สำนักราชบัณฑิตยสภา

«“√ “√ √“™∫— ≥±‘ µ¬ ∂“π ªï ∑’Ë Û ©∫— ∫∑’Ë Ù µ.§.-∏.§. ÚıÙ¯ 1135 Scientific Instrumentation in Research capillary columns’ carrier flow is 25 or 30 times less and therefore easier to “pump down”. That said, GC/MS interfaces have been developed for packed column systems that allow for analyte molecules to be dynamically extracted from the carrier gas stream at the end of a packed column and thereby selectively sucked into the MS for analysis. For one type interface, using a silicone membrane, the selectivity for organic molecules (the analyte) over helium (the carrier gas) is 50,000. I have previously mentioned the analytical technique belongs to the class of hyphenated analytical instru- mentation and cited gas chromatograhy/mass spectrometry (GC/MS) as an example. Recently this technique has been used extensively especially the coupling of HPLC with other instruments including NMR, UV, MS and commonly known as LC-hyphenated techniques. HPLC originally stands for High Pressure Liquid Chromatography which is in common use today for the separation of various compounds. The technique is evolved from the low pressure liquid chromatography and medium liquid chromatography. Today the word HPLC is commonly referred as High Performance Liquid Chromatography. HPLC or commonly called LC nowadays is a very versatile separation technique thus the instrument is coupled with other instruments for identification of organic compounds especially natural products. LC-hyphenated techniques are playing an increasingly important role as a strategic tool to support phytochemical investigations. Indeed, these techniques provide a great deal of preliminary information about the content and nature of constituents of crude plant extracts. This is very useful when large numbers of samples must be processed since unnecessary isolation of known compounds is avoided. Once the novelty or utility of a given constituent is established, it is then im- portant to process the plant extracts in the usual manner, to obtain samples for full structure elucidation and biological or pharmacological testing. The recent introduction of LC/NMR for crude plant extract screening will probably make another breakthrough in the on-line structural determination of natural products. This hyphenated method allows the recording of precious complementary on-line structure information when LC/UV/MS data are insufficient for unambiguous peak identification. Indeed, LC/NMR has proven to be very effective in obtaining 1-D spectra on both flowing and non-flowing samples, as well as stop flow 2-D spectra. However, compared with DV or MS, NMR remains a rather insensitive detection method and the need for solvent suppression in conventional LC/NMR restricted the observable NMR range. LC/MS analysis of crude plant extracts is not straightforward due to the great variety of their constituents. No interface allows an optimum ionisation of all the metabolites within a single crude plant extract since the response is compound dependent. Often, different ionisation modes or different interfaces are necessary to obtain a complete picture of the extract composition. With the full set of spectroscopic information obtained by LC/UV, LC/MS and LC/NMR, the phytochemist will be able to characterise rapidly the main constistuents of a given plant and to choose carefully which metabolites are to be isolated for indepth structural or pharmacological studies. The chemical screening of