Laser-induced-fluorescence inspection of jet fuels

An optical inspection system for using laser-induced luminescence to detect the quality of materials, such as fuel. The inspection system comprises an excitation means for illuminating a specimen to cause it to produce fluorescent radiation. The spectral representation of the fluorescence produced by the specimen is compared to a reference spectrum to obtain an indication of the physical characteristics of the specimen.

FIELD OF THE INVENTION 
The present invention relates generally to the field of optical inspection 
of materials. More specifically, the present invention provides a method 
and apparatus for utilizing laser-induced-fluorescence techniques to 
determine the quality of jet fuels. The laser-induced-fluorescence 
techniques of the present invention can be used to detect the existence of 
gums and other deposits in quantity of jet fuel. 
BACKGROUND 
It is well known that fuel impurities can adversely affect the performance 
of jet engines. The mechanism of the formation of deposits from thermally 
stressed jet fuels is a complex process involving several consecutive 
reactions steps. In the most general case, deposits form as the result of 
a fuel oxidation, of which the first step in the mechanism is the 
formation of peroxides. The thermal decomposition of peroxides and the 
ensuing chain propagation reactions lead to the formation of several 
oxygenated compounds and free radical species. The free radicals react 
with dissolved oxygen in the fuel to form relatively stable alkylperoxyl 
radicals; because these less reactive radicals tend to build up in the 
fuel, they have a greater probability of recombining with each other to 
form species that have slightly more than twice the molecular weight of 
the average fuel molecule. These higher molecular weight species, commonly 
known as gums, which are relatively insoluble in the fuel, contain high 
concentrations of oxygen and other heteroatoms such as sulfur and nitrogen 
that may be present in the fuel. 
It is generally believed that the insolubles/gums are the precursors to 
deposit formation. Since deposit formation depends on parameters other 
than chemistry such as the flow conditions, mass transport and surface 
activity, thermal stability of a fuel seems to be best indicated by the 
fuel's tendency to form gums. A temperature dependent global rate constant 
for the formation of gums could then become a defined fuel property for 
thermal stability and could be used for predicting the potential deposit 
formation in aircraft fuel systems at specified temperature and flow 
conditions. 
To determine the global rate constant for the formation of insolubles in 
fuels, the rate of gum formation needs to be measured under controlled 
conditions, specifically, constant temperature. However, the actual 
measurement of the rate of formation of gums hinges upon the current 
capability to measure gum concentration in jet fuels. Currently, the ASTM 
D381 method using the stream jet evaporation technique is the only 
established method of measuring the concentration of gums in jet fuels. 
This analysis method involves a large sample size, lengthy analysis, and 
yields poor accuracy. Furthermore, its intrusiveness tend to discourage 
the use of the D381 method in an examination of the kinetics of gum 
formation in jet fuels. 
The deposits that form in aircraft fuel systems include soft gums, strongly 
adhering lacquers, and varnishes. Sometimes there are very hard, brittle 
substances resembling coke. When deposits are formed by autooxidation of 
jet fuel, they have oxygen concentrations much greater than deposits from 
the thermally unstressed fuel, and their hydrogen-to-carbon ratio is lower 
than that of the original fuel. If the unstressed fuel contains hetroatoms 
such as sulfur and nitrogen, these relatively polar species are 
concentrated in the deposit. 
There is very little quantitative data on the composition of fuel-system 
deposits formed from jet fuels. Furthermore, there is little data 
available on the compositions of gums formed by the autooxidation of 
fuels. The data on gums are important because they are precursors of 
deposit formation and thus provide an indication of deposit composition. 
Data pertaining to the compositions of gums formed by the autooxidation of 
heating oil and gasoline show that the polar components, including the 
hetroatoms oxygen, sulfur, and nitrogen, are highly concentrated in the 
gum, although they are usually present in only minute amounts (&lt;&lt;1%) in 
the original fuel. 
In view of the difficulties in measuring gums in jet fuels, spectroscopic 
methods of analysis have been explored. In the examination of fuels that 
contain gums, it has been found that the gums seem to always cause the 
fuel to have a slight brandy color that is characteristic of light 
absorption in the blue region of the visible spectrum. Other similar 
observations have been reported, such as the work by Bhan et al. reported 
in "Color Change/Sediment Formation in Marine Diesel Fuels, Task II," 
NIPER-B06710-2 (1986), which suggested a correlation between color change 
and sediment formation in marine diesel fuels. 
The fact that pristine hydrocarbon fuels do not absorb light in the visible 
region of the spectrum, but gums have a finite absorption there, suggests 
that a spectroscopic method of measuring gums is possible. The first 
method that comes to mind is simply light absorption in the blue region 
(ca. 450 nm) of the spectrum as mentioned above. Although absorption would 
appear to work in principle, it does not appear to be strong enough to 
indicate that a highly sensitive method of measuring gums could be 
developed. A more sensitive method requires a laser diagnostic technique, 
such as that discussed hereinbelow. 
SUMMARY OF THE INVENTION 
The present invention provides a method of inspection and quality detection 
which can be used for analyzing fuel, particularly jet fuel having gums 
with a molecular composition which provides fluorescence characteristics. 
The fluorescent radiation from a desired quantity of fuel has a specific 
characteristic spectrum which can be compared to and differentiated from 
the spectrum radiated from an undesired quantity of fuel. Through the use 
of laser-induced luminescence, or more particularly laser-induced 
fluorscence, it is possible to detect minor differences in the 
characteristics of the fuel which might not be detected using standard 
detection techniques. 
The invention inspection and sorting system comprises an excitation source 
which illuminates the fuel to be examined in order to cause that fuel to 
produce fluorescent radiation. In a preferred embodiment of the invention, 
the excitation source is a laser. A light detection means is operable to 
detect the fluoresent light produced by the fuel under examination and is 
operable to produce a spectral representation of the fluorescent light 
produced by the fuel. This spectral representation is processed in a 
processing means capable of differentiating between the spectrum (or 
portion of the spectrum) of a desired fuel and that of an undesired fuel. 
The invention method for inspecting fuel comprises the steps of 
illuminating the fuel with light from an excitation source, thereby 
causing the fuel to fluoresce; detecting the fluorescent light reradiated 
by the fuel and producing an output signal in response thereto; processing 
the output signal to obtain a spectral representation of the reradiated 
light; and comparing the spectrum, or portion thereof, of the fuel under 
examination to the corresponding spectrum of a desired fuel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a schematic representation of the preferred embodiment of the 
invention system for optical inspection of a fuel product. Light from an 
excitation source 10 is passed through a filter 12 and reflected by 
mirrors 14 and 16 to illuminate a fuel sample 18 to be tested for 
fluorescence. In the preferred embodiment of the invention the excitation 
source 10 is a laser having a suitable wavelength to cause fluorescence. 
All subsequent discussion of the excitation source 10 will refer generally 
to a laser. 
The fluorescence characteristics of the test sample 18 are determined by 
analysis of the light reradiated by the sample. The reradiated light 
passes through lens 20, filter 22 and is used as input for the 
spectrometer 24. The filter 22 can be selected to minimize the scattered 
light from the laser 10. The spectrometer 24 disperses the light which is 
then detected by the diode array detector 26 and amplified to provide 
input for a suitable display device 28, such as the minicomputer display 
28 shown in FIG. 1. 
In order to understand the principles of operation of the present 
invention, it is important to understand the meaning of luminescence, as 
well as the historical evolution of the definition of luminescence. 
Historically, materials were said to exhibit characteristics of 
"luminescence" if they emitted photons after being irradiated with light 
having a wavelength in the range of approximately 1800 to 3700 Angstroms 
(ultraviolet). Prior art definitions of this phenomenon have included two 
categories: fluorescence and phosphorescence. A material was said to 
exhibit fluorescence if the luminescence ceased after termination of the 
irradiation. However, if the luminescence persisted after irradiation, the 
phenomena was termed phosphorescence. 
The above-mentioned definitions evolved at a time when observations of the 
persistence of luminescence were made with the unaided eye. The 
development of sophisticated instruments capable of measuring the 
persistence of luminescence for very short time periods, e.g., 
nanoseconds, has led to a more precise definition of the above-mentioned 
terms and has changed the definition of luminescence for some materials. 
For example, it is now known that many materials which have been 
characterized in the literature as being fluorescent emit luminescence for 
as long as 1000 microseconds after termination of excitation. This 
luminescence offers significant information regarding the physical 
characteristics of the illuminated material and in the present invention 
can be used to distinguish between desired fuel and fuel containing 
contaminants, as will be discussed in greater detail below. 
It is well known that certain materials luminesce in the presence of 
ultraviolet or blue light and that the variation of the visible light 
luminescence can be used to determine certain features of the material. An 
example of an apparatus for using these phenomena to detect the presence 
of caries in human teeth is shown in U.S. Pat. Nos. 4,290,433 and 
4,479,499 issued to Alfano. The luminescence in human teeth which is 
essential to the methods shown in these patents is dependent on the 
recognition of total visible luminescence. Further, the detection of the 
caries as shown therein relies on a visual recognition of differences in 
the color of the reradiated light from the teeth. While this luminescence 
technique is useful for detecting certain types of characteristics of 
materials, it is not suitable for an application such as that shown in the 
present invention because the technique is dependent on visual recognition 
of color differences in the luminescence of the material. 
Quantitative measurements show that there is sometimes a very strong 
correlation between the laser-induced luminescence and physical 
characteristics of the material. Many times this is due to the fact that 
fluorescence reveals relationships between molecular functional groups, 
for instance conjugation. (This is in contrast to infrared absorption 
techniques, which are mainly used to reveal the presence of individual 
molecular functional groups.) Due to alteration of the relationships 
between certain molecular functional groups during degradation, 
laser-induced luminescence can be used to monitor degradation processes. 
With the monochromaticity and power density available with lasers, 
transitions can be probed in molecules that are not normally thought of as 
fluorescent. For instance, chromophores that exhibit ultraviolet 
absorption can sometimes be induced to fluorescence with laser excitation 
in the visible region of the spectrum; representative examples include 
esters, ethers, and amines. This effect can be very important from the 
viewpoint of practical implementation. 
One would not normally think of a fuel gum as a fluorescent material 
because, under standard room light conditions, the dominant process is 
simple light scattering and absorption. The wavelength dependence of these 
processes gives the fuel its characteristic color. Each photon of light is 
either absorbed or scattered by the fuel, but the wavelength remains 
essentially the same. Since room light contains all visible wavelengths, 
any fluorescence effects are completely masked to the unaided eye. The 
desired fluorescent effects can be observed, however, by illuminating the 
fuel with laser light at an appropriate wavelength, e.g., 488 nm, and 
looking at it through a filter that only passes longer wavelengths. 
For a given excitation spectrum, samples of a material can have different 
fluorescence or phosphorescence spectra, even though they appear visually 
similar. The method and apparatus of the present invention differs from 
standard ultraviolet fluorescence techniques in that it takes advantage of 
the complicated excitation-luminescence spectra of the gums. The present 
invention is based on the discovery that fuel and gums have distinctive 
characteristic responses to radiation at certain frequencies. In 
particular, these characteristic responses can be used to differentiate 
between various grades of fuel and can also be used to differentiate 
between desired fuel and fuel which is contaminated with gums. 
The invention method overcomes the shortcomings of previous optical 
inspection systems because it takes advantage of complex 
excitation-luminescence spectra of jet fuels. Thus, two quantities of jet 
fuel which both absorb approximately the same spectrum can have different 
fluorescence characteristics which can be distinguished to differentiate 
between desired fuel and fuel containing contaminants. 
In the preferred embodiment of the present invention, the excitation source 
10 and detector 24 may implemented using a Turner model 430 
spectrofluorometer with a xenon lamp light source. Alternatively, the 
laser 10 may be implemented using Argon Ion laser source at 488 nm. The 
filter 22 may be implemented using a Schott-glass OG530 blocking filter, 
and in the preferred embodiment, scattered laser light, except for that at 
488 nm, is removed by the filter. The system of the present invention 
could be implemented using fiber optics, known in the art. In addition the 
spectrometer used for the detector 24 could be replaced with a simple 
filter and photomultiplier tube system with appropriate supporting 
electronics. 
Experimental results from embodiments using a Turner model 430 
spectrofluorometer showed that there was a distinct, but very weak, 
fluorescence at about 550 nm from a solution of acetonitrile containing 
about 80 mg/dL of gum. The fluorescence was most intense when the incident 
xenon lamp light source was blue (ca. 450 nm). However, when an Argon Ion 
laser source at 488 nm was used as the incident light source 10, the 
fluorescence was considerably stronger since the laser light source was 
several orders of magnitude more intense at 488 nm than the xenon lamp 
light source. 
A typical output displayed using the invention system is shown in FIG. 2. 
The upper curve 30 represents the spectrum observed for gum in the jet 
fuel, while the lower curve 32 represents the spectrum observed for a 
neat, or pure, fuel sample. Alternatively, the signals produced by the 
system could be modified such that the upper curve could represent the 
desired material, while the lower curve could represent the fuel to be 
discarded. The main requirement is that there must be difference between 
the observed spectra of the materials. 
In some cases, additives may be used to enhance the effect of the 
laser-induced-fluorescence. Additionally, treatment of some fuels may be 
necessary to give very precise results, such as degassing of excess free 
oxygen (in some fuels). 
While the method and apparatus of the present invention have been described 
in connection with the preferred embodiment, it is not intended to limit 
the invention to the specific form set forth herein, but on the contrary, 
it is intended to cover such alternatives, modifications, and equivalents 
as may be included within the spirit and scope of the invention as defined 
by the appended claims.