Chromatography Forum

Hi,

I have a problem with my biodiesel analysis. I have followed the procedure in ASTM D6584. All the standards were run individually. My problem is when I'm running low concentration of the triolein standard, the peak cannot be detected. This is fresh standards because every day I prepare it. Please help me.

TQ.

Your column is showing activity and causing the losses of the peak.

This is not unusual. Different columns show different performance when new and as they age.

The best solution is to replace the column with a new column unless your levels are low enough in your sample to give you a satisfactory answer to your analysis.

best wishes,

Rod

It's also possible that the retention gap ( in front of the column ) needs replacing. Was the decline gradual or sudden?

If gradual, you may also be able to condition the column at temperature to reduce the activity that is destroying the triglycerides, but ensure the following first....

If sudden, it's really important to ensure that you have good quality, well maintained gas purifers on your carrier gas lines, and you protect the column from trace oxygen by having good cylinder changeover regimes - if you use cylingers of gases.

I strongly recommended that any laboratory performing routine QC have replacement columns, retention gaps, injector inserts. When ther is a problem, then the new column can be sustituted once simple problem solving suggests the root cause is the column.

Please keep having fun,

Bruce Hamilton

Bruce is correct as always.

The retention gap is a critical component.

One other thing comes to mind, do you have your injector following the oven temperature profile? Or is it remaining at a cool temperature?

A setting may have been changed by accident.

best wishes,

Rod

Hi,

Thank you for the reply.

My Cool on column injector is new. This is the first analysis for this inlet. The retention gap also new. Before this we don't have this gap for our GC. We seldom used for this column which is DB-1HT. This analysis used very high temperature.

I think maybe because the concentration is too low inside my standards, just 10 ul & 20 ul. But before this for standards glycerol & monoolein, I can detect the peak although it is 10 ul & 20 ul. It is possible because of this concentration it cannot detect any peak? This 10 ul of standard was mixed with 8 ml n-Heptane and 100 ul MSTFA. The chromatograms shows all the peak from 0 minutes until 1. 9 minutes only. The retention times for triolein is about 22-26 minutes.


Instrument conditions:
Cool on column inlet
Mode Ramped
Initial temperature oven track, approx 50 Deg C
Pressure 7.6 psi helium
Injection amount 1 ul
Initial column flow 3.0 ml/min, constant pressure mode
FID temperature 380 Deg C
Oven temperature program 50 Deg C for 1 min
15 Deg C/min to 180 Deg C, hold 0 min
7 Deg C/min to 230, hold 0 min
30 Deg C/min to 380, hold 10 min


Thank you.

You still have to determine why you can't see the peaks - so you can fix the problem.

My suggestion would be to prepare a triolein standard that has 5 times your current concentration in n-heptane and no MSTFA. Ensure that you can see the triolein peak, and it appears where it should, even if it's smaller than expected. Then repeat the injection and analysis 5 times, and see whether the peak changes in area with more injections.

If you have some tripalmitin or tristearin, try those as well, as they are less susceptible to activity and degradation. Triolein can polymerise and oxidise quite quickly, so it's useful to have a spare stock of it to confirm your solutions are OK.

Please keep having fun,

Bruce Hamilton