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ISOTEC INC.
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Source for STABLE ISOTOPES Isotec Inc. offers the scientific community one of the world's largest selections of stable isotopes and labelled compounds Ό N cD • Amino Acids • Fatty Acids • Sugars • Substrates (for breath tests and other metabolic studies) • > 99% chemical purity • High isotopic enrichment • Competitive prices • Fast delivery Plus . . . metal stable isotopes, helium-3 and other noble gas isotopes, other enriched stable isotopes, multiply-labelled compounds and custom synthesis Call or write for your NEW complete price list today! INSTRUMENTATION Figure 2. Photomicrographs of laser-induced craters for (left) free-running and (right) Q-switched laser pulses (Nd:YAG laser, 1064 nm). > 1 pulse/s to establish steady-state analytical signals. In Q-switching, the efficiency or quality factor, Q, of the laser cavity is initially main- tained at a very low level for typical- ly a few hundred microseconds. This inhibits laser amplification, allowing the population inversion of the Nd !+ to be maximized. At this point the ef- ficiency of the cavity is "switched" from low to high, thereby tapping the maximum energy available in the la- ser medium in a very short period of time. The result is a much shorter la- ser pulse, on the order of 10 ns, con- taining nearly all of the energy. In terms of output power delivered within the laser pulse, an increase of approximately 10 4 is achieved by Q-switching (13). In the absence of Q-switching, oth- erwise referred to as "free-running" or "fixed Q" operation, the envelope of the laser pulse is about 120-150 μβ in duration. Detailed examination of this long pulse has shown the pres- ence of an irregular sequence of sharp, narrow pulses, each less than a microsecond wide, that dampen out with time {13). Because the energy contained in the pulse is delivered over a much longer time period than that produced by Q-switched opera- tion, the power is considerably less. These two pulse modes of a Nd: YAG laser can lead to significantly different analytical outcomes. The physical characteristics of the sample "crater," or spot, of Q-switched and free-running lasers are significantly different. Consequently, the two pulse modes can be used to achieve very different sampling objectives. Both modes can be executed as ei- ther a single pulse or a series of pulses. A single laser pulse provides a transient signal typically having a full width at half maximum (fwhm) of -5-10 s (i, 2), although shorter times are possible (8). On the other hand, multiple pulsing can be used to produce a steady-state signal analo- gous to that obtained with continu- ous solution aspiration (2, 3). The pulse mode and the repetition rate both affect not only the intensity and duration of the analytical signal, but also the size and shape of the sample crater. A single free-running pulse produces a deep, narrow crater, whereas the single Q-switched pulse creates a wider, more shallow crater. Photomicrographs of such craters are shown in Figure 2. Several factors may contribute to the differences observed. In the Q-switched mode, the intense plasma generated early in the pulse (1-2 ns) can absorb some of the laser energy itself. This laser-induced plasma then transfers energy to the sample (5). As a result of this secondary en- ergy transfer, the dimensions of the sample crater are determined to a larger extent by the dimensions of the plasma rather than by the diam- eter of the focused laser beam. Thus effective spot size may be several times greater than the diameter of the laser beam itself. In the free-running mode, little or no plasma is generated (5). The free- running laser pulse is, in fact, an en- velope of many pulses interacting with the sample for a much longer time than the Q-switched pulse. 452 A · ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991 Make THE SOURCE Your You Get More Front THE SOURCE ISOTEC INC. A Matheson, USA Company 3858 Benner Road, Miamisburg, OB 45342 513 859-1808 (800)448-9760 Telex: 288278 fax: (513) 8594878 CIRCLE 63 ON READER SERVICE CARD
Transcript of ISOTEC INC.
Source for STABLE ISOTOPES Isotec Inc. offers the scientific community one of the world's largest selections of stable isotopes and labelled compounds
Ό N cD
• Amino Acids • Fatty Acids • Sugars • Substrates (for
breath tests and other metabolic studies)
• > 99% chemical purity • High isotopic enrichment • Competitive prices • Fast delivery
Plus . . . metal stable isotopes, helium-3 and other noble gas isotopes, other enriched stable isotopes, multiply-labelled compounds and custom synthesis Call or write for your NEW complete price list today!
INSTRUMENTATION
Figure 2. Photomicrographs of laser-induced craters for (left) free-running and (right) Q-switched laser pulses (Nd:YAG laser, 1064 nm).
> 1 pulse/s to establish steady-state analytical signals. In Q-switching, the efficiency or quality factor, Q, of the laser cavity is initially maintained at a very low level for typically a few hundred microseconds. This inhibits laser amplification, allowing the population inversion of the Nd ! +
to be maximized. At this point the efficiency of the cavity is "switched" from low to high, thereby tapping the maximum energy available in the laser medium in a very short period of time. The result is a much shorter laser pulse, on the order of 10 ns, containing nearly all of the energy. In t e rms of ou tput power delivered within the laser pulse, an increase of approximately 104 is achieved by Q-switching (13).
In the absence of Q-switching, otherwise referred to as "free-running" or "fixed Q" operation, the envelope of the laser pulse is about 120-150 μβ in duration. Detailed examination of this long pulse has shown the presence of an i r regula r sequence of sharp, narrow pulses, each less than a microsecond wide, that dampen out with time {13). Because the energy contained in the pulse is delivered over a much longer time period than that produced by Q-switched operation, the power is considerably less.
These two pulse modes of a Nd: YAG laser can lead to significantly different analytical outcomes. The physical characteristics of the sample "crater," or spot, of Q-switched and free-running lasers are significantly different. Consequent ly , the two pulse modes can be used to achieve very different sampling objectives.
Both modes can be executed as either a single pulse or a series of pulses. A single laser pulse provides a transient signal typically having a full width at half maximum (fwhm) of -5 -10 s ( i , 2), although shorter times are possible (8). On the other hand, multiple pulsing can be used to produce a steady-state signal analogous to that obtained with continuous solution aspiration (2, 3).
The pulse mode and the repetition rate both affect not only the intensity and duration of the analytical signal, but also the size and shape of the sample crater. A single free-running pulse produces a deep, narrow crater, whereas the single Q-switched pulse creates a wider, more shallow crater. Photomicrographs of such craters are shown in Figure 2.
Several factors may contribute to the differences observed. In the Q-switched mode, the intense plasma generated early in the pulse (1-2 ns) can absorb some of the laser energy itself. This laser-induced plasma then transfers energy to the sample (5). As a result of this secondary energy transfer, the dimensions of the sample crater are determined to a larger extent by the dimensions of the plasma rather than by the diameter of the focused laser beam. Thus effective spot size may be several times greater than the diameter of the laser beam itself.
In the free-running mode, little or no plasma is generated (5). The free-running laser pulse is, in fact, an envelope of many pulses interacting with the sample for a much longer time than the Q-switched pulse.
452 A · ANALYTICAL CHEMISTRY, VOL. 63, NO. 8, APRIL 15, 1991
Make THE
SOURCE Your
You Get More Front
THE
SOURCE
ISOTEC INC. A Matheson, USA Company
3858 Benner Road, Miamisburg, OB 45342 513 859-1808 (800)448-9760
Telex: 288278 fax: (513) 8594878
CIRCLE 63 ON READER SERVICE CARD
