Frequently Asked Questions
Reporting Limit (RL)—is the lowest concentration at which an analyte can be detected in a sample and its concentration can be reported with a reasonable degree of accuracy and precision. A criterion of ± 20% accuracy and 20% RSD for replicate determinations is often used to define “reasonable”. The acceptable ranges depend somewhat on the analytical methodology used. For samples that do not pose a particular matrix problem, the RL is typically about three to five times higher than the MDL. Similar to the MDL, the RL is a laboratory-specific number, which may change with time. When a sample has to be diluted before analysis, either because of matrix problems or to get the instrument response within the linear dynamic range, the RL is raised by a factor corresponding to the dilution factor. The RL should generally be below any regulatory limits, but notifying the lab of reporting limit requirements is highly recommended.
The method detection limit (MDL) is defined as the minimum concentration of a substance that can be measured and reported with 99% confidence that the analyte concentration is greater than zero and is determined from analysis of a sample in a given matrix containing the analyte.
The MDL procedure now uses method blanks to calculate an MDL, in addition to the spiked samples that have always been used to calculate the MDL. As a result, the new definition of the MDL is: “The method detection limit (MDL) is defined as the minimum measured concentration of a substance that can be reported with 99% confidence that the measured concentration is distinguishable from method blank results.” The value calculated from the spiked samples is called the MDLS. The method blank samples are used to calculate the MDLb, which is a very similar calculation that also calculates the 99% confidence level that the result is derived from the sample rather from contamination/noise. The MDL is the higher of the two values (either the MDLS calculated using spiked samples or the MDLb calculated using method blanks). EPA considers this change important because as detector sensitivity improves, the background contamination of the laboratory, consumable supplies, and equipment can be more important in determining the detection limit than the sensitivity of the instrument.
The MDL now requires that the samples used to calculate the MDL are representative of laboratory performance throughout the year, rather than on a single date.
A laboratory has the option to pool data from multiple instruments to calculate one MDL that represents multiple instruments.
Precision is the level of agreement between duplicate measurements of the same characteristic. Laboratory precision, or analytical error, is assessed by determining the agreement of results for replicate measurements of the same sample. Field precision is assessed by determining the agreement for results for two independent samples collected from the same site at the same time. Precision may be evaluated using LCS/LCSD samples, MS/MSD samples, analytical duplicate samples and/or field duplicate samples. The comparison is made by calculating the relative percent difference (RPD) as given by:
where: S1 = measured sample concentration; and
S2 = measured duplicate concentration.
Accuracy is the degree of difference between the measured or calculated value and the true value. Data accuracy and analytical bias are often evaluated by the analysis of LCS and MS samples, with results expressed as a percentage recovery measured relative to the true (known) concentration.
The percentage recovery for LCS samples is given by:
where: A = measured concentration of the surrogate or LCS; and
T = known concentration.
The percentage recovery for MS samples is given by:
where: A = measured concentration of the spiked sample;
B = concentration of unspiked sample; and
T = amount of spike added
The Clean Water Act (CWA) requires the 200 series methods. The SW846 methods (for solids, wastes, process waters, etc.) follow the 6000/7000 series.
TCLP (Toxicity Characteristics Leaching Procedure) extractions are generally performed on samples that require disposal. This is why the extraction fluid contains acetic acid (which mimics a landfill site). TCLP extractions reflect leaching potential under acetic conditions. SPLP (Synthetic Precipitation Leaching Procedure) extractions are generally performed to understand the potential for metals leaching under rainfall conditions.
Total vs. Total Recoverable digestion has been the focus of many arguments over the years, with each camp claiming applicability of its selected digestion procedure to a non-filtered sampled aliquot. The EPA has taken the discussion and simplified it so that Permit Writers no longer need to request digestion procedures in order to use the best possible digestion. The term “best” is subjective to the individual nature of environmental samples and how well samples were taken to digestive completion. The 2012/2016 Method Update Rule has put forth a footnote to explain the current requirements on how a “Total” or “Total Recoverable” sample will be digested, below is the verbiage as relates to ICP/ICP-MS requirements:
For the determination of total metals (which are equivalent to total recoverable metals) the sample is not filtered before processing. A digestion procedure is required to solubilize analytes in suspended material and to break down organic-metal complexes (to convert the analyte to a detectable form for colorimetric analysis). The approved total recoverable digestion is described as Method 200.2 in Supplement I of “Methods for the Determination of Metals in Environmental Samples” EPA/600R- 94/111, May, 1994, and is reproduced in EPA Methods 200.7, 200.8, and 200.9 from the same Supplement. For analyses using inductively coupled plasma-atomic emission spectrometry (ICP-AES), the direct current plasma (DCP) technique or EPA spectrochemical techniques (platform furnace AA, ICP-AES, and ICP-MS) use EPA Method 200.2 or an approved alternate procedure (e.g., CEM microwave digestion, which may be used with certain analytes as indicated in Table IB); the total recoverable digestion procedures in EPA Methods 200.7, 200.8, and 200.9 may be used for those respective methods. Regardless of the digestion procedure, the results of the analysis after digestion procedure are reported as “total” metals.
The above digestion requirements were listed in both the 2012 and 2016 Method Update Rules. The rules supersede older EPA rules and digestion procedures and as such will be SVL’s default digestion (EPA 200.2). SVL upon request will use alternative digestive methods but will still report out results as “Total Metal.”
Control Charts are a tool to monitor ongoing sampling. If samples are collected in the sample place with the same sampling method, the data collected may be monitored for trends and outlier data points. Continuous monitoring can provide insight into long term trends whose gradual change may go unnoticed without statistical analysis that control charting provides. Control charting can provide peace of mind when an outlier data point occurs and it can forewarn of potential problems creeping into monitored sampling sites. SVL’s LIMS software is capable of creating control charts for samples, however, the laboratory does not have access to all of the information that the sampler has available (especially variations at the sample site).
SVL supplies a Cation/Anion (C/A) Balance as part of our routine report when certain cations and anions are requested. Cations are the positively charged species, and anions are the negatively charged species. In a perfectly balanced sample of water, the milli-equivalent (meq) ratio of Cations to Anions is 1.00. This is referred to as the C/A balance. For many samples—especially those that are complex, acidic, or contain sediment and solids—the C/A balance is often skewed with either cation or anions being significantly higher or lower than the other. In order to calculate a C/A balance, the following analytes must be analyzed: Ca, K, Mg, Na, alkalinity, Cl, and SO4. These represent the major cations and anions in a typical sample. There are other cations and anions that can also contribute to the C/A balance such as other metals, F, and NO3. High levels of NO3 and P often give rise to poor C/A balances. Many clients submit metals (cations) as “dissolved” (filtered) and a raw unfiltered bottle for anions. We often see less than optimal C/A balances for a filtered vs. non filtered analytes. Another interesting thing to check is Total Dissolved Solids (TDS). The total for the concentrations of cations and anions (mg/L) should add up to the TDS value in mg/L.
If you have historical data about your site and you have never had a result above your permit level, then something may have occurred during sampling or at the lab. This leaves you with two choices: either you re-sample, or you have the lab re-run the sample you submitted. Re-running the sample submitted to the lab may account for errors at the lab but not those that may have happened during sampling. Re-analyzing samples already submitted is not foolproof, sometimes errors can not be recreated or determined. If you do not have historical data and you believe that the results don’t make sense, by all means request a re-analysis or re-sample (after all that is why we are here, we want you to have accurate, legally defensible data in order for you to make the correct decisions for you project).
IF your historical average is near your action level, a result just above the action level may fall within acceptable statistical variation. This means if you tested the sample multiple times, some results would be above your action level and some would be below. SVL has procedures in place for how we re-analyze your samples and how a re-analysis shows up on your report. If there is justification for a re-analysis you will receive a report with only the new results; if not, then you will receive a report with both sets of results. We do this so that we can show that we are not in collusion with our clients and are doing what is known as result shopping, this is where a lab keeps running a sample until the client has the result they require.
In all cases the best policy is to re-sample, but we understand this is not always possible.
It depends upon which way you are traveling from the decimal point. When you are going to smaller and smaller numbers, significant figures are determined by weighing and dispensing devices and sometimes instruments are limited in the numbers used in their read-out displays. When numbers start to get large, significant figures are limited because being very specific really has no meaning in relationship to the action level or reporting limit (for example: 1243 mg/kg would be reported as 1240 mg/kg when the reporting limit is 10 mg/kg, because the result is so large there is no reason to report every number possible from a read out).
For a more scientific explanation, this link has a great explanation using a graduated cylinder « scale » https://chem.libretexts.org/Core/Analytical_Chemistry/Quantifying_Nature/Significant_Digits/Significant_Figures
SVL uses three to four significant figures in the reporting of your results. We base this upon the error potential associated with our support equipment and analytical instrumentation.
Now you have a report and know more or less what your action level is, so how do you read our report?
Front Page—will contain your company name and address, the project name and SVL work order number that we assigned to your project. Receipt date of your samples. The samples that you submitted to us and the condition we received them in (we qualify any non-conformences). We also list case narratives with information that we feel needs to be explained further than our catalog of qualifiers allows.
Client Sample Results Page(s)—identifies client sample ID and SVL’s corresponding ID. The date sample was taken, when we received it and who it was sampled by. We identify the sample method used, the analyte tested for, the result of that testing, the units used, the reporting limit for the method/analyte, the MDL for the method/analyte, if there was a dilution performed, what batch your sample was in, who the analyst was that performed the test, what date it was analyzed, and any qualifiers assigned. The following break down the column headers:
Result—This is the number we analytically determined, you can compare it to your action level.
Units—Liquids will usually be in mg/L or µg/L. Soils will be in mg/kg or µg/kg.
RL—Is the reporting limit, this can be a level above our MDL or it can be a limit assigned by the client (it is the level that we are confident you are seeing the actual concentration of the analyte tested for give or take any error associated with the analysis).
MDL—Is the level at which we determined a positive result to be a value above zero. Meaning we are 99% confident that there is analyte in the sample.
Dilution—Depending on the amount of analyte in the sample or matrix interferences on the analyte, a sample may be diluted to bring the analytical result back to within a known range. If a sample is diluted for any reason, the RL and MDL will be multiplied by that same factor (any value below these limits will be an estimation of what could be in the sample).
Batch—SVL uses analytical batches which means that we can combine multiple work orders until we are at 20 samples per batch to maximize efficiency. Batch QC is applied to all samples within the batch.
Notes—Show the abbreviations that we use for our qualifiers. We assign qualifiers to provide you with information involved with the running of your sample.
Method, Analyte, Analyst, and Analyzed are all self explanatory.
Quality Control Page breaks down potential errors that can be applied to your sample results. We run batch QC so this page is applied to every work order in the batch. The BLANK and LABORATORY CONTROL SAMPLE (LCS) are used to show any possible contamination that could be introduced to your sample by undergoing the same preparation steps as your sample. The LCS recovery indicates if we recovered more or less of a known spiked amount of the tested analyte. The MATRIX SPIKE (MS) and MATRIX SPIKE DUPLICATE (MSD) are more of an indication on a single sample’s interferences and should not be applied to the whole analytical batch. Spike recovery and reproducibility of sample analysis are indicated. The following breaks down what to look for in each QC sample:
Blank—Most of our blank criteria fits either < RL or < ½ RL. When you see a number that is not immediately preceeded by a < sign you know there is contamination in your sample. Your results should be qualified. If there is 10 times the blank result in your sample it will not have a qualifier because the EPA finds that level of contamination to be acceptable. MDL and RL listed are SVL’s default values. LCS—The LCS (Laboratory Control Sample) is the most useful of all the batch QC, it shows you how well a lab can recovery a known amount of spike in a pristine medium (de-ionized water or an inert solid material). LCSs show how well the analyst prepared the analytical batch. You can take the +/- % Recovery based on the LCS True column and apply it to your sample result. The adjusted value will give you a better idea if you are potentially above your action level. We say “potentially” because there is still a scientifically acceptable range around the reported result that the LCS recovery may not account for.
MS—The MS(Matrix Spike) is QC that really only applies to the source sample spiked. Poor recovery shows that there is an interferent that is not allowing the instrument to indicate the true value of the source sample. MSs are measured by an acceptance range around a true or known value. Labs use MS data to show if their prepatory methods work on real world samples.
MSD—MSDs (Matrix Spike Duplicate) are actually more useful than an MS because the results show that the lab has the skills necessary to repeat like measurements. You will have an acceptance range and an RPD column. The RPD shows reproducibility, any value under 20 is acceptable.
Notes and Definitions Page—Our qualifiers are explained on this page. The abbreviations from the Notes column are defined here. Qualifiers explain when there is a deviation from normal procedures or testing ranges. Qualifiers are a way for the lab to explain what was going on with your sample at anytime after we receive them. Sometimes an explanation is outside of the qualifiers available to us or may not be acceptable by your state, in this situation we will have to write a case narrative (which shows up on first page of the report).
It depends on what you are testing for and the action levels associated with your needs. Drinking Water analysis is different than waters under the Clean Water Act, which is different than requirements under SW-846 program. So you need to determine what action level you need to be above or below.
For Drinking Water you can visit: www.epa.gov/dwstandardsregulations
For NPDES under the Clean Water Act it is a little more involved you will have to work in conjunction with your regulatory permit writer to know where your action levels are. The following document is a good start: www3.epa.gov/npdes/pubs/final_local_limits_guidance.pdf
For solid waste under The Resource Conservation and Recovery Act there are no set levels other than hazardous waste determinations. Determinations are used to assign where waste will go (landfill or repository). The hazardous waste program is complicated to navigate. Start at the EPA’s Hazardous Waste home page: www.epa.gov/hw. Most of this program is based upon guidelines, the department issuing a permit will assign your action level.
Smell: Before you do anything, you should run the water for a few minutes (this will help with determining where the smell is coming from). If the smell is coming from all of your water faucets then there is an issue with your water supply. Most smells come from the presence of bacteria or metals (iron, copper, and lead) in your water.
If odor comes from one faucet and it goes away after a few minutes (or not) then you have plumbing-related issues. There is somewhere in your plumbing where the water flow is not smooth and organic material has allowed bacteria to grow, or the pipes are made from a material that releases material slowly and is trapped until it builds up to a level you can detect by scent. If the smell is detectable in the hot water side only, then you potentially have an issue with the magnesium rod in your hot water heater. For these issues you will need a plumber.
Looks: This is almost always due to sedimentation or scale (the material that collects on the insides of the pipes bringing water to your house). Sedimentation comes when water systems are flushed by the water provider (or well system) or when the water system is not used very often, allowing the sedimentation to build up. Opening your faucets and letting them flush out will usually take care of the problem. The release of scaling is due to a change in the chemistry (pH) of your water and will continue to look funny for a longer period of time. We can test your water for the amounts of sediment present and also its pH. If there is a change, you need to inform your water provider because contaminants can be released into your water.
Taste: is the most difficult to determine because of the natural amount of minerals in the water and the additives that your water provider may add to your water (such as chlorine). Taste is an individual preference—what bothers you may not bother others. Most tastes are actually a reaction to the odor of the water (the tests for which are mentioned above). Bacteria, algae, metals, and chlorine can all lead to different tastes in your water. There are many filtration systems available that will change the flavor of your water by removing the above (which may also remove potential contaminants).