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I am confused about what a Total Ion Chromatogram (TIC) means. The definition I have says:

The total ion current (TIC) chromatogram represents the summed intensity across the entire range of masses being detected at every point in the analysis.

Does this mean it is a (mixture) sample injected into a MS without the preceding chromatographic separation (as is typical in, say, a GC-MS or an LC-MS)? Are there MS instruments sold today that are not GC-MS's or LC-MS's etc. (i.e. mass spec. sans the preceding seperation?

What's the point in doing this? Wouldn't the resultant spectrum be very noisy since it's essentially the sum over all components of a mixture of unknown proportions.

Is the GC eliminated for speed of analysis?

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    $\begingroup$ Not a full answer but yes, there are many applications of MS that don't involve a GC system. The more obvious like HPLCMS and LCMS but also just MS systems alone. $\endgroup$ – NotEvans. Dec 31 '16 at 13:10
  • $\begingroup$ @NotCorey LCMS / HPLCMS are analogous to a GCMS. You chromatographically separate peaks & then use the MS on essentially pure component peaks. Direct MS of a non-pure sample seems different. $\endgroup$ – curious_cat Dec 31 '16 at 13:38
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Does this mean it is a (mixture) sample injected into a MS without the preceding chromatographic separation (as is typical in, say, a GC-MS or an LC-MS)?

No. A chromatogram is the measurement of something after separation by chromatography. So you got chromatography beforehand. In case of direct injection into MS the term is usually used too, if you record over time like you would do using chromatography.

What's the point in doing this?

Ok so let's imagine I'm running a sample of an unknown mixture and I'm recording m/z 100-1000. Now I don't know at all what to expect, so selecting randomly a specific mass for my chromatogram would, most likely, lead to nothing. What we can do is just integrate over the whole mass range and take the total number of all ions, so we see a peak this means there are more ions than for baseline noise. That should correspond to a substance reaching the detector.

Wouldn't the resultant spectrum be very noisy since it's essentially the sum over all components of a mixture of unknown proportions.

Depending on the method: yes that might be a problem. And if the noise is very high then even a strong signal of a single m/z wouldn't really contribute to the total number of ions, so we won't see a peak in the TIC.

A way to circumvent this problem is using a base peak chromatogram. Base peak is the highest peak in your spectrum, and you are plotting the intensity of the highest peak over time. Now for noise there might be a large amount of total ions, but the intensity of each m/z is usually small. If you got a specific signal from a substance you will get much higher signals there, so the base peak chromatogram will show a peak.

What's the alternative?

Collecting data in TIC mode is an alternative to collecting data in SIM mode. SIM stands for "single ion monitoring" or "selected ion monitoring". Let's say that you know your sample contains acetophenone, and the four largest peaks for acetophenone are at 120 (molecular ion), 105 (base), 77, and 51 m/z. Instead of integrating over all ions, you could monitor only those masses (or only one of them). You would know where the acetophenone eluted and could quantify it relative to a standard, but you would know little about the rest of the sample.

By analogy, consider a LC system with a UV/Vis photodiode array detector. The detector can monitor single wavelengths or it can capture a full absorbance spectrum at each time interval. A mass spectrometer collecting data in TIC mode is like a PDA detector collecting a full spectrum. A mass spectrometer collecting data in SIM mode is like a PDA monitoring individual wavelengths.

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  • $\begingroup$ Great answer! Thanks. Can you elaborate more about the bit on "direct injection into MS if you record over time " When is this done? I thought the time dimension only comes from the GCMS or a TOF sensor. When would one record a directly injected MS over time? $\endgroup$ – curious_cat Jan 1 '17 at 8:10
  • $\begingroup$ Time doesn't stop, even when the injection port is directly connected (with no column) to the mass spectrometer. "Flow injection analysis" or FIA is a fancy name that analytical chemists use when they inject a small volume of fluid from a sample into an analyzer. The window of time in which the sample is actually going into the analyzer (e.g. the mass spec) is finite, and furthermore varies according to the flow rate and dead volume of the instrument's tubing. The easiest solution: record spectral data over a long period of time, wider than when the sample goes in. $\endgroup$ – Curt F. Jan 2 '17 at 2:45
  • $\begingroup$ @curious_cat in case of MS systems used for liquid chromatography (HPLC most of the case), for example ESI-MS systems you can just inject an analyte solution by using a syringe. Since you need very little volume over time you usually use a syringe pump to do so. In this case the solution going into the MS doesn't change over time, like it would in case of chromatography, but you would still get a "chromatogram" with measurement over time. $\endgroup$ – DSVA Jan 2 '17 at 8:26

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