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nleslie321
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Advice on buying a GC/MS
So im hoping to get a GC/MS off ebay for around $2,500 to $4,000 seen some in working condition with pump etc for that price. im weighing up between a
GC/MS and an IR Spec at the same price. What are the pros and cons of each? I will be using them for identifying/confirming chemical intermediates,
impurities and finished products in synthesis in a home lab. Im only in the second year of my chemistry degree at uni and have not had much hands on
except for basic operation with either IR spec or GC/MS machines. I have 7 years of self taught lab experience also before starting study. im leaning
towards GC/MS as its the gold standard in identifying compounds but as its more complex than the IR Spec im worried it will be a lot more of a big
hassle if it breaks down rather than the IR spec.
Any guidance would be great.
[Edited on 18-5-2020 by nleslie321]
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Ubya
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go for GC/MS, if you want to know the compounds of a mixture that is the easiest way, with IR you just get a sum of all the active groups of your
sample.
after a synthesis if you want to check for example the purity of your product you could make an IR spectrum and look for not expected bands. if your
product is an alcohol and you see a C=O band you know something is wrong, but you won't know what's the impurity.
with CG/MS you get the separation of a mixture thanks to the GC, and then you can make a mass spectrum to see which peak is your product for example.
plus IR has a different sample preparation depending on the state of your sample, if it is a pure liquid, a solution, or a solid.
in gas chromatrography you just need to be sure about the volatility of it.
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Syn the Sizer
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Honestly I was thinking IR was the way to go but after reading Ubyas opinion I fully agree with them. GC/MS is the way to go, just because GC was the
most boring class doesn't mean it's useless.
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Nonexistent
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if I could get a gc/ms or hplc for that price in good working condition, I definitely would do so. You could also start some form of basic testing and
make an income from it. As Ubya said IR is just groups, great for seeing if you have your compound, and purity, but gc/ms and once you have a method,
you have a nice separation and %'s, + your ms library of compounds, so many things you could look at and discover.
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Metacelsus
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GC-MS is better, but the maintenance and operating costs will be higher. With IR you don't need gas cylinders or fancy pumps.
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VSEPR_VOID
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I would recommend buying a visible light spec. They are cheap and will be able to identify any compound you prepare that is colored. They also are
easy to upkeep, and you only need to buy new kimwipes. I really like using them. Nanodrops are too easy to fuck up.
A GC will require an expensive mobile phase. Hydrogen and helium are both expensive. If the machine breaks, good luck fixing it. However, if you do
buy one, pay special attention to what sort of columns it uses and can accept. If you clog one up, or cant get a sample to elute, you would have to
buy a new one.
Also, you can use a DIY syringe pump to send your product through a chromatography column, and measure each fraction with whatever you buy
Within cells interlinked
Within cells interlinked
Within cells interlinked
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Ubya
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Quote: Originally posted by VSEPR_VOID  | I would recommend buying a visible light spec. They are cheap and will be able to identify any compound you prepare that is colored. They also are
easy to upkeep, and you only need to buy new kimwipes. I really like using them. Nanodrops are too easy to fuck up.
A GC will require an expensive mobile phase. Hydrogen and helium are both expensive. If the machine breaks, good luck fixing it. However, if you do
buy one, pay special attention to what sort of columns it uses and can accept. If you clog one up, or cant get a sample to elute, you would have to
buy a new one.
Also, you can use a DIY syringe pump to send your product through a chromatography column, and measure each fraction with whatever you buy
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most organic compounds are white powders, so at least an uv-vis is necessary, plus if IR just shows functional groups, UV-Vis just shows conjucated pi
systems and a few low energy orbital transitions, so identification is even harder.
a GC/MS no doubt has a higher maintenance cost, but the amount of informations you get is way higher than other techniques.
hydrogen and helium could be substituted with a nitrogen generator, we used that in my university for the lower tier instruments.
there's no better instrument in absolute, it depends on what informations you want to get from it, uv-vis is the cheapest option, you can buy one from
ebay for 200 dollars, IR needs special cells depending on the sample and it is not great at analyzing mixtures, CG/MS you get separation of complex
mixtures from the CG, and then you get a mass spectra that you can read yourself or match with one from a database and get a better idea of your
compound/impurity.
next is getting a 300MHz superconducting NMR c:
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Dr.Bob
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For small molecules, a GC/MS is a great tool. If you can get a FID or thermal detector, that will help even more, as some molecules do not ionize
well. GC is a powerful tool for checking the purity of a molecule, it can detect amounts to several orders of magnitude to get purities to a high
precision. And depending on the detectors, some are very universal. The MS is the part most likely to give issues, but the ones on GS are simpler
than any other type in my experience. But they will take some work to perform.
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RogueRose
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I've been considering getting something to do this for awhile as my goal is to be able to test the purity of compounds and see what the impurities
are. I'd also like to be able to test water samples to see what contaminants they have, and I'm wondering if GC/MS is the best option for this.
The biggest question I have with the GC/MS is the database that is needed to make the comparison between the sample and the known compounds. Are
these database's freely available, do they come with the machine or do you have to buy them or subscribe?
I've seen a number of machines sold with I guess what are called "columns" which look like small diameter metal hoses with a screw connection on each
side and they vary in length. I see that they are really expensive when new but often see them sold, used, in lots for pretty low prices (like 5-10%
of new price, sometimes lower when a large lot). How many of these do you need and what are the differences in them? Keep in mind IDK the full
process of this, I've never done it myself, so any explanation I'm grateful!.
So i see that people listed H2 and He as consumables needed for GC/MS, is there anything else and does it have to be a special gas, like ACS grade, or
can it be purchased the same as gases from welding suppliers? I also thought that methanol is used in this or is that for LC/MS? Are there any other
consumables that are needed?
Does anyone know what the average cost would be to run a sample - meaning what the cost is of consumables?
As for the pump, is this for vacuum or to push the solution through the tube?
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Ubya
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| Quote: Originally posted by RogueRose | I've been considering getting something to do this for awhile as my goal is to be able to test the purity of compounds and see what the impurities
are. I'd also like to be able to test water samples to see what contaminants they have, and I'm wondering if GC/MS is the best option for this.
The biggest question I have with the GC/MS is the database that is needed to make the comparison between the sample and the known compounds. Are
these database's freely available, do they come with the machine or do you have to buy them or subscribe?
I've seen a number of machines sold with I guess what are called "columns" which look like small diameter metal hoses with a screw connection on each
side and they vary in length. I see that they are really expensive when new but often see them sold, used, in lots for pretty low prices (like 5-10%
of new price, sometimes lower when a large lot). How many of these do you need and what are the differences in them? Keep in mind IDK the full
process of this, I've never done it myself, so any explanation I'm grateful!.
So i see that people listed H2 and He as consumables needed for GC/MS, is there anything else and does it have to be a special gas, like ACS grade, or
can it be purchased the same as gases from welding suppliers? I also thought that methanol is used in this or is that for LC/MS? Are there any other
consumables that are needed?
Does anyone know what the average cost would be to run a sample - meaning what the cost is of consumables?
As for the pump, is this for vacuum or to push the solution through the tube?
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oh boi many questions, i'll try to explain my best.
there are tons of MS databases many for many you need to buy a pricey subscription, others are free but have less data. my mass spectroscopy professor
explained to us how they work (i haven't had the occasion of using one myself), a mass spectra heavily depends on the instrument, most of the
databases use electron ionization mass spectra as they give the most reproducilble data, different combinations of ionization sources and detectors
will give you a different spectrum. so each database is not universal, you need to pick the one best suited for your instrument.
the columns. columns are used in the CG part of the instrument, they "simply" are a tube with something inside, when the carrier gas with the analyte
flows through it, the different interactions of the compunds with the stationary phase (the thing inside the tube) will gradually separate them. the
only way to change the interactions of the sample with the stationary phase is 1) change the nature of the stationary phase (change column) or 2)
change the temperature (higher the temperture the faster the interactions with the stationary phase) or 3) change the flow of carrier gas.
gas chromatography columns can be packed or capillary (there are more types but this two are the broadest)
packed columns are usually stainless steel tubes of a few mm in diameter filled with an adsorbant or a support material covered in a liquid film (so
solid-gas interactions or liquid-gas interactions)
capillary columns are thin, like micrometer thin tubes of silica covered with a liquid film inside.
how many columns do you need? it depends strongly on the type of analysis you want to perform. every column has different stationary phases with
different polarities and different interactions with a family of substances. so to get the best possible separation you need the best suited column
(and temperature and flow rates xD)
for example one of my many projects is building a gas chromatograph, i just need to check hydrocarbons so i found i could make my own media for a
packed column covering some silica gel powder (80-100 mesh) with a layer of paraffin wax.
the difference is that some give better results with a class of compounds and give shitty separations with another class, so it depends on what you
are testing.
yes the carrier gas for gas chromatography must be pure, like 99.995% pure or more, a welding supplier won't work
consumables are: the inlet septum, after many injections it starts to leak,
the graphite ferrule holding the capillary column (it should be changed every time you change a column but maybe you can keep it as we are not aiming
for extra precise data), the detector also has consumables, a GC/MS has a mass spectrometer as detector, but if you only want a gas chromatograph you
need to pick a detector, for example a FID (flame ionization detector) needs high purity hydrogen to work, TCD (thermal conductivity detectors) use
heated filaments and like any incandescent light bulb they can burn.
yeah methanol is usually used to dilute the sample before injection, and yeah, heet methanol won't be good enough xD
the cost is pretty relative, universities buy everything from Sigma so prices are not low, but it also depends on the instrument. if you use a packed
column, with N2 as carrier gas (from a nitrogen generator so no high pressure tanks), with a TCD detector(an universal detector) you can run an
analysis for pennies, but then if you use 7N helium as carrier gas with a chiral column and a nickel-63 electron capture detector wee ohh costs are
over the roof.
push. if you have a gas cylinder you just use a regulator, i mean 1 from the tank, and another fine tuning-pc controlled regulator before the inlet
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nleslie321
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after reading all the comments i think im going to get a GC/MS. What about LC/MS is that similar in usefulness to GC/MS. I can get ultra high purity
hydrogen gas bottles delivered to my house for carrier gas for GC/MS and i will buy one with the vacuum pump included. I am hoping to find someone on
here that has a copy or can get me a copy of the software needed for the GC/MS. I can buy high purity carrier solvents from chemical supply near me
also. What about the columns that were talked about? do i need to worry getting different ones for testing different types of substances?
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DavidJR
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HPLC is overall just much more versatile than GC. You have more options of separation modes, and you aren't limited by the need for analytes to be
volatile and generally there is no need for derivatization.
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nleslie321
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You think HPLC is more versatile than LC/MS or GC/MS?
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Dr.Bob
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Straight HPLC (using a UV or RI detector) is good for determining what componnents are in a mixture, and comparing a compound to a starndard for
identification. But without a MS on it, it will not tell you anything about the identity of the components. It is the liquid version of a GC.
The costs of running each will be similar, as a GC requires a carrier gas, either helium (which is getting pricey) or hydrogen (dangerous, but cheap).
For FID detector, H2 is needed. For an HPLC, you will need solvents, usually water and an organic, typically methanol (cheap) or acetonitrile
(pricey), both can work OK. Both GC and HPLC require a column of some sort to separate components into peaks for detection, most columns are
expensive.
Any MS, whether on an HPLC, or a GC, will be somewhat expensive, hard to maintain, and require a very good vacuum pump. So the difference between
GC and HPLC is that GC is mostly ideal for compounds that volitalize at some temp below 200 or so (typically liquids or organics with low mps), but
HPLC works best for larger mw liquids and most solids. If you don't know what each is good for, then buying one might be a bad idea, unless you have
a lot of time and money. All of these tools are great for doing analytical work, but each has its costs and limitations. While I have heard of a
few individuals buying these, even many small chemical companies can't afford to buy or use them, due to the operating costs.
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DavidJR
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Sorry, I was strictly just talking about the chromatography part... Obviously HPLC/MS is much more useful than HPLC/UV. But HPLC/UV is undeniably more
versatile than GC/FID etc, and HPLC/MS is similarly more versatile than GC/MS. GC has its place, of course, but if I had to choose one, it would be an
HPLC. (In fact, I did - I have an HPLC in my garage, but no MS (yet)).
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Ubya
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another thing, all of these instruments are not "plug and play", every analysis is different from the others, and to get the best (or even just
usable) results you need to tweak many factors, just installing the right column and pressing the start button is not nearly enough.
to run a CG or an HPLC you need to be very familiar with the van deemter equation, because by looking at the spectrum you can then modify the
temperature/flow rate/eluent to get good separation.
then you are adding a mass spectrum instrument to it, i think that most machines you can find on the web at a low enough price are quadrupole mass
spectrometers.
then another thing, HPLC/MS and GC/MS interfaces are different. while GC/MS can use an electron ionization source, HPLC uses elelectrospray
ionization.
problem solving also is not easy without training, how do you know that your column is at the end of its life? how do you know that a bubble in the
HPLC is messing with your specteum? did you degass your solvents?
the base principle of each system is not hard at all to understand, it's the many many little things that can mess your spectrum, so my advise is
before buying anything, HPLC, GC, HPLC/MS, GC/MS and all the flavours of systems with different detectors, analyzers etc etc, study well the theory
behind to understand well what each can do, because you need that angway to run the instrument.
i know you expect us to just say "buy that it's better" but honestly there isn't a better, each one has its pros and cons, only you know what you need
it for, and by understanding what each is capable of you can decide accordingly.
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Nonexistent
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| Quote: Originally posted by RogueRose | I've been considering getting something to do this for awhile as my goal is to be able to test the purity of compounds and see what the impurities
are. I'd also like to be able to test water samples to see what contaminants they have, and I'm wondering if GC/MS is the best option for this.
|
It really comes down to a matter of what you want to test. GC for compounds that are volatile, HPLC for most things, but For water (especially from
pools, ponds, etc), I'd go for a Flow injection analyser, the salts and potential metals would destroy a GC column and also over time ruin hplc
columns.
| Quote: Originally posted by RogueRose |
The biggest question I have with the GC/MS is the database that is needed to make the comparison between the sample and the known compounds. Are
these database's freely available, do they come with the machine or do you have to buy them or subscribe?
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There are free ones available, but smaller databases, you can of course buy/subscrive to a company for larger and more often updated ones.
| Quote: Originally posted by RogueRose |
I've seen a number of machines sold with I guess what are called "columns" which look like small diameter metal hoses with a screw connection on each
side and they vary in length. I see that they are really expensive when new but often see them sold, used, in lots for pretty low prices (like 5-10%
of new price, sometimes lower when a large lot). How many of these do you need and what are the differences in them? Keep in mind IDK the full
process of this, I've never done it myself, so any explanation I'm grateful!.
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I don't know how game I would be to use 2nd hand columns, if they were still working well, they'd likely be kept, unless the company shut down/was no
longer doing that form of testing. But I'd still be wary, an old column can have many problems, pressure being too high, missing overlapping compounds
and not having clean peaks for example.
That all depends on the type of sample you want to run. Your mobile phase, any derivitizarions, sample preparation, all adds up and all varies from
one test to another.
| Quote: Originally posted by DavidJR | | but if I had to choose one, it would be an HPLC. (In fact, I did - I have an HPLC in my garage, but no MS (yet)). |
So jealous, I'd be putting all sorts of stuff through, what do you mostly run through it?
if you have the space and money, LC/MS
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DavidJR
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| Quote: Originally posted by Nonexistent |
I don't know how game I would be to use 2nd hand columns, if they were still working well, they'd likely be kept, unless the company shut down/was no
longer doing that form of testing. But I'd still be wary, an old column can have many problems, pressure being too high, missing overlapping compounds
and not having clean peaks for example.
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I've actually had pretty good luck with second hand C18 columns. I think part of that is that there are a lot of regulated labs that have to replace
columns as soon as they start showing any sign of degraded performance - but that doesn't mean the column is not usable any more.
It's worth going through a cleaning process, cycling through progressively more non-polar solvents and back, just to make sure anything strongly
retained is flushed out. I.e. methanol, acetonitrile, isopropanol, dichloromethane, hexane, isopropanol, methanol (10 column volumes each).
Depends what I'm working on at the time! Mostly using it for reaction monitoring. But I also was doing air analyses for aldehyde pollution with
2,4-DNPH (on hold due to coronavirus situation meaning I can't go out and take samples).
[Edited on 20-5-2020 by DavidJR]
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Bmoore55
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I currently work with both and if you want something that has much less maintenance to mess with go for the IR. IR is useful for identifying most
organic intermediates and a good rule of thumb with regards to identifying a material is A) the thing you are looking to analyze should have as little
water as possible since that strongly absorbs in the MID IR range and B) the concentration of what you are interested in should be at least 5% in most
cases to even have a chance of identifying. Don't get me wrong GCMS is an amazing tool and if you have the skills to do the regular maintenance
properly and you can afford the parts and software then it is also a great tool. I just find that a great compliment to anyone doing chemistry is a
relatively quick and easy analysis, and when I think quick and easy I think of IR. Just for your reference I am currently running and Agilent GC 7890
and mass spec 5975C and I currently have two Perkin Elmer Spectrum 100 FTIRs.
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DavidJR
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I often see old FTIRs sell on ebay for more than old HPLCs...
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nleslie321
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what about something like this? https://www.ebay.com/itm/Varian-CP-3800-3800-Saturn-2000-MS-...
or i was looking at an IR spec like this https://www.ebay.com/itm/THERMO-NICOLET-IR300-SPECTROMETER-W...
I think after a lot of thinking im going to go with a IR Spec learn that inside and out then look at buying a LC/MS in a year or so.
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Ubya
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eh but they usually don't have a PC controller, they have a roll of paper where the spectrum is drawn by the instrument, it could still be used, but
having to measure peaks and bands like 40 years ago with a caliper mh
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monolithic
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I've always wanted to buy HPLC or GC/MS but the learning curve seems absurdly steep. So much to go wrong with the equipment, concerns about
proprietary software, and so much to learn just to set it up. Then even more to learn when it comes to keeping it running and tweaking things to
provide usable data... and then learning to actually interpret the data. 
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DavidJR
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| Quote: Originally posted by monolithic | | I've always wanted to buy HPLC or GC/MS but the learning curve seems absurdly steep. So much to go wrong with the equipment, concerns about
proprietary software, and so much to learn just to set it up. Then even more to learn when it comes to keeping it running and tweaking things to
provide usable data... and then learning to actually interpret the data.
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Yeah, it can definitely be a bit of a time and money pit... But I think it's worth it.
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Bmoore55
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Be weary of buying an FTIR that is sitting in what looks like a garage or a storage area. Often the optics are KBr windows that get destroyed with
high moisture and can make your purchase a bust.
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