Benzene and naphthalene: huge differences in FID response

[christiancarioca]

I am analyzing a gaseous stream containing 0.5 % of benzene and 0.015 % of naphthalene in methane. This mixture is made by injecting a 5 % wt solution of naphthalene in benzene by means of a syringe pump into a methane stream. Transfer lines to the GC are held at 150°C. Sampling is made automatically.
The chromatogram shows the following area values:

(area C6H6) : (area C10H8) = 520000/144000 = 3.61

while the molar ratio is 0.5/0.015 = 33.3

So naphthalene shows a molar response factor almost 10 times higher than that of benzene !!!

Take a look at the naphthalene peak (http://mihd.net/eglzh7). It has a shoulder on the right...

Does anybody know what's going on?

Thank you. Christian

[Peter Apps]

Hi Christian

To a good approximation the FID responds to mass of carbon, not moles of whole molecules. In this case you have a ratio of 19:1 mass:mass, which still does not resolve the discrepancy.

Please give all the details for the set-up - concentration of benzene and naphthalene in the methane, volume injected, column, flow rates etc etc.

Does the ratio of methane peak area to benzene or naphthalene conform to the expected ratio ?

The link to the chromatogram does not open, the shoulder might point to something eluting with the naphthalene and increasing its area, what happens if you run just the methane ?.

Another possibility is that the benzene peak is off the top of the detector or electronics linear range.

Peter

[christiancarioca]

Please give all the details for the set-up - concentration of benzene and naphthalene in the methane, volume injected, column, flow rates etc etc.

Chrompack 9001
CP-Sil 5CB, 0.32 mm, 40 m
35 kPa H2 carrier
loop volume appr. 50 microL (gas)
0.5 % C6H6, 0.015 % C10H8, balance CH4
analysis: 8 min at 40°C, ramp 50°C/min tp 260°C, 10 min at 260°C

Does the ratio of methane peak area to benzene or naphthalene conform to the expected ratio ?

CH4 is measured in one range (less sensitive) because of its concentration, and the rest of hydrocarbons in a more sensitive range, but both C6H6 and C10H8 are measured in the SAME range.

The link to the chromatogram does not open, the shoulder might point to something eluting with the naphthalene and increasing its area, what happens if you run just the methane ?.

Please, download the picture.
Methane alone nor benzene/CH4 mixtures shows such a feature.
Benzene-to-naphthalene area ratio is hold aproximately constant at various dilutions in the methane stream.
Area ratios benzene-to-CH4 and naphthalene-to-CH4 seem to be linear with respect to molar ratios in the sample.

Another possibility is that the benzene peak is off the top of the detector or electronics linear range.

Neither C6H6 not C10H8 peaks reach the saturation level at that range.

I cannot imagine why this is happening !!!

[Peter Apps]

Hi Christian

Puzzling for sure.

The methane is 200 times more concentrated than the benzene if your % concentrations are mole:mole, which is about 45 times on a mass:mass basis, which is what the FID responds to. Therefore you should easily be able to get both the methane and the bezene (and probably the naphthalene as well) on scale at the same range because the FID had a linear range of 5 to 6 orders of magnitude. Why do you need to change the range ?

Is this effect reproducible with different concentrations of naphthalene in benzene ?

Are there any other volatiles in the methane ? - your temperature programme is dramatically fast, when do the peaks elute ? - the link to the chromatogram is completely dead (i.e. it is not a link), please post again if you want me to have a look at it.

Peter

[MikeD]

Christian

A couple of thoughts come to mind. To confirm the masses reaching the FID is it a splitless injection of the 50 uL ? (naphthalene 0.04 ug, benzene 0.8 ug, methane 31 ug). If not what is the split ratio?

Secondly the vapour pressure of naphthalene at around 20-25oC temperature is equivalent to about 0.01 mol% and you say you are vaporizing 0.015 mol%. If you were actually vaporising 0.01 mol% it would make your found response ratio even worse, nevertheless I think there might be a connection. Perhaps there is liquid naphthalene somewhere in your inlet system.

[MikeD]

meaning your sample gas stream consists of methane, benzene vapour, naphthalene vapour and maybe naphthalene aerosol particulate.

[christiancarioca]

Thank you guys for answering!

Is this effect reproducible with different concentrations of naphthalene in benzene ?

Yes, it is.

A couple of thoughts come to mind. To confirm the masses reaching the FID is it a splitless injection of the 50 uL ? (naphthalene 0.04 ug, benzene 0.8 ug, methane 31 ug). If not what is the split ratio?

It is aproximately 5:1

Are there any other volatiles in the methane ? - your temperature programme is dramatically fast, when do the peaks elute ? - the link to the chromatogram is completely dead (i.e. it is not a link), please post again if you want me to have a look at it.

No, there are no other volatiles, except for traces of ethane.
Here is the new link: http://z31.zupload.com/download.php?fil ... path=15852

Secondly the vapour pressure of naphthalene at around 20-25oC temperature is equivalent to about 0.01 mol% and you say you are vaporizing 0.015 mol%. If you were actually vaporising 0.01 mol% it would make your found response ratio even worse, nevertheless I think there might be a connection. Perhaps there is liquid naphthalene somewhere in your inlet system.

Transfer lines are heated to 200°C from injection point of the benzene/naphthalene solution to the sampling valves of the chromatograph. Should be entirely in the vapous phase.

A new finding: I injected 0.5 microL of solution with a syringe, obtaining a area ratio C6/C10 = 0.030, which match with the reponse factor based on the mass of carbon in each compound:

(% wt C10H8)/10 : (% wt C6H6)/6 = (0.048/10) : (0.952/6) = 0.030

So the problem must be in the sampling !!!
Any idea?

[Peter Apps]

Hi Christian

The shoulder on the naphthalene is a major peak distortion, and given the very rapid temperature programme 13 min is a long retention time unless the column has a thicker than usual stationary phase. What is the retention time for the methane, the carrier gas head pressure, and the temperatures of all the heated zones, what is the pressure of the methane stream that you sample into the 50 mml loop, and what is the pressure in the loop. This whole troubleshooting process will go much quicker if you give us all the details in one go.

How are you connecting the sampling valve (6-port ?) to the column, and how are you doing the 5:1 split. Does changing the split ratio make any diffeence to the peak area ratios ?

Can you post the whole chromatogram from methane to naphthalene.

Peter

[christiancarioca]

Here is the whole chromatogram:
http://mihd.net/ui8e5v

The shoulder on the naphthalene is a major peak distortion, and given the very rapid temperature programme 13 min is a long retention time unless the column has a thicker than usual stationary phase

Why is this shoulder not shown when injected as liquid with a syringe?

What is the retention time for the methane, the carrier gas head pressure

45 kPa

and the temperatures of all the heated zones,

injector: 200°C
detector: 200°C
gas sampling valve block: 220°C

what is the pressure of the methane stream that you sample into the 50 mml loop, and what is the pressure in the loop.

atmospheric

How are you connecting the sampling valve (6-port ?) to the column,

via a capillary tube, heated over 200°C, to the injection port

and how are you doing the 5:1 split. Does changing the split ratio make any diffeence to the peak area ratios ?

It will check it.

The big question is: Why do I get the right relationship between areas with a syringe injection (and no shoulder), but naphthalene appears in much greater quantity with a loop injection? Where does all this naphthalene come from?

[Peter Apps]

Hi Christian

My best guess at the answer to the big question is that there is some kind on pressure or flow disruption during injection from the valve, which is changing the split ratio.

I have two major concerns. With hydrogen at 35 or 45 KPa head pressure (which is it ?) I would expect the methane to elute faster than 2.88 minutes, unless the column has a thick film - you have not posted what the film thickness is. The optimum linear flow rate for hydrogen is 50 cm/s, which would mean the methane eluting at 80s.

Also you say that there are no other volatiles in the sample except some traces of ethane, and yet there are two other peaks at 11.7 and 12.2 min that are similar in size to the naphtalene.

The problem is not that you are getting extra naphthalene, but that you are losing benzene - that is why I wanted to know how the methane area relates to the benzene and naphtalene areas. I am still not clear why you change the sensitivity between methane and benzene - naphthalene (you do not have to have all the peaks on scale in order to integrate them). At what point on the chromatogram (time) does the sensitivity change ?.

How is the capillary line to the inlet connected to the inlet - via the carrier gas line or through the septum ? If the latter how far down into the inlet does the tip of the line extend. What kind of liner have you got in the inlet ?

Have you checked everything for leaks ?

Regards Peter