The Sample Vial

Sad to say, but an attractive package can sell an inferior product. So why not sell yours. Dress it up in a neat new label. Put on 

Your name. Just in case the sample gets lost on the way to camp. 

Product name. So everyone will know what is in the vial. What does Product from part C mean to you Nothing Funny, it doesn t mean anything to instructors either. 

Melting point (solids only). This is a range, like M.P. 96-98°C (see Chapter 9, The Melting Point Experiment ). 

---

Although chloroform is an analyte, it also can be interferent. Due to its volatility, chloroform present in the laboratory air may diffuse through the sample vial s Teflon septum, contaminating the samples. How can we determine whether samples have been contaminated in this manner ... 

Why is it necessary to collect samples such that there is no headspace (layer of air overlying the liquid) in the sample vial ... 

Injecting the Sample The mechanism by which samples are introduced in capillary electrophoresis is quite different from that used in GC or HPLC. Two types of injection are commonly used hydrodynamic injection and electrokinetic injection. In both cases the capillary tube is filled with buffer solution. One end of the capillary tube is placed in the destination reservoir, and the other is placed in the sample vial. 

Hydrodynamic injection uses pressure to force a small portion of the sample into the capillary tubing. To inject a sample hydrodynamically a difference in pressure is applied across the capillary by either pressurizing the sample vial or by applying a vacuum to the destination reservoir. The volume of sample injected, in liters, is given by the following equation... 

Electrokinetic injections are made by placing both the capillary and the anode into the sample vial and briefly applying an electric fleld. The moles of solute injected into the capillary, nj, are determined using... 

Special attention needs to be given to collecting samples for VOC analysis. The VOC vial should be filled slowly until a convex meniscus is present above the sample vial lip. Carefully cap the sample vial, invert and tap to check for bubbles. If bubbles are present, then a new sample vial should be filled. 

If your sample is reluctant to dissolve in the chosen solvent, avoid adding more solvent for the reasons outlined above. Instead, try warming the sample vial carefully on a hotplate or with a... 

Remove the sample vial from the Processor, recap it, and save it for 7 days. 

The SPME process, adapted for solid or viscous matrix, is shown in Figure 10.1. A fused silica fibre, coated with a polymer, is installed inside a stainless steel hollow needle. In the first step, the needle is introduced in the sample vial through the septum. The fibre is then exposed to the headspace above the sample and the organic analytes adsorb to the coating of the fibre. After a variable sampling time, the fibre is drawn into the needle and the needle is withdrawn from the sample vial. Finally, in the same way, the fibre is introduced into the chromatograph injector where the analytes are thermally desorbed. 

Wet solids. Press out on filter paper, break up, let dry. The solid shouldn t stick to the sides of the sample vial. Tacky ... 

Electrokinetic (also called electromigration) injection is performed by placing the inlet of the capillary and an electrode in the sample vial. Following this a voltage is applied during a defined period of time. The sample constituents are actively carried into the capillary, and when present, the EOF also passively carries them into the capillary. For this reason, neutral compounds are also injected. The active migration is due to the effective electrophoretic mobilities of the constituents. The amount (B), in units of concentration injected into the capillary is expressed by [2,38]... 

Hydrodynamic injection can also be performed by using gravity to generate AP [41]. This injection mode is also called hydrostatic injection. The inlet of the capillary is placed in the sample vial and this is then raised during a period of time, creating a difference in height (Ah, in cm) between the inlet and the outlet of the capillary. The sample enters the capillary by siphoning. The amount and volume injected are derived from Eqs. 17.46 and 17.47, respectively, after substitution of AP with Eq. 17.48. 

The sample is usually introduced into the capillary by dipping it into the sample vial and applying a slight positive pressure. This results in a small volume of the sample entering the capillary. Depending upon the pressure and the... 

To prevent more volatile samples from sudden evaporation a circulation water cooling is often incorporated in the DIP, and refrigerated probes for more volatile samples have also been developed. [43] Sometimes, glass wool is placed into the sample vial to increase the surface for adsorption, and thus to slow down evaporation of the sample. Sudden evaporation causes distorted spectra and may even result in a temporary breakdown of the high vacuum. 

Vial levels Difference in liquid levels between vials will create a hydrodynamic flow and disturb injection precision. Fill the sample vials to the same level and make sure when programming the instrument that there is a vial with a constant level of BGE at the capillary outlet when injecting. 

When analyzing very low conductivity samples, it is necessary to add some ions with slow mobility to allow the tension applied to generate current in the sample vial. Eor example, we may add heptanesulfonate (CAS 22767-50-6 at a level of 1.2 mM in the sample) in the sample to thus generate current and EOF. 

Valve V[ had the column connected within the sample loop by using two 2S0 mm lengths of tubing, so that sample loading was done in the opposite flow direction to elution. The injection valve V3 incorporated a 287 pi sample loop. Pump Pj is switched off by the autosampler when the sample probe travels between the sample vial and probe washpot this prevents air from entering the column. Pump P2 runs continuously at a fixed speed. 

All essential identification information related to the sample is either printed on the sample vial s bar code label or exists within a look up table resident in controller memory. This approach avoids operator entry of this information through the computer keyboard and also facihtates implementation of a random order processing scheme. 

It is now possible to purchase commercially available systems for achieving something akin to LPME. In this instance, instead of a capillary microflbre, the extraction solvent is contained in a porous insert that sits inside the sample vial. This can be sampled automatically using the autosampler syringe after an appropriate incubation time. 

Injection parameters. If the injection reproducibility or linearity results are problematic, ensure that the sample vial cap is put on correctly. Sometimes, if the cap is put on incorrectly, the vial cannot be pressurized and injection either fails or is irreproducible. Also, check to make sure that no air bubbles are present in the sample vial. If air is injected into the capillary, poor results will be obtained. 

To extract the liberated fatty acids, 1.5 ml of 0.1 mM carbonate-bicarbonate buffer, pH 10.5 is added and the mixture is shaken for 10 s. Centrifugation for 45 min at 1500 xgin a swing-out rotor will separate the water from the lower organic phase. In a scintillation vial, 2 ml of the upper water phase are mixed with 50 pi of glacial acetic acid containing 500 pg ferric acid before the scintillation liquid is added (16 ml of Ecoscint toluene (7 1 volume). Liquid scintillation counting is done for 5 min and the LPL activity is calculated from the difference in counts between the blank and the sample vials [40]. 

Due to the volatility of some of the compounds present in food, it is very important to utilize cryogenic cooling when the sample is introduced onto the GC column. This helps to prevent the loss of low-molecular weight volatiles and also tends to focus volatiles on the initial portion of the column, thus allowing for improved separation and quantification. The use of a film thickness of 1.0 mm will also aid in the retention of the aforementioned compounds. In the static headspace procedure, the 4-min pressurization step is also crucial, in that equal pressures between the sample vials and the GC must be attained to ensure reproducible sample injections. Forboth the static and SPME procedures, heating the samples for 30 min prior to injection is important to ensure proper equilibration between the sample and the head-space. 

Add 5.0 ml injection solvent (50% acetonitrile) to each 4-dram vial and tighten the Teflon-lined screw cap. Vortex the sample vials every 10 to 15 min until no residue is found on the wall of the vials ( l hr). 

---