I Understand There is Something New in Enviro-chemical TEQA When the Sample Matrix is Polluted Air: What is It?
It is the application of Selected Ion Flow Tube-Mass Spectrometry (SIFT-MS)l Recall that near the end of Chapter 3, the topic of conventional sample preparation techniques available for sampling air pollutants was introduced. These techniques are still quite valuable and routinely used. SIFT-MS however, holds the promise of direct air sampling combined with direct quantitative analysis via quadrupole mass spectrometry, without the need for sample prep! This discussion on SIFT-MS is largely drawn from a recent book that was brought to the author’s attention.135 SIFT-MS originated in the pioneering work of Spanel and Smith in 1996 which followed from selected ion tube flow tube (SIFT) technology introduced 20 years ago by Adams and Smith. SIFT, in turn, was an extension of the early work of adapting flow tubes to investigate ion- molecule reactions begun in the late 1960s by Ferguson and collaborators. SIFT-MS is unique in that the reagent (or precursor) ions are well characterized and are mass-selected, and undergo known ion-molecule reactions with the analytes. In principle, an analyte such as an air pollutant can be quantitated without the need for analyte calibration!
SIFT-MS appears to complement the existing determinative techniques that measure environmental contaminants such as GC-MS and LC-MS-MS as shown in Figure 4.131 where analyte polarity is plotted against analyte molecular weight (MW). SIFT-MS is seen to “fill in the gap” left by these two major determinative techniques introduced and discussed earlier. One can then envision that environmental laboratories in the future may have all three determinative techniques. Figure 4.132 shows a schematic outline of the principle operation of a SIFT-MS instrument. The ion source region is a microwave discharge of moist air. The dominant terminal ions from the discharge in air are: H,0+, NO+, and 0,+. The mixture of ions is transmitted to the lower pressure upstream quadrupole mass filter where mass selection takes place at a typical pressure in the 10-4 torr range (1 atmosphere of pressure = 760 torr). The selected reagent ion (for positive ion, one of the following: H,0+, NO+, or 0,+) is then transmitted into the flow tube where the reagent ion-analyte reaction occurs under controlled conditions. The flow tube pressure is 0.6 torr so that the ions from the upstream quadrupole enter the flow tube against a pressure gradient. The entry is assisted by means of a Venturi orifice which facilitates the transmission of ions against the pressure gradient. The reagent ions are then carried along the flow tube by the carrier gas (usually Fie although N, is being used in an increasing number of applications). It is in the flow tube where the diagnostic reagent ion-analyte reaction occurs. All ions within the flow tube are then sampled through a small orifice at the downstream end of the flow tube and are mass analyzed
by the second quadrupole mass filter. The ion number densities are then counted by the pulsecounting electronics. These three reagent ions can be interchanged within a few milliseconds to obtain a complete analysis in real time of analytes (from contaminated air for example) from all three reagent ions.
Most of the commercial units made by Syft Technologies Ltd. (the Voice 200 and the Voice 200Ultra SIFT-MS instruments) operate at a flow tube temperature of 110°C and a carrier gas
pressure of 0.6 torn In some reactions where association reactions compete with electron transfer, the ratio of the product ion peaks are influenced by the conditions of temperature and pressure selected.135
I Understand There is Something Called “ion Chemistry” in SIFT-MS. What is This?
The term “ion chemistry” refers to the variety of reagent ions that are generated in the micro- wave plasma and of the chemical reactions with analytes that yield new chemical products. Figure 4.133 shows a somewhat different way to view a SIFT-MS instrument. The SIFT-MS determinative technique is seen to consist of three major parts: 1) reagent ion selection, 2) analyte ionization, and 3) analyte quantitation. Note the abundant negative ions shown in Figure 4.133 are also produced in the plasma namely O', O,, OH, NO,, NO,'. We focus here only on the three positive ions, H,0+, NO and 0,+ as reagent ions:
When H,0+ ions are injected into the flow tube, clusters of water ions are also formed. They often react in the same way as the H,0+ ion does with the VOC providing an exoergic pathway for proton transfer as shown below:
The predominant reaction of the H,0+ reagent ions with VOCs (e.g. from contaminant air) represented by A (see below) is exothermic proton transfer;
When proton transfer is quite exothermic (more than about 1 eV) then dissociative proton transfer may occur from AH+ as in some reactions with organic compounds such as alcohols, aldehydes, and carboxylic acids. This leads to elimination of H,0. The specific aldehyde shown in the example is hexanal (an aliphatic aldehyde) which serves as an example of a VOC drawn from contaminated air. The molecular structure for hexanal is shown below:
The reaction that occurs in the SIFT-MS is shown below:
Hydronium ions can also simply add to the analyte molecule either as a single product or in conjunction with other product channels:
In some cases the weakly bound association complexes denoted by H,0+(A) may switch out the H,0+ to water and if this happens the use of H,0+ as a diagnostic reagent is lost.
Greater variety exists in the reactions of NO compared to the reactions of HO in SIFT-MS.
There are usually one or two product ions in its reactions with a given analyte. These differences make NO a very useful and important reagent ion in SIFT-MS analysis. Electron transfer can occur if the ionization potential of the analyte is less than the ionization potential for NO which is 9.26 eV. Numerous VOCs have lower ionization potentials when compared to NO with electron transfer as the common reaction pathway for example:
H (hydride ion) is also a common pathway of reactions with NO+ with aldehydes, ethers and alcohols with the exception of tertiary alcohols w'hen hydroxide ion transfer occurs. Consider the VOC acetaldehyde:
0,+ is the most energetic of the common SIFT-MS reagent ions having an ionization potential (IP) of 12.06 eV. This value is larger than the IPs of most VOCs with the result that O, generally reacts rapidly either by electron transfer (ET) or dissociative electron transfer (DET) and produces multiple product fragment in some cases. O,- is particularly useful in monitoring analytes that are unreactive with H,0+ and NO+ reagent ions such as the lower MW hydrocarbons. The O, reaction with isoprene is a typical example in that there are three product ion channels:
Methane which is unreactive with H,0+ and NO+ reagent ions, reacts with the 02+ reagent ion. The detection limit is not as low as for other analytes. The reaction is as follows:
The small rate coefficient for methane means that the limit of detection for methane are in the mid- to high-ppb range by volume.135
The real-time monitoring of atmospheric VOCs has been important for years. Using the SIFT- MS determinative techniques enables Diesel engine exhaust gases such as NO, NO,, HNO,, aldehydes, and ketones. Peroxyacetyl nitrate (PAN) is known to be a precursor to photochemical smog. PAN can be measured using SIFT-MS with a limit of detection of 20 ppt by volume.135 The molecular structure of PAN is shown below:
A bright future is ahead for SIFT-MS in direct analysis air monitoring.