Sample introduction is a very complex process. There are many parameters that effect the injection. Using incorrect parameters can result in bad chromatography.. The parameters that are relevant to sample injection are shown here in blue. It is very important to get the sample injected quickly. The goal in GC is to get a chromatogram with well resolved narrow peaks.. If you don't get your sample onto the column quickly enough, you are gong to end up with wide poorly resolved peaks which will be difficult to integrate. Injecting the sample so that it moves onto the column as a narrow band is very important. .
Two conditions we want to avoid is carryover and discrimination. We will discuss carryover in the following slide. Discrimination can occur when we have a complex samples with wide ranges of boiling points and different volatilities. Discrimination can be commonly measured by analyzing a wide range of hydrocarbons from C-10 to C-40. Discrimination can occur when the response of the heavier hydrocarbons such as C39 or C40 response is much different from your lighter hydrocarbons. The large difference in response means that you are treating the lighter compounds different then the heavier compounds. <break time="2s" /> Another critical parameter is the injection volume. It is important to inject the minimum amount of sample necessary to accomplish the application. Sample cleanup before injection is very important. When the sample is not cleaned up properly, non-volatile buildup occurs in the liner and the head of the column that will interact with analytes in your future injections. In sample introduction, limiting the sample volume to only what is needed helps to eliminate this problem along with better sample cleanup. .
It is important to understand the anatomy of a GC autosampler syringe. There are a lot of different features and choices to be considered when choosing a syringe.. One very important thing to consider is the needle tip. A point of confusion is that vendors call the needle tip different names . For an example,
some venders call the cone tip needle a Point style A S tip. <break time="2s" /> The first type of syringe needle is the cone tip needle. Cone tip syringes are tapered at the end. Also, the needle hole is in the center. This is the best type of syringe for a GC autosampler since it minimizes the septa particles that end up in the liner and plug the syringe needle.
Another type of needle is the beveled needle or PS2 needle syringe. This syringe is a better choice when used for transferring liquid from one vial to another. It is also good to use when doing manual injections to help penetrate the septa without coring it. <break time="2s" /> The last type of needle is the side hole needle where the hole is located on the side of the needle instead of the end. These are most commonly used with a headspace analysis or when doing a large volume injection that are commonly used with a CTC headspace application.
With this needle, the sample is injected on the side of the liner. With a large volume injection, a slow injection is needed, so this needles works well with this type of application.
Another important property of a syringe is how the needle is fixed to the syringe barrel, also called the needle termination. Fixed needles, or FN abbreviation, use an epoxy to cement the barrel to the syringe. This is the termination type used in most autosampler syringes, and is the preferred termination for trace level analysis or volatile analysis since they are less likely to leak. One drawback is that if the needle breaks or bends, the entire syringe will need to be replaced. A second type of needle termination is the replaceable type which has a separate needle. This type is recommended for chlorinated solvents since the threaded connection does not contain an epoxy in which the chlorinated solvent could be dissolved. They can also be heated to higher temperatures so they can be heated for applications requiring syringe heating.
Now let's talk about plungers, there are two styles -- fitted syringe plungers and PFTBE tipped plungers. Fitted syringes are individually fitted to the syringe barrel. They limit loss of volatile samples, and are not replaceable or interchangeable with barrels from other syringes. They are very sensitive to air actuation, so if your wash vial runs dry, a fitted syringe should not be used. They are recommended for general analysis of liquid samples. PTFE syringes on the other hand make a gas tight seal, so you can use them for gas as well as liquid samples. They are recommended for chlorinated solvents since the Teflon seal prevents the corrosion of stainless steel area on the plunger. They are also recommended for volatile samples since the gas tight seal prevents the loss of highly volatile compounds. The PTFE seal also helps sweep sample residue that has been absorbed on the inner syringe barrel surface, so they are very good for dirty samples. Maintaining the PFTE seal does require a little maintenance.
The next item we will discuss is sample injection volumes. The first item is the sample syringe capacity. In Mass Hunter acquisition method, the maximum sample injection volume is capped at 50% of the syringe capacity. In Mass Hunter, once the syringe is configured in the ALS tab, the sample injection volumes are automatically configured. The usable range of a syringe is the middle 80% of it's volume. For a 10 micro liter syringe, using injection volumes of 1 to 9 micro liters is still in the safe performance zone from a precision viewpoint. Once you move below the 1 micro liter level, the reproducibility and precision will not be as good. It is up to your application to determine if the loss of precision is acceptable. We want to avoid large injection volumes when possible. When there is not enough sample clean up such as when you have a very complex matrix such as soil, you may have non-volatile material build up in many places along the sample flow path. This can cause many problems in chromatography. We need to inject as little sample as possible, and still be able to meet our method detection limit..
Now, we will list some general starting points. Your best bet for GCMS analysis is to start out with a 10 micro liter syringe, cone tip, tapered needle, and a PTFE tipped plunger.
One of the more confusing items is the syringe gauge. The bigger the gauge the smaller or thinner the needle. The 26 gauge needle is the thinnest available, and result in less septa coring, however, thin needles have less strength, and are prone to bending or snapping. The larger 23 gauge needles diameter needles have higher strength, but can lean to reduced septa lifetime. Tapered needles are have the best features. The upper section of the needle is 23 gauge which gives the needle more strength, the part the pierces the septa is 26 gauge which leads to longer septa life. Note - the "S" number in the gauge number , denotes a smaller inner needle diameter as well as a thicker outer wall, for better durability. .
Many people want to know the difference between the Agilent gold and blue syringes. One is not better then the other, they just work differently. Both have a wide range of volumes, but since they are made differently, blue is better for trace level samples, or if plunger lifetime is a concern, or if excessive wear on the septa is also of concern,
the gold syringes are a better economic option, they work well for dirty or high concentration samples and still maintain long lifetime. .
Now, lets talk about sample washes and sample pumps. In the autosampler turet there are solvent A washes and sample B washes. The main reason for solvent washes is to prevent sample carryover or bonus peaks. It is not straight forward to provide specific set points since these are application specific. For example, if your customer is running highly concentrated samples, it is highly important to flush sample residue from the syringe barrel. Trace level analysis may not require as many solvent flushes to prevent carryover. The number and type of washes needed for a given application depends on a couple of factors. The first is analyte solubility. If the analyte of interest is only partially soluble in the solvent, the number of washes will be higher. Analyte and solvent viscosity are two other parameters that can effect the number of solvent washes needed. More viscous solvents will require more solvent washes. Syringe barrel wear is another factor. More solvent washes, result in increased plunger cycles which may reduce the syringe lifetime. The right balance is key. Finally, the wash volume itself can effect the number of washes needed. A good starting point is 4 pre and 4 post injection washes, at max volume. Mass Hunter acquisition defaults the Max volume to 80% of the syringe volume. So, for a 10 micro liter syringe, this would result in a 8 micro liter wash volume used. As we will see from the next slide, consideration must be given to the remaining solvent left in the solvent vials. We should never let the wash vials run dry, because this will destroy our syringe plunger. You should take into consideration the number of vials you are injecting in your sequence and the number of pre and post washes and the wash volume. The calculated amount is shown in this slide. If the number calculated is less then 2 milliliters, that is acceptable. If you are over 2 ml, use a 2nd wash vial, wash vial B, or reduce the wash volume. This is because the solvent vials have a minimum level. The syringe will not move below thus level. This is important to think bout when setting up your sequence. Lets take an example. You are running a 22 line sequence, for wash vial A they have 3 pre and post injection washes, at 5 micro liters solvent wash. When we do the calculation, we find we are using .66 milli liters of solvent, so if we fill up to the 4 milli liter mark we will have 3.3 milli liters left at the end of the sequence, so this is acceptable. .
But for B we have 6 pre and 6 post washes at the max volume, which is 80%. So injection wash B is 22 injections times 12 total washes at micro liters which calculates to 2.1 milli liters. This means there will be 1.8 milli liters remaining at the end of the sequence. This is too low and would generate a carryover condition or shortening the syringe lifetime.
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