Category - Mercury-testing
UOP 938 is a mercury detection method for determining mercury levels in low boiling hydrocarbons. This method is used for detection levels from 0.1 to 10,000 ng/mL. This is a specific method that specifies a Nippon instrument.
The sample is decomposed by heating it at 700c. The mercury is vaporised and collected on a gold trap for measurement.
Within the PS Analytical instruments we use a similar process but without combustion to create a similar result.
See: Sir Gallahad
When measuring the concentration of mercury within a sample it is important to understand the effect of different measurement units on the accuracy of the results. When looking at gas phase mercury concentrations there are a wide range of different units encountered including pg/l, ng/l, ppm, ppb and more. Unfortunately all of these units are ambiguous without more detailed information about how they are created.
As we are looking at mercury in gas samples, when we are measuring mass volume units it is important to look at specific factors that will effect the gas measurement such as the temperature and pressure of the gas. There are a number of factors that can affect the measurement basis, such as the contract requirements or the standard being followed.
Simply altering the measurement temperature from 0c to 20c will affect the measurement result by approximately 7%, which can be highly significant in certain environments.
With measurements of PPM and PPB, the variance can be significantly higher, depending on the measurement basis. Whether you are measuring mass/mass, mass/volume or volume/volume can result in differences of up to 1000 times.
In this video we investigate the factors that are important to ensure that your measurements are accurate.
When measuring mercury at sub pg/g level, there is a strict quality control procedure you need to comply.
1) You need to prepare at least 5 calibration standards, with the lowest standard being 0.5 pg/g or 1 pg/g depending on you are in America or Europe.
2) The linearity of the calibration curve needs to be in a accepted range
3) The intercept of the calibration curve needs to be less than half of the slope
4) The recovery of the lowest standard need to be within certain range
5) An ‘on going standard’ needs to be used for controlling the drift
6) An ‘independent standard’ needs to be prepared by another person or from independent source to ensure the accuracy of the calibration standards
7) The usage of reference material is highly recommended
8) Random sample needs to be spiked in duplicate, the spike recoveries of these two sample need to be within certain range
It is imperative to keep the background level under control otherwise you fail the QC automatically because of it. The high mercury background can come from the following sources: Deionized water Hydrochloric acid Tin(II) chloride Bromine solution Hydroxylamine hydrochloride Containers Carrier gas — argon Your lab environment Personal hygiene
How to lower the background Use highest purity available for the chemicals Keep the concentrations as low as possible Use clean containers Boil your reductant and then purge it Filter your argon carrier gas Operate in environment Change gloves often to avoid cross contamination.
I have seen many users preparing low level Hg standards in many different ways, some worked and many didn’t. I can recommend one way of doing it which proves to be working for me every time.
• Firstly, an intermediate Hg stock solution should be prepared. The concentration can vary depending on the user’s preference, say 1 to 5 ppb.
• Place a pre-cleaned container on an analytical balance with at least two decimal places.
• Weight in around 95 grams of the reagent blank, spike mercury standard
• Dropping pipette in reagent blank until it reaches 100.0 grams
• Cap the container tightly and shake well before use
Generally speaking, the container shall not contaminate the sample as well as absorb mercury from the sample. Clean, and less porous is the criteria when it comes to choosing the containers. There are many types of containers such as quartz, glass, PE, HDPE, PP, PET and PTFE. They can all be used when treated properly. Our preference is however PP especially PTFE.
Nitric acid is commonly used to stabilise trace metals however it is not particularly helpful for mercury at low levels. Halogens such as chloride or bromide are more preferable in that they form a very stable complex with mercury therefore to keep mercury in the solution. Acidify your sample with hydrochloric acid is one of the easiest way to stabilise your water samples. If you combine hydrochloric acid and bromine which forms BrCl in the solution, it not only stabilise mercury, also digest the sample at the same time. We would recommend to use these two methods to preserve mercury in your water sample.
In order to measure the mercury levels in your sample correctly and reliably, the mercury level in your reagents should be at least less than half of the mercury concentrations in the sample. For water analysis, mercury is usually found at 1 ng per litre level, which means the mercury background value in all the chemical we use combined needs to be less than 0.5 ng per litre. That shows the purity of the chemical is absolutely critical for mercury analysis in water samples.
Very often I hear users complaining about things like “why does my Hg signal keeps going down”, “unstable signal” or “the background is too high or too noisy”, they could be all coming from one source — your reductant. You probably haven’t prepared your reductant correctly. If your gas liquid separator is turning white or even yellow which means you are almost definitely doing it wrong. A key point of preparing the reductant is to boil Ti(II) chloride in concentrated hydrochloric acid so that it can dissolve properly. After boiling, the solution should be diluted with deionized water then purge with argon, for the best results, overnight.