Better Ways to Work High Matrix Samples
Posted on May 15th, 2012
The current generation of GC-triple quadrupole mass spectrometers, provides a great means for the detection and quantification of target compounds in complex samples.
In this context, “complex” is a polite way of describing samples that are utterly filthy, full of high boiling, involatile material and liable to contaminate your inlet and column pretty quickly.
Extensive clean-up can be used, but this is expensive, extremely time-consuming and usually results in very low analyte recoveries, so an increasing number of analysts use simplified sample preparation methods and choose to live with the instrument down-time that results from a build-up of dirt within the system.
GC-MS suppliers recognize this reality and it is common to fit a back-flush capability to the system, so that the high boiling gunk can be back-flushed into the hot injector, preserving the integrity of the main analytical column.
In our experience, the approach offers only a partial solution:
- Samples often contain involatile material that remains in the injector, and injector contamination will build up rapidly to a point where it becomes a problem. Back-flushing is no help in solving this problem.
- Material that is back-flushed into the injector will accumulate somewhere in the injector or its associated pneumatics and will often give rise to carry-over and eventually, blockages. Cleaning-out the injection system at this point is a big job.
There is another way of approaching the problem.
ALEX works like this:
- Use the lowest injector temperature that will transfer enable your least volatile target analyte to the column completely (easy to find with a few scouting runs using your standard mix).
- This temperature will ensure that all of the involatile material stays within the injector and that the only material to enter the column and detector is certain to come out at the other end.
- As you run successive samples, you will begin to accumulate muck within the injector and experience will soon tell you how frequently you need the system to swap out the liner automatically.
- Once you have this sorted out, you should then be able to maintain you data quality while running for extended periods.
More up time, means that you can run more samples with less hassle.
It is pretty easy to clean-up and deactivate the used liners, and I know of analysts that recycle their ALEX liners continuously.
If you wanted a real belt-and-braces approach, you could combine ALEX with a column back-flush.
With ALEX, you are restricted with the normal injection volumes (1-2 microlitres), however, there is a variation on this concept that enables you to work with much bigger samples (GERSTEL ATEX).