Sample clean

Tandem mass spectrometry or ms/ms was first introduced in the 1970s and gained rapid acceptance in the analytical community. The technique has been used for stmcture elucidation of unknowns (26) and has the abiUty to provide sensitive and selective analysis of complex mixtures with minimal sample clean-up (27). Developments in the mid-1980s advancing the popularity of ms/ms included the availabiUty of powerhil data systems capable of controlling the ms/ms experiment and the viabiUty of soft ionisation techniques which essentially yield only molecular ion species. 

The main advantages of the ms/ms systems are related to the sensitivity and selectivity they provide. Two mass analyzers in tandem significantly enhance selectivity. Thus samples in very complex matrices can be characterized quickly with Htde or no sample clean-up. Direct introduction of samples such as coca leaves or urine into an ms or even a gc/lc/ms system requires a clean-up step that is not needed in tandem mass spectrometry (28,29). Adding the sensitivity of the electron multiplier to this type of selectivity makes ms/ms a powerhil analytical tool, indeed. It should be noted that introduction of very complex materials increases the frequency of ion source cleaning compared to single-stage instmments where sample clean-up is done first. 

Because of the extreme surface sensitivity of static SIMS, care should always be exercised not to handle the samples. Clean tools and gloves should be used always to avoid the possibility of contaminating the surface. While it is possible to remove surface contamination with solvents like hexane, it is always desirable not to clean the surface to be analyzed. 

Coupled liquid chromatography-gas chromatography is an excellent on-line method for sample enrichment and sample clean-up. Recently, many authors have reviewed in some detail the various LC-GC transfer methods that are now available (1, 43-52). For the analysis of normal phase eluents, the main transfer technique used is, without doubt, concurrent eluent evaporation employing a loop-type interface. The main disadvantage of this technique is co-evaporation of the solute with the solvent. 

Trace enrichment and sample clean-up are probably the most important applications of LC-LC separation methods. The interest in these LC-LC techniques has increased rapidly in recent years, particularly in environmental analysis and clean-up and/or trace analysis in biological matrices which demands accurate determinations of compounds at very low concentration levels present in complex matrices (12-24). Both sample clean-up and trace enrichment are frequently employed in the same LC-LC scheme of course, if the concentration of the analytes of interest are Sufficient for detection then only the removal of interfering substances by sample clean-up is necessary for analysis. 

Chlorophenoxy acids are relatively polar pesticides which are usually determined by LC because volatile derivatives have to be prepared for GC analysis. This group of herbicides can be detected by multiresidue methods combined with automated procedures for sample clean-up, although selectivity and sensitivity can be enhanced by coupled-column chromatographic techniques (52). The experimental conditions for Such analyses are shown in Table 13.1. 

E. A. Hogendoom, G. R. van der Hoff and P. van Zoonen, Automated sample clean-up and fractionation of organochlorine pesticides and polychlorinated biphenyls in human milk using NP-HPEC with column-switching , J. High Resolut. Chromatogr. 12 784-789 (1989). 

The injector temperature should be determined by the nature of the sample and the volume injected, not by the column temperature. When analyzing biological or high-boiling samples, clean the injector body with methanol or other suitable solvent once per week. Install a clean packed injector liner and a new septum, preferably near the end of a workday. Program the column to its maximum temperature, then cool the column and run a test mixture to check the system using standard conditions. 

One approach to the problem of matrix effects is to prevent the matrix materials reaching the electrospray source by carrying out some form of clean-up prior to analysis and/or to employ chromatographic separation. It is not always possible, however, to develop a simple procedure for sample clean-up and since this approach involves further work-up with the associated increase in analysis time and potential for sample loss it is therefore not ideal. 

GC/ECD or a halogen-specific detector (HSD) (Method 8080) is the technique recommended by EPA s Office of Solid Waste and Emergency Response for determining a- and [3-endosulfan and endosulfan sulfate in water and waste water at low-ppb levels (EPA 1986a). At these low concentrations, identification of endosulfan residues can be hampered by the presence of a variety of other pesticides. Consequently, sample clean-up on a Florisil column is usually required prior to analysis (EPA 1986a). 

GC/ECD and GC/MS (EPA Method 608) are the methods recommended for determining a-endosulfan, (3-endosulfan, and endosulfan sulfate in municipal and industrial discharges (EPA 1991a). Sample clean-... 

Bartok, T., Borsok, G., and Sagi, F., RP-HPLC separation of polyamines after automatic FMOC-C1 derivatization and precolumn sample clean-up using column switching, /. Liq. Chromatogr., 15, 777, 1992. 

Urine Sample cleaned-up by coprecipitation, treated with HN03 and H202, wet ashed Biphasic liquid scintillation 1 pCi/200 cm3 84% Bomben et al. 1994... 

Urine Spiked sample clean-up by co-precipitation, purified by TRU-spec column and electrodeposition a -Spectrometry 0.016 pCi/800 cm3 95% at 0.1-100 pCi/sample Goldstein et al. 1991... 

Post-extraction manipulation (minimisation of sample clean-up procedures)... 

A single SFE/ESE instrument may perform (i) pressurised C02 (SFE), (ii) pressurised C02/modifier and (iii) pressurised modifier (i.e. ASE /ESE , organic solvent) extractions. The division between SFE and ASE /ESE blurs when high percentages of modifier are used. Each method has its own unique advantages and applications. ESE is a viable method to conduct matrix/analyte extraction provided a solvent with good solvating power for the analyte is selected. Sample clean-up is necessary for certain matrix/analyte combinations. In some circumstances studied [498], SFE may offer a better choice since recoveries are comparable but the clean-up step is not necessary. 

Table 3.46 compares SPME and SPE. Although SPME has in common with SPE that the analytes are concentrated by adsorption into a solid phase, SPE involves absorbing the analyte from the sample onto a modified solid support. In practice, the two techniques are quite different. SPME differs from conventional SPE in that SPE isolates the majority of the analyte from a sample (>90%) but injects only about 1 to 2% of the sample onto the GC. SPME isolates a much smaller quantity of analyte (2-20%), but that entire sample is injected into the GC. SPME is easy-to-perform and often significantly more rapid and simpler than SPE, but its quantitative aspect is exacting. Both conventional SPE and SPME minimise the use of solvents for sample preparation and free analysts from tedious sample clean-up. Where SPE can replace LLE... 

Function/objective Sample clean-up Sampling method... 

Hinman et al. [492] have compared SFE and ASE in the extraction of antioxidants from LDPE. Comparable extraction yields were obtained with both techniques. However, sample clean-up was necessary after ASE , while with SFE the extract could be analysed directly without any post-extraction clean-up. Supercritical fluid extraction of 15 polymer additives (AOs, UVAs, process lubricants, flame retardants and antistatic agents) from eight PS formulations was compared to dissolu-tion/precipitation extractions [557], Additive recoveries were comparable. Numerous additional comparisons can be found under the specific headings of the extraction techniques (Sections 3.3 and 3.4). 

Table 4.45 shows the main features of SEC. This technique has become an indispensable tool for polymer characterisation. SEC has some advantages over other LC methods, such as the predictability of the end of a chromatographic run and of the retention times in a calibrated chromatographic system. SEC is an attractive technique for prefractionation or sample clean-up prior to a more sensitive RPLC technique. This intermediate step is especially interesting for experimental purposes whenever polymer matrix interference cannot be separated from the peak of interest [647]. Disadvantages are that the whole separation must be eluted within the... 

Determination of caffeine in soft drinks was undertaken using the aerosol alkali flame ionization detector.24 Soft drinks studied were Coke, Diet Coke, Pepsi, Diet Pepsi, Dr. Pepper, and Mountain Dew. A sample clean-up and concentration procedure is employed followed by GC separation with alkali flame ionization detection. Results showed that Coke, Diet Coke, Pepsi, Diet Pepsi, Dr. Pepper, and Mountain Dew contained 41 2, 52 2, 43 4, 35 9, 46 6, and 60 15 mg caffeine per 355-ml serving. These values compared favorably with levels reported in the literature. 

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