Mass spec fundamentals - quad parameters
Posted: Wed Oct 09, 2019 6:22 pm
I'm trying to get a better understanding of how quadrupole parameters affect MSD function. Namely, I'm trying to better understand what parameters such as scan speed, frequency, cycle time are and how they are related.
The parameters of a particular method for a GC-MS (Agilent 7890B and 5975) that I use are as follows:
Segment: 2.5
Mass range: 35-260
threshold: 150
scan speed (u/s): 1,562[N=2]
frequency (scans/sec): 6
cycle time (ms): 166.66
step size (m/z): 0.1
Here's what I gather about those parameters, please correct accordingly:
Segment: I think relates to the solvent delay; Time in minutes at which point the filaments turn on and the MS starts collecting data.
Mass Range: m/z range that quad scans over, not too complicated
Threshold: Minimum counts for a peak to be recognized
Scan Speed (u/s): Not sure about this. Software manual describes as "scan speed in u/s" - what is the unit "u"? "N" is the number of "sample points"
Frequency - Number of scans "per cycle" (a cycle is 1 second, I guess?), calculated from cycle time.
Cycle Time (ms): time for one scan to complete
Here's my attempt to tie it all together (this is where I really need help): At the end of the segment, the quad begins changing RF/DC in such a way that each m/z in the range is sequentially allowed to the detector, in intervals corresponding to the step size (so 2250 steps total, for my method). The scan speed is the speed at which the entire range is scanned, so the faster the speed, the higher the frequency and the shorter the cycle time. The cycle time describes the actual time it takes to scan the entire range one time.
The ultimate question I have is, what is the utility of increasing or decreasing scan speed? It seems like there would need to be some compromise between scan speed and how much time is allowed to acquire counts at each mass step. What would be the benefit of increased scan speed? I'm having trouble understanding conceptually how these parameters are related to data parameters like resolution, abundance, etc.
Please confirm or correct my understanding, and please pass along any useful references for this subject.
The parameters of a particular method for a GC-MS (Agilent 7890B and 5975) that I use are as follows:
Segment: 2.5
Mass range: 35-260
threshold: 150
scan speed (u/s): 1,562[N=2]
frequency (scans/sec): 6
cycle time (ms): 166.66
step size (m/z): 0.1
Here's what I gather about those parameters, please correct accordingly:
Segment: I think relates to the solvent delay; Time in minutes at which point the filaments turn on and the MS starts collecting data.
Mass Range: m/z range that quad scans over, not too complicated
Threshold: Minimum counts for a peak to be recognized
Scan Speed (u/s): Not sure about this. Software manual describes as "scan speed in u/s" - what is the unit "u"? "N" is the number of "sample points"
Frequency - Number of scans "per cycle" (a cycle is 1 second, I guess?), calculated from cycle time.
Cycle Time (ms): time for one scan to complete
Here's my attempt to tie it all together (this is where I really need help): At the end of the segment, the quad begins changing RF/DC in such a way that each m/z in the range is sequentially allowed to the detector, in intervals corresponding to the step size (so 2250 steps total, for my method). The scan speed is the speed at which the entire range is scanned, so the faster the speed, the higher the frequency and the shorter the cycle time. The cycle time describes the actual time it takes to scan the entire range one time.
The ultimate question I have is, what is the utility of increasing or decreasing scan speed? It seems like there would need to be some compromise between scan speed and how much time is allowed to acquire counts at each mass step. What would be the benefit of increased scan speed? I'm having trouble understanding conceptually how these parameters are related to data parameters like resolution, abundance, etc.
Please confirm or correct my understanding, and please pass along any useful references for this subject.