Time drift

Upon acceleration through an electric potential of V volts, ions of unknown m/z value reach a velocity v = f2zeV/m]" ). The ions continue at this velocity (drift) until they reach the detector. Since the start (to) and end (r) times are known, as is the length d of the drift region, the velocity can be calculated, and hence the m/z value can be calculated. In practice, an accurate measure of the distance d is not needed because it can be found by using ions of known m/z value to calibrate the system. Accurate measurement of the ion drift time is crucial. 

The IMS response for a compound is strongly dependent on temperature, pressure, analyte concen-tration/vapour pressure, and proton affinity (or elec-tron/reagent affinity). Pressure mainly affects the drift time, and spectral profiles are governed by concentration and ionisation properties of the analyte. Complex interactions among analytes in a mixture can yield an ambiguous number of peaks (less, equal to, or greater than the number of analytes) with unpredictable relative intensities. IMS is vulnerable to either matrix or sample complexity. 

In later measurements, Tewari and Freeman (1968,1969) measured the ion mobilities from drift-time measurement and obtained k/u values from the current decay following a pulse of X-rays of 1 ms duration. The purpose was to find the dependence of Gfl on molecular structure. It was found that Gf. increased with the sphericity of the molecule. In liquid argon Gf. 5 was measured, which indicated that all ionized electrons in argon are free. However, this... 

The drift time as calculated by means of Eq. 4.7 is not fully identical to the total time-of-flight. Obviously, the time needed for acceleration of the ions has to be added. Furthermore, a short period of time Iq may be attributed to the laser pulse width and the process of desorption/ionization, which is typically in the order of a few nanoseconds. Thus, the total time-of-flight itotai is given by... 

Decomposition of the adduct ion can also lead to the loss of NO2, which can be accompanied by the retention of charge to NO2 (an ion with m/z 46 and a high mobihty usually faster in drift time than the reactant ion peak) as shown in Eq. (4) or to (M-NO2). data not shown. 

Figure 12 Topographic plots of a DMS-IMS analyzer response to 2,4-DNT (left frame) and 2,4,6-trinitrotoluene (right frame). Positions ofions in plots are circled the reactant ion peak is seen at compensation voltage of 8V and drift time of 2 ms. Source (C.R. White et al., unpublished data. New Mexico State University, September 2005.)...

Also noted in Figure 7A are three small ion mobility peaks at drift times of about 28,30 and 33 ms. These unwanted ions are formed in the ion source by the clustering of the Cr ion to HCl, HCOOH, and CHjCOOH. These impurities are not introduced with the buffer gas, but are formed by the Ni-induced radiation chemistry that is continuously occurring in the ion source. Because these ions do not interfere with the IM reaction of interest in Figure 7, their presence can be ignored as long as their... 

Figure 10. Plot of In [(Acr + Ab,-)/A -1 versus Cl" drift time as determined from the reaction-modified IMS spectra shown in Figure 9. The slope of the line shown (obtained by least-squares analysis) is expected to be equal to k2(CH3Br].

An interesting aspect of the IMS approach to reaction rate measurements is that fast equilibration in the reactions of an ion with an added clustering agent will be quantitatively reflected in the apparent drift time of the ion. For example, in Figure 11 is shown the observed drift time of a partially clustered Cr ion packet as... 

Figure 13. Reaction-modified IMS spectra for the reactions of chioride ion with (A) n-butylbromideand (B) /-propylbromide in nitrogen buffer gas at atmospheric pressure and 125 °C. The concentrations (molecules/cm ) of the alkylbromide added to the drift gas are indicated. The peaks at drift times greater than 0.050 seconds are due to impurities formed in the ion source and do not affect the kinetic measurements of interest.

We expect that the IMS approach will be applicable to the study of most positive and negative IM reaction systems for which a stable reactant ion can be made in an atmospheric pressure buffer gas. However, the need for conditions in which the reactant ion is unaffected by side reactions with trace impurities can present a formidable problem in the IMS approach. This is because the timescale of the experiment is relatively long and the absolute concentration of impurities can be relatively high even though they are only minor components of the buffer gas. Consider, for example, if a reactive impurity (with = 2 x 10 cm s ) is present in an atmospheric pressure drift gas at a concentration of only 1 part per billion, that impurity will consume 80% of a set of reactant ions having a drift time of 30 ms. Therefore, very pure buffer gases and reagents are required. Since it is extremely difficult to reduce residual water to levels below 10 parts per billion, the IM reactions of ions that react readily with water probably cannot be studied by the IMS method described here. 

The Average Drift Time or the time it takes the monitor to respond to changes in concentration can be calculated from Equation 2 ... 

For a path length of. 65 cm and diffusion rate of 0.12, the response time (Average Drift Time) is less than two seconds. In other works, the monitor responds to changes in concentration of most organics in the atmosphere it is sensing, within two seconds. 

The ion mobility spectrum in Figure 22-16b is a graph of detector response versus electrophoretic terms, see problem 26-46. drift time for several explosives. Peak area is proportional to the number of ions. Peaks are... 

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