Chromatography Forum

hi,

i'm trying to develop a method in which i'm going to use PTV to inject a large sample volume (50µl) on a GC-column.

now, are there any limitations on the diameter of the column in terms of peak broadening...

Best regards,

Willem

Yes

Do you mean smaller limitations, or larger limitations?

peter so concisely responded [100% correctly as always, of course,] there are limitations.

With too big a column diameter, you can get broadening of the plug, as also with too small a diameter. Phase coating type and the type of carrier solvent of your sample also has a lot to do with your successful use of LVI, and/or diameter and film thickness for capillary columns.

As always, holding back details allows only the most brief and non-helpful responses. Remember, the more you give, the more you get.

Now, if you want to get a more detailed answer you should provide a LARGER amount of details about what you are doing.

best wishes,

Rod

ps

if your project is secret, why are you discussing it on a public forum?

Ok, The components we are trying to analyze are around 50 pesticides (OCP and OPP).
The PTV-program is optimized and ready to use but we have a rather bad separation.
So we want to achieve more efficiency with a column with a smaller diameter.
Now I’m working with an HP-5MS column (30mx0.250mmx0.25µm).
I would like to keep the injection volume, because I need my sensitivity for the trace analysis and I’m already working in SIM-mode.


Best regards!

By reducing column diameter you reduce the loadability of the column, which can lead to even worse separations because peaks front-tail, and to retention shifts of target analytes caused by major components that you do not even see on the chromatogram if you are running SIM.

On the plus side, at optimum conditions the peaks from a narrower column are themselves narrower, and so you get better lower limits of detection, and better signal:noise.

Volume flow rates into narrow bore columns are lower than into normal bore, which can cause peak broadening due to poor flushing of the inlet. To an extent this can be improved by stationary phase focussing and temperature programming.

With columns narrower than the one you are using inlet pressures have to be high to overcome the flow resistance of the column, and because linear optimum flow rates are higher in narrower columns. These higher pressures affect the evaporation of solvent so you might need to adjust some of your inlet conditions.

Before going down in column diameter I would make certain that the separation problems that you have at the moment are caused by column diameter per se. If they are due to column deterioration for example you can solve the problem by replacing the column with one of the same dimensions.

Peter

Are you sure you need an other column?
We are using a 50µl CH2Cl2 injection, a RXI5 sil-MS 30m 0.25mm 0.25µm with 5m pre-column to analyse +/- 140 OCP-OPP-PCB on a Thermo TSQ
could you please tell us what solvent you are using and what the PTV-settings are?

kind regards
BMU

The program of the PTV is the following:

initial T°: 60 C --> 350 C (in 4 min)
pressure 10 psi
vent time:0.75 min
vent flow: 50ml/min
purge flow: 50 ml/min


we are using CH2CL2 as well, so we can keep the risk of losing volatile components to a minimum.
what is the program you are using?

The more you don't tell us the more we can't tell you, feeding little scraps of information is not the fastest route to an answer.

For how long does the inlet stay at the initial temperature ?

For how long do you transfer sample to the column ?, is this transfer splitless ?

Are you doing a fast injection of the 50ul ?

What kind of inlet liner do you have, and does it have any packing in it ? What kind of packing ?

What is the concentration of the analytes in the solution that you are injecting ?

Is your problem that the different pesticides do not separate, or that the pesticides do not separate from interferences ? I presume the former since you are working in SIM. How are you programming your SIM ?

Peter

The inlet starts at 60°C and stays at the temperature for 1 min., then the temperature rises to 340° in 4 min.
the injection speed is 70 µl/min and the mode we use is a solvent vent method (splitless).
The liner is an empty insert with swirl holes, so no packing.
And for now the analyte concentration is 50pg/µl.

My problem is that I cannot develop a good SIM method without a good separation of my components. With the program you can cut the analysis time into parts and in each part you can decide which fragments that are measured. Too much analytes in a part give too few points per peak and with a few analytes the windows are getting very small which will probably give trouble in the future.

If you need more info, feel free to ask because I’m not really familiar with this technique.

What is the column temperature programme ?

You might just be using the wrong type of column. Do you know that the column can separate the pesticides under ideal conditions ? Have you been able to separate them on this column using ordinary splitless injections, or have you seen an application where they are separated on the same stationary phase ?

Are your peaks tailed ?

Peter

our PTV method (ocp-opp-pcb +/-140 components) on Thermo Trace GC-ultra +TSQ quantum:

mode: PTV large volume
start temp 50°C
splitflow 20ml/min
splitless time 1 min

injecting at once (50µl / sec I think)
- flow: 100ml/min for 0.3 min
- temp ramp: 2.5 °C/sec -> 240°C
- cleening fase 14.5°C/sec -> 320°C

We do not use a seperate evaporation fase, but simply keep the injector at a higher splitflow for 0.3min.
We use a LV-liner with sintered glass-beads.

Oven temp starts at 40°C

Has this analysis worked before? Have you tried to inject 50µl into the liner when it is not in the ptv, just to see if you are not flooding the liner when injecting 50µl of CH2Cl2 at this speed into a liner without packing?

Have you tried to inject 50µl into the liner when it is not in the ptv, just to see if you are not flooding the liner when injecting 50µl of CH2Cl2 at this speed into a liner without packing?

With a 70 ul/min injection at 60C Willem's method looks like a simultaneous vaporization method. If it wasn't the solvent would flood the liner for sure, but that would give poor peak size repeatability rather than (or as well as) poor separation.

Just out of interest - do you see a drop in inlet temperature while you are venting solvent ? Many years ago I did some crude measurements of evaporative cooling with fast gas flows and volatile solvents and it was easy to get temperature drops of more than 10 C.

Peter

Peter,

We have never done any measurements on inlet temp, but there has to be a drop.
DM has a boiling point of 40°C and our inlet temp is 50°C.
If it wasn't cooling down the liner would flood.

I see I misread Willems post of an injection speed of 70µl/min I assumed it was 70µl/sec

i got the method from an application.

For the temperature program of the column:

First I start at 60° and keep it that ways as long as the PTV program is running.
Then
40°/min till 160°C (first peak T°C -10)
3°/ min till 220° C
20°/min till 300°

But I’m still fine tuning a bit to get rid of the gaps and non-seperated peaks.