Chromatographic sampling methods

Sampling methods in SFC are far more restricted than in the case of GC (Section 4.3 and Table 7.5). Not surprisingly, supercritical fluid extraction is an obvious choice. 

The number of reports on on-line TLC analysis of extracts is quite limited. Stahl [16,29] described a device for supercritical extraction with deposition of the fluid extract on to a moving TLC plate. On-line SFE-TLC provides rapid and simple insight into the extraction performance. Its strength is that the extract is deposited on a plate, which means that detection is a static process. Limitations of SFE-TLC are that quantification is difficult, and that the stability of components on the support material or in the presence of oxygen may be a problem. For additives in beverages (such as benzoic 

Overviews of sample preparation for chromatographic separations are available [31-33]. See also Table 4.15. Trapping methods for GC were critically reviewed [34]. 

Applications The potential of a variety of direct solid sampling methods for in-polymer additive analysis by GC has been reviewed and critically evaluated, in particular, static and dynamic headspace, solid-phase microextraction and thermal desorption [33]. It has been reported that many more products were identified after SPME-GC-MS than after DHS-GC-MS [35], Off-line use of an amino SPE cartridge for sample cleanup and enrichment, followed by TLC, has allowed detection of 11 synthetic colours in beverage products at sub-ppm level [36], SFE-TLC was also used for the analysis of a vitamin oil mixture [16]. 

Sandra and David [37] have reported on validation studies for SHS-HSGC, Py-HSGC, on-line LVI-GC, on-line SPME-GC, on- and off-line SPE-GC, on-line and off-line derivatisation-GC, SBSE-TD-GC, and PTV-LC(SEC)-GC. 

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An on-line concentration, isolation, and Hquid chromatographic separation method for the analysis of trace organics in natural waters has been described (63). Concentration and isolation are accompHshed with two precolumns connected in series the first acts as a filter for removal of interferences the second actually concentrates target solutes. The technique is appHcable even if no selective sorbent is available for the specific analyte of interest. Detection limits of less than 0.1 ppb were achieved for polar herbicides (qv) in the chlorotriazine and phenylurea classes. A novel method for deterrnination of tetracyclines in animal tissues and fluids was developed with sample extraction and cleanup based on tendency of tetracyclines to chelate with divalent metal ions (64). The metal chelate affinity precolumn was connected on-line to reversed-phase hplc column, and detection limits for several different tetracyclines in a variety of matrices were in the 10—50 ppb range. 

Various aspects of the chromatography of vitamin B 2 and related corrinoids have been reviewed (59). A high performance Hquid chromatographic (hplc) method is reported to require a sample containing 20—100 p.g cyanocobalamin and is suitable for premixes, raw material, and pharmaceutical products (60). 

The most common precolumn chromatographic techniques discussed here are SFE (Section 3.4.2), SPE (Section 3.5.1) and SPME (Section 3.5.2). However, sampling methods such as thermal desorption, pyrolysis and headspace (Section 4.2.2) may also be classified in this category. 

Kobylinska et al. [62] described a high performance liquid chromatographic analytical method for the determination of miconazole in human plasma using solid-phase extraction. The method uses a solid-phase extraction as the sample preparation step. The assay procedure is sensitive enough to measure concentrations of miconazole for 8 h in a pharmacokinetic study of Mikonazol tablets and Daktarin tablets in human volunteers. The pharmacokinetics of the two formulations was equivalent. 

A gas liquid chromatographic (GLC) method was described for determining residues of Bayer 73 (2-aminoethanol salt of niclosamide) in fish muscle, aquatic invertebrates, mud, and water by analyzing for 2-chloro-4-nitroaniline, a hydrolysis product of Bayer 73 [83]. Residues were extracted with acetone-formic acid (98 + 2), and partitioned from water samples with chloroform. After sample cleanup by solvent and acid base partitioning, the concentrated extract was hydrolyzed with 2N NaOH and H202 for 10 min at 95°C. The 2-chloro-4-nitroaniline was then partitioned hexane ethyl ether (7 + 3) and determined by electron capture GLC. Average recoveries were 88% for fish, 82% for invertebrates, 82% for mud, and 98% for water at 3 or more fortification levels. 

Micellar electrokinetic capillary chromatography with photodiode array detection was used for the determination of polyaromatic hydrocarbons in soil [65]. A detection limit of lOpg and linear calibration over five orders were observed. Compared to a standard gas chromatographic analysis method, the miscellar electrokinetic chromatographic method is faster, has a higher mass sensitivity and requires smaller sample sizes. 

The system shown in Fig. 4.11 is designed to dehver optimal performance when coupled to sample-preparation, sample-introduction and data-handhng products available. The HP 6890 series GC offers a smooth transition for users of HP 5890 series gas chromatographs through methods compatibihty, extremely useful for modern laboratories whose methodology is costly to develop. 

At the Department for Chemical Ecology we have developed adsorption techniques for sampling and analyzing of volatile components in air. These gas chromatographic (GO methods have been used in allelochemical research, i.a. for analyses of volatiles emitted from plant leaves (1,2). We believed that such an adsorption method could be adapted for sampling volatiles in the soil by allowing for the high humidity in the soil and its effect on the adsorbent. 

Quantitative analyses were made on samples which originally contained ca lOOmg of Centr. The chromatographic-spectrographic method can detect quantitatively less than O.lmg of substance... 

Today the situation is the reverse. Multiple measurements are often made on each subject. By taking as many measurements as possible, it is hoped that the complete picture of each subject will be less error-prone than each individual measurement considered separately. As an illustration, the development of chromatographic separation methods and detection techniques like spectrophotometry has made it possible to identify many substances in one blood sample from a patient. At the same time, the cost of each experiment has increased. Consequently, new methods for data analysis are needed, methods that can utilize the information in the data also when the subjects are few and the measurements are many. 

EDTA has been determined in a wide variety of sample matrices by HPLC. These matrices include waste waters, natural waters, sediments, fertilizers, chemical cleaning solutions, radioactive waste solutions, and pharmaceutical preparations. Chinnick reported the separation and identification of EDTA and other aminopolycarboxylic acid sequestrants by a high performance liquid chromatographic (HPLC) method [57]. 

The variety of sampling methods that are available, ie dilute and concentrated samples, suspensions, solids, surfaces and combination with chromatographic methods, such as that used in the high performance liquid chromatography separation of o-phthalyl dialdehyde derivatised amino acids in natural and sea water samples. 

Sereda et al. developed an electron-capture gas chromatographic detection method for the detection of benazepril and other angiotensinconverting enzyme (peptidyl-dipeptidase) inhibitors [26]. Sample solutions (0.001-0.1 mL) containing the inhibitor or metabolite were... 

Alcohol Content Alcohol levels in some gel preparations are determined by gas chromatographic (GC) methods. Desoxymetasone gel USP and naftifine hydrochloride gel USP contain 18-24% and 40-45% (w/w) of ethyl alcohol, respectively. In a desoxymetasone gel, the sample is dissolved in methanol and injected into a gas... 

Because of the favorable sorptive properties of the reversed-phase supports, batch adsorption and desorption can be a very effective way to desalt a chromatographed sample or to partially fractionate a peptide mixture during a purification procedure. For example, 1-2 gm of an oc-tadecyl silica packed into a silanized glass or plastic pipette can be used for the batch fractionation of small amounts of a crude peptide extract from tissues, such as the pancreas or pituitary, or from a synthetic experiment. A number of commercial products, such as the Waters Sep-Pak, have found use in this manner 10) as a purification or sample preparation aid. Protocols for batch extraction procedures on alkyl silicas have been discussed 17a,b) and applied to neuropeptides 10, 158, 166) and other hormonal peptides 88, 162, 167, 168). With these methods recoveries of peptides present in a tissue extract are generally higher than those found with classical fractionation techniques due in part to the fact that proteolytic degradation is minimized. 

A gas chromatographic (GC) method has been described in the literature. GC is based on the oxidation of microcystins which splits the Adda side chain to produce 3-methoxy-2-metlyl-4-pheitylbu-tyric acid (MMPB), which is then determined, either by GC or GC/MS (as its methyl ester) (Sano 1992 Kaya and Sano 1999) or by HPLC/fluoiescence detection (after conversion to a fluorescent derivative) (Sano 1992). GC/MS has been used to monitor microcystins in Japanese lakes (Tanaka 1993) and in sediments (Tsuji 2001). A similar method was developed by Harada (1996), but in this case the MMPB was determined directly without derivatization using GC/MS or LC/MS. The results of this approach ate given in terms of total toxin concentration, which then can be expressed in terms of microcystin-LR. However, individual toxins ate not determined and consequently it is not possible to produce a result in terms of microcystin-LR toxicity equivalents. This procedure cannot therefore be used to monitor water samples in relation to the proposed guideline. 

Thermal Desorption Thermal desorption is an alternative GC inlet system particularly used for VOC analysis. However, the analytes subjected to thermal desorption must be thermally stable to achieve successful analysis. Otherwise, decomposition occurs. This technique is mainly used for determination of volatiles in the air. Such a methodology requires sample collection onto sohd sorbents, then desorption of analytes and GC analysis. Traditionally, activated charcoal was used as a sorbent followed by extraction with carbon disulfide. However, solvent desorption involves re-dilution of the VOCs, thus partially negating the enrichment effect. Therefore, the sampling method is to pump a sample of gas (air) through the sorbent tube containing certain sorbents in order to concentrate the VOC. Afterwards, the sample tube is placed in thermal desorber oven and the analytes are released from the sorbent by application of high temperature and a flow of carrier gas. Additionally, desorbed compounds are refocused in a cold trap and then released into the GC column. Such a two-step thermal desorption process provides a narrow chromatographic band at the head of the column. 

Bemy, P.J., Buronfosse, T., Lorgue, G. (1995). Anticoagulant poisoning in animals a simple new high-performance thin-layer chromatographic (HPTLC) method for the simultaneous determination of eight anticoagulant rodenticides in liver samples. J. Anal. Toxicol. 19(7) 576-80. 

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