GC-MS instruments can be sensitive and precise; it really better be both or you should be using a different instrument. Sensitivity without precision is like driving a car at 100 miles/hr (160 km/hr) without using the steering wheel. How sensitive and how precise does the instrument need to be? How does the sensitivity and precision carry over to the method that I am running in the lab?
Reasons for the need for low levels of sensitivity with great precision;
- Regulatory limits to report to.
- Amount of sample used in the extraction. Save money on shipping costs associated with the samples.
- Length of time desired to be spent in sample prep. More sensitivity means less time to concentrate after extraction, save time.
If you do not have precision you do not have any of this.
Like what you are learning?
Now what really matters is this sensitivity and precision in the method you are running. There are a lot of variables to consider. You do not want the GC-MS instrument you are running to be the weak part in your lab workflow. There are several specific functions that can further help the performance of the method and the lab. One of these is the use of SRM, also known as MRM, with a triple quad GC-MS. SRM provides the ability to remove the matrix background from the datafile, increasing the selectivity. However, this is not always perfect. Making the SRM even more selective for a specific ion will increase the selectivity, increasing the method sensitivity, and increasing the method precision. USRM, Ultra Selective Reaction Monitoring is the key here.
We can demonstrate the use of this USRM tool with the TSQ Quantum XLS Ultra by analyzing 400 pesticides in produce samples that are known to have a lot of background. USRM can further eliminate the background and dramatically improved method sensitivity and method precision. This will allow us to analyze more compounds in one injection, save money on sample shipment, and spend less time in sample prep.
For most of the pesticide compounds included in the method, the complete list of the compounds with their respective SRM transitions have been downloaded from the Pesticide Analyzer Reference into the instrument acquisition method. Each transition has been determined for optimal sensitivity and selectivity, with the complete list documented for TSQ Quantum XLS users. Over 400 pesticides have been monitored in several matrices such as wheat, blackcurrants and cucumber; the results of the most challenging pesticides in terms of activity and response are highlighted, showing calibration curves, repeatability and ion ratio stabilities. The TSQ Quantum XLS Ultra is able to perform SRM with a higher mass resolution or 0.1 Da (USRM) setting thus allowing for better selectivity. Not all pesticides in all matrices benefit from a higher mass resolution setting, but depending on the matrix and the compound analyzed, there can be a significant improvement on the signal to noise ratio.
Download application note here; Pesticides in Produce Using USRM