Sample Introduction Modes

Volume-based sample injections are usually made by injecting samples into a non-segmented carrier stream which is subsequently segmented by the extractant downstream. Dispersion of the injected sample before segmentation is therefore a matter of concern if loss of sensitivity through dispersion should be avoided. Toei [22] suggested segmentation of the carrier stream prior to the sample injection to reduce the dispersion of 

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Sample introduction mode (extraction thimble loading)... 

The aforementioned injection of a solution plug is termed as plug injection. Another sample introduction mode is stack injection. Stack and plug injections used in DNA separation have been compared (see Figure 4.2). Under similar conditions, the plug injection produced a better resolution, whereas the stack injection produced a higher sensitivity (see Chapter 6, section 6.2 for more on CE separation) [315]. 

Viktorova, O.S., Kogan, V.T., Manninen, S.A., Kotiaho, T., Ketola, R.A., Dubenskii, B.M., Parinov, S.P., Smirnov, O.V. Utilization of a multimembrane inlet and a cyclic sudden sampling introduction mode in membrane inlet mass spectrometry. J. Am. Soc. Mass Spectrom. 15, 823-831 (2004)... 

The chemiluminescent detection method has also been employed for monitoring the glucose content of dialysate. This detection mode requires the addition of reagents such as luminol to obtain the analytical signal, and this requires a more complex system with additional pump lines, mixing coils, etc. Flow injection is considered the ideal sample introduction mode for chemiluminescence [134], but this makes the measuring system rather cumbersome and not easy to miniaturize. 

Generally, samples are injected in the chromatographic system without any dilution or pretreatment step, using the split mode (i.e., with sample division), which is suitable for the analysis of the major compounds in beverages. When the objective of analysis is the determination of compounds present in small quantities ( xg/L), some extraction and/or concentration step is necessary, followed by the sample injection in the splitless mode (without sample division). This last sample introduction mode is usually... 

One unique type of MS, ICP-MS, needs to be discussed separately because it does not deal with molecular species, but with atomic ones. The inductively coupled plasma is a common atomization source for atomic spectrometry. This sample preparation/ sample introduction mode has been coupled with an MS to yield an instrument capable of trace level elemental analysis. Each element has a unique set of isotopes in known proportions. These can be used to quantify the element. In the case of elements with overlapping isotopic mass numbers, simple deconvolution can be used to give results for each. ICPMS has very low detection limits. 

Programmed-temperature vaporizers are flexible sample-introduction devices offering a variety of modes of operation such as spHt/sphtless, cool-sample introduction, and solvent elimination. Usually the sample is introduced onto a cool injection port liner so that no sample discrimination occurs as in hot injections. After injection, the temperature is increased to vaporize the sample. 

Wide choice of sample introduction, mass analysers and ionisation modes in relation to application... 

Figure 4.2 shows the complete CBMS II system. The main unit is comprised of three modules, the Biosampler Module, the Sample Introduction Module (SIM), and the Mass Spectrometer Module. The Biosampler Module houses the virtual impactor air particle concentrator and is only needed for the biological agent monitoring mode. The Sample Introduction Module contains the multiport sampling valve with its three input connections ... 

Fig. 11.1. Conceptual diagrams of a mass spectrometer showing the various functional components. The top diagram represents instruments that employ conventional modes of ionization such as El or Cl. In such instruments, the sample introduction process (for example, direct insertion probe) bridges the atmospheric pressure/high-vacuum interface. The bottom diagram represents instruments that employ the recently developed API techniques such as ESI. Ions are formed outside the vacuum envelope of the instrument and transported into the instrument through the API interface.

TTie pneumatics also optimize the performance of the spht/sphtless inlet. Its forward pressure control in sphtless injection mode significantly reduces the risk of sample loss and maximizes accuracy and reproducibihty. TTie provision of mass flow control coupled with back pressure control in the spht injection mode maximizes reproducibihty and accuracy and also allows electronic adjustment of spht ratios. The net result is that sample-introduction conditions are optimized individually for the two most popular injection techniques. In addition, parameters are recorded in the methods file. 

Direct injection is the most commonly used technique for sample introduction in GC, typically using combined spht/splitless injectors. In split mode, a portion of the sample passes onto the column and the rest is directed to waste. After sufficient split time to completely flush the injector the split vent may be closed to save gas, although this is optional. The injector is set to a sufficiently high temperature to eliminate discrimination between analytes. The sensitivity of the technique is inversely proportional to the split ratio. 

The optimization of the process in recent years, led to defined ion formation with solvent evaporation and complete desolvatation of analyte ions, which are then accelerated towards the mass separator. Analyte molecules often form multiply charged ions. ESI can be carried out both in positive and in negative mode. The sample introduction can be performed with microscale tips mainly made of fused silica capillaries, which are inexpensive and available in various sizes and geometric forms. Recently, nanospray technologies as microvariants of ESI with increased sensitivity were developed, which allowed the analysis of extremely small sample amounts [57]. 

HPLC has more or less supplanted GC as a method for quantifying drugs in pharmaceutical preparations. Many of the literature references to quantitative GC assays are thus old and the precision which is reported in these papers is difficult to evaluate based on the measurement of peak heights or manual integration. It is more difficult to achieve good precision in GC analysis than in HPLC analysis and the main sources of imprecision are the mode of sample introduction, which is best controlled by an autosampler, and the small volume of sample injected. However, it is possible to achieve levels of precision similar to those achieved using HPLC methods. For certain compounds that lack chromophores, which are required for detection in commonly used HPLC methods, quantitative GC may be the method of choice, for analysis of many amino acids, fatty acids, and sugars. There are a number... 

On line additions of aqueous standard solutions for the calibration of LA-ICP-MS including a comparison of wet and dry plasma conditions are discussed by O Connor et al.ls For solution calibration of standard solutions the authors used a 100 (xl PFA nebulizer together with a cyclonic spray chamber or a MCN-6000 sample introduction system with desolvator, to study the wet and dry plasma, respectively. A polypropylene Y piece was applied to mix the laser ablated material and the nebulized standard solutions. The authors found that the on line addition of water is the preferred mode of operation for quantification by LA-ICP-MS, i.e., wet plasma is more stable (improved standard deviation of sensitivity ratios). 

Figure 20.4—Static mode of headspace sample analysis. The sampling phial is pressurised with the carrier gas of the chromatograph. After equilibrium, a small volume of the gas containing the volatile compounds is inserted into a sample loop. Rotation of the six-way valve allows introduction of the sample into the injector of the chromatograph. Consequently, this set-up combines sample preparation with sample introduction into the chromatographic column. (Reproduced by permission of Tekmar.)...

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. 

In most cases, sample introduction on-chip is achieved using electrokinetic (EK) flow [3]. Two important EK injection modes, namely, pinched injection and gated injection, have been developed. Furthermore, some alternative injection methods are described. 

The liner may also be inverted, reversing the location of its radial restrictions. If the narrower restriction is placed at the top and a wide-bore column into the bottom restriction, the column will enter into the expansion chamber and butt to the narrower end. In this mode, a 26-gauge needle can be used to perform wide-bore on-column injections. The advantage of this mode of sample introduction is that exposure to active sites of the glass liner is eliminated. The disadvantage is that small samples must be used relative to the direct flash. The effect of depositing particles directly on the column is more severe for the on-column procedure, along with wider solvent peaks. 

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

Problems related to sample introduction, high voltage isolation, or other typical of other gradient methods are avoided. Samples can be injected in both electro-kinetic and hydrodynamic mode without affecting other aspects of the instrument operation or causing sample waste. 

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