PTR-MS and SIFT-MS are both flow-tube based mass spectrometric techniques that are used to detect and quantify VOCs in whole air in real time. This article provides a detailed comparison of selected ion flow tube mass spectrometry (SIFT-MS) and proton transfer reaction mass spectrometry (PTR-MS). Both techniques offer real-time analysis for volatile organic compounds (VOCs) in whole air and both claim to use soft chemical ionization.
While SIFT-MS and PTR-MS have similarities, the fundamental difference is that SIFT-MS applies three reagent ions (H3O+, NO+ and O2+) as standard. Application of three reagent ions gives SIFT-MS a significant advantage in the discrimination of isomeric compounds.
Recently a “switchable” reagent ion (SRI-PTR-MS) has become available. However, SRI-PTR-MS lacks the instantaneous reagent ion switching capabilities, stability, usability, specificity, and true soft ionization of SIFT-MS instruments.
In its original form, PTR-MS used just protonated water ions (H3O+) as the reagent ion. The “switchable reagent ion” (SRI) source now allows some models of PTR-MS instrument to use the other SIFT-MS reagent ions as well, though the timescale for switching reagent ions is a number of seconds for PTR-MS rather than milliseconds for SIFT-MS. The reagent ions are created in a hollow cathode discharge. They are not mass selected and are injected immediately into a drift tube where reactions occur with VOCs in the gas sample. An electric field gradient is used to carry the ions along a drift tube to the detection region. Depending on the instrument model, detection is either via a quadrupole and particle multiplier tube system, or a time-of-flight mass spectrometer. Identification of the analyte is by means of the resulting product ion mass(es) arising from the reagent ion – analyte reaction.
SIFT-MS uses soft chemical ionization reactions coupled with mass spectrometric detection to rapidly quantify VOCs in real time from whole-gas samples. Three standard chemical ionization agents (or reagent ions) are used (H3O+, NO+, and O2+). These reagent ions are mass selected and react with trace VOCs in very well controlled ion-molecule reactions but do not react with the major components of air, allowing SIFT-MS to analyze whole air for trace VOCs to pptv levels.
Soft chemical ionization yields a smaller number of product ions per compound than electron impact mass spectrometry (as used in GC-MS, for example), so gas chromatographic separation is unnecessary. This speeds sample throughput and provides instantaneous quantification of VOCs. Use of multiple reagent ions also greatly reduces interferences, markedly increasing the specificity of SIFT-MS compared with most other whole-gas analysis technologies.
Technical advantages of SIFT-MS over SRI-PTR-MS
|Reagent ion selection||Real-time switching of reagent ions means all ions can be used in a single analysis of the sample||Switching is offline, requiring multiple scans on one sample, reducing efficiency|
|Reagent ion energy||Consistent thermal reagent ion energies yield reliable identification and quantitation||Higher (non-thermal) reagent ion energies lead to spectral confusion and poorer quality of quantitation|
|Detection sensitivity across mass range||Use of an advanced ion optics system gives much-improved response to compounds at the higher end of the mass range||Quadrupole instruments struggle to achieve sufficient signal for product ions greater than about 120 m/z|
Reagent ion selection:
Switching of reagent ions on a millisecond timescale enables real-time, single-scan resolution of interfering compounds. Only SIFT-MS offers users this capability, with reagent ion changes made in less than 10 milliseconds. This characteristic allows synchronous concentration measurements of one analyte to be made with several reagent ions and/or various compounds to be detected with different reagent ions.
Synchronous measurement is illustrated in the figure below, where breath analysis has been made while a volunteer is chewing peppermint-flavored chewing gum. In this real-time analysis, menthol was analyzed using the H3O+ reagent ion, while menthone, menthofuran and acetone were analyzed using NO+. (Acetone is produced by the body through metabolism of lipids, so it acts as a useful marker for breath exhalations.) All data are acquired in a single scan.
A PTR-MS fitted with a switchable reagent ion (SRI) source can utilize the three SIFT-MS reagent ions, but since it lacks a quadrupole mass filter to switch these ions, it must use a very much slower change of source gas and/or ionization conditions in its hollow cathode discharge source. At best, a modified PTR-MS can achieve a change in the order of seconds, by switching from one reagent gas to another. This means that it is impractical to monitor real-time changes with multiple reagent ions.
|Water to make H3O+||Yes||Yes|
|Bottled gases to make O2+||No||Yes|
|Time to switch reagent ions||10 ms (uses quadrupole)||10 s (changing of gas supply lines)|
|Reagent ion purity||High||High|
Reagent Ion Energies:
Consistency of reagent ion energy is one of the most critical factors in controlling analyte ionization, which in turn provides very consistent product formation and reliable, stable quantitation. In SIFT-MS, use of a carrier gas enables the chemical ionization process to be controlled much more effectively than in PTR-MS or other variations of chemical ionization mass spectrometry. The carrier gas plays two very important roles in controlling ionization:
- It thermalizes the reagent ions prior to introduction of sample, which means that the energies of the reagent ions are as low and consistent as possible, providing predictable, precise, and ultra-soft chemical ionization. In contrast, reagent ions of elevated and widely distributed energies are injected into the reaction region of the PTR-MS, resulting in reduced stability of the ionization process, with consequent effects on resolving and quantifying compounds.
- It transports the product ions and unreacted reagent ions down the flow tube to the detection region without addition of excess energy that is inherent in PTR-MS, where an electric field is used to accelerate ions down the drift tube to the detection region. Adding additional energy further complicates mass spectra, reducing specificity and ability to uniquely quantify compounds.
The left-hand side of the figure illustrates the comparative reagent ion energy distributions of PTR-MS and SIFT-MS. These distributions apply to any of the reagent ions used. On the right-hand-side of the figure, an example of non-thermal H3O+ reagent ion impact is evident. These spectra were generated from the data cited in the next table.
Example 1: Product ion distributions with the H3O+ reagent ion
The table shows published product ion distributions for H3O+ reagent ions in SIFT-MS and PTR-MS instruments. Higher (non-thermal) energies of the PTR-MS H3O+ ions are evident from the much higher fragmentation. Further, since H3O+ reactions are based on the proton transfer mechanism, compounds with proton affinities close to water (for example, formaldehyde, hydrogen sulfide, and phosphine) are difficult to detect in PTR-MS, due to higher ion energies.
|Compound [molar mass; g mol-1]||SIFT-MS product masses in m/z (branching ratio in %)||PTR-MS product masses in m/z (branching ratio in %)|
|Acetaldehyde ||45 (100%) 1||61 (26%), 45 (47%), 43 (20%), 41 (6%), 39 (1%) 2|
|Ethyl acetate ||89 (100%) 3||89 (5%), 61 (74%), 43 (21%) 2 83 (57%), 55 (43%)2|
|Hexanal ||101 (50%), 83 (50%) 1||61 (26%), 45 (47%), 43 (20%), 41 (6%), 39 (1%) 2|
|1. Spanel et al., Int. J. Mass Spectrom. Ion Processes, 165/166 (1997), 25-37.|
2. Blake et al. Int. J. Mass. Spectrom. Ion Processes, 254 (2006), 85-93.
3. Spanel et al., Int. J. Mass Spectrom. Ion Processes, 172 (1998), 137-147.
Example 2: Product ion distributions with the NO+ reagent ion
The non-thermal energies of NO+ reagent ions in SRI-PTR-MS is evident from published marketing material where it is stated that the dominant mode of reaction of ketones is simple charge transfer:
Ketone + NO+ → [Ketone]+ + NO
However, based on the ionization potentials reported in the NIST on-line “webbook” database this should not be the case since most small ketones have a higher ionization potential (IP) than NO (see Table 4). The observed SIFT-MS products, which are almost all the result of termolecular association processes:
Ketone + NO+ + “third body” → Ketone.NO+ + “third body”
Here the “third body” represents another atom or molecule that participates in the three-body collision, and though not reacting is critical because it carries away excess energy allowing the Ketone.NO+ adduct to stabilize. The resulting thermal product ions are shown in Table 4 in the SIFT-MS column. Formation of charge transfer products in SRI-PTR-MS demonstrates the reagent ions are non-thermal and not “soft”.
|Compound (synonyms) [molar mass in g mol -1]||Ionization Potential (electron volts; eV)||Predicted Charge Transfer Product with Thermal NO+ Ions||Actual Product Ions with Thermal NO+ Reagent Ions (SIFT-MS)1|
|Acetone (propanone) ||9.70||No||88 (100%)|
|Butanone (methyl ethyl ketone; MEK) ||9.52||No||102 (100%)|
|2-Pentanone (methyl propyl ketone) ||9.38||No||116 (100%)|
|3-Pentanone (diethyl ketone) ||9.31||No||116 (100%)|
|2-Hexanone (methyl butyl ketone) ||9.35||No||130 (100%)|
|3-Hexanone (methyl butyl ketone) ||9.12 – 9.30 2||Maybe 2||130 (85%); 100 (15%)|
|1. Reference: Spanel, Ji, Smith, Int. J. Mass Spectrom. Ion Processes, 165/166 (1997), 25-37.|
2. Two values for the IP are given in the NIST database and the IP of NO+ lies between them. The occurrence of some charge transfer product suggests the IP of NO+ is similar to that of 3-hexanone.
With SIFT-MS, reagent ions are thermalized in the carrier gas stream prior to reaction with analyte in the sample. This allows use of analyte rate coefficients and products directly from a generic SIFT-MS reaction database that is applicable to thermal ion-molecule reactions. Therefore SIFT-MS provides absolute quantitation of analyte in all these situations – without calibration standards – from the ratio of the product peaks to reagent peaks and the knowledge of the chemical kinetics.
However, because PTR-MS reagent ions are not at thermal energies, the reaction products and rate coefficients are dependent on the energies of the reagent ions produced for given instrument conditions. This means that there is no generic PTR-MS database of rate coefficients and branching ratios (as there is for SIFT-MS), because any database applies to a very specific set of drift-tube operating conditions. Consequently, PTR-MS can only undertake absolute quantitation in real time if the products and rate coefficients have been determined using exactly the same conditions in which the drift tube of the PTR-MS is currently operating. Hence in most situations, a series of calibration standards must be run for PTR-MS measurements (in a similar way to GC-MS).
Detection mass spectrometers:
Product ions and unreacted reagent ions are detected using a mass spectrometer in both SIFT-MS and PTR-MS instruments. Commercial PTR-MS instruments are available with quadrupole or time-of-flight (TOF) mass spectrometers, whereas SIFT-MS instruments only use quadrupoles at the present time. In this section, a brief comparison of detection technologies is provided.
“Traditional” quadrupole PTR-MS instruments
Quadrupole detection systems are not all equal. In the Voice200 instrument, Syft introduced ion optics technology that has provided significantly enhanced detection of volatiles with product masses over approximately 100 m/z. The significance of this breakthrough was recently demonstrated in a head-to-head comparison of a Syft Voice200 and a commercial “standard” PTR-MS instrument. The two instruments simultaneously analyzed sample streams delivered from a permeation oven containing permeation tubes (one or several at a time). Although the instruments had similar sensitivities for the H3O+ reagent ion reaction with benzene at m/z = 79 (the Voice200 was 1.27 times higher), Figure 3 shows that SIFT-MS has far superior sensitivity to product ions above about 100 m/z.
PTR-Time of Flight-MS (PTR-TOF-MS)
In recent years two manufacturers of PTR-MS instruments have offered time-of-flight (TOF) mass analyzers. TOF mass spectrometers can offer improved selectivity and detection of higher mass compounds, however this must be balanced against the main disadvantages of the TOF-based analysis, which are reduced sensitivity, larger size, and significantly higher instrument price than the equivalent quadrupole-based instruments.
Finally, it is important to remember that a TOF-based PTR-MS retains these key disadvantages with respect to SIFT-MS:
- High and variable reagent ion energies, causing increased fragmentation and reduced stability
- Switching of reagent ions is off-line, not real time.
Operational advantages of Syft SIFT-MS compared to PTR-MS
In addition to the technical aspects of SIFT-MS and PTR-MS, commercial aspects such as ease of use and serviceability must also be considered. Table 5 provides a summary of these operational considerations.
|Characteristic||SIFT-MS (Voice series)||PTR-MS|
|Ease of use||Suitable for novice and non-technical users through to expert users||Suitable only for expert users|
|Software||Intuitive, easy-to-use software for both non-technical and expert users||Very basic, scientist-focused software|
|Configurability||Designed to provide the results a particular level of operator requires||Minimal|
|Automation||High – including sophisticated self- checks and fully automated validation||Moderate – no performance validation|
|Integration||Designed for easy integration into existing operations and workflow||Designed as standalone|
|Analytical Stability||Long term, via SIFT-MS process and automated validation||Short term, due to PTR-MS process (use of drift voltage)|
Syft Technologies’ SIFT-MS instruments provide very clear technical and operational advantages compared to the various commercial forms of PTR-MS. Significant advantages include:
- Rapid switching of reagent ions, allowing multiple reagent ions to be used in a single scan, saving time and sample, and allowing instantaneous results
- True soft ionization with lower energy and less fragmentation, meaning less spectral confusion and more precise compound identification and quantitation
- Enhanced usability, especially:
- Network compatibility, allowing remote operation, monitoring, fault diagnostics, and support
- Turnkey applications and a touchscreen interface, allowing users with minimal training to operate the instrument
- Advanced LabSyft software, allowing research-level users to access the technology’s full capabilities
- Online support and backup, meaning help is never more than a click away.
Before purchasing a PTR-MS, contact Syft or your local distributor for more information on our analytical solutions.