Instrument Calibration

September 11th, 2019 in Pond's Pondering

"So Pat, what is a..."

Before I could get the words out of my mouth, he answered me.

"A "mass spec" or "MS" is an instrument that allows a certain size molecule to pass through a filter at a given moment in time. Yes, this includes an ion trap. In my days, we called it "the gun". I loved it because you were like a big game hunter looking for a specific particle in a mass of s**t.

I immediately reflected on my graduate studies research.

"There are four general types of mass spec:

1. Time of flight (TOF)

2. Magnetic sector

3. Quadrupole

4. Ion trap (including orbiTrap)

"Early on, we reacted the compounds in the electric field and monitored spectra. Side-story: The first MS was a ToF. Imagine having to count 988 dots for a 988 amu compound. We looked for better ways.


(Photo above: A mass spec in 1919)

Any MS needs tuning. Without proper tuning, you cannot assign an accurate m/z ratio.

"So if you are doing qualitative full scan work, you want to stick to your classical concepts of tuning and settings for reproducible MS (i.e. how we shattered something to be able to refer back to known references.) However, most compounds are monitored for very specific pieces that we process, either by SIM or SRM through a combined MS/MS. 

"For this, whichever instrument you are using, you want to set your controls to maximize the number of molecules hitting the detector in the range you are looking for."

"Explain," I dryly requested.

"Example: If you're using hi-res MS (i.e. 4 decimal places at delta mass -- look it up) for dioxins, you want to maximize your ion beam for the range of atomic mass between 300 and 500 amu (questions about amu? look it up). Whereas if you're doing nitrosamine work, we want max sensitivity between 40-170 amu. This is all done by being skilled at the electronic control of your ion beam, which allows the specific charged particles to be detected at a given moment in time. 

"So, that means that through the molecular structure of your compounds of interest, if you are doing targeted analysis, knowing the makeup allows you to put controls on the ratios at m/z of ions you generate. And that is what we do as a private lab: Provide accurate info, quantitate."

Pat reminded me that secretaries are more efficient than scientists: Stop overthinking it. 

An excellent reminder for an analyst who suffers from paralysis by analysis.

~ Filtered by Dave Hope ~