1. (a) Why do open tubular columns provide greater resolution than packed column
ID: 510332 • Letter: 1
Question
1. (a) Why do open tubular columns provide greater resolution than packed columns in gas chromatography?(b) How can you improve resolution between two closely spaced peaks in gas chromatography without changing the column?
(c) Describe split, splitless, and on-column injection techniques. When would you use each? l. (a) Why do open tubular columns provide greater resolution than packed columns in gas chromatography? (b) How can you improve resolution between two closely spaced peaks in gas chromatography without changing the column? (c) Describe split, splitless, and on-column injection techniques. When would you use each?
Explanation / Answer
a) Open tubular columns provide greater resolution than packed columns in gas chromatography because of their large retention time.
b)
This question would be much easier to answer if you provided details of your current operating conditions. Lacking those I can give only general advice.
With hydrogen as carrier gas the linear flow velocity for maximum resolution is 50 cm/s. Increasing the flow will decrease resolution slightly, but will save time. Decreasing the flow will degrade resolution and waste time, so is pointless.
The optimum programme rate for a 120m column is 1 C/min - you should see peak widths at half height the same for late peaks as for early ones when the programme rate is optimal.
A column delivers the maximum number of separated peaks when the peaks are more or less evenly spread across the chromatogram. If your peaks are bunched together but you have areas of empty baseline also then you are not using all the separating power of the column. If you are trying to separate compounds with similar boiling points on a column whose selectivity is based on molecular weight, volatility and boiling point, then you will inevitably have bunched, overlapping peaks. The only solution to this is a change in stationary phase - and a solution is not guaranteed; you would need compounds with structural differences for the different selectivity of another stationary phase to make any difference to their relative retentions.
In a crowded chromatogram, no matter what column you use, there will always be peak overlaps - the best that you can hope for is to get clean peaks for the subset of components that you are particularly interested in.
Since increased peak widths at baseline compromise separation you need to eliminate extraneous causes of peak distortion - concentration overloading, dirt in the inlet, poor choice of solvent, wrong inlet liner etc etc
c)
There are four types of injectors: split, splitless, megabore and "on-column". The first two types use a common injector called "split/splitless injector". Each type of injector is well suited for a particular type of sample.
A split/splitless injector consists of a glass tube ("liner") which creates an inert environment inside the injector and where the sample is vaporized. A syringe is used to pierce the septa and to introduce the sample into the injector. The injector temperature should be high enough to ensure instant vaporization without degradation of the sample. For most samples, 200-250°C as injector temperature is high enough. The sample vapor is mixed with the carrier gas and is transported into the column. Components of the sample that are not vaporized remain in the injector. The septum purge is a low flow which minimizes the amount of septum bleed materials which could contaminate the GC system. Septum purge gas sweeps the bottom of the septum and the top of the liner (labeled "T" for top at the GC) out through the purge vent. A typical septum purge flow is between 0.5 and 5 mL/min.
Split injection is a most common technique which is used for highly concentrated samples (concentrations between 0.1 and 10 µg/µL per single component). In this technique the sample is vaporized immediately, mixed with carrier gas and a small amount enters the column. The remaining sample leaves the injector via the split vent at the bottom of the liner (labeled "B" for bottom at the GC). Split ratios (resp.: "split") between 1:10 and 1:100 are typical.
Splitless injection is used with a closed split flow for lower concentrated samples.
Megabore direct injection is used with packed columns (0.45 and 0.53 mm I.D.).
The on-column injection is not a vaporization technique. The sample is deposited directly into the column without vaporization. It is used for high boiling compounds like petroleum waxes or thermally labile compounds.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.