Super inertia/Low running off/High efficiency/Long life
During the gas chromatography, the chromatographic column may be called the heart of the gas chromatograph and it plays the important role for analyzing the group to be tested. Today, in order to choose chromatographic columns well, let’s learn about relevant specification parameters of the gas chromatography column.
There are 2 types of GC which are the packed column and the capillary column, the filler which is inside of the column with the adsorbent function is called the stationary phase. Based on differences of stationary phases, there are 2 modes which are gas-liquid distribution and gas-solid distribution. In application, compared with packed column, the capillary column is more efficient and widespread using. And let’s see some relevant parameters and meanings.
Stationary phase types
Relevant parameters are on the package and the label
WM-5, WM-35, WM-InoWAX are signed on the label and the package. WM means the brand of the chromatographic column, like Elite, TG, DB, Rtx, SH and etc. The numbers and letters mean columns types.
Let’s see upper picture, 3 parts are in the capillary column: the surface polyimide coating(strength the tube wall), the middle fused silica layer and the inside stationary phase.
Presently, popular stationary phases are polysiloxane stationary liquid, polyethylene glycol and solid particles with adsorbent function.
WM-1 (100% dimethyl polysiloxane)
WM-5 (5% phenyl, 95% dimethyl polysiloxane)
WM-624/1301 (6% Cyanopropyl phenyl, 94% dimethyl polysiloxane)
WM-1701 (14% Cyanopropyl phenyl, 86% dimethyl polysiloxane)
WM-INOWAX (polyethylene glycol)
WM-FFAP (Polyethylene glycol modified by nitroterephthalic acid)
WEL-PLOT Al2O3 (Sodium sulfate modified alumina)
Besides, the analytical lab now uses GC-MS, GC-MS/MS analyzer which need good inertia, low running off and high temperature resistant chromatographic columns. According to this demands, the MS mass spectrum column comes out, like WM-1MS, WM-5MS, these type of columns have low flowing off in high temperature.
Main types of the capillary columns:
- WCOT (wall coated open tube column)
After pre-treating the inside wall, coating the stationary liquid or crosslinking bonding on the inside wall. It’s the most type.
- SCOT (supporter coated open tube column)
Coating the supporter on the inside wall, then coating the stationary liquid. This column has thick liquid film and big column capacity.
- PLOT (Porous layer open tube column)
Coating adsorbent function solid particles instead the stationary liquid. It belongs to a gas-solid chromatographic open tube column. This column is used for separating permanent gas and low molecular weight organic compounds.
Porous polyvinylphenyl, deactivated alumina and molecular sieve are common PLOT stationary phases.
Length, internal diameter, liquid film thickness
Longer the column, higher the total efficiency, better the component separation, but longer the analyzing time, higher the column cost. If sample groups are less, and easy to separate, we can choose short column, like 10~15m.
The column internal diameter: the capillary column’s theoretical plate height and the internal diameter are direct ratio. On the same length, less the internal diameter, less theoretical height, more the theoretical plate number and better the column efficiency. Small caliber column is fitting for analyzing multiple groups pesticide residue regarding to high separating degree. Wide caliber column has high volume and low efficiency fitting for direct feeding, upper column feeding and undivided feeding.
The thick liquid film, the high column volume, the target compound keeps long time staying and it’s very good to separate and analyze efficiently. We can select thin liquid film column for weak volatility and high boiling point grouping.
For example: Column 30m×0.25mm×0.25μm, means the column length is 30m, the internal diameter is 0.25mm, the thickness of liquid film is 0.25μm.
The column using temperature limit.
The capillary generally has 3 temperature limit values, for example -60 to 325/350℃.
- -60℃: low limit temp.. When the temp. is lower than this value, the column’s stationary liquid is quite thick with low efficiency. If the temp. is lower, the sample is easy to condensate and isn’t normal to separate.
- 325℃: constant up limit temp.. That means the column can be used for a long time in this temperature.
- 350℃: procedure heating temp. upper limit. The procedure heating can’t be over this temperature, and can’t stay on this temperature for a long time, or the stationary phase will be hot damaged.
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