Trapping trace analysis

An important property of the MOT is the ability to catch atoms whose optical frequencies are shifted from the laser frequency by only a few natural linewidths. This property has been applied for ultrasensitive isotope trace analysis. Chen et al. (1999) developed the technique in order to detect a counted number of atoms of the radioactive isotopes Kr and Kr, with abundances 10 and 10 relative to the stable isotope Kr. The technique was called atom trap trace analysis (ATTA). At present, only the technique of accelerator mass spectrometry (AMS) has a detection sensitivity comparable to that of ATTA. Unlike the AMS technique based on a high-power cyclotron, the ATTA technique is much simpler and does not require a special operational environment. In the experiments by Chen et al. (1999), krypton gas was injected into a DC discharge volume, where the atoms were excited to a metastable level. 2D transverse laser cooling was used to collimate the atomic beam, and the Zee-man slowing technique was used to load the atoms into the MOT. With the specific laser frequency chosen for trapping the Kr or Kr isotope, only the chosen isotope could be trapped by the MOT. The experiment was able to detect a single trapped atom of an isotope, which remained in the MOT for about a second. 

Gas Chromatography. Gas chromatography is a technique utili2ed for separating volatile substances (or those that can be made volatile) between two phases, one of which is a gas. Purge-and-trap methods are frequently used for trace analysis. Various detectors have been employed in trace analysis, the most commonly used being flame ioni2ation and electron capture detectors. 

When columns of the same polarity are used, the elution order of components in GC are not changed and there is no need for trapping. However, when columns of different polarities are used trapping or heart-cutting must be employed. Trapping can be used in trace analysis for enrichment of samples by repetitive preseparation before the main separation is initiated and the total amount or part of a mixture can then be effectively and quantitatively transferred to a second column. The main considerations for a trap are that it should attain either very high or very low temperatures over a short period of time and be chemically inactive. The enrichment is usually carried out with a cold trap, plus an open vent after this, where the trace components are held within the trap and the excess carrier gas is vented. Then, in the re-injection mode the vent behind the trap is closed, the trap is heated and the trapped compounds can be rapidly flushed from the trap and introduced into the second column. Peak broadening and peak distortion, which could occur in the preseparation, are suppressed or eliminated by this re-injection procedure (18). 

Complex matrixes typically cannot be analysed directly to obtain the selectivity and sensitivity required for most trace analysis applications. To circumvent this problem, solid-phase micro extraction techniques were used to preconcentrate analytes selectively prior to gas chromatography/ion trap mass spectrometry analysis. 

Splitless Trace analysis (ppb) possible Cold trapping and solvent effects provide sharp peaks More complex than split Limited to temperature programming Several parameters to optimize Loss of low-volatility, labile analytes... 

Solid-phase microextraction (SPME) for preconcentration, followed by GC/ Ion Trap MS, was used for trace analysis of explosives and their metabolites in seawater [9]. NICI was used with methane as reagent gas. Compounds of interest included RDX, TNT and two of its metabofrtes 2-amino-4,6-dinitrotoluene (2ADNT) and 4-amino-2,6-dinitrotoluene (4ADNT). Although the instrument sensitivity was in low-ppb range, the detection limits for SPME with GC/ITMS... 

All of the techniques developed were amenable to the analysis of the concentrated compounds in collection devices (traps). Because these compounds were expected to be present in neat form in the traps, it was not anticipated that the sample preparation aspect of the analysis would pose any difficulty (i.e., it was assumed that any adsorption to glass walls, etc., that might occur would affect only a proportionately very small amount of the collected sample). As discussed later, this situation did not prove to be the case for some of the compounds studied. In addition, the difficulties inherent in trapping trace quantities of organics in the effluent C02 stream were not obvious during the early stages of this program. 

The linear ion trap (Fig. 15.6) is essentially a quadrupole detector with an electrically controlled ion lens at either end. It can trap a much larger volume of ions in its trap, allowing much higher sensitivity in fragment ion detection for trace analysis as well as MSn-type of experiments in which fragmentation ions can be trapped and further fragmented to aid in structure studies. 

Until recently, another clear advantage of GC/MS has been the lower limits of detection that are generally achievable for trace analysis. Examples are thiodiglycol, thiodiglycol sulfoxide, and methyl alkylphosphonic acids. GC capillary columns usually provide sharp narrow peaks, and techniques such as NICI in combination with fluorinated derivatives are inherently extremely sensitive. However, the latest generation of LC/MS/MS instruments, including some ion traps, triple sector quadrupole and TOF systems, are at least an order of magnitude more sensitive than their predecessors and may eventually provide equally low limits of detection. In terms of initial cost, they are still likely to be more expensive than GC/MS instruments. 

J. Riches, I. Morton, R.W. Read and R.M. Black, The trace analysis of alkyl alkylphosphonic acids in urine using gas chromatography-ion trap negative ion tandem mass spectrometry, 7. Chromatogr. B, accepted for publication. 

Oostdijk, J.P., Degenhardt, C.E., Trap, H.C., Langenberg, J.P. (2007). Selective and sensitive trace analysis of sulfur mustard with thermal desorption and two-dimensional gas chromatography-mass spectrometry. J. Chromatogr. A. 1150(1-2) 62-9. 

Barshick, S.-A. and W.H. Griest. 1998. Trace analysis of explosives in seawater using solid-phase microextraction and gas chromatography / ion trap mass spectrometry. Anal. Chem. 70 3015-3020. 

Joos, P. E., Godoi, A. F. L., De Jong, R., De Zeeuw, J., and Van Grieken, R., Trace analysis of benzene, toluene, ethylbenzene and xylene isomers in environmental samples by low-pressure gas chromatography-ion trap mass spectrometry, J. Chromatogr. A, 985(1-2), 191-196, 2003. 

Gonzalez-Rodriguez, J., Garrido-Frenich, A., Arrebola, F. J., and Martinez-Vidal, J. L., Evaluation of low pressure gas chromatography linked to ion-trap tandem mass spectrometry for the fast trace analysis of multiclass pesticide residues. Rapid Commun. Mass Spectrom., 16, 1216-1224, 2002. 

Purity of the carrier gas is very important in modern GC equipment designated for trace analysis. Consequently, it is essential that the gas purifiers, such as the traps containing various adsorbents, be inserted in the gas tine before the injection port. The same requirement usually applies for purification of the combustion gases for the flame ionization detector. The role of these adsorbent traps is to remove even the trace quantities of water, oxygen and organic impurities present in commercial gas cylinders, and thus minimize both the system contamination and chemical alteration of an injected sample. 

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