Sample Introduction Injector

There are several types of sample introduction systems available for GC analysis. These include gas sampling valves, split and splitless injectors, on-column injection systems, programmed-temperature injectors, and concentrating devices. The sample introduction device used depends on the application. 

Injector A common term for the method of sample introduction into a chromatographic system. 

Principles and Characteristics As mentioned already (Section 3.5.2) solid-phase microextraction involves the use of a micro-fibre which is exposed to the analyte(s) for a prespecified time. GC-MS is an ideal detector after SPME extraction/injection for both qualitative and quantitative analysis. For SPME-GC analysis, the fibre is forced into the chromatography capillary injector, where the entire extraction is desorbed. A high linear flow-rate of the carrier gas along the fibre is essential to ensure complete desorption of the analytes. Because no solvent is injected, and the analytes are rapidly desorbed on to the column, minimum detection limits are improved and resolution is maintained. Online coupling of conventional fibre-based SPME coupled with GC is now becoming routine. Automated SPME takes the sample directly from bottle to gas chromatograph. Split/splitless, on-column and PTV injection are compatible with SPME. SPME can also be used very effectively for sample introduction to fast GC systems, provided that a dedicated injector is used for this purpose [69,70],... 

As mentioned previously, introducing the sample to the flowing mobile phase at the head of the column is a special problem in HPLC due to the high pressure of the system and the fact that the liquid mobile phase may chemically attack a rubber septum. For these reasons, the use of the so-called loop injector is the most common method for sample introduction. 

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 whole atomizer may be water cooled to improve precision and increase the speed of analysis. The tube is positioned in place of the burner in an atomic absorption spectrometer, so that the light passes through it. Liquid samples (5-100 mm ) are placed in the furnace, via the injection hole in the centre, often using an autosampler but occasionally using a micro-pipette with a disposable, dart-like tip. Solid samples may also be introduced in some designs, this may be achieved using special graphite boats. The sample introduction step is usually the main source of imprecision and may also be a source of contamination. The precision is improved if an autosampler is used. These samplers have been of two types automatic injectors and a type in which the sample was nebulized into the furnace prior to atomization. This latter type was far less common. 

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.)...

Splitless injection is used when the sample is dilute and cannot be introduced into the GC system with stream splitting. In practice, the column temperature is set 10° to 30°C below the boiling point of the solvent at the time of injection. When sample is introduced into the injector inlet, vaporized solvent together with the FAME condense at the beginning of the column along with the carrier gas flow. The condensed solvent plus the stationary phase of the column forms a diluted stationary phase that traps the FAME in it. After the initial sample introduction period, the column temperature is raised to normal operating conditions, and chromatographic separation starts from there. 

Sample-loop injectors, although usually the most expensive, are generally applicable and give the best reproducibility. Their operation involves the introduction of die sample into a loop of a certain volume in a valve system. Hie valve system is then adjusted so that the sample in the loop is flushed on to the column by the mobile phase. The volume of sample loops varies from 1.0 jil to 2 ml. 

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). 

In one report, bidirectional FTP was achieved on a PMMA chip. A common terminating electrolyte (TE) was employed to achieve simultaneous cationic and anionic separations. Without a complex injector design, sample introduction was achieved hydrodynamically for FTP separation [638]. 

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. 

H.G.J. Mol, H.-G. Janssen and C.A. Cramers, Large volumes sample introduction using temperature programmable injectors implication of linear diameter, 7. High Resolut. Chromatogr., 18, 19-27 (1995). 

Solid-Phase Microextraction. Solid-phase microextraction (SPME), used as a sample introduction technique for high speed gc, utilizes small-diameter fused-silica fibers coated with polymeric stationary phase for sample extraction and concentration (33). The trapped analyte can be liberated by thermal desorption. By using a specially designed dedicated injector, the desorption process can be shortened to a fraction of a second, producing an injection band narrow enough for high speed gc. A modified system has been investigated for the analysis of volatile compounds listed in EPA Method 624. Separation of all 28 compounds by ion trap mass spectrometric detector is achieved in less than 150 seconds. 

The vast majority of ICP-based analyzes are performed on liquid samples that are introduced to the plasma in the form of an aerosol. In this case, sample introduction system consists of four parts (i) a nebulizer, which generates an aerosol (ii) a spray chamber, which filters the aerosol and transports it to the plasma (hi) a desolvation system to reduce the mass of solvent reaching the plasma (iv) an injector tube to introduce the aerosol into the plasma base. 

---