Ion mobility spectrometer apparatus and method, incorporating air drying

An ion mobility spectrometer for detecting substances such as narcotics and explosives, has inlets for a sample gas and a drift gas. The gas can be ambient air, bottled air, or another gas source. To ensure accuracy and prevent drifting of analyte peaks, the air is dried. A two stage dryer is provided comprising a first dryer, preferably a dryer which chills the air and removes water by condensation. This removes the bulk of the water. The second dryer includes a suitable absorbent, and reduces the water content to around 1-10 .mu.g/L, i.e. a level which will not substantially affect the performance of the IMS apparatus. The first dryer substantially reduces the load on the second dryer, and enables an extended period of use before the absorbent material in the second dryer either needs to be replaced or regenerated. The increased stability of calibrant and analyte peak positions allows detection windows to be narrowed, resulting in significantly lower false alarm rates.

FIELD OF THE INVENTION 
This invention relates to chemical detection based on the collection and 
analysis of surface residues and particles. It particularly relates to 
analysis in an ion mobility spectrometer (IMS) indicating drying of air. 
It more particularly relates to the detection with low false alarms of 
explosives and various contraband items, such as drugs and narcotics in 
baggage and cargo and on passengers using various modes of transportation, 
using the technique of ion mobility spectroscopy (IMS). 
BACKGROUND OF THE INVENTION 
There is currently an increasing problem in many countries with the 
smuggling of illicit or illegal substances, such as drugs and narcotics, 
and also various substances which are legal but subject to high tariffs 
making smuggling attractive; generally the illicit or illegal substances 
are the major concern. A further problem is the transportation of 
explosives, either illegally in the nature of smuggling, or with the 
intention of being used as part of a terrorist threat or attack on a ship, 
aircraft or the like. 
Consequently, there is increasing demand for detection equipment for use at 
airports, seaports, border crossings, etc. to enable authorities of 
individual countries to detect such substances, whether carried by 
individuals, in baggage carried by an individual, or in large, commercial 
transportation containers and the like. 
Such detection equipment is used increasingly in the screening of baggage 
for explosives, and commonly relies on the collection of vapors which are 
subsequently analyzed by mass spectrometric or chromatographic techniques. 
Detection of modern plastic explosives and illicit drugs, such as heroin 
and cocaine, by such vapor collection is difficult, and frequently 
impossible, because of factors such as: the extremely low vapor pressures 
of explosives and narcotics; the small amounts of vapors emanating from 
these explosives and drugs requiring high volume sampling and very high 
sensitivity of detection; these explosives and drugs being easily 
concealed in a variety of baggage or personal articles inhibiting the 
collection of vapors; the frequent presence of toilet articles, perfumes, 
cosmetics and the like on persons, belongings, and baggage, producing 
vapors containing molecules with some properties similar to the targets of 
interest, thus further complicating the analysis. 
Handling of explosives and/or drugs etc. and attempts at their concealment 
results in minute surface contamination from trace residues of these 
substances. Similarly, the packaging of drugs, such as cocaine and heroin, 
for concealment in baggage or cargo is equally difficult to achieve 
without similar surface contamination. With exceptional measures being 
taken, such traces will be present, and are in sufficient quantity, 
although minute, to enable them to be collected through a variety of 
means, and to be subsequently analyzed by IMS. 
Inherent in the detection of small quantities is the possibility of 
unacceptable False Alarm Rates (FAR). 
In general, the smaller the quantity that can be detected, i.e. the more 
sensitive the detection equipment, the greater the possibility of a False 
Alarm. This can be caused by detection of spurious trace quantities 
present from another source, e.g. because the user has just handled a 
contaminated article; residue from a previous test that was lodged in the 
apparatus, but becomes dislodged and drawn through the apparatus. 
Thus, IMS affords a well-established technique for the detection of drugs 
and explosives, but because of its high sensitivity interferences can 
occur and cause false alarms. If FAR are excessive, the instrument can 
have little practical value to security screeners and customs officers, 
and users will have little confidence in the instrument. It would enhance 
the value of the equipment if the FAR is reduced to the minimum possible. 
A major cause of interference and instability in IMS is the presence of 
water vapor and other contaminants in drift gas. For obvious operational 
convenience, air is generally used as the drift gas in customs and 
security screening locations. Ambient air has to be dried to a high degree 
before use, to avoid these problems. Additionally, chemical scrubbing with 
charcoal or the like is used to remove other contaminants from the air, 
such as trace hydrocarbons often present in urban atmospheres from 
automotive traffic, airport traffic, or industrial operations. In 
laboratory situations IMS instrumentation can be run from compressed, 
dried air, called zero air, but this is impractical and too costly for 
actual use. 
Another source of interference is the presence of cosmetics, toilet 
articles and other articles on travellers and in their belongings. These 
may cause spurious alarms due to chemical signature interferences in IMS, 
unless the IMS incorporates superior peak discrimination capabilities. 
Some legal substances can give chemical signatures that are difficult to 
differentiate from illegal substances of interest. 
SUMMARY OF THE INVENTION 
In accordance with a first aspect of the present invention there is 
provided a method of detecting the presence of an analyte in an ion 
mobility spectrometer apparatus having a drift region, a first inlet for 
drift gas at one end of the drift region, an outlet for exhaust gas 
connected to the drift region, a second inlet, and means for applying an 
electric field across the drift region including a collector electrode, 
the method comprising: 
(a) drying a supply gas to remove a major portion of the water vapor in a 
first drying device capable of generally continuous operation; 
(b) subjecting the supply gas to secondary drying in a second drying device 
capable of only a finite period of operation, to reduce water vapor 
content to a concentration level sufficient to prevent any substantial 
effect on at least one predetermined analyte peak position wherein removal 
of a major portion of the water vapor in step (a) extends the effective 
operating period of the second drying device; 
(c) passing the supply gas through the drift region, as a drift gas; 
(d) introducing vapor of at least one analyte through the second inlet into 
the drift region; and 
(e) detecting the presence of each analyte from the drift time through the 
drift region. 
Preferably, the IMS device includes an ionization region, and a gating grid 
separating the ionization region from the drift region. The second inlet 
then opens into the ionization region and the outlet is connected to the 
ionization region, preferably adjacent the gating grid. The dried supply 
gas is then also supplied to the second inlet as a carrier gas for 
entraining the analyte vapor. 
Preferably, the gas is first dried by chilling to a low temperature to 
remove water vapor by condensation. This technique can be carried out 
continuously, without any degradation in its performance. The gas can be 
chilled to 2.degree. to 6.degree. C., by a thermo-electric cooler. 
The second drying step is preferably carried out by an absorbent, capable 
of reducing the water content to about 1 to 10 .mu.g/L or minus 60.degree. 
C. to minus 100.degree. C. dew point. The absorbent could be calcium 
sulphate, phosphorus pentoxide, or molecular sieve, or proprietary 
material based on these substances, and containing appropriate color 
indicators, to indicate the moisture content of the absorbent material. 
The present invention also provides a corresponding ion mobility 
spectrometer apparatus, the apparatus comprising: 
an ion mobility spectrometer having a drift region, a first inlet for a 
drift gas at one end of the drift region, an outlet for exhaust gas 
connected to the drift region, means for applying an electric field across 
the drift region including a collector electrode, means for introducing a 
sample vapor into the drift region, and means for analysing an output of 
the collector electrode; 
a first dryer having a main gas inlet for a supply gas, for removing the 
bulk of the water vapor in the gas; and a second dryer having an inlet 
connected to the first dryer and outlet connected to the first inlet. 
Again, the spectrometer preferably includes an ionization region. The first 
dryer preferably chills the gas to remove water vapor by condensation, 
while the second dryer includes an absorbent material for absorbing water 
vapor. 
The two-stage dryer efficiently removes water vapor from the drift gas over 
an extended period of time, thereby producing IMS ion peaks that remain 
very stable in position over long periods of time, and thereby allowing 
the use of very narrow IMS time intervals in which to locate the ion peak 
of the species of interest, to more effectively discriminate that peak 
from close by ion peaks of chemical interferences. 
The invention can be used in the IMS analysis of minute particulate 
contamination collected from the surfaces of objects being inspected by 
security personnel in situations where real-time detection with low FAR is 
essential. The better discrimination provided by the invention provides 
lower FARs than hitherto possible. The better ion stability provided by 
the invention allows many weeks of instrument operation without 
recalibration. 
The present invention utilises the improved ion and IMS peak position 
stabilities to effectively and efficiently select, discriminate, and 
confirm the presence of IMS peaks for the ionic species of interest, 
through the use of algorithms and associated electronics, and optionally 
through the use of the IMS peak position of a calibrant ion. This aspect 
of the invention results in faster data processing, thereby allowing more 
data to be used and more peaks to be monitored simultaneously, all within 
the time frame required for real-time detection of surface contamination. 
This increased data processing capability results in greater accuracy, 
lower FARs, and the capability of monitoring for more species of interest. 
In contrast, most conventional IMS instruments using air as drift gas 
experience a rapid deterioration in air-drying efficiency, causing the 
position of IMS peaks to change significantly with time, thereby requiring 
the use of wider detection intervals, resulting in increased interference 
from other peaks appearing within these intervals, thereby producing 
higher FARs.

DESCRIPTION OF PREFERRED EMBODIMENT 
An apparatus in accordance with the present invention includes an IMS 
detector 4, which is depicted in FIG. 1, and which makes use of ion 
mobility principles to respond selectively to substances of interest. 
The apparatus has an inlet 3 for the sample carrier gas, in this case air, 
including a reactant if required. A sample substrate 1 is placed adjacent 
a desorber heater 2, and the sample carrier gas is processed through the 
desorber heater 2 and sample 1. 
A conduit guides the gas flow to a reaction region 6 of the IMS device. 
This includes a repelling ring 70 at the inlet for gas, and an ionizing 
source 5. An outlet for an exhaust gas flow is indicated at 72. 
A gating grid 8 separates the ionization or reaction region 6 from a drift 
region 7. The drift region 7 has a series of focusing rings 9 around it, 
and a collector electrode 11 at its end remote from the repelling ring 70. 
An inlet 10 is provided for a drift gas flow, including a calibrant. 
The collector electrode 11 is connected through an amplifier 12 to a 
digitizer 13. This in turn is connected to a microprocessor 14, connected 
to a display 15 and key pad 74, in a known manner. 
The detailed electronics of the IMS device do not form part of the present 
invention, and can be conventional. It is sufficient to note that an 
electric field is applied between the collector electrode 11 and repelling 
ring 70, to cause certain species of ions to tend to drift towards the 
collector electrode 11. Passage of ions from the ionization or reaction 
region 6 is controlled by the gating grid 8, as detailed below. 
In use, a sample of microscopic dust is collected from the surface under 
investigation through the process of either swabbing, wiping, or vacuum 
suction and/or abrasion via a sampling head, with the sample being 
collected on an inert substrate indicated at 1. The substrate 1 is placed 
in the IMS device 4, as shown. Vapors are liberated from the substrate 1 
by application of the desorber heater 2, and are subsequently carried into 
the reaction region 6 by the carrier gas flow 3. In the reaction or 
ionization region 6, the carrier gas and trace vapors are ionized by the 
weak radioactive source 5. As a result of complex interchange reactions 
which take place in the reaction region 6, the molecules of certain 
species in the vapor form ions and ionic clusters, both of which are 
hereafter designated as ions, while others do not. The ions are prevented 
from entering the drift region 7 by the potential of the charged gating 
grid 8. When the gating charge is changed to a lower potential, the ions 
can enter the drift region 7; with the higher gating charge or potential 
present, the ions are prevented from entering the drift region 7 and exit 
through exhaust 72. After entering the drift region 7, the ions are 
accelerated, under the influence of a strong electric field applied 
through focusing rings 9, through the drift region 7 against a flow of 
drift gas 10 towards the collector electrode 11. Their arrival time at the 
collector electrode 11, the "drift time", is a function of each ion's 
characteristic mobility and is a characteristic of the individual species. 
These species are therefore classified according to their ability to be 
ionized, and to the relative mobilities of the ions produced. 
The weak ionic current through the collector electrode 11 is amplified in 
the amplifier 12, digitized at 13 and processed by the microprocessor 14, 
employing algorithms for discrimination of the desired species from any 
interfering vapor present in the sample as background. The resulting 
identification of the drug or explosive compound is reported on a liquid 
crystal display 15 and also as a visual and audio alarm. 
Ambient air is used to provide supply gas for use as the drift and sample 
carrier gases for IMS operation. Other gas sources, e.g. bottled air or 
another gas, can be used. Acceptable operation of an IMS is obtained when 
the water content of air is very low, about 1-10 .mu.g/L (-60.degree. C. 
to -100.degree. C. dew point). Although this water content level is 
achievable by means of a drying tube containing a proprietary dryer based 
on calcium sulphate or similar material, it is well recognized that the 
water-removing efficiency of such tubes becomes increasingly less 
efficient as the material within the tube is gradually deactivated, 
leading to an increasing water content in the drift gas. Water molecules 
cluster with sample and calibrant ions in the IMS drift region. The extent 
of clustering increases with water content, and causes the drift times of 
the sample and calibrant peaks to change to varying extents. Gradually 
increasing water content in the drift gas therefore results in an ongoing 
instability in drift times, and to a considerable degree of uncertainty in 
the sought analyte peak positions. 
Referring to FIG. 2, efficient drying of the ambient air, the supply gas, 
used as drift and carrier gases is achieved over an extended period by 
means of a two-stage drying process. The ambient air 16 is pumped through 
a prefilter 17, to remove large particulate contamination, by a membrane 
pump 18 with the flow being stabilized by a surge tank 19. The first stage 
of drying is by means of a thermo-electric (TE) cooler 20 which chills the 
moist incoming air to 2.degree. to 6.degree. C. and thus removes a major 
portion of the water by condensation which is removed at 21. The predried 
air then proceeds to the second stage in a large capacity drying tube 22 
containing calcium sulphate 23 or other drying agents such as phosphorous 
pentoxide or molecular sieves, where the water content is reduced to the 
1-10 .mu.g/L level. A small portion at the end of the drying tube 22 
contains activated charcoal 24 to remove organic contaminants from the 
air. The dry air then passes through another filter 25, to remove any 
extraneous contamination, before being split at 26 into drift gas and 
sample carrier gas flows. 
The flows of carrier gas and drift gas are controlled by respective mass 
flow controllers 27, 28, after which each proceeds by alternate routes, 
selected by three way switches 29, 30, dependent on whether drugs or 
explosives are being analyzed. A reactant 31 is added to the sample 
carrier gas to assist in ionization reaction chemistry when explosives are 
being analyzed; such a reactant is not needed for drug analysis and the 
carrier gas flows through line 34. Appropriate calibrants are added to the 
drift gas for the analysis of drugs 32 and explosives 33 by means of a 
permeation tube bleed. The drift and carrier gases then proceed to 
opposite ends of the IMS detector. The drift gas passes through the drift 
tube 7 and the gating grid 8 before exiting through the exhaust 52 at the 
grid end of the ionization chamber 6. The carrier gas carries the desorbed 
sample vapors into the ionization chamber 6 before going to the common 
exhaust 52. Typical gas flows are 250 to 350 cc/min for drift gas and 150 
to 350 cc/min for carrier gas. The drift gas always flows through the IMS 
detector when the unit is in operation, even when not in the sample 
analysis mode. This constant flow purges the unit between sample analysis, 
thus eliminating memory effects, and provides a means of constantly 
monitoring the calibrant ion peak stability. 
The exhaust gas is drawn from the IMS by a second membrane pump 39 with the 
flow controlled by a surge tank 40 and a mass flow controller 41. The 
exhaust gas flow, typically 500 cc/min, is the sum of the drift and 
carrier gas flows. The exhaust is cleaned up by a filter 42, typically 
packed glass wool, to remove pollutants. A three way switch 43 allows 
purge air 44 to be introduced into the exhaust gas line when the IMS is 
not in the sample analysis mode. 
Removal of water by the first-stage TE chiller 20 significantly reduces the 
load on the second-stage drier material, thereby significantly increasing 
its life as an efficient provider of dry air at the required level of 
water content. Furthermore, the second-stage drier material can be 
reactivated by in situ heating, shown at 35. In known manner, the 
absorbent or drying agent in the tube 22 can include a color indicator, to 
indicate the state of the absorbent, which in turn enables the activation 
to be carried out at appropriate times. This two-stage drying results in 
an up to 10-fold increase in effective life of drying material compared to 
a tube of similar dimensions without an upstream chiller. The two-stage 
drying process results in several weeks of IMS operation with a drift gas 
of essentially constant, low water content, before the drying material 
requires replacement. As detailed below, the increased stability permits 
halving of the width of the IMS detection time windows, and results in a 
five- to ten-fold improvement in the FAR. 
FIG. 3 depicts a typical plasmagram generated by an IMS as configured in 
FIG. 1. The vertical axis 45 is the amplitude of the detected ion current, 
and is proportional to the amount of material desorbed, ionized and 
collected. The vertical axis can be scaled in mA of ion current or more 
usually digital units of signal detected. The horizontal axis 46, in 
milliseconds, represents the drift times for the various ion analytes. It 
should be remembered that a drift time for a sought analyte depends not 
only on analyte ion factors such as molecular weight and shape but also 
upon various instrument parameters such as accelerating voltage across the 
drift tube, length of the drift tube, pressure within the drift tube, 
temperature, amongst others. FIG. 3 shows the results obtained from 1 
.mu.l of a 600 pg/.mu.l solution of cocaine in methanol, as indicated at 
the top of the Figure. The plasmagram presentation identifies the time 
location of sought analytes listed at the left of FIG. 3. For example peak 
15 is the calibrant, and peak 2 is the sought cocaine peak. The plasmagram 
additionally shows the settings of various instrument parameters such as 
timing, signal averaging arrangements, temperature settings (desorbing, 
inlet and drift tube), gas flows, and accelerating voltage 47. 
In FIG. 3, the window or time during which the sample is taken is indicated 
as "Wind" in the top right-hand corner. The drift time is given in 
millisec. and the reduced mobility is indicated as Red Mobil, in units of 
cm.sup.2 /volt.sec. 
Along the bottom, the Signal Range gives the vertical scale. 
The total number of windows making up the complete window is indicated as 
"Wnds". Each of these windows comprises a certain number of sweeps 
indicated as "Swps" and each spaced at an interval .delta.T. Each sweep in 
turn comprises a number of points indicated as "Pts", spaced by time 
interval indicated at .delta.T. Thus, in this case, the spacing of the 
points was 25 .mu.s. and there are 776 points in each sweep, for a total 
time of 20 ms. With 16 sweeps in each window and a .delta.t of 20 ms, this 
gives an individual window length of 0.32 seconds. With 14 windows, this 
gives a total window of 4.48 seconds. 
FIG. 4 depicts the method of selecting and discriminating sought analyte 
ion peaks. The detection window 48, the preset drift time interval in 
which the IMS detection system looks for the sought ion peak, is shown set 
over an analyte peak 49 which is the sought target. Two neighboring peaks, 
50 and 51, also appear as may be experienced when chemical interferences 
are present from perfumes, toilet articles, or the like. The detection 
requirement is to select the analyte peak 49 and reject the interfering 
peaks 50, 51. Evidently, the narrower the detection window that can be set 
over the analyte peak 49, the better is the discrimination achieved in 
that peak, by the rejection of nearby interfering peaks 50, 51. 
In an IMS employing single-stage drying, the detection windows are 
typically 200 to 250 .mu.s to ensure adequate capture of the analyte peak 
within a 10-20 ms mobility range, i.e. the window is approximately 1% of 
the mobility. Under the condition of two-stage drying, when very stable 
peaks are achieved, the detection windows can be reduced typically to 80 
to 120 .mu.s i.e. to approximately 0.5% of the mobility. Experiments show 
that halving of the detection windows reduces the FAR for explosives 
detection by a factor of 5- to 10-fold, this being a significant 
improvement over operations in a more conventional IMS with the wider 
detection windows. 
Various methods can be used in IMS instruments to achieve peak detection. 
They vary dependent upon the use of calibrants, detector electronics, and 
algorithms. By way of example, consider a three class or step peak 
detection system, comprising: 
a) After start-up, the detection algorithm first searches for the calibrant 
ion within a relatively wide preset window; the peak is "found" when 
positions in successive data cycles are within a preset "discriminant" 
value. 
b) Once found, the calibrant ion peak position is monitored, within a 
narrower window, and updated, by running average, throughout the entire 
operation, the calibrant monitoring being carried out before and between 
individual analyses. 
c) Target analyte ion peaks are detected at a defined drift time within a 
narrow window (peak position .+-.a preset variability); the analyte peak 
position occurs at a ratio to that of the calibrant ion, based on the 
ratio of their characteristic reduced ion mobilities, and this is used to 
set the narrow window for the analyte peak. 
If the windows are made too wide, to compensate for ion peak instability, 
data processing times are longer, incorrect peaks may be selected, and 
false positive results may be generated. If windows are made reasonably 
narrow, the detection algorithm may never find the calibrant peak, and may 
miss target analyte ion peaks, both due to ion peak instability, and this 
can generate false negative results. Ensuring drift air of low and 
essentially constant water content over an extended period by means of 
two-stage drying results in ion peak stability, thereby allowing narrow 
windows to be used for better selection and discrimination of peaks 
without incurring the risks described above. The significance of providing 
stable ion peaks by control of the air drying process can thus be more 
fully appreciated. Furthermore to one skilled in the art it will be 
appreciated that whatever detection scheme is applied, accurate detection 
is dependent on stable analyte peaks; if the detection system has to 
compensate for excessive peak instability, then accuracy deteriorates. 
FIG. 5 shows schematically collection of data and implementation of the 
detection process. Each scan of the IMS spectrum starts when the gating 
grid 8 opens, and ends just before the moment the gating grid opens again 
some time later, as shown by pulses 52. This interval between gating grid 
pulses, the scan interval 53, is operator adjustable. During each of these 
scans, the amplified signal from the drift tube collector electrode is 
digitized at a constant rate, and the resulting values are stored in a 
buffer in a RAM 54 of the microprocessor 14. Two of these buffers are 
used, with the data from each consecutive scan stored alternately between 
the two buffers; while data is being stored in one buffer, the data from 
the previous scan in the other buffer is processed. 
The data for several scans are added together before further processing to 
improve the signal to noise ratio and hence the sensitivity of the 
detector. To carry this out, the processor uses a second pair of summing 
buffers 55. Once a scan is complete, the processor takes the data for each 
time period from the beginning of the scan and adds it to the appropriate 
value in a summing buffer. This is carried out for a user-selectable 
number of scans which constitutes a complete data collection cycle for one 
sample. As shown in FIG. 3, for a scan interval (.delta.T) of 20 ms and an 
integration value (number of sweeps or scans) of 16, a complete sample 
analysis cycle takes 320 ms. 
After a complete data cycle, the processor 14 has a sum buffer filled with 
summed data from all the scans of that cycle. It now starts summing the 
subsequent scans from the next data collection cycle into the other 
summing buffer 55 and simultaneously starts processing the filled summed 
data buffer. The signal processing 56 in the microprocessor 14, consists 
of basically two steps carried out for the calibrant and each of the 
various target channels that are being monitored. In the first step, the 
data is treated by algorithms 57 of a finite positive fit function that 
indicate the presence of correctly shaped peaks near the expected 
positions. The expected positions are determined from electronic look-up 
tables, which give expected positions relative to calibrant peak 
positions. If a possible peak is found, the algorithm returns and inspects 
the raw spectral data to confirm and define the position of that peak. 
FIG. 6 shows the detailed application of a detection algorithm to the data 
of FIG. 5. The better the fit between the data and the expected peak shape 
and, to some extent, the larger the amplitude of the peak and the larger 
the background amplitude, the large the value of the shape/position 
function. Fit threshold values 58 in calibrant and channel control menus, 
see Table 1, are set to minimum acceptable values by means of which the 
algorithm decides if the peak is acceptable. The positive fit function has 
a finite positive value as high as 3.0, even with a flat background; hence 
there is a minimum acceptable threshold value below which the algorithm 
will produce meaningless results since it will detect peaks in a flat 
background. If the threshold is set too high, the algorithm will not 
identify any peak, no matter how good the fit. A value of 7 to 10 gives a 
strong spectral peak free of interferences and with the FWHM (full width 
at half maximum) value within 10% of the actual peak width. 
If a possible peak is found from the above positive fit function, it is 
checked at 59 to see if it is acceptable as to peak shape and position. If 
so, the algorithm returns to the raw spectral data 60, and inspects the 
data at the indicated position for the presence of a local maximum 61. If 
one is identified close enough to the position indicated by the above 
function, then a polynomial fit 62 is carried out to calculate the local 
maximum 63 and the position of the center of the peak and its amplitude 
64. If the amplitude is above a minimum, i.e. above a preset intensity 
threshold 65, and the peak is close enough to the expected position 66, 
which presets the allowed variability, then the peak is marked as "found" 
for subsequent processing--be it averaging for the calibrant, or 
activating an alarm 67 for the target compounds. 
As noted, the time available to the detector to complete the data 
processing before the next data cycle starts is typically 320 ms. The 
amount of processing, and therefore the time required for processing, 
increases with the number of target channels activated, the width of the 
window or variability associated with each channel including the calibrant 
(since a wider window requires more time to search), and the FWHM (since a 
wider peak requires a wider window). A lower threshold value also 
increases data processing time since this generally means that more 
possible peaks need to be investigated. If data processing requires longer 
than the available time, data will be missed in the subsequent cycle, 
leading to less accurate or less timely results. Choosing instrument and 
analysis parameters that ensure fast processing will not only obviate 
these risks, but will also enable more target ion peaks to be covered in 
each scan. Additional channels could include the detection of a greater 
number of analytes thus increasing the scope of the analysis, or the 
detection of multiple peaks for the same analyte thus enhancing result 
validation. The major potential for faster processing is by reducing peak 
variability, for which greater ion stability, achieved through control of 
drift gas water content, is required. 
It will now be seen that all instrumental measures should be taken to 
minimize variability in IMS peak positions, for both calibrant and 
analytes, so that the measurement window for each peak is as narrow as 
possible. Some variability in peak position is inevitable because 
ionization chemistry processes within the reactant region of the IMS 
produce effects that are not entirely controllable. However, one 
significant and controllable contributor to variability in peak drift 
times is the water vapor content of the drift air. With a single stage 
drying process, the drying material will soon deteriorate, causing the 
water content in the drift gas to gradually increase and peak positions to 
change with time. A low and essentially constant water content over an 
extended period of time is achieved by means of a two-stage drying process 
involving TE predrying by condensation at 2.degree. to 6.degree. C., 
followed by further reduction of water content to a low controlled level 
with a large capacity drying tube containing calcium sulphate or 
equivalent material, thereby achieving a significant improvement in peak 
stability, which allows the search algorithms to be set to narrower widths 
than would be otherwise achievable.