Why not use nitrogen instead of helium or hydrogen?
Nitrogen has a minimum plate height that is about 10% lower (better) than helium or hydrogen (Figure 2). This means that if we were to operate at the optimum velocity, then nitrogen would deliver around 10% more theoretical plates. The effect on peak resolution is negligible, however, because resolution is related to the square root of the plate number. What is not so good about nitrogen is the much narrower velocity range over which the plate height remains close to optimum, from about 8 to 20 cm/s in this case. The optimum linear velocity depends upon the solute and temperature, so for different solutes, and as the temperature changes during programming, the optima for each solute will appear at velocities somewhat different than the theoretical minimum. With nitrogen it is more likely that, for a given solute, the plate height will be farther from optimum than with carrier gases like helium or hydrogen for which the plate height does not depend as strongly upon the linear velocity.
Will hydrogen react with my unsaturated or aromatic solutes?
Hydrogen is a reactive compound that will hydrogenate unsaturated and aromatic compounds under the influence of temperature, pressure, and a catalyst. Sufficient conditions for hydrogenation do not exist inside conventional wall-coated fused-silica capillary columns, but the use of nickel columns with hydrogen carrier gas at higher temperatures might better be avoided if reactive solutes are to be separated.
How pure does hydrogen carrier gas need to be?
Hydrogen carrier gas should be of the same purity level as helium carrier — "research" grade, or 99.9999% purity for trace work under 1 ppm, or at least 99.9995% for most normal analyses. The manufacturer's literature will specify the purity and suitable applications.
Do I need to filter hydrogen carrier gas from a generator or a tank?
Hydrogen carrier is just like any other high-purity carrier gas and requires the same types of filters to be installed close to each supplied GC system. The filters are not needed to purify hydrogen as it leaves the generator or tank; they are needed to trap any contaminants introduced from regulators, connecting tubing, and fittings, as well as to isolate multiple instruments from each other.
If I use a hydrogen generator, will there be water or oxygen in the carrier gas?
Ultrahigh-purity hydrogen carrier gas generators remove water and the oxygen that is formed by passing the generated hydrogen through a palladium membrane that prevents passage of substances other than hydrogen.
Do hydrogen generators need specifically purified water and chemical consumables?
Yes. Hydrogen generators use deionized water that will need to be refilled regularly. Continuous flow water supplies are also available. Some carrier-gas grade generators will need periodic replacement of a deionizer bag, but normally this only needs to be done once or twice a year. Refer to the product manual for exact maintenance requirements.
How much hydrogen flow does a GC system consume?
A single-channel GC system with FID and split–splitless injection will consume 100–500 cm3 /min of hydrogen carrier and fuel gases, depending upon the split flow rate and column consumption. Gas-saver operation — turning off the split flow when not required — will reduce carrier gas consumption considerably. In general, a hydrogen generator should run at under 80% of its rated capacity, so for example, a generator capable of delivering 1 L/min could operate two single-channel GC systems with split flows up to 300 cm3 /min each, or possibly three GC systems at lower split flows. It is better to over-specify a generator and, thus, be sure not to run out of generating capacity for future planned applications.
I already have hydrogen cylinders that supply detector fuel gas in my laboratory: Can I use these for carrier gas as well?
Hydrogen for detector fuel gas might not be pure enough for carrier gas use. However, UHP or research-grade purity hydrogen cylinders or generators can be used for fuel gas as well.
Most GC laboratories now use flame ionization or other detectors, such as flame-photometric or nitrogen–phosphorus types, that consume hydrogen as a fuel gas. If your laboratory now uses or plans to use hydrogen as fuel or carrier gas, you should review safety equipment and procedures related to the use of hydrogen. It is also a very good idea to consider the hazards of non-flammable compressed gases such as nitrogen, air, and helium. Consult with instrument and hydrogen generator manufacturers, local safety authorities such as the fire marshal in your city, and in the U.S. with federal and state regulatory bodies. Many companies will have a safety policy that must be adhered to as well. In all cases, prepare the laboratory suitably before introducing any flammable, compressed, or otherwise hazardous gases. The use of electronic carrier gas controls is strongly recommended because such automated systems will detect many potentially unsafe fault situations that could result in leakage, and will shut off carrier gas flow.
A safety plus point for hydrogen generators is that they only store a small amount of hydrogen. The exact amount depends upon the generator capacity but typically is less than 0.20 L. Compared with the 6000–8000 L of gas that is stored in a full-size laboratory gas cylinder when new, it should be obvious that a hydrogen generator presents a considerably lower hazard in terms of the amount of stored flammable gas. Even so, this does not change the rate and total amount of hydrogen that is consumed nor does it remediate the hazards of using flammable gases, it just minimizes the total amount that is present in the laboratory at any one time.
The cost of a hydrogen generator, when used to replace helium carrier gas, will pay for itself in a relatively short time. It is not possible to review exact purchase-versus-lease figures or return on investment payback times in this article: these will vary considerably depending upon individual circumstances and usage, and on future helium costs.
In our lab we use Helium which is expensive - I want to look at changing to Hydrogen, however, would this would render the EI libraries useless?
Hydrogen is a little under half as viscous as Helium (Figure x). This makes it slightly more difficult to pump away for high vacuum equipment. Secondly, hydrogen shows minimum plate heights at higher linear velocities than helium. Thirdly, when the outlet pressure drops to almost zero (i.e. a vacuum) the flow of hydrogen into the instrument may be large.
A large volumetric gas flow into the instrument when using a direct interface will mean a lower vacuum level is attainable. This may increase the number of background molecules which can collide with the ions formed, leading to a potential reduction in sensitivity and a change in the relative abundance of ions within the mass spectrum. Care should be taken when analysing “fragile compounds, compounds at trace levels, and reactive compounds (alcohols, aldehydes, ketones).
This all being said, if we use smaller internal diameter columns (0.15, 0.18 or 0.20mm), higher linear velocities are achievable at lower volumetric flow rates – thus we can maintain the vacuum levels within the system and there should be no wholesale changes in the appearance of the spectra.
Hydrogen is a reactive gas whereas helium is not, and there is always an exception to the rule which states that we do not typically see ion / carrier gas reactions in GCMS, however, one that regularly appears is the dehalogenation of chlorinated compounds.
Figure 2: Influence of carrier gas and temperature on viscosity.
The full article by John V. Hinshaw our GC department editor can be read here »
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