FID Limits of Detection

[ChrisR]

I used search and couldn't find a good enough answer. I am fairly new to GC and am currently preparing standards to calibrate the FID response for a series of reactions we are performing. Our GC (CP-3800, 1177 Injector, FID Detector, VF-1 FactorFour column) has been offline for several years and anyone who knew how to use it has moved on, but I've got it cleaned and running with a clean baseline (ghost peaks were a big problem for a few weeks!).

So my question is this, what concentrations should I be injecting for a FID detector? Specifically, what concentrations should work for when we analyze conjugated aromatics (azulenes, anthracenes, stilbenes, etc). I read a suitable range is 0.2 - 2.0 ug/uL, so I prepared several standards and started making calibration curves. When I inject these solutions (1 uL injections), the peaks show significant fronting and the area per peak at a given concentration seems to vary too much. For example, 5 1uL injections of trans,trans-1,4-diphenyl-1,3-butadiene in hexane (0.2 ug/uL) gave an area response of [1444 1167 544 1444 1032] (units in mV), which gives a standard deviation of 371.3115 mV.

To me, this seems too large. When a 0.4 ug/uL standard is injected, I am seeing responses as low as 1567 mV, which to me says that quantification is going to be an issue at the moment. As our primary use of the GC is for reaction quantum yield measurements, I will need to get this sorted out before I can get back to all the fun chemistry.

So, what do you think? Which concentrations should work, and is there a way to deal with the significant peak fronting? I should mention, when the crude compound is injected before purification (contains impurities), the minor impurity peaks show good shape (no fronting or tailing) and have peak heights from 1 - 15 mV (the compound of interest gives peak heights over 100mV).

Other pertinent information : FID Gases : H2, Air, He. Carrier gas: He (2mL/min). Injections are split (I don't really understand what is going on here, so I would appreciate it if someone could explain what happens in the method! I took this split method directly from the manual). In the method, the split ratio is set like this : 0.0 min, split off | 0.5 min, split on, split ratio 100 | 1.5 min, split on, split ratio 5.

Thanks for your time, let me know if you any more info would help.

edit: fixed units

[CE Instruments]

You injections are splitless based on the details you give re your method. Ignoring the units it appears that your compounds of interest are around 1-10mA your standards 1000mA. Whilst FID output is in pA are you actually talking about mV from the data system ? It seems to me that you are about 2-3 orders of magnitude to concentrated with your standards, hence the fronting and poor reproducability.

Try reducing the concentration ?

[ChrisR]

Thanks CE, I did mean mV for the units (not mA). I am going to try lowering the concentration today. Thanks for the input.

[JMcK]

Chris,
You might want to purchase a pre-made mixed standard and compare retention times, peak heights, resolution, etc. to values reported by the vendor. You can probably find a mix that contains compounds that are similar to those that you are trying to analyze. Also, you can ask a vendor to prepare these for you. I usually go through Restek, but there are other vendors out there as well.
Good luck,
John

[aldehyde]

For a split injection say you inject 1 uL of sample with a 100:1 split ratio, then for every 'part' of sample that passes onto the column 100 'parts' are vented out as waste. So if you have a column flow of 1 mL min, you will have a split flow of 100 mL.

There is also septum purge which is usually restricted to 3 mL/min, so if you look at the inlet's total flow it would be column flow + split flow + septum purge, or in this case 1 + 100 + 3 = 104 mL/min of total flow.

For a splitless injection rather than setting a split ratio that is on all the time you essentially a timed split, called a purge flow.

In this case a common setup would be to turn the purge on at 0.75 min for 50 or 75 mL/min of purge flow. Enough gas to purge the inlet volume (somewhere around 2 mL) several times within a minute or so.

Split can be used to control how much sample you inject on column, so say you set up a few different methods that each inject 0.5 uL of sample you could make methods of decreasing split flow (say 1000:1, 500:1, 250:1, 100:1, 50:1, 20:1. Below 5:1 is not recommended.) This way you can evaluate how much response you get and gauge what an appropriate split ratio would be.

Once you're looking at a good amount of signal inject it several times (like you did) to look at reproducibility/repeatablility. As you continue to validate your method you'll need to look at how quickly your inlet consumables get contaminated and set up a maintenance schedule that reflects your needs.

I recommend using a log book to record gas gauge values daily (this way you'll avoid running a tank empty and you'll catch leaks faster), you should also verify your FID baseline and either do some form of signal to noise check. Or a check of response factor, tailing factor, and separation factor. This is called a system suitability check and is used to verify consistent response.

With the area counts you posted I would first try a new syringe, if that doesn't help replace your inlet liner, gold seal, and split vent trap. You should also cut an inch or two off the front end of the column.

[AICMM]

ChrisR,

Nanograms at the detector. The FID will detect about 1 - 5 ng of material (depends a little on the material but this is a good rule of thumb) at the detector. So, wherever dilutions and split ratios take you, you want to be shooting at least 50 or 100 ng at the detector for a nice strong signal.

Best regards,

AICMM