Practical Considerations for Quantitative Gas Analysis with Quadrupole Mass Spectrometers
Many factors must be considered when comparing the overall suitability of different quadrupole-based gas analyzers for any given application and the list can sometimes appear daunting and confusing. This can be due to inconsistencies in the way that different manufacturers choose to define specifications or, in some cases, omit them altogether.
These factors can be categorized into two main areas: (i) inlet interface suitability and (ii) quadrupole mass analyzer suitability. This article aims to remove some of this confusion and define and present those practical specifications which are critical for repeatable and reliable quantitative gas analysis.
The suitability of the inlet and interface determines how well the gas analyzer can capture, condition or transfer the gas sample without altering it and for it to be measured on an appropriate timescale, which could be milliseconds or hours. The inlet and interface can include both the upstream transfer elements and the downstream pumping and gas handling elements.
Quadrupole Mass Spectrometer
Assuming the inlet and interface are properly designed and equal between systems, then the quadrupole mass spectrometer is the critical element determining the overall precision, stability, and detection limits of the gas analyzer. The quadrupole mass spectrometer includes the ionization method, the transmission characteristics, and the quality of the driving electronics.
Precision, stability, and detection limit are often mis-represented in commercial literature. This misrepresentation can be addressed and clarified by directly comparing two different classes of quadrupole analyzers: a 6mm rod diameter, RGA type instrument, typical of many currently on the market, and a higher performance 19mm rod diameter instrument, used in more demanding research and industrial applications. These two systems are compared with nominally identical inlet/transfer conditions, so that only the mass spectrometer performance is under consideration. This presents a direct comparison of the practical range of precision, stability and detection limit in each case so potential users of this powerful analytical technique may be better equipped to make meaningful comparisons between different suppliers.
The MAX300-CAT is typical of the high-end RGA based gas analyzers, based upon 6mm quadrupole rod technology, whereas the MAX300-LG is a higher performing analyzer based on 19mm quadrupole rod technology and more sophisticated electronics.
Detection Limit Comparison
The specified figure of detection limit can be very misleading. Often it will be a calculated figure, or it may reflect data that has been averaged and smoothed for long periods of time to give a best possible case which is often not achievable in practical situations. Nonetheless, the ultimate detection limit is a good starting point to begin to define the practical capabilities of the analyzer.
Speed of Analysis
Analysis speed is a key factor in quantitative gas analysis. Applications such as catalysis, reaction monitoring or kinetics, and evolved gas monitoring all require faster capture of process changes than QA/QC applications, while a breath measurement application needs to report quantitative differences on the millisecond scale. Note that this refers to the ability of the analyzer to measure, with the desired level of accuracy, raw signals and then analyze these in a given timeframe, taking into account spectral interferences, in order to output the result of a single analysis. The rate at which an analyzer scans directly influences this data quality. Slower scanning or more averaging yields more repeatable results and lower detection limits.
Analysis precision (or short-term repeatability) represents the standard deviation of analysis results over short time periods. Repeatability can be improved by slowing analysis scan speed or averaging more scans.
Analysis stability is a representation of drift or fluctuations over long-term data collection. It is a critical factor which influences longer analyses such as process control, slow heating TGA and thermal analysis, and air monitoring, but also impacts general instrument operation. Stability allows for accurate results over time, less calibration frequency, and confidence in the day-to-day repeatability of the analyzer.
Large dynamic measurement range is an essential requirement of quantitative gas analysis and becomes especially apparent in applications such as solvent drying, where species must be monitored from high to low concentrations with accuracy and repeatability.
The MAX300-CAT, a high-end RGA based gas analyzer using 6mm quadrupole rod technology, can demonstrate low detection limits of approximately 5 ppb, using slow scan speeds. The scan speed on this instrument can be increased to a typical quantitative analysis rate of 2 seconds per component, resulting in an increase of detection limits to 0.5 ppm. The MAX300-CAT has a maximum speed of approximately 2-seconds per component in quantitative scans. While this changes the instrument precision, the stability remains constant. The dynamic range of the MAX300-CAT allows for an analysis range from 1×10-6 to 5×10-13 Torr (100% to 0.5 ppm), when scanning at a rate of 2 seconds per analysis component.
The MAX300-LG, a higher performing analyzer based on 19mm quadrupole rod technology and more sophisticated electronics, displays extremely low detection limits of <1 ppb, using slow scan speeds. The scan speed on this instrument can be increased to a typical quantitative analysis rate of 400 milliseconds per component, resulting in a moderate increase of detection limits to <10 ppb. The MAX300-LG has a maximum speed of 5 milliseconds per component in quantitative scans. This instrument has incomparable precision and stability, a result of the large quadrupole and high-performance electronics combination. The MAX300-LG demonstrates a very large dynamic range from the dual detector setup, allowing an analysis range of 1×10-6 to <1×10-14 Torr (100% to <10ppb), while scanning at a rate of 400 milliseconds per component.