Sample Preparation of Liquids
Liquid samples can be the easiest or most difficult samples to work with depending on the composition and stability. A sample cup is filled about ¾ full and then presented to the analyzer.
The problems with liquid samples are that they evaporate, stratify, and precipitate. The liquid may attack or be absorbed by the window film, wick up and out of the cup.
Because of these issues liquid samples should be freshly prepared, preferably immediately prior to analysis, although some liquids are stable for a day or more. Solutions should be well mixed prior to pipetting them into a sample cup. The sample should be taken from the center of the container since some components may concentrate on the walls. There are some techniques that can be used to overcome most of the other problems.
All liquids have some vapor pressure at standard pressure and temperature. Some low vapor materials such as water or mineral oil may be stable for a day or more, while gasoline and other volatile liquids may only be stable for a few seconds. Highly volatile samples should be prepared and analyzed one at a time and the time from pouring to starting the analysis should be consistent within a few seconds.
A cover or cap may be placed over the cup to reduce evaporation, but the film will bulge causing poor reproducibility. Single end cups and caps for double-ended cups can be punctured with a pin, and some have a snap off device that leaves a hole. Window film covers can also be punctured to relieve pressure. Most cup manufactures also make baffled cups that are designed to minimize evaporation and in theory the sample.
Liquids stratify when they either contain two or more immiscible liquids like oil and water, or if they contain several molecules that have different density such as a crude oil. Various organo-metalics are also higher in density and tend to settle out over a long period of time. Most liquids need only be well mixed prior to pipetting it into a cup, and they will be stable during analysis. Others will stratify during analysis and must be analyzed quickly.
Immiscible liquids are a special challenge. Sometimes it is possible to produce a meta-stable emulsion. Others times it may be necessary to analyze the two components separately. Lastly solidification techniques can be attempted.
As with stratification, liquid samples that precipitate may be well mixed and analyzed one at a time if the particles stay suspended long enough. Normally the supernatant liquid is pipetted off the top and measured separately from the precipitate, which should be dried, weighed, and measured as a powder. Solidification techniques have also been used successfully with rapidly precipitating samples.
Wicking happens when the liquid is hydrophilic with respect to the window film. The liquid is drawn along the film, out, up, and over the bottom retaining ring. This can also happen even with a hydrophobic liquid because of gravity, when the liquid level is above the height of the bottom retaining ring. One cup vender attacks this problem by having a taller bottom ring, while others use collars that slide up the outside of the cup holding the window film above the sample height. Another technique that works in a pinch is to use tape around the outside of the cup to hold the window film up.
Alternative Liquid Sample Preparation Methods
Since many liquid samples are inherently unstable there are a number of alternative methods for stabilizing the samples. There are also a number of pre-concentration techniques that are available that have been used with some success.
Liquid Sample Solidification
Solidification is one useful method when the sample contains immiscible liquids such as oil and water, or it the precipitates rapidly. Several materials including, cellulose, carbon, gelatin, and alumina, have been used successfully as solidifying agents. Lower atomic number solidification agents are preferred when analyzing low Z elements. The ideal mixing ratio must be determined experimentally.
Thin Film Preparations
Popular method to attempt, is to deposit a sample on a thin sample support and then measure it either wet or after drying. By reducing the sample thickness it is possible to reduce the background while keeping the elemental net intensities high, thus improving the detection limits. Drying the sample helps even more since much of the background due to backscatter is due to the hydrogen and other low atomic elements in the base matrix.
Common support materials include filter paper and IR cards. A few, typically 5-50, microliters of sample are be pipetted onto the support, and allowed to soak in and distribute. The method has also been done using an atomizer to spray the sample. It can be presented wet provided the moisture content and distribution is consistent from sample to sample or else it can be dried. The primary problem with these techniques is that they are often not reproducible enough for routine laboratory work.
There are several concentration/thin films techniques that have been used for XRF analysis. The simplest involves filling up a sample cup and dying it. This works best when the matrix is a highly volatile solvent.
If the elements of interest are particulates in suspension then a filter can be used to filter a large volume of fluid. Then the filter can be analyzed wet or dried. One instrument vender uses this method to analyze trace metals such as iron in nuclear reactor coolant.
Another method involves the use of ion exchange filters or resins. A large amount of fluid can be moved through an ion exchange medium removing the ions of interest. The medium can be measured to determine concentrations of the elements of interests. Filters can be analyzed wet or dry. It is important to select an ion exchange medium that does not interfere with the XRF analysis.