Sample introduction liquid chromatography

During their passage through the column, sample molecules spend part of the time in the mobile phase and part in the stationary phase. All molecules spend the same amount of time in the mobile phase. This time is called the column dead tine or holdup time (t.) and is equivalent to the tine required for an unretained solute to reach the detector frsolute retention time (t,) is the time between the instant of saiq>le introduction and when the detector senses the maximum of the retained peak. This value is greater than the column holdup time by the amount of time the solute spends in the stationary phase and is called the adjusted retention time (t, ). These values lead to the fundamental relationship, equation (1.1), describing retention in gas and liquid chromatography. 

In addition to GC/MS, high performance liquid chromatography (HPLC/MS) has been used to analyse natural resins in ancient samples, particularly for paint varnishes containing mastic and dammar resins [34]. A partial limitation of chromatographic techniques is that they do not permit the analysis of the polymeric fraction or insoluble fraction that may be present in the native resins or formed in the course of ageing. Techniques based on the direct introduction of the sample in the mass spectrometer such as direct temperature resolved mass spectrometry (DTMS), direct exposure mass spectrometry (DE-MS) and direct inlet mass spectrometry (DI-MS), and on analytical pyrolysis (Py-GC/MS), have been employed as complementary techniques to obtain preliminary information on the... 

The //PLC system described in this chapter is equipped with 24 parallel columns for liquid chromatography, each with its own sample introduction port and exit port for connection to detectors of choice (UV absorbance and/ or fluorescence). Flow from a binary solvent delivery system is divided evenly across 24 channels and results in 1/24 of the programmed pump flow rate through each column (i.e., total flow of 300 /zL/ min will produce a flow of 12.5 /zL/ min in each column). Samples are introduced to the columns by a multichannel autosampler configured to sample from either 96-or 384-well SBS standard plates. Figure 6.2 depicts a general view of the system. 

Sample preparation for analysis by hyphenated methods requires some additional planning when compared to nonhyphenated methods. All steps, extraction, concentration, and final solvent selection must take into consideration and be compatible with all the components of the hyphenated instrumentation. For gas chromatographic methods, all the components in the mixture must be in the gaseous state. For liquid chromatography (LC) or high-performance liquid chromatography (HPLC), the samples of the analytes of interest can be solids or liquids, neutral or charged molecules, or ions, but they must be in solution. If the follow-on analysis is by MS, then each of the analytes may require a different method of introduction into the MS. Metals and metal ions may be introduced by HPLC if they are in solution but commonly are introduced via AAS or inductively coupled plasma (ICP). Other analytes may be directly introduced from HPLC to MS [2],... 

Whilst these methods are informative for the characterisation of synthetic mixtures, the information gained and the nature of these techniques precludes their use in routine quantitative analysis of environmental samples, which requires methods amenable to the direct introduction of aqueous samples and in particular selective and sensitive detection. Conventionally, online separation techniques coupled to mass spectrometric detection are used for this, namely gas (GC) and liquid chromatography (LC). As a technique for agrochemical and environmental analyses, high performance liquid chromatography (HPLC) coupled to atmospheric pressure ionisation-mass spectrometry (API-MS) is extremely attractive, with the ability to analyse relatively polar compounds and provide detection to very low levels. 

Besides liquid samples, gases and solids can be analysed after making the appropriate modifications to the sample introduction system. The application of plasma sources as detectors for gas chromatography of metal complexes have teen reviewed by Uden Literature dealing with the analysis of gas and liquid chromatographic effluents have been surveyed by Carnahan et al. 

LIQUID CHROMATOGRAPHY. An analytical method based on separation of the components of a mixture in solution by selective adsorption. All systems include a moving solvent, a means of producing solvent motion (such us gravity or a pump I, a means ol sample introduction, a fractionating column, and a detector. Innovations in functional systems provide the analytical capability for operating in three separation modes (1) liquid-liquid partition in which separations depend on relative solubilities of sample components in two immiscible solvents (one of which is usually water) 12) liquid-solid adsorption where the differences in polarities nf sample components and their relative adsorption on an active surface determine tile degree ol separation (2) molecular size separations which depend on the effective molecular size of sample components ill solution. 

Samples submitted for analysis can be introduced into the mass spectrometer in many forms. Probably the most common means are (1) direct introduction, (2) gas chromatography (GC), and (3) liquid chromatography (LC). 

In contrast, both gas and liquid chromatography enable the samples of interest to be separated into individual components prior to introduction into the mass spectrometer ion source. Gas chromatography involves sample introduction with the requisite that the sample components must be volatilized prior to separation, and results in a gas sample being introduced to the mass spectrometer (i.e. El, Cl). Figure 5-2 shows the chromatogram obtained after a mixture of three simple phenolic compounds - phenol,... 

FAB and PD have been replaced by electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) in the analytical mass spectrometry laboratory, because both of these newer techniques have a wider mass range of analysis and have lower detection limits. ESI and MALDI have become invaluable ionization techniques for nonvolatile components. This is particularly true for a wide range of biological molecules including proteins, peptides, nucleic acids, etc. Samples can be analyzed by ESI using either direct injection or introduction through liquid chromatography. 

J. Staniewski, H.G. Janssen, C. A. Cramers and J. A. Rijks, Programmed-temperature injector for large-volume sample introduction in capillary gas chromatography and for liquid chromatography-gas chromatography interfacing , J. Microcolumn Sep. 4 331-338(1993). 

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