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Technical Service "Red Flags" - GC

30 Jul 2015

For those of you not familiar with the term “red flag”, or phrase “It raised a red flag”, it basically means that something does not sound correct, or is very unusual or uncommon. In tech service, we listen for these “red flags” and if we hear one, we will either offer a cautionary statement if we consider something is dangerous or offer unsolicited professional advice if we believe it is needed. In many cases, it may simply be asking a customer to repeat a statement which is questionable.

red flag

So what are examples of “Red Flags” in technical service? A few of the more common ones are below.

GC capillary columns which are connected in series (using a simple union-type connector) rather than to a switching device. If one needs to connect two capillary columns in series (not including a guard column), I usually ask why this needs to be done. This is especially true if the columns contain dissimilar phases. If you are connecting multiple columns in series, you may want to review the following:

Restek Searchable Chromatogram Library

EZGC Chromatogram Modeler

How to choose the correct GC column – Part 1

How to choose the correct GC column – Part 2

How to choose the correct GC column – Part 3

How to choose the correct GC column – Part 4

GC columns – when one is not enough


Requests for packed/micropacked columns which are longer than 5-meters. Although packed columns longer than 5-meters are used, columns less than 4-meters are much more common. Carrier gas head-pressures needed for long packed/micropacked columns tend to be very high, and unless a valve/sample loop (or other switching device) is used, loss of sample through the GC injection port septa via blow-back is common if using a (Gas-Tight) syringe injection. To read more about packed column basics, see "Packed Column information for the beginner".

If interested in ordering a packed/micropacked column, or to obtain a quote, you may want to review:

Things to Consider Before Ordering a Packed Column


Using a Uniliner in split-only mode. I even commented on it several years ago in this post:  Liners Every Lab Should Own (in my opinion)

"Liners Every Lab Should Own (in my opinion)" Just remember, these liners are designed for splitless injections only (if you experience carryover, try turning on the split flow at 5mL/min after the last compound elutes).

Injecting 1µL (or more) of an aqueous (water) sample in splitless mode. Sample back-flash can be a real concern. Capillary GC Column Killers – Part 4


Splitting a sample after the GC column and sending part of the sample to an atmospheric detector (like a FID) and the other part to a mass spectrometer (mass spec). Remember that the mass spec is under vacuum, so it will want to pull more of the sample/carrier gas into it. Because different mass specs have different pumping capacities, there is no kit or product that we sell which will guarantee equal flow to each detector.

Using air as a carrier gas. Please remember that with almost all GC columns, oxygen will cause irreparable damage to the column’s phase (through oxidation). While it is true that at room temperature very little damage/oxidation will occur, at higher temperatures damage will occur almost immediately.

red flag

So the next time one of us in technical service asks you to repeat yourself, it may be because we thought we heard a "Red Flag".  Thanks for understanding.