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" Adapted from Pfab (1995). '' Campos et al. (1990).

b Hippler and Pfab (1995). ' McDiarmid and Sabljic (1988).

" Adapted from Pfab (1995). '' Campos et al. (1990).

b Hippler and Pfab (1995). ' McDiarmid and Sabljic (1988).

separation using these techniques, see standard analytical chemistry texts such as Skoog et al. (1998).

A potential limitation in the application of MS to near-surface measurements is the tremendous number of compounds in the atmosphere, particularly organics, and hence the increased complexity of interpretation of the single mass spectrum. In the MS ion source, the use of particular ion-molecule reactions to form the ions of interest or the ionization of one selected com pound through resonant multiphoton absorption discussed earlier provides one means of specificity. A second method applied in the analyzer region is tandem mass spectrometry.

Figure 11.20, for example, is a schematic diagram of a tandem MS used for both surface and airborne measurements in the troposphere (Spicer et al., f 994a). Air is drawn into the sample inlet and ions are formed by a corona discharge generated by high voltage be

Corona discharge needle

Sample inlet

Sample exhaust

RF focusing collision cell

Pump

Channel electron multiplier

Channel electron multiplier

Corona discharge needle

Sample exhaust

RF focusing collision cell

Pump

FIGURE 11.20 Schematic diagram of tandem mass spectrometer (adapted from Spicer et al., 1994a).

FIGURE 11.20 Schematic diagram of tandem mass spectrometer (adapted from Spicer et al., 1994a).

tween a needle and plate. Ions such as H30 + (H20)„ are generated and undergo the ion-molecule reactions with trace gases (T) as described earlier. The ions then enter the interface, where water is stripped from the cluster by a stream of dry N2, leaving the ion TH+. Negative ions are generated and sampled by reversing the voltages on the needle and plate.

After being focused, the ions enter the first quadrupole (Ql), which can be used as a single mass spectrometer. However, the peaks observed using Ql may not be parent ions. While the degree of fragmentation of ions formed using chemical ionization is generally much less than that using electron impact, it does occur. Hence observation of a particular peak corresponding to TH+ in the positive ion mode, for example, does not guarantee that the trace gas T is responsible for the signal at this mass rather than a fragment from a larger molecule.

Tandem MS provides a powerful approach to this problem. In this mode, an ion exiting Ql enters a cell containing a low pressure of a gas such as Ar or N2 where it is collisionally dissociated. The fragments are then detected using the second quadrupole (Q2). The fragments are characteristic of the ion selected using Ql and provide confirmation of the identity of the parent ion.

Figure 11.21, for example, shows the MS-MS of the peaks at m/e 70, 72, and 74, respectively, when a calibration sample of Cl2 is sampled into the instrument shown in Fig. If.20. The peak at m/e 70 fragments only to 35 amu, that at m /e 72 to both 35 and 37 amu, and that at m/e 74 only to 37 amu. Clearly, such fragmentation is consistent with the peaks in the Ql scan being attributable to Cl2 with isotopes 35CI and 37C1.

32 40 48 56 64 72

32 40 48 56 64 72

32 40 48 56 64 72

FIGURE 11.21 MS-MS of peaks at m/e 70, 72, and 74 due to CI 2 (spectra taken by K. Oum).

32 40 48 56 64 72

FIGURE 11.21 MS-MS of peaks at m/e 70, 72, and 74 due to CI 2 (spectra taken by K. Oum).

The instrument can be run in various combinations of fixed or scanning modes for Qf and Q2 (e.g., see Johnson and Yost, 1985). Particularly useful is the continuous mode, where particular peaks in the Ql scan and certain fragments in the Q2 mass spectrometer are followed, rather than scanning one (or both) of the quadrupoles. Indeed, this method has been used to measure Cl2 specifically in the marine boundary layer (Spicer et al., 1998). In these studies, Cl2 was generated as described earlier and the mass combinations (Q1/Q2) for 70/35, 72/35, 72/37, and 74/37 were followed. The combination of MS-MS and the isotope ratios provided unique confirmation that the species being measured was indeed Cl2. Concentrations down to 15 ppt CI 2 could be measured using this approach, with slightly better sensitivity for Br2.

Table 11.6 shows some compounds that can be measured using this technique and estimated detection limits (Spicer et al., 1994a).

d. Detectors

The detectors used in mass spectrometers for atmospheric applications are essentially the same as for other MS applications and are commonly electron multipliers, either channeltrons or multichannel plate

TABLE 11.6 Estimated Detection Limits for Some Species of Atmospheric Interest by Atmospheric Pressure Ionization Mass Spectrometry"

Estimated detection

TABLE 11.6 Estimated Detection Limits for Some Species of Atmospheric Interest by Atmospheric Pressure Ionization Mass Spectrometry"

Estimated detection

Class

Example

limit (ppt)

Alcohols

Methanol

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