Injection in splitless mode

Injection in the splitless mode is used to introduce analytes into a diluted solution. The valve is closed generally for 1 to 2 min after the injection to allow a maximum amount of analyte to enter the column. The valve is then opened to purge the injector of any possible residues. The sample is injected at such a temperature that the solvent and solutes are instantly vaporized in the glass insert. 

During the injection, the temperature of the oven is 20 to 30°C below the solvent boiling point in order to condense it and to trap the molecules in the column header. First, the solvent plays the role of stationary phase in relation to the different constituents of the mixture. The solvent polarity must therefore be compatible with that of the stationary phase for the solvent to spread homogeneously in the column header. Because of its high retention power, this condensed phase allows the slowing of the volatile molecules until they are swept by the carrier gas. 

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Derivatization was conducted by the addition of a 10% H-ethyl-diiso-propylethylamine solution and a-bromo-2,3,4,5,6-pentafluorotoluene. Sample obtained from the derivatization procedure were dissolved in ethyl acetate prior to injection in splitless mode using a DB-1 capillary column. Helium was used as the mobile phase, and the injector temperature was set at 290 °C with a transfer line temperature of 270 °C. Sample detection used ion trap MS for detection, with the detector being set at negative chemical ionization with m/z = 262 (for CCA) and m/z = 286 (for the internal standard). The limit of quantitation was 5 ng/ml, and the average recovery ranged from 92.0% to 114%. In addition, the extraction efficiency ranged from 48.2% to 55.6% for concentrations of 5, 50, and 250 ng/ml. Samples were reported to be stable for up to 6 months when stored at 18 °C. 

The analyses was performed with a 30m x 0.25 mm fused silica capillary column coated with a 0.25 pm thick film (DBS J W Scientific). Helium was used as carrier gas for analyte separation. An aliquot of 1 pi was injected in splitless mode. The temperature of the column was held at 70 C for 1 min, rate 7 C min to 310 C for 5 min. 

For GC/MS analysis of 3-MH and 3-MHA, a fused-silica apolar column (10m x 0.32mm i.d. 0.25-gm film thickness) connected to the PEG-like polar column can be used with the following operative conditions carrier gas He (flow rate 1.2mL/min), transfer line temperature 220 °C, MS source temperature 150 °C, GC injector temperature 250 °C, injection in splitless mode, oven temperature program from a 35°C isotherm for 5 min, l°C/min to 40 °C, 10°C/min to 250°C. 

The extracts are analysed using a single quadrupole GC-MS system. As analytical column a 12 m short non polar capillary column with 0.2 mm ID and a thick film of 0.33 pm has been used. One microlitre of the extract has been injected in splitless mode. 

The residue levels of 46 pesticides, including oxyfluorfen in soil, were determined using GC/ITDMS as described in S ection 3.2.1. The conditions for GC/ITDMS were as follows column, fused-silica capillary (30 m x 0.25-mm-i.d.) with a0.25- am bonded phase ofDB-5 column temperature, 50 °C (1 min), 30 °Cmin to 130 °C, 5 °C min to 270 °C inlet and transfer temperature, 270 and 220 °C, respectively He gas with column head pressure, 12psi injection method, splitless mode. The retention time and quantitation ion of oxyfluorfen were 23.9 min and mjz 252, respectively. ... 

In the above-mentioned method by Hodgeson el acifluorfen was determined by GC/ECD after the extraction with a 47-mm PS-DVB disk and derivatization with diazomethane. The conditions for GC/ECD were as follows column, DB-5 fused silica (30 m X 0.32-mm i.d., 0.25-p.m film thickness) He carrier gas velocity, 25 cm s (210°C), detector makeup gas, methane-argon (5 95), 30mLmin column temperature, 50 °C (5 min), 10°Cmin to 210 °C (5 min) and to 230 °C (10 min) injection port and detector temperature, 220 and 300 °C, respectively injection method, splitless mode. The recovery of acifluorfen from purified water, dechlorinated tap water and high humic content surface water fortified at 0.5-2.0 ug was 59-150% and the LOD was 25 ngL Acifluorfen after derivatization with various chlorofor-mates was also determined by GC/MS using an SE-54 column (25 m x 0.20-mm i.d., 0.32- um Aim thickness), and the average recovery of acifluorfen fortified between 0.05 and 0.2 ugL was 78%. ... 

We use a GC Top 8000 gas chromatograph coupled with a PolarisQ ion-trap mass spectrometer and equipped with an AI3000S autosampler (Thermofinnigan www. thermo.com). The steroids are separated on a DB-1 crosslinked methyl-silicone column, 15 mx 0.25 mm i.d., film thickness 0.25 pm (J W Scientific marketed by Agilent). Helium is used as a carrier gas at a constant pressure of about 35 kPa. A 1-pl aliquot of the final derivatized extract is injected into the system operated in splitless mode (valve opened at 2 min). The GC temperature program is the same described before for the quadrupole GC-MS system. The injector and transfer lines are kept at 260°C and 280°C, respectively. The ion source temperature is 225°C. A damping gas flow of helium is applied to the ion trap. 

Figure 5.2.1. Simplified diagram of a Py-GC system (not to scale). The pyrolyser is schematized as a heated filament type. A piece of a deactivated fused silica line is passed through the injection port of the GC and goes directly into the pyrolyser. This piece of fused silica is connected to the column, which is put in the GC oven. The pneumatic system consists of (1) a mass flow controller, (2) an electronic flow sensor, (3) a solenoid valve, (4) a backpressure regulator, (5) a pressure gauge, and (6) septum purge controller. The connection (7) is closed when working in Py-GC mode, and connection (8) is open. (Connection (7) is open when the system works as a GC only.) Connection (9) is closed and connection (10) is open when the GC works in splitless mode (purge off). Connection (10) is closed and connection (9) is open when the GC works in split mode (purge on). No details on the GC oven or on the detector are given.

Researchers should therefore study the performance of the derivatization reactions used with their samples to avoid the possibility that biases are introduced due to the derivatization reactions (QC samples of the type employed in LC-MS can provide much useful data in this respect) (44). Another issue is the need for extensive column cleanup. In most cases, injection occurs in splitless mode. Introduction of excess derivatization reagents may damage the injection liner and the analytical column. Frequent replacement of the liner is thus necessary. Cleanup of the column can be done by back-flush (when this feature is available) or by application of high temperatures (bake-out). 

Figure 2.5 Injectors, (a) Above left, injection chamber. The carrier gas enters the chamber and can leave by three routes (when the injector is in split mode). A proportion of carrier gas (1) flows upward and purges the septum, another (2) exits through the split outlet (a needle valve regulates the split) and finally a proportion passes onto the column, (b) Above right, cold injection onto the column, (c) Below, a typical chromatogram obtained in splitless mode. For solvent peaks which are superimposed upon those of the compounds, a selective detector which does not see the solvent is recommended.

A split/splitless injector is more frequently used than on-column injection. When this injection system is used in splitless mode, the initial column temperature should be at least 20°C below the boiling point of the solvent and the injection temperature should be over 200°C. To increase reproducibility, the sample is injected slowly by setting a high carrier gas flow rate during the injection time. On-column injection is also often used, in which case the initial temperature is 10°C to 15°C lower than the boiling point of the solvent. 

Injector SpUt/splitless operated in splitless mode. The liner SPME injection sleeve 0.75 mm i.d. (Supelco) is recommended for SPME injection. 

Withdraw 0.5 mL of the gas phase by using 1-mL gaslight syringe, and inject this volume in GC/MS in splitless mode equipped with the specific liner for syringe injections. 

The extracted degradation products were identified and quantified by a ThermoFinnigan GCQ (San Jose, CA, USA). The column used a wall-coated open tubular (WCOT) fused silica CP-WAX 58 (FFAP)-CB column from Varian (25 m x 0.32 mm id, od 0.2 pm). Helium of scientific grade purity from AGA (Stockholm, Sweden) was used as carrier gas at the constant velocity of 40 cm/s. The initial oven temperature was 40 "C, which was held for 1 minute. The oven was heated to 250 C for 15 minutes. Electron impact mode (El) detection was used with an electron energy of 70 eV. The mass-range scanned was 35-400 m/z and the ion source and transfer line temperatures were 180 and 250 C, respectively. The injector temperature was set to 250 C. A sample (1 pi) was injected in splitless injection mode and two blanks were run between each sample by injecting clean methanol (0.15 M HCl). 

GC-MS was performed on a Hewlett-Packard 6890 GC wifli 7683 autosampler and 5973 mass-selective detector (Agilent Technologies, Palo Alto, CA). Electron impact (El) MS was obtained at 70 eV. A split / splidess injector was used in splitless mode with a purge flow to split at 2.0 min after injection. Chromatograms were run at a constant flow of 0.5 mL /min of He gas. TTie inlet tenq>erature was set at 250 C. An HP-5MS (5% diphenyl-95% dimethylsiloxane) capillary column (30 m x 0.25 mm, 0.25 pm < ) was used with temperature programming from 60 C (1 min hold) to 300 C at 10 C/min witii a al 10 min hold. Solvent samples (1 pL) were injected by the autosampler. SPME sanqrles were manually injected by insertion of the fiber into the mass spectrometer inlet until after the purge flow to split occurred. Mass spectra were recorded from m/z 40 to m/z 550. 

Figure 2S7 Hot needle thermo spray injection technique (in splitless mode).

For GC, the injector is most frequently a small heated space attached to the start of the column. A sample of the mixture to be analyzed is injected into this space by use of a syringe, which pierces a rubber septum. The injector needs to be hot enough to immediately vaporize the sample, which is then swept onto the head of the column by the mobile gas phase. Generally, the injector is kept at a temperature 50 C higher than is the column oven. Variants on this principle are in use, in particular the split/splitless injector. This injector can be used in a splitless mode, in which the entire injected sample goes onto the column, or in a split mode, in which only part of the sample goes onto the column, the remainder vented to atmosphere. For other less usual forms of injector, a specialist book on GC should be consulted. 

For capillary GC, the split/splitless inlet is by far the most common and provides an excellent injection device for most routine applications. For specialized applications, there are several additional inlets available. These include programmed temperature vaporization (PTV) cool on-column and, for packed columns, direct injection. PTV is essentially a split/splitless inlet that has low thermal mass and a heater allowing rapid heating and cooling. Cool injection, which can be performed in both split and splitless mode with the PTV inlet, reduces the possibility of sample degradation in the inlet. Capabilities of the commonly available inlets are summarized in Table 14.3. 

One method that prevents overloading of narrow-bore capillary columns is split injection. The inlets are usually bimodal split/splitless inlets, and either mode can be selected for a given analytical method. In the split mode, the sample is rapidly vaporized in the inlet and a portion is introduced into the column in a narrow band with carrier gas, while the rest of the sample is vented (Dybowski and Kaiser, 2002). The amount introduced can vary for each method and is chosen as a ratio, e.g., 1 100. Easily vaporized compounds may preferentially vent, leading to the introduction of a nonrepresentative sample to the column (Watson, 1999). When the splitless mode is chosen, the entire sample is introduced into the column and the vent is opened after a predetermined period of time, to flush the excess solvent from the injector (Dybowski and Kaiser, 2002). 

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. 

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