FID versus FTIR, total hydrocarbons

[polartownjunkie]

im using a FID to measure total hydrocarbons from a diesel engine, I want to be able to create a method in which an FTIR would measure as a FID

the FID measures significantly higher than the FTIR, once ive added up the entire CH and CC bonds of the FTIR as well as inplementing FID response factors of individual HC species

I am mainly focused on measuring long chain alkanes, though it appears as though the FID's response is proportional mainly to ethylene, as I see very little in the FTIR's CH and CH2 absorption

am I neglecting a fundamental behavior of the FID, I understand it is a 'carbon counter', though I calibrate and sample at the same flow rates, shouldnt I be seeing a concentration measurement unaffected by 'amount of hydrocarbons'?

thanks for your insight, im very excitied to have stumbled across such a collection of GC knowledge

[AICMM]

polartownjunkie,

An interesting question since the FID is sort of the considered the gold standard for THC analysis. I set one of these up on a GC and got linearity from about 1 to 100% so my suspicion is that the issue is somewhere in the response factors. I am puzzled by your comment about "mainly proportional to ethylene" as well as by "concentration measurement unaffected by 'amount of hydrocarbons'". Could you please expound a bit?

Best regards,

AICMM

[Peter Apps]

An FID is not a "carbon counter" although it comes fairly close to it for straight alkanes with chain lengths of 6 carbons and upwards. Even structural isomers have different response factors, and unsaturation (including aromatic rings) has quite a strong effect. So, as with other detectors, you need to calibrate with standards that are chemically similar to your analytes.

You may have a problem with the FTIR rather than the FID - it's hard to imagine a diesel engine exhausting mainly ethylene.

Peter

[polartownjunkie]

it is generally pressumed the primary hydrocarbons from a diesel engine is unbrunt diesel fuel, such that I would expect changes in the FID to correspond closely to changes in the 2800-3050 -cm region on the FTIR

though this is not always the case, I often see the longhchain alkane absorption seemingly unchanged from one engine setpoint to another, though the change on the FID does follow changes in the 'light' hydrocarbon region

though ECN and FID response of my identified 'light' hydrocarbons (ethylene methane formaldehyde/acetaldehyde) does not explain why the FID's measurement has increased, too low to account for the FID's increase

perhaps i am at fault in considering my FID measurement as a concentration, but i dont know how to deduce it otherwise

ive thought perhaps the ratio of CH to CH2 bonds in the alkane region would help me understand the ECN of the 'incompletely combusted diesel fuel', though i believe im seeing other influences on the FID's measurement than what ive thus far accounted for

should i suspect the FID's measurement to be proportional to the hydrocarbon concentration, after calibrating with propane at the same flows and temperature as I sample?

THANK YOU! i am trying to imporve my communicating these thoughts, and i greatly appreciate you feedback

[polartownjunkie]

im noticing some further distinguishing absorptions in the 1350 - 1500 -cm region of FTIR, the methyl and methylene groups, perhaps these areas will help identify changing ECN of remaining longchains in sample

H2O absorption adds a weee bit of noise

when i see measurements from the FID of THC, what am I seeing? concentration, mass flow rate of carbon atoms? some complicated entanglement of not so ideal gases?

[Peter Apps]

The signal from an FID is proportional to the mass of analyte flowing through it per unit time. Thus the area of a chromatographic peak with an FID detector is proportional to the mass of analyte comes out of the end of the column - which should relate straightforwardly depending on injection technique etc to the mass of analyte in the sample injected to the inlet. To relate signal to mass needs calibration with known masses of a substance similar to your target analytes.

If you are feeding the exhaust gas straight to the FID (i.e. no chromatography) then the standing signal on the FID is proportional to the concentration of organics in the gas, provided that there are no major changes in the relative proportions of different components of the exhaust - for e.g. low MW oxygenates like acetaldehyde and formaldehyde have much less response per mass than the equivalent hydrocarbons. Once again you have to calibrate with a mixture which is similar to what you intend to measure.

Also; an FID is remarkably robust, but changes in the flow rates and composition (in terms of permanent gasses) of input flows can change its response characteristics.

Peter

[Don_Hilton]

I would also be careful of sampling technique. If you are looking for long chain hydrocarbons, you need be sure that you have not lost them to the walls of your sampling containers or plumbing used to move gasses around.

[hellyesidid]

I have to follow up on Don Hilton's comment. It is difficult to make this comparison unless you are working with identical samples extracted with identical solvents. For example, if you are sampling diesel exhaust with charcoal tubes, I would desorb the tubes with either Freon or carbon disulfide. Then you can run the extracts by FTIR and by GC/FID. Note that you should adjust your GC program if you were using a different solvent system such as methylene chloride.

[Jumpshooter]

You ought to be able to demonstrate some semblance of correlation between the FID and FTIR measurement since you are essentially measuring the "same" thing--but arriving at it by different means/analytical instruments. In essence, the final analytical results should concur.

[Don_Hilton]

While the molecues are the same in both measurements, there is a very significant difference in the measurement. In FID you are close to counting carbon atoms with a response factor that varies somewhat based on some structural differences - aromatic vs. aliphatic is the strongest that comes to mind for hydrocarbons. In IR you depend on measuring absorbance strength of various bonds - with the wavelength chaning based on bond strength and mass of connected parts. The strength of absorbance depends on things like molecuar symetry. (Thus a weaker IR signal for ethylene, which is highly symetrical.) And the presence of things like carbonyl, hydroxyl, and caroxyl compunds (which adds a nice degree of complexity as there are some strong absorbances associated with these compunds and one would expect some of these to be present in engine exhaust - and to provide signals which could interfere with the measurement of hydrocarbons.


With careful use of a sufficiently diverse training set, it is not unreasonable to expect to be able to use chemometrics to predict the FID signal. And it should be an interesting project.