Trace Sensors

Bulk sensors certainly have a role in chemical sensing of explosives, but the subject of this book is the other basic type sensor, one that seeks molecules released from the bulk of the explosive material in an object. We will refer to these as trace chemical sensors. They are sometimes called vapor sensors, but that seems a less accurate description when they are applied to explosive molecules, which may not always be found in a vapor state. As we shall see in Chapter 5, that requires us to understand where and how to look for these molecules. It will become apparent upon a little reflection that the two types of sensors are complementary and are best used in different situations. Furthermore, even when trace sensors are used, in some situations sampling of particles of soil or vegetation or sampling from surfaces may prove to be more productive that vapor sampling. For underwater sources the term vapor sensing is also inappropriate. 

In fact, with the very recent addition of differential reflection spectroscopy (DRS) to the suite of applicable technologies, as described in Chapter 15, we now have the possibility of sensing trace quantities of explosives where they are most often found in the environment, adsorbed to solid surfaces. Technologies that can, like DRS, locate these traces in situ offer a very different way to approach the problem. There have been several recent attempts to do this in situ detection from some distance away. To date the DRS seems the most successful. It has demonstrated detection at a range of a few meters. 

The marked advantage of an in situ detection system is that it does not ingest the molecules hence it does not disturb the area, does not require as close approach to an explosive device, and it should produce a faster response time than ingesting systems. The disadvantage of in situ systems is also that they do not ingest the molecules hence they have no means of concentrating the sample 

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Mercury layers plated onto the surface of analytical electrodes serve as Hquid metal coatings. These function as analytical sensors (qv) because sodium and other metals can be electroplated into the amalgam, then deplated and measured (see Electro analytical techniques). This is one of the few ways that sodium, potassium, calcium, and other active metals can be electroplated from aqueous solution. In one modification of this technique, a Hquid sample can be purified of trace metals by extended electrolysis in the presence of a mercury coating (35). 

Derivation of simple and unambiguous quantitative relations between the signal amplitude of a sensor, i.e., the value of the change of electric conductivity, work function, etc. and concentration of detected traces of admixture in the medium under study is also important for successful development of the sensor measuring technique. Theoretical considerations given in this book show that such relations exist in most simple form. The purpose of experiment consists in statistical substantiation that these dependencies rigorously hold at proper conditions. 

Fig.4.8. Oscilloscope traces of variation of the electric conductivity of a ZnO sensor upon admission of isopropyl alcohol vapor to the vessel (the initial vapor pressure is 0.01 Torr) at the temperature of 390 C (/), 370 C (2), 350 C (5), 320 C (4), and upon admission of H2 at the temperature of 390 C (5).

Fig.4.9. Oscilloscope traces of temporal variation of the electric conductivity of a ZnO sensor for different initial pressures of the isopropyl alcohol vapor 5-2-t0-2 Torr (0, 3-6.10-2 (2), 1.65-10-1 0 and 3.25 10- Torr. The temperature of the ZnO film is 390 C.

Atoms of metals are more interesting tiian hydrogen atoms, because they can form not only dimers Ag2, but also particles with larger number of atoms. What are the electric properties of these particles on surfaces of solids The answer to this question can be most easily obtained by using a semiconductor sensor which plays simultaneously the role of a sorbent target and is used as a detector of silver adatoms. The initial concentration of silver adatoms must be sufficiently small, so that growth of multiatomic aggregates of silver particles (clusters) could be traced by variation of an electric conductivity in time (after atomic beam was terminated), provided the assumption of small electric activity of clusters on a semiconductor surface [42] compared to that of atomic particles is true. 

The overall objective of the system is to map from three types of numeric input process data into, generally, one to three root causes out of the possible 300. The data available include numeric information from sensors, product-specific numeric information such as molecular weight and area under peak from gel permeation chromatography (GPC) analysis of the product, and additional information from the GPC in the form of variances in expected shapes of traces. The plant also uses univariate statistical methods for data analysis of numeric product information. 

In addition, the GPC trace, an example of which is shown in Fig. 42, reflects the composition signature of a given product and reflects the spectrum of molecular chains that are present. Analysis of the area, height, and location of each peak provides valuable quantitative information that is used as input to a CUSUM analysis. Numeric input data from the GPC is mapped into high, normal, and low, based on variance from established normal operating experience. Both the sensor and GPC interpretations are accomplished by individual numeric-symbolic interpreters using limit checking for each individual measurement. 

The quartz crystal microbalance has a long history of application as a means of determining film thickness in vacuum deposition techniques and more recently as a means of detecting trace constituents in the gas phase. In essence, it is an extremely sensitive sensor capable of measuring mass changes in the nanogram range. 

Most probably, the first - but non-fiberoptic - sensors for continuous use where those for pH and for oxygen. It has been known for decades that cellulosic paper can be soaked with pH indicator dyes to give pH indicator strips which, however, leached and thus were of the "single-use" type. The respective research and development is not easily traced back since it is not well documented in the public literature. However, in the 1970s, indicator strips became available where they pH indicator dye was covalently linked to the cellulose matrix, usually via vinylsulfonyl groups. These "nonbleeding" test strips allowed a distinctly improved and continuous pH measurement, initially by visual inspection. In the late 1980 s instruments were made available that enabled the color (more precisely the reflectance) of such sensor strips to be quantified and related to pH. Respective instruments are based on the use of LEDs and are small enough to be useful for field tests in that they can be even hand-held. This simple and low cost detection system is still superior to many of the complicated, if not expensive optical pH sensors that have been described in the past 20 years. 

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