Loop-type interfaces

LOOP-TYPE INTERFACES (CONCURRENT ELUENT EVAPORATION)... 

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. 

K. Grob and J. M. Stoll, Loop-type interface for concuirent solvent evapoi ation in coupled HPLC-GC. Analysis of raspbeiry ketone in a raspbeiry sauce as an example , 7. High Resolut. Chromatogr. Chromatogr. Commun. 9 518-523 (1986). 

The analysis of sterols, sterols esters, erythrodiol and uvaol, and other minor components of oils and fats, is usually carried out by normal-phase HPLC-HRGC by using a loop-type interface and the concurrent eluent evaporation technique, as reported in the papers cited by Mondello et al. (48) (up to 1995) and in more recent papers (49, 50). More recently, reversed-phase LC-GC methods have been... 

In order to achieve the widest application range, partially concurrent solvent evaporation (PCSE) with an on-column interface is normally used during the transfer of analytes from the LC-type precolumn to the GC system. Eully concurrent solvent evaporation (ESCE), with a loop-type interface, is used in some cases, although the... 

One of the first examples of the application of reverse-phase liquid chromatography-gas chromatography for this type of analysis was applied to atrazine (98). This method used a loop-type interface. The mobile phase was the most important parameter because retention in the LC column must be sufficient (there must be a high percentage of water), although a low percentage of water is only possible when the loop-type interface is used to transfer the LC fraction. The authors solved this problem by using methanol/water (60 40) with 5% 1-propanol and a precolumn. The experimental conditions employed are shown in Table 13.2. 

Figure 13.16 LC separation of urban air particulate exrtact (a), along with the GC/FID cliro-matogram (b) of an oxy-PAC fraction (transfeired via a loop-type interface). Reprinted from Environmental Science and Technology, 29, A. C. Lewis et al., On-line coupled LC-GC-ITD/MS for the identification of alkylated, oxygenated and nirtated polycyclic aromatic compounds in urban air particulate exti acts , pp. 1977-1981, copyright 1995, with permission from the American Chemical Society.

A more sophisticated method, giving a much more detailed characterization, involves the on-line coupling of EC and GC (LC-GC). Analysis schemes for middle distillates (kerosine, diesel and jet fuels) combining EC and GC have been reported by various authors (25-31). However, only Davies et al. (25) andMunari et al. (27) have reported on the required automatic transfer of all of the individual separated fractions from the EC unit the GC system. Davies used the loop-type interface and Munari the on-column interface. Only Beens and Tijssen report a full quantitative characterzation by means of LC-GC (31). 

Figure 8.20 Scheaatic representation of process of (A) pairtial concurrent evaporation and (B) concurrent evaporation using a loop-type interface for the transfer of eluent fron an VC colunn to a 6C column. (Reproduced with permission from ref. 224. Copyright Elsevier Scientific Publishers).

The on-column interface is the one which is most often used in LC-GC of aqueous samples because it can be applied to a wider range of compounds.The loop-type interface is limited for determining volatile compounds that are volatilized together with the solvent and not retained in the retention gap. Several attempts at solving this problem have been made. One option is to add a co-solvent which enters the retention gap before the analytes and thus forms a co-solvent film in front of the eluate. 

On-Line Systems Flowing MMLLE systems have been established in different layouts with automation and on-line hyphenation to GC and HPLC analysis. An automated on-line FS-MMLLE-GC system with a loop-type interface compatible with LVI was used for the extraction of pesticides and PAHs in surface waters.86 In another study, pressurized hot water extraction (PH WE) was coupled on-line to a FS-MMLLE-GC-FID system and applied to the analysis of PAHs in soil, where MMLLE was used as a cleanup and concentration step of the PH WE extract prior to final GC analysis.87 In addition, an HF-MMLLE setup was incorporated in PHWE and GC, resulting in an online PHWE-HF-MMLLE-GC system, where the HF membrane module contained 10-100 HFs. The system served for the extraction and analysis of PAHs in soil and sediments ... 

The application of the loop-type interface for LC-GC for multifraction introduction has been introduced [134]. The use of microbore LC columns have been used as a means to reduce the injection volumes of solvent [135,136]. 

Figure 3.11. Schematic diagram of a loop type interface with concurrent eluent evaporation for LC-GC. (From ref. [100] Elsevier).

A first method simplified the classical analysis of the sum of free and esterified sterol instead of saponification and tedious extraction from a soap solution, the sample is transesterified at ambient temperature and the mixture (containing the fatty acids as methyl esters) is preseparated by NPLC. The LC-UV chromatogram enables one to select the type of sterols (A5, A7, methyl-, dimethyl sterols) to be analyzed by GC (after online transfer through the loop-type interface). 

Figure 4 Design of the loop-type interface. Easy to use for large fraction volumes, but not suitable for components eluted less than 50-80°C above the column temperature during transfer. (Reproduced with permission from Grob K (2000) Efficiency through combining HPLC and HRGC progress 1995-1999. Journal of Chromatography A 892 407-420.)...

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