Chromatography Forum

Hi all,

I'm interested in active compound in natural product.

As I was told GCMS is more suitable for quantitative work since there is library to search the possible present compound.

GC-TOF-MS is more suitable for qualitative work to study the unknown. But, I saw that the GC-TOF-MS also installed with library(such as NIST). Thus, is it this equipment also capable to perform GCMS job and run NIST library?

Please share your valuable opinion comparing the two different chromatograh approach.

Cheers,
Kan

Hi Kan,

You can do both, quantitative and qualitative work, with GC TOFMS.

See the link below for an example how this works for allergenes in perfume:

http://www.leco.com/resources/applicati ... e/-236.pdf

Best Regards

The type of MS system is not the most relevant regarding Quantitation and qualitative. The Ion source type has far more bearing. a GC EI source gives a library searchable identification. If a good major ion is idenifyable this should give excelent reproducable data for quantitative work. An LC ESi/ApCI source will not necessarily give you much more than molecular mass data and will only give identification in the event you know what you are looking for and/or you have a high resolution system (TOF/Sector). A good strong ion should again be applicable for quantitative data.
Beware for quantitative analysis you must collect enough data points accross a peak to get good data. Some MS protocols such as high resolution reduce significantly the number of resultant scans you can get. If looking a wide LC peaks this is not an issue. If looking at narrow GC peaks a few seconds wide you will not get good data. Aim for minimum 10 scans per peak for quantification ??

TOF-MS can achieve higher mass resolution compared to a quadrapole (although not as high as FT-MS or magnetic sector instruments). This is useful in determining the identity of unknown compounds. Here an informative article on the subject:

http://www.lcgceurope.com/lcgceurope/da ... rticle.pdf

There are two kinds of comercial TOFMS instrumets out there for GC work. One is high resolution, which can give accurate mass. The other is integer mass, which will have a greater dynamic range. Both will give a full range spectrum without sacrificing sensitivity.

In a quadropole (trap, or sector) what comes off the column is ionized in the ion source and then the ions are filtered to allow ions of a particular mass to charge ratio be counted. In a TOF instrument, all the ions are pulsed into the analyser and separate by mass, lighter ions flying faster than heavier ions. Thus, in a TOF, everything you ionize, you count. There is not filtering.

Thus, I have a TOFMS in my lab where I can obtain a library searchable ( with a correct match) spectrum of 2 pg hexachlorbenzene on column over a mass range from 35 to 300. (That's the instrument installation specification.)

So, if that helps on understanding TOF...

You can do qualitative analysis on either a quadrupole mass spectrometer or a TOF, they will both give good library searchable mass spectra. In addition, the TOF will have better mass accuracy, which can aid in compound identification.

If you are also doing quantitative analysis, the quadrupole instrument will be better suited to this. In the post above, 2 pg of hexachlorobenzene on the TOF gives a signal to noise ratio of >10:1. A modern quadrupole instrument will give a signal to noise well in excess of 100:1 under these conditions. The sensitivity of the quadrupole is much better than the TOF, and in addition the linear range is greater.

TOFs are nice for some applications, and ion traps as well, but if you only have one mass spectrometer the quadrupole instrument would be my choice as the general purpose instrument.

Watch the comparison of specs on Quad vs. TOF. If the Quad has a specification of getting that wonderful signal to noise in SIM mode, then you may be getting that signal to noise on only one mass signal. If you are going to do quantification with quant plus two (confirming ions), you have to divide the acquisition time across three ions - and if the signal to noise was 100:1 in acquiring a single ion - the singal to noise limit is cut by about 1/3. And if you want to quantify two coeluting compunds - 6 ions, then the quadropole's signal to noise is reduced even more.

On TOF you get all the ions all the time. So, you meet that 10:1 specification no matter if you look at one ion or many.

Bottom line: If you are looking to purchase a machine, get a demo from the vendor. I work for a vendor. I love to do the demos because the customer knows what he/she will get from that instrument when they get it home.

TOFs are wonderful scanning instruments. If it can be ionized with the mechanism that you are using, then you can detect it with a TOF. TOF is a high resolution, scanning instrument. Very great sensitivity in the scanning mode. Resolution is accurate mass to 4 decimal places.

Quads are wonderful for SIM, SRM, MRM, etc. You do not detect all ions because you are using them as mass filtering devices. Very great sensitivity in the SIM, SRM, MRM modes. Not as good sensitivity in the scan mode as a TOF. Resolution is unit mass.

Sorry to be pedantic but TOFs do not scan. They always look at the full mass range, unlike Quads which are filters and look at a single mass at any time. TOFs are not necessarily high resolution some give only unit resolution for some modes. They only give greater sensitivity if the ion source is the same, typically 10 times better vs quads with same source and usually quads will offer better ultimate sensitivity if you can use SIM. Quads can give accurate mass info. e,g Thermo TSQ Quantum.

If you have funds to buy a system, GET A DEMO ! The biggest differences in systems are often the software and the local support. Make sure you get your samples run for your application

To clarify the sensitivity issue, all published specification for quadrupole mass spectrometer are in scan mode using 1 pg of octafluoronaphthalene. Signal to noise is typically in the range of 150:1 to 400:1. SIM mode signal to noise should be about 10 times higher.

Modern quadrupoles scan much faster than older instruments, so getting 10 mass spectra per second is not difficult, and you can go as high as 50 scans per second over a limited mass range for targe compound detection.