Chromatography Forum

Hi all,

we have a puzzling situation here on our GCMS. We inject a Grob mix to test system suitability. We decided to switch the solvent from ethyl acetate to acetonitrile and to double the concentration of the analytes.

Surprisingly, we see a decrease in MS-Signal for all analytes across the board when we make injections of both solutions. I.e. there is about 1/2 the signal for the ACN injeciton despite it being twice as concentrated.

Peak shapes are good in both cases and are actually improved a little with ACN.

Any ideas how this could be explained? Thank you!

Injection parameters: 250°C, Split: 1:25, Inj. Vol: 1 µL, Column flow: 1.25 mL/min, Pressure: 10 kPA, Liner: 3.5mm.

The molecular weights and hence the volume the vaporised solvent occupies must have some effect. ACN would in effect dilute the sample down when vaporised compare to ETAC.

The vapor volume of an injection of ACN is roughly double the vapor volume of an equal volume injection of ethyl acetate. However, for a split injection I would not expect this to cause a dramatic effect because no matter how the solvent expands when vaporized you will still get 1 part of sample onto the column for every 25 parts of carrier gas.

Contrarywise, if the injection were splitless and the split vent time was unchanged for both ACN and ethyl acetate injections, and the split vent time allowed most of the ethyl acetate injection to load onto the column, the same split vent time would not be enough for a similar amount of ACN vapor to load on the column.

Curious if you were able to solve this one.

Thank you both for your insights. I also disagree with the rational that since ACN exapands more and we get a smaller concentration of the analytes in the gas, there is lless signal. If everything is injected we woudl still have the same amount on column.

.....Unless we get backflash, which after consultign the Restek solvent expansion calculator may very well be the case. 1 µL of ACN ist quite a bit for a 95x3.5mm liner. I never worried about this with solvents other than water or maybe MeOH. I do get a warning in the calculator:

EXCESSIVE BACKFLASH!
Under these conditions, the size of the vapor cloud exceeds the liner's effective internal volume.
Generally, reduce backflash by injecting a smaller amount of liquid. Temperature, pressure, and choice of solvent also affect vapor cloud size.
Volumes (µL)
457µL Liner (effective*)
486µL Vapor cloud
* Presence of carrier gas in liner diminishes volume available by approximately ½.

Thank you both for your insights. I also disagree with the rational that since ACN exapands more and we get a smaller concentration of the analytes in the gas, there is lless signal. If everything is injected we woudl still have the same amount on column.

.....Unless we get backflash, which after consultign the Restek solvent expansion calculator may very well be the case. 1 µL of ACN ist quite a bit for a 95x3.5mm liner. I never worried about this with solvents other than water or maybe MeOH. I do get a warning in the calculator:

EXCESSIVE BACKFLASH!
Under these conditions, the size of the vapor cloud exceeds the liner's effective internal volume.
Generally, reduce backflash by injecting a smaller amount of liquid. Temperature, pressure, and choice of solvent also affect vapor cloud size.
Volumes (µL)
457µL Liner (effective*)
486µL Vapor cloud
* Presence of carrier gas in liner diminishes volume available by approximately ½.

For this I try using a pulsed pressure injection. Increase the pressure at time of injection to compress the vapor cloud, then return to normal once twice that volume had been passed through the inlet. If the inlet is 457ul, then if increased pressure results in 1000ul/minute flow rate in column I hold pressure for 1 minute. Raise pressure to give 2000ul/minute flow into column and hold for 0.5-0.6 minutes, ect. Normally it gives very good peak shapes and eliminates problems with backflash.

Thank you for the advice! We will try this. I think it is called high pressure injection on the Shimadzu.