Mineral Resources On-Line Spatial Data
Frequently Asked Questions
- What is the difference between "L" and "N" qualifiers?
For many users of these data, there may not be a real difference between "L" and "N" qualifiers. In all of the determinations by different analytical methods, the qualifiers "<", "L", and "N" can be defined as "less than the value given" in the associated element field. Some analytical methods, analysts, or laboratories preferred to use an "N" and others preferred to use an "L".
The only analytical determinations for which there was a distinct and precise difference between "L" and "N" values were the older multielement optical emission spectrography methods. In optical emission spectrophotometry, the sample was placed in a small carbon electrode and burned (usually in a direct-current arc). The light emitted from burning the sample was passed through a grating or a prism to produce a spectrum (or series of spectral lines) that was then recorded photographically on a glass plate. The intensity of each spectral line on the glass plate represents the concentration of one or more elements in the sample. The analyst would then visually compare the spectral lines with standards of known concentration to determine the unknown amounts of each element in the sample. When the spectral line for a specific element was missing, the analyst would record an "N" for "not detected" and the lowest standard value (i.e., In one optical emission spectrography method the lowest standard value for gold (Au) was 20 parts-per million (ppm). Thus if the spectral line for gold was missing, the analyst would record the concentration of gold as "20N"). When the spectral line for a specific element was present but at an intensity that was weaker than the lowest standard value, the analyst would record an "L" for "detected but less than the lowest standard ( also know as the lower determination limit)" and the lowest standard value (i.e., "20L" ppm Au).
For elements of great interest (like gold, silver, copper, lead, arsenic, etc.) but whose lower determination limits were quite high, values designated with an "L" were considered more significant than those with an "N". Note that many modern gold mines have average gold concentrations less than 20 ppm Au and so a value of "20L" ppm gold could be very significant.
With the advent of more modern quantitative analytical methods with much lower determination limits, the convention of distinguishing "N" and "L" values was discontinued.
- Why are there multiple lower determination limit values for an element within a single analytical method?
There are several reasons for differences in lower determination limits. The four most common reasons are as follows:
- What appears to be a single analytical method within the NGDB is actually a general category of closely related methods. Within a category the methods all use the same type of analytical instrumentation but they differ by specific instrument brand or set up, by variations in the analytical procedures, by laboratory, by location, by analyst, and over time. In general all of the methods in a category provide comparable analytical results but the most obvious difference is seen in the variation of their lower determination limits.
- There are a number of different factors that affect lower determination limits for an element within a specific method. These factors include instrumental limitations, sample matrix differences, interference from other elements, speed of analysis, and instrumental set up for multielement analytical packages. Thus the reported lower determination limit for a specific element is often set by a compromise of these various factors. In many cases, an analyst may be able to lower the determination limit for a specific desired element but this may take more time and effort and often raises the lower determination limits for some other elements reported at the same time. When geologists work closely with analysts on research projects, an analytical method may be tweaked to provide better results for elements critical to that particular project.
- Element interference is a large factor in determining lower determination limits. Very high concentrations of one or more elements often interfere with the determination of other elements. For example, in the optical emission spectrography methods, samples with high concentrations of iron often had problems due to interference such that some elements would be reported with higher lower determination limits and other elements could not even be determined. Often the lower determination limits were automatically doubled in samples of ores containing high concentrations of various metals. Some of the more recent computerized methods actually calculate a different lower determination limit for each element in each sample based on the amount of cross element interference. Thus samples analyzed for zinc within the same analytical batch could have qualified values ranging from <6 ppm Zn to <277 ppm Zn.
- Another factor that contributes to variations in lower determination limits within the NGDB is human error. In the early days of computerized geochemical databases, each of the analytical determinations were recorded by hand on a form. The original form was filed and copies were sent to the sample submitter and to a keypuncher for entry into the database. Thus common transcription errors (mistranslations from handwritten numbers, excess numbers, extra zeros, reversed numbers, missing or misplaced decimal points, etc.) have been found and fixed in the database. It is certain that more such errors exist. Additionally, many analysts would often simply record qualified values with just a "L" or "N" and not bother to write out the associated lower determination limit value. Thus it was up to the user or data entry person to determine and enter the correct associated value. In many cases, these qualified values were entered into as 0 (zero). Many qualified determinations of "0N" or "0L" can be found in the NGDB. Efforts are underway to research and replace these 0 values but this is a large and time intensive task.