“It’s the lit­tle things that kill”–Bush

OK… The band Bush may not be the great­est rock band ever, but their lyric above is indeed remark­able. In foren­sic sci­ence, it is very true that it is the lit­tle things that kill.

Three of the most often over­looked and hid­den sources of ana­lyt­i­cal error are the “lit­tle things:”  (1)  Pipet­ting, (2)  use of Vol­u­met­ric glassware/flasks, and (3)  mass mea­sure weights/balances.

(The above video is sim­ply the best sin­gle video after scour­ing the inter­net that graph­i­cally demon­strates error by improper tech­nique of pipettes and improper use of vol­u­met­ric glassware/flasks).

There is also the proper han­dling of mass mea­sure­ments and stan­dards.  Even the sim­ple deposit of oil by using one’s hands can com­pro­mise the valid­ity of the weight itself.  Per NIST Hand­book 105–1, all sur­faces shall be smooth and free of scratches.

Mass Weight mea­sures hid­den source of error in foren­sic science

The proper method of han­dling these impor­tant vol­u­met­ric and mass mea­sures used in foren­sic sci­ence is impor­tant, but does not con­sti­tute the com­plete pic­ture.  It is cru­cial to note that there are dif­fer­ent types of classes of Vol­u­met­ric glassware/flasks and mass weights.  The best and most pre­cise of these are trace­able to NIST stan­dards and are trace­able to ISO/IEC 17025 lab­o­ra­to­ries. One cuh exam­ple is a des­ig­na­tion known as “Type A glassware”.

There are typ­i­cally three sources of weight clas­si­fi­ca­tions used in the United States:

• National Insti­tute of Stan­dards and Tech­nol­ogy (NIST) Class F;
• ASTM (orig­i­nally known as the Amer­i­can Soci­ety for Test­ing and Mate­ri­als) E 617–97, Classes 0 through 7; and
• Inter­na­tional Orga­ni­za­tion of Legal Metrol­ogy or Organ­i­sa­tion Inter­na­tionale de Metrolo­gie Legale (OIML) R111, classes: E1, E2, F1, F2, M1, M2, M3.

Accord­ing to balances.com which is one provider of mass measures:

OIML E2 and Ulti Mass Class U Mass Stan­dards.  These mass stan­dards pro­vide the great­est pre­ci­sion we offer.  They are ideal for use as ref­er­ence stan­dards to cal­i­brate other weights or for cal­i­brat­ing high-resolution ana­lyt­i­cal bal­ances.  Ulti Mass weights are iden­ti­cal to E2 weights except for the nom­i­nal value mark­ings.  E2 weights have no mark­ings on the weights, many cus­tomers pre­fer the weights be marked with the nom­i­nal value.  The OIML E2 tol­er­ance is approx­i­mately two times the pre­ci­sion of ANSI/ASTM Class 1.

ANSI/ASTM Class 1 can be used as ref­er­ence stan­dard in cal­i­brat­ing other weights and appro­pri­ate for cal­i­brat­ing high-precision ana­lyt­i­cal bal­ances (from 0.01mg to 0.1mg).

ANSI/ASTM Class 2 Appro­pri­ate for cal­i­brat­ing high-precision topload­ing bal­ances with read­abil­i­ties rang­ing from 0.001g to 0.01g.

ANSI/ASTM Class 3 Appro­pri­ate for cal­i­brat­ing bal­ances with mod­er­ate pre­ci­sion, rang­ing from 0.01g to 0.1g.

ANSI/ASTM Class 4 Appro­pri­ate for stu­dent use and semi-analytical weighing.

NIST Class F Used to test com­mer­cial weigh­ing devices by state and local weights and mea­sures offi­cials, device installers and ser­vice tech­ni­cians. Class F weights may be used to test most accu­racy class III scales, all scales of class III L or IIII, and scales not marked with a class designation.

Each mass mea­sure and other stan­dard device such as Pipettes, Vol­u­met­ric glassware/flasks, and mass mea­sure weights/balances should have a report of trace­abil­ity.  For exam­ple in mass mea­sures, these reports should identify:

• Tol­er­ance class
• Type of construction
• NIST test number
• Date of applic­a­ble Sar­to­rius pri­mary standard(s)
• Sec­ondary set number
• Includes technician’s signature
• Descrip­tion of the weight(s)
• Test Num­ber
• Ser­ial Number
• Nom­i­nal Value
• Uncer­tainty of the mea­sure­ment tolerance
• Infor­ma­tion on weigh­ing procedures
• Date the pri­mary stan­dards were last tested at NIST and the NIST test number
• Date and test num­ber for the cal­i­bra­tion place’s work­ing standards.
• State­ment of com­pli­ance to ANSI/NCSL Z540 and ANSI/ISO/IEC 17025
• NVLAP Accred­i­ta­tion Logo and lab­o­ra­tory code.
• State­ment of trace­abil­ity to the Inter­na­tional Pro­to­type Kilo­gram (IPK) this meets the inter­na­tional require­ment for trace­abil­ity to the Inter­na­tional Sys­tem of Units.

In addi­tion, there should be cer­tifi­cates of cal­i­bra­tion.  One pos­si­ble way to deter­mine whether not a weight is one that has a cer­i­tifi­cate of cal­i­bra­tion is if there is a “C” at the end of the part num­ber.  The Cer­tifi­cate of Cal­i­bra­tion includes every­thing found in a Report of Trace­abil­ity listed above with the addi­tional infor­ma­tion listed below:

• 

  A Sam­ple of a Cer­tifi­cate of Calibration

• Uncer­tainty value
• Test con­di­tions tem­per­a­ture, baro­met­ric pres­sure, humidity.
• Con­ven­tional Mass Correction
• Uncer­tainty of the measurement
• Iden­ti­fi­ca­tion of the bal­ance used in the calibration
• as found value if an adjust­ment is performed
• assumed den­sity of the weight
• mate­r­ial weight is made from
• Pass/Fail result of mag­netic testing
• Vol­ume of the weight
• Cubi­cal Coef­fi­cient of expansion
• True Mass correction

When we look at these “lit­tle things” we have to insist upon their pre­ci­sion and the accu­racy as they are indeed the foun­da­tion of proper mea­sure­ment.  There are many dif­fer­ent pos­si­ble trace­able sources of cer­ti­fi­ca­tion and/or trace­abil­ity back to stan­dards includ­ing the National Insti­tute of Stan­dards & Tech­nol­ogy (NIST), the Amer­i­can Soci­ety for Test­ing & Mate­ri­als (ASTM), the Inter­na­tional Orga­ni­za­tion of Legal Metrol­ogy (OIML) and ISO 17025. Inter­na­tion­ally, there is accred­i­ta­tion by the National Vol­un­tary Lab­o­ra­tory Accred­i­ta­tion Pro­gram (NVLAP) and the United King­dom Accred­i­ta­tion Ser­vice (UKAS), for­merly known as NAMAS.

What we have to remem­ber is the old lab­o­ra­tory adage:  Quan­ti­ta­tive analy­sis is clean analysis.