Trace analyses

As a general guide items for trace analysis should be enclosed in three layers of packaging these are removed in sequence as the item is moved from the uncontrolled general environment into the controlled trace laboratory environment. This approach is intended to avoid the possibility of any contamination on the outside of the packaging being transferred into the trace laboratory. 

A good general purpose screening technique for organic explosive traces, albeit often undervalued, is thin-layer chromatography (TLC). The advantages of TLC are that it requires only Hmited capital equipment, litde sample preparation other than dissolution in a suitable solvent, and that it provides rapid results that are easily interpreted and explained [16]. 

Conversely, it lacks the selectivity and sensitivity of the best instrumental methods of analysis results from TLC analysis should always be supported with confirmation by other analytical techniques or appropriate supplementary evidence. 

Colleagues at the Forensic Explosives Laboratory have found the following procedure suitable pre-activated sfrica gel coated plates containing an ultraviolet fluorescent indicator are used. A mixed standard solution is run in one of the channels on the plate, alongside solutions of the samples and blanks. Table 1 shows the recommended eluent systems. 

Normally only mixed explosive standards should be taken into, or used in, a trace laboratory this has the advantage that in the unlikely event of an error the presence of multiple species matching the mixed standard will clearly show what has happened. If circumstances dictate the use of a single standard, it is important to document any extra precautions taken to prevent confusion between samples and standards. It may also be wise to review the cleaning and quality assurance 

First let us distinguish between trace analysis and microsampling. Trace analysis by FT-IR spectrometry is the measurement of the spectmm of a minor component of a sample. Microsampling is the measurement of the spectmm of a very small amount of a pure material or major component of a mixture. It is not too difficult to obtain the spectmm of 1 ng of a pure sohd or hquid, but it can be exceedingly difficult to obtain the spectmm of the same amount of the same material when it is present at a level of 1 ppm in 1 mg of a strongly absorbing matrix. The measurement is almost impossible if the most characteristic bands in the spectmm of the matrix and the analyte absorb at similar wavelengths. 

Let us recall the mle of thumb given at the start of this chapter (i.e., a strong band in a 10-pm film of a pure material has a typical absorbance of about 1 AU). Furthermore, we will assume that the strongest band in the spectmm of the analyte must have an absorbance of at least 0.001 AU (1 mAU) for the analyte to be identified above the noise level, interference by atmospheric water vapor, and small variations of the baseline. These constraints imply that the thickness of a sample containing 1 ppm of the analyte must be 1 cm if a trace component is to yield a recognizable spectmm. However, the transmittance of most samples that are 1 cm thick is well below 0.1% in those spectral regions where chemically useful information is to be found (3100 to 2700 and 1800 to 400 cm ). Thus, windows in the spectmm of the matrix material must be found where at least one characteristic band of analyte absorbs. An example of just such a case is found in the determination of antioxidants in polyolefins. Many polyolefins have weak absorption between 

3300 and 4000 cm, making it possible for the O H stretching bands of hindered phenolic antioxidants present at the ppm level to be observed, even though the rest of their spectrum is masked completely. 

These considerations should be bome in mind if the spectrum of a trace solute in a liquid solvent is to be obtained. In this case, the stracture and polarity of the solvent must be considered. Most classical texts on infrared spectroscopy recommend that carbon tetrachloride or carbon disulfide be used as the solvent if the spectmm of a species in solution is to be measured. CCLi and CS2 are both small, nonpolar molecules that have very simple infrared spectra containing wide window regions, and the transmittance of much of the spectrum of a 1-mm-thick film of both molecules is greater than 10%. 

on the other hand, n-hexane is to be used as a solvent, the cell thickness must be reduced to about 100 pm. Even though CCLi, CS2, and n-hexane are all nonpolar, the increased structural complexity of n-hexane and the fact that it contains lighter atoms than CCLt and CS2 mean that its fundamental absorption bands cover a greater region of the mid-infrared spectrum than those of CCLt or CS2. Thus, the window regions, where useful information on the solute can be found, are much shorter. 

If the analytical task is the qualitative or quantitative determination of an analyte at low concentration it is first necessary to optimize the chromatographic system. Under isocratic conditions the concentration at the peak maximum Cn,ax is lower than in the injection solution c,  

10 Ul of a solution with 1 ppm (1g ml ) of the analyte of interest are injected. The retention value of its peak is 14 ml. With this peak a plate number of 10000 can be calculated. What is the peak maximum concentration  

For trace analysis it is necessary to obtain as high a signal, i.e. a maximum concentration, as possible. How can this be done  

It follows that the lowest dilution or the highest peaks are obtained if a short and, most importantly, thin column with as high a theoretical plate number as possible, is used. The origin of the separation efficiency of the column must come from its fine, 

therefore c max depends on 1/y/L. With a given sample concentration Cj, the peak 

The equation given above can also the written in a different manner (because Vr = UdlTTe k + l)/4 and V = U/H)  

Although a short column is to be preferred, its separation performance must be high enough to solve the separation problem. The method needs to be optimized with regard to the trace component. 

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Siebert D R, West G A and Barrett J J 1980 Gaseous trace analysis using pulsed photoacoustic Raman spectroscopy Appl. Opt. 19 53-60... 

Heydorn, K. Detecting Errors in Micro and Trace Analysis by Using Statistics, Anal. Chim. Acta 1993, 283, 494M99. 

Clement, R. E. Environmental Sampling for Trace Analysis, Anal. Chem. 1992, 64, 1076A-1081A. 

Ghristian, G. D. Gallis, J. B. eds. Trace Analysis and Spectroscopic Methods for Molecules. Wiley-lnterscience New York, 1986. 

Raman measurements [INFRARED TECHNOLOGY AND RAMAN SPECTROSCOPY - RAMAN SPECTROSCOPY] (Vol 14) -trace analysis of [IHACE AND RESIDUE ANALYSIS] (Vol 24)... 

Although the book on reagent chemicals contains many tests for the determination of trace impurities in reagents, it is not intended to be a text on the techniques of trace analysis but rather to provide tests that are reproducible in various laboratories, and which are accurate, economic, and feasible (see... 

There are many colorimetric methods used for trace analysis of peroxides using reagents such as ferrous ion, leuco base of methylene blue, yy -diphenylcarbohydrazide, titanium(IV), iodide ion, and Ai,A7-dimethyl- -phenylenediamine. The latter two are the most commonly used reagents... 

The extreme sensitivity and high resolving power of trace analysis techniques encourage the quest for single atom detection. 

A number of techniques have been developed for the trace analysis of siUcones in environmental samples. In these analyses, care must be taken to avoid contamination of the samples because of the ubiquitous presence of siUcones, particularly in a laboratory environment. Depending on the method of detection, interference from inorganic siUcate can also be problematic, hence nonsiUca-based vessels are often used in these deterrninations. SiUcones have been extracted from environmental samples with solvents such as hexane, diethyl ether, methyl isobutylketone, ethyl acetate, and tetrahydrofuran (THF)... 

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