Environmental Water Analysis: Expand Your Toolbox

Environmental analysis of water samples is often an underappreciated aspect of monitoring or processing infrastructure. It certainly isn’t easy: whether it’s checking for pollutants in a river or the quality of drinking water, water samples frequently have complicated, dirty matrices. They can foul up instruments, requiring downtime to clean ion sources, and often have heavy co-elution of analytes, making identification difficult at best. While many countries have regulations in place to maintain safe levels of various chemicals, no written list can contain every last signal that might be concerning. Where can you start if you don’t even know what you’re looking for?

Gas Chromatography (GC) coupled with Time-of-Flight Mass Spectrometry (TOFMS) is an excellent tool for water analyses. With TOFMS, the ions’ mass-to-charge ratio (m/z) is determined via time measurement. The first TOFMS was developed in the 1950s, by Dow, and as the technology has been refined, the popularity and usefulness of this technique has grown. With TOFMS, one sample run results in the full mass range data, with no compromises needed in acquisition rates. In addition, there is no spectral skewing with changes of analyte concentration, unlike with scanning mass spectrometers like quadrupoles.
这两个优点允许在非常窄的峰上收集到足够数量的高质量的质谱数据点，无论浓度如何，这对于发生准确可靠的反褶积至关重要。它允许目标分析和非目标筛选同时进行，从而能够进行额外的调查，也许可以监测其他化学品类型和水平的趋势和模式。因为所有的数据都已经收集了，这也允许建立一个历史记录，因此，如果一个新的污染物被发现，例如，以前的样本可以检查它什么时候开始出现。
图1。TOFMS和扫描质谱在整个GC峰宽上的比较。随着分析物浓度的变化，质谱与TOFMS保持一致。
TOFMS提供的不仅仅是高质量的数据:它还可以快速、廉价地提供高质量的数据。根据所选择的仪器，GC-TOFMS可以设置为以氢作为载气而不是氦气运行。在氦气价格上涨的情况下，这样做更划算，而且不需要太多的初始投资。
Method transfers can be handled easily with online tools, and by swapping out the column for a narrower, shorter one, the same sort of spectral resolution can be obtained in a fraction of the time.
Using a GC-TOFMS with an open ion source is another way to ease some strain on your analyses: an open ion source minimises the interaction of sample ions with the surfaces of the instrument, eliminating the need for cleaning.
This is especially useful when combined with hydrogen as the carrier gas, which is frequently seen as problematic for robustness and mass spectral continuity with typical ion source designs. An open ion source design in your instrument eliminates that concern entirely.
Figure 2. A comparison of chromatograms for the same sample run with helium and hydrogen carrier gases and a close-up of trichlorophenol with helium and hydrogen carrier gases. Chromatogram 1 shows helium carrier gas and a run time of more than 30 minutes with a resolution of 1.66. The library match confidence is 941. Chromatogram 2 shows the same sample run with a direct method transfer to hydrogen carrier gas. The run time has dropped to less than 10 minutes with a resolution of 1.58 and a library match confidence of 935. Chromatogram 3 uses the same sample with the method optimised for speed. The run time is now less than 8 minutes, and the resolution is 1.51.
Even with all the benefits of GC-TOFMS, sometimes it isn’t enough for challenging water matrices. This is where GCxGC comes into play as an additional tool for your laboratory. The original two-dimensional GC was called ‘Heart-Cut’ chromatography, where just a specific portion of the effluent from one column is diverted to a different column for a wide-peaked slice of additional information. However, true comprehensive GCxGC traps the entire effluent from the first column with a modulator, which then re-injects the entire sample into a secondary column for a huge increase in chromatographic peak capacity and increased sensitivity. Various modulators can change the level of results you can obtain, allowing you to customise your instrument to your budget, but in all cases, using GCxGC-TOFMS results in improved deconvolution and more confidence in your samples.
The tools of TOFMS don’t stop with comprehensive two-dimensional GCxGC analysis. As the name Time-of-Flight implies, the flight time is very important to TOFMS. The longer the ions’ flight path is, the higher the resolution of the spectral results can be. High Resolution TOFMS (HR-TOFMS) is actually possible with innovative techniques such as a folded flight path using gridless mirrors and periodic ion lenses to reflect the ions back and forth, leading to mass resolution modes of 25,000 or 50,000. HR-TOFMS can be done with both one-dimensional GC or comprehensive GCxGC analyses.

还有许多其他工具可以与TOFMS协同工作。多模式离子源允许离子模式切换，无需手动调整仪器，电子电离(EI)，正化学电离(PCI)和负化学电离(ECNI)模式使您比以往任何时候都更方便地获得完整的化学剖面。这对于非目标筛选和环境分析(如硫丹)尤其有用，无论用EI模式还是ECNI模式分析，硫丹的反应都非常不同。
The technology isn’t limited to just the hardware. TOFMS software has developed immensely since those early 1950s days, with automated tools allowing for confident deconvolution, library matching, and data set comparison. Data can be outputted to a variety of plots and charts and heat maps to enable you to zero in on the areas of your data that are the most interesting without spending significant time just trying to find those areas in the first place.
When all you have is a hammer, everything looks like a nail. Make sure you have more tools at hand for the most efficient and thorough environmental water analysis, so you aren’t just brute-forcing your samples to give you results.