Handbook of Food Analysis Instruments

More documents

Recommendations

Info

TABLE 7.1 Expan sion Vol ume of So lvents Used in GC Solvent 1 mL at 2508C and 69 kPa (10 psig) To overcome, or at least partly compensate for these problems, pulsed splitless injection can be applied. Increased column head pressure for a short period during the sample injection (usually 1–2min) leads to a higher carrier gas flow rate through the injector (8–9 versus0.5–1 mL=min during classical splitless injection), thus faster transport of sample vapors onto the GC column. In this way, the residence time of analytes and, consequently, their interaction with active sites in the GC inlet is fairly reduced [3]. The detection limits of troublesome compounds obtained with pulsed splitless injection are thus lower and their further improvement can be obtained by injection of higher sample volumes (for most liners up to 5 mL) without the risk of backflash (Table 7.1) [4]. It should be noted that for injections > 1–2 mL, a retention gap prior to the analytical column is generally required to avoid excessive contamination of separation column and peak distortion (Figure 7.3). 7.2.2 C OLD ON-COLUMN INJECTION Expansion Volume (mL) 1 mL at 2508C and 345 kPa (50 psig) 5 mL at 2508C and 345 kPa (50 psig) Water 1414 540 2700 Methanol 631 241 1205 Acetonitrile 487 186 929 Acetone 347 133 663 Ethyl acetate 261 100 498 Toluene 241 92 460 Hexane 195 75 373 Isooctane 155 59 295 Source: From Hewlett-Packard FlowCalc 2.0 software. Available at http:==www.chem.agilent. com=cag=servsup=usersoft=files=GCFC.htm via the Internet. Accessed July 1, 2007. Note: Calculated using Hewlett-Packard FlowCalc 2.0 software. In COC injectio n, a sample aliq uot is directly introduced by a speci al syri nge onto the analyt ical colum n or a reten tion gap at tem peratures lower (608 C –80 8C) than those typicall y used in hot Pulsed splitless Pulsed splitless 3 µL 4 µL 2 µL 1 µL Splitless 1 µL (A) (B) FIGURE 7.3 Peak shapes obtained by pulsed splitless injections of different volumes of standard solution onto the GC column (A) without a retention gap; (B) with an installed retention gap. (Reproduced from Godula, M., Hajslova, J., and Alterova, K., J. High Resolut. Chromatogr., 22, 395, 1999. With permission.) ß 2008 by Taylor & Francis Group, LLC. 5 µL 3 µL 2 µL 1 µL
split=splitless mode (2008C–3008C). COC is therefore expected to cause less thermal stress on analytes during the injection process. This low-temperature injection eliminates both syringe needle and inlet discrimination and is suitable namely for high-boiling analytes. On the other hand, the introduction of the entire sample (both analytes and matrix components co-isolated from food matrix) into the GC system is associated with increased demands for cleaning and maintenance when such complex samples as food is analyzed [5]. There are two alternative approaches available to perform on-column injection. 1. Small volume on-column injection: In this approach, a small volume of the sample (up to 1–2 mL) is injected onto the separation column, or preferably, in the case of dirty samples, onto a retention gap [6]. 2. Large volume on-column injection: In this mode, a large volume of the sample (up to 1000 mL) can be introduced into the GC system [7]. The bulk of solvent is usually eliminated via a special solvent vapor exit. Once the venting is finished, the solvent vapor exit is closed and analytes, together with remaining traces of solvent, are transferred onto the analytical column. However, a modification of the GC system is required in this case to include a large diameter retention gap (10–15 m 0.53 mm), connected to a retaining precolumn (3–5 m 0.32 mm) assisting in retention of volatile analytes. In addition, injection speed has to be slowed down to prevent flooding of the system during large volume injections (LVI) with injection speeds in LVI-COC of 20–300 mL=min as compared to LVI-PTV at 50–1500 mL=min [8]. 7.2.3 PROGRAMMABLE TEMPERATURE VAPORIZATION INJECTION A programmable temperature vaporization (PTV) injector represents the most versatile GC inlet offering significant reduction of most problems typically present when using hot vaporizing devices (splitless and=or cool on-column inlets) in trace analysis. The most important fact is that a PTV injector chamber is cool at the moment of injection. A rapid temperature increase, following withdrawal of the syringe from the inlet, allows efficient transfer of the volatile analytes onto the GC column while leaving behind nonvolatiles in the injection liner. With regard to these operational features, PTV is ideally suited for thermally labile analytes and samples with a wide boiling range (when needed, PTV operating temperature can be programmed even higher than the usual column temperature allowing injection of analytes that would not be vaporized through a classic split=splitless inlet). In addition eliminating a discrimination phenomenon and diminishing adverse affects of nonvolatile matrix deposits on the recovery of injected analytes, PTV enables to introduce large sample volumes (up to hundreds of microliters) into the GC system. No retention gaps or precolumns are needed for this purpose; instead of that, the liner size is increased. This feature makes PTV particularly suitable for trace analysis and also enables its online coupling with various enrichment and cleanup techniques such as automated solid-phase extraction (SPE) approaches. From practical point of view, PTV is compatible with any capillary GC column diameter including microbore columns. However, to attain optimal PTV performance in particular application, many parameters have to be optimized (e.g., initial and final injector temperature, inlet heating rate, venting time, flow and pressure, transfer time, injection volume, type of liner). Due to the inherent complexity of this inlet, method development might become on some occasions a rather demanding task. Despite that, the use of PTV in food analysis is rapidly growing. The paragraphs below describe two most commonly used PTV operation modes. 1. PTV splitless injection. The sample is introduced at a temperature below or close to the boiling point of the solvent. A split exit is closed during the sample evaporation and solvent vapors are vented via a GC column. PTV splitless injection can be employed for both LVI of up to 20 mL of sample and for conventional small volume injections [9]. The ß 2008 by Taylor & Francis Group, LLC.
Page 1 and 2: 7 CONTENTS Gas Chromatography in Fo
Page 3: Mass and concentration Splitless, d
Page 7 and 8: 7.2.4 DIRECT SAMPLE INTRODUCTION=DI
Page 9 and 10: TABLE 7.3 Characterization of Stati
Page 11 and 12: Intensity (A) Intensity (B) 50000 4
Page 13 and 14: 7.3.3. 1 GC GC Setup The hea rt of
Page 15 and 16: of stationar y phase focusing (com
Page 17 and 18: Abundance (A) 410 4 310 4 210 4 110
Page 19 and 20: and produce chromatogr ams co nsist
Page 21 and 22: into the GC syst em. Unfortu nately
Page 23 and 24: TABLE 7.7 Overview of Typical Condi
Page 25 and 26: 10. Grob, K. and Läubli, Th., Spli

Handbook of Food Analysis Instruments

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?