This question split's in three parts

1) can You get those compounds in the gas phase

2) can You get them separated out of your column

3) detectable with FID

1): Choosing the right sampling and preparation technique helps a lot. With high injection temperatures or derivatisation it is possible to push the boundary's but the higher the BP the more problems

2):The kind of separation you need will determine the column you use. each column has a maximum operating temperature so you need to get your compounds separated and out of the column before you reach this temperature. Not all columns can be used up to 250°C. Derivatisation my help

3): All compounds witch are ionized in the flame (burnable) will be detected. CCL4 has a low BP but is difficult to detect Dodecane on the other hand yields a huge response. The use of other detectors (like ECD) is possible

No, they have to be volatile too. It is not a matter of boiling point. For example, methyl searate (FAME) has a boiling point of 442 - 443 C and is readily analysed by GC-FID. It is more a matter of vapor pressure of the compounds. FOr FID, of course your compounds must bear carbons ...

Kind regards,

It also depends on the column phase, carrier gas flow etc.: if the retention time of a compound is short enough to overlap with the solvent peak, you won't see it.If separation is not good enough, again you will not be able to distinguish the overlapping compounds. Then, you won't be able to analyze thermolabile compounds even if their boiling point is below 250°C. On the cntrary, certain substances boiling at higher t° could still be analysed with GC after derivatization (e.g. in form of TMS derivatives). It is good idea to make certin what column exactly you have and to consult teh manufacturer's (online) guide/catalog.

This question split's in three parts

1) can You get those compounds in the gas phase

2) can You get them separated out of your column

3) detectable with FID

1): Choosing the right sampling and preparation technique helps a lot. With high injection temperatures or derivatisation it is possible to push the boundary's but the higher the BP the more problems

2):The kind of separation you need will determine the column you use. each column has a maximum operating temperature so you need to get your compounds separated and out of the column before you reach this temperature. Not all columns can be used up to 250°C. Derivatisation my help

3): All compounds witch are ionized in the flame (burnable) will be detected. CCL4 has a low BP but is difficult to detect Dodecane on the other hand yields a huge response. The use of other detectors (like ECD) is possible

Dear Vijay,

FID detector of GC can analyse volatile liquids and volatile liquids are"acetone,ammonia,dichloromethane,diethylether,alcohols etc compounds that have low boiling point and high vapour pressure at constant temperature."

Regards

Amir

Another guide is to check the boiling point of the compound If it decomposed after certain temperature before boiling, then it cannot be analysed by GC unless they can be derivatised. Normally compounds like that will be written as follows: bp 249 (dec). This means it decomposed at 249C.

Water and carbon disulphide are not detected by FID. The sensitivity is related to how many carbon atoms are in the molecule as compared to oxygen and the presence of chlorine sensitizes the detection by forming ions. That is the key to the answer - how many ions are formed on combustion.

As you have probably gathered, the simple answer is NO, you cannot detect all compounds that boil below 250C by GC-FID.

A better answer is that you can detect most compounds that boil below 250C by GC-FID. As Kris said, there are three main things to worry about:

1-can you get the analyte into the column?

2-can you get the analyte to move down the column (without destroying it and while getting a nice, peak)?

3-will the analyte give a good response?

Another thing that you must consider is how to identify your analyte with only FID detection. If you have a relatively simple mixture with well-resolved peaks, reproducible injection timing, and good control of pressure/temperature, this will not be a problem as the retention times of your compounds will be reproducible and there won't be any other analytes nearby to confuse you during data processing.

However, if you have a complex mixture with peaks that elute very closely and/or overlap extensively, or if you have a matrix that is highly variable (extra peaks from your matrix that are not your analytes of interest show up frequently in a variety of unpredictable locations in the chromatogram) then you will be doomed to uncertainty with just an FID as you will not be able to say for certain that a peak is due to one and only one compound in your mixture.

Some information about the analysis that you are trying to do would go a long way towards getting truly helpful answers.

PS- Thank you for providing ideas for exam questions! :)

Compound boiling points are not always good indicators of volatility. There are many high boiling compounds that can be analyzed by GC. As a general rule, the greater the molecular weight or polarity of a compound, the lower its volatility. Both factors have to be considered. a compound to be suitable for GC analysis, it must possess appreciable volatility at temperatures below 350–400 °C. In other words, all or a portion of the compound molecules have to be in the gaseous or vapor state below 350–400 °C. Another characteristic is the compound must be able to withstand high temperatures andbe rapidly transformed into a vapor without degradation or reacting with other compounds. There are no realistic, absolute guidelines that can be used to determine whether a compound can be analyzed by GC. Overall, it has been estimated that only about 10% of all compounds can be analyzed by GC.

The presence of polar functionalities such as hydroxyl and amine groups decrease compound volatility. Molecules such as sugars and amino acids can not be easily analyzed by GC because of the large number of polar groups. Inorganic compounds are not suitable for GC analysis. But most organic compounds are suitable for GC analysis.

Anything that will burn other than hydrogen will be detected. Even fluorocarbons can be detected

Requirements for analysing compounds in GC are the following: possibility to have substance in vapour phase and stability of compounds in this state for all the analysis in capillary column. For example, it is not possbile to analyse by GC carbohydrates because they are not stable in vapour phase, but it is possible to derivatise them as acetals and analyse derivatives by GC. Moreover you can analyse by GC-FID only compounds containing carbon atoms.

You have to derivatise your compound to make products which have lower boiling point.

All compound below 250 deg C can be detected by GC. We have to select non polar capillary columns for it. These column can operate upto 300 deg C. So higher boiling point compounds are thus eluted from it and will detected by detectors.

Why not? At a higher temperature, your analytes may be degraded. You need to check on the stability of the analytes by using standards. Usually, the GC column can withstand up to 330C (depending on the type). If you use a higher temperature, your column will wear out very fast..

First of all the ultimate criteria for compounds to be determined by GC is that they are volatile without any decomposition!

So the boiling point alone is not a good criteria to judge if you can bring a substance over the GC or not. Polarity is another important factor: the more polar your substance is the more likely it will adsorb mainly in the injection system or on the column. So check the condition of your liner because if your inlet is not clean many substances (acids, alcohol, thiols, amines....) will adsorb on a non deactivated surface.

The second story is the detector: by theory a FID needs -CH fragments to form the electrons during combustion NOT only carbon (it wont work with CCl4) and NOT only hydrogen (you can´t see water either).

Not necessarily. Derivatization may help you run compounds which are les volatile and a high temp may decompose you analyte into a more volatile species (dehyration for example)

Adding to my previous post, if your compounds are volatile, then go for a GC. Now, what matters is the injector temperature. Make sure the one you choose is sufficient for your compounds and yet it will not 'change' your compounds. How to do? Flip through the relevant studies and see what temperature they used. Or, run your standards on the GC and optimize it. If the compounds are not new to chemistry, please do not start your work from scratch. Literature helps you save your time and cost. Based on the previous reports, you can modify the conditions according to your needs. If the compounds are too large and heavy, then HPLC is a choice. Commonly, the analysts perform their trial-and-error with TLC to understand the basic conditions for the compounds. Then they optimize the conditions using LC. Please bear in mind, HPLC uses pressure while TLC uses gravity. To play on a safe side, LC can detect almost all compounds but this is very tedious and expensive.

I think same that BP is not the only criteria to be considered. There are other factors like polarity etc.

The second thing is choice of a column, not all compounds you could detect with FID whose boiling point is less than 250