Page:NIOSH Manual of Analytical Methods - 2018.pdf/5

ALIPHATIC ALDEHYD ES: METHO D 2018, Issue 1, dated 15 March 2003 - page 5 of 10 EVALUATION OF METHOD: The backup data report for this method development contains the data on recovery and breakthrough stud ies for eac h of th e alde hydes.[1] This method was evaluated with Supelco S10 LpDNPH sam plers and free (underivatized) acetaldehyde, propionaldehyde, valera ldehyde, an d isov aleraldehyde. Recoveries were determined after fortification of six sam plers in each set with known quantities of free aldehyde (see Table 5). W hile acetaldehyde, propionaldehyde and valeraldehyde showed average recoveries greater than 90%, average recoveries of isovaleraldehyde in Table 5 were in the 80% range; the reason for these relatively low recoveries is unclear. For sto rage stu dies, s ix sam plers in each set were fortified with 3.00-µg of free aldehyde and stored at 5 oC in the dark (see Table 6). Also, liquid standards (standard solutions of aldehyde-DNPH de rivatives in acetonitrile) were stored at 5 °C in the dark in vials with airtight caps (see Table 6). Limits of detection and quantitation were determined by least squares calculations with series of s ix standard solutions of aldehydeDN PH d erivatives in acetonitrile (see T able 4). This method also was evaluated for butyraldehyde-DNPH and free butryaldehyde on Supelco S10 LpDNPH samplers. Recoveries decreased from 98% to 79% when quantities applied increased from 1.5 :g to 20 :g. The negative slope caus ed difficulty or amb iguity in applying recove ry factors to quantities found. This method is not re com m end ed fo r butyraldehyde. Crotonaldehyde and acrolein were also studied. The recoveries of crotonaldehyde (crotonaldehyde-DNPH) and acrolein (acrolein-DNPH ) were found to be very low (less than 30%). This method is not recomm ended for either of these co m pou nds [12]. In each bre ak through stu dy, a pair of sam plers in series was connecte d to an air pump. A glass U-tube (a Schwa rtz drying tube) preceded the front sampler. For dry air, an impinger of dried, indicating silica gel preceded the U-tube. For humid air, an impinger of water preceded the U-tube. Known quantities of free aldehyde in solution were placed into the U-tube, and the pump drew air through the system at 1 L/min. Upper limits of the method for acetaldehyde and propionaldehyde were calculated as two-thirds of the smallest quantities of aldehyde placed into the U-tube which gave rise to 5% breakthrough. For these two aldehydes, breakthrough took p lace earlier in hum id air than in dry air or laboratory air; this humidity effect was reversed in the c ase of form aldehyde [13]. How ever, quantities of valeraldehyde and isovaleraldehyde generally did not break through the front samplers, and excessive quantities of valeraldehyde and isovaleraldehyde remained on the front samplers without reaction with D NP H. T hus , upper lim its of the method for valeraldehyde and isovaleraldehyde could not be determined from quantities of aldehyde which ga ve rise to 5% breakthrough. Upper limits of valeraldehyde and isovaleraldehyde were calculated as two-thirds of the largest quantities of aldehydes place d into the Utube which gave rise to a mas s balance or a n ear m ass balanc e (see Ba ckup Data R eport [1]). Since the sampler does not have a backup section for determination of breakthrough, the worker conducting sampling in the field may connect two samplers in series in the cases of acetaldehyde and propionaldehyde. The back pressure of the sampling train will be higher and a lower flow rate may be required. Use of backup sam plers in the field for valeraldehyde and isovaleraldehyde wou ld be m eaningless bec ause ex cessive quantities of these aldehydes are tra ppe d on the fro nt sam pler without re action with DNPH and breakthrough to backup samplers may never occur. Alternatively, sampling without a backup sampler may be conducted even whe n high con cen trations of aldehydes and ketones are anticipated if the sampling period is minimal and the flow rate of the pump is low. Although this method m ay be useful for determ ining aroma tic aldehydes in the air, it would be very difficult or im possible to evaluate th is m eth od with free (u nderivatized) aro m atic aldehydes. A rom atic aldehydes, such as benzaldehyde, undergo rap id oxidation in air to form the corresponding carboxylic acids, su ch as benzo ic acid. The capability to separate valeraldehyde-DNPH from isovaleraldehyde-DNPH by HPLC was investigated. Twenty microliters of a single acetonitrile solution containing valeraldehyde-DNPH at 2 µg/mL and isovaleraldehyde-DNPH at 1 µg/mL (equivalent concentrations of und erivatized alde hydes) was inje cted into NIOSH Manual of Analytical Methods (NMAM), Fourth Edition