Page:NIOSH Manual of Analytical Methods - Chapter C.pdf/6

 d. Blind Samples

Blind samples are prepared by someone other than the analyst performing the measurement and are to provide an independent check on the accuracy and precision of the measurement.

If blind samples cannot be prepared with confidence, their use should be avoided. In these cases, confusion may result when discrepancies occur and it will not be possible to say for certain whether the measurement of the blind sample was in error.

The results should be used in conjunction with control charting techniques to identify errors or malfunctions in the system [2,12,13]. To accomplish this goal, quality assurance personnel should work closely with laboratory personnel to quickly identify and eliminate trouble spots.

It will not always be possible to isolate the source of error in the results of a blind sample. In these instances, it should be recognized that it will not be possible to defend quantitative results for that particular sample set; therefore, reporting of results where these discrepancies occur should be avoided.

e. Recovery Studies

Recovery Studies should be performed as part of the measurement whenever the analyte of interest must be liberated or separated from the sampling media. The analayte of interest should be added to the media at levels consistent with the field samples. These "spiked" samples should then be treated in the same manner as the field samples. Corrections for recovery should be made whenever the measured recovery is significantly different from 100%. Even if recovery has historically been 100%, recovery studies can be used as additional analytical and calculation checks. It is often helpful for a laboratory to maintain a record of past recovery studies so that current data may be compared for discrepancies. Sample for which estimated recovery is less than 75% should be reported as "semiquantative."

f. Duplicates

Duplicate preparations of bulk materials are useful as an indication of the uniformity of the bulk material. Duplicate injections or measurements from air samples are of lesser importance since preparations from air samples are generally uniform in nature. True duplicates of air samples are useful as an an indication of both the reproducibility of the entire sampling and measurement method and as an indication of the uniformity of the atmosphere being sampled.

5. INTERLABORATORY TESTING

Assignable analytical uncertainty can be considered as consisting of interlaboratory and intralaboratory 1/15/98