SIFT-MS Instrumentation

A schematic outhne showing the principle of operation of a SIFT-MS instrument is shown in Fig. 8.5 and Fig 8.6. 

The SIFT-MS instrument shown here may be viewed as consisting of four distinct regiorts. The ion source region is a microwave discharge of moist air. As noted in Sect. 8.1.2, the dominant terminal ions from a discharge in air are H3O+ NO% and O/. This mixture of ions is transmitted to the lower pressure upstream quadrupole 

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The NO+ ion, an ionic reagent often used in SIFT-MS instrument, is used to distinguish several isobaric aldehydes and ketones. The ionic reaction of NO tends to lead to more fragmentation and the formation of complex that provides a specific mass spectral characteristics to discriminate between aldehyde and ketone molecules [8]. 

One other influence of the drift field is the effect that it has on association reactions. These are used commonly to help distinguish between isobaric compoimds in SIFT-MS instruments. But in PTR-MS, the influence of the electric field can disrapt the more weakly bound cluster ions and this is very evident in the reagent ion mass spectrum generated from HjO. ... 

NO, and 2 was developed with sensitivities of up to 800 cps/ppbv for aromatics, aldehydes, and ketones and this instrument has been denoted as PTR-SRI-MS [61], The HjO source in this instrument was again water vapor, the source gas for NO" was ambient air that is passed through a charcoal filter and the source gas for 02" was pure O2. The switching time in the PTR-SRI-MS instrument between reagent ions is around 10 s which when combined with the drift field does not allow the same separation of isobaric ions that are found using SIFT-MS instruments. 

Fig. 8.5 A schematic representation of the essential regions in a SIFT-MS instrument. (Reprinted by permission from Syft Technologies Ltd [101])...

In summary, the SIFT-MS instrument provides a value for [A] in molecules cm on the basis of the ratio of product ion coimts to reagent ion coimts with appropriate corrections for mass discrimination and differential diffusion and the known reagent-analyte ion-molectrle reaction kinetics (8.50). This value is converted to the analyte concentration in ppbv, [AnalyteJppj, by combining Eqs. (8.51), (8.52), and (8.53). 

As noted earlier in Sect 8.5.1, SIFT-MS instruments can operate at different conditions of temperature and pressure. For example, most of the Syft Technologies Ltd Voice200 SIFT-MS instruments operate at a flow tube temperature of 110 °C and a carrier gas pressnre of 0.6 Torr. In some reactions where association reactions compete with electron transfer, the product ion peak ratios of association/electron transfer are sensitive to temperature and pressure. For example, the reaction of the terpenoid carvone, with NO+ has product ions of reaction C,(,H,40 and... 

Due to the ease of application of SIFT-MS to research in the medical area and particularly for those studies that require direct breath analysis in real time there has been a major focus on medical applications. Quite a number of reviews of the medical area have been undertaken and some of these will be referenced in the appropriate section. For example, shghtly more than two thirds of all SIFT-MS investigations pubhshed in 2013 had a medical focus. This does not mean that there are fewer opportunities for SIFT-MS in say, the environment and food technology areas but the preponderance of medical studies is merely a legacy of the early research focus of the technology. Most conunercial SIFT-MS instruments are in fact operating in the enviromnental and food areas doing routine analyses. 

Fig. 8.10 a Aldehydes and b hydrocarbons monitored (using a Syft Technologies Voice200 SIFT-MS instrument) in a car exhaust that had been fitted with a catalytic converter. The concentrations shown in the figure were based on the known reagent ion kinetics with the analytes listed... 

Fig. 8.7 A comparison of the mass spectrum of carvone (Cj H j O) = 150 generated by a 70 eV eleetrons with the three reagent ions in SIFT-MS as found in a eommercial Voice200 Syft instrument operating with a nitrogen earrier gas. b Chemical ionization (Cl) by HjO reagent ion. c Cl by NO+ reagent ion. d Cl by reagent ion. In b, c, and d the reagent ion signals for at m/z 19, NO at m/z 30 and at m/z 32 are represented by hollow bars and the product ions by filled bars...

PTR-MS and SIFT-MS have a clear advantage over most other techniques for direct analysis of gas mixtures as no derivatization, adsorption onto traps followed by desorption steps, or other sample pretreatment steps are required before the sample is admitted to the analytical instrument. A comparison between the two techruques relevant to breath-sampling has been presented recently [220]. Both technologies allow irmnediate quantification of an analyte in a gas rruxture in real time whether it be a breath sample or an envirormrental sample. As discussed in Sect. 8.4 (PTR-MS) and 8.5 (SIFT-MS), there are three main differences between these two techruques ... 

Fig. 8.12 SIFT-MS mass spectra on a Syft Technologies Ltd Voicc200 instrument with a helium carrier gas. The unfilled bars represent reagent ions and the filled bars are product ions. Figure 8.12a is the mass spectra generated by HjO ions in which the product ion peaks are superimposed. Figure 8.12b is the mass spectra generated by NO+ ions which show clear separation of the isobaric ions...

Fig. 8.13 The ratio of ion intensities obtained by a Syft Voice200 SIFT-MS and a standard PTR-MS measuring VOC samples from the same sample stream with the HjO reagent ion in both instruments. (Courtesy Syft Technologies Ltd)...

The principal weaknesses of SIFT-MS when compared with PTR-MS are its lower sensitivity and the larger size of the instrument. In terms of sensitivity, it rarely achieves better than a few ppbv, although there has been a report of the detection of phosphine down... 

There are various soft chemical ionization mass spectrometric analytical techniques being used for breath analysis, and after PTR-MS the most popular is SIFT-MS (and most notably the work by Smith and Spanel [74] and their co-workers). In this book, we will only concentrate on those studies that have been undertaken with PTR-MS. However, for the interested reader who wishes to know more about the results from a wide range of analytical instruments, we comment that the International Association for Breath Research (lABR), which was founded in 2005, is establishing a database of volatile substances found in breath (both for humans and animals), as well as from skin, urine, faeces and flatulence. Once available, this database will be accessible through the website http //iabr.voc-research.at. Furthermore, disease markers in breath and their potential diagnostic properties are also comprehensively discussed in a book edited by Marczin and Yacoup [75]. 

However, although PTR-MS is loosely based on estabhshed measurements of proton transfer rate coefficients these cannot be applied directly as an absolute method for quantification of an analyte because of the presence of the electric field in the drift mbe. For exothermic proton transfer, a reaction rate coefficient of 2 X 10 cm s is often used in PTR-MS to estimate trace analyte concentrations. However, the rate coefficients for proton transfer have been measured under thermal conditions (e.g., SIFT) and vary between 1 x 10" and 8 x 10 cm s. It is therefore necessary to resort to calibration proceditres using permeation mbes or calibrated mixtures of analytes at the operational field strength of the instrument to determine the absolute analyte concentration in a sample. 

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