Analysing for Unknowns – Sample Prep Considerations

In an earlier post by Paul Roberts on the analysis of unknown analytes by GC-MS, Paul touched upon a point concerning sample preparation that is worth considering in more detail – the requirement that this kind of analytical work demands from your sample preparation.

There are three vital points that you have to consider…

1. The importance of non-selective sample preparation. When analysing for unknowns, since you don’t know what you are looking for, the only sample preparation techniques worth considering are ones that are non-selective. In targeted analysis, it is perfectly OK and indeed positively helpful, if the sample preparation method that you use is selective with respect to the class of analyte you are testing for, but when analysing for unknowns, you will be doomed by the curse of false negatives.
2. Any sample clean-up applied can only function on a basis of component volatility. Whatever the capabilities of your mass-spec, when you receive a dirty sample, injecting the sample in an untreated form will create serious problems with the inlet and column. The problem materials will usually comprise, involatile solids or solutes that stick in the inlet or on the head of the column and intefere with subsequent runs. Point number 1 limits your options in terms of sample clean-up, however, it is perfectly feasible, by careful selecton of injector conditions, to apply a “volatility cut-off” so that the only compounds that are able to enter the GC column are of sufficient volatility to be able to transit the column. This gives you a way of cleaning-up the sample (as seen by the column and MS) that is consistent with point 1. The only things we are removing from the sample are things that a GC-MS can’t measure anyway.
3. Maximise your sample introduction options. Your operational effectiveness will be seriously compromised if  you end-up limited by your choice of sample introduction. Go for the most adaptable instrumentation that your budget will allow for and you will get the greatest benefit from your expensive mass-spec. The minimum should include a cryo-cooled PTV injector. PTV’s are incredibly useful tools that can be used to solve all kinds of sample introduction problems. The three top two advantages in my book are; 1 – a much wider range of sample injection volumes. 2 – the ability to work with solid samples directly and 3 – a cryo-trapping capability.

I will explore each of these key points in more detail in future posts.

We have installed a GERSTEL MPS MultiFlex sampling system onto our new Agilent 7200 GCqTOF, and the MultiFlex fulfills all of these requirements perfectly!

If you wish to discuss this in more detail, please do not hesitate to contact me at enquiries@anatune.co.uk, or call the office on +44 (0)1223 279210.