Received: 25 February 2019 | Revised: 28 May 2019 | Accepted: 29 May 2019
DOI: 10.1002/ffj.3516
REVIEW ARTICLE
Comprehensive odorant analysis for on‐line applications using
selected ion flow tube mass spectrometry (SIFT‐MS)
Vaughan S. Langford1 | Diandree Padayachee1 | Murray J. McEwan1,2 |
Sheryl A. Barringer3
1
Syft Technologies Limited, Christchurch,
New Zealand Abstract
2
Department of Chemistry, University of Selected ion flow tube mass spectrometry (SIFT‐MS) is a direct mass spectrometry
Canterbury, Christchurch, New Zealand
technique developed for volatile organic compound (VOC) analysis. In this compre‐
3
Department of Food Science and
Technology, The Ohio State University,
hensive review of the SIFT‐MS flavour literature, its flexibility and utility are evi‐
Columbus, OH, United States of America dent. SIFT‐MS has found wide application in the analysis of formation and release
Correspondence
of odorants in fruits, vegetables, nuts, and seeds – including flavour and sensory
Vaughan S. Langford, Syft Technologies, 3 optimization during cooking. Dairy products and poultry, red meat, and seafood also
Craft Place, Middleton, Christchurch 8024,
New Zealand.
comprise a significant number of studies across processing, sensory, and shelf‐life
Email: vaughan.langford@syft.com studies. Edible oils, initially investigated for flavour and oxidative status, have re‐
cently been very successfully classified according to origin using a combined chemo‐
metrics and SIFT‐MS approach. Other recent developments – across different food
groups – have been growth in applications to packaging (including meats in modi‐
fied atmosphere packaging), and correlations of SIFT‐MS data with various types of
sensory testing. The continuing growth of food odorant publications indicates that
the SIFT‐MS technique has a significant role to play in understanding and managing
favourable and unfavourable aroma formation in both academia and industry.
KEYWORDS
aroma analysis, flavour release, food processing, SIFT‐MS, spoilage
1 | I NTRO D U C TI O N static and dynamic headspace approaches), 2 various extraction
and preconcentration methods have been applied over the years.
Food aromas are primarily derived from volatile organic compounds These include solvent extraction, distillation, purge and trap, solid‐
(VOCs) and semi‐VOCs.1‐3 Aroma, together with taste and mouth phase extraction (SPE), and solid‐phase microextraction (SPME).
feel, is critical to acceptability of food to the consumer. However, Reineccius provides excellent introductions to these techniques. 2
the aromas of many foods are very complex, containing not only However, all of these preparatory techniques discriminate in some
large numbers of compounds but also a mix that is chemically very way. Further complicating factors for certain GC/MS applications
diverse. Examples often cited are coffee and cheese. include the choice of a chromatographic column that resolves all
Chemical characterization of food aromas is the first step to‐ compounds responsible for a particular aroma, and a generally
ward understanding how they are perceived by consumers. The slower analysis time.
key analytical tool that greatly advanced this field was the intro‐ In addition, as a purely chemical detector, GC/MS detection (in‐
duction of gas chromatography coupled with mass spectrometry cluding the more comprehensive two‐dimensional variants) does not
(GC/MS). 2 Because the sensitivity to many important aroma com‐ provide any indication of the sensory importance of the many VOCs
pounds was insufficient via direct headspace analysis (both for that it identifies in the food or ingredient. This problem lead to the
Flavour Fragr J. 2019;00:1–18. wileyonlinelibrary.com/journal/ffj © 2019 John Wiley & Sons, Ltd. | 1
,2 | LANGFORD et AL.
advent of GC/Olfactometry (GC/O), where trained human noses are With these opportunities in view, this article provides the first
used as the detector with the aim of identifying and quantifying the comprehensive review of the emerging applications of SIFT‐MS in
important aroma compounds. As Reineccius2 points out, although the food industry, although we note that the SIFT‐MS technique has
it is not without some weaknesses, when made in parallel with GC/ been the subject of numerous general and breath analysis review ar‐
MS detection, GC/O is a powerful aid to identifying key odorants in ticles. Like APCI‐MS and PTR‐MS,9 SIFT‐MS offers advantages over
foods. GC/MS in three main areas, viz:
Having identified the key odour‐active volatiles using GC/MS
and GC/O, researchers encountered a problem: they lacked the time 1. high sensitivity to aroma compounds without preconcentration/
resolution to study the release of aroma compounds from the food extraction, simplifying analysis and eliminating some of the
when it is consumed. This is a very important consideration, because discriminatory issues;
while the taste and mouth feel sensations are localized to the mouth, 2. broad‐spectrum analysis of chemically diverse flavour compounds
the aroma sensation is experienced through chemoreceptors lo‐ in a single analysis (including compounds of very different polarity
cated at the back of the nose. 2 Because the food does not physically and thermally labile compounds); and,
touch these receptors, the flavour sensation is directly related to the 3. high sample throughput due to elimination of the time‐limiting
release of flavour compounds from the food. Release of flavour from chromatography step.
food is a complex and only partially understood phenomenon for a
number of reasons. These include the complexity of the food ma‐ In this article, we focus on the SIFT‐MS technique and its benefits for
trix that consists of different ratios of key food ingredients that bind odorant analysis, then illustrate its research and industrial applications
odorants differently, the wide range of physicochemical properties from the literature, before concluding with some comments on emerg‐
of aroma compounds, and the complexity of the human chewing ing industry trends for SIFT‐MS. It should be emphasized that many of
process (and its variability from person to person, together with age these flavour applications have also been investigated using APCI‐MS
and health effects). and PTR‐MS,9,10 but review of that literature lies outside the scope of
Analytical techniques with sufficient time resolution, sensitiv‐ this review.
ity, selectivity, and robustness to water vapour are necessary to
investigate aroma release. Several direct mass spectrometry meth‐
odologies emerged in the 1990s that offered the potential to in‐ 2 | S E LEC TE D I O N FLOW T U B E M A S S
vestigate not only in situ flavour release, but also other applications S PEC TRO M E TRY (S I F T‐ M S)
that needed faster response times. In order of emergence, these
techniques were atmospheric pressure chemical ionization mass The SIFT‐MS technique emerged as an analytical technique in the
spectrometry (APCI‐MS) whose flavour application was pioneered mid‐1990s7 and saw its first food‐related application in 1999.11 The
by Taylor and Linforth,4 proton transfer reaction mass spectrom‐ theoretical basis of SIFT‐MS lies outside the scope of this review;
etry (PTR‐MS) introduced by Lindinger and co‐workers, 5,6
and the reader is instead referred to the review by Smith and Španěl.8
selected ion flow tube mass spectrometry (SIFT‐MS) introduced Instead, we briefly describe the SIFT‐MS technique, choosing rather
by Smith and Španěl. 7,8
These techniques have contributed signifi‐ to highlight its attributes that offer important benefits across a di‐
cantly to a deeper understanding of flavour release, though the verse range of food applications.
complexity of the process means that it remains wide open to fur‐
ther study.
2.1 | An overview of the SIFT‐MS technique
Direct mass spectrometry has also made other important con‐
tributions in the food industry. Applications include monitoring SIFT‐MS applies precisely controlled chemical ionization (CI) re‐
aroma compounds during dynamic processes, such as those in‐ actions to detect and quantify trace amounts of volatile organic
volving enzymes, or occurring in roasting, fermentation and dry‐ compounds (VOCs) and inorganic gases in air in real‐time. In the mid‐
ing. Real‐time monitoring has the potential to optimizing aroma on 1990s, detection limits were in the mid‐part‐per‐billion by volume
every production run. Real‐time techniques also offer an unprece‐ (ppbv) range, but the latest commercial instruments can achieve low
dented opportunity to economically monitor critical odour‐active part‐per‐trillion by volume (pptv) detection limits.12,13
volatiles through the entire process – from manufacturing facility There are three main elements of the SIFT‐MS technique
to the eventual release of flavour in the mouth of the consumer. (Figure 1):
Real‐time, direct mass spectrometry techniques applying soft 1. Reagent ion generation and selection. The traditional SIFT‐
chemical ionization (CI) offer a faster option for the manufacturer MS reagent ions (also called precursor ions) – H3O+, NO+ and O2+ –
to ensure that a product from their facility will meet the expecta‐ are all formed by microwave discharge through moist air. No bottled
tions of the consumer throughout the product's entire shelf‐life. In gases are required, and microwave ionization is very clean, yielding
the medium‐ to long‐term these may become the dominant indus‐ minimal maintenance of the ion source region.
try applications since they can benefit quality control and process The reagent ion is selected using a quadrupole mass filter. Since
monitoring. selection is electrical, not mechanical, instrument software can use
, LANGFORD et AL. | 3
FIGURE 1 Schematic representation of the SIFT‐MS technique. For explanation of the diagram, see the text
this to a significant advantage by providing additional selectivity in compound library,15 instrument software can instantaneously cal‐
real‐time. In SIFT‐MS, reagent ions can be changed within 10 ms, culate each analyte's absolute concentration in real time. This op‐
eliminating the delays and slow rise times resulting from the much erational mode of SIFT‐MS has been denoted variously as multiple
slower switching in mechanical source gas systems. We address ion monitoring (MIM) or selected ion mode (SIM) in the literature,
these benefits in detail in Section 2.2. with the latter name used to indicate the loose relationship that
Recently, negatively charged reagent ions (O ‐, O2‐, OH‐, NO2‐, exists between targeted, quantitative analysis modes in SIFT‐MS
‐
and NO3 ) have been added into commercial SIFT‐MS instrumen‐ and GC/MS.
tation.14 These ions were introduced for environmental and indoor Like GC/MS, SIFT‐MS can also be operated in full mass scan
air quality monitoring, where target compounds include inorganic mode to detect all ionization products. Here, for each reagent
species (such as the acid gases, SO2 and HF) that are not detect‐ ion, the second quadrupole mass filter is scanned across a pre‐
able using positively charged reagent ions. However, addition of selected mass range. Full scan mode data can be used to obtain a
extra reagent ions with different reaction mechanisms also facil‐ detailed record of volatiles in a sample for later reprocessing, to
itates more selective analysis of odorants (especially for oxygen‐ aid identification of compounds, or for volatile ‘fingerprinting’ of
ates and sulphur‐containing species). To date this advance has not a sample. It must be noted, however, that the absence of chro‐
been exploited by users in the food and fragrance industries, as matographic separation means that the SIFT‐MS technique is not
far as the authors are aware. Hence this article focuses on appli‐ as well suited to compound identification as is GC/MS. In subse‐
cations of SIFT‐MS that utilize the conventional positive mode of quent references to SIFT‐MS full scan mode in this article, we will
ionization. refer to it as SCAN mode because of the analogy to SCAN mode
2. Analyte ionization. The selected reagent ion is injected into in GC/MS.
the flow tube and excess energy is removed through collisions with
the carrier gas (helium or nitrogen are options on commercial instru‐
2.2 | SIFT‐MS: multiple rapidly switchable reagent
ments) over a period of about 1 ms. These collisions yield reagent
ions for enhanced selectivity
ions of extremely consistent thermal energies, giving SIFT‐MS a
unique ability to precisely control the sample ionization conditions. The three standard positive reagent ions (H3O+, NO+ and O2+) used
Next, the sample is introduced at a known flow rate and the re‐ in SIFT‐MS exhibit a variety of reaction mechanisms, which means
active compounds it contains are ionized simultaneously by reagent that they usually react differently with each compound. The domi‐
ions to form well‐characterized product ions. A further benefit of nant mechanisms of these ions are summarized in Table 1. In ad‐
using a carrier gas is realized at this point: it transports reagent and dition to H3O+, NO+ and O2+ have also been available in PTR‐MS
product ions along the flow tube without the need for adding extra instruments for a decade.16 However, the additional energy in the
energy. This means that fragmentation is minimized, simplifying drift tube of the PTR‐MS technique means that the three‐body as‐
spectra compared to PTR‐MS, where product ions are transported sociation reaction of NO+ is not observed.16,17 This eliminates a
along a drift tube under an electric field gradient. very useful, selective reaction mechanism for many carbonyls from
3. Analyte quantitation. Product ions and unreacted reagent PTR‐MS.
ions are sampled into a second quadrupole mass spectrometer at Different reaction mechanisms yield different spectral
the downstream end of the flow tube. The ions are mass‐filtered ‘fingerprints’ for the analytes in the sample – often with a dif‐
and detected using a particle multiplier. By utilizing an integrated ferent ‘fingerprint’ for each reagent ion. This provides greater
