at least it did when I was in that game) which is a headache of its own accord.

May I make a suggestion? Go visit with an accredited environmental lab to see how they do this to give you a much better picture of what you are proposing.

Sorry for the long note.

Best regards,

AICMM