Chromatographic analysis disadvantages

The various ELISA assays give both semi-quantitative and reliable results more rapidly than chromatography. They are particularly practised to eliminate all of the non-useful negative samples, which need not be submitted to the extraction steps and to a subsequent chromatographic analysis. There are, however, several disadvantages ... 

MAE has found its place in the field of chromatographic analysis of environmental samples. From 1986 till now it has been accepted as an interesting alternative to the conventional extraction methods, as well as to some of the newly developed ones. Some of the main benefits and disadvantages of the MAE are summarized in Table 2.5, together with the Soxhlet and the ultrasound assisted extraction techniques. 

In an online SPE configuration, extraction is done in a solid phase packed in a precolumn placed at the loop of a six-port switching valve. This method is easily automated, and preconcentration and chromatographic analysis can be done in the same run. It allows high throughput analysis, timesaving, improved precision, and accuracy. Its disadvantages are systematic errors and recovery error. It can be used in combination with GC or LC. ... 

Quantitative infrared spectroscopy suffers certain disadvantages when compared with other analytical techniques and thus it tends to be confined to specialist applications. However, there are certain applications where it is used because it is cheaper or faster. The technique is often used for the analysis of one component of a mixture, particularly when the compounds in the mixture are alike chemically or haye very similar physical properties, e.g. structural isomers. In these cases, analysis by using ultraviolet/visible spectroscopy is difficult because the spectra of the components will be almost identical Chromatographic analysis may be of limited use because the separation of isomers, for example, is difficult to achieve. The infrared spectra of isomers are usually quite different in the fingerprint region. Another advantage of the infrared technique is that it is non-destructive and requires only a relatively small amount of sample. 

A.W. Westerberg (1969) published, almost 40 years ago, the report of a chromatographic analysis with overlaid peaks [4]. In this work, Westerberg describes the disadvantages of vertical skimming with superimposed peaks and recommends a curve fitting, which is discussed in the following section. 

The practice In the chromatographic analysis of porous solids 1s Identical to the ordinary column calibration procedure using a series of fully characterized standard polymers. In fact, many of the works cited above aimed at the evaluation of the packing materials for SEC. The disadvantage of the SEC method Is that It cannot be applied to those materials unsuitable for packing in a chromatography column. Gel materials of fibrous or bulky form, or of insufficient rigidity, cannot form a stable gel bed In a column, and only the static solute exclusion method can be applied to these materials. 

A major disadvantage of gradient elution in terms of fast analysis remains the time to adequately equilibrate the chromatographic column between two experiments. However, Carr et al. recently demonstrated an excellent repeatability ( 0.002 min in retention time) obtained with two column volumes of re-equilibration instead of the usual 10 column volumes when a small amount of ancillary solvent (1-3% of 1-butanol or 1-propanol) is added to the mobile phase [45, 58]. 

Trace analysis is particularly attractive for SFE-HPLC since quantitative transfer of all analytes extracted to the chromatographic system becomes possible. At present, on-line SFE-HPLC appears to be feasible for qualitative analysis only quantitation is difficult due to possible pump and detector precision problems. Sample size restrictions also appear to be another significant barrier to using on-line SFE-HPLC for quantitative analysis of real samples. On-line SFE-HPLC has therefore not proven to be a very popular hyphenated sample preparatory/separation technique. Although online SFE-HPLC has not been quantitatively feasible, SFE is quite useful for quantitative determination of those analytes that must be analysed by off-line HPLC, and should not be ruled out when considering sample preparatory techniques. In most cases, all of the disadvantages mentioned with the on-line technique (Table 7.15) are eliminated. On- and off-line SFE-HPLC were reviewed [24,128]. 

On the basis of the preceding discussion, it should be obvious that ultratrace elemental analysis can be performed without any major problems by atomic spectroscopy. A major disadvantage with elemental analysis is that it does not provide information on element speciation. Speciation has major significance since it can define whether the element can become bioavailable. For example, complexed iron will be metabolized more readily than unbound iron and the measure of total iron in the sample will not discriminate between the available and nonavailable forms. There are many other similar examples and analytical procedures that must be developed which will enable elemental speciation to be performed. Liquid chromatographic procedures (either ion-exchange, ion-pair, liquid-solid, or liquid-liquid chromatography) are the best methods to speciate samples since they can separate solutes on the basis of a number of parameters. Chromatographic separation can be used as part of the sample preparation step and the column effluent can be monitored with atomic spectroscopy. This mode of operation combines the excellent separation characteristics with the element selectivity of atomic spectroscopy. AAS with a flame as the atom reservoir or AES with an inductively coupled plasma have been used successfully to speciate various ultratrace elements. 

---