Sample dilutions for splitless injections

Discussions about GC and other "gas phase" separation techniques.

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Hi,

I am using a Gas chromatograph (CP-3800) with a FID detector and a capillary column (VF-5 ms, 30 m × 0.25 mm with 0.25 μm film thickness of 5% phenyl-methylpolysiloxane column) to analyse fatty acid methyl esters (FAMEs).

For the analysis, 1 ml of total sample volume is prepared using 100 µL of FAMEs + 20 µL of internal standard + 880 µL of heptane. 1 µL from this 1 ml is injected into the column with a split ratio of 100.

However, split injection is not working properly now. Thus, i am thinking to do the injections using splitless mode by doing the manual dilution instead of on column dilution.
can somebody suggest me, how to prepare the sample of the same composition (same as with split ratio of 100) to do the injections with splitless mode.

Thank you!
Use 1ul of FAMES instead of 100 ul.

Peter
Peter Apps
Hey Peter!

Thank you for your reply. 1 uL of FAMEs is fine but the solvent composition would be very high in this case if i make up the total sample to 1 ml.
I mean to say that i will get a broader peak for solvent if i inject 0.9 uL of heptane. In the split mode only 9 nL of heptane was going (but it here it is much concentrated in splitless mode).

Kindly, help me in making same the composition as in the split mode for all the sample components (FAMEs, internal standard and the solvent).

Thank you!
nehalamba wrote:
...I mean to say that i will get a broader peak for solvent if i inject 0.9 uL of heptane. In the split mode only 9 nL of heptane was going (but it here it is much concentrated in splitless mode).
...

That's the nature of splitless injection. The whole amount of injected solvent goes through the column.
Stay with splitt injections for other reasons as well.
dblux_ wrote:
nehalamba wrote:
...I mean to say that i will get a broader peak for solvent if i inject 0.9 uL of heptane. In the split mode only 9 nL of heptane was going (but it here it is much concentrated in splitless mode).
...

That's the nature of splitless injection. The whole amount of injected solvent goes through the column.
Stay with splitt injections for other reasons as well.


Hey,

So, we can not dilute it to make it to the same composition as in case of split injection?

Thank you!
nehalamba wrote:
So, we can not dilute it to make it to the same composition as in case of split injection?

You may dilute your sample as much as you want.
But the aim of dilution is to inject less FAME to the column. However you will still inject 1 µL or 0.5 µL and it will be mostly heptane.
nehalamba wrote:
dblux_ wrote:
nehalamba wrote:
...I mean to say that i will get a broader peak for solvent if i inject 0.9 uL of heptane. In the split mode only 9 nL of heptane was going (but it here it is much concentrated in splitless mode).
...

That's the nature of splitless injection. The whole amount of injected solvent goes through the column.
Stay with splitt injections for other reasons as well.


Hey,

So, we can not dilute it to make it to the same composition as in case of split injection?

Thank you!


By definition, if you dilute something you change its composition.

Probably you should fix the splitter.

Peter
Peter Apps
I am assuming that the broadening of the solvent peak is interfering with some of your earlier eluting analytes and if this is the case and main issue for you, could you use an earlier eluting solvent such as hexane or pentane? You would still get the broader solvent peak but it would elute earlier and might not be an interference quite as far into the chromatogram.
Gizmo wrote:
I am assuming that the broadening of the solvent peak is interfering with some of your earlier eluting analytes and if this is the case and main issue for you, could you use an earlier eluting solvent such as hexane or pentane? You would still get the broader solvent peak but it would elute earlier and might not be an interference quite as far into the chromatogram.


Hey Gizmo,

You got it correctly. I will try with hexane once and check. If this does not interfere with my analyte peak, i will proceed with this.
Are there any invisible solvents also that can be used here?

Thank you!
A solvent that would be "invisible" to your analysis would be one that falls outside of the retention index range of your analytes or falls within a large window of irrelevance within the chromatogram. And choosing something would depend upon the retention indices of your analytes. There could be something you could use that would elute after all of your analytes and in that case how broad the solvent peak is would be irrelevant. If you have the RI's of your analytes handy maybe someone might have some ideas. But hopefully the hexane or pentane works for you.
Gizmo wrote:
A solvent that would be "invisible" to your analysis would be one that falls outside of the retention index range of your analytes or falls within a large window of irrelevance within the chromatogram. And choosing something would depend upon the retention indices of your analytes. There could be something you could use that would elute after all of your analytes and in that case how broad the solvent peak is would be irrelevant. If you have the RI's of your analytes handy maybe someone might have some ideas. But hopefully the hexane or pentane works for you.


I would find out some of the solvents which would elute after my analytes after trying with hexane. Thank you so much for providing this useful information.
nehalamba wrote:
Gizmo wrote:
A solvent that would be "invisible" to your analysis would be one that falls outside of the retention index range of your analytes or falls within a large window of irrelevance within the chromatogram. And choosing something would depend upon the retention indices of your analytes. There could be something you could use that would elute after all of your analytes and in that case how broad the solvent peak is would be irrelevant. If you have the RI's of your analytes handy maybe someone might have some ideas. But hopefully the hexane or pentane works for you.


I would find out some of the solvents which would elute after my analytes after trying with hexane. Thank you so much for providing this useful information.


A solvent that elutes late in the chromatogram is unlikely to be anywhere near volatile enough for flash injections, it will recondense on the column, and cause all sorts of horrible peak shapes.

If you are really serious about this, carbon disulphide is nearly invisible to an FID, but it stinks and is toxic.

Fixing the splitter still seems like the best option - what is the problem with it ?

Peter
Peter Apps
Another option is to use a hold on the column oven temperature that would allow the solvent to pass through but retain the early eluting targets.

Something similar to doing solvent focusing would work well I think, start at just below the boiling point of the heptane, then after about 0.2minutes ramp to about 15C above the boiling of heptane, then hold for 2 minutes then continue the ramp as normal. You may need to add the appropriately deactivated retention gap to maintain peak shapes, but that isn't too difficult.
The past is there to guide us into the future, not to dwell in.
Peter Apps wrote:
nehalamba wrote:
Gizmo wrote:
A solvent that would be "invisible" to your analysis would be one that falls outside of the retention index range of your analytes or falls within a large window of irrelevance within the chromatogram. And choosing something would depend upon the retention indices of your analytes. There could be something you could use that would elute after all of your analytes and in that case how broad the solvent peak is would be irrelevant. If you have the RI's of your analytes handy maybe someone might have some ideas. But hopefully the hexane or pentane works for you.


I would find out some of the solvents which would elute after my analytes after trying with hexane. Thank you so much for providing this useful information.


A solvent that elutes late in the chromatogram is unlikely to be anywhere near volatile enough for flash injections, it will recondense on the column, and cause all sorts of horrible peak shapes.

If you are really serious about this, carbon disulphide is nearly invisible to an FID, but it stinks and is toxic.

Fixing the splitter still seems like the best option - what is the problem with it ?

Peter


Hey Peter,

The pressure is not getting stabilized with split on. Thus, the status is always equilibrating and not ready. However, there is no such problem when split is off.
James_Ball wrote:
Another option is to use a hold on the column oven temperature that would allow the solvent to pass through but retain the early eluting targets.

Something similar to doing solvent focusing would work well I think, start at just below the boiling point of the heptane, then after about 0.2minutes ramp to about 15C above the boiling of heptane, then hold for 2 minutes then continue the ramp as normal. You may need to add the appropriately deactivated retention gap to maintain peak shapes, but that isn't too difficult.


Thank you James, i would try this. What should be the provided ramp rate?
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