Laboratory Analysis Demystified

By Patrick Vowell, Water Quality Engineer

Most people have no idea how critical getting a good sample is, and how hard it is in general to get really good data.  The analytical process for almost anything we test for has so many steps – each of which compounds any deviations or discrepancies made in the previous steps – that it’s vitally important to be as accurate and precise in each step along the way in order to get good, meaningful data in the end. Every drinking water and wastewater treatment operator out there knows how important it is to take samples correctly.  But what happens after the sample is taken, the chain of custody is all filled out, and everything is safely packed away and ready for pickup or shipment to the lab?  For many people, the laboratory is a mysterious place filled with serious people in white lab coats running large complicated instruments that somehow sniff out whatever is in our water.  Having worked in laboratories for a few years in positions from a bench-level chemist to a laboratory director, I wanted to try and make the whole process a bit less mysterious by walking through what happens to your samples after they leave your hands. 

Chain Of Custody (COC)

The COC is a legal document that establishes an unbroken trail or chain of who had the sample, when they had it, and who they signed it over to.  It is a legal document that can be used in a court of law to establish or refute the validity of the sampling and analysis procedures.  As such, it is an incredibly important piece of paper and needs to be filled out correctly, completely, and with great care.  The date and time of sampling as well as the name of the sampler are just a few of the things that are important to make note of on the COC.  Without that information, the samples could be invalidated and the samples might have to be taken all over again.  In the laboratory, additions or addendums to the COC are created that are used to track where the sample goes within the lab, who handles it, and what tests are run.  These addendums become a part of the legal document that started with the sampler in the field.

After the samples get to the laboratory, the first thing the laboratory will do is begin the log in procedure that will enter the sample information contained on the COC into the laboratory information management system, or LIMS. This involves cross-checking the information on the COC with that on the bottle labels; making sure the COC is filled out completely and correctly; checking for any special handling requirements, notes, or rush turn-around-times; and entering this information, along with the requested analysis information, into the LIMS. If there are any discrepancies, errors, or omissions, the laboratory will contact you to get those issues rectified. Common issues include forgetting to put the sample date or time, or not writing down some piece of field data such as a chlorine residual on the COC. Once the sample is finished being logged in, the COC is filed appropriately and the sample is stored in a specific area depending upon what analysis is called for. For example, with volatile organic compound (VOC) analysis, the sample must be maintained between 0oC (freezing) and 4oC at all times. Special refrigerators are used to maintain these tight temperature requirements that have chart recorders for the temperature, and they are connected to the labs version of a SCADA system so that if the temperature strays outside those boundaries, an alarm is sounded and someone gets a phone call. At the other extreme, samples for metals analysis are stored on regular shelves at room temperature. In either case, however, the samples will be stored in a room that has controlled access so the chain of custody can be maintained.

Lab Analysis

Once all the log-in procedures are completed, the laboratory analysts will be notified through the LIMS system that the samples are available for analysis.  One of the most important steps the analyst takes at this point is to check to see how old the samples are.  Any sample has a specified hold time in which the sample must be analyzed or it is no longer considered valid, and this hold time varies widely depending upon the analysis.  For example, drinking water samples for nitrate only have a hold time of 48 hours; VOC hold time is 14 days; and for most metals analysis, the hold time is 6 months.  Of course, if the lab waited 6 months to analyze your metals samples you wouldn’t be very happy with them, so they get them done much quicker than that.  But if a re-analysis of such a sample is ever needed, it can be done and still be legally and analytically valid up to 6 months from the time the sample was taken.

After the analyst has checked all the hold times, he or she will go to the sample custody room where the samples are stored and check them out, which is another step in maintaining that all important chain of custody. In most large modern labs these days, that means scanning them with a barcode reader and electronically signing for them before removing them from the sample custody area and taking them to the appropriate section of the laboratory for analysis. Laboratories are generally split up into sections based upon the type of analysis being done. This is not only important from a general organization standpoint, but to prevent the samples from being contaminated. For example, the analysis of semi-volatile organic compounds (SVOCs) can involve the use of a solvent called methylene chloride. When a sample is analyzed for volatile organic compounds (VOCs), a related test, one of the compounds being analyzed for is methylene chloride. So if VOC samples accidentally make their way into the SVOC section of the laboratory, contamination of those samples is very likely, and that’s very bad. That’s why most labs have a VOC section that is completely separate from the rest of the lab, in its own sealed room, with special positive pressure air filtration systems to prevent that kind of contamination from occurring. Each analysis also requires its particular analytical instrumentation, and each instrument has its own requirements. For example, VOC and SVOC are run on gas chromatographs (GC), which require a supply of ultra-high purity helium; and many metals are run on some variation of an inductively coupled plasma (ICP) instrument, which require high purity argon gas.

All samples are analyzed along with quality assurance (QA) checks to make sure the instruments are running correctly.  Only after the analyst reviews the sample data for any anomalies and confirms that all the QA checks have passed the criteria listed in the analytical method can the sample data be entered into the LIMS.  The data commonly then undergoes at least two further reviews by other laboratory personnel to make sure no mistakes have been made before it is entered into the final report which goes to the customer.

And that’s it!  That’s what happens to all those samples you take after they get to the laboratory.  It’s a very complex process that requires many checks and double-checks to make sure you get the good data you need to make appropriate operational decisions.