Total sample transfer technique

With the total sample transfer technique, the sample is injected into the hot injector with the split valve closed (Figure 2.58). The volume of the injector insert must... 

The splitless technique therefore requires working with temperature programs. Because of the almost complete transfer of the sample on to the column, total sample transfer is the method of choice for residue analysis (Grob, 1994, 1995). 

The purpose of this injection technique is to introduce the entire injected sample into the column and use it for trace determination. Different techniques can be used, but the most common is the solvent effect technique, which uses the same instrumentation as used for spht injection (Figure 2.4). In splitless injection, the sample is introduced into the heated liner as in split injection and brought into the gas phase. Contrary to the spht injection, the splitter outlet valve is now dosed. Hence, the total sample volume (1-2 ml of gas) is transferred to the column. When splitiess injection is carried out, the column inlet temperature is kept at a temperature that is 20-50 °C lower than the solvent Bp. Hence, when the sample arrives at the column inlet, the solvent condenses as a thick film on the column wall. This film will act as a plug of stationary phase into which the sample components will be dissolved. Following the sample transfer to the column, which will take 2 min when 2 pi is injected and the carrier gas flow rate is 1 ml min , the column oven temperature is increased. The solvent evaporates first from the column entrance and thereafter the analytes, which will subsequently be separated in the column. The sphtter valve is opened when the whole sample has been transferred to the column in order to wipe out remains of the sample before the next injection. This injection technique is used for trace determinations and can only be carried out in combination with temperature programming. 

With respect to resolving power (theoretical number of plates) and separation time, no doubt, capillary electrophoresis (CE) is the ultimate separation technique for complex peptide samples, and its combination with ESI MS online as well as MALDI MS off-line has been demonstrated many times [222-229]. The main reason why CE-MS, in contrast to nano-LC-MS, has not become a widespread method for protein and peptide analysis is the maximum total sample volume that can be separated by CE. In contrast to nano-LC, where many himdred microhters of dilute sample can be loaded without compromising separation power, the performance of CE directly depends on the sample volume and works best if only 50 nL or less is loaded. Recently, however, it has been reahzed that this requirement of CE is perfectly matched by nano-LC, which provides efficient sample concentration, and that the two techniques can be combined online upfront ESI or MALDI MS. For this purpose, a microfluidic chip was developed that enables, on demand, on-hne transfer (loading) of nano-LC fractions to an orthogonal CE separation channel, the effluent of which is either analyzed online by ESI MS or off-line by MALDI MS [230-232]. 

These techniques are dependent on an electrolysis that attempts to oxidize or reduce completely all of an electroactive analyte species in a volume of sample solution. These methods have the distinct advantage of requiring no calibration since they are dependent on total electron transfer, which can be quantitatively related to the total mass of the analyte. Coulometry refers to the integration of electric current with time as electrolysis proceeds either at a constant current or at a constant voltage. The latter parameters are chosen to optimize analysis time and limit interferences. For constant current, the number of coulombs of electrons consumed or produced, Q, is related to the current I and time t by... 

One example is sampling for S02- Liquid sorbents for SO2 depend on the solubility of SO2 in the liquid collection medium. Certain liquids at the correct pH are capable of removing ambient concentrations of SO2 with 100% efficiency until the characteristics of the solution are altered so that no more SO2 may be dissolved in the volume of liquid provided. Under these circumstances, sampling is 100% efficient for a limited total mass of SO2 transferred to the solution, and the technique is acceptable as long as sampling does not continue beyond the time that the sampling solution is saturated (1). A second example is the use of solid sorbents such as Tenax... 

The simplest method to measure gas solubilities is what we will call the stoichiometric technique. It can be done either at constant pressure or with a constant volume of gas. For the constant pressure technique, a given mass of IL is brought into contact with the gas at a fixed pressure. The liquid is stirred vigorously to enhance mass transfer and to allow approach to equilibrium. The total volume of gas delivered to the system (minus the vapor space) is used to determine the solubility. If the experiments are performed at pressures sufficiently high that the ideal gas law does not apply, then accurate equations of state can be employed to convert the volume of gas into moles. For the constant volume technique, a loiown volume of gas is brought into contact with the stirred ionic liquid sample. Once equilibrium is reached, the pressure is noted, and the solubility is determined as before. The effect of temperature (and thus enthalpies and entropies) can be determined by repetition of the experiment at multiple temperatures. 

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