GCT versus GC/MS in scan mode

[Elena A.G.]

Hello,

We need to screen plastic additives until 5 ppb levels in liquid samples and sometimes also in food. The list of additives is huge, we usually don’t have information about what additives could be found and we don’t have standards of many of these additives.

We are looking for an equipment good for this purpose and some people told us that GCT could be the solutions but we are not sure that it will achieve these sensitivity levels. We don’t know if a GC/MS will give us similar sensitivity in scan mode or only slightly lower.

Can you advice me with this subject?

Thank you in advance

[jh1]

5ppb will probably be tough on either instrument unless you can concentrate the samples heavily before injection or have a large volume injector.

If you need to screen a large number of compounds on a limited number of samples one option can be to create multiple SIM methods and run all the samples multiple times, not fun but I know of a couple labs that keep low detection limits for 120+ compounds on 5973s by doing this.

[bhuvfe]

I agree with jh1 a sample concentration step will be needed (either online or offline).

Consider also that a GCT is 2.2-2.5 times the price of a single quad (considering the price of a 5975+LVI option). Since you are going to
search for known compounds do you really need 5 ppm mass accuracy provided by a GCT?
If you want to use TOFs and you want a screening instrument an alternative might be LECO (be aware that it’s a unit resolution TOF instrument).

I don’t know where you are based but I’m quite sure that any of these suppliers will be glad to run 1-2 samples spiked with your target analytes.

[Ron]

Quadrupole instruments tend to be more sensitive than TOFs and less expensive. For general purpose use a quadrupole instrument is generally preferred.

Is the 5 ppb concentration you mention in the sample or in the extracted sample? Many extraction methods include a concentration step, so if the detection limit in the sample is 5 ppb the concentration in the extract injected is 100 to 1000 times greater.

I am assuming SIM mode is not an option since you need to identify compounds, modern top of the line quadrupoles can collect good quality mass spectra at the 20 to 50 ppb level routinely.