The 7000 triple quadrupole GC/MS is the first quadrupole MS/MS system
designed specifically for the unique requirements of gas chromatography applications.
These requirements include: enhanced selectivity, consistent low detection limits, and robust day-to-day operation.
The typical MS/MS description focuses on the extraction of ions from the source
isolation of the precursor ion in the first quadrupole
dissociation of precursor ion in collision cell
and mass filtering of the product ions in the second quadrupole before detection.
But let’s return to the ion source to discuss processes that are often overlooked.
Even for a high performance source, ionization efficiency - as a percentage of molecules entering the source - is very low.
This means there are many more neutrals than ions exiting the source, so a thorough description must include:
Analytes neutrals . . . and ions
Helium neutrals . . . and ions
Column bleed neutrals and ions
And many more neutrals and ions from the matrix – at concentrations typically higher than the analyte.
To simplify the description, let’s start with the analyte molecules and ions
Analyte molecules enter the vacuum source from the GC capillary column and begin bouncing off the hot surfaces of the source
Analyte ions are immediately extracted from the source and focused into the quadrupole mass filter.
But what about the non-ionized molecules?
The electrical fields within the MS have no influence on these molecules.
They follow rapid, random motion until removed by the vacuum pump.
Parallel processes are occurring simultaneously for the molecules from the sample matrix
and instrument background like column bleed.
The quadrupole analyzer performs the critical task of mass filtering
to isolate the analytes ions from the majority of these ions.
Chemical noise often remains after the first quadruple, but the selectivity of MS/MS removes this final source of interference.
Again we ask, what about the large population of non-ionized molecules.
Although the turbo pump removes many neutrals quickly, some will randomly diffuse into the quadrupole.
At the end of the GC separation, the EI source will be flooded with the highest boiling molecules from the sample matrix
Even in a vacuum, these high boiling molecules can contaminate surfaces in the mass spectrometer.
A cool surface will accelerate this contamination process.
As we think about the potential for contamination, we must not forget helium.
The intense flux - high flow - of helium ions helps to ‘burn’ the high boiling molecules
onto the surfaces of the source and the analyzer.
The cooler the surfaces the faster the rate of contamination and the faster performance is lost.
For a LC atmospheric pressure source like ESI,
metal quadrupoles at a temperature of 100 C are perfectly adequate and commonly used.
But for GC/MS/MS analyses, metal surfaces at this temperature will quickly become contaminated.
The Agilent 7000 uses a combination of proprietary and patented technologies
to avoid contamination problems and ensure robust,high performance operation.
These include: an inert source operating at up to 350 C and quartz quadrupole operating at up to 200 C.
With 35 years of MS experience, Agilent built the 7000 around the unique, hot quartz analyzer of the MSD,
not the cooler metal quadrupole rods used for LC/MS/MS.
Before continuing to the collision cell, one more process must be mentioned.
An EI source produces a large number of highly energetic, metastable helium atoms.
These highly energetic atoms follows a random path like any other neutral until removed by the pump.
Some will follow a path parallel to the analyzer and have a high probability
of traversing the entire analyzer assembly to the HED-EM detector.
During the development of the 7000, Agilent engineers realized a small flow of helium added to the nitrogen collision gas
was very effective way to reduce the transmission of metastable helium through the collision cell.
We call this patent-pending solution: “Helium Quenching”.
A comparison of signal to noise ratio with and without Helium Quenching shows to benefit of this technology.
Helium Quenching has no affect on Product Ion signal, but the neutral noise component is reduced by a factor of four.
This technical breakthrough means lower limits of detection and quantitation.
The 7000 MS/MS, an MS/MS system optimized for gas chromatography
from the classical distribution of ions produced in the inert, high performance source
to the precise isolation of the precursor ion through the path of the hot, quartz, hyperbolic quadrupole
into the high pressure, hexapole collision cell for efficient precursor dissociation,
product ion transmission and simultaneous reduction of metastables with “Helium Quenching”
To the final, accurate mass analysis in a second, hyperbolic quadrupole field before ion collection at the Triple-Axis detector
The 7000 system has been carefully designed to deliver robust sensitivity and reliable quantitative results
The Agilent 7000 quadrupole GC/MS/MS system
an MS/MS optimized for trace analysis of your most complex GC samples
the new standard for GC/MS/MS performance