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A Conversation with 7000A Tandem Mass Spec Users
Lately, Phil Wylie (Sr. Applications Chemist for Agilent’s Chemical Analysis Group) has been spending a lot of time talking about the many features and benefits of Agilent’s new 7890A/7000A tandem quadrupole mass spec system. So it is not surprising that he started dreaming about it as well. Phil wrote down everything he could remember and swears that his dream was inspired by conversations with customers who bought this new tandem quadrupole GC/MS/MS…
Phil: What made you decide to invest in Agilent’s new tandem quadrupole GC/MS/MS?
Dr. Molly Cule: We have a number of Agilent GC/MS systems that we run day and night. They’ve been extremely reliable and productive. And, we get great service when we need it.
Dr. Sal Monella: We liked the fact that your GC/MS/MS system is based on the 5975C MSD. We have several 5975s and have been really happy with them. You used the same gold plated quartz quadrupole design, the same inert source, and the same triple axis detector. Even the quads swing out from the manifold just like on the 5973 and 5975 MSDs. The 7890A/7000A is so similar to the 5975C MSD that we assumed we would be just as happy with it.
Ms. Polly Ester: Our colleagues down the hall have two Agilent LC triple quads and they showed us the MassHunter software. We really like the GC/MS ChemStation and have been using versions of it for years, so we were pleased to see that the MassHunter’s GC instrument control looks just like ChemStation. That similarity will help us lower our training costs. Plus, MassHunter has a lot of useful features that are not in ChemStation.
Dr. Sal Monella: Yeah, we really love the parameterless integrator. You don’t have to set any parameters at all to make it work.
Dr. Molly Cule: I like the way that you’ve imbedded deconvolution into MassHunter.
Phil: You know, the first GC tandem quadrupole MS that we built in R&D was based on our 6000 series LC/MS/MS. If we had continued down this path, we could have brought our GC/MS/MS to market much sooner and would have saved a lot of money. That first system was really sensitive and worked very well – until we started analyzing dirty samples. That’s when we discovered that both the source and the first set of quads got dirty very quickly and had to be cleaned a lot more often than we expected. It made us realize that the fundamental design of a GC/MS/MS system has to be different from an LC/MS/MS.
Ms. Polly Ester: My colleagues swear by your LC/MS/MS. Why can’t you modify it for GC use?
Phil: An LC/MS generates the ions outside of the tandem quadrupole system and pumps the neutrals away, along with the solvent. In GC/MS, everything goes into the source and only a fraction of the neutrals are actually ionized. So, you must have the source and the quads hot enough to prevent neutral molecules from condensing on them and dirtying the system. That’s why we use gold plated quartz quadrupoles instead of metal ones. Quartz quads can be heated up to 200°C – hot enough so that they never have to be cleaned. Metal quads expand and contract too much when the temperature changes, so you can’t heat them enough and they get dirty.
Dr. Sal Monella: We mostly analyze food extracts for pesticide residues and we really appreciate the sensitivity of the 7000A. With a 1 µL injection we can see almost all of our target pesticides below the 10 ppb (10 µg/Kg) level and many below 1 ppb. Does that mean that the 7000A is lot more sensitive than the single quad?
Phil: Well, not really. The 7000A and 5975C are similar in their inherent sensitivity. The difference is in the selectivity of the two instruments. With a single quad, you get a lot of interference from the matrix background which can hurt your detection limits. GC/MS/MS uses SIM in the first quad to isolate a precursor ion which fragments in the collision cell. The second quad can then isolate the most abundant and/or diagnostic reaction products. The chance that the matrix will have the same transition at the same retention time is very small, so you don’t see much chemical noise in GC/MS/MS.
Ms. Polly Ester: I read two Agilent application notes that really made good points regarding lack of chemical noise. One of them talked about the analysis of pesticides in marine biota and the other one gave a method for the analysis of 175 pesticides in fruits and vegetables. The samples in both these applications were very dirty – especially the mussel tissue extract – and you could still detect a lot of pesticides at 1 ppb or lower.
Dr. Molly Cule: I noticed in your application notes that you recommend backflushing when running dirty samples. Does this really work?
Phil: Believe me, it really helps to backflush when you run dirty samples. We use Capillary Flow Technology to reverse the flow through the column at the end of the run rather than baking out the column at a high temperature. Instead of sending the high boilers into the mass spec source, they are backflushed out of the inlet’s split vent. We find that it virtually eliminates carryover and we rarely have to clip the column or clean the source.
Dr. Sal Monella: Are there any other papers we can read that discuss the 7000A?
Phil: Sure. There’s one that shows the analysis of nitro-PAHs in air and another that talks about analyzing pesticides at low ppb levels in traditional Chinese medicine. If you are interested in toxicology, we also have a paper that discusses the analysis of drugs of abuse in whole blood extracts. Oh! Excuse me. I think I hear something ringing…
It looks like Phil’s alarm clock has rung and his dream is over. He’s discussing the many advantages of the Agilent 7890A/7000A tandem quadrupole mass spec system in real life. Now it’s your chance to stop dreaming of greater productivity and learn more about this remarkable new GC/MS/MS.
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