Sampling loops, manual

The maximum injection volume depends on the volume of the sample loop in the injection valve. The reproducibility of manual injection depends on the skill of the operator. The use of a small sample loop and an overflow injection of the sample solution so that the loop is fully flushed with sample are basic requirements for quantitative analysis. The highest injection reproducibility can be obtained by an auto-injector with a fixed sample loop. The smallest reasonable injection volume is 1 (A. A nl-scale injection valve can be constructed however, the memory effect at the surface of contact parts affects quantitative analysis compared with the use of a /d-scale injection valve. For a semi-micro system, a low hold-up volume injection valve is desired. The minimum injection volume is 80 nl. For a preparative-scale injection, the sample loop can be easily replaced with a larger-volume loop, such as a 200 jA, instead of the standard 20 /A loop. 

An HPTC injector allows the introduction of a precise sample volume onto the column. A typical manual injector consists of a 6-port valve with a rotor, a sample loop and a needle port (Eigure 9). A sample solution is introduced into the sample loop using a 22-gauge blunt tip syringe in the TOAD position. The sample is then injected into the column by switching the valve to INJECT. The typical external sample loop size ranges from 6 pT to 2 mT. For many years, the Rheodyne 7125 injector was the industry-standard. In the early 1990s, it was replaced by the Rheodyne 7725 injector, which injects samples without momentary flow disruptions. ... 

An injector valve operates in two modes— the fixed-loop mode or the partial-loop mode. In the fixed-loop mode, a sample is overfilled into the loop at 2-4 times the loop volume and the entire loop content is injected. In the partial-loop fill mode, a variable sample aliquot, measured precisely by a syringe at <50% of the loop volume, is injected. Note that the sample slug is introduced into the end of the sample loop and is back flushed onto the column to minimize band dispersion by the sample loop (Figure 9). Due to the emphasis on productivity, manual injectors are seldom used in the pharmaceutical laboratory except for preparative applications. 

High-performance liquid chromatograph (HPLC e.g., Waters Chromatography) equipped with column heater, solvent pump, UV detector (set at 210 nm), integrator, autosampler, and (for manual injection) a 10-pl sample loop 15 x 0.46-cm YMC-ODS-AQ analytical column (AQ12S031546WT, Waters Chromatography)... 

The sample loop size can be varied depending on the volume of sample. Typical sample loops range from 5 /A to 5 ml, and each must be manually removed before another can be put in place. The replacement of one loop size for another is one of the major disadvantages of the injector design. The accuracy of this type of injector varies with sample loop size, from about 5% for a 2-ml loop to as much as 30% for a 5-pl loop. 

Good analytical precision is achieved with these valves, which can be manually, electrically or pneumatically operated. Moreover, they are capable of long-term, maintenance-free operation. The selected volume to be introduced is dependent on the length and inner diameter of the sampling loop, which can be readily changed. 

In early applications [2], a by-pass transmission line was incorporated in the valve in order to avoid disruption in the manifold during the time interval needed for the sampling loop to be displaced from the load to the inject position. With currently available valves and commutators, this bypass line is no longer necessary, even if manual sample insertion is required. 

Loop-based injection is used for sample insertion and a similar procedure is used for displacing the tubular detector D between monitoring sites a and b (Figs 7.9 and 7.10, lower). Consequently, this strategy has usually been accomplished using two loop-based injectors or a double injection valve/commutator [81], where a flow cell with two small transmission lines replaces one of the sampling loops. Although the flow system can be manually operated, computer-controlled valves or commutators have been preferred. 

Samples can be introduced manually into the valve with a syringe to fill the sample loop. Automated sampling valves are routinely used today in which samples are taken from an autosampler for unattended operation. The major limitation of valve injectors is that the sample size is fixed, and the loop must be changed in order to vary the injected sample size. There are automated motor-driven adjustable syringes that provide enough pressure to inject the sample past a check valve that prevents backflow. 

Samples can be injected via a sample loop using a valve (Figure 3.12). Once the sample has been introduced into the loop, the valve is switched (manually or by electric actuation) to deliver the sample plug onto the analytical column. Autosamplers allow more precise injection. In this case, a carousel or tray of sample vials can be left unattended for automatic injection (by compressed air or nitrogen) at predetermined intervals (Figure 3.13). The volumes injected are usually in the range 5-500)tL but lower and higher volumes can be injected under certain conditions if required. 

As with HPLC, samples can be injected manually by syringe, via a sample loop or utilising autosamplers. 

Sampling Automation. In the terminology given in the first part of the chapter, instruments that emulate manual sample handling and processing without the use of control loops are automatic (mechanized), but not automated. Among the automatic functions are ... 

The HPLC analysis was performed on an Agilent 1100 series LC (Santa Clara, CA), equipped with a quaternary pump (G1311A) and a UV multiwave length detector (G1365B), set at 220 and 280nm. The injector was a Rheodyne manual injector valve, model 77251 (Santa Clara, CA), equipped with a 20-1 sample loop. The column temperature was controlled using a ThermaSphere TS-130 oven from Phenomenex set at 40 °C. 

Sample loop calibrators are commonly used for low concentration analytes that are difficult to prepare or are unstable in gas bottles. They are also extremely flexible, in that new compounds can be calibrated upon acquisition of a sample of the pure compound in liquid form. Sample loops are basically small heated known-volume vessels into which a small amount of liquid standard is injecSecL The sample vaporizes, and the entire volume is then pumped past the inlet, into which a small amount of the vapor is drawn for calibration. Drawbacks of sample loops include the lack of automation for unattended calibration, and inaccuracies inherent in making manual injections of nanoliter-scale liquid volumes. For ambient air monitoring of toxic compounds, one should bear in mind the safety imphcations of handling syringes that contain potentially hazardous analytes. 

Analytes are introduced into GC colmtms with several techniques. An ahquot of a relatively concentrated vapor or air sample— for example, from a plastic bag or a canister—can be introduced into a short section of tubing of known volume, called a sample loop, and subsequently pinged with carrier gas into the GC colurmt Volatile analytes in ambient air samples in a canister or trapped on a solid phase adsorbent are usually concentrated and focused in a cryogenic trap or a secondary adsorbent trap, then thermally vaporized into the GC carrier gas stream. However, in some analytical methods, volatiles trapped on an absorbent are thermally desorbed directly into the GC colunm. Aliquots of organic solvent extracts from various aqueous and solid samples are usually injected with a syringe into the carrier gas stream in a heated injection port. Both manual and antomated syringe injection systems (autoinjectors) are used and the latter are generally very reliable, precise, and have the capacity to process many samples unattended. 

The sample injector The sample injector can be a simple septum injector and a hypodermic syringe that can place the sample directly on to the column or a high-pressure sample valve (that can be manually or mechanically operated) fitted with an appropriate sample loop. 

Figure 2.4 (a) Diagram of a six-port loop injector being filled by injection of the analyte solution from a syringe. In this mode the loop is bypassed and the mobile phase flows directly from pump to column through the injector valve via ports 2 and 3. In order to inject the sample in the loop onto the column the valve is rotated so that internal connections are now made between ports 1 and 2, and between ports 3 and 4. Reproduced from literature of Rheodyne Corporation, Technical Note 5 (2001), Achieving Accuracy and Precision with Rheodyne Manual Sample Injectors, with permission (www.rheodyne.com). 

---