Variable Volume Injection

Partial filling is chosen when the available amount of sample is limited. Partial filling can be combined tvith backfiushing the loop in order to reduce the dilution in the loop. The injected amount is then determined by the volume metered by the injection syringe. Small injected volumes in large loops will result in considerable sample dilution. 

---

The majority of injection systems are only capable of delivering a fixed-volume sample plug into the analysis channel and are limited in function by their configuration. Fu and his co-workers have proposed the multifunction injection system and a variable-volume-injection system within the microfluidic chip as described below. 

Electrokjnetlc Sample Injection, Figure 8 Variable-volume-injection system (a) microtluidic chip, (b) focusing ratios f/0 = 1, (c) F/4> = 1.4, and (d)F/0 = 1.8... 

Precision The precision of a gas chromatographic analysis includes contributions from sampling, sample preparation, and the instrument. The relative standard deviation due to the gas chromatographic portion of the analysis is typically 1-5%, although it can be significantly higher. The principal limitations to precision are detector noise and the reproducibility of injection volumes. In quantitative work, the use of an internal standard compensates for any variability in injection volumes. 

An alternative form of split injection is the timed split technique, Figure 6.11 [130,131,133]. In this case the column is connected directly to the valve and the valve actuator is controlled electronically to turn the valve to the inject position and back very rapidly with only a portion of the sample in the loop displaced to the column. Timed split allows variable volumes to be injected by changing the valve actuator tine and provides more reproducible splitting than the dynamic split technique. However, it suffers from many of the same problems as dynamic split, namely, poor accuracy, split ratios that depend on pressure, and high detection limits. 

Figure 30.4, displays the operation of a sample loop in two different modes i.e., (a) sampling mode and (b) injection mode. Here, the sample is loaded at atmospheric pressure into an external loop in the microvolumesampling valve, and subsequently injected into the mobile-phase by a suitable rotation to the valve. However, the volume of sample introduced usually ranges between 2 p/ to over 100 p/ but can be varied either by altering the volume of the sample loop or by employing specific variable-volume sample valves. 

In bioanalysis, extracted samples are usually stored in either autosampler vials or wells in a plate (such as 96-well plate) sealed with pierceable caps or covers. During injection, the autosampler needle has to pierce the caps or covers to load samples. The debris may completely or partially block the autosampler needle, which would result in no sample or variably low sample volumes injected. Accordingly, no or randomly low IS responses are observed. As most autosamplers have a built-in needle flushing mechanism, the debris in the needle might be flushed out later partially or completely. Therefore, the injected volume can be back to normal at a later time without an operator s intervention. Apparently, when a needle will be blocked and when the blocked needle will be cleared by flushing, as well as how it will be blocked (completely or partially) are difficult to predict. Hence, there would be no clear pattern for this type of IS variations. However, the affected injections normally have lowered IS responses (Fig. 9). Despite lowered IS responses, the accuracy of quantitation can usually be maintained except for situations where no or very low amount of samples are injected, resulting in responses outside the limit of linear range or unacceptable S/N. 

These results will be used to construct a standard response curve for phenacetin. Since precision is a function of the volume injected, there will be a small variability as the sample size is changed. This approach is used for economy of time, and the error should not be significant. 

Figure 1-3 shows two different types of batch reactors used for gas-phase reactions. Reactor A is a constant-volume (variable-pressure) reactor and Reactor B is a constant-pressure (variable-volume) reactor. At time r = 0, the reactants are injected into the reactor and the reaction is initiated. To see clearly the different forms the mole balance will take for eadi type of reactor, consider the follovring examples, in which the gas-phase decomposition of dimethyl ether is taking place to form methane, hydrogen, and carbon monoxide ... 

H.L. Wu, Y. Hayashibe, Y. Sayama, M. Shibukawa, K. Oguma, A computer-controlled variable-volume injector for flow injection analysis, Anal. Sci. 11 (1995) 401. 

In an analysis to determine the ethanol content of a wine by gas chromatography, an internal standard of isopropanol is used to account for the variability in the volume injected between tests. In the measurement of a four-point calibration curve and the repeated analysis of the wine sample, six injections in all are performed. Each injection contained 1% v/v of the internal... 

The adsorption of a surfactant at an interface between CO2 and a second fluid, such as water, may be determined directly from measurement of the interfacial tension (change in Gibbs free energy with surface area), y, versus surfactant concentration. A novel tandem variable-volume pendant drop tensiometer has been developed to measure equilibrium and dynamic values of y as a function ofT.p and time (Figure 2.4-1) [21]. An organic [21] or aqueous phase [18] is preequilibrated with CO2 in the first variable-volume cell (drop-phase cell). A droplet of this liquid is injected into the second variable-volume cell, with two windows at 180° mounted on a diameter, containing either pure CO2 or CO2 and surfactant. 

Recently, a unique variable-volume injector (10-20D ul), based on a principle different from that of common injection valves, has been developed (Hewlett-Packard) (see Fig. 10). 

The attractive features of splitless injection techniques are that they allow the analysis of dilute samples without preconcentration (trace analysis) and the analysis of dirty samples, since the injector is easily dismantled for cleaning. Success with individual samples, however, depends on the selection of experimental variables of which the most important sample size, sample solvent, syringe position, sampling time, initial column temperature, injection temperature and carrier gas flow rate, often must be optimized by trial and error. These conditions, once established, are not necessarily transferable to another splitless injector of a different design. Also, the absolute accuracy of retention times in splitless injection is generally less than that found for split injection. For splitless injection the reproducibility of retention times depends not only on chromatographic interactions but also on the reproducibility of the sampling period and the evaporation time of the solvent in the column inlet, if solvent effects (section 3.5.6.2) are employed. The choice of solvent, volume injected and the constancy of thermal zones will all influence retention time precision beyond those for split injection. For quantitative analysis the precision of repeated sample injections is normally acceptable but the method is subject to numerous systematic errors that may... 

The sample introduction system must be capable of introducing a known and variable volume of sample solution reproducibly into the pressurized mobile phase as a sharp plug without adversely affecting the efficiency of the column. The superiority of valve injection has been adequately demonstrated for this purpose and is now universally used in virtually all modern instruments for both manual and automated sample introduction systems [1,2,7,31,32]. Earlier approaches using septum-equipped injectors have passed into disuse for a several reasons, such as limited pressure capability, poor resealability, contamination of the mobile phase, disruption of the column packing, etc., but mainly because they were awkward and inconvenient to use compared with valves. For dilute sample solutions volume overload restricts the maximum sample volume that can be introduced onto the column without a dramatic loss of performance. On-column or precolumn sample focusing mechanisms can be exploited as a trace enrichment technique to enhance sample detectability. Solid-phase extraction and in-column solid-phase microextraction provide a convenient mechanism for isolation, concentration and matrix simplification that are easily interfaced to a liquid chromatograph for fully or semi-automated analysis of complex samples (section 5.3.2). 

Once the compound of interest has been isolated from the sample matrix, it is often necessary to dilute the sample in order to analyse it within the validated assay range (see Chapter 8). A test injection may have to be performed in order to assess the sample concentration in the first instance. Once it has been established that the sample does require dilution, this can easily be performed using calibrated glass (larger volumes) or variable volume pipettes (smaller volumes). 

Many studies have demonstrated that the injection system oti a CE microchip is one of the most fundamental components of the sample handling process and that its characteristics determine the quality of the separation. Practical applications, such as separation or detection, require that the dispensed sample volume be precisely controlled and that sample leakage to the separation channel be prevented. Hence, developing viable low-leakage injection techniques with the ability to dispense samples of variable volume is... 

---