Splitting injectors

Figure 14.12 Schematic diagram of the Refomiulyser system Inj, split injector Cl, polar capillary column C2, packed column to retain the alcohols C3, packed Porapak column for the separation of the oxygenates C4, non-polar capillary column C5, packed 13X column A/E cap, Tenax trap to retain the ar omatics Olf. trap, cap to retain the olefins Pt, olefins hydrogenatOT A cap, cap to retain the -alkanes FID, flame-ionization detector.

Split Injection2 In the split injector, the injected sample is vaporized into the stream of carrier gas, and a portion of the sample and solvent, if any, is directed onto the head of the GC column. The remainder of the sample is vented. Typical split ratios range from 10 1 to 100 1 and can be calculated from the equation ... 

Figure 6.11 Schenatic diagran of a split/splitless valve injector (A) and a timed-split injector (B) for open tubular column SFC.

The chromatographic procedure was carried out using a 0.53 mm x 30 m fused silica analytical column coated with 1.0 p,m Supelcowax 10 stationary phase, a flame-ionization detector, and a split injector (1 30). The carrier gas was helium at a flow rate of 3 mL/min. The temperature of the injection port and detector was set up at 250 and 280°C, respectively. The column temperature program was programmed as follows ... 

GC equipped with a split injector, flame ionization detector or mass selective detector, and suitable chromatography data station... 

The first experimental investigation and performance demonstration of an integrated liquid chromatography chip was carried out by Ocvirk et al. [84]. The device is shown schematically in Fig. 15 and comprises a split injector, a smallbore separation column, a frit, and a detector cell, all integrated in a monolithic manner. An electron micrograph of the silicon chip is also depicted in Fig. 15. The whole device was composed of two 350 pm Si chips and a 50 pm interme-... 

Fig. 15. Schematic layout of the HPLC chip a layout of the channel system, b cross section along the channel axis, c cross section along the detector cell axis. IS sample and mobile phase inlet, S split injector, OS outlet for rejected sample and mobile phase, C separation channel, F frit, D optical detector cell, OD outlet to waste, P positioning grooves for optical fibers (reprinted with permission from [84]. Copyright 1995 John Wiley)...

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. 

Ideally, a sample is introduced into a chromatograph as a perfect plug. In practice, this is not the case, and diffusion occurs because of the injector. For narrow-bore and microbore applications, injectors capable of introducing the required sample volumes are commercially available and optimized to reduce dispersion. This is not the case for capillary LC, and homemade injection systems include the sample tube technique, in-column injection, stopped-flow injection, pressure pulse-driven stopped-flow injection (PSI), groove injection, split injection, heart-cut injection, and the moving injection technique (MIT). Of the injection techniques, only the split injector, MIT and PSI approaches can introduce subnanoliter sample volumes accu-... 

Procedure (See Chromatography, Appendix IIA) Use a gas chromatograph equipped with a split injector and an electron-capture detector and fitted with a 50-m x 0.2-mm (id), fused-silica column (Carbowax 20M, or equivalent) coated with dimethylpolysiloxane, or equivalent. Use nitrogen as the carrier gas at a flow rate of 8 mL/min. Before use, precondition the column by heating it at 200° and the detector at 300° for 24 h. Set the injector temperature at 250° and the electron-capture detector at 300°, and program the column temperature as follows Maintain for 10 min at 115°, raise rapidly at 307 min to 200°, and maintain at 200° for 12 min. 

Apparatus (See Chromatography, Appendix IIA.) Use a suitable gas chromatograph (HP 6890, or equivalent) equipped with a split injector port, a flame-ionization detector (FID), and a 30-m x 0.25-mm (od) GC capillary column (DB-5MS, or equivalent) having a film thickness of 0.25 p,m. 

Injectors Sample injection devices range from simple syringes to fully programmable automatic injectors. The amount of sample that can be injected into a capillary column without overloading is small compared with the amount that can be injected into a packed column, and may be less than the smallest amount that can be manipulated satisfactorily by syringe. Capillary columns are therefore used with injectors able to split samples into two fractions, a small one that enters the column and a large one that goes to waste (split injector). Such injectors may also be used in a splitless mode for analyses of trace or minor components. 

Procedure Inject 1-pL aliquots of the Standard Preparation and the Sample Preparation into a gas chromatograph equipped with a split injector, a flame-ionization detector, and a 25 -m fused silica capillary column coated with a 2-pm film of 7% cyanopropyl-7% phenyl-85% methyl-1% vinylpolysiloxane (CP-Sil 19 CB, Chrompack Middelburg, or equivalent). Maintain the column at 100°, raising the temperature at 8°/min to a final temperature of 300°. Set the injector temperature to 270°and the detector temperature to 270°. Use a mixture of helium and methane, at a split ratio of 1 100, as the carrier gas, flowing at 120 mL/ min. Run the chromatogram for 27 min. 

Figure 4.14 Sample injection ports, (a) Rash-vaporizer, (b) Split injector with septum purge for capillary columns, (c) Direct -cold on-column injection onto a capillary column showing rotating valve and insertion of needle into the base of the column. (Reproduced by permission of Dr Alfred I liithig Verlag from J. High Res. i hromuiogr., Chromalogr. Contniun., 2. 35.X (1979).)...

Split/splitless injector in the split mode only a portion of the injected sample (typically, 1 part in 50) reaches the column. The rest is vented to waste. A split injector is used for concentrated samples (> 0.1 mgmL for FID see p. 215). In the splitless mode all the sample volume injected passes through to the column. It is used, in this mode, for trace samples (< 0.1mgmL for FID). 

The split/splitless detector has been designed for use with open-tubular columns or solid-coated open-tubular (SCOT) columns. Due to the small dimensions of such columns, they have very limited sample load capacity and, thus, for their effective use, require sample sizes that are practically impossible to inject directly. The split injector allows a relatively large sample (a sample size that is practical to inject with modern injection syringes) to be volatilized, and by means of a split-flow arrangement, a proportion of the sample is passed to the column while the remainder is passed to waste. A diagram of a split/splitless injector is shown in Fig. 1. 

The product is nearly 98% pure as determined by GC using a Carlo Erba 5160 Mega instrument, flame ionization detection, with H2 as the carrier gas (50 cmysec). The column was a 10-m x 200- im i.d. fused silica column coated at a 0.15-pm film thickness with SE-33 (methyl silicone). The split injector was kept at 300°C,... 

In addition to the availability of different methods of sample introduction into the column in gas chromatography, there are two different types of injectors that can be used to input the sample. These are the splitless injector and the split injector. 

However, for capillary columns, the amount of sample is very important. This is why there is the second type of injectors split injectors. With these injectors, it is possible to input just a fraction of the evaporated sample in the column. The other part of the evaporated sample can be pushed directly over a special split into the atmosphere. This type of injector is used only for capillary columns. 

The capillaries are 0.10 to 0.53 mm i.d., with lengths of 15 to 100 m and can have several hundred thousand plates, even a million. They are sold as coils of about 0.2 m diameter (Figure 20.4). Capillary columns offer advantages of high resolution with narrow peaks, short analysis time, and high sensitivity (with modem detectors) but are more easily overloaded by too much sample. Split injectors by and large alleviate the overload problem. 

---