All Conditions Used for Measuring S/N Should Be the Same for Both the S/N Calculations and for All Other Routine Analyses
by Mark Taylor, Richard Whitney, John Monti, and Denny Miser, Shimadzu Scientific Instruments
The combined technique of gas chromatography/mass spectrometry (GC-MS) provides a powerful tool for separation, identification and quantification of compounds in complex mixtures. Increasingly sensitive instruments are invaluable to experts in a variety of fields who were previously unable to identify trace compounds in complicated or difficult samples.
Achieving Sensitivity
There are a number of ways to optimize GC-MS sensitivity. When sensitivity is defined in terms of signal-to-noise ratio, or S/N, increasing sensitivity is achieved by increasing signal, decreasing noise, or a combination of both. Because S/N is a simple ratio, factors affecting signal or noise have equivalent influence on sensitivity (S/N).
Intensity
Optimizing the transfer of analyte from the injector through all the components of the GC-MS system to the detector can increase the signal. Optimizing the response of the detector to obtain adequate signal without excessive noise is a complementary approach to increasing signal.
Many components of the GC-MS system can contribute to background, or chemical, noise. In addition, noise can originate from the sample itself, from the injection technique, or from the type of chromatographic column. Significant noise can also emanate from the electrical power source.
Alternate data acquisition modes (SIM and recently introduced Scan/SIM techniques) have dramatic effects on GC-MS sensitivity. In a similar manner, alternate ionization techniques (chemical ionization, negative ion chemical ionization) can enhance sensitivity, especially for selected analytes. The present discussion of sensitivity is limited to the full-scan electron impact mode of operation and applies primarily to single-quadruple instruments.
Increasing Signal
Optimizing the signal can be considered in three phases: maximizing the injection, or transfer, to the GC column; optimizing the GC
column for maximum sensitivity; and adjusting the detector (MS) conditions.
GC Injection
Efficient sample transfer to the GC column is dependent upon optimizing the injector conditions. Minor inefficiencies in injector operation become major obstacles when trace quantities of analyte are injected. The selection of injection-port liner, split- and septum-purge flow rates, syringe needle penetration, column positioning in the injector and other parameters affect the efficient transfer of analyte to the GC column. Using alternate injector types can enhance analyte transfer to the GC column.
GC Column Selection
For most GC-MS applications, the GC typically employs relatively long narrow-bore capillary columns (0.18- to 0.320-mm OD; 15-
to 75-M long). Sensitivity increases as the GC column diameter decreases because narrower columns give taller, narrower peaks. If noise remains constant, S/N is greater with tall, narrow chromatographic peaks.
Optimizing the MS Signal
Signal generation in the MS comprises three processes:
• Ion creation (from the ion source)
• Ion transmission (through the lenses and mass filter)
• Ion detection (with the electron multiplier)
The combined efficiency
of these processes results in optimizing signal intensity. In some cases, inefficiencies in these processes also affect noise. These efficiencies are affected by instrument design and condition (cleanliness, age), as well as operating parameters.
Ionization and Ion Transmission
Sensitivity is affected considerably by ion source condition (cleanliness) and optimum tuning. Ion source operating parameters have a significant effect on sensitivity because these parameters control ion production.
Various design improvements in mass spectrometer sources in recent years have resulted in significant improvements in sensitivity compared to previous designs. Filament construction and placement is one such design improvement that results in enhanced sensitivity. Filament shielding can ensure efficient ion transport to the detector while protecting thermally labile compounds. Thus, a high-efficiency ion source provides more uniform temperature control for increased sensitivity.
The vacuum system significantly affects ion transmission. Differentially pumped vacuum systems, in some mass spectrometer designs, maintain a higher level of vacuum in the mass analyzer region. An increased vacuum results in longer mean-free paths for ions, which allows for more efficient ion transmission through the mass
filter. This, in turn, enables greater sensitivity.
Detectors
Electron multipliers detect and convert ions to a signal. When impacted by a charged particle (ion), the surface of the multiplier emits several electrons in a process called secondary emission. This process repeats several times to give up to a million electrons for each ion impact on the electron multiplier surface. The gain, or signal amplification, is determined by the voltage applied across the entire electron-several hundred to several thousand volts.
Most GC-MS instruments employ two types of electron multipliers: continuous dynode types and discrete dynode types. Discrete dynode electron multipliers typically operate at lower voltages, so they show slightly less noise than continuous dynode types. Typically, the electron multiplier's sensitivity can be optimized to obtain a maximum signal with minimum noise.
Decreasing Noise
Challenges and solutions in the GC that affect the S/N ratio directly affect the sensitivity of the GC-MS system as a whole. Lowering the background signal originating from the GC and improving the chromatographic
resolution can improve GC sensitivity. As the quantity of a specific analyte to be detected is decreased, the effects of minor interferences on the ability to detect the specific analyte become an increasingly significant problem.
Carrier Gas
Achieving the highest possible purity in the carrier gas helps to decrease noise, specifically "chemical noise." For example, carbon dioxide gives a background signal at m/z 44 that increases chemical noise. Oxygen degrades column phase, which results in increased bleed (i.e. background signal at m/z 207). Using pure carrier gas and eliminating contamination and leaks in carrier gas lines are important factors affecting instrument sensitivity as well as overall chromatographic performance. Use of high-purity gases with appropriate filters can significantly lower the chromatographic background signal and therefore increase sensitivity.
Injection Ports
Chromatographers face
the continuing challenge of assuring complete sample volatilization and transfer
of sample to the GC. Using appropriate injection-port operating parameters and maintenance procedures ensures optimum sample transfer to the GC column.
Background noise often arises from siloxanes from the GC septa, glass wool used in injection port liners, deactivation of injection port liners and other sources. These peaks are commonly seen in normal GC-MS backgrounds. Use of low-bleed septa, preconditioning of septa and liners, and use of injection-port septum purge can minimize this source of contamination.
GC Columns
Another common source of background noise originates from the GC column stationary phases. Traces of oxygen in the carrier gas can degrade the liquid phase. To minimize this problem, the GC column, injection port, and transfer line temperatures should never exceed the maximum-rated temperature of the
liquid phase of the capillary column. Thin liquid phases result in lower column bleed and decreased chemical noise. In addition, several proprietary low-bleed column phases, designed specifically for GC-MS use, have been introduced in recent years.
Electronic and Vibrational Noise
Electronic noise is determined largely by the instrument design and manufacture. Minimizing overall electronic noise is a major consideration in instrument design and selection of electronic components. Noise
is frequently minimized by supplying "clean" or conditioned electrical power for instrument operation. In addition, minimizing vibration from motors and other devices, such as mechanical pumps, is an important
consideration in minimizing overall instrument noise.
This article is an excerpt
of Shimadzu Scientific Instruments' white paper on Sensitivity in GC-MS, which is available online for free download at www.ssi.shimadzu.com/sens.
AT A GLANCE
• Alternate data acquisition modes have dramatic effects on GC-MS sensitivity
• Sensitivity increases as the GC column diameter decreases because narrower columns give taller, narrower peaks
• Sensitivity is affected considerably by ion source condition (cleanliness) and optimum tuning
• Lowering the background
signal originating from the GC and improving the chromatographic resolution can improve GC sensitivity
ONLINE
For additional information on
the technology discussed in this article, see Laboratory Equipment magazine online at www.LaboratoryEquipment.com or the following Web site:
• www.ssi.shimadzu.com Laboratory Equipment Advantage Business Media
