liquid hydrocarbon analysis by GC

[amrnasser87]

hello all

My problem is while analysis of liquid hydrocarbon after converting it to gas by convertor it does not give accurate and constant result along many runs . what is the accurate way to analysis liquid hydrocarbons by GC ?

best regard
amr

[chromatographer1]

Direct volumetric injection is the best method of analyzing liquid hydrocarbons.

But INFORMATION IS MISSING.

Which hydrocarbons? What levels? What matrix?

The best way to travel over water is a boat......... but over a pond, a river, a lake, or an OCEAN?

What are you doing and what is your goal?

Help us help you. Give some details, ok?

best wishes,

Rod

[amrnasser87]

instrument name :
HP Gas Chromatography 6890 II

Column used:
AL2O3NA2SO4

Column tempraure:
50 - 180 c

Carrier gas:
nirogen

Questions :
1-is it possible to inject dirctly the sample to GC ( i think carrier gas can not cary liquid also liquid will harm the column )
2- what is the general princple of liquid analysis by GC ?

amr

[DSP007]

Its crasy method, no comments.
For hydrocarbons analisys use a column which low polar phase such as OV-1, OV-17, SE-54
Nowdays use calpillar column.
See the ASTM from benzene if you in USA or GOST Бензины if you "in USSR".

[Don_Hilton]

Amr,

What analytes are you looking for? This is not a particularly common type of column.
Is this a method you are devloping or is it a method that has been operating and you are maintaining/modifying?

There are a number of very reproducable methods for hydrocarbons that have been published. One comes to mind for petroleum hydrocabons ranging from methane through C9: ASTM D5134 - very good and well known.

On the question of injecting a liquid directly in the GC. This is commonly done. The GC inlet is hot (250 - 300 deg. C) and will quickly vaporise a sample. If you are looking at things that will elute from your column at 180 deg or below, the GC inlet will take care of this.

As far as liquid on the column. With the column you are using, if there is water present in the liquid, it will cause a problem. Other polar compounds can also cause a problem. But, if you inject hydrocarbons that boil a temperatures close to 180 deg. They will strongly partition to the stationary phase at 50 deg. They may be adsorbed on the column or may be present as liquid condensed on the column. Whichever way the vapors partion to give separation, any significant quantity of compunds boiling above 50 deg will condense when they reach the relatively cool column and then spread from there.

[amrnasser87]

dear all,
thank you so much for reply .however i wrote this method data from my lab supervisor i am going to discuiss him regarding your replies

best regard
Amr

[AICMM]

amrnasser87,

If you are talking about something like liquid propane, this is not difficult to do. If you have a Valco type valve (4 port liquid sampling) and you plumb it into the injection port (as Don suggested) it will do a great job of vaporizing your sample. You can get very good reproducibility with this type of configuration. Use a split injection so that your alumina column does not get over-loaded. Is this the sort of analysis you are talking about?

Best regards,

AICMM

[larkl]

The alumina columns are still quite common in the petrochem labs measuring light organics (C1 - C5) that do NOT have cryogenics for cooling. Can't get the required separation on something like a methyl silicone (SPB1, DB1) column.

Flashing the liquid to a gas is problematic, especially if the components differ in boiling point and/or vapor pressure. You'll get discrimination in the vaporization. If you're doing low-boilers, the Valco valve is the way to go.