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CHEMICAL-INTEGRITY TESTING WITH A MASS SELECTIVE DETECTOR
Using the "Electronic Nose" to Sniff Out
Food Spoilage
By Miriam Adechy, Vince Shiers, and Adrian Squibb, Leatherhead Food Research Association, Consultancy and Research Group,
Leatherhead, Surrey (UK)
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The Leatherhead Consulting and Research Group has been evaluating electronic-nose instruments for the past six years and
now has a worldwide reputation as an independent expert in electronic nose evaluation and applications. Along with a
number of solid-state sensor arrays assessed initially,1, 2 the Agilent 4440A mass-spectrometry-based electronic nose
has been evaluated at Leatherhead for over a year.3
This instrument, designed primarily as a rapid-screening tool, was successfully used for a number of applications in the
food, pharmaceutical, and chemical industries. Food applications included spoilage detection in meat4 and fish, trace
contaminants in soft drinks (i.e., benzene5), taint in packaging,6 rancidity and authenticity study of edible oils and
fats, and other applications carried out for the quality control of flavor or product blends.
The Technology: Headspace Profiling
The Agilent 4440A is a quadrupole mass spectrometer designed for headspace profiling with the possibility of specific
key components identification. It operates by detecting fragment ions in the range 25 to over 150 m/z from volatiles
present in the headspace. The data are processed using a variety of multivariate statistical methods.
This instrument, which is interfaced to the Agilent 7694 headspace sampler using 10- or 20-mL-capacity vials, offers a
number of advantages over such conventional analytical techniques as GC/MS. It is designed for rapid headspace profiling
and screening. Easy to use, it requires neither expertise in mass spectrometry nor complex sample preparation. It can
elucidate the vast array of fragment ions present in the headspace and identify compounds in simple matrices. Although
it cannot absolutely confirm compound identification as GC/MS does, it is more sensitive and more stable than solid-state
sensor array instruments. The data analysis software package allows easy data processing and
interpretation.
In this article we present two applications from our recent experience.
Study 1: Trace Benzene Analysis in Soft Drinks
In 1998, there was an alert in the UK concerning a possible contamination of soft drinks with benzene, caused by
contaminated carbon dioxide. Because established methods of trace benzene analysis can be time-consuming and laborious,
a rapid method for soft drinks screening was needed. This work was an initial feasibility study to show how the Agilent
4440A sensor could be applied to a relevant industrial issue.
Aqueous solutions of benzene at levels of 8, 22, and 44 ppb were prepared and 5 mL of each placed into 10-mL-capacity
vials, then incubated at 80ºC for 5 minutes to allow headspace equilibration. The instrument was initially set up to
run samples in the scan mode from 50 to 150 m/z. In order to improve sensitivity, selective ion monitoring (SIM) mode
was used, focusing only on the major fragments of benzene (77 and 78 m/z).
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Figure 1. Agilent 4440A chemical sensor in SIM mode. The principal-components analysis is a 3D graphical
representation of the volatile fingerprints of each sample class. A line plot of mass abundance shows replicates
of m/z for benzene; 8.8 ppb (lowest lines), 22 ppb (middle lines), and 44 ppb (highest lines).
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Figure 2. Agilent 4440A; benzene calibration plot.
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Benzene Detection at 5 ppb
Figure 1 illustrates the mass abundance of the main fragments (ranging from 76 to 78 m/z) for samples having a benzene
concentration of 8, 22, and 44 ppb. Five replicates were analyzed, and the figure shows that the response increases with
increased concentration. Figure 2 shows the benzene calibration plot used to determine the limit of detection of benzene
for the most significant fragment at 78 m/z. Results demonstrate that benzene could be detected at a level of 5 ppb and
accurately quantitated at a level of 16 ppb.
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Study 2: Spoilage Detection in Meat
There is no quick, reliable method of determining the freshness or rancidity of minced beef in a meat factory. Currently,
time-consuming methods are used, such as microbiological plate counts after incubation, extraction of the fat and
subsequent chemical measurement, and sensory tests, particularly concentrating on appearance and odor. Buyers and
sellers of minced beef, and meat in general, therefore require a rapid technique that could assess the age, freshness
and organoleptic appeal of meat so that its quality and likely shelf-life can be assessed.
Leatherhead investigated the possibility of detecting spoilage in meat using the Agilent 4440A chemical sensor.
Minced beef samples were packed individually under modified-atmosphere packaging (20% CO2, 80% O2) in sufficient numbers
to complete the length of storage. Samples were stored at 4ºC for 17 days and exposed to light for 12 hours per day
to simulate retail conditions. The samples were analyzed at selected time points for microbiological, sensory (color and
off-odor) and mass-spectrometry headspace profiling. For the MS, one gram of sample in a 10-mL-capacity vial was
incubated at 50ºC for 5 minutes. Samples were run in scan mode, from 25 to 150 m/z. It is essential to run standard
materials in order to monitor the stability of the instrument. Hexanal (1 ppm in water) and fresh samples were analyzed
at each time point.
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Figure 3. Minced-meat storage trial. Agilent 4440A in scan mode (25-150 m/z), excluding fragments 28, 32, 40, and 44;
PCA of modified-atmosphere-packed minced meat analyzed on Days 0, 4, 8, 11, 14, and 17 (averaged data).
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Figure 4. Minced-meat storage trial. Agilent 4440A in scan mode (25-150 m/z), excluding
fragments 28, 32, 40, and 44; PLS of modified-atmosphere-packed minced meat on Days 0, 4, 8, 11, 14, and 17,
without data processing.
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Predicting Freshness
Figure 3 is a principal component analysis of minced beef stored for different periods of time. Aging meat samples
were easily discriminated under the test conditions. A PLS (partial least-square) presentation in Figure 4
illustrates the possibility to predict the freshness of the meat.
Processing the Data
A number of options are available with the Pirouette® (Infometrix, Inc.) data analysis software package. To
optimize the data processing, it is important to use the appropriate tool. For example, one option permits finding
the most discriminant fragments by using soft independent modeling of class analogy (SIMCA). Other analytical
tools allow unknown-sample identification and prediction (e.g. PLS). A common method of presenting multivariate
data, principal component analysis (PCA), is also available. Many other advanced options available require
appropriate training.
Fast Analysis at ppb Level
The Agilent 4440A chemical sensor offers a new approach to fast and flexible headspace analysis or classification.
Many variables can be optimized for a wide range of applications. The sensor is easy to use and may allow the
rapid analysis for specific compounds down to ppb levels.
Leatherhead is now investigating other applications and methods to reach lower limits of detection, concentrating
on quality control issues that would benefit from a rapid screening technique. Current work includes the analysis
of trichloroanisole (TCA) in cork, and early results suggest that concentrations in the ppt range are attainable.
If confirmed, this capability could be of major benefit to cork suppliers, wine producers, and retailers as part
of their quality control procedure.
The Agilent 4440A chemical sensor is currently available in North America, Western Europe, and Australia.
Click for additional information.
Or telephone or e-mail: Yann Filaudeau, +33 1 69 82 63 48 / yann_filaudeau@agilent.com (Western Europe, except Germany); Gerhard Tressl, +49 7243 602 596
/ gerhard_tressl@agilent.com (Germany); Mona Burke +1 (302) 633-8133 / mona_burke@agilent.com (North America);
Ron Minett +61 3 9210 546 / ron_minett@agilent.com (Australia).
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Electronic Nose Technology: The Sixth Sense
The Agilent 4440A chemical sensor is a screening tool designed for both R&D and QA/QC laboratories. It combines
the technologies of headspace, mass spectrometry and pattern recognition and "digitizes" the volatile
fingerprint of samples to assess their chemical integrity and/or conformity in typically less than 4 minutes.
Because it is fast (no GC separation) and easy to operate, the Agilent 4440A chemical sensor helps laboratories
break the analytical bottleneck and reduce delays in making decisions. Many more analyses can be performed without
the extra workload required by conventional methodologies, reducing the risk of missing critical information. But
traditional quantitation/identification techniques, like GC/MS, can be used for further investigation when
out-of-spec samples are detected.
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References
1. Adechy, M., Shiers, V., and Rossell, J. B. Study of rancidity and resistance to oxidation in edible oils and
fats using electronic nose technology in comparison with conventional analytical techniques. Leatherhead Food
RA Research Report No. 751 (1998).
2. Shiers, V. and Adechy, M. Use of Multi-Sensor Array Devices to Attempt to Predict Shelf-Lives of Edible Oils.
Seminars in Food Analysis, Vol. 3 (1), ISSN (1998).
3. Shiers, V., Adechy, M., and Squibb, A. A new mass spectrometry-based electronic nose for headspace
characterisation.
5th Symposium on Olfaction and Electronic Nose, Hunt Valley, Baltimore, USA, September 1998.
4. Shiers, V. and Squibb, A.D. Evaluation of the Hewlett-Packard chemical sensor HP 4440A - Minced beef storage
trial.
Leatherhead Food RA Technical Notes No. 130 (1999).
5. Offen, C.P., Shiers, V., and Squibb, A.D. Rapid methods for the determination of benzene in carbonated drinks.
Leatherhead Food RA Technical Notes No. 127 (1998).
6. Shiers, V. and Squibb, A.D. Rapid methods for the determination of benzene in carbonated drinks. Leatherhead
Food RA Technical Notes No. 131 (1999).
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